Method for preparing poly(phenylene sulfide) membranes

A process is disclosed for preparing a poly(phenylene sulfide) (PPS) separation membrane in which a PPS is treated with oxygen to increase its bulk viscosity to at least about 20,000 poise. The treated PPS is then dissolved in a solvent and cast or extruded, followed by a precipitation step.

BACKGROUND OF THE INVENTION 
This invention relates to a method for making separation membranes from 
poly(phenylene sulfide) resins. 
Poly(phenylene sulfide) (PPS) polymers are known to have excellent thermal 
stability and chemical resistance. For these reasons, it has been 
attempted to make membranes from these polymers. 
Membranes are often made by extruding a solution of a polymer in a suitable 
solvent. With most polymers, this technique is readily applied because the 
polymers form high viscosity solutions which can be extruded to form 
membranes having useful properties. However, it has been found to be 
difficult to prepare membranes in this manner from poly(phenylene 
sulfide). Unless the polymer concentration is very high, PPS solutions 
have viscosities so low that they do not maintain their shape as, for 
example, sheet, tubules or hollow fibers, after they are extruded. These 
low viscosity solutions are also difficult to feed and convey using the 
typical screw extrusion equipment since this equipment is designed to use 
the melt viscosity of the extrudate to provide a pumping action. For these 
reasons, it is difficult to extrude these solutions into a membrane. As a 
result, the membrane is either impossible to form, or contains 
imperfections such as pinholes which destroy the membrane's ability to 
perform good separations. 
By increasing the PPS concentration, the viscosity of PPS solutions can be 
increased enough to extrude them. However, membranes made from highly 
concentrated PPS have very low porosity. The fluxes of these membranes are 
therefore too low to be suitable for commercial use. 
It it therefore desirable to provide a method by which poly(phenylene 
sulfide) polymers can be formed into membranes which are substantially 
free of imperfections and which exhibit fluxes which are high enough to be 
useful commercially. 
SUMMARY OF THE INVENTION 
In one aspect, this invention is a process for preparing a poly(phenylene 
sulfide) separation membrane, comprising: 
(a) treating a poly(phenylene sulfide) polymer with oxygen under conditions 
sufficient that the poly(phenylene sulfide) polymer attains a viscosity of 
at least about 20,000 poise, measured as neat polymer at 320.degree. C. by 
extrusion plastometry at a shear rate of 150 sec.sup.-1 ; 
(b) dissolving the treated polymer from step (a) in a solvent at an 
elevated temperature; 
(c) casting or extruding the solution from step (b); and 
(d) precipitating the poly(phenylene) sulfide polymer from the cast or 
extruded solution to form a membrane. 
In another aspect, this invention is a process for preparing a 
poly(phenylene sulfide) separation membrane, comprising: 
(a) heating a poly(phenylene sulfide) polymer to a temperature of at least 
200.degree. C., but below the temperature at which the poly(phenylene 
sulfide) polymer decomposes, in the presence of oxygen for a period of 
time sufficient that the poly(phenylene sulfide) polymer attains a 
viscosity of at least about 20,000 poise, measured as neat polymer at 
320.degree. C. by extrusion plastometry at a shear rate of 150 sec.sup.-1 
; 
(b) dissolving the treated polymer from step (a) in a solvent at an 
elevated temperature; 
(c) casting or extruding the solution from step (b); and 
(d) precipitating the poly(phenylene sulfide) polymer from the cast or 
extruded solution to form a membrane. 
In either aspect of this invention, poly(phenylene sulfide) membranes which 
are substantially free of defects and which exhibit good fluxes can be 
economically prepared. 
DETAILED DESCRIPTION OF THE INVENTION 
In the first step of the process of this invention, a poly(phenylene 
sulfide) polymer is treated with oxygen under conditions which cause an 
increase to the solution viscosity of the polymer. The oxygen may be in 
the form of O.sub.2, and may be used as a mixture with other gasses, with 
air being a preferred oxygen source. The oxygen may also be wholly or 
partially in the form of ozone. The conditions for treatment are not 
themselves critical provided that the required increase in solution 
viscosity is obtained. Among suitable treatment techniques are irradiation 
in the presence of oxygen (O.sub.2) and thermal treatment in the presence 
of oxygen. Thermal treatment in the presence of oxygen is especially 
preferred. 
Commercial grades of poly(phenylene sulfide) can be used in this invention, 
provided that they exhibit increased solution viscosity after treatment 
with oxygen as described herein. Thus, certain thermally treated 
commercial grades of poly(propylene sulfide), such as RYTON.RTM. V-1, P-4 
and P-6 poly(phenylene sulfide) resins sold by Phillips Chemical Company, 
are less preferred because their solution viscosity is inadequate despite 
this thermal treatment, and further treatment usually does not 
sufficiently increase the solution viscosity. More linear, higher 
molecular weight grades of poly(phenylene sulfide) having melt flows (as 
neat resins) of less than about 150 g/10 min, preferably less than about 
100 g/10 min and which exhibit viscosity increases following thermal 
treatment are therefore preferred. Examples of such PPS resins include the 
extrusion grades of PPS resins sold by Hoechst Celanese Corporation under 
the trade name FORTRON.RTM.. A particularly preferred resin is 
FORTRON.RTM. polyphenylene sulfide resin, available from Hoechst Celanese 
Corporation. In general, it is preferred that the PPS is in a finely 
divided form, so that more uniform treatment of the resin is easily 
achieved. 
The PPS is treated with oxygen under conditions sufficient that the treated 
PPS exhibits a viscosity as a neat polymer of at least about 20,000 poise, 
preferably at least about 50,000 poise, more preferably at least about 
200,000 poise, most preferably at least 1,000,000 poise. Neat polymer 
viscosities reported herein are measured at 320.degree. C. on an Instron 
Capillary Rheometer using a capillary die which is 1 inch long and having 
a hole diameter of 0.05 inch. The sample is permitted to melt for 10 
minutes at 320.degree. C. before beginning the test. The crosshead speed 
is such that the shear rate is 150 sec.sup.-1. 
Treatment with irradiation can be done with any radiation source which 
provides activation energy sufficient to permit oxygen to react with the 
PPS. Suitable radiation sources include ultraviolet, electron beam and 
gamma radiation. 
The most preferred method is to heat treat the PPS in the presence of 
oxygen. The heat treating is advantageously done at a temperature in 
excess of 200.degree. C. but below a temperature at which measurable 
degradation of the PPS occurs. It is preferred that the temperature is no 
more than slightly (10.degree. C.) above the crystalline melting point of 
the PPS. It is preferred to treat the PPS in the solid state, because the 
surface area of the solid particles is greater than that of the molten 
polymer. Preferably, the temperature is from about 240.degree. C., more 
preferably about 260, up to about 340.degree. C., more preferably up to 
about 320.degree. C., most preferably up to about 300.degree. C. The 
heating is conducted in the presence of oxygen, at a partial pressure of 
from 1 Torr up to atmospheric pressure or above. Mixtures of oxygen and 
other gases, such as oxygen/nitrogen mixtures, can be used. It is most 
preferred from an economic standpoint to use air. 
The heating is conducted for a time sufficient to increase the solution 
viscosity as described above. Although the time required depends somewhat 
on the temperature used and the partial pressure of oxygen, in general a 
heating time of about 5 minutes to about 6 hours, preferably about 10 
minutes to about 5 hours, is sufficient. The time required also depends on 
the size of the PPS particles and the manner of heating. Larger particles 
require longer heating times, so it is preferred to use a finely 
particulate resin in order to minimize the time and expense of heat 
treatment. Heating the PPS resin using a fluidized bed or similar 
technique which provides for efficient heat transfer and oxygen contact to 
the resin particles can greatly reduce the needed treatment time, and is 
therefore preferred. 
It has been found that as heat treating continues, the solution viscosity 
of the resin reaches a point at which it remains more or less constant. 
After this point, continued heating increases the gel content (i.e., the 
proportion of insoluble crosslinked material) but does not result in a 
further significant increase in solution viscosity. Since it is difficult 
or impossible to process the insoluble gels into a membrane, the gelled 
material is largely wasted. For this reason, it is preferred to terminate 
the heat treatment while the gel content is below 50% by weight, 
preferably below about 30% by weight of the PPS. For the purposes of this 
invention, gel content is measured according to ASTM D-2765 (Test Method 
A) with the following modifications. The sample is contained in a pouch 
made from 400.times.400 mesh 304 stainless steel mesh (McMaster-Carr No. 
9236T28, wire diameter 0.0010 inch, mesh opening width 0.0015 inches, open 
area 36%)(instead of the less stringent 120 size mesh specified by the 
ASTM test). The solvent is diphenyl terephthalate, and the extraction is 
carried out at 260.degree. C. for 2 hours. After the extraction, the 
sample is leached in methylene chloride for several hours, dried and the 
gel fraction calculated from the amount of polymer remaining in the pouch. 
The treated resin is then dissolved into a solvent in order to prepare the 
membrane. The solvent is any which (a) dissolves at least 10% by weight of 
the treated PPS, (b) is stable (i.e. does not significantly degrade or 
react with the PPS) and (c) does not boil at the temperature required to 
dissolve the PPS resin and to prepare a membrane therefrom. The solvent 
does not have to be a liquid at room temperature. Suitable such solvents 
are described in U.S. Pat. No. 5,043,112 to Beck, issued Aug. 27, 1991, 
and U.S. Pat. No. 5,246,647 to Beck et al., issued Sep. 21, 1993, 
incorporated herein by reference. Preferred solvents include m-terphenyl, 
o-terphenyl, p-terphenyl, mixtures of o-, m- and p-terphenyls, such as 
sold under the trade name Santowax.RTM.; hydrogenated and partially 
hydrogenated terphenyls, such as are sold by Monsanto Chemical Company 
under the trade designation HB-40; phenanthrene, 1,2,3- and 
1,3,5-triphenylbenzene, diphenyl sulfone, diphenyl phthalate, 
epsilon-caprolactam, N-cyclohexyl-2-pyrrolidone, diphennyl isophthalate, 
diphenyl terephthalate, mixtures thereof and the like. 
Methods of forming a solution of PPS in a solvent and extruding a membrane 
therefrom are described in U.S. Pat. No. 5,246,647, and those methods are 
suitable for use in this invention. In general, the solvent and the 
treated PPS are blended and heated to a temperature sufficient to melt 
both of them. The required temperature in any given case depends on the 
particular solvent used, but in general, a temperature of about 
150.degree. to about 340.degree. C. is sufficient. Agitation may be and 
preferably is used to facilitate dissolution of the PPS. 
The relative proportions of PPS and solvent used are selected to provide a 
solution having a workable viscosity and from which the polymer can be 
precipitated to form a porous membrane. 
Solution viscosity increases with increasing polymer concentration. It is 
normally desirable to provide a solution which, at the temperature of 
extrusion, has a viscosity of at least about 100 poise, preferably at 
least about 200 poise, more preferably at least about 400 poise. At lower 
viscosities, the solution usually does not maintain its shape after 
extrusion and is difficult to process through an extruder. For making 
hollow fiber membranes, it is especially preferred that the solution has a 
viscosity at the temperature of extrusion of at least about 300 poise, 
preferably at least about 400 poise, more preferably at least about 800 
poise. On the other hand, very viscous solutions require excessive energy 
to extrude, making the process less efficient. Therefore, it is preferred 
that the solution has a viscosity of less than about 5000 poise. 
Suitable solution viscosities are generally obtained when the solution 
contains from about 5, preferably from about 10, more preferably from 
about 15 weight percent, up to about 60, preferably up to about 50, more 
preferably up to about 40 weight percent PPS, based on the total weight of 
the solvent and PPS. Solutions containing these amounts of PPS can also be 
precipitated to provide a membrane exhibiting good flux values. 
The resulting solution may be immediately cast or extruded, or stored until 
a later time. 
If desired, the PPS solution may contain additional components such as 
antioxidants, plasticizers, pore forming agents, diluents, stabilizers and 
the like. It is also within the scope of this invention to employ one or 
more additional polymers in the PPS solution, if desired to form a 
membrane having particular properties. Membranes from from blends of PPS 
and another polymer are described, for example, in copending U.S. patent 
application Ser. Nos. 12,584, now abandoned, filed Feb. 3, 1993 and 
12,872, filed Feb. 3, 1993. 
The PPS solution may also contain a pore forming component. Such a pore 
forming component is advantageously a material which is miscible with the 
solvent, is a non-solvent for the PPS, boils at a temperature above about 
280.degree. C., and does not undesirably react with the PPS and the 
solvent. The use of such a pore forming component is described in U.S. 
Pat. No. 5,246,647. Preferred pore forming components include, for 
example, tetra-phenylsilane, diphenyl sulfoxide, diphenic acid, 
4-acetylbiphenyl, bibenzyl, diphenyl methyl ketone, mineral oil, butyl 
stearate, phenyl benzoate, 1-phenyldecane, 1,3-diphenoxybenzene, 
1,8-dichloroanthraquinone, polyphosphoric acid, dioctyl phthalate, 
5-chlorobenzoxazolone, bis-(-4-chlorophenyl sulfone), diphenyl 
chlorophosphate, sulfolane, methyl myristate, methyl stearate, hexadecane, 
dimethyl phthalate, tetraethylene glycol dimethyl ether, diethylene glycol 
dibutyl ether, docosane, dotriacontane, tetraphenylene, pentafluorophenol, 
paraffin oil, 1-methyl-2-pyrrolidinone, 4,4'-dihydroxybenzophenone, or 
mixtures thereof. 
If a membrane is to be cast from the PPS solution, any suitable technique 
can be used, including those described in U.S. Pat. No. 5,246,647, 
incorporated herein by reference. 
Preferably, the PPS solution is extruded. Extrusion may be performed in any 
convenient manner. The solution is heated to a temperature sufficient to 
form a melt having a viscosity in the ranges described above, and extruded 
though a suitable die. The die is advantageously a slit die, which 
extrudes a thin film of the solution for making a sheet membrane, or a 
spinneret suitable for preparing hollow fiber membranes. 
When a slit die is used, the solution may be extruded onto a rolling drum, 
a belt, or other means for supporting the extrudate so it maintains its 
shape and integrity until the phase inversion step is complete. 
When a slit die is used, it is preferred that the die produce an extrudate 
having a thickness of about 1 to about 100 microns, preferably about 2 to 
about 50 microns, more preferably about 2 to about 20 microns. 
The extrusion conditions may be manipulated in order to adjust the pore 
size of the membrane. For example, when the membrane is cast onto a 
rolling drum or a belt, the linear speed of the drum or belt may be 
increased or decreased relative to the linear rate of extrusion. The rate 
of cooling the extrudate also affects pore size, with faster cooling 
tending to favor the formation of smaller pores. A convenient way of 
controlling the cooling rate is by heating or cooling the drum or belt on 
which the PPS is extruded. The drum or belt may be maintained at a 
temperature from about -80.degree. C. to about the melting point of the 
PPS solution, preferably from about 25.degree. to about 260.degree. C. 
In making a hollow fiber membrane, the solution is extruded through a 
spinneret including a means for supplying a fluid to the core of the fiber 
to prevent it from collapsing upon itself. Suitable fluids include gasses 
such as air, nitrogen, carbon dioxide and the like as well as liquids 
which do not boil at the temperatures of extrusion and which do not react 
with the PPS solution in any adverse manner. The liquid is preferably not 
a solvent for the PPS. Such liquids include dioctyl phthalate, methyl 
stearate, polyglycols, mineral oil, paraffin oil, petroleum oil, heat 
transfer fluids and silicone oils. 
The outside diameter of a hollow fiber membrane made according to this 
invention may vary depending on the separations for which it is intended, 
and the conditions, including pressures, under which it will operate. In 
general, however, the size of the hollow fiber is not critical to this 
invention. Outer diameters of hollow fiber membranes made according to 
this invention advantageously range from about 5 to about 3,000, 
preferably about 50 to about 2,000 microns, with wall thicknesses 
advantageously in the range from about 5 to about 500 microns, preferably 
about 20 to about 400 microns. 
After the polymer solution is extruded, a precipitation of the PPS from the 
solution is effected. This precipitation is conducted in such a way that a 
continuous polymer phase is formed in the desired shape. Within the 
continuous polymer phase is formed a continuous solvent phase which, when 
leached out, forms pores or channels through the resulting membrane, 
thereby imparting flux to the membrane. This precipitation step is 
conveniently conducted simply by permitting the extruded solution to cool. 
As the solution cools, it normally forms a phase rich in polymer and a 
phase which is lean in polymer. As the polymer-rich phase cools, the 
polymer molecules form a continuous phase which incorporates the solvent 
as a separate continuous phase. The polymer-lean phase is advantageously 
removed from the membrane. 
The solvent droplets formed in the precipitation step are advantageously 
leached out with a leaching agent which dissolves the solvent used in 
making the membrane but does not dissolve the PPS. Suitable leaching 
agents include toluene, xylene, acetone, methyl ethyl ketone, N-methyl 
pyrrolidone, chlorinated hydrocarbons such as methylene chloride, carbon 
tetrachloride, trichloroethylene and 1,1,1-trichloroethane. Suitable 
leaching conditions include a temperature from about 0.degree. C. to about 
200.degree. C. for a period of a few minutes up to several hours. In this 
way, pores are formed in the membrane which correspond in size and shape 
to the solvent droplets. The number and size of the pores determine the 
porosity of the membrane. The membrane advantageously has a void volume of 
from about 20, preferably from about 30% by volume, up to about 90, more 
preferably up to about 80% by volume. The pore size can be estimated by 
several techniques, including a bubble point test as described in ASTM 
F316-86. The average pore size is preferably between about 
1.times.10.sup.-3 microns to about 5 microns, more preferably between 
about 3.times.10.sup.-3 microns to about 1 micron in diameter. 
The leaching solvent may be exchanged with a drying solvent. The drying 
solvent permits the removal of the leaching solvent without causing the 
membrane to collapse. Suitable drying solvents include ethanol, 
isopropanol, methanol, lower hydrocarbons such as octane, hexane and 
cyclohexane, mixtures thereof, and the like. 
Following leaching, the membrane may be dried if desired to remove the 
leaching agent or drying agent. Drying can be accomplished by heating 
below the glass transition temperature of the PPS, or by treating the 
membrane with a chemical drying agent. 
The membrane may be drawn down or stretched if desired in order to adjust 
its size to a desired thickness, to induce axial orientation and/or 
manipulate pore size. The conditions for draw down or stretching are not 
critical provided that the membrane maintains its integrity. Suitable such 
conditions are described in U.S. Pat. No. 5,246,647, incorporated herein 
by reference. Draw down or stretching may be performed before, during or 
after leaching. 
Because of their porosity, membranes prepared in accordance with this 
invention are useful for a variety of ultrafiltration and microfiltration 
applications, such as filtering from liquids and gasses solid and 
colloidal particles having diameters in the range from about 0.005 microns 
to about 5 microns, preferably about 0.07 to about 1 micron. Examples of 
such filtrations applications include removal of pigments from liquids, 
filtration of smoke particles from gasses, removal of dust particles from 
gasses, filtration of bacterial and/or viruses from liquids and gases, and 
other filtration applications. Because the PPS has excellent temperature 
stability and solvent resistance, the membranes of this invention is 
particularly suitable in high temperature applications and in applications 
involving exposure of the membrane to organic solvents. The membrane is 
also useful in non-filtration applications such as breathable fabrics, 
porous catalyst supports, membrane reactors, membrane contactors and 
bioreactors. 
The membrane made in accordance with this invention can be used in any 
convenient manner. In general, the fluid being filtered is applied to one 
side of the membrane, and a permeate withdrawn from the opposing side, 
with a pressure drop occurring across the membrane.

The following examples are provided to illustrate the invention but are not 
intended to limit the scope thereof. All parts and percentages are by 
weight unless otherwise indicated. 
EXAMPLE 1 
A commercial grade particulate PPS resin (FORTRON.RTM. 300 BO 
(polyphenylene sulfide resin, from Hoescht-Celanese) is heated for three 
hours in air at 280.degree. C. in an air circulating oven. The treated PPS 
exhibits a solution viscosity in excess of 300 poise. 
Portions of the treated resin are then dissolved in various solvents by 
heating a mixture of the treated resin and the solvent to about 
300.degree. C. with stirring until a clear solution is obtained. The 
resulting solutions (Sample Nos. 1-5, respectively) are then cooled to 
allow them to solidify, and granulated. The solvents and proportions of 
treated PPS and solvent are as described in Table 1 following. 
A membrane is made from Sample No. 1 by extruding it at a temperature of 
277.degree. C. through a film die onto a heated godet roll stack. The die 
gap is 12 mils, the die width is six inches and the linear extrusion rate 
is 146 cm/min. The godet temperature is 160.degree. C. and its speed is 
146 cm/min. The air gap between the godet and the die is 1 cm. 
Sample No. 2 is extruded into a membrane at a temperature of 279.degree. C. 
through a film die onto a godet roll stack which is heated at 130.degree. 
C. The die gap is 12 mils, the die width is two inches and both the linear 
extrusion rate and the godet speed are 171 cm/min. The air gap between the 
godet and the die is 1 cm. 
Sample No. 3 is extruded into a membrane at a temperature of 270.degree. C. 
through a film die onto a godet roll stack which is heated at 160.degree. 
C. The die gap is 12 mils, the die width is six inches and both the linear 
extrusion rate and the godet speed are 137 cm/min. The air gap between the 
godet and the die is 1 cm. 
Sample No. 4 is extruded into a membrane at a temperature of 275.degree. C. 
through a film die onto a godet roll stack which is heated at 160.degree. 
C. The die gap is 12 mils, the die width is six inches and both the linear 
extrusion rate and the godet speed are 146 cm/min. The air gap between the 
godet and the die is 1 cm. 
Sample No. 5 is extruded at a temperature of 280.degree. C. through a film 
die onto a heated godet roll stack. The die gap is 12 mils, the die width 
is 10 inches, and the linear extrusion rate is 97 cm/min. The godet 
temperature is 180.degree. C. and its speed is 91 cm/min. The air gap 
between the godet and the die is 0.5 cm. 
After extrusion of Sample Nos. 1-5, the PPS rapidly phase separates to form 
a PPS polymer film. The film is cooled and leached for four hours with 
methylene chloride. The leached films are then wetted with isopropanol in 
order to exchange the methylene chloride for isopropanol, and then are air 
dried. 
The resulting membranes are then tested for nitrogen flux, water flux, 
maximum pore size and, in some instances, mean size. The results are as 
reported in Table 1. These tests are done on disc samples mounted in a 
model UHP-25 membrane holder sold by MFS, Dublin, Calif. Water flux tests 
are done at 20-40 psi, with the results normalized for active membrane 
area and pressure. Feed water for the water flux tests is HPCL grade. 
Maximum pore size is determined by the bubble point method of ASTM 
F-316-86, modified as follows to accommodate the membrane holder. The 
membrane is held in the membrane holder and wetted with ethanol. The 
bubble point is determined by connecting a pipette to the cell output and 
watching for bubbles from the pipette tip submerged in a beaker of water 
as the nitrogen pressure is increased. Maximum pore size is determined in 
this manner, mean pore size is determined per ASTM--F-316-86, except that 
the sample is held in the membrane holder model UHP-25 as previously 
described. 
TABLE 1 
______________________________________ 
Sample No. 
Property 1 2 3 4 5 
______________________________________ 
PPS Conc., 
38 38 35 35 35 
%.sup.1 
Solvent.sup.2 
A B B C D 
Viscosity 733/124 2133/43 487/224 
330/270 
N/D 
of the PPS 
solution, 
poise/ 
Shear 
rate, 
sec.sup.-1 
Nitrogen 0.0070 0.020 0.029 0.030 0.014 
Flux 
(cc/cm.sup.2 / 
sec/cmHg) 
Membrane 60 109 218 216 67 
Water Flux 
(L/M.sup.2 /Hr/ 
Bar) 
Maximum 0.55 0.18 0.16 0.16 0.06 
Pore Size, 
Membrane 
(.mu.m) 
Mean Pore ND ND .06 ND ND 
Size, 
Membrane 
(.mu.m) 
______________________________________ 
.sup.1 Concentration of poly(phenylene sulfide) in the extruded solution, 
in weight percent. 
.sup.2 A-m-terphenyl; B-diphenyl isophthalate; C 32.5/32.5 blend of 
diphenyl isophthalate and mterphenyl; D-50/15 blend of diphenyl 
isophthalate and a hydrogenated terphenyl sold by Monsanto Chemicals as 
HB40. 
ND means not determined. 
As can be seen from the data in Table 1, membranes having useful filtration 
properties can be prepared in accordance with this invention using a 
variety of solvents. 
EXAMPLE 2 
A portion of the heat treated PPS from example 1 is dissolved in diphenyl 
isophthalate to form a solution containing 32% by weight PPS. The 
resulting solution is then separated into Sample Nos. 6 and 7. 
Sample No. 6 is extruded at a temperature of 275.degree. C. through a film 
die onto a heated godet roll stack. The die gap is 12 mils, the die width 
is 6 inches, and the linear extrusion rate is 174 cm/min. The godet 
temperature is 102.degree. C. and its speed is 149 cm/min. The air gap 
between the godet and the die is 0.5 cm. 
Sample No. 7 is extruded at a temperature of 273.degree. C. through a film 
die onto a heated godet roll stack. The die gap is 12 mils, the die width 
is 6 inches, and the linear extrusion rate is 189 cm/min. The godet 
temperature is 200.degree. C. and its speed is 149 cm/min. The air gap 
between the godet and the die is 0.5 cm. 
Following extrusion, Samples Nos. 6 and 7 are cooled, leached, wetted and 
dried as described in Example 1. In both cases, the extrudate is visually 
observed as it contacts the godet to determine the length of time before 
it becomes opaque, indicating that precipitation of the PPS has begun. The 
water flux and maximum pore size of the resulting membranes are measured 
and are as reported in Table 2. 
TABLE 2 
______________________________________ 
Time to Maximum Mean 
Godet Phase Water Flux 
Pore Pore 
Sample 
Temp. Inversion, 
(L/M.sup.2.Hr. 
Size Size 
No. .degree.C. 
s.sup.1 Bar) (.mu.m) (.mu.m) 
______________________________________ 
6 102 0.2 118 0.08 0.04 
7 200 6 409 0.20 0.10 
______________________________________ 
.sup.1 The time required for the extrudate to become opaque after 
contacting the godet roll. 
The data in Table 2 demonstrates that the pore size and flux properties of 
the PPS membranes may be affected by controlling extrusion variables such 
as godet temperature. Pore size may also be increased or decreased by 
other techniques, such as by blowing air or nitrogen across the surface of 
the hot extrudate, contacting the extrudate with a liquid or gaseous 
coolant, and the like. 
EXAMPLE 3 
A portion of FORTRON.RTM. 300 BO PPS resin is heated in air for 6 hours at 
280.degree. C. in an air circulating oven. The solution viscosity of the 
resulting heat treated PPS is found to be 818 poise at a shear rate of 139 
sec.sup.-1. At a shear rate of 100 sec.sup.-1, the viscosity would be even 
higher. A solution of 40 weight percent of the heat treated resin and 60% 
DPIP is prepared by melting PPS and solvent together with mixing at 
320.degree. C. The resulting solution is solidified by cooling and 
granulated. The granulated solution is loaded into a ram extruder equipped 
with a hollow fiber spinneret and extruded at 280.degree. C. Nitrogen is 
fed to the fiber lumen as a core gas. The resulting fiber is cooled, taken 
up on a room temperature godet roll, extracted with methyl ethyl ketone to 
remove the solvent, and air dried. The resulting hollow fiber membrane has 
a nitrogen flux of 1.76.times.10-2 cm.sup.3 /cm.sup. 2 /sec/cmHg, a 
maximum pore size of 0.15 .mu.m, and a water flux of 100 L/M.sup.2 
/Hr/Bar. The fiber dimensions are approximately 370 .mu.m on the inside 
and 600 .mu.m on the outside.