Asymmetric semipermeable poly(aryletherketone) membranes and method of producing same

Asymmetric, semipermeable membranes cast from a solution comprised of poly(aryletherketone) dissolved in a strongly protic non-reactive acid.

FIELD OF INVENTION 
This invention relates to asymmetric (anisotropic), semipermeable 
membranes. In particular, it relates to asymmetric, semipermeable 
membranes derived from poly(aryletherketones) and a method of producing 
same. 
DESCRIPTION OF PRIOR ART 
Asymmetric, or anisotropic, semipermeable membranes are well-known. Such 
membranes are characterized by a porosity gradient such that at least one 
outer surface of the membrane consists of a relatively thin, dense, finely 
porous layer supported by a relatively thicker and progressively less 
dense and more openly porous interior portion. (See "Handbook of 
Industrial Membrane Technology", M. C. Porter, ed., Noyes Publications, 
1990, pp 12ff.) Due to the relatively thin "skin" layer, such membranes 
have high hydraulic permeabilities and are useful for separating the 
components of gaseous and liquid mixtures. The use of such membranes in 
dialysis, microfiltration, ultrafiltration, reverse osmosis, 
pervaporation, and membrane-based gas separation processes is commercially 
well-established. 
Asymmetric, semipermeable membranes may be produced from various materials 
by appropriate methods. Polymers are often preferred materials. 
Asymmetric, semipermeable membranes have been fabricated from a wide 
variety of polymers, including cellulose and its derivatives, 
polyacrylonitrile, polyvinyl chloride, polyvinylidene fluoride, 
polyamides, polyimides, polysulfones, and poly(arylethersulfones). 
Preferred membrane polymers are those which produce membranes with high 
hydraulic permeabilities, well-defined and controllable separation 
characteristics, good mechanical properties, and resistance to physical 
and chemical degradation. None of the polymeric materials enumerated above 
or currently employed in the fabrication of asymmetric, semipermeable 
membranes embody an optimum combination of physical and chemical membrane 
properties. 
The poly(aryletherketones) constitute a class of engineering thermoplastics 
with an exceptional combination of desirable membrane properties including 
excellent combination of heat, physical distortion, and chemical 
degradation. The insolubility of poly(aryletherketones) in all common 
solvents has precluded their fabrication into asymmetric, semipermeable 
membranes by solution casting methods. 
Certain substituted derivatives of poly(aryletherketones) (PAEK) do have 
sufficient solubility to be solution cast into semipermeable membranes. In 
Offenlegungsschrift DE 3,321,860, a poly(etherketone)(PEEK) is sulfonated 
in concentrated sulfuric acid, and thereafter the semipermeable membrane 
is obtained by precipitating the sulfonated poly(etheretherketone) from 
the sulfuric acid solution. In UK Patent Application GTB 2,216,134A 
semipermeable membranes are prepared by solution casting blends of 
sulfonated poly(etheretherketone) and at least one other compatible 
polymer. In Chimicaoggi, volume 7, number 11, 1989, pp 59-63, the 
preparation of semipermeable membranes by casting solutions of PEEKWC in 
organic solvents is disclosed. PEEKWC is a soluble poly(aryletherketone) 
derivative with repeating units of: 
##STR1## 
Microporous articles consisting of insoluble semicrystalline 
poly(aryletherketones) are disclosed in U.S. Pat. No. 4,721,732. Said 
articles are prepared from a molecularly compatible blend of a 
poly(aryletherketone) and a poly(etherimide) by solvent-leaching of the 
poly(etherimide) component from the blend. Microporous membranes are 
claimed to be produced by the same method. Said membranes are claimed to 
have a thickness of from about 1 to about 500 microns. It is clear that 
membranes prepared according to the teachings of U.S. Pat. No. 4,721,732 
consist of a symmetric voided structure, do not possess a porosity 
gradient from at least one outer finely porous surface into a 
progressively more openly porous interior portion, and have hydraulic 
permeabilities which are insufficient to be economically useful in 
membrane-based separation processes. 
SUMMARY OF THE INVENTION 
It has now been discovered that semi-crystalline poly(aryletherketones) 
form stable solutions in strongly protic non-reactive acids, and said 
solutions possess sufficient viscosity and polymer concentration so that 
semipermeable membranes derived from said poly(aryletherketones) may be 
prepared by solution casting procedures. It has further been discovered 
that the resulting semipermeable membranes are asymmetric in 
cross-sectional structure, possess high hydraulic permeabilities, possess 
a superior combination of physical and chemical membrane properties 
including insolubility in all non-acidic, organic solvents, and are highly 
efficient when employed in conventional, pressure-driven membrane 
processes.

DETAILED DESCRIPTION OF THE INVENTION 
The semipermeable membranes of this invention are comprised of 
poly(aryletherketones), and are asymmetric in cross-sectional structure. 
Such membranes may be prepared by solution casting methods, from stable 
solutions of said poly(aryletherketones) in strongly protic, non-reactive 
acids. 
According to the present invention there is provided an asymmetric, 
semipermeable membrane comprised of a poly(aryletherketone) or blend of 
poly(aryletherketones) having repeating units of the formula: 
EQU CO--Ar.sup.1 --CO--Ar.sup.2 -- 
wherein Ar.sup.1 and Ar.sup.2 are aromatic moieties, wherein at least one 
aromatic moiety contains a diaryl ether functional group which is a part 
of the polymer backbone, and wherein both Ar.sup.1 and Ar.sup.2 are 
covalently linked to the carbonyl groups through aromatic carbon atoms. 
Preferably, Ar.sup.1 and Ar.sup.2 are independently selected from 
unsubstituted phenylene and polynuclear aromatic moieties. More 
preferably, said phenylene and polynuclear aromatic moieties are 
covalently linked to the polymer chain through 1, 4, or all para, 
linkages. Poly(aryletherketones) having the following repeating units are 
preferred: 
##STR2## 
The preferred poly(aryletherketones) of this invention are 
semi-crystalline, and are insoluble in all non-acidic organic solvents. 
The membranes of this invention may be prepared from stable solutions of 
poly(aryletherketones) in strongly protic, non-reactive acids. For the 
present invention, strongly protic acids are defined as Bronsted acids of 
Bronsted acid-Lewis acid mixtures whose Hammett acidity function H.sub.o 
is more negative than -5.00. Preferred acids are those which do not react 
with, modify, or degrade the poly(aryletherketone) for example, 
methanesulfonic acid, ethanesulfonic acid, trifluoromethanesulfonic acid, 
2,2,2-trifluoroethanesulfonic acid, and the like. Mixtures of strongly 
protic acids may also be employed. Particularly preferred is a mixture of 
methanesulfonic acid and trifluoromethanesulfonic acid. Said solutions may 
contain from about 5% to about 60%, and preferably from about 10% to about 
30% poly(aryletherketone) by weight. Said solutions may also contain 
optional additives. Preferred additives are those which can alter the 
average pore size of the resulting membrane, the total porosity of the 
resulting membrane, or both. Such optional additives are known to the art 
as "pore forming agents". Suitable additives would be, for example, metal 
or quaternary ammonium salts of strong protic acids, sulfoxides, sulfones, 
nitriles, carboxylic acids, sulfides, disulfides, and aralkyl ethers. 
The semipermeable, asymmetric membranes of this invention may be prepared 
by solution-casting methods from stable solutions of 
poly(aryletherketones) in strongly protic, non-reactive acids. Such 
solution-casting methods are known in the art, and may comprise, for 
example, coating said poly(aryletherketone) solution onto a supporting 
material, and then immersing said coated material into a fluid which 
causes the poly(aryletherketone) to precipitate and form the membrane. 
Suitable supporting materials include, for example, glass, paper, metal, 
woven and non-woven fabrics, and porous tubes. Alternatively, the above 
said poly(aryletherketone) solution may be extruded through an annular 
orifice into a precipitation fluid so that the resulting membrane is in 
the form of a self-supporting hollow fiber. Any fluid which is miscible 
with the strongly protic acid and optional additives, and is not a solvent 
for the poly(aryletherketone), will be a suitable precipitation fluid. A 
preferred precipitation fluid is water. 
The membranes of the present invention possess an asymmetric, or 
anisotropic, cross-sectional structure. Such membranes are characterized 
by a porosity gradient such that at least one outer surface of the 
membrane consists of a relatively thin, dense, finely porous layer 
supported by a relatively thicker and progressively less dense and more 
openly porous interior portion. Such a structure is depicted in the 
FIGURE. Asymmetric membranes possess significantly greater hydraulic 
permeabilities than symmetric membranes of the same thickness since the 
resistance to flow in an asymmetric membranes is largely confined to the 
ultra-thin, finely porous skin layer. The asymmetric membranes of the 
present invention possess finely porous skin layers of about 0.05-1.0 
microns, and preferably from about 0.1-0.5 microns as determined by 
scanning electron microscopy. 
The following examples are representative of the present invention but are 
not intended to be limiting: 
EXAMPLE 1 
A solution was prepared from 20 g Victrex.RTM. poly(etherketone)(PEK)(grade 
220G; ICI Advanced Materials; structure I), 120 mL methanesulfonic acid 
and 10 mL of trifluoromethanesulfonic acid. The resulting dark red, 
viscous, homogeneous solution was coated onto a woven polyester fabric at 
a thickness of 6 mils. The thus coated fabric was immediately immersed 
into water at 49.degree. F. for 2 minutes, and then immediately 
transferred into water at 125.degree. F. for 10 minutes. The resulting 
off-white membrane was uniform in appearance with occasional minor pinhole 
defects. Sample disks (44 mm diameter) were cut randomly from the membrane 
and tested in an Amicon model 8050 stirred cell for pure water 
permeability (PWP). The membrane of this example had a PWP of 2010.+-.308 
GFD at 50 psi. Sample disks were also tested for macrosolute rejection in 
the same stirred cell. Test solutions of individual macrosolutes were made 
up to 1000 ppm and then concentrated by a factor of 2 by filtration 
through the membrane. The results are shown in Table 1. 
TABLE 1 
______________________________________ 
Macrosolute MW (Daltons) 
% Rejection* 
______________________________________ 
Gamma-Globulin 160,000 4 .+-. 5 
Apoferritin 443,000 17 .+-. 2 
Blue Dextran 2,000,000 41 .+-. 11 
______________________________________ 
*% Rejection determined from remaining macrosolute in the retentate. 
EXAMPLE 2 
The general preparative and testing procedure of Example 1 were repeated 
using instead a solution of 30 g Victrex.RTM. PEK, 15 mL 
trifluoromethanesulfonic acid, and 120 mL methanesulfonic acid. This 
solution was coated at a thickness of 5 mils onto a non-woven polyolefin 
fabric and immediately immersed in 62.degree. F. water for 10 minutes. The 
resulting membrane was extremely uniform in appearance and did not contain 
any visible defects. This membrane had a PWP of 1348.+-.62 GFD at 50 psi. 
Macrosolute rejection data are shown in Table 2. 
TABLE 2 
______________________________________ 
Macrosolute MW (Daltons) 
% Rejection* 
______________________________________ 
Myoglobin 17,400 93 .+-. 2 
BSA 67,000 99 .+-. 1 
Blue Dextran 2,000,000 99.1 .+-. 0.2 
______________________________________ 
*% Rejection determined from macrosolute concentration in the filtrate. 
The membrane of example 2 shows the high PWP and high rejection of 
macrosolutes characteristic of asymmetric ultrafiltration membranes. 
EXAMPLE 3 
The procedures of Example 1 were repeated using instead a solution of 30 g 
Victrex.RTM. poly(etherketone) (PEEK)(grade 450 P; ICI Advanced Materials; 
structure II), 15 mL trifluoromethanesulfonic acid, and 120 mL of 
methanesulfonic acid. The homogeneous, mahogany solution was coated at a 
thickness of 5 mils onto a non-woven polyester fabric and immediately 
immersed into 60.degree. F. water for 2 minutes and 125.degree. F. water 
for 5 minutes. The resulting membrane was very uniform in appearance and 
contained no visible defects. This membrane had a PWP of 240.+-.13 GFD at 
50 psi. Macrosolute rejection data are shown in Table 3. 
TABLE 3 
______________________________________ 
Macrosolute MW (Daltons) 
% Rejection* 
______________________________________ 
Myoglobin 17,400 78 .+-. 5 
Bacitracin 1,422 -7 .+-. 0.9 
______________________________________ 
*% Rejection determined from macrosolute concentration in the retentate. 
The negative rejection indicates that none of the material was rejected b 
the membrane, and a portion of the material was absorbed by the membrane. 
EXAMPLE 4 
The procedures of Example 1 were repeated using instead a solution of 22.5 
g Victrex.RTM. PEK (I), 7.5 g Victrex.RTM. PEEK(II), 1.0 g sodium 
methanesulfonic, 18 mL of trifluoromethanesulfonic fluoromethanesulfonic 
acid, and 145 mL methanesulfonic acid. This solution was coated at a 
thickness of 5 mils onto a non-woven polyolefin fabric and immediately 
immersed into a 55.degree. F. water for 8 minutes. The resulting membrane 
was uniform in appearance with an occasional pinhole defect. This membrane 
had a PWP of 3151.+-.243 GFD at 50 psi. Macrosolute rejections were 
determined from the filtrates: Myoglobin=55%.+-.17%; BSA=95%.+-.2%; Blue 
Dextrean=98.5%.+-.0.5%. 
EXAMPLE 3 
The procedures of Example 1 were repeated using instead a solution of 30 g 
Ultrapek.RTM. KR4176 (BASF; Structure V), 30 mL trifluoromethanesulfonic 
acid, and 125 mL of methanesulfonic acid. This solution was coated at a 
thickness of 6 mils onto a non-woven polyolefin fabric and immediately 
immersed into 68.degree. F. water for 20 minutes. The resulting off-white 
membrane was uniform in appearance and contained no visible defects. This 
membrane had a PWP of 1558.+-.174 GFD at 50 psi. 
EXAMPLE 6 
The membranes from Examples 2, 3, and 4 of this invention were immersed in 
various solvents for 24 hours and then retested for PWP. For comparative 
purposes, a commercial poly(ethersulfone) membrane(PTTK; Millipore Corp.) 
was subjected to the same procedures. Results are shown in Table 4. 
TABLE 4 
______________________________________ 
PWP (GFD/50 psi after 
24 hour solvent immersion) 
N-methyl- 
Membrane Acetone Dixylylethane 
pyrrolidone 
______________________________________ 
Present Invention: 
Example 2 1239 .+-. 89 
906 .+-. 104 
996 .+-. 98 
Example 3 200 .+-. 31 
163 .+-. 40 
148 .+-. 18 
Example 4 2912 .+-. 203 
2489 .+-. 291 
1826 .+-. 324 
Millipore PTTK 
&lt;10 0 dissolved 
(polylethersulfone) 
______________________________________ 
It can be seen that the PAEK membranes of this invention are substantially 
unaffected by 24-hour immersion in the above selected solvents, while a 
commercial poly(ethersulfone) membrane is completely plasticized or 
dissolved by the same treatments. 
While some embodiments of the present invention have been shown described 
herein, it will be apparent to those skilled in the art that various 
modifications and changes can be made without departing from the spirit 
and scope of the present invention. All such modifications and changes 
coming within the scope of the appended claims are intended to be covered 
thereby.