Isotropic microporous syndiotactic polystyrene membranes and processes for preparing the same

This invention relates to semi-permeable isotropic syndiotactic polystyrene microporous membranes and processes for preparing such membranes.

BACKGROUND OF THE INVENTION 
This invention relates to isotropic mioroporous membranes comprised of 
syndiotactic polystyrene and processes for preparing such membranes. 
Microporous membranes have long been used in the liquid membrane separation 
processes of ultrafiltration, microfiltration, membrane distillation, and 
membrane stripping. Ultrafiltration and microfiltration are pressure 
driven filtration processes using microporous membranes in which particles 
or solutes are separated from solutions based on differences in particle 
size, particle shape, and/or molecular weight. Membrane distillation and 
membrane stripping are separation processes using microporous membranes in 
which certain components of the liquid to be treated which are more 
volatile permeate through the membrane as vapor more rapidly than 
components which are less volatile due to differences in chemical 
potential across the membrane. In membrane distillation, the permeated 
components are condensed on the permeate side of the membrane and removed 
as liquid, while in membrane stripping the permeated components are 
removed from the permeate side of the membrane as vapor. 
Such microporous membranes may be isotropic or anisotropic (asymmetric). 
Isotropic microporous membranes possess a morphology in which the pore 
size within the membrane is substantially uniform throughout the membrane. 
Anisotropic (asymmetric) microporous membranes possess a morphology in 
which a pore size gradient exists across the membrane: that is, the 
membrane morphology varies from highly porous, larger pores at one 
membrane surface to less porous, smaller pores at the other membrane 
surface. Such anisotropic membranes thus possess a microporous "skin" of 
smaller pores. The term asymmetric is often used interchangeably with the 
term anisotropic. 
In the past, such microporous membranes have been fabricated from aliphatic 
polyolefins such as polyethylene and polypropylene, or from high 
performance polymers such as sulfonated polyetheretherketone. 
However, the aliphatic polyolefin polymers presently used, while 
inexpensive and easy to process, exhibit relatively low heat distortion 
temperatures. The high performance polymers, such as sulfonated 
polyetheretherketone, are derived from polymers which are difficult to 
process and quite expensive. 
What is needed are isotropic microporous membranes useful for 
ultrafiltration, microfiltration, membrane distillation, and/or membrane 
stripping which possess good solvent resistance and heat distortion 
temperatures, are easily processed, and are prepared from low-cost 
materials. 
SUMMARY OF THE INVENTION 
The invention is a semi-permeable membrane comprising a thin isotropic 
syndiotactic polystyrene microporous membrane. 
In another aspect, the invention is a process for preparing a 
semi-permeable isotropic syndiotactic polystyrene microporous membrane 
comprising the steps of: 
A. forming a mixture comprising: 
(i) syndiotactic polystyrene, 
(ii) at least one solvent for the syndiotactic polystyrene, 
(iii) optionally at least one non-solvent for the syndiotactic polystyrene: 
B. heating the mixture to a temperature under conditions such that a 
homogeneous fluid is formed which possesses sufficient viscosity to be 
formed into a membrane: 
C. extruding or casting the homogeneous fluid into a membrane: 
D. quenching or coagulating the membrane by passing the membrane through 
one or more zones under conditions such that the membrane solidifies: 
E. simultaneously or consecutively leaching the membrane by passing the 
membrane through one or more zones under conditions such that at least a 
substantial portion of the solvent and optional non-solvent for the 
syndiotactic polystyrene is removed from the membrane: and 
F. optionally drawing the membrane before, during, and/or after leaching at 
a temperature at or above ambient temperature and below the melting point 
of the syndiotactic polystyrene or the depressed melting point of the 
polystyrene/solvent/optional non-solvent mixture to elongate the membrane 
and to induce orientation of the syndiotactic polystyrene in the membrane: 
wherein the semi-permeable membrane so formed possesses isotropic 
microporous structure. The solvents used in said process may be protic or 
aprotic. 
The semi-permeable membranes so formed are useful in ultrafiltration, 
microfiltration, membrane distillation, and/or membrane stripping, or as 
supports for composite gas or liquid separation membranes. The 
semi-permeable membranes of this invention possess good solvent resistance 
and heat distortion properties. The semi-permeable membranes may be 
prepared with relative ease and low cost. 
DETAILED DESCRIPTION OF THE INVENTION 
The membranes of this invention are prepared from syndiotactic polystyrene. 
Polystyrene is represented by the formula: 
##STR1## 
in which the phenyl group is pendant to the polymer backbone. Polystyrene 
may be isotactic, syndiotactic, or atactic, depending upon the positional 
relationship of the phenyl groups to the polymer backbone. Syndiotactic 
polystyrene is polystyrene wherein the phenyl groups which are pendent 
from the polymer backbone alternate with respect to which side of the 
polymer backbone the phenyl group is pendent. In other words, every other 
phenyl group is on the same side of the polymer backbone. Isotactic 
polystyrene has all of the phenyl groups on the same side of the polymer 
backbone. Standard polystyrene is referred to as atactic, meaning it has 
no stereoregularity; the placement of the phenyl groups with respect to 
each side of the polymer backbone is random, irregular, and follows no 
pattern. For further definition and description of stereoregular polymers, 
see Leo Mandelkern, An Introduction to Macromolecules, 2nd edition, 
Springer-Verlag, New York, N.Y., 1983, pp. 49-51, the relevant portions 
incorporated herein by reference. 
The properties of polystyrene vary according to the tacticity of the 
polystyrene, that is, the positional relationship of the pendent phenyl 
groups to the polymer backbone. Atactic polystyrene is generally 
unsuitable for membrane formation because of its lack of crystallinity and 
poor solvent resistance. Isotactic polystyrene possesses improved solvent 
and temperature resistance over atactic polystyrene. However, isotactic 
polystyrene is somewhat slow to produce suitable gels from its solutions 
at ambient conditions necessary to form membranes. Isotactic polystyrene 
crystallizes very slowly to form crystalline species that melt at about 
190-240.degree. C.; nucleating agents often must be added to facilitate 
its crystallization. Syndiotactic polystyrene exhibits excellent heat 
distortion temperature properties. Syndiotactic polystyrene rapidly 
crystallizes to form crystalline species that melt at about 
269-276.degree. C; nucleating agents are generally not needed to 
facilitate its crystallization. 
Syndiotactic polystyrene may be prepared by methods well known in the art. 
See Japanese Patent 104818 (1987): Ishihara et al., U.S. Pat. 
No.4,680,353; and Ishihara, Macromolecules, 19 (9), 2464 (1986); the 
relevant portions incorporated herein by reference. The term syndiotactic 
polystyrene includes polystyrene in which syndiotactic stereoregularity 
predominates. Syndiotactic polystyrene, as defined herein, preferably 
possesses a degree of syndiotactic tacticity of at least about 55 percent, 
more preferably of at least about 70 percent, even more preferably of at 
least about 80 percent. The percent tacticity of syndiotactic polystyrene 
may be determined from nuclear magnetic resonance spectra and X-ray 
diffraction patterns by methods known in the art. See "Stereoregular 
Linear Polymers" and "Stereoregular Styrene Polymers", Encyclopedia of 
Polymer Science and Engineering, Vol. 15, John Wiley & Sons, New York, 
N.Y., 1989, pp. 632-763 and "Nuclear Magnetic Resonance," Encyclopedia of 
Polymer Science and Engineering, Vol. 10, John Wiley & Sons, New York, 
N.Y., 1987, pp. 254-327: the relevant portions incorporated herein by 
reference. The syndiotactic polystyrene of this invention may optionally 
contain small amounts of inert compounds, inhibitors, stabilizers, or 
other additives which do not adversely affect the membrane-forming ability 
of the syndiotactic polystyrene/solvent/optional non-solvent mixture. 
Preferably the presence of these minor impurities in the syndiotactic 
polystyrene is less than about 10 weight percent, more preferably less 
than about 5 weight percent, most preferably less than about 1 weight 
percent. 
The invention comprises a semi-permeable isotropic syndiotactic polystyrene 
microporous membrane and processes for preparing the same. 
Solvents useful in this invention are those compounds which are a liquid at 
membrane fabrication temperatures and which dissolve a sufficient amount 
of the syndiotactic polystyrene to result in a solution viscous enough to 
form membranes. Solvents at the membrane fabrication temperature dissolve 
at least about 5 weight percent of the syndiotactic polystyrene, 
preferably at least about 10 weight percent, more preferably at least 
about 25 weight percent, even more preferably at least about 50 weight 
percent. The boiling points of the solvents useful in this invention are 
preferably above the membrane fabrication temperature so that a 
significant portion of the solvent is not flashed off during the extrusion 
or casting step but is instead retained in the membrane until the 
quenching/coagulating and/or leaching steps. The solvents preferably 
possess a boiling point of at least about 130.degree. C., more preferably 
of at least about 140.degree. C. The solvents useful in this invention may 
be protic or aprotic. A protic solvent as used herein refers to a solvent 
which is capable of dissolving the polystyrene, in which one of the steps 
of dissolution involves the generation of protons or hydrogen ions. An 
aprotic solvent as used herein refers to a solvent which is capable of 
dissolving the polystyrene, in which one of the steps of dissolution does 
not involve the generation of proton or hydrogen ions. 
Preferred solvents for syndiotactic polystyrene include substituted 
benzenes of the formulas: 
##STR2## 
wherein R.sup.1 is hydrogen, alkyl, cycloalkyl, halo, or nitro: 
R.sup.2 is alkyl: 
R.sup.3 is alkyl, aryl, carboxyaryl, or alkoxy: 
a is an integer of from 1 to 3; 
b is an integer of from 0 to 3; and 
c is an integer of from 1 to 2. 
Other preferred solvents include alkyl, cycloalkyl, aryl, or aralkyl 
substituted pyrrolidinones; chloronaphthalenes: hydrogenated and partially 
hydrogenated naphthalenes; aryl substituted phenols; ethers of the 
formula: 
##STR3## 
wherein R.sup.4 is alkyl, cycloalkyl, or aryl, diphenyl sulfone; benzyl 
alcohol; bisphenol A; caprolactam; caprolactone; alkyl aliphatic esters 
containing a total of from 7 to 20 carbon atoms; alkyl aryl substituted 
formamides; dicyclohexyl; terphenyls; partially hydrogenated terphenyls; 
and mixtures of terphenyls and quaterphenyls. 
Preferred substituted benzene solvents include o-dichlorobenzene, 
1,2,3-trichlorobenzene, 1,2,4-trichlorobenzene, xylene, nitrobenzene, 
acetophenone, methyl benzoate, ethyl benzoate, diphenyl phthalate, benzil, 
methyl salicylate, benzophenone, cyclohexyl benzene, n-butylbenzene, 
n-propylbenzene, phenol, and dimethyl phthalate. Examples of preferred 
ethers include phenetole (phenyl ethyl ether), diphenyl ether, and 
anisole. Examples of preferred pyrrolidinone solvents include 
1-benzyl-2-pyrrolidinone, 1-cyclohexyl-2-pyrrolidinone, 
1-ethyl-2-pyrrolidinone, 1-methyl-2-pyrrolidinone, and 
1-phenyl-2-pyrrolidinone. More preferred pyrrolidinone solvents include 
the alkyl and cycloalkyl substituted pyrrolidinones. Even more preferred 
pyrrolidinone solvents include 1-cyclohexyl-2-pyrrolidinone, 
1-ethyl-2-pyrrolidinone, and 1-methyl-2-pyrrolidinone. Preferred ether 
solvents include anisole and diphenyl ether. Preferred hydrogenated 
naphthalene solvents include decahydronaphthalene (deoalin) and 
tetrahydronaphthalene (tetralin). Examples of terphenyls and partially 
hydrogenated terphenyls preferred include partially hydrogenated 
terphenyls, available from Monsanto under the tradename Therminol.RTM. 66; 
mixed terphenyls and quaterphenyls, available from Monsanto under the 
tradename Therminol.RTM. 75; and mixed terphenyls available from Monsanto 
under the Santowax.RTM. R tradename. More preferred aliphatic esters are 
those methyl aliphatic esters with a total of from 10 to 14 carbon atoms, 
with methyl laurate being most preferred. 
More preferred solvents include 1,2,3-trichlorobenzene, 
1,2,4-trichlorobenzene, 1-ethyl-2-pyrrolidinone, 1-methyl-2-pyrrolidinone, 
1-cyclohexyl-2pyrrolidinone, acetophenone, anisole, benzil, benzophenone, 
benzyl alcohol, bisphenol A, caprolactam, caprolactone, 
decahydronaphthalene, tetrahydronaphthalene, diphenyl ether, ethyl 
benzoate, methyl salicylate, dichlorobenzene, mixed terphenyls, and 
partially hydrogenated terphenyls. 
Water miscible solvents are a preferred class of solvents which include 
1-cyclohexyl-2-pyrrolidinone, 1-methyl-2-pyrrolidinone, caprolactam, 
caprolactone, N,N-diphenylformamide, and sulfolane. 
Alkali miscible solvents are a preferred class of solvents which include 
alcohols and phenols. 
Protic solvents preferred for use in this invention include 4-phenylphenol, 
benzyl alcohol, bisphenol A, caprolactam, phenetole, and phenol. 
Aprotic solvents preferred for use in this invention include 
o-dichlorobenzene, 1,2,3-trichlorobenzene, 1,2,4-trichlorobenzene, xylene, 
nitrobenzene, acetophenone, methyl benzoate, ethyl benzoate, diphenyl 
phthalate, benzil, methyl salicylate, benzophenone, cyclohexyl benzene, 
n-butylbenzene, n-propylbenzene, anisole, 1-benzyl-2-pyrrolidinone, 
1-cyclohexyl-2-pyrrolidinone, 1-ethyl-2-pyrrolidinone, 
1-methyl-2-pyrrolidinone, 1-phenyl-2-pyrrolidinone, decahydronaphthalene 
(decalin), tetrahydronaphthalene (tetralin), methyl laurate, and 
caprolactone. 
The solvents useful in this invention may optionally contain small amounts 
of inert compounds, inhibitors, stabilizers, or other additives which do 
not adversely affect the membrane-forming ability of the syndiotactic 
polystyrene/solvent/optional non-solvent mixture. Preferably the presence 
of these minor impurities in the solvent is less than about 10 weight 
percent, more preferably less than about 5 weight percent, most preferably 
less than about 1 weight percent. 
The optional non-solvents useful in this invention are those compounds 
which are a liquid at membrane fabrication temperatures and which dissolve 
less than about 5 weight percent of the syndiotactic polystyrene at the 
membrane fabrication temperature. The boiling points of the non-solvents 
useful in this invention are preferably above the membrane fabrication 
temperature so that a significant portion of the non-solvent is not 
flashed off during the extrusion or casting step but is instead retained 
in the membrane until the quenching/coagulating and/or leaching steps. The 
non-solvents preferably possess a boiling point of at least about 
100.degree. C., more preferably of at least about 120.degree. C. 
Non-solvents preferred for use in this invention include dimethylsulfoxide, 
ethylene carbonate, methyl caproate, methyl caprylate, methyl enanthate, 
methyl valerate, mineral oil, and paraffin oil. 
The non-solvents useful in this invention may optionally contain small 
amounts of inert compounds, inhibitors, stabilizers, or other additives 
which do not adversely affect the membrane-forming ability of the 
syndiotactic polystyrene/solvent/optional non-solvent mixture. Preferably, 
the presence of these minor impurities in the non-solvent is less than 
about 10 weight percent, more preferably less than about 5 weight percent, 
most preferably less than about 1 weight percent. 
The concentration of syndiotactic polystyrene in the casting or extrusion 
mixture may vary. The concentration of the syndiotactic polystyrene in the 
mixture is dependent upon the solvent and optional non-solvent, the 
molecular weight of the syndiotactic polystyrene, and the viscosity of the 
mixture. The syndiotactic polystyrene molecular weight should be 
sufficient such that membranes with reasonable physical integrity can be 
formed. The weight-average molecular weight (Mw) of the syndiotactic 
polystyrene is preferably at least about 200,000 daltons, more preferably 
at least about 400,000 daltons. The preferred upper limit on the 
weight-average molecular weight is about 5,000,000 daltons, with about 
3,000,000 daltons being more preferred. The mixture must possess 
sufficient viscosity to enable casting or extruding the mixture into a 
membrane. If the viscosity is too low, the membranes will lack physical 
integrity; if the viscosity is too high, the mixture cannot be formed into 
membranes. Preferably, the lower limit on viscosity at the membrane 
casting or extrusion step is about 20 poise, more preferably about 40 
poise. The upper limit on viscosity at the membrane casting or extrusion 
step is preferably about 1,000,000 poise, more preferably about 500,000 
poise, most preferably about 100,000 poise Preferably, the mixture 
contains between about 5 and about 90 weight percent syndiotactic 
polystyrene, more preferably between about 10 and about 80 weight percent 
syndiotactic polystyrene, even more preferably between about 15 and about 
70 weight percent syndiotactic polystyrene. The amount of optional 
non-solvent used in the mixture is such that the solvent and non-solvent 
together dissolve at least about 5 weight percent of the syndiotactic 
polystyrene present at the membrane fabrication temperature, preferably at 
least about 10 weight percent, more preferably at least about 25 weight 
percent. Preferably the amount of optional non-solvent in the mixture is 
less than about 20 weight percent, more preferably less than about 15 
weight percent. 
The membranes of this invention may be prepared by solution casting or 
extrusion. In the solution casting process, the syndiotactic polystyrene 
is contacted with at least one solvent and optionally at least one 
non-solvent for the syndiotactic polystyrene at elevated temperatures. The 
elevated temperature at which the mixture is contacted is that temperature 
at which the mixture is a homogeneous fluid, and below that temperature at 
which the syndiotactic polystyrene degrades and below that temperature at 
which the solvent and optional non-solvent boils. The upper temperature 
limit is preferably below about 325.degree. C., more preferably below 
about 300.degree. C. The minimum temperature limit is preferably at least 
about 25.degree. C. The contacting takes place with adequate mixing or 
agitation to ensure a homogeneous solution. 
In the case of casting, a membrane may be cast into flat sheet form by 
pouring the mixture onto a smooth support surface and drawing down the 
mixture to an appropriate thickness with a suitable tool such as a doctor 
blade or casting bar. Alternately, the mixture may be cast in a continuous 
process by casting the mixture onto endless belts or rotating drums. The 
casting surface may be such that the membrane may thereafter be readily 
separated from the surface. For example, the membrane may be cast onto a 
support having a low surface energy, such as silicone, coated glass, 
Teflon, or metal, or a surface to which the membrane will not adhere. The 
mixture may also be cast onto the surface of a liquid with which the 
syndiotactic polystyrene is immiscible, such as water or mercury. 
Alternately, the mixture may be cast onto a support surface which may 
thereafter be dissolved away from the finished membrane. The membrane may 
also be cast onto a porous support surface. The cast membrane is 
thereafter subsequently quenched or coagulated, leached, and optionally 
drawn as described hereinafter for isotropic microporous membranes formed 
by the extrusion process. 
Extrusion is the preferred process for the fabrication of flat sheet, 
tubular, or hollow fiber membranes. In the case of extrusion, the 
components of the extrusion mixture may be combined prior to extrusion by 
mixing in any convenient manner with conventional mixing equipment, as for 
example, in a Hobart mixer or a resin kettle. Alternately, the extrusion 
mixture may be homogenized by extruding the mixture through a twin screw 
extruder, cooling the extrudate, and grinding or pelletizing the extrudate 
to a particle size readily fed to a single or twin screw extruder. The 
components of the extrusion mixture may also be combined directly in a 
melt-pot or twin screw extruder and extruded into membranes in a single 
step. 
The mixture of syndiotactic polystyrene/solvent/optional non-solvent is 
heated to a temperature at which the mixture becomes a homogeneous fluid. 
The homogeneous fluid is then extruded through a sheet, hollow tube, or 
hollow fiber die (spinnerette). Hollow fiber spinnerettes are typically 
multi-holed and thus produce a tow of multiple hollow fibers. The hollow 
fiber spinnerettes include a means for supplying fluid to the core of the 
extrudate. The core fluid is used to prevent collapse of the hollow fibers 
as they exit the spinnerette. The core fluid may be a gas such as 
nitrogen, air, carbon dioxide, or other inert gas, or a liquid with which 
the syndiotactic polystyrene is substantially immiscible, such as water. 
Following casting or extruding, the membrane is passed through at least one 
quench or coagulation zone under conditions such that the membrane 
solidifies. The environment of the quench or coagulation zone may be a 
gas, a liquid, or a combination thereof. Within the quench or coagulation 
zone, the membrane is subjected to cooling and/or coagulation to cause 
gelation and solidification of the membrane. In one preferred embodiment, 
the membranes are first quenched in a gas, preferably air. Within the gas 
quench zone, the membranes gel and solidify. A portion of the solvent and 
optional non-solvent may evaporate and the membrane pore structure may 
begin to form. The temperature of the gas quench zone is such that the 
membrane gels and solidifies at a reasonable rate. The temperature of the 
gas quench zone is preferably at least about 0.degree. C., more preferably 
at least about 15.degree. C. The temperature of the gas quench zone is 
preferably less than about 100.degree. C., more preferably less than about 
50.degree. C. Ambient temperatures are particularly convenient and 
suitable for the gas quench zone. Shrouds may be used to help control gas 
flow rates and temperatures in the gas quench zone. The residence time in 
the gas quench zone is such that the membrane gels and solidifies. The 
residence time in the gas quench zone is preferably at least about 0.01 
seconds, more preferably at least about 0.05 seconds. The residence time 
in the gas quench zone is preferably less than about 10 seconds, more 
preferably less than about 8 seconds. 
Following or instead of the gas quench, the membranes may be quenched or 
coagulated in a liquid which does not dissolve the syndiotactic 
polystyrene. The primary function of the quench or coagulation liquid may 
be to provide a sufficient heat transfer media to solidify the membrane. 
However, the quench or coagulation liquid may optionally also be a solvent 
for the syndiotactic polystyrene solvent and optional non-solvent so as to 
enable removal of at least a portion of the syndiotactic polystyrene 
solvent and optional non-solvent from the membrane during the quenching 
and/or coagulation step. Preferred liquid quench or coagulation zone 
materials include water, lower alcohols, phenols, halogenated 
hydrocarbons, and perhalogenated carbon compounds. Perhalogenated carbon 
compounds are materials with a carbon backbone wherein all of the hydrogen 
atoms have been replaced by halogen atoms. More preferred liquid quench or 
coagulation materials include water, chlorinated hydrocarbons, and lower 
alcohols, with lower alcohols being most preferred. Preferred lower 
alcohols are C.sub.1-4 alcohols. The lower temperature limit on the liquid 
quench or coagulation zone is that temperature at which the liquid quench 
material freezes. The lower temperature limit on the liquid quench or 
coagulation zone is preferably at least about 0.degree. C. The upper 
temperature limit on the liquid quench or coagulation zone is either the 
boiling points of the syndiotactic polystyrene solvent and optional 
non-solvent or that temperature above which the membrane does not undergo 
solidification when in contact with the liquid quench or coagulation 
material, whichever is lower. The upper temperature limit on the liquid 
quench or coagulation zone is preferably less than about 100.degree. C., 
more preferably less than about 50.degree. C. Ambient temperatures are 
suitable and convenient. The residence time in the liquid quench or 
coagulation zone is such that the membrane gels and solidifies. The 
residence time in the liquid quench or coagulation zone is preferably at 
least about 0.01 seconds, more preferably at least about 0.05 seconds. The 
residence time in the liquid quench zone is preferably less than about 200 
seconds, more preferably less than about 100 seconds. 
Following quenching and or coagulation, the membrane is passed through at 
least one leach zone under conditions such that at least a substantial 
portion of the solvent and optional non-solvent for the syndiotactic 
polystyrene is removed from the membrane. The leach zone material is a 
solvent for the syndiotactic polystyrene solvent and optional non-solvent 
but does not dissolve the syndiotactic polystyrene. The materials which 
may be used in the leach zone are the same as the materials which may be 
used in the liquid quench or coagulation zone. The minimum temperature of 
the leach zone is that temperature at which the syndiotactic polystyrene 
solvent and optional non-solvent are removed from the membrane at a 
reasonable rate. The minimum temperature of the leach zone is preferably 
at least about 0.degree. C., more preferably at least about 15.degree. C. 
The maximum temperature of the leach zone is below that temperature at 
which membrane integrity is adversely affected. The maximum temperature of 
the leach zone is preferably less than about 150.degree. C., more 
preferably less than about 100.degree. C. The residence time in the leach 
zone is that which is sufficient to remove at least a portion of the 
syndiotactic polystyrene solvent and optional non-solvent from the 
membrane. Preferably, a substantial portion of the remaining syndiotactic 
polystyrene solvent and optional non-solvent is removed from the membrane 
in the leach zone. The residence time in the leach zone is preferably 
between about 1 second and about 24 hours, more preferably between about 
30 seconds and about 8 hours. The leach step may be performed as a 
continuous or batch process. The residence time is dependent upon the 
particular solvent and optional non-solvent, the membrane size and 
thickness, and the kinetics for removing the solvent and optional 
non-solvent from the membrane. 
Before, during, and/or after leaching, the membranes may be drawn down or 
elongated to the appropriate size and thickness. Drawing down or 
elongating means the membranes are stretched such that the length of the 
membrane is longer and the diameter is smaller at the end of the drawing 
or elongation process. Drawing increases the mechanical strength of the 
membrane by inducing orientation in the membrane. The draw temperature is 
dependent upon whether the membrane contains solvent and optional 
non-solvent at the time of drawing. For substantially solvent and optional 
non-solvent free membranes, the membrane is heated to a temperature 
between the glass transition temperature of syndiotactic polystyrene and 
the melting point of syndiotactic polystyrene, with preferred lower 
temperatures being at least about 90.degree. C., more preferably at least 
about 100.degree. C., and with preferred upper temperatures being less 
than about 280.degree. C., more preferably less than about 270.degree. C. 
For membranes containing solvent and optional non-solvent, the membrane is 
heated to a temperature between ambient temperature and the melting point 
of syndiotactic polystyrene or the depressed melting point of the 
polystyrene/solvent/optional non-solvent mixture, with preferred lower 
temperatures being at least about 10.degree. C., more preferably at least 
about 25.degree. C., and preferred upper temperatures being less than 
about 10.degree. C. below the depressed melting point. The membrane is 
drawn by stretching the membrane under tension. Flat sheet membranes may 
be uniaxially or biaxially drawn, while hollow fiber membranes are 
uniaxially drawn. This is generally performed by running the membranes 
over a pair of godets in which the latter godets are moving at a faster 
rate than the former godets. The draw down or elongation ratio is the 
ratio of the beginning length of the membrane to the final length of the 
membrane. Preferably the lower limit on the draw down or elongation ratio 
is about 1.05, more preferably 1.1. Preferably the upper limit on the draw 
down or elongation ratio is about 10. The membrane may be drawn in one or 
more stages with the options of using different temperatures, draw rates, 
and draw ratios in each stage. Line speeds are generally not critical and 
may vary significantly. Practical minimum preferred line speeds are at 
least about 10 feet/minute, more preferably at least about 30 feet/minute. 
Practical maximum preferred line speeds are less than about 2000 
feet/minute, more preferably less than about 1000 feet/minute. 
Optionally before or after leaching and/or drawing, the membranes may be 
annealed by exposing the membranes to elevated temperatures. The membranes 
may be annealed at temperatures above the glass transition temperature 
(Tg) of the syndiotactic polystyrene or syndiotactic 
polystyrene/solvent/optional non-solvent mixture and about 10.degree. C. 
below the melting point of the syndiotactic polystyrene or depressed 
melting point of the syndiotactic polystyrene/solvent/non-solvent mixture 
for a period of time between about 30 seconds and about 24 hours. 
The transport rate through the membrane for an isotropic membrane is 
inversely proportional to the membrane thickness. The thickness of the 
membrane is such that the membrane possesses adequate mechanical strength 
under use conditions and good separation characteristics. In the case of 
flat sheet membranes, the minimum thickness is preferably at least about 
10 microns, more preferably at least about 15 microns; the maximum 
thickness is preferably less than about 500 microns, more preferably less 
than about 400 microns. In the case of hollow fibers, the outer diameter 
of the membrane is preferably at least about 50 microns, more preferably 
at least about 70 microns; the outer diameter of the membrane is 
preferably less than about 5000 microns, more preferably less than about 
4000 microns. The inside diameter of the hollow fiber membranes is 
preferably at least about 30 microns, more preferably at least about 40 
microns; the inside diameter of the hollow fiber membranes is preferably 
less than about 4980 microns, more preferably less than about 3980 
microns. The hollow fiber membrane thickness is preferably at least about 
10 microns, more preferably at least about 15 microns; the membrane 
thickness is preferably less than about 500 microns, more preferably less 
than about 400 microns. 
The final solvent and optional non-solvent free membranes preferably 
exhibit a glass transition temperature of at least about 80.degree. C., 
more preferably of at least about 90.degree. C., most preferably of at 
least about 100.degree. C. 
The membranes are fabricated into flat sheet, spiral, tubular, or hollow 
fiber devices by methods described in the art. Spiral wound, tubular, and 
hollow fiber devices are preferred. Tubesheets may be affixed to the 
membranes by techniques known in the art. The membrane is sealingly 
mounted in a pressure vessel in such a manner that the membrane separates 
the vessel into two fluid regions wherein fluid flow between the two 
regions is accomplished by fluid permeating through the membrane. 
Conventional membrane devices and fabrication procedures are well known in 
the art. 
The membranes are useful in ultrafiltration, microfiltration, membrane 
distillation, and/or membrane stripping, and as supports for composite gas 
or liquid separation membranes. 
In ultrafiltration or microfiltration, the membranes are used to recover or 
isolate solutes or particles from solutions. The membrane divides the 
separation chamber into two regions, a higher pressure side into which the 
feed solution is introduced and a lower pressure side. One side of the 
membrane is contacted with the feed solution under pressure, while a 
pressure differential is maintained across the membrane. To be useful, at 
least one of the particles or solutes of the solution is selectively 
retained on the high pressure side of the membrane while the remainder of 
the solution selectively passes through the membrane. Thus the membrane 
selectively "rejects" at least one of the particles or solutes in the 
solution, resulting in a retentate stream being withdrawn from the high 
pressure side of the membrane which is enriched or concentrated in the 
selectively rejected particle(s) or solute(s) and a filtrate stream being 
withdrawn from the low pressure side of the membrane which is depleted in 
the selectively rejected particle(s) or solute(s). 
In membrane distillation or membrane stripping, the membranes are used to 
remove or recover more volatile components and less volatile components. 
For example, membrane distillation and membrane stripping are useful for 
desalinating water and/or removing volatile organics from aqueous streams. 
The membrane divides the separation chamber into two regions. The feed 
stream containing more volatile and less volatile components is contacted 
with the non-permeate side of the membrane, while contacting the permeate 
side of the membrane with a sweep gas such as nitrogen, carbon dioxide, 
air, or other inert gas, a vacuum, or a combination thereof, under 
conditions such that the more volatile components permeate through the 
membrane as a vapor. A chemical potential gradient is thus established 
across the membrane due to the difference in vapor pressure of the more 
volatile components across the membrane. When a sweep gas is used, in some 
embodiments it may be advantageous to maintain the pressure on the 
permeate side of the membrane at a pressure greater than the pressure on 
the non-permeate side of the membrane in order to prevent leakage of 
liquid from the non-permeate side of the membrane through defects in the 
membrane to the permeate side of the membrane. In membrane stripping, the 
more volatile components which permeate through the membrane as vapor are 
removed from the permeate side of the membrane as vapor; in membrane 
distillation, the more volatile components which permeate through the 
membrane as vapor are condensed on the permeate side of the membrane and 
removed as liquid. 
The separation processes described hereinbefore should be carried out at 
pressures which do not adversely affect the membrane, that is, pressures 
which do not cause the membrane to mechanically fail. The pressure 
differential across the membrane is dependent upon the membrane 
characteristics, including pore size, porosity, the thickness of the 
membrane, and in the case of hollow fiber membranes, the inside diameter. 
For the membranes of this invention, the pressure differential across the 
membrane is preferably between about 5 and about 500 psig, more preferably 
between about 10 and about 300 psig. The separation processes described 
hereinbefore should be carried out at temperatures which do not adversely 
affect membrane integrity. Under continuous operation, the operating 
temperature is preferably between about 0 and about 150.degree. C., more 
preferably between about 15 and about 130.degree. C. 
The isotropic microporous membranes of this invention may be characterized 
in a variety of ways, including porosity, mean pore size, maximum pore 
size, bubble point, gas flux, water flux, and molecular weight cut off. 
Such techniques are well known in the art for characterizing microporous 
membranes. See Robert Kesting, Synthetic Polymer Membranes, 2nd edition, 
John Wiley & Sons, New York, N.Y., 1985, pp. 43-64; Channing R. Robertson 
(Stanford University), Molecular and Macromolecular Sieving by Asymmetric 
Ultrafiltration Membranes, OWRT Report, NTIS No. PB85-1577661EAR, 
September 1984; and ASTM Test Methods F316-86 and F317-72 (1982); the 
relevant portions incorporated herein by reference. 
Porosity refers to the volumetric void volume of the membrane. Porosity may 
be determined gravimetrically from the density of the void-free polymer or 
from the differences between the wet and dry weights of the membrane. The 
membranes of this invention preferably have a porosity of at least about 5 
percent, more preferably at least about 10 percent; the membranes of this 
invention preferably have a porosity of less than about 90 percent, more 
preferably of less than about 80 percent. 
Pore size of the membrane may be estimated by several techniques including 
scanning electron microscopy, and/or measurements of bubble point, gas 
flux, water flux, and molecular weight cut off. The pore size of any given 
membrane is distributed over a range of pore sizes, which may be narrow or 
broad. 
The bubble point pressure of a membrane is measured by mounting the 
membrane in a pressure cell with liquid in the pores of the membrane. The 
pressure of the cell is gradually increased until air bubbles permeate the 
membrane. Because larger pores become permeable at lower pressures, the 
first appearance of bubbles is indicative of the maximum pore size of the 
membrane. If the number of pores which are permeable to air increases 
substantially with a small increase in pressure, a narrow pore size 
distribution is indicated. If the number of air-permeable pores increases 
gradually with increasing pressure, a broad pore size distribution is 
indicated. The relationship between pore size and bubble point pressure 
can be calculated from the equation; 
##EQU1## 
where r is the pore radius 
G is the surface tension (water/air), and 
P is the pressure. 
See ASTM F316-86, the relevant portions incorporated herein by reference. 
The membranes of this invention useful for ultrafiltration preferably 
possess a maximum pore size which exhibits a bubble point with denatured 
alcohol in the range of about 90-100 psig or greater, more preferably in 
the range of about 180-190 psig or greater. 
The mean pore size of the membranes of this invention useful for 
ultrafiltration is preferably between about 5 and about 1000 Angstroms, 
more preferably between about 10 and about 500 Angstroms; the maximum pore 
size of such membranes is preferably less than about 1000 Angstroms, more 
preferably less than about 800 Angstroms. The mean pore size of the 
membranes of this invention useful for microfiltration, membrane 
distillation, and/or membrane stripping is preferably between about 0.02 
and about 10 microns, more preferably between about 0.05 and about 5 
microns; the maximum pore size of such membranes is preferably less than 
about 10 microns, more preferably less than about 8 microns. 
Gas flux is defined as 
##EQU2## 
A standard gas flux unit is 
##EQU3## 
abbreviated hereinafter as 
##EQU4## 
where STP stands for standard temperature and pressure. The membranes of 
this invention preferably have a gas flux for nitrogen of at least about 
##EQU5## 
more preferably of at least about 
##EQU6## 
Water flux is defined as 
##EQU7## 
under given conditions of temperature and pressure. The water flux is 
commonly expressed in gallons per square foot of membrane area per day 
(GFD). The membranes of this invention preferably exhibit a water flux at 
about 50 psig and about 25.degree. C. of at least about 1 GFD, more 
preferably of at least about 10 GFD. See ASTM 317-72 (1982), the relevant 
portions incorporated herein by reference.

SPECIFIC EMBODIMENTS 
The following Examples are included for illustrative purposes only and are 
not intended to limit the scope of the invention or Claims. All 
percentages are by weight unless otherwise indicated. 
EXAMPLE 1 Solubility Of Syndiotactic Polystyrene In Various Compounds 
Mixtures consisting of approximately five weight percent polymer in various 
organic compounds are prepared in two dram-capacity glass vials that are 
subsequently sealed with aluminum foil liners. The mixtures are weighed to 
a precision of one milligram. The vials are placed in an air-circulating 
oven at about 125 - 140.degree. C. Dissolution behavior is observed by 
transmitted light at close range from an AO universal microscope 
illuminator at progressively increasing temperatures until complete 
dissolution is observed, until the boiling point of the solvent is closely 
approached, or until 300.degree. C. is reached (the approximate ceiling 
temperature of the syndiotactic polystyrene). The temperature is increased 
in about 25.degree. C. increments. The mixtures are allowed to remain at a 
given temperature for at least about 30 minutes before the temperature is 
increased further. The hot mixtures are cooled to room temperature; their 
appearance is noted after they are allowed to stand undisturbed overnight 
at room temperature. The results are compiled in Table 1. The polymer 
noted as "SYNDI02" is a sample of syndiotactic polystyrene with a 
weight-average molecular weight of about 5.6 x 105 daltons. The polymer 
noted as "SYNDIO" is a sample of syndiotactic polystyrene with a lower 
molecular weight. 
TABLE I 
__________________________________________________________________________ 
CONC. APPROX. TEMP. APPEARANCE 
POLYMER 
WGT. % 
SOLVENT B.P., DEG. C. 
DEG. C. 
SOLUBILITY 
AT ROOM 
__________________________________________________________________________ 
TEMP 
SYNDIO 4.86 1-chloronaphthalene 
250 211 Soluble Firm hazy gel 
SYNDIO 5.08 1-cyclohexyl-2-pyrrolidinone 
301 200 Soluble Amber soft gel 
SYNDIO 4.95 1-phenyl-2-pyrrolidinone 
345 200 Soluble Opaque hard solid 
SYNDIO 4.97 4-phenylphenol 321 211 Soluble Opaque hard solid 
SYNDIO 25.12 
4-phenylphenol 321 221 Soluble Opaque solid 
SYNDIO 5.16 benzil 347 211 Soluble Yellow hard solid 
SYNDIO 5.02 benzophenone 305 200 Soluble Clear firm gel 
SYNDIO 4.70 caprolactam (epsilon) 
271 211 Soluble Opaque hard solid 
SYNDIO 24.94 
caprolactam (epsilon) 
271 221 Soluble Opaque hard solid 
SYNDIO 5.29 diphenyl ether 259 211 Soluble Firm hazy gel 
SYNDIO 5.35 diphenyl sulfone 
379 231 Soluble Opaque hard solid 
SYNDIO 5.08 N,N-diphenylformamide 
337 200 Soluble Opaque hard solid 
SYNDIO 5.21 o-dichlorobenzene 
180 171 Soluble Firm hazy gel 
SYNDIO 4.77 sulfolane 285 217 Not soluble 
SYNDIO 4.77 sulfolane 285 231 Soluble Liquid slush 
SYNDIO2 
5.09 1,2,3-trichlorobenzene 
218 150 Soluble White opaque hard 
solid 
SYNDIO 4.72 1,2,4-trichlorobenzene 
214 211 Soluble Cloudy Soft gel 
SYNDIO 5.19 1-benzyl-2-pyrrolidinone 
420 211 Soluble Amber clear firm gel 
SYNDIO2 
5.14 1,2,4-trichlorobenzene 
214 136 Soluble Cloudy stiff gel 
SYNDIO2 
5.58 1-benzyl-2-pyrrolidinone 
420 224 Soluble Amber hazy stiff gel 
SYNDIO2 
5.58 1-benzyl-2-pyrrolidinone 
420 200 Partly soluble 
SYNDIO2 
5.26 1-chloronaphthalene 
258 136 Soluble Hazy stiff gel 
SYNDIO2 
5.16 1-cyclohexyl-2-pyrrolidinone 
301 136 Partly soluble 
SYNDIO2 
5.16 1-cyclohexyl-2-pyrrolidinone 
301 150 Soluble Amber soft hazy gel 
SYNDIO2 
5.13 1-ethyl-2-pyrrolidinone 
296 161 Soluble Pale yellow opaque 
slush 
SYNDIO2 
5.15 1-methyl-2-pyrrolidinone 
202 136 Soluble Cloudy stiff gel 
SYNDIO2 
5.04 1-phenyl-2-pyrrolidinone 
345 200 Soluble Tan opaque hard solid 
SYNDIO2 
5.09 4-phenylphenol 321 225 Soluble White opaque hard 
solid 
SYNDIO2 
5.09 4-phenylphenol 321 200 Almost soluble 
SYNDIO2 
5.13 acetophenone 202 165 Soluble Cloudy gel above solid 
SYNDIO2 
5.13 acetophenone 202 150 Almost soluble 
SYNDIO2 
5.01 anisole 154 153 Soluble Cloudy stiff gel 
SYNDIO2 
5.04 benzil 347 200 Soluble Yellow opaque hard 
solid 
SYNDIO2 
5.04 benzil 347 150 Partially soluble 
SYNDIO2 
5.05 benzophenone 305 188 Soluble Clear stiff gel 
SYNDIO2 
5.05 benzophenone 305 165 Partly soluble 
SYNDIO2 
5.67 benzyl alcohol 205 190 Almost soluble 
SYNDIO2 
5.67 benzyl alcohol 205 204 Soluble White opaque soft gel 
SYNDIO2 
5.12 butyl stearate 343 273 Soluble White opaque fluid 
SYNDIO2 
5.12 butyl stearate 343 250 Partly soluble 
SYNDIO2 
5.09 caprolactam (epsilon) 
271 200 Soluble Hard solid 
SYNDIO2 
5.10 cyclohexanone 155 150 Soluble soft gel 
SYNDIO2 
5.20 decahydronaphthalene (decalin) 
190 188 Almost soluble 
Moderately stiff slush 
SYNDIO2 
5.18 dimethyl phthalate 
282 200 Partly soluble 
SYNDIO2 
5.18 dimethyl phthalate 
282 224 Soluble White opaque slush 
SYNDIO2 
5.02 diphenyl ether 259 150 Soluble Clear stiff gel 
SYNDIO2 
5.02 diphenyl ether 259 136 Partly soluble 
SYNDIO2 
5.28 diphenyl sulfone 
379 225 Soluble Pale tan hard solid 
SYNDIO2 
5.19 ethyl benzoate 212 165 Almost soluble 
SYNDIO2 
5.19 ethyl benzoate 212 188 Soluble Stiff pale yellow hazy 
gel 
SYNDIO2 
5.34 HB-40 (Monsanto) 
325 151 Partly soluble 
SYNDIO2 
5.34 HB-40 (Monsanto) 
325 200 Soluble Slightly hazy pale 
yellow 
firm gel 
SYNDIO2 
5.13 Mesitylene (1,3,5-trimethyl 
163 161 Almost soluble 
Stiff heterogeneous 
gel 
benzene) 
SYNDIO2 
4.97 methyl benzoate 199 150 Soluble Cloudy stiff gel 
SYNDIO2 
5.04 methyl laurate 262 250 Soluble White opaque slush 
SYNDIO2 
5.04 methyl laurate 262 224 Almost soluble 
SYNDIO2 
4.96 methyl myristate 
323 241 Hazy & soluble?? 
SYNDIO2 
4.96 methyl myristate 
323 255 Soluble Opaque white slush 
SYNDIO2 
5.07 methyl salicylate 
222 175 Soluble Cloudy stiff gel 
SYNDIO2 
5.07 methyl salicylate 
222 150 Not soluble 
SYNDIO2 
5.06 methyl stearate 359 273 Soluble Opaque solid 
SYNDIO2 
5.06 methyl stearate 359 250 Partly soluble 
SYNDIO2 
5.13 nitrobenzene 211 151 Soluble Yellow cloudy firm gel 
SYNDIO2 
4.82 N,N-dimethylacetamide 
165 165 Not soluble 
white slush 
SYNDIO2 
5.04 N,N-diphenylformamide 
337 225 Soluble Brown hard solid 
SYNDIO2 
5.04 N,N-diphenylformamide 
337 200 Almost soluble 
SYNDIO2 
5.13 o-dichlorobenzene 
180 150 Soluble Cloudy stiff gel 
SYNDIO2 
5.13 9-dichlorobenzene 
180 136 Partly soluble 
SYNDIO2 
5.00 Santowax R (Monsanto) 
364 166 Partially soluble 
SYNDIO2 
5.00 Santowax R (Monsanto) 
364 200 Soluble Tan hard solid 
SYNDIO2 
5.00 sulfolane 285 200 Not soluble 
SYNDIO2 
5.00 sulfolane 285 249 Soluble Light tan opaque firm 
gel 
SYNDIO2 
5.00 sulfolane 285 225 Partially soluble 
SYNDIO2 
5.27 tetrahydronaphthalene (tetralin) 
207 136 Soluble Stiff hazy gel 
SYNDIO2 
5.15 Therminol 66 (Monsanto) 
340 200 Soluble Slightly hazy pale 
yellow 
soft gel 
SYNDIO2 
5.15 Therminol 66 (Monsanto) 
340 151 Partly soluble 
SYNDIO2 
4.99 Therminol 75 (Monsanto) 
385 200 Soluble Yellow opaque firm 
solid/gel 
SYNDIO2 
5.25 xylene 141 136 Soluble Moderately stiff white 
opaque gel 
SYNDIO2 
4.98 cyclohexylbenzene 
239 181 Soluble Cloudy firm gel 
SYNDIO2 
4.98 cyclohexylbenzene 
239 158 Almost Soluble 
SYNDIO2 
4.99 dicyclohexyl 227 200 Mostly soluble 
SYNDIO2 
4.99 dicyclohexyl 227 225 Soluble Homogeneous slush 
SYNDIO2 
4.98 methyl caproate 151 151 Not soluble 
Clear liquid with 
solid 
polymer sediment 
SYNDIO2 
5.01 methyl caproate 194 194 Not soluble 
Milky liquid with 
solid 
sediment 
SYNDIO2 
4.94 methyl enanthate 
172 172 Not soluble 
Water-clear liquid 
with 
polymer sediment 
SYNDIO2 
4.99 Methyl valerate 128 128 Not soluble 
Water-clear liquid 
with 
solid sediment 
SYNDIO2 
4.96 n-butylbenzene 182 183 Mostly soluble 
White opaque soft gel 
SYNDIO2 
4.96 n-butylbenzene 182 169 Heavily swollen 
SYNDIO2 
4.96 n-butylbenzene 182 151 Not soluble 
SYNDIO2 
5.00 n-propylbenzene 159 158 Soluble White opaque firm gel 
SYNDIO2 
5.04 phenetole 169 128 Swollen 
SYNDIO2 
5.04 phenetole 169 150 Soluble Hazy pink firm gel 
SYNDIO2 
5.35 phenol 182 155 Swollen 
SYNDIO2 
5.35 phenol 182 158 Almost soluble 
SYNDIO2 
5.35 phenol 182 181 Soluble Opaque white firm 
__________________________________________________________________________ 
gel 
EXAMPLE 2 Syndiotactic Polystyrene Hollow Fiber Membranes Prepared Without 
Drawing From A Protic Solvent 
Hollow fibers are prepared from an extrusion blend containing about 35 
percent syndiotactic polystyrene (Mw of about 450,000 daltons) and about 
65 percent caprolactam by weight. The blend is mixed while heating to 
about 270.degree. C. homogenize the blend and the blend is then 
transferred to a Ram extruder. Hollow fibers are extruded at about 
235-240.degree. C. using a single hole spinnerette and nitrogen as the 
core gas. The fibers are taken up on a chilled godet roll at a rate of 
about 30 feet/minute. The fibers as extruded have an internal diameter of 
about 539 microns and a wall thickness of about 61 microns. 
Samples of the fibers are leached in water at ambient temperature and about 
80.degree. C. respectively for about 21/2 hours to remove the caprolactam. 
The fibers are vacuum dried at about 50.degree. C. and fabricated into 
test cells for the evaluation of gas flux. The fibers leached at ambient 
temperature possess an internal diameter of about 397 microns and a wall 
thickness of about 46 microns. The fibers leached at about 80.degree. C. 
possess an internal diameter of about 366 microns and a wall thickness of 
about 52 microns. 
The fibers are internally pressurized with nitrogen at room temperature and 
about 15 psig and the rate of permeating nitrogen measured. Fibers leached 
in water at ambient temperature possess a nitrogen flux of of about 
0.051.times.10.sup.-6 cm.sup.3 (STP)/(cm.sup.2 sec cmHg) and fibers 
leached in water at about 80.degree. C. possess a nitrogen flux of about 
2.times.10.sup.-6 cm.sup.3 (STP)/(cm.sup.2 sec cmHg). 
EXAMPLE 3 Syndiotactic Polystyrene Hollow Fiber Membranes Prepared With 
Drawing Before Leach From A Protic Solvent 
Hollow fibers are extruded as described in Example 2. The fibers have an 
internal diameter of about 569 microns and a wall thickness of about 48 
microns as extruded. 
The fibers, which still contain caprolactam, are placed in an air oven at 
about 80.degree. C. for about 3 minutes, and then drawn about 100%. After 
drawing, the fibers are restrained at their drawn length, cooled, and then 
fabricated into test cells. The inside and the outside of the potted 
fibers are flushed with running water for about 5 minutes to remove the 
caprolaotam. The final drawn fibers possess an internal diameter of about 
339 microns and a wall thickness of about 35 microns. 
The fibers are dried in a vacuum oven prior to testing for gas flux. The 
fibers possess a nitrogen flux of about 1.1.times.10.sup.-3 cm.sup.3 
(STP)/cm.sup.2 sec cmHg). 
The pores of the fibers are wetted with ethanol and the ethanol then 
replaced with water. The fibers possess a water flux of about 6 ml/(hr 
m.sup.2 cmHg). 
Bubble point testing indicates that the pores of the membrane are smaller 
than about 0.2 microns. 
EXAMPLE 4 Syndiotactic Polystyrene Hollow Fiber Membranes Prepared With 
Drawing During Leach From A Protic Solvent 
The fibers as extruded are the same as described in Example 3. 
The fibers are leached in a first glycerine bath at ambient temperature for 
about 1 hour and then transferred to a second glycerine bath at about 
130.degree. C. The fibers are immediately drawn about 100% of their 
original length. After cooling, the fibers are fabricated into test cells. 
The outside of the fibers are washed with ethanol for about 25 minutes to 
remove the caprolactam and glycerol. The fibers are then dried. The fibers 
have an internal diameter of about 356 microns and a wall thickness of 
about 32 microns. The fibers possess a nitrogen flux of about 
0.2.times.10.sup.-3 cm.sup.3 (STP)/cm.sup.2 sec cmHg). 
The pores of the fibers are wet with ethanol and the ethanol then replaced 
with water. The fibers possess a water flux of about 4.8 ml/(hr m.sup.2 
cmHg). 
Bubble point testing indicates that the pores of the membrane are smaller 
then about 0.25 microns. 
EXAMPLE 5 Syndiotactic Polystyrene Hollow Fiber Membranes Prepared With 
Drawing After Leach From A Protic Solvent 
The fibers as extruded are the same as described in Example 3. 
The fibers are leached in water at ambient temperature for about 2 hours. 
The fibers are then vacuum dried at ambient temperature for about 2 hours. 
The fibers are placed in a glycerine bath at about 130.degree. C. and drawn 
to about 100% of their original length after a residence time in the bath 
of about 1 sec. The fibers are then washed with water to remove any 
remaining glycerol. The drawn fibers have an internal diameter of about 
327 microns and a wall thickness of about 29 microns. 
The fibers are fabricated into test cells and the nitrogen flux determined 
to be about 0.7.times.10.sup.-5 cm.sup.3 (STP)/cm.sup.2 sec cmHg). 
The flat sheet membranes of Examples 6-8 are evaluated as follows. The 
membranes are dried in a vacuum oven at room temperature for at least 24 
hours. The membranes are then placed in an Amicon test cell on top of a 
macroporous support disk. The effective membrane area is about 12.5 
cm.sup.2 The membrane is first checked for leaks by filling the cell with 
water, pressurizing the cell to about 5 psi, and measuring any decay in 
pressure once the pressure source is cut off. The water is then emptied 
from the cell and the membrane dried. The nitrogen flux through the dry 
membrane is then measured. The membrane is then wetted with isopropyl 
alcohol, followed by water. The water flux through the membrane is then 
measured. The membrane is then again wetted with alcohol and dried prior 
to measuring the bubble point, mean pore size, and maximum pore size in 
accordance with ASTM F316-86. 
EXAMPLE 6 Syndiotactic Polystyrene Flat Sheet Membrane Prepared From An 
Aprotic Solvent 
Syndiotactic polystyrene with a weight-average molecular weight of about 
5.6.times.10.sup.5 daltons is used to prepare a solution containing about 
30.0% syndiotactic polystyrene and 70.0% o-dichlorobenzene. The mixture is 
dissolved by heating at about 176.degree. C. for about 3 hours with 
repeated inversion of the vial containing the solution to ensure 
homogeneity. 
The hot solution is cast between glass plates at about 157.degree. C. The 
glass plates are quenched to room temperature. The film is extracted in 
isopropyl alcohol for about 4.5 hours after being removed from the glass 
plates. The film is dried under vacuum at room temperature for about 17.5 
hours. 
The nitrogen flux of the membrane at room temperature is about 
##EQU8## 
EXAMPLE 7 Syndiotactic Polystyrene Flat Sheet Membrane Prepared From An 
Aprotic Solvent 
The syndiotactic polystyrene described in Example 6 is used to prepare a 
solution consisting of a mixture of about 30.0% syndiotactic polystyrene 
and about 70.0% nitrobenzene. The mixture is dissolved by heating at about 
200.degree. C. for about 3 hours with repeated inversion of the vial 
containing the solution to ensure homogeneity. 
The hot solution is poured between two glass plates at about 158.degree. C. 
The glass plates are quenched to room temperature. After removing the film 
from the plates, it is soaked in isopropyl alcohol at room temperature for 
about 3 hours. The film is dried under vacuum at room temperature 
overnight. 
The membrane exhibits a nitrogen flux at room temperature of about 
##EQU9## 
EXAMPLE 8 Syndiotactic Polystyrene Flat Sheet Membrane Prepared From An 
Aprotic Solvent 
A solution of about 30.0% of syndiotactic polystyrene with a molecular 
weight in excess of 3.times.10.sup.6 daltons and about 70.0% 
o-dichlorobenzene is cast, quenched, extracted, and dried as described in 
Example 7. 
The membrane characteristics are measured according to ASTM F316-86 and 
F317-86 and F317-72 (1982). The porosity is about 47.3%, the mean pore 
size is about 460 Angstroms, and the maximum pore size is about 500 
Angstroms. The membrane exhibits a water flux of about 100.1 GFD at 50 psi 
.