Abstract:
A method of making nanostructured polymeric film includes: providing a tool having a porous anodized aluminum surface or a metallic replica thereof, wherein the average depth of the pores and the average pore width fall with specified ranges; forming a continuous layer of thermoplastic polymer selected from cellulose esters, poly alpha-olefins, and combinations thereof on at least a portion of the surface of the tool such that the continuous layer extends into the pores of the surface; separating the continuous layer from the tool as a film having nanofibrils formed on a major surface of the tool, the surface of the tool having a fluorocarbon release agent thereon, or the continuous layer comprising a fluorocarbon melt additive, or both. Nanostructured films are also disclosed.

Description:
BACKGROUND 
     Surface nanostructures have attracted much attention because they provide unique properties for many potential applications. For example, in some cases surface nanostructure can dramatically change the apparent surface energy, which is particularly useful in articles wherein fluid control would be either essential or advantageous. 
     Surface nanostructures also considerably increase surface area, which is typically important in such areas as making catalysts and sensors. 
     SUMMARY 
     In one aspect, the present invention provides a method of making nanostructured polymeric film comprising: 
     providing a tool having a porous anodized aluminum surface or a metallic replica thereof, wherein the average depth of the pores is in a range of from about 0.1 micrometer to about 5 micrometers and the average pore width is in a range of from about 10 to about 500 nanometers; 
     forming a continuous layer of thermoplastic polymer on at least a portion of the surface of the tool such that the continuous layer extends into the pores of the surface, wherein the thermoplastic polymer is selected from cellulose esters, poly alpha-olefins, and combinations thereof; and 
     separating the continuous layer from the tool as a film having first and second opposed major surfaces with nanofibrils formed on the first major surface, the nanofibrils have an average width of from 5 to 200 nm and an aspect ratio of at least 10, 
     wherein the surface of the tool has a fluorocarbon release agent thereon, or the continuous layer comprises a fluorocarbon release agent, or both. 
     Methods according to the present invention are useful for preparing exotic surface structures that are useful, for example, for modifying wetting characteristics (e.g., increasing or decreasing wetting) of fluids on polymeric surfaces. 
     As used herein, 
     “anodize” refers to anodic oxidation; 
     “fluorocarbon” refers to any organic species containing at least four —CF 2 — groups; and 
     “separating the continuous layer from the tool as a film” refers to a process in which the tool remains substantially intact (e.g., is not dissolved). 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         FIG. 1  is a simplified schematic representation of an exemplary method according to the present invention; 
         FIG. 2  is a simplified schematic representation of another exemplary method according to the present invention; 
         FIG. 3  is a scanning electron micrograph of the nickel surface of Example 1, corresponding to the inverse image of an anodized aluminum film; 
         FIG. 4  is a scanning electron micrograph of an exemplary nanostructured polymeric film made according to one embodiment of the present invention Example 2; and 
         FIG. 5  is a scanning electron micrograph of an exemplary nanostructured polymeric film made according to one embodiment of the present invention Example 3. 
     
    
    
     DETAILED DESCRIPTION 
     As used herein, the term “aluminum” refers to aluminum and alloys of aluminum in which a majority by weight of the alloy is aluminum. Examples of other metals which may be present in such aluminum alloys are silicon, magnesium, bismuth, copper, nickel, zinc, chromium, lead, iron, titanium, and manganese. 
     The tool has a porous surface (characterized by a network of pores) that is formed directly or indirectly by anodizing aluminum. For example, if the surface of the tool is aluminum, the porous surface may be formed directly by anodizing the surface of the tool. 
     In one indirect method, a sacrificial metal (e.g., copper) may be electroformed onto the anodized aluminum surface such that it forms a metal plate or foil with the inverse surface structure of the anodized aluminum. Subsequently the aluminum is dissolved, for example, by aqueous alkali, to leave the sacrificial metal plate or foil. Next, the desired metal (e.g., nickel) is electroformed on the textured surface (i.e., that surface having the inverse surface structure of the anodized aluminum) of the sacrificial metal plate or foil original anodized surface, followed by dissolving the sacrificial metal with a suitable etchant. Of course, other methods of replicating the anodized aluminum surface onto another metallic substrate may also be used. 
     Conditions for anodizing aluminum are well known in the art, and generally involve applying a positive voltage to an aluminum workpiece in the presence of an appropriate oxidant such as, for example, sulfuric acid, oxalic acid, choric acid, boric acid, sulfonic acid, or the like. Further details of exemplary aluminum anodizing processes may be found in reference works such as, for example, the  Aluminum Finishes Process Manual : Reynolds Metal Company, Richmond, Va, © 1973, pages 62-113. 
     While the precise surface features may be varied depending on the anodizing conditions used, useful anodized aluminum surfaces have an average pore depth of from about 0.1 to about 5 micrometers, and average pore widths in a range of from about 10 to about 500 nanometers. 
     The tool may have any form suitable for embossing or solvent casting a film, including, for example, a roll, plate, belt, or sleeve. 
     A continuous layer of thermoplastic polymer is formed on a portion of the porous surface of the tool such that the thermoplastic polymer fills the pores in that portion of the surface. The thermoplastic polymer may spontaneously flow into the pores, for example, if dissolved or dispersed in a solvent. In some cases, the thermoplastic polymer may be forced into the pores, by pressure, heat, or a combination of the two. 
     The thermoplastic polymer should be selected such that it is not so sticky or brittle that it leaves material in the pores upon separation of the film from the tool. Useful thermoplastic polymers include cellulose esters (e.g., cellulose acetate, cellulose butyrate), poly alpha-olefins (e.g., polyethylene, polypropylene, and ethylene-propylene copolymers), and combinations thereof. 
     The thermoplastic polymer may include additives such as stabilizers, anti-oxidants, fragrances, colorants, and the like, but conventional fillers should typically be avoided as their size interferes with formation of the nanofibrils. 
     Using these thermoplastic polymers it is possible under normal conditions to obtain nanofibrils having an average width of from 10 to 200 nanometers and an aspect ratio of at least 10, 20, 30, 50, or more. 
     Further, the density of the nanofibrils typically ranges from about 1 to about 500 per square micrometer, although higher and lower densities may also be obtained. In some embodiments, the nanofibrils are discrete and continuously extend away from the first surface of the film. In other embodiments, the nanofibrils are bonded to one another at points where they contact each other, and are generally disposed parallel to the first surface of the film. 
     In order to obtain a clean release of the continuous layer from the porous surface of the tool, it is generally necessary to use a fluorinated organic release agent. 
     In some embodiments, the surface of the tool may be treated with a fluorocarbon release agent. In some embodiments, fluorocarbon release agents are characterized by a polar group that bonds to the metal surface and a fluorinated segment having at least four —CF 2 — groups (e.g., —CF 2 CF 2 CF 2 CF 2 — or —CF 2 CF 2 OCF 2 CF 2 —). Examples of useful fluorocarbon release agents include fluorocarbons having the trade designation “FLUORAD” (e.g., “3M FLUORAD SURFACE MODIFIER”) marketed by 3M Company, St. Paul, Minn.; fluorochemical trichlorosilanes, and fluorochemical monophosphates, for example, as described in U.S. Pat. Publ. No. 20040043146 (Pellerite et al.); and fluorocarbon benzotriazoles as described in U.S. Pat. No. 6,376,065 (Korba et al.); the disclosures of which are incorporated herein by reference. The fluorocarbon release agent may be applied to the porous surface, for example, as a solution or a vapor. Typically, the mold release is applied in sufficient quantity as to achieve at least monolayer coverage of the porous surface. 
     In some embodiments, a fluorocarbon melt additive may be combined with the thermoplastic polymer, such that the continuous layer comprises both thermoplastic polymer and the fluorocarbon melt additive. Suitable fluorocarbon release materials that may be combined with the thermoplastic polymer include, for example, fluorochemical oxazolidinones as described, for example, in U.S. Pat. No. 5,025,052 (Crater et al.) and U.S. Pat. No. 5,099,026 (Crater et al.); fluoroaliphatic group-containing non-ionic compounds as described, for example, in U.S. Pat. No. 5,244,951 (Gardiner) and U.S. Pat. No. 5,300,357 (Gardiner); fluorochemical aminoalcohols as described, for example, in U.S. Pat. No. 5,380,778 (Buckanin); fluorochemical piperazine compounds as described, for example, in U.S. Pat. No. 5,451,622 (Boardman et al.); fluorochemical and hydrocarbon surfactant blends as described, for example, in U.S. Pat. No. 5,804,625 (Temperante et al.); fluoroaliphatic compounds as described, for example, in U.S. Pat. No. 5,882,762 (Goeman), U.S. Pat. No. 6,127,485 (Klun et al.) and U.S. Pat. No. 6,262,180 (Klun et al.); fluorochemical oligomers as described, for example, in U.S. Pat. No. 6,174,964 (Jariwala et al.), U.S. Pat. No. 6,284,843 (Jariwala et al.), U.S. Pat. No. 6,288,157 (Jariwala et al.), U.S. Pat. No. 6,391,807 (Jariwala et al.) and U.S. Pat. No. 6,586,522 (Jariwala et al.); and fluoroaliphatic radical-containing surface-modifying additives as described, for example, in U.S. Pat. No. 6,380,289 (Thompson et al.); the disclosures of which are incorporated herein by reference. If used the level of fluorocarbon melt additives included with thermoplastic polymer in the continuous layer is typically in a level of from about 0.01 to about 10 percent based on the total weight of the continuous layer, although amounts outside this range may also be used. 
     Fluorocarbon release agents and fluorocarbon melt additives may be used individually or in combination. 
     The method of the present invention is better understood by reference to the Figures. 
     In one exemplary method shown in  FIG. 1 , a film is embossed to generate nanofibrils. Referring now to  FIG. 1 , in exemplary method  100  thermoplastic polymer film  150  is passed between backup roll  110  and tool  120 . Tool  120  has porous surface  125  formed by anodic oxidation. As polymer film  150  is squeezed between backup roll  110  and tool  120 , it forms continuous layer  130  such that continuous layer  130  extends into the pores of the surface  125 . Upon emergence from roll  110  and tool  120 , film  140  is formed. Film  140  has first and second opposed major surfaces  145  and  147 , respectively. As film  140  separates from porous surface  125 , nanofibrils  160  are formed that extend .between porous surface  125  and first major surface  145 . During formation nanofibrils  160  stretch in length until they finally are released from porous surface  125 . 
     Typically, during such an embossing method, the thermoplastic polymer film should be heated to a temperature above the glass transition temperature of the thermoplastic polymer immediately prior to embossing. 
     In another exemplary method  200  shown in  FIG. 2 , molten thermoplastic polymer  250  is passed between backup roll  210  and tool  220 . Tool  220  has porous surface  225  formed by anodic oxidation. As molten polymer  250  is squeezed between backup roll  210  and tool  220 , it forms continuous layer  230  such that continuous layer  230  extends into the pores of surface  225 . Upon emergence from roll  210  and tool  220 , molten polymer  250  is sufficiently cooled that polymer film  240  is formed. Film  240  has first and second opposed major surfaces  245  and  247 , respectively. As film  240  separates from porous surface  225 , nanofibrils  260  are formed that extend between porous surface  225  and first major surface  245 . During formation nanofibrils  260  stretch in length until they finally are released from porous surface  225 . 
     The continuous film may also be formed by casting a solution of thermoplastic polymer onto a continuous belt or plate, removing the solvent, e.g., by evaporation, and separating the continuous film from the tool to form a film having nanofibrils. 
     While various peel angles may be used when separating the thermoplastic film from the tool, it has been found that peel angles of about 90 degrees reliably yield fibrils having high aspect ratio while still releasing from the porous surface, however other peel angles may also be used. In general, the peel rate should be adjusted to a sufficiently slow speed such that thermoplastic polymer does not remain in the pores of the tool. While wishing not to be bound by theory, the choice of peel rate is not believed to be important, as long as thermoplastic polymer does not remain in the pores of the tool. 
     The output thermoplastic polymer film may have any thickness, width, and length. The film may be converted into strips, sheets, rolls, and the like. 
     Methods according to the present invention may be practiced in step-wise or continuous manner. 
     Films made according to the present invention have unique features that make them useful as substrates for display graphics, and in some cases for protective films. 
     Objects and advantages of this invention are further illustrated by the following non-limiting examples, but the particular materials and amounts thereof recited in these examples, as well as other conditions and, details, should not be construed to unduly limit this invention. 
     EXAMPLES 
     Unless otherwise noted, all parts, percentages, ratios, etc. In the examples and the rest of the specification are by weight, and all reagents used in the examples were obtained, or are available, from general chemical suppliers such as, for example, Sigma-Aldrich Company, Saint Louis, Mo., or may be synthesized by conventional methods. 
     Contact angles reported in the following examples were determined using deionized water at a temperature of 72° F. (22° C.). 
     Example 1 
     A piece of aluminum plate of dimensions 305 mm by 305 mm by 0.76 mm (composed of aluminum 1100 alloy and obtained from McMaster-Carr, Inc., Chicago Ill.) was cleaned using acetone and a paper towel to remove surface contaminants. The plate was placed in a tank containing 13 percent by weight of phosphoric acid and anodized in this solution at a temperature of 68° F. (20° C.) at a 60-volt electrical potential for 21.5 hours. The resulting anodized plate was removed from the tank, rinsed with deionized water and dried with compressed air. 
     The anodized aluminum plate was then used as a template for replication by nickel electroforming as follows. In a polypropylene tank an electroforming solution was prepared that consisted of 600 g/L of nickel sulfamate, 30 g/L of boric acid and 0.3 g/L of lauryl sodium sulfate. The anodized aluminum plate was placed in the electroforming solution, and electroforming proceeded at 140° F. (60° C.) at a current density of 20 amp/ft 2  (215.3 amp/m 2 ) for 20 hours. The nickel deposited on the anodized aluminum plate had an average thickness of 0.508 mm. A 5 molar solution of sodium hydroxide was prepared and heated to 180° F. (82° C.). The anodized aluminum plate with the deposited nickel was then dipped in the sodium hydroxide solution until the nickel surface appeared completely black. 
     The surface of the resulting nickel structure was examined with a scanning electron microscope (SEM) at a magnification of 80,000×, as shown in  FIG. 3 . The SEM showed that the aluminum had been completely dissolved, and that what remained was a nickel surface with nanostructures ranging in diameter from 10 to 230 nm. 
     Example 2 
     An anodized aluminum plate was prepared as in the first part of Example 1. A benzotriazole fluorochemical ester (0.1 percent by weight ester of benzotriazole-5-carbonxylic acid and 1H, 1H, 2H, 2H-perfluorododecyl alcohol, made by 3M Company, St. Paul, Minn. and disclosed in U.S. Pat. No. 6,376,065 (Korba et al.)) was used as a release agent. The release agent was applied as follows. A piece of filter paper (available from Whatman International, Ltd., Maidstone, England), having about the same dimensions as the anodized aluminum plate, was dipped into a tray containing the release agent until it was saturated and then positioned on top of the plate. The plate and filter paper were then placed in an oven set to a temperature of 120° C. After 30 minutes the plate and filter paper were removed from the oven, the filter paper was lifted from the top of the plate and the plate was allowed to cool to room temperature. 
     A sheet of cellulose acetate (CAS No. 9004-35-7, available from SPI Supplies Division, West Chester, Pa.), 25 micrometers in thickness, was placed on the anodized aluminum plate. Four drops of acetone were distributed on the surface of the cellulose acetate film using a 5 ¾ inch (14.6 cm) Pasteur pipette (available from VWR International, West Chester, Pa.), thereby dissolving the film and allowing the cellulose acetate to flow into the porous anodized aluminum surface of the plate. After the acetone evaporated, the thin cellulose acetate film that remained was removed from the plate by peeling. First, the film was freed from the edges of the plate by lifting a corner of the film with tweezers, and pulling the film normal to the plate until the entire film released from the plate. Viewed via SEM at a magnification of 20,000×, as shown in  FIG. 4 , nanostructures were observed having aspect ratios ranging from about 10 to about 50. 
     Contact angles were measured for both the nanostructured cellulose acetate film of this example and a comparable unstructured cellulose acetate film. A goniometer obtained under the trade designation “MODEL A-100 GONIOMETER” from Ramé-Hart, Inc., Mountain Lakes, N.J., was used for the measurements. Both advancing and receding contact angles with deionized water were measured. For the nanostructured film of this example, the advancing angle was 61° and the receding angle was 0°. For a comparable unstructured cellulose acetate film, the advancing angle was 61° and the receding angle was 27°. 
     Example 3 
     An anodized aluminum plate was prepared as in the first part of Example 1, except that the electrical potential for anodizing was 70 volts and the time was 15 minutes. A polypropylene (available as PP3445 from Exxon Mobil Corporation, Irving Tex.) was combined with 1 percent of a fluorochemical additive (trade designation “FC1801”, obtained from 3M Corporation, St. Paul, Minn.) to make pellets. The pellets were formed into a film using thermal compression molding. The film was then embossed using the anodized aluminum plate as an embossing tool. A Wabash Compression Molding Machine, Model V75H-24-CLX (available from Wabash MPI, Wabash, Ind.) was used both to form the film and to emboss it. The film was embossed by applying a pressure of 1600 psi (11.03 MPa), at a temperature of 190° C. for 3 minutes. When the embossed film had cooled to about 70° C., it was separated from the embossing tool as in Example 2. An SEM photomicrograph at a magnification of 15,000, shown in  FIG. 5  of the resulting embossed surface structure in the polypropylene film. 
     Contact angles were measured as in Example 2. For the embossed polypropylene film of this example, using deionized water, the advancing contact angle was 167° and the receding angle was 125°. For a comparable polypropylene film without the embossed nanostructure, the advancing contact angle was 104° and the receding angle was 78°. 
     Various modifications and alterations of this invention may be made by those skilled in the art without departing from the scope and spirit of this invention, and it should be understood that this invention is not to be unduly limited to the illustrative embodiments set forth herein.