Patent Description:
Polytetrafluoroethylene is an attractive material due to one or more properties such as inertness to many chemicals, biocompatibility, thermal stability, low surface energy, low coefficient of friction, and the ability to be processed into a variety of form factors, such as membranes, fibers, tubes, and the like. Expanded polytetrafluoroethylene (ePTFE) may be used alone or used in composites and/or laminates to produce articles for use in a variety of applications. Many of these applications may benefit from using materials that are lighter, thinner, stronger, and/or have improved optical properties. As such, there is an ever present need to provide ePTFE articles with improved properties. <CIT> relates to a balloon catheter comprising a catheter shaft having a longitudinal axis and an inflatable balloon affixed to said shaft, said balloon having an uninflated length which remains relatively unchanged upon inflation and comprising at least two helically oriented layers oriented at a balanced force angle. <CIT> relates to a method of forming and mounting a distensible balloon catheter, and sleeves used to construct such balloons. One embodiment of the method comprises forming a distensible balloon sleeve with non-distensible ends. The sleeve is mounted on the catheter shaft at the non-distensible ends while retaining the distensibility of the balloon as a whole. Alternatively or additionally, the balloon may be mounted to the catheter shaft using non-distensible tape that also retains the distensibility of the operative portion of the balloon.

The present invention is defined by an expanded polytetrafluoroethylene (ePTFE) membrane according to claim <NUM>. Preferred embodiments are claimed in the dependent claims.

Area Weighted Fibril Width (AWFVη: wAWFW (nm).

Area Weighted Fibril Width was calculated utilizing the following Equation: <MAT>.

Specific Surface Area (SSA) <MAT> (m<NUM>/g) was calculated with the following Equation: <MAT> where:.

Specific Surface Area (based on wm) (m<NUM>/g) was calculated with the following Equation: <MAT>.

Specific Surface Area (based on wAWFW) (m<NUM>/g) was calculated with the following Equation: <MAT>.

Areal Density (Mass per area) (g/m<NUM>):.

Area Ratio (AR) was calculated by the following Equation: <MAT>.

Persons skilled in the art will readily appreciate that various aspects of the present disclosure can be realized by any number of methods and apparatus configured to perform the intended functions. It should also be noted that the accompanying figures referred to herein are not necessarily drawn to scale, but may be exaggerated to illustrate various aspects of the present disclosure, and in that regard, the drawing figures should not be construed as limiting.

In addition, the terms "adjacent" and "adjacent to" as used herein are meant to denote that when an element is "adjacent" to another element, the element may be directly adjacent to the other element or intervening elements may be present. As used herein, the singular forms "a", "an", and "the" include plural reference unless the context clearly dictates otherwise. The term "on" as used herein is meant to denote that when an element is "on" another element, it can be directly on the other element or intervening elements may also be present. It is to be appreciated that the terms "fine powder" and "powder" may be used interchangeably herein. Also, the terms "ePTFE membrane(s)" and "membrane(s)" may be used interchangeably herein. Further, in this application, the term "ePTFE membrane" is meant to include a single layer or multiple layers of ePTFE membrane(s). It is to be understood that the machine direction and the longitudinal direction are the same and may be interchangeably used herein. In addition, the terms "microporous ePTFE membrane" and "ePTFE membrane" may be used interchangeably herein.

In one aspect, the present invention is directed to thin, self-supporting biaxially oriented polytetrafluoroethylene (ePTFE) membranes that have a high crystallinity index, high intrinsic strength, low areal density (i.e., lightweight), and high optical transparency. The ePTFE membrane has a crystallinity index of at least about <NUM>%, a matrix modulus of at least about <NUM> GPa at a temperature of <NUM> and a matrix tensile strength of at least about <NUM> MPa in the machine direction. The ePTFE membrane may also have an areal density less than about <NUM>/m<NUM> and a total luminous transmittance of at least <NUM>%. In addition, the ePTFE membrane is transparent or invisible to the naked eye. Further, the ePTFE membrane is stackable, which may be used to control permeability, pore size, and/or bulk mechanical properties. The ePTFE membrane may be used to form composites, laminates, fibers, sheets, tubes, or other three-dimensional objects, which may or may not be subsequently diced or otherwise cut or sectioned into smaller portions. Additionally, the biaxially oriented ePTFE membranes may be used in filtration applications. In another aspect, the biaxially oriented ePTFE membrane may be further uniaxially expanded, which aligns the fibrils in one direction (hereafter a uniaxially oriented ePTFE membrane). Such an ePTFE membrane may have a tenacity greater then about <NUM> Newtons grams per <NUM> meters (Ng/<NUM>)(<NUM> grams force per denier (gf/d)) and a bulk denier less than about <NUM> grams per <NUM> meters (g/<NUM>).

With polytetrafluoroethylene (PTFE) polymers, the particle size, shape, and distribution thereof are important to obtain desired porous structures. These particle characteristics affect the packing density as well as connection density, thereby affecting the porous structures that can be produced from the particles. The PTFE resin is provided in a particulate form, for example, in the form of a fine powder. PTFE fine powders are formed of primary particles.

In forming the ePTFE membrane, the PTFE fine powder is first mixed with a lubricant, such as a light mineral oil. One particular example of a suitable lubricant is an isoparaffinic hydrocarbon, such as ISOPAR™ K (ExxonMobil Chemical, Spring, TX). Other suitable lubricants include aliphatic hydrocarbons, aromatic hydrocarbons, halogenated hydrocarbons, and the like, and are selected according to flammability, evaporation rate, and economic considerations. It is to be appreciated that the term "lubricant", as used herein, is meant to describe a processing aid that includes (or consists of) an incompressible fluid that is not a solvent for the polymer at the process conditions. The fluid-polymer surface interactions are such that it is possible to create a homogenous mixture. It is also to be noted that that choice of lubricant is not particularly limiting and the selection of lubricant is largely a matter of safety and convenience. The lubricant may be added to the PTFE powder in an amount from about <NUM>/kg to about <NUM>/kg.

In at least one embodiment, the PTFE fine powder and lubricant are mixed so as to uniformly or substantially uniformly distribute the lubricant with the PTFE powder. It is to be appreciated that various times and mixing methods may be used to distribute the PTFE powder in the lubricant. Once the lubricant and PTFE powder are sufficiently distributed, the lubricated powder is compressed into a cylindrical form (i.e., a pellet). The pellet may then be ram extruded (e.g., typically called paste extrusion or paste processing when lubricant is present) through an extruder die to produce a cohesive, flexible PTFE tape. As used herein, the term "cohesive" is meant to describe a tape that is sufficiently strong for further processing. The ram extrusion occurs below the melting temperature PTFE polymer (e.g., below <NUM>). The tape formed has an indeterminate length and a thickness less than about <NUM>, less than about <NUM>, less than about <NUM>, or less than about <NUM>. The cohesive, flexible tapes are referred hereafter simply as "tape".

In a subsequent step, the lubricant is removed from the tape. In instances where ISOPAR™ K is the lubricant, the tape may be heated to about <NUM>. In other embodiments, the lubricant may be removed by washing the tape in hexane or other suitable solvent. If the lubricant is of sufficient volatility, the lubricant may be removed without a washing step, or it may be removed by heat and/or vacuum. It is to be appreciated, however, that any conventional drying method may be used.

The tape is then expanded in the longitudinal and transverse directions simultaneously (i.e., biaxially expanded). As used herein, the terms "biaxially expanded", "biaxial expansion", and "biaxially oriented" are meant to describe a polymer, membrane, preform, or article that is expanded in at least two orthogonal directions such that the fibrils are substantially oriented in-plane. In one embodiment, the tape is subsequently expanded only in the machine direction (i.e., uniaxially expanded). As used herein, the term "uniaxial", "uniaxially oriented" or "uniaxial expansion" is meant to describe a polymer, membrane, preform, or article that is expanded in only one direction (e.g., either the machine direction (MD) or the transverse direction (TD)). The expansion may be conducted with or without heat at strain rates up to about up to about <NUM>,<NUM> %/second, up to about <NUM>,<NUM> %/second, up to about <NUM>,<NUM> %/second, up to about <NUM>,<NUM> %/second, up to about <NUM> %/second, up to about <NUM> %/second, up to about <NUM> %/second, up to about <NUM> %/second, up to about <NUM> %/second, up to about <NUM> %/second, up to about <NUM> %/second, up to about <NUM> %/second, up to about <NUM> %/second, up to about <NUM> %/second, up to about <NUM> %/second, up to about <NUM> %/second, or up to about <NUM> %/second. Additionally, the tape may be expanded (with or without heat) from about <NUM>%/second to about <NUM>,<NUM> %/second, from about <NUM> %/second to about <NUM>,<NUM> %/second from about <NUM> %/ second to <NUM>,<NUM> %/ second, from about <NUM> %/second to about <NUM>,<NUM> %/ second, from about <NUM> %/ second to about <NUM> %/second, from about <NUM> %/second to about <NUM> %/second, from about <NUM> %/ second to about <NUM> %/ second, from about <NUM> %/ second to about <NUM> %/ second, from about <NUM> %/ second to about <NUM> %/ second, from about <NUM> %/ second to about <NUM> %/ second, from about <NUM> %/ second to about <NUM> %/ second, from about <NUM> %/ second to about <NUM> %/ second, from about <NUM> %/ second to about <NUM> %/ second, from about <NUM> %/ second to about <NUM> %/ second, from about <NUM> %/ second to about <NUM> %/ second, from about <NUM> %/ second to about <NUM> %/ second, or from about <NUM> %/ second to about <NUM> %/ second. It is to be appreciated that an increase in intrinsic strength concurrently occurs upon expansion. The increase in intrinsic strength of the PTFE polymer is dependent upon the strength of the tape prior to expansion, the quality of the PTFE resin (e.g., particle size, molecular weight, distribution of particle size and/or molecular weight, degree of crystallinity, composition of polymer, and the like), the temperature at which expansion is performed, the rate of expansion, and/or the total amount of expansion.

The tape is biaxially and in some embodiments, additionally uniaxially expanded, to form an ePTFE membrane. The tape may be expanded at the same or different strain rates and at the same or different temperatures to achieve the microporous ePTFE membrane. As used herein, the term "microporous" is meant to define an article, such as a membrane, that has pores that are not visible to the naked eye. It has been discovered that the material properties of an ePTFE membrane produced in this manner exceed comparative properties of conventional membranes through an efficient and complete conversion of the PTFE primary particles (i.e., PTFE fine powder) into fibrils. Advantageously, the ePTFE membranes discussed herein retain the properties of conventional ePTFE membranes, such as, but not limited to, chemical inertness, thermal stability, low surface energy, low coefficient of friction, biocompatibility, and a wide range of use temperatures. The ePTFE membrane may optionally be heat treated at a temperature up to about <NUM>. Uniaxially stretching the ePTFE membrane creates an ePTFE membrane with uniaxially-oriented fibrils, a high crystallinity index, and a high matrix tensile strength in the direction in which it was stretched (i.e., the machine direction (MD) or the transverse direction (TD)). Hereafter, the ePTFE membrane is described with respect to expansion in the machine direction, but it is to be appreciated that expanding in the transverse direction is considered to be within the purview of the invention.

The biaxially oriented ePTFE membrane is very thin, and may have a total membrane thickness less than about <NUM>, less than about <NUM>, less than about <NUM>, less than about <NUM>, less than about <NUM>, less than about <NUM>, less than <NUM>, less than <NUM>, less than <NUM>, less than about <NUM>, less than about <NUM>, less than about <NUM>, less than about <NUM>, less than about <NUM>, less than about <NUM>, less than about <NUM>, less than about <NUM>, less than about <NUM>, or less than about <NUM>. The biaxially oriented ePTFE membrane may be formed to have a total membrane thickness from about <NUM> to about <NUM>, from about <NUM> to about <NUM>, from about <NUM> to about <NUM>, from about <NUM> to about <NUM>, from about <NUM> to about <NUM>, from about from about <NUM> to about <NUM>, from about <NUM> to about <NUM>, from about <NUM> to about <NUM>, from about <NUM> to about <NUM>, or from about <NUM> to about <NUM>.

In at least one embodiment, the biaxially oriented ePTFE membrane has a thickness per layer less than about <NUM>, less than about <NUM>, less than about <NUM>, less than about <NUM>, less than about <NUM>, less than about <NUM>, or less than about <NUM>, less; than about <NUM>, less than about <NUM>, less than about <NUM>, less than about <NUM>, less than about <NUM>, less than about <NUM>, less than about <NUM>, less than about <NUM>. In some embodiments, the ePTFE membrane has a thickness per layer from about <NUM> to about <NUM>, from about <NUM> to about <NUM>, from about <NUM> to about <NUM>, from about <NUM> to about <NUM>, from about <NUM> to about <NUM>, from about <NUM> to about <NUM>, from about <NUM> to about <NUM>, from about <NUM> to about <NUM>, from about <NUM> to about <NUM>, from about <NUM> to about <NUM>, from about <NUM> to about <NUM>, from about <NUM> to about <NUM>, from about <NUM> to about <NUM>, or from about <NUM> to about <NUM>. The biaxially oriented ePTFE membrane, unlike conventional ePTFE membranes, is so thin as to be invisible to the naked eye.

The "invisibility" of the biaxially oriented ePTFE membranes is also at least partially due to the fibril microstructure of the ePTFE membranes. Generally, the fibrils are substantially cylindrical in shape. The term "substantially cylindrical" as used herein is meant to denote that the fibrils in the biaxially oriented ePTFE membranes have an aspect ratio in cross section of about <NUM>:<NUM> to about <NUM>:<NUM>. In addition, the fibrils in the biaxially oriented ePTFE membranes are thin, and have a median fibril width that is not greater than about <NUM>. In some embodiments, the median fibril width is less than about <NUM>, less than about <NUM>, less than about <NUM>, less than about <NUM>, less than about <NUM>, less than about <NUM>, or less than about <NUM>. In some embodiments, the median fibril width is from about <NUM> to about <NUM>, from about <NUM> to about <NUM>, from about <NUM> to about <NUM>, from about <NUM> to about <NUM>, from about <NUM> to about <NUM>, from about <NUM> to about <NUM>, or from about <NUM> to about <NUM>. In some embodiments, the median fibril width is from about <NUM> to about <NUM>, from about <NUM> to about <NUM>, from about <NUM> to about <NUM>. In other embodiments, the median fibril width is from about <NUM> to about <NUM>, from about <NUM> to about <NUM>, from about <NUM> to about <NUM>, from about <NUM> to about <NUM>, or from about <NUM> to about <NUM>. The intersection or overlap of two or more fibrils are called "crossover points" herein. In some embodiments, the thickness of a biaxially oriented ePTFE membrane may be the thickness of the crossover point of two fibrils.

In addition, the biaxially oriented ePTFE membranes are extremely light, having an areal density per layer less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>) or less than about <NUM>/m<NUM> (<NUM>/m<NUM>). In some embodiments, the areal density per layer is about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>), from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>), from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>), from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>), from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>), from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>), from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>), from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>), from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>), from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>), from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>), from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>), from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>), from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>), from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>), from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>) or from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>). In some embodiments, the areal density per layer is from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>), from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>), or from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>).

Further, the biaxially oriented ePTFE membrane has an area ratio from about <NUM>:<NUM>:<NUM> to about <NUM>,<NUM>,<NUM>:<NUM>. In some embodiments, the biaxially oriented ePTFE membrane has an area ratio from about <NUM>,<NUM>:<NUM> to about <NUM>,<NUM>,<NUM>:<NUM>, from about <NUM>,<NUM>:<NUM> to about <NUM>,<NUM>,<NUM>:<NUM>, from about <NUM>,<NUM>:<NUM> to about <NUM>,<NUM>,<NUM>:<NUM>, from about <NUM>,<NUM>:<NUM> to about <NUM>,<NUM>,<NUM>:<NUM>, from about <NUM>,<NUM>:<NUM> to about <NUM>,<NUM>,<NUM>:<NUM>, from about <NUM>,<NUM>:<NUM> to about <NUM>,<NUM>,<NUM>:<NUM>, from about <NUM>,<NUM>:<NUM> to about <NUM>,<NUM>,<NUM>:<NUM>, from about <NUM>,<NUM>,<NUM>:<NUM> to about <NUM>,<NUM>,<NUM>:<NUM>, or from about <NUM>,<NUM>,<NUM>:<NUM> to about <NUM>,<NUM>,<NUM>:<NUM>.

Additionally, the biaxially oriented ePTFE membranes may have a total areal density less than about <NUM>/m<NUM>, less than about <NUM>/m<NUM>, less than about <NUM>/m<NUM>, less than about <NUM>/m<NUM>, less than about <NUM>/m<NUM>, less than about <NUM>/m<NUM>, less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>) less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), or less than about <NUM>/m<NUM> (<NUM>/m<NUM>). In some embodiments, the biaxially oriented ePTFE membranes have a total areal density from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM>, from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM>, from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM>, from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM>, from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM>, from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>), from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>), from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>), from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>), from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>), from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>), from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>), from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>), from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>), from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>), from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>), from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>), from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>), from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>) or from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM>.

The uniaxially oriented ePTFE membranes are also extremely light, having an areal density per layer less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), less than about <NUM>/m<NUM> (<NUM>/m<NUM>), or less than about <NUM>/m<NUM> (<NUM>/m<NUM>). In some embodiments, the areal density is from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>), from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>), from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>), from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>), from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>), from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>), from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>), from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>), from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>), from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>), from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>),from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>), from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>), from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>), or from about <NUM>/m<NUM> (<NUM>/m<NUM>) to about <NUM>/m<NUM> (<NUM>/m<NUM>).

Despite being thin and lightweight, the ePTFE membranes that are biaxially expanded possess high intrinsic strength properties. The ePTFE membranes have a matrix tensile strength (MTS) of at least about <NUM> MPa in both the longitudinal and transverse directions, at least about <NUM> MPa, at least about <NUM> MPa, at least about <NUM> MPa, at least about <NUM> MPa, at least about <NUM> MPa, at least about <NUM> MPa, or at least about <NUM> MPa in both the longitudinal and transverse directions. In at least one embodiment, the biaxially oriented ePTFE membranes have a matrix tensile strength (MTS) from about <NUM> MPa to about <NUM> MPa, from about <NUM> MPa to about <NUM> MPa, from about <NUM> MPa to about <NUM> MPa, from about <NUM> MPa to about <NUM> MPa, from about <NUM> MPa to about <NUM> MPa, from about <NUM> MPa to about <NUM> MPa, or from about <NUM> MPa to about <NUM> MPa in both the longitudinal and transverse directions.

Additionally, ePTFE membranes having been additionally uniaxially expanded have even higher intrinsic strength properties. The ePTFE membranes have a matrix tensile strength (MTS) greater than about <NUM> MPa in the machine direction, greater than about <NUM> MPa in the machine direction, greater than about <NUM> MPa in the machine direction, greater than about <NUM> MPa in the machine direction, greater than about <NUM> MPa in the machine direction, or greater than about <NUM> MPa in the machine direction. In some embodiments, the uniaxially oriented ePTFE membrane has a matrix tensile strength from about <NUM> MPa to about <NUM> MPa in the machine direction, from about <NUM> MPa to about <NUM> MPa in the machine direction, from about <NUM> MPa to about <NUM> MPa in the machine direction, from about <NUM> MPa to about <NUM> MPa in the machine direction, or from about <NUM> MPa to about <NUM> MPa in the transverse direction. It is to be appreciated that although the matrix tensile strength is given herein for the machine direction, it is equally applicable for ePTFE membranes expanded in the transverse direction.

In addition, uniaxially oriented ePTFE membrane has a matrix storage modulus of at least <NUM> GPa at ambient temperature (i.e., about <NUM>). In some embodiments, the uniaxially oriented ePTFE membrane has a matrix storage modulus at ambient temperature (i.e., about <NUM>) from about <NUM> GPa to about <NUM> GPa, from about <NUM> GPa to about <NUM> GPa, from about <NUM> GPa to about <NUM> GPa, from about <NUM> GPa to about GPa, from about <NUM> GPa to about <NUM> GPa, from about <NUM> GPa to about <NUM> GPa, from about <NUM> GPa to about <NUM> GPa, from about <NUM> GPa to about <NUM> GPa, from about <NUM> GPa to about <NUM> GPa, from about <NUM> GPa to about <NUM> GPa, or from about <NUM> GPa to about <NUM> GPa. The uniaxially oriented ePTFE membrane also has a bulk denier less than about <NUM>/<NUM>. In some embodiments, the uniaxially oriented ePTFE membrane has a bulk denier from about <NUM>/<NUM> to about <NUM>/<NUM>, from about <NUM>/<NUM> to about <NUM>/<NUM>, from about <NUM>/<NUM> to about <NUM>/<NUM>, from about <NUM>/<NUM> to about <NUM>/<NUM>, from about <NUM>/<NUM> to about <NUM>/<NUM>, from about <NUM>/<NUM> to about <NUM>/<NUM>, from about <NUM>/<NUM> to about <NUM>/<NUM>, from about <NUM>/<NUM> to about <NUM>/<NUM>, from about <NUM>/<NUM> to about <NUM>/<NUM>, from about <NUM>/<NUM> to about <NUM>/<NUM>, from about <NUM>/<NUM> to about <NUM>/<NUM>, from <NUM>/<NUM> to about <NUM>/<NUM>, from about <NUM>/<NUM> to about <NUM>/<NUM>, from about <NUM>/<NUM> to about to about <NUM>/<NUM>, from about <NUM>/<NUM> to about <NUM>/<NUM>, from about <NUM>/<NUM> to about <NUM>/<NUM>, or from about <NUM>/<NUM> to about <NUM>/<NUM>.

In addition, the uniaxially oriented ePTFE membranes have a tenacity of at least about <NUM> Ng/<NUM> (<NUM> gf/d). In some embodiments, the uniaxially oriented ePTFE membranes have a tenacity from about <NUM> Ng/<NUM> (<NUM> gf/d) to about <NUM> Ng/<NUM> (<NUM> gf/d) from about <NUM> Ng/<NUM> (<NUM> gf/d) to about <NUM> Ng/<NUM> (<NUM> gf/d), or from about <NUM> Ng/<NUM> (<NUM> gf/d) to about <NUM> Ng/<NUM> (<NUM> gf/d). Further, the uniaxially oriented ePTFE membranes have a <P2> orientation greater than or equal to <NUM>.

In addition, biaxially oriented ePTFE membranes have little air resistance. In some embodiments, the air resistance of the ePTFE membrane may be less than about <NUM>,<NUM> Pa•s/m, less than about <NUM>,<NUM> Pa•s/m, less than about <NUM>,<NUM> Pa•s/m, less than about <NUM>,<NUM> Pa•s/m, less than about <NUM>,<NUM> Pa•s/m, less than about <NUM>,<NUM> Pa•s/m, less than about <NUM> Pa•s/m, less than about <NUM> Pa•s/m, less than about <NUM> Pa•s/m, less than about <NUM> Pa•s/m, less than about <NUM> Pa•s/m, less than about <NUM> Pa•s/m, less than about <NUM> Pa•s/m, or less than about <NUM> Pa•s/m. In some embodiments, the air resistance is from about <NUM> Pa•s/m to about <NUM> Pa•s/m, from about <NUM> Pa•s/m to about <NUM> Pa•s/m, from about <NUM> Pa•s/m to about <NUM> Pa•s/m, from about <NUM> Pa•s/m to about <NUM> Pa•s/m, from about <NUM> Pa•s/m to about <NUM> Pa•s/m, from about <NUM> Pa•s/m to about <NUM> Pa•s/m, or from about <NUM> Pa•s/m to about <NUM> Pa•s/m. The low air resistance in combination with the high surface area of the ePTFE membrane provides for a high performance filtration device.

The biaxially oriented ePTFE membranes are also highly optically transmissive, with a total luminous transmittance (measured from <NUM> to <NUM>) that is greater than or equal to about <NUM>%, greater than or equal to about <NUM>%, greater than or equal to about <NUM>%, greater than or equal to about <NUM>%. In exemplary embodiments, the biaxially oriented ePTFE membrane may have a total luminous transmittance from about <NUM>% to about <NUM>%, from about <NUM>% to about <NUM>%, or from about <NUM>% to about <NUM>%. In some embodiments, the ePTFE membrane has a total luminous transmittance of nearly <NUM>%.

The fibrils of the ePTFE membrane (biaxially oriented and uniaxially oriented) may optionally be coated such that the ePTFE is porous or such that the ePTFE is non-porous with at least one coating composition, such as, but not limited to, a polymer or a biologic coating. A coating composition can be applied to the ePTFE membrane by any conventional coating method such as solvent coating, spray coating, spin coating, vapor deposition, atomic layer deposition (ALD), or dip coating. Additionally, a coating may be applied to the ePTFE membrane by applying compression with heat between sheets of a component, such as, but not limited to, fluorinated ethylene propylene (FEP), polyfluoroacrylate (PFA) and silicone.

In some embodiments, the coating composition occupies or fills at least a portion of the spaces through the thickness of the biaxially or uniaxially oriented ePTFE membrane. Suitable polymers and/or biologic coatings that may be coated and/or imbibed on or into the ePTFE membrane include, but are not limited to, polyesters; polystyrene; polyamides; polyphthalamides; polyamideimides; polycarbonates; polyethersulphones; polysulfones; polyphenylenesulfides; liquid crystalline polymers; polyetherketones; polyetheretherketones; polysiloxanes; epoxies; polyurethanes; polyimides; polyetherimides; polyacrylates; polyparaxylylene; terpolymers of tetrafluoroethylene (TFE), VDF (vinylidenefluoride), and HFP (hexafluoropropylene); copolymers of tetrafluoroethylene (TFE) and perfluoroalkylvinylethers (PAVEs); a copolymer of tetrafluoroethylene and perfluoro-<NUM>,<NUM>-dimethyl-<NUM>,<NUM>-dioxole; perfluoroalkylvinylethers; perfluoroalkylethers; polyvinylidenefluoride (PVDF); ethylene tetrafluoroethylene (ETFE); polychlorotrifluoroethylene (PCTFE); fluorinated ethylene propylene (FEP); perfluoroalkoxyalkanes (PFA), polyvinyl alcohol (PVA), CBAS®/Heparin coating (commercially available from W. Gore & Associates, Inc. ), antimicrobial agents, antibodies, pharmaceuticals, biologic entities, vascularization stimulators, and any combination thereof. The amount of coating applied will be dependent upon the desired application.

The biaxially or uniaxially oriented ePTFE membranes are self-supporting, and in some embodiments the ePTFE membrane is used to reinforce a polymer film such as porous polymers, non-porous polymers, fluoropolymers, polyolefins, films, tapes, and other membranes. By "self supporting" is it meant that the ePTFE membrane does not require a backing or carrier layer. However, because the ePTFE membrane is so thin, the edges of the ePTFE membrane are often restrained at macroscale lengths. In other words, the ePTFE membrane is restrained around the perimeter of the membrane (e.g., "picture framed") to maintain the integrity of the ePTFE membrane. The intrinsic strength of the membrane is connected across distances and holds itself together without a backing or supporting layer behind or under the membrane.

The biaxially oriented and uniaxially oriented ePTFE membranes may be formed as a single layer of ePTFE membrane. In other embodiments, the biaxially and uniaxially oriented ePTFE membranes may have tens, hundreds, or thousands of layers of ePTFE membrane in the ePTFE membrane. In some embodiments, <NUM> layers to <NUM> layers may be present. In other embodiments, <NUM> layers to <NUM> layers may be present in the ePTFE membrane. In further embodiments, <NUM> layers to <NUM> layers, <NUM> layers to <NUM>,<NUM> layers, <NUM> layers to <NUM>,<NUM> layers, <NUM> layers to <NUM>,<NUM> layers, <NUM> layers to <NUM>,<NUM> layers, <NUM> layers to <NUM>,<NUM> layers, <NUM> to <NUM>,<NUM> layers, <NUM> to <NUM>,<NUM> layers, <NUM> to <NUM>,<NUM>,<NUM> layers (or more) may be present in the ePTFE membrane. Although not wishing to be bound by theory, it is believed that the only limiting factor for the number of ePTFE layers present in the ePTFE membrane is the time spent to stack the layers and expand. Typically, the ePTFE membrane stack "grows" four-fold each time the ePTFE membrane is biaxially expanded. It is to be appreciated that no adhesive or other bonding agent is typically used to connect the individual ePTFE membranes in the stacked ePTFE membranes, although the inclusion of an adhesive or other bonding material is not precluded from use herein and is considered to be within the purview of the invention.

In another embodiment, the ePTFE membrane (both biaxially oriented and uniaxially oriented) may include ePTFE membranes having the same mechanical properties, ePTFE membranes having differing mechanical properties, and/or a spacing layer (e.g. a different polymeric layer such as, a porous polymer, a non-porous polymer, a fluoropolymer, a porous polyolefin, or a non-porous polyolefin. In other words, the ePTFE membrane may be engineered to include different polymeric and/or non-polymeric layer(s) in the ePTFE membrane. Additionally, one ePTFE membrane layer may differ from another ePTFE layer by the amount of expansion and/or strain rate, and/or total work it has been through. By varying the membrane type, expansion, mechanical properties of any additional layers within a ePTFE membrane, the ePTFE membrane may be formed to meet specific bulk properties while maintaining transport, filtration, or separation requirements.

Through the formation of the ePTFE membranes (biaxially and uniaxially oriented) and/or the optional additional spacing layers within the ePTFE membrane, the permeability, the pore size, and bulk mechanical properties may be controlled. As used herein, the term "permeability" means the ability to transmit fluids (i.e., liquid or gas) through the pores of a membrane or filter material when the material is subjected to a differential pressure across it. In one instance, the ePTFE membranes allow for a range of pore sizes, such as, for example, pore sizes less than about <NUM> microns in diameter. As used herein, the term "pore size" means the size of the pores in ePTFE membrane. The pore size may range from about <NUM> to about <NUM> microns. Additionally, the ePTFE may have a specific surface area (SSA) as measured by area weighted fibril width (AWFW) from about <NUM><NUM>/g to about <NUM><NUM>/g, from about <NUM><NUM>/g to about <NUM><NUM>/g, from about <NUM><NUM>/g to about <NUM><NUM>/g, from about <NUM><NUM>/g to about <NUM><NUM>/g, from about <NUM><NUM>/g to about <NUM><NUM>/g, from about <NUM><NUM>/g to about <NUM><NUM>/g, from about <NUM><NUM>/g to about <NUM><NUM>/g, from about <NUM><NUM>/g to about <NUM><NUM>/g, or from about <NUM><NUM>/g to about <NUM><NUM>/g.

In some embodiments, the biaxially oriented ePTFE membranes may be used in air filtration applications. In such applications, the ePTFE membrane demonstrates a quality factor at a face velocity of <NUM> with a <NUM> micron diameter challenge particle of at least <NUM> (kPa-<NUM>). It is to be appreciated that the strength to weight ratio (intrinsic strength) of the ePTFE membrane is higher than that of conventional ePTFE membranes. Higher quality factor values are associated with better filtration performance. In certain embodiments, the biaxially oriented ePTFE membrane may have a quality factor from about <NUM> (kPa-') to about <NUM> (kPa-<NUM>), from about <NUM> (kPa-') to about <NUM> (kPa-<NUM>), from about <NUM> (kPa-<NUM>) to about <NUM> (kPa-<NUM>), from about <NUM> (kPa-<NUM>) to about <NUM> (kPa-<NUM>), from about <NUM> (kPa-<NUM>) to about <NUM> (kPa-<NUM>), from about <NUM> (kPa-<NUM>) to about <NUM> (kPa-<NUM>), from about <NUM> (kPa-') to about <NUM> (kPa-<NUM>), from about <NUM> (kPa-') to about <NUM> (kPa-<NUM>), from about <NUM> (kPa-') to about <NUM> (kPa-<NUM>), from about <NUM> (kPa-') to about <NUM> (kPa-<NUM>), from about <NUM> (kPa-') to about <NUM> (kPa-<NUM>), or from about <NUM> (kPa-') to about <NUM> (kPa-').

The biaxially oriented ePTFE membranes may be used in applications where it is desirable to filter nanoparticles (e.g., from about <NUM> to about <NUM>) from a liquid media, even when the liquid media is traveling at high flow rate. Thus, the ePTFE membrane may be used as a filtration material, and by the nature of polytetrafluoroethylene, is resistant to chemical attack, is biocompatible, and demonstrates a high matrix tensile strength (MTS). The filterable matrix may be selected from a solution, a suspension, a colloid, a biological fluid, a component of a biological fluid, an aqueous material, or a nonaqueous material. To filter a filterable matrix, the matrix is passed through the ePTFE membrane and the resulting filtrate is collected. In one embodiment, the biaxially oriented ePTFE membrane includes a nanoparticle retention percent (%) equal to or greater than the line defined by Equation (<NUM>). <MAT> where.

The non-contact thickness of the membranes was measured using a KEYENCE LS-<NUM> laser system (commercially available from KEYENCE America).

Samples were cut to form square sections <NUM> by <NUM>. Each sample was weighed using a MettlerToledo AT20 balance. Using the thickness calculated by the KEYENCE laser, the densities of the samples were calculated using Equation (<NUM>). <MAT> where:.

To determine the MTS of biaxial ePTFE membranes, a sample ePTFE membrane was cut in the longitudinal and transverse directions using an ASTM D412-Dogbone Die Type F (D412F). To determine MTS of uniaxial membranes, a sample ePTFE membrane was loaded in the longitudinal direction. Tensile break load was measured using an INSTRON® <NUM> (Illinois Tool Works Inc. , Norwood, MA) tensile test machine equipped with flat-faced grips and a "<NUM> lb" (-<NUM> N) load cell. The gauge length for the grips was set to <NUM> and the strain rate was <NUM>/s. After placing the sample in the grips, the sample was retracted <NUM> to obtain a baseline followed by a tensile test at the aforementioned rate. The peak force measurements were used for the MTS calculation. The longitudinal and transverse MTS were calculated using Equation (<NUM>): <MAT>.

To determine the MTS of uniaxial ePTFE membranes, a sample ePTFE was loaded in the longitudinal direction using cord and yarn grips. Tensile break load was measured using an INSTRON® <NUM> (Illinois Tool Works Inc. , Norwood, MA) tensile test machine equipped with cord and yard grips and a "<NUM> lb" (-<NUM> N) load cell. The gauge length for the grips was set to <NUM> and the strain rate was <NUM>/s. After placing the sample in the grips, the sample was retracted <NUM> to obtain a baseline followed by a tensile test at the aforementioned rate. The peak force measurement was used for the MTS calculation.

Low voltage STEM (scanning transmission electron microscopy) is a technique used to visualize thin samples by accelerating a focused beam of electrons through the sample and collecting the transmitted electrons with a suitable detector. Low voltage refers to the use of a beam accelerating voltage of less than 100kV (<30kV as exemplified herein). The image contrast is due to differences in the electron absorption by the membrane due to either composition or thickness.

A scanning electron microscope (Hitachi, SU8000; Hitachi Ltd, Tokyo, Japan) with a transmission adapter (STEM) was used and operated at an accelerating voltage no higher than 30kV. No prior or additional treatment (staining) was applied to the samples. The samples for the analysis of the thin porous films were prepared on a copper grid (PELCO® Center-Marked Grids, <NUM> mesh, Copper, Product # 1GC400, Ted Pella Inc. , Redding, CA) with a carbon support layer (Carbon Type-B, <NUM> mesh, Copper, Product # <NUM>, Ted Pella, Inc.

Two dimensional (<NUM>-d) x-ray diffractograms were obtained using the X27C beamline of the National Synchrotron Light Source at Brookhaven National Laboratory (Upton, NY). The beamline provided a well collimated, monochromatic x-ray beam with a wavelength of <NUM>, a nominal flux of <NUM><NUM> photons/s, and a diameter of <NUM>. The detector was a Rayonix MAR-CCD <NUM>-d Image System (Rayonix LLC, Evanston, IL). The system was set with a sample-detector distance of <NUM> and calibrated using a Al<NUM>O<NUM> powder standard. The samples were mounted between the beam and detector and the transmission geometry scattered/diffracted x-ray image collected for between <NUM> and <NUM> seconds. In addition, a background image without the sample present was recorded for the same time period immediately after each sample was imaged. The background image was then subtracted from the sample image to remove the effect of air scattering and create the desired diffractogram.

The wide-angle x-ray scattering experiments were carried out on a Xenocs brand Xeuss <NUM> SAXS/WAXS Laboratory Beamline system (Xenocs SAS, Sassenage, France). The instrument uses a GeniX3D Cu ka source (<NUM> wavelength) operating at <NUM> kV and <NUM> mA, and a Dectris brand Pilatus <NUM> detector (Dectris Ltd. , Baden-Daettwil, Switzerland). The beam was collimated with <NUM> in-line slits, each open to an area of <NUM> x <NUM>. The sample-detector distance was <NUM> (calibrated by lanthanum hexaboride standard). The "Virtual Detector" feature of the Xeuss <NUM> system was used to erase blind spots in the detector as well as to extend its angular range. This is achieved by translating the detector in the horizontal direction, then averaging multiple scans. Here, <NUM> scans were taken at varying horizontal detector offsets, each with exposure times of <NUM> minutes. Averaging these four scans provided a scattering profile. The orientation was quantified from the I vs. ϕ Azimuthal scans utilizing Equation (<NUM>).

As <P2> approaches <NUM>, nearly perfect orientation in the machine direction is achieved as determined by Equation (<NUM>).

The crystallinity index was obtained by peak fitting of Intensity vs. q scans using JMP® <NUM>. <NUM> statistical analysis software (SAS institute). The range of integration was limited to q = <NUM> to <NUM> (nm-<NUM>), and a linear background was defined such that it coincided with the measured intensity from about q = <NUM> to <NUM> (nm-<NUM>) and q = <NUM> to <NUM>(nm-<NUM>). The Pearson VII function was used to fit both peaks after the linear background was subtracted.

As defined in <CIT>, the crystallinity index was calculated from the area under the fitted <NUM> crystalline peak (A<NUM>) and the area under the fitted amorphous peak (Aamorphous) according to the following Equation (<NUM>): <MAT>.

The bubble point was measured according to the general teachings of ASTM F31 <NUM>-<NUM> using a Capillary Flow Porometer (Model CFP <NUM> AE from Porous Materials, Inc. , Ithaca, N. The sample membrane was placed into a sample chamber and wet with SilWick Silicone Fluid (commercially available from Porous Materials, Inc. ) having a surface tension of <NUM> N/m (<NUM> dynes/cm). The bottom clamp of the sample chamber consists of a <NUM> micron porous metal disc insert (Mott Metallurgical, Fannington, Conn. ) with the following dimensions (<NUM> diameter, <NUM> thickness). The top clamp of the sample chamber consists of an opening, <NUM> in diameter. Using the Capwin software version <NUM>. <NUM>, the following parameters and set points were used:.

The values presented for bubble point were the average of two measurements.

The ATEQ airflow test measures laminar volumetric flow rates of air through membrane samples. Each membrane sample was clamped between two plates in a manner that seals an area of <NUM><NUM> across the flow pathway. An ATEQ® (ATEQ Corp. , Livonia, MI) Premier D Compact Flow Tester was used to measure airflow rate (L/hr) through each membrane sample by challenging it with a differential air pressure of <NUM> kPa (<NUM> mbar) through the membrane.

A Textest FX <NUM> Air Permeability Tester device manufactured by Textest AG (Zurich, Switzerland) was used to test airflow resistance. The Frazier permeability reading is the rate of flow of air in cubic feet per square foot of sample area per minute at a differential pressure drop across the test sample of <NUM> water column. Where noted the pressure drop was reduced for the characterization of lightweight unsupported membranes. Air permeability was measured by clamping a test sample into a circular, flanged fixture which provided a circular opening of <NUM> diameter (<NUM><NUM> area). The upstream side of the sample fixture was connected to a flow meter in line with a source of dry compressed air.

Optical transmittance measurements were carried out using a spectrophotometer (Jasco V-<NUM>; JASCO Deutschland GmbH, Pfungstadt, Germany) with double-beam integrating sphere attachment (<NUM> diameter, ILN-<NUM>). The spectrophotometer is comprised of a deuterium & tungsten-halogen lamps, a single Czerny-Turner type monochromator (<NUM> line/mm diffraction grating) and a photomultiplier tube (PMT) detector. The light from the monochromator is split into a sample and reference beam before entering the integrating sphere. The integrating sphere is configured for unidirectional illumination and diffuse detection. The sample beam illuminates, at normal incidence, a <NUM> x <NUM> sample mounted on the integrated sphere entrance port; while the reference beam passes through an open port on the integrating sphere. The sample and reference beams are alternately incident upon the PMT detector and converted to a digital signal after being subjected to synchronous rectification.

The monochromator bandwidth was set to <NUM> and the grating wavelength was scanned from <NUM> to <NUM> at a scan rate of <NUM>/min. The source was changed from the deuterium to the tungsten-halogen lamp at <NUM>. The signal was recorded at intervals of <NUM>. A 'dark correction' spectrum (blocking the sample beam) and a 'baseline correction' spectrum (sample beam passes through an open port) was collected: these spectra were used to report the transmittance spectrum expressed as a percentage of the incident light.

The total luminous transmittance was calculated by weighting the transmittance spectrum by a CIE Standard Illuminant and CIE Standard Colorimetric Observer (see ASTM D1003-<NUM>: Standard Test Method for Haze and Luminous Transmittance of Transparent Plastics. ) The D65 illuminant and the <NUM><NUM>-deg standard observer were used in the calculation presented here. The % Transmittance in the UVA and UVB ranges were calculated by computing the average % Transmittance for the <NUM>-<NUM> and <NUM>-<NUM> wavelength ranges, respectively.

Selected samples were imaged by STEM and characterized manually by <NUM> measurements of the projected width of the fibrils (e.g., <FIG>). Uniform sampling was facilitated by using a random number generator to highlight <NUM> regions, then the operator would trace the outline of the nearest fibril - preferably a fragment of a fibril which has not already been characterized. In general, the marked fibril was rectangular in shape, with an aspect ratio greater than <NUM>. The nominal projected width of the fibril was calculated from the ratio of the area divided by the length of the object. This is thought to be more representative and informative than a single width measurement as it naturally forces the projected width measurement to be perpendicular to the major axis of the rectangular shape. To confirm the method, lines were drawn through the centroid of the manually identified region, orthogonal to the major axis at the calculated width as calculated according to Equation (<NUM>).

Measurement of the matrix storage and loss moduli was carried out using a TA Instruments Q800 system (TA Instruments, New Castle, DE) outfitted with tension sample clamps. The DMA was calibrated according to standard TA Instruments procedures. Samples dimensions were obtained using a 10x microscope with a <NUM> graduated reticule for width and a KEYENCE LS7010 high accuracy non-contact micrometer (Keyence Corp. , Itasca, IL) for thickness. The sample mass was measured using a Mettler-Toledo A120 micro-balance (Mettler-Toledo, LLC, Columbus, OH). The sample was then mounted in the instrument and a <NUM> mN pre-load applied. The sample length was obtained from the calibrated DMA clamp position at <NUM>. A sinusoidal strain with a true strain amplitude of <NUM> and frequency of <NUM> was applied with an additional constant load just sufficient to maintain the sample in tension throughout the imposed sinusoidal strain. The sample was equilibrated at -<NUM> for <NUM> minutes and then the temperature was ramped at <NUM>/min to <NUM>. The magnitude and phase angle of the resulting sinusoidal force acting on the sample was measured once a second throughout the heating ramp and used to calculate the storage and loss moduli. The desired matrix moduli were obtained by multiplying the storage and loss moduli by the ratio ρtrue/ρsample. ρtrue was assumed to be that of crystalline poly(tetrafluoroethylene), <NUM>/cm<NUM>, while ρsample was calculated from the measured sample dimensions and mass.

Particle Filtration Efficiency Membrane filtration efficiency testing was performed using dioctyl phthalate (DOP) aerosol on a TSI CERTITEST® Model <NUM> Automated Filter Tester (TSI Incorporated, St. Paul, Minn. ), according to the procedure specified in the CERTITEST® Model <NUM> Automated Filter Tester Operation and Service Manual. The sample test area was <NUM> and face velocity was <NUM>/sec.

The Quality Factor Qf was determined using Equation (<NUM>): <MAT>.

The penetration, P, is the fraction of particles penetrating or passing through the sample and Δp is the pressure drop in kPa at an air velocity of <NUM>/s. Higher quality factor values are associated with better filtration performance (see <NPL>)). Comparisons of the quality factor are made using the same face velocity and test aerosol particle size. Quality factor is defined in units of reciprocal pressure (kPa-<NUM>). The efficiency, E (%) = <NUM> * (<NUM>-P).

The bead test measures permeability and bead retention of the membrane sample. The membrane sample was restrained in a <NUM> filter holder. The membrane was first wetted with an isopropyl alcohol (IPA)-DI water solution (<NUM>:<NUM> v/v !PA:water). Air pressure was used to force this solution through the membrane. <NUM> grams of solution was flowed through the sample, followed by <NUM> grams of aqueous solution made of <NUM> % by volume of the nonionic surfactant TRITON™ X-<NUM> (<NPL>; Sigma Aldrich, St. Louis, MO) in DI water. The membrane was then challenged with a solution of <NUM> diameter polystyrene latex beads (Fluoro-Max R25 red fluorescent polymer microspheres; Thermo Fisher Scientific, Waltham, MA) dispersed in an aqueous solution made of <NUM> % by volume of TRITON™ X-<NUM> in DI water, such that the membrane was challenged with a quantity of beads sufficient to cover the membrane surface area with a single monolayer of beads. The concentration of the beads in the challenge solution and filtrate was determined using an Agilent Technologies Cary Eclipse Fluorescence Spectrophotometer (Agilent Technologies, Santa Clara, CA).

The permeability of the membrane was calculated using Equation (<NUM>): <MAT>.

In Equation (<NUM>), k is the permeability of the membrane, g is the mass of an aliquot of filtrate, A is the physical area of the membrane sample in the filter holder, t is the time require to collect the aliquot of filtrate, and P is the pressure differential across the membrane. In Equation (<NUM>), g/t is the mass flow rate through the membrane and g/At is the mass flux through the membrane.

The percent of beads in the solution that were retained by the membrane was calculated using Equation (<NUM>): <MAT>.

In Equation (<NUM>), Cchallenge is the concentration of beads in the challenge solution, and Cfiltrate is the concentration of beads in the filtrate.

Unless defined otherwise herein, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The present invention is further defined in the following Examples. It should be understood that these Examples, while indicating preferred embodiments of the invention, are given by way of illustration only.

The following example discloses the production of a single layer PTFE membrane having very low areal density (e.g., areal densities of less than <NUM>/m<NUM>).

Polytetrafluorethylene (PTFE) fine powder (E. DuPont de Nemours; Wilmington, DE) was blended with ISOPAR™ K isoparaffinic hydrocarbon lubricant (ExxonMobil Chemical; Spring, TX) at a target ratio of <NUM> per pound (~ <NUM>) of fine powder (<NUM> lube/g total) (grams lube/mass total mixture). The lubricated powder was compressed into a cylinder and was ram extruded at <NUM> to provide a tape. The tape was <NUM> wide and <NUM> thick. The ISOPAR™ K was removed by heating to approximately <NUM> to form dried tape (the "initial tape"). A <NUM> square was cut from the initial tape. The initial tape areal density (prior to pantograph expansion) was determined to be <NUM> grams per square meter (g/m<NUM>). As used herein, all initial tape areal densities are meant to denote <NUM> +/- <NUM>/m<NUM>. A summary of the process parameters used in Example <NUM> is provided in Table <NUM>.

Using a pantograph machine, the <NUM> square of dried tape was heated in an oven set to <NUM> (set point) for <NUM> seconds and then expanded in the longitudinal direction (machine direction (MD)) and transverse direction (TD) simultaneously (biaxial expansion) at a target ratio of about <NUM>:<NUM> in each direction while maintaining a temperature of about <NUM>. The average engineering strain rate target was set to <NUM>%/second. The pantograph opened at a constant velocity target for approximately <NUM> seconds. The ePTFE membrane was allowed to cool to room temperature (~<NUM>) under restraint of the pantograph.

A piece of the cooled ePTFE membrane from the first pass was harvested for further expansion, i.e. a "second pass. " Using the same pantograph machine, the selected membrane was heated in an oven set to <NUM> for a target of <NUM> seconds and then expanded in the longitudinal direction and transverse direction simultaneously at a target ratio of <NUM>:<NUM> in each direction while maintaining a temperature of about <NUM>. The average engineering strain rate target was set to <NUM>%/second. The pantograph opened at a constant velocity target for approximately <NUM> seconds. The second pass ePTFE membrane was allowed to cool to room temperature (~<NUM>) under restraint of the pantograph.

A piece of the cooled ePTFE membrane from the second pass was selected for further expansion, i.e. a "third pass. " Using the same pantograph machine, the selected membrane was again heated in an oven set to <NUM> for <NUM> seconds and then expanded in the longitudinal direction and transverse direction simultaneously at a target ratio of <NUM>:<NUM> in each direction while maintaining a temperature of about <NUM>. The constant acceleration rate set point was1%/second. The pantograph opened at a constant acceleration rate set point for approximately <NUM> seconds.

The ePTFE membrane was allowed to cool to room temperature (~<NUM>) under restraint of the pantograph. A summary of Example <NUM> process parameters is provided in Table <NUM>.

The cooled expanded ePTFE membrane from the third pass was harvested from the pantograph and placed onto (<NUM> x <NUM>) adhesive backed frames. Using the frame as a cutting guide, the ePTFE membrane was weighed and the average areal density was calculated to be <NUM>/m<NUM> with the lightest sample weighing <NUM>/m<NUM> (Table <NUM>). The area ratio is defined as the ratio of the areal density before and after the series of expansion operations. The ePTFE membrane from the third pass exhibited area ratios ranging from <NUM>,<NUM>:<NUM> to <NUM>,<NUM>:<NUM>; depending upon process conditions (Table <NUM>). <FIG> are from the same sample (<NUM>/m<NUM>; Sample E1G) at <NUM> different magnifications. No remaining primarily particles can be observed. <FIG> is from a second piece where the same strain path was used, while the oven was set to <NUM> (Sample E1H). A STEM image of Sample E1I is provided as <FIG>. Table <NUM> consolidates the process parameters.

The following examples disclose the production of an ePTFE membrane having very low areal density per layer with layering up to <NUM> layers and area ratios up to about <NUM> million to <NUM>.

PTFE fine powder (E. DuPont de Nemours) was blended with ISOPAR™ K isoparaffinic hydrocarbon lubricant at a target ratio of <NUM> per pound (∼<NUM>) of fine powder (<NUM> lube/g total) (grams lube/mass of total mixture). The lubricated powder was compressed into a cylinder and was ram extruded at <NUM> to provide a tape. The tape was <NUM> wide and <NUM> thick. The ISOPAR™ K was removed by heating the tape to approximately <NUM>. A <NUM> square was cut from the dry tape. A summary of the process parameters used in Example <NUM> is provided in Table <NUM>.

Using a pantograph machine, four squares of tape were heated in an oven set to <NUM> for <NUM> seconds and then expanded in the longitudinal direction and transverse direction simultaneously at a target ratio of about <NUM>:<NUM> in each direction while maintaining a temperature of about <NUM>. The average engineering strain rate target was set to <NUM>%/second. The pantograph opened at a constant velocity target for approximately <NUM> seconds. The ePTFE membrane was allowed to cool to room temperature (~ <NUM>) under restraint of the pantograph. Four pieces (<NUM> layers each) were harvested from the cooled ePTFE membrane and set aside for further expansion, i.e. a second pass. The first pass process was repeated <NUM> more time to create another <NUM> layers. Both <NUM> layer samples were combined to make a <NUM>-layer sample.

Using the same pantograph machine, after both stacks of <NUM> layers (<NUM> layers total) were heated in an oven set to <NUM> for a target of <NUM> seconds and then expanded in the longitudinal direction and transverse direction simultaneously at a target ratio of <NUM>:<NUM> in each direction while maintaining a temperature of about <NUM>. The average engineering strain rate target was set to <NUM>%/second. The pantograph opened at a constant velocity target for approximately <NUM> seconds. The ePTFE membrane was allowed to cool to room temperature (~<NUM>) under restraint of the pantograph.

Four samples (<NUM> layers each) were harvested from the cooled ePTFE membrane and layered (<NUM> total layers) for further expansion, i.e. a third pass. Using the same pantograph machine, the membrane was again heated in an oven set to <NUM> for a target of <NUM> seconds and then expanded in the longitudinal direction and transverse direction simultaneously at a target ratio of <NUM>:<NUM> in each direction while maintaining a temperature of about <NUM>. The simultaneous expansion was conducted at a constant acceleration rate set point of <NUM>%/s for a target ratio of <NUM>:<NUM> in each direction. The pantograph opened for approximately <NUM> seconds for the third pass. The ePTFE membrane was allowed to cool to room temperature (~<NUM>) under restraint of the pantograph.

A sample (<NUM> layers) was harvested from the cooled ePTFE membrane and layered (<NUM> total layers) for further expansion, i.e. a fourth pass. Using the same pantograph machine, the membrane was again heated in an oven set to <NUM> for a target of <NUM> seconds and then expanded in the longitudinal direction and transverse direction simultaneously at a target ratio of <NUM>:<NUM> in each direction while maintaining a temperature of about <NUM>. The simultaneous expansion was conducted at a constant acceleration rate set point set point of <NUM>%/s for a target ratio of <NUM>:<NUM> in each direction. The pantograph opened for approximately <NUM> seconds for the fourth pass. The expanded ePTFE membrane was allowed to cool to room temperature (~<NUM>) under restraint of the pantograph.

The ePTFE membrane was harvested from the machine onto (<NUM> x <NUM>) adhesive backed frames. Using the frame as a cutting guide, the ePTFE membrane was weighed to calculate an areal density of <NUM>/m<NUM>/layer, while the ePTFE membrane weighed <NUM>/m<NUM> (Sample E2A; Table <NUM>). Area ratios and areal density (for both ePTFE membrane and stacks of the ePTFE membrane) for other ePTFE membranes were set and are set forth in Table <NUM>.

Additionally three more ePTFE membranes were generated (E2B-D) for Example <NUM>, each consisting of <NUM> layers using the first <NUM> passes described above. Each ePTFE membrane was individually loaded for a fourth and final expansion. Using the same pantograph machine, the membrane was again heated in an oven set to <NUM> for a target of <NUM> seconds and then expanded in the longitudinal direction and transverse direction simultaneously at a target ratio of <NUM>:<NUM>(E2B), <NUM>:<NUM>(E2C) or <NUM>:<NUM>(E2D), in each direction while maintaining a temperature of about <NUM>. The simultaneous expansion was conducted at a constant acceleration rate set point of <NUM>%/s for Examples E2B-D. The pantograph opened for approximately <NUM> (E2B), <NUM> (E2C) or <NUM> (E2D) seconds for the fourth pass. At the end of each expansion (E2B-E2D), the expanded ePTFE membrane was allowed to cool to room temperature (~<NUM>) under restraint of the pantograph. The ePTFE membrane was harvested from the machine onto (<NUM> x <NUM>) adhesive backed frames. Using the frame as a cutting guide, the ePTFE membrane was weighed. Table <NUM> includes the fourth pass ratio settings, area ratio, areal density of the stack of ePTFE membranes, the areal density of each layer, and translation time during the final pass.

Additionally three more ePTFE membranes were generated (E2E-G) for Example <NUM>, primarily for observations of images from STEM (<FIG>).

Example E2E was processed using the same steps as Example E2D with the following <NUM> exceptions. The dwell time prior to expansion was reduced from <NUM> (E2D) to <NUM> (E2E) seconds prior to the third pass and the fourth pass target ratio was increased from a set point of <NUM>:<NUM> in both directions (E2D) to <NUM>:<NUM> in both directions (E2E) for an area ratio set point for the last pass. The pantograph opened for approximately <NUM> (E2E) seconds for the fourth pass. The expanded ePTFE membrane was allowed to cool to room temperature (~<NUM>) under restraint of the pantograph. The ePTFE membrane was harvested from the machine onto (<NUM> x <NUM>) adhesive backed frames.

Example E2F was processed using the same steps as Example E2E with the following <NUM> exceptions. The number of layers loaded for the fourth expansion was increased from <NUM> (E2E) to <NUM> (E2F) and the fourth pass target ratio was increased from a set point of <NUM>:<NUM> in each direction (E2E) to <NUM>:<NUM> in each direction (E2F). The expanded ePTFE membrane was allowed to cool to room temperature (~<NUM>) under restraint of the pantograph. The ePTFE membrane was harvested from the machine onto (<NUM> x <NUM>) adhesive backed frames.

Example E2G was processed using the same steps as Example E2E with the following <NUM> exceptions. The number of layers loaded for the second expansion was decreased from <NUM> (E2E) to <NUM> (E2G) and no fourth pass was used. The expanded ePTFE membrane was allowed to cool to room temperature (~<NUM>) under restraint of the pantograph. The ePTFE membrane was harvested from the machine onto (<NUM> x <NUM>) adhesive backed frames. Using the frame as a cutting guide, the ePTFE membrane was weighed to calculate an areal density of <NUM>/m<NUM>/layer, while the ePTFE membrane weighed <NUM>/m<NUM> (Sample E2G; Table <NUM>). Area ratios and areal density (for both ePTFE membrane and stacks of the ePTFE membrane) for these and other ePTFE membranes were calculated and are set forth in Table <NUM>.

The following example discloses the production of an ePTFE membrane having very low areal density per layer having up to <NUM> ePTFE layers and area ratios of up to nearly <NUM> million to <NUM>.

Using a pantograph machine, the four squares of tape were stacked upon each other and these four squares (layers) of tape were heated in an oven set to <NUM> (set point) for <NUM> seconds and then expanded in the longitudinal direction and transverse direction simultaneously at a target ratio of about <NUM>:<NUM> in each direction while maintaining a temperature of about <NUM>. The average engineering strain rate target was set to <NUM>%/second. The pantograph opened at a constant velocity target for approximately <NUM> seconds. The ePTFE membrane was allowed to cool to room temperature (~ <NUM>) under restraint of the pantograph. The first pass process was repeated <NUM> more time to create another <NUM> layers.

Using the same pantograph machine, both stacks of <NUM> layers (<NUM> layers total) were heated in an oven set to <NUM> for a target of <NUM> seconds and then expanded in the longitudinal direction and transverse direction simultaneously at a target ratio of <NUM>:<NUM> in each direction while maintaining a temperature of about <NUM>. The average engineering strain rate target was set to <NUM>%/second. The pantograph opened at a constant velocity target for approximately <NUM> seconds. The expanded membrane was allowed to cool to room temperature (~<NUM>) under restraint of the pantograph.

Four samples (<NUM> layers each) were harvested from the cooled ePTFE membrane and stacked (<NUM> total layers) for further expansion, i.e. a third pass. Using the same pantograph machine, the stacked ePTFE membrane layers were again heated in an oven set to <NUM> for a target of <NUM> seconds and then expanded in the longitudinal direction and transverse direction simultaneously at a target ratio of <NUM>:<NUM> (E3B) or <NUM>:<NUM> (E3A) in each direction while maintaining a temperature of about <NUM>. The simultaneous expansion was conducted at a constant acceleration rate target of <NUM>%/s until <NUM>% strain (λsp = <NUM>:<NUM> in both directions) as the pantograph accelerated to a velocity target of <NUM>/s, and the expansions were completed at a constant velocity set point of <NUM>%/s (<NUM>/s in this specific case based on the original length input of <NUM> ("r/s" rate mode)). The pantograph opened for approximately <NUM> (E3B) or <NUM> (E3A) seconds for the third pass. The expanded membrane was allowed to cool to room temperature (~<NUM>) under restraint of the pantograph.

A sample (<NUM> layers) was harvested from the cooled ePTFE membrane and stacked (<NUM> total layers) for further expansion, i.e. a fourth pass. Using the same pantograph machine, the membrane was again heated in an oven set to <NUM> for a target of <NUM> seconds and then expanded in the longitudinal direction and transverse direction simultaneously at a target ratio of <NUM>:<NUM> in each direction while maintaining a temperature of about <NUM>. The simultaneous expansion was conducted at a constant acceleration rate set point of <NUM>%/s until <NUM>% strain (λsp = <NUM>:<NUM> in both directions) as the pantograph accelerated to a velocity set point of <NUM>/s, and the expansions were completed at a constant velocity set point of <NUM>%/s (<NUM>/s in this specific case based on the original length input of <NUM>) ("r/s" rate mode). The pantograph opened for approximately <NUM> seconds for the fourth pass to a (λsp = <NUM>:<NUM> in both directions). The expanded membrane was allowed to cool to room temperature (~<NUM>) under restraint of the pantograph.

The ePTFE membranes were harvested from the machine onto (<NUM> x <NUM>) adhesive backed frames. Using the frame as a cutting guide, the ePTFE membranes were weighed to calculate an areal density ranging from <NUM> to <NUM>/m<NUM>/layer, while the ePTFE membrane had a mass per area (MPA; areal density) ranging from <NUM>/m<NUM> to <NUM>/m<NUM> (Table <NUM>). Two different locations were measured for sample E3A (i.e., E3A-<NUM> and E3A-<NUM>) while three different locations were measured for sample E3B (i.e., E3B-<NUM>, E3B-<NUM>, and E3b-<NUM>). STEM imaging was conducted for samples E3A (<FIG>) and E3B (<FIG>).

Area ratios of up to <NUM>,<NUM>,<NUM>:<NUM> were possible (Table <NUM>). The lowest areal density for a complete stack of ePTFE membranes was ~<NUM>/m<NUM>.

The following example discloses production of ePTFE membranes with areal densities on the order of <NUM> - <NUM> grams/m<NUM> to facilitate membrane thickness measurements (per layer).

PTFE fine powder (E. DuPont de Nemours) was blended with ISOPAR™ K isoparaffinic hydrocarbon lubricant at a target ratio of <NUM> per pound (∼<NUM>) of fine powder (<NUM> lube/g total) (grams lube/mass of total mixture). The lubricated powder was compressed into a cylinder and was ram extruded at <NUM> to provide a tape. The tape was <NUM> wide and <NUM> thick. The ISOPAR™ K was removed by heating to approximately <NUM>. A <NUM> square was cut from the dry tape. A summary of the process parameters used in Example <NUM> is provided in Table <NUM>.

Using a pantograph machine, four squares of tape were heated in an oven set to <NUM> for a target of <NUM> (set point) (Sample E4B) or <NUM> (set point) (E4A, E4C and E4D) seconds and then expanded in the longitudinal direction and transverse direction simultaneously at a target ratio of about <NUM>:<NUM> in each direction while maintaining a temperature of about <NUM>. The average engineering strain rate target was set to <NUM>%/second. The pantograph opened at a constant velocity target for approximately <NUM> seconds. The ePTFE membrane was allowed to cool to room temperature (~<NUM>) under restraint of the pantograph.

Using the same pantograph machine, stacks (<NUM>, <NUM> or <NUM> layers total - details in Table <NUM>) were heated in an oven set to <NUM> for a target of <NUM> (E4B) or <NUM> (E4A, E4C and E4D) seconds then expanded in the longitudinal direction and transverse direction simultaneously at a target ratio of <NUM>:<NUM> in each direction while maintaining a temperature of about <NUM>. The average engineering strain rate target was set to <NUM>%/second. The pantograph opened at a constant velocity target for approximately <NUM> seconds. The expanded membrane was allowed to cool to room temperature (~<NUM>) under restraint of the pantograph.

Four samples were harvested from the cooled ePTFE membrane and stacked <NUM> (E4A-C) or <NUM> (E4D) total layers for further expansion, i.e. a third pass. Using the same pantograph machine, stacks of <NUM> or <NUM> layers were loaded. The ePTFE membrane was again heated in an oven set to about <NUM> for a target of <NUM> (E4A-C) or <NUM> (E4D) seconds then expanded in the longitudinal direction and transverse direction simultaneously at a target ratio of <NUM>:<NUM> in each direction while maintaining a temperature of about <NUM>. The simultaneous expansion was conducted at a constant acceleration rate set point of <NUM>%/s until <NUM>% strain (λsp = <NUM>:<NUM> in both directions) as the pantograph accelerated to a velocity set point of <NUM>/s, and the expansions (E4A, E4C-D) were completed at a constant velocity set point of <NUM>%/s (<NUM>/s in this specific case based on the original length input of <NUM>). The pantograph opened for approximately <NUM> seconds during the expansions of E4A and E4C-D. The simultaneous expansion of example (E4B) was conducted at a constant acceleration rate target of <NUM>%/s to a target ratio of <NUM>:<NUM> in each direction. The entire third pass translation took approximately <NUM> seconds for Example E4B. The ePTFE membranes were allowed to cool to room temperature (~<NUM>) under restraint of the pantograph. The ePTFE membranes were harvested from the machine onto (<NUM> x <NUM>) adhesive backed frames.

Using the frame as a cutting guide, ePTFE membranes E4A-E4D were weighed and measured. ePTFE membrane E4A was weighed to calculate an areal density of <NUM>/m<NUM>/layer, while the membrane weighed <NUM>/m<NUM>. The average total thickness measurement of the <NUM> layer ePTFE membrane was <NUM> microns which corresponded to approximately <NUM> per layer. Table <NUM> includes the process details for this and a similar sample which was exposed to approximately <NUM> for a target of <NUM> minutes (i.e., the "thermal treatment") to promote dimensional stability. ePTFE membrane E4B was weighed to calculate an areal density of <NUM>/m<NUM>/layer, while the membrane weighed <NUM>/m<NUM>. The average total thickness measurement of the <NUM> layer ePTFE membrane was <NUM> microns which corresponded to approximately <NUM> per layer. Table <NUM> includes the process details for two additional, similar samples which were compressed to reduce thickness using the methods described herein. ePTFE membrane E4C was a compressed region of ePTFE membrane E4A. ePTFE membrane E4C was placed in a laboratory press under <NUM> MPa (<NUM> psi) for approximately <NUM> minutes at approximately <NUM>. ePTFE membrane E4D was placed in an autoclave at <NUM> MPa (<NUM> psi) for <NUM> minutes under pressure at approximately <NUM>. ePTFE membrane E4C was weighed to calculate an areal density of <NUM>/m<NUM>/layer, while the membrane weighed <NUM>/m<NUM>. The average total thickness measurement of the <NUM> layer ePTFE membrane was <NUM> microns which corresponded to approximately <NUM> per layer. ePTFE membrane E4D was weighed to calculate an areal density of <NUM>/m<NUM>/layer, while the membrane weighed <NUM>/m<NUM>. The average total thickness measurement of the <NUM> layer ePTFE membrane was <NUM> microns which corresponded to approximately <NUM> per layer.

Table <NUM> indicates that the <NUM> and <NUM> layer ePTFE membranes were heavy enough and thick enough for wall thickness measurements. The calculated thickness per layer of each uncompressed ePTFE membrane is consistent with approximately twice the width of a typical fibril as measured from microscopic images from the STEM to be approximately <NUM>-<NUM>. The solid volume fraction and porosity are calculated using <NUM>/cc as the density of PTFE. The compressed ePTFE membrane indicated a reduced porosity and thickness per layer.

ePTFE membrane E4C was placed in a Caver Laboratory Press Model M (Fred S. Carver Inc. , Menomonee Falls, Wl). The laboratory press was operated at room temperature (~<NUM>) with a <NUM>" diameter (~<NUM>) anvil on top of to generate approximately <NUM> psi (∼<NUM> MPa) for approximately <NUM> minutes.

ePTFE membrane E4D was placed in an autoclave bag assembled from KAPTON® polyimide film (E. DuPont de Nemours Inc. , Wilmington, DE). The assembly was placed in an Econoclave™ <NUM> meter x <NUM> meter (<NUM> feet x <NUM> feet) laboratory autoclave (ASC Process Systems; Valencia, CA) using a temperature set point of <NUM> with an applied pressure of <NUM> psi (∼<NUM> MPa) for approximately <NUM> minutes.

The following example discloses the production of stacked ePTFE membranes (stacks of up to <NUM> layers by layering and co-expansion) and the measurement of various membrane parameters including: mean fibril width, area weighted fibril width (AWFW), median fibril width, specific surface area, bubble point, airflow resistance, and areal density.

High permeability is shown by relatively high air flow at a particular pressure, or stated another way, less pressure is required for higher flow. Airflow resistance is a function of structure and most simple models use the solid volume fraction and a representative fibril radius as the primary factors. More sophisticated models address slip as fibril radii decrease such that they are a small fraction of the mean free path of air at standard conditions, taken here to be <NUM>. Other contributing factors to generate a membrane with high air flow are the uniformity of the distribution of the fibrils, fibril shape and orientation. Uniform distribution of fibrils would be maximized if each fibril was separated by an identical distance, where a less uniform distribution would be represented by clumped or aggregated collections of fibrils, the later exhibiting higher permeability. Fibril shape can alter air flow resistance as well.

One way to determine the average fibril width is to the manually measure the width of fibrils within a representative sample. <FIG> (ePTFE membrane E1H) was used to manually measure the fibril widths (<NUM> fibrils measured) to calculate average width and median width (<FIG>). The fibril measurements were in nanometers (nm). From <FIG> (ePTFE membrane E1H), it is obvious due to grey scale intensity changes across the width of the fibril, that indeed, the projected width is an over simplification as smaller fibrils are observed aggregating or clumping on larger fibrils. A histogram of the fibril measurements from <FIG> is provided as <FIG> where the data was fitted to a lognormal distribution.

PTFE fine powder (E. DuPont de Nemours) was blended with ISOPAR™ K isoparaffinic hydrocarbon lubricant at a target ratio of <NUM> per pound (∼<NUM>) of fine powder (<NUM> lube/g total) (grams lube/mass of total mixture). The lubricated powder was compressed into a cylinder and was ram extruded at <NUM> to provide a tape. The tape was <NUM> wide and <NUM> thick. The ISOPAR™ K was removed by heating to approximately <NUM>. The dry tape was cut in to <NUM> squares. A summary of the process parameters used in Example <NUM> is provided in Table <NUM>.

Using a pantograph machine, up to four squares of tape were heated in an oven set to <NUM> (set point) for between <NUM> (E5A-G) or <NUM> (E5H-J) seconds and then expanded in the longitudinal direction and transverse direction simultaneously at a target ratio (λsp) of <NUM>:<NUM>, <NUM>:<NUM>, or <NUM>:<NUM> (Table <NUM>) while maintaining a temperature of about <NUM>. The average engineering strain rate target was set to <NUM>%/second. The pantograph opened at a constant velocity target for approximately <NUM>, <NUM> or <NUM> seconds based on the target ratio (Table <NUM>). The expanded ePTFE membrane was allowed to cool to room temperature (~<NUM>) under restraint of the pantograph.

Samples were harvested from the cooled ePTFE membrane for further expansion, i.e. a second pass. The specific number of layers stacked for the second pass of each sample are provided in Table <NUM>. Using the same pantograph machine, stacks of layers of ePTFE were heated in an oven set to <NUM> for a target of <NUM> (E5A-G) or <NUM> (E5H-J) seconds and then expanded in the longitudinal direction and transverse directions simultaneously at a target ratio of <NUM>:<NUM> (E5J), <NUM>:<NUM> (E5H-I) or <NUM>:<NUM> (E5A-G) in each direction while maintaining a temperature of about <NUM>. The average engineering strain rate target ranged from <NUM> %/s to <NUM> %/s (Table <NUM>). The pantograph opened at a constant velocity target for approximately <NUM> (E5A-BG), <NUM> (E5H), <NUM> (E5I) and <NUM> (E5J) seconds (Table <NUM>). The ePTFE membrane was allowed to cool to room temperature (~<NUM>) under restraint of the pantograph.

Samples were harvested from the cooled ePTFE membrane and stacked if needed for further expansion, i.e. a third pass. Using the same pantograph machine, the membrane was again heated in an oven set to <NUM> for a target of <NUM> (E5A-G), <NUM> (E5I) or <NUM> (E5H and E5J) seconds and then expanded in the longitudinal direction and transverse directions simultaneously at a target ratio of <NUM>:<NUM> (E5H and E5J), <NUM>:<NUM> (E5I) or <NUM>:<NUM> (E5A-G) (Table <NUM>) in each direction while maintaining a temperature of about <NUM>. The biaxial expansions were conducted at a constant acceleration rate set point of <NUM>%/s (E5A-E5H and E5J). For Example E5I, the biaxial expansion was conducted at a constant acceleration rate set point of <NUM>%/s until <NUM>% strain target (λsp = <NUM>:<NUM> in both directions) as the pantograph accelerated to a velocity set point of <NUM>/s, and the expansion was completed at a constant velocity set point of <NUM>%/s (<NUM>/s in this specific case based on the original length input of <NUM>) ("r/s" rate mode) to the target ratio, λsp = <NUM>:<NUM> in both directions. The pantograph opened for approximately <NUM> (E5I), <NUM> (E5A-G) and <NUM> (E5H and E5J) seconds). Selected samples (E5E-G and E5I) were thermally conditioned (heat treated) in an oven at a set point of <NUM> for a target of <NUM> seconds while restrained on the pantograph. The ePTFE membrane was allowed to cool to room temperature (~<NUM>) under restraint of the pantograph.

The cooled expanded ePTFE membrane from the third pass was harvested from the pantograph and placed onto (<NUM> x <NUM>) adhesive backed frames. The mean fibril width, area weighted fibril width (AWFW), median fibril width, specific surface area, bubble point, airflow resistance, and areal density are set forth in Table <NUM>.

The following example discloses the production of ePTFE membranes and the measurement of various membrane parameters including: Quality Factor, airflow resistance, areal density, particle capture efficiency and penetration. Air filtration performance was measured as described in Test Methods section.

PTFE fine powder (E. I DuPont de Nemours) was blended with ISOPAR™ K isoparaffinic hydrocarbon lubricant at a target ratio of <NUM> per pound (-<NUM>) of fine powder (<NUM> lube/g total) (grams lube/mass of total mixture). The lubricated powder was compressed into a cylinder and was ram extruded at <NUM> to provide a tape. The tape was <NUM> wide and <NUM> thick. The ISOPAR™ K was removed by heating to approximately <NUM>. The dry tape was cut in to <NUM> squares. A summary of the process parameters used in Example <NUM> is provided in Table <NUM>.

Using a pantograph machine, a single (E6A-C) layer or up to four (E6D) squares of tape were layered and heated in an oven set to <NUM> (set point) for <NUM> (E6A-C) or <NUM> (E6D) seconds and then expanded in the longitudinal direction and transverse directions simultaneously at a selected target ratio (λsp) of <NUM>:<NUM>, <NUM>:<NUM>, or <NUM>:<NUM> (Table <NUM>) in each direction while maintaining a temperature of about <NUM>. The average engineering strain rate target was set to <NUM>%/second. The pantograph opened at a constant velocity target for approximately <NUM>, <NUM>, or <NUM> seconds based on the target ratio (Table <NUM>). The ePTFE membrane was allowed to cool to room temperature (~<NUM>) under restraint of the pantograph.

Samples were harvested from the cooled membrane for further expansion, i.e. a second pass. The specific number of ePTFE membranes loaded for the second pass of each sample is provided in Table <NUM>. Using the same pantograph machine, stacks of ePTFE membranes were heated in an oven set to <NUM> for a target of <NUM> (E6A-C) or <NUM> (E6D) seconds and then expanded in the longitudinal direction and transverse directions simultaneously at selected target ratio of <NUM>:<NUM> or <NUM>:<NUM> in each direction while maintaining a temperature of about <NUM>. The average engineering strain rate target was <NUM> %/s, <NUM> %/s or <NUM> %/s (Table <NUM>). The pantograph opened at a constant velocity target for approximately <NUM> (E6A-B), <NUM> (E6C) and <NUM> (E6D) seconds. The expanded membrane was allowed to cool to room temperature (~<NUM>) under restraint of the pantograph.

Samples were harvested from the cooled membrane and layered if needed for further expansion, i.e. a third pass. Using the same pantograph machine, the membrane was again heated in an oven to <NUM> for a target of <NUM> (E6A-C) or <NUM> (E6D) seconds and then expanded in the longitudinal direction and transverse direction simultaneously at target ratios of <NUM>:<NUM> (E6A-B), <NUM>:<NUM> (E6C) or <NUM>:<NUM> (E6D) in both longitudinal and transverse directions while maintaining a temperature of about <NUM>. The average strain rate target was <NUM>%/s. The pantograph opened at a constant acceleration rate target for approximately <NUM> (E6A-B), <NUM> (E6C) or <NUM> (E6D) seconds. Two samples, E6B and E6D were exposed to heat (approximately <NUM>) for <NUM> minutes. The expanded membrane was allowed to cool to room temperature (~<NUM>) under restraint of the pantograph.

Samples of the ePTFE membrane were harvested from the machine onto (<NUM> x <NUM>) adhesive backed for further testing. Samples were tested for air flow resistance and filtration efficiency as described in the Test Method section. The air filtration results are provided in Table <NUM>. A plot of particle diameter versus quality factor (Qf) for samples E6A, E6B, E6C, E6D, and E6E (Comparative Example <NUM>) is provided as <FIG> shows the improvement in quality factor for Samples E6A through E6D relative to the comparative example, E6E.

The ePTFE samples were cut free from the tape and weighed on a Mettler Toledo AT <NUM>. The fibril width was measured for Samples E6A and E6B and shown in <FIG> and <FIG>. <FIG> and <FIG> show Samples E6A and E6B at lower magnification, respectively. Fibril width measurement results are provided in Table <NUM>.

An ePTFE membrane was manufactured according to the general teachings described in <CIT>. The ePTFE membrane (Sample E6E) had a mass per area of <NUM>/m<NUM>, an air flow resistance of <NUM> H<NUM>O, and a particle capture efficiency of <NUM> % for <NUM> micron DOP challenge particles tested with a face velocity of <NUM>/s (Table <NUM>).

The following example discloses the production of ePTFE membranes subsequently used for optical transmittance measurements.

PTFE fine powder (E. I DuPont de Nemours) was blended with ISOPAR™ K isoparaffinic hydrocarbon lubricant at a target ratio of <NUM> per pound (-<NUM>) of fine powder (<NUM> lube/g total) (grams lube/mass of total mixture). The lubricated powder was compressed into a cylinder and was ram extruded at <NUM> to provide a tape. The tape was <NUM> wide and <NUM> thick. The ISOPAR™ K was removed by heating to approximately <NUM>. The dry tape was cut into <NUM> squares. A summary of the process parameters used in Example <NUM> is provided in Table <NUM>.

Using a pantograph machine, one or four squares of tape were heated in an oven set to <NUM> (set point) for <NUM> (E7A) or <NUM> (E7B) seconds and then expanded in the longitudinal direction and transverse directions simultaneously at various target ratios (Table <NUM>). The average engineering strain rate target was determined for samples E7A and E7B (Table <NUM>). The pantograph opened at a constant velocity target for approximately <NUM> (E7A) or <NUM> (E7B) seconds. The ePTFE membrane was allowed to cool to room temperature (~<NUM>) under restraint of the pantograph.

Samples were harvested from the cooled ePTFE membrane for further expansion, i.e. a second pass. Using the same pantograph machine, a single layer (E7B) or a stack of <NUM> layers (E7A) were heated in an oven set to <NUM> for a target of <NUM> (E7B) or <NUM> (E7A) seconds and then expanded in the longitudinal direction and transverse directions simultaneously at a target ratio of <NUM>:<NUM> (E7A) or <NUM>:<NUM> (E7B) in each direction while maintaining a temperature of about <NUM>. The average engineering strain rate target was set to <NUM> %/s (E7A) or <NUM> %/s (E7B). The pantograph opened at a constant velocity target for approximately <NUM> (E7A) or <NUM> (E7B) seconds. The ePTFE membrane was allowed to cool to room temperature (~<NUM>) under restraint of the pantograph.

Samples were harvested from the cooled ePTFE membrane and stacked as needed for further expansion, i.e. a third pass. Using the same pantograph machine, a <NUM> layer (E7B) and a <NUM> (E7A) layer sample were heated in an oven set to <NUM> for a target of <NUM> seconds and then expanded in the longitudinal direction and transverse directions simultaneously at a target ratio of <NUM>:<NUM> (E7B) or <NUM>:<NUM> (E7A) (Table <NUM>) in each direction, while maintaining a temperature of about <NUM>. The average strain rate target was set to <NUM>%/s. The pantograph opened at a constant acceleration rate target for approximately <NUM> (E7A) or <NUM> (E7B) seconds. The ePTFE membrane was allowed to cool to room temperature (~<NUM>) under restraint of the pantograph.

The ePTFE membranes were harvested from the machine onto (<NUM> x <NUM>) adhesive backed frames for further testing. Samples were tested for air flow resistance as described in the Test Method section. The ePTFE membrane was cut free from the tape and weighed on a Mettler Toledo AT <NUM>. Selected samples were also tested for optical transmission as described in the Test Method section. The results of optical transmission testing are provided in Table <NUM> and <FIG> is a plot of wavelength vs. % transmission for both a <NUM>-layer sample (E7B; black line) and a <NUM>-layer sample (E7A; grey line).

This example highlights the enhanced strength to weight ratio for relatively balanced ePTFE membranes consisting of extremely slender and similar fibrils exhibiting exceptionally high crystallinity index that is at least <NUM>%. Stacking and co-expansion was employed to generate ample mass for bulk mechanical characterization and to reduce the time in the synchrotron for structural characterization. The amorphous content and relative strength balance was determined using x-ray diffraction (XRD).

PTFE fine powder (DuPont) was blended with ISOPAR™ K isoparaffinic hydrocarbon lubricant at a target ratio of <NUM> per pound (-<NUM>) of fine powder (<NUM> lube/g total) (grams lube/mass of total mixture). The lubricated powder was compressed into a cylinder and was ram extruded at <NUM> to provide a tape. The tape was <NUM> wide and <NUM> thick. The ISOPAR™ K was removed by heating to approximately <NUM>. The dry tape was cut in to <NUM> squares. A summary of the process parameters used in this example is provided in Table <NUM>.

Using a pantograph machine, up to four squares of tape were heated in an oven set to <NUM> (Samples E8A and E8B) or <NUM> (Samples E8C and E8D) for <NUM> seconds and then expanded in the longitudinal direction and transverse directions simultaneously at a target ratio of either <NUM>:<NUM> (E8C-D) or <NUM>:<NUM> (E8A-B) (Table <NUM>) in each direction. The average engineering strain rate target was set to <NUM> %/s. The pantograph opened at a constant velocity target for approximately <NUM> (E8C-D) and <NUM> (E8A-B) seconds. The ePTFE membrane was allowed to cool to room temperature (~<NUM>) under restraint of the pantograph.

Samples were harvested from the cooled ePTFE membrane for further expansion, i.e. a second pass. Table <NUM> lists the specific number of layers loaded for the second pass of each condition. Using the same pantograph machine, samples having <NUM> (E8B) or <NUM> (E8A and E8C-D) layers were heated in an oven set to <NUM> (E8A-B) or <NUM> (E8C-D) for <NUM> seconds and then expanded in the longitudinal direction and transverse directions simultaneously at a ratio <NUM>:<NUM> each in direction while maintaining the set point temperature. The average engineering strain rate target was <NUM>%/s (E8A-B) or <NUM> %/s (E8C-D) (Table <NUM>). The pantograph opened at a constant velocity target for approximately <NUM> (E8A-B) or <NUM> (E8C-D) seconds. The expanded membrane was allowed to cool to room temperature (~<NUM>) under restraint of the pantograph.

Samples were harvested from the ePTFE membrane and stacked if needed for further expansion, i.e. a third pass. Using the same pantograph machine, the ePTFE membrane (using <NUM> (E8B) or <NUM> (E8A and E8C-D) layers) was again heated in an oven set to <NUM> (E8A-B) or <NUM> (E8C-D) for <NUM> (E8C-D) or <NUM> (E8A-B) seconds, respectively, and then expanded in the longitudinal direction and transverse directions simultaneously at a target ratio of <NUM>:<NUM> (E8A-B) or <NUM>:<NUM> (E8C-D) in each direction while maintaining the set point temperature (Table <NUM>). The average strain rate target was <NUM>%/s. The pantograph opened at a constant acceleration for approximately <NUM> (E8A-B) or <NUM> (E8C-D) seconds. Samples E8B and E8D were thermally conditioned in an oven at a set point of <NUM> for a target of <NUM> seconds while restrained on the pantograph. The ePTFE membrane was allowed to cool to room temperature (~<NUM>) under restraint of the pantograph.

The ePTFE membranes were harvested from the machine onto (<NUM> x <NUM>) adhesive backed frames for further testing. Tensile test results included in Table <NUM> show the intrinsic strength to weight metrics exceed values previously reported in the art (see Comparative Examples - Table <NUM>). Samples E8C and E8D were further characterized by X-ray diffraction (XRD) (<FIG>, Sample E8C (not heat treated) and <FIG>, Sample E8D (heat treated) and the results are consistent with isotropic orientation in the MD-TD plane. These results are consistent with the balanced strength results. <FIG> is a plot of q(nm-<NUM>) vs. intensity over a range of <NUM>-<NUM>-<NUM> for both a heat treated sample (Sample E8D; top trace) and a sample that was not heat treated (Sample E8C; bottom trace). <FIG> is a plot of q (nm-<NUM>) vs. intensity (<NUM>-<NUM>-<NUM>) over a range of <NUM>-<NUM>-<NUM> for samples E8D (heat treated, top trace) and E8C (not heat treated, bottom trace). <FIG> and <FIG> demonstrate that the ePTFE membranes have a very high crystallinity index. In addition, the narrowness of the peaks centered at q=<NUM>-<NUM> (<FIG>) suggest that the crystalline packing of these ePTFE membranes have few defects. Example E8C had a crystallinity index of <NUM>%. Example E8D had a crystallinity index of <NUM>%.

The matrix tensile strength of Comparative ePTFE Examples <NUM>-<NUM> in the art are set forth in Table <NUM>.

The following example describes the preparation and analysis of uniaxially orientated ePTFE membranes having low mass with high intrinsic strength in the fibril direction.

PTFE fine powder (E. I DuPont de Nemours) was blended with ISOPAR™ K isoparaffinic hydrocarbon lubricant at a target ratio of <NUM> per pound (-<NUM>) of fine powder (<NUM> lube/g total) (grams lube/mass of total mixture). The lubricated powder was compressed into a cylinder and was ram extruded at <NUM> to provide a tape. The tape was <NUM> wide and <NUM> thick. The ISOPAR™ K was removed by heating to approximately <NUM>. A set of <NUM> square was cut from the dry tape. A summary of the process parameters used in Example <NUM> is provided in Table <NUM>.

Using a pantograph machine, two different samples, each having four layers of tape were heated in an oven set to <NUM> (set point) for <NUM> seconds and then expanded in the longitudinal direction (machine direction) and transverse direction simultaneously at a target ratio of about <NUM>:<NUM> in each direction while maintaining a temperature of about <NUM>. The average engineering strain rate target was set to <NUM>%/second. The pantograph opened at a constant velocity target for approximately <NUM> seconds. The ePTFE membranes were allowed to cool to room temperature (~ <NUM>) under restraint of the pantograph.

Four pieces (<NUM> layers each) were harvested from the cooled membrane for further expansion, i.e. a second pass. Using the same pantograph machine, samples containing a stack of <NUM> layers were heated in an oven set to <NUM> for a target of <NUM> seconds and then expanded in the longitudinal direction and transverse direction simultaneously at a target ratio of <NUM>:<NUM> in each direction while maintaining a temperature of about <NUM>. The average engineering strain rate target was set to <NUM>%/second. The pantograph opened at a constant velocity target for approximately <NUM> seconds. The ePTFE membrane was allowed to cool to room temperature (~<NUM>) under restraint of the pantograph.

Four samples (<NUM> layers each) were harvested from the cooled membrane and <NUM> stacks of <NUM> layers (<NUM> layers total) were loaded for further expansion, i.e. a third pass. Using the same pantograph machine, the membrane was heated in an oven set to <NUM> for a target of <NUM> seconds and then stretched in the longitudinal direction and transverse directions simultaneously at a target ratio of <NUM>:<NUM> (Example E9A) or <NUM>:<NUM> (Example E9B) in each direction (specific details for each example are included in Table <NUM>) while maintaining a temperature of about <NUM>. The average strain rate target was set to <NUM>%/second. The pantograph opened at a constant acceleration for approximately <NUM> (E9A) or <NUM> (E9B) seconds. The ePTFE membrane was allowed to cool to room temperature (~<NUM>) under restraint of the pantograph.

Using the same pantograph machine, for Sample E9B, the <NUM> layer sample was again heated in an oven set to <NUM> for a target of <NUM> seconds and then stretched in the longitudinal direction and transverse direction simultaneously at a target ratio of <NUM>:<NUM> in each direction while maintaining a temperature of about <NUM>. The simultaneous expansion was conducted at a constant acceleration rate set point target of <NUM> %/s. The pantograph opened at a constant acceleration for approximately <NUM> (E9B) seconds.

Using the same pantograph machine, the ePTFE membrane is freed from constraint in the transverse direction, while remaining fixed in the machine direction. The ePTFE membrane was heated in an oven set to <NUM> for a target of <NUM> seconds and then stretched only in the longitudinal direction at a target ratio of <NUM>:<NUM> (in the machine direction) while the ePTFE membrane in the transverse direction was allowed to neck down (i.e., narrow) freely. The pantograph opened with a constant acceleration rate set point for approximately <NUM> seconds (E9A-B). The expansion was conducted with a constant acceleration rate set point of <NUM>%/s.

Using the same pantograph machine the <NUM> layer sample was heated in an oven set to <NUM> for a target of <NUM> seconds and then stretched at a target ratio of <NUM>:<NUM> (Sample E9A) or <NUM>:<NUM> (Sample E9B) only in the machine (longitudinal) direction while maintaining a temperature of about <NUM>. The expansion was conducted with a constant acceleration rate set point of <NUM>%/s. The pantograph opened with a constant acceleration rate set point for approximately <NUM> (E9A) or <NUM> (E9B) seconds.

The membranes were harvested from the machine onto adhesive backed frames of known dimensions (<NUM> X <NUM>). Using the frame as a cutting guide, the ePTFE membranes were weighed to calculate the linear density (bulk denier) and mechanical data was collected using the Matrix Tensile Test from the Test Methods section set forth above. Sample E9A was also characterized using dynamic mechanical analysis (DMA) indicating a matrix modulus of <NUM> GPa ambient temperature (i.e., about <NUM>) (<FIG>). Sample E9A was further characterized by XRD (<FIG>) which is consistent with an extremely high degree of crystalline orientation, where the <P2> orientation function is <NUM> where <NUM> would be consistent with perfect parallel alignment (<FIG>). The crystallinity index was determined to be <NUM>%. An SEM of Sample E9A is provided as <FIG>.

The following example discloses the production of multilayered ePTFE membranes having very low mass with high intrinsic strength and the measurement of nanoparticle retention. Nanoparticle retention is tested using the bead test disclosed in the methods section which measures permeability and bead retention of the membrane sample.

Using a pantograph machine, four squares of tape were layered and heated in an oven set to <NUM> for a target of <NUM> (E10A-C) seconds and then expanded in the longitudinal direction and transverse direction simultaneously at a selected target ratio (λsp) of <NUM>:<NUM> (E10A and E10C) or <NUM>:<NUM> in each direction for E10B, while maintaining a temperature of about <NUM>. The average engineering strain rate target was set to <NUM>%/second. The pantograph opened at a constant velocity target for approximately <NUM> (E10A and E10C) or about <NUM> (E10B) seconds based on the target ratio (Table <NUM>). The expanded membrane was allowed to cool to room temperature (~<NUM>) under restraint of the pantograph. The first pass is repeated until <NUM> (E10A), or <NUM> (E10B) or <NUM> (E10C) layers are available for the second pass.

The specific number of layers, <NUM> (E10A), <NUM> (E10B) or <NUM> (E10C) were loaded for the second pass of each condition as provided in Table <NUM>. Using the same pantograph machine, stacks of layers were heated in an oven set to <NUM> for a target of <NUM> (E10A and B) or <NUM> (E10C) seconds and then expanded in the longitudinal direction and transverse direction simultaneously at a selected target of <NUM>:<NUM> (E10A) or <NUM>:<NUM> (E10B) or <NUM>:<NUM> (E10C) while maintaining a temperature of about <NUM>. The average engineering strain rate target was <NUM>%/s (E10A and E10C) or <NUM>%/s (E10B) (Table <NUM>). The pantograph opened at a constant target velocity for approximately <NUM> (E10A), <NUM> (E10B) and <NUM> (E10C) seconds. The expanded membrane was allowed to cool to room temperature (~<NUM>) under restraint of the pantograph.

Samples were harvested from the cooled membrane and layered if needed for further expansion, i.e. a third pass. The specific number of layers, <NUM> (E10A), <NUM> (E10B) or <NUM> (E10C) were loaded for the second pass of each condition as provided in Table <NUM>. Using the same pantograph machine, the membrane was again heated in an oven set to <NUM> for a target of <NUM> (E10A-C) seconds and then expanded in the longitudinal direction and transverse direction simultaneously at target ratios of <NUM> (E10A), <NUM> (E10B) or <NUM>:<NUM> (E10C) in both longitudinal and transverse directions while maintaining a temperature of about <NUM>. The average constant acceleration strain rate set point was <NUM>%/s. The pantograph opened at a constant acceleration rate target for approximately <NUM> (E10A), <NUM> (E10B) or <NUM> (E10C) seconds. The expanded membrane was allowed to cool to room temperature (~<NUM>) under restraint of the pantograph.

The samples were harvested from the machine onto (<NUM> x <NUM>) adhesive backed frames for further testing (E10A and E10B) or further expansion (E10C).

Samples were harvested from the cooled membrane and layered if needed for further expansion, i.e. a fourth pass. The specific number of layers, <NUM> (E10C) were loaded for the fourth pass of each condition as provided in Table <NUM>. Using the same pantograph machine, the membrane was again heated in an oven set to <NUM> for a target of <NUM> (E10C) seconds and then expanded in the longitudinal direction and transverse direction simultaneously at target ratios of <NUM>:<NUM> (E10C) in both longitudinal and transverse directions while maintaining a temperature of about <NUM>. The constant acceleration strain rate set point was <NUM>%/s. The pantograph opened at a constant acceleration rate target for approximately <NUM> (E10C) seconds. The expanded membrane was allowed to cool to room temperature (~<NUM>) under restraint of the pantograph.

The samples (E10A-C) were densified while restrained in the MD and TD plane, by gently pouring Isopropyl Alcohol (IPA) onto the restrained membrane and allowing the IPA to evaporate.

Each membrane sample (samples E10A, E10B, and E10C) was evaluated for mean filtrate permeability (See Equation (<NUM>) above), and bead retention (See Equation(<NUM>) above) according to the Determination of Permeability and Retention with Bead Test set forth above in the Test Methods section. The results are provided in Table <NUM> and in <FIG>.

Three comparative liquid filtration samples were prepared as follows.

A fine powder of polytetrafluoroethylene polymer made in accordance with the teachings of <CIT>was combined with <NUM> N/N (<NUM> Ib/lb) of an isoparaffinic hydrocarbon lubricant (ISOPAR™ K, Exxon, Houston, Texas). The resultant mixture was then blended, compressed into a cylindrical pellet, and thermally conditioned for at least <NUM> hours at a temperature of <NUM>. The cylindrical pellet was then extruded through a rectangular orifice die at a reduction ratio of <NUM>:<NUM> to form a tape. The tape was then calendered between rolls at a calendering ratio of <NUM>:<NUM>. The calendered tape was then transversely stretched at a ratio of <NUM>:<NUM> and dried at a temperature of <NUM>.

The dried tape was then expanded at <NUM> in the machine direction to an expansion ratio of <NUM>:<NUM>. The resulting material was subsequently expanded in the transverse direction to an expansion ratio of <NUM>:<NUM> at temperature of about <NUM>.

This biaxially expanded membrane was compressed between rollers (at <NUM>) at a speed of <NUM>/minute and with a compression force of <NUM> N/mm.

A fine powder of polytetrafluoroethylene polymer made in accordance with the teachings of <CIT>was combined with <NUM> N/N (<NUM> Ib/lb) of lubricant (ISOPAR™ K, Exxon, Houston, Texas). The resultant mixture was then blended, compressed into a cylindrical pellet, and thermally conditioned for at least <NUM> hours at a temperature of <NUM>. The cylindrical pellet was then extruded through a rectangular orifice die at a reduction ratio of <NUM>:<NUM> to form a tape. The tape was then calendered between rolls at a calendering ratio of <NUM>:<NUM>. The calendered tape was then transversely stretched at a ratio of <NUM>:<NUM> and dried at a temperature of <NUM>.

The dried tape was then expanded at <NUM> in the machine direction to an expansion ratio of <NUM>:<NUM>. The resulting material was subsequently expanded in the transverse direction to an expansion ratio of <NUM>:<NUM> at temperature of about <NUM>. The membrane was then heat treated at a temperature of approximately <NUM> for a target of <NUM> seconds.

Each Comparative Example membrane sample (samples E10D1, E10D2, and E10D3) was evaluated for mean filtrate permeability (See Equation (<NUM>)), and bead retention (See Equation (<NUM>)) using the testing procedures described above. The results are provided in Table <NUM> and in <FIG>.

Claim 1:
An expanded polytetrafluoroethylene (ePTFE) membrane comprising:
a matrix tensile strength of at least about <NUM> MPa in the machine direction
a matrix modulus of at least about <NUM> GPa at a temperature of <NUM>; and
a crystallinity index of at least about <NUM>%, the matrix tensile strength, matrix modulus and crystallinity index being determined as indicated in the description.