Poly(ether sulfone) membranes for desalination: membrane preparation and characterization

Desalination, 56 (1985) 381--394

381

Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

POLY(ETHER MEMBRANE

Teresa Dept.

SULFONE)

MEMBRANES

PREPARATION

P. V e n t o z a of C h e m .

FOR DESALINATION:

AND CHARACTERIZATION.

and

Douglas

R.

Eng., T h e Univ.

Lloyd

of T e x a s ,

Austin,

TX, USA.

ABSTRACT Desalination membranes have been prepared from sulfonated poly(ether sulfone). These a s y m m e t r i c s t r u c t u r e s w e r e p r e p a r e d by casting from a ternary system comprised of t h e p o l y m e r , a nonsolvent, and a swelling a g e n t (that is, a c o s o l v e n t casting solution). Phase inversion was induced by a combination of evaporation and gelation in a s e c o n d nonsolvent. Membrane performance (flux and salt rejection) was related to t h e c o m p o s i t i o n of the initial c a s t i n g solution, the c o m p o s i t i o n of the c a s t f i l m at the t i m e of g e ] a t i o n , a n d the s o l u t i o n history. Partial solubility parameters were used to q u a n t i f y the compositions a n d to f a c i l i t a t e a v i s u a l i z a t i o n of the m e m b r a n e p r e p a r a t i o n process. INTRODUCTION The classic skinned,

method

[I] ,for p r e p a r i n g

phase-inversion

membranes

comprised

of a polymer,

a volatile

nonsolvent

or s w e l l i n g

on a s u p p o r t period

immersed medium

with

of partial

is cast as a thin

knife. F o l l o w i n g the

precipitation

method

and

membranes,

Casting

can be a d a p t e d

solvents

at room

None

are not s o l v e n t s

0011-9164/85/$03.30

Elsevier Science Publishers B.V.

volatility

of these

membrane

is the use of a c o m b i n a t i o n

that i n d i v i d u a l l y

Details

For s o m e p o t e n t i a l l y

the a p p l i c a t i o n

economical

for

a suitable

can be found.

are of l i m i t e d

temperatures.

this

developed

from such s o l v e n t s

temperature,

in t e r m s of e f f i c i e n t

alternative

[2-5].

are

for the p r e p a r a t i o n

provided

nonsolvent

membranes

or the use of e l e v a t e d

One v i a b l e

and p o l y m e r

elsewhere

the a v a i l a b l e

evaporation

is d e s i r a b l e

inversion

originally

film

an optional

and support It is w i t h i n

method,

less v o l a t i l e

presented

are

temperature.

prolonged

This

film

medium.

from a v a r i e t y of p o l y m e r s

solvent

useful polymers,

vacuum,

solution

evaporation,

take place.

solution

and a less v o l a t i l e

the aid of a casting

cellulosic

of m e m b r a n e s volatile

with a polymer

solvent,

that the final stages of phase

preparing

at r o o m

This

begins

or i n t e g r a l l y

solvent

in a n o n s o l v e n t

solidification

of this

agent.

asymmetric

for the p o l y m e r

requires of options

production.

of t w o liquids but

in

382

combination

in a p p r o p r i a t e

the p o l y m e r

proportions

[6-8]. S e l e c t i n g

a volatile

component

advantages

of this a p p r o a c h

The c o m p o n e n t s rel a t i v e

solvent

system

of the r e s u l t i n g

interactions

These

in solution,

the rate at w h i c h

polymer

the p r e c i p i t a t i o n and

performance

medium,

~ with

contributions

to c o h e s i v e

procedures

upon and further

solution

illustrates

components,

variables

used

supplied

City,

is a c t u a l l y

exchange

in

structure

%' ~p' and ~h

bonding

This p a r a m e t e r

for material

[20,21].

the u s e f u l n e s s

of

variables

solution

is the

parameters

membrane

has

selection

This

paper

expands

this p a r a m a t e r

set

such as casting

composition,

and m e m b r a n e

set

preparation

and d u r a t i o n

a relationship

between

performance

of

these

is f a c i l i t a t e d

parameters.

in this s t u d y w a s

by I m p e r i a l

Hartfordshire, a copolymer

interactions

science

Establishing

by the use of s o l u b i l i t y

Garden

and the

the m e m b r a n e

[10-15].

preparation

casting

period.

casting p r o c e d u r e

sulfone)

energy)

and p e r m e a t i o n

membrane

the e v a p o r a t i o n

The p o l y m e r

conformation

- nonsolvent

and h y d r o g e n

of e x p l a i n i n g

[6-9,18,19]

in d e t e r m i n i n g

polar,

use in m e m b r a n e

and as a m e a n s

these

its partial

the dispersive,

[16,17]

of the

and its

evaporates

and, u l t i m a t e l y ,

(representing

found p r e v i o u s

The choice of

[5].

paramater

solubility

the p o l y m e r

the solvent

for characterizing

One choice

and their

on the structure

[7-9].

the rate of solvent

precipitates,

in this paper.

g o v e r n macromolecular

interactions

includes

and

the nature and s t r e n g t h

between

environment.

that

solution

influence

membrane

determines

system

The v a l i d i t y

the c a s t i n g

have a strong

components

physicochemical

easy.

are d e m o n s t r a t e d

comprise

concentrations

and p e r f o r m a n c e

such a c o s o l v e n t

is r e l a t i v e l y

that

are capable of d i s s o l v i n g

a sulfonated

poly(ether

Industries,

PLG ( W e l w y n

Chemical

AL7

IHD,

United Kingdom).

of the f o l l o w i n g

The p o l y m e r

repeat units:

oO Essentially units

are d i s u b s t i t u t e d

stituted evidently units.

all of the b e n z e n e d i o l

[22].

while

The degree

refers

The d e g r e e

units are sulfonated;

o- and p-diol

of s u b s t i t u t i o n

to the n u m b e r

f r a c t i on

of s u b s t i t u t i o n

m-diol

units are m o n o s u b -

stated by the

of s u b s t i t u t e d

of the p o l y m e r

supplier II repeat

considered

here

383

was given

as 1:10 indicating

a substituted

II

EXPERIMENTAL

that one in every

ten repeat

units

was

unit.

PROCEDURES

Materials The polymer shaped for

beads

24 h o u r s

liquids

used

stored

liquid

grade

liquids

ethyl

studies

certified

to use.

The partial

obtained

from

of d r o p l e t -

They

were

dried

in a d e s i c c a t o r .

were

of

further

casting

solubility

were

spectro-

formamide,

these

All

the p u r i s t

ACS grade

E a c h of

Reference

mm).

used without

for m e m b r a n e

alcohol.

in t h e f o r m

vacuum

and were

used

formate,

received

= 3.74±0.45

under

isopropyl

prior

was

and

the s o l u b i l i t y

The

grade

degassed

150°C

available

grade

was

at

purification.

reagent

above

diameter

for

commercially

scopic

described

(approximate

was

and

filtered

parameters

and

for e a c h

15.

MeThods. Solubility

Studies.

experimentally polymer.

This

a preweighed container,

was

the b e a d s

size

remained

or a h e t e r o g e n e o u s

~

increased

(bead size,

components. estimated

In some

In s u c h c a s e s

using

= ~i ~il

the

beads was

or

at a 95%

t-test

was a mixture

of two

parameters w e r e

solubility

[23,24]:

the

~'s are

the volume

and i and 2 represent

the two

liquids

the m i x t u r e .

compositions nonsolvent

a

a

+ ~2 ~ i 2

of the two liquids,

Asymmetric

the

of a

formed),

occured),

amount

relationship

i = d, p, o r h; m = m i x t u r e ,

fractions

some

size

whether

and a clear

suspension

the liquid

the p a r t i a l

following

to

the

to a n accuracy of ± l O - 4 m m ,

significant cases

was

35 m a s s - %

of l i q u i d

to d e t e r m i n e

(either

in b e a d

measured

by a statistically level).

solvent

change

mass

sealing

dissolved

a partial

(no s i g n i f i c a n t

15 a n d

temperature for a m i n i m u m

then evaluated

(all of

for

accuracy),

at t h e d e s i r e d

The sample

of the p o l y m e r

the a p p r o p r i a t e

(±lO-4g

formed),

confidence

forming

by adding

was

swellinq

where

region

at 4 ° , 25 ° , a n d 3 7 ° C

sample

a solvent

nonsolvent

~im

done

and agitating

liquid was

reduced

was

polymer

of 48 hours.

solution

The solubility

determined

Membrane were

ethyl

Preparation.

prepared formate

by adding

and

Casting

solutions

a mixture

the less v o l a t i l e

of

of v a r i o u s

the v o l a t i l e

swelling

agent

384

formamide

to a p r e w e i g h e d p o l y m e r sample.

The s o l u t i o n w a s cast a t

25°C onto a g l a s s p l a t e u s i n g a G a r d n e r k n i f e (0.381 mm). A f t e r

an a p p r o p r i a t e

p l a t e and the f i l m w e r e

immersed

evaporation

set

at 0.015

period,

in the n o n s o l v e n t

alcohol,

IPA. The IPA w a s s u b s e q u e n t l y e x c h a n g e d

membrane

characterization.

inches

the g l a s s

isopropyl

for w a t e r p r i o r

W h i l e the c o m p o s i t i o n of the initial

c a s t i n g s o l u t i o n w a s k n o w n a priori,

the o v e r a l l

c o m p o s i t i o n of the

cast s o l u t i o n at the t i m e of i m m e r s i o n in the g e l a t i o n m e d i u m to be d e t e r m i n e d e x p e r i m e n t a l l y . weight

to

had

This w a s d o n e by m o n i t i o r i n g the

loss d u r i n g the e v a p o r a t i o n s t e p and a t t r i b u t i n g

the w e i g h t

c h a n g e to the loss of the s i g n i f i c a n t l y m o r e v o l a t i l e ethyl formate. Membrane Performance Characterization.

T w o m e m b r a n e s w e r e cut

f r o m e a c h f i l m and m o u n t e d in h i g h - p r e s s u r e r a d i a l - f l o w membranes

w e r e o r i e n t e d s u c h that the s k i n s u r f a c e w a s

high-pressure

feed and

the s p o n g y u n d e r s i d e

filter p a p e r and a p o r o u s steel plate. p r e s s u r i z e d at

12,400 kPa

The

f a c i n g the

s u p p o r t e d by a

The m e m b r a n e s w e r e

(1800 psi) until

became essentially constant

was

cells.

the pure

(requiring a m i n i m u m

water

flux

of three hours).

E a c h e x p e r i m e n t c o n s i s t e d of three steps w i t h a feed f l o w rate of 420±7 ml/min

and a fixed feed pressure. First a pure water feed

w a s u s e d to d e t e r m i n e input w a s

switched

the initial p u r e w a t e r p e r m e a b i l i t y .

pressure. B e f o r e a c t u a l l y m e a s u r i n g concentration,

at

original

the

the flux and p e r m e a t e

least i0 ml of p e r m e a t e w a s g a t h e r e d and

d i s c a r d e d to a s s u r e m e m b r a n e solution.

Next

to the d e s i r e d salt feed s o l u t i o n at the s a m e

equilibrium

w i t h the e l e c t r o l y t e

The feed c o n c e n t r a t i o n c h a n g e d by less than I% of its value during

the e x p e r i m e n t .

Finally,

the p u r e w a t e r

p e r m e a b i l i t y w a s d e t e r m i n e d a g a i n to m o n i t o r any c h a n g e that m a y h a v e occurred. volume.

The fluxes w e r e m e a s u r e d

The feed and p e r m e a t e

YSI M o d e l 10,000 and

31 c o n d u c t i v i t y meter. 35,000 p p m

NaCI

Experiments

s o l u t i o n s and

CaCI 2 and Na2SO 4 at p r e s s u r e s of 2000, kPa

both g r a v i m e t r i c a l l y and by

concentration were determined using a

4000,

( d o w n s t r e a m p r e s s u r e w a s atmospheric).

c o n d u c t e d at r o o m

were conducted using

10,000 p p m 6000,

apparatus

and

of 10,000

The e x p e r i m e n t s w e r e

t e m p e r a t u r e and all data w e r e

D e t a i l s of the e x p e r i m e n t a l

solutions 8000,

converted

are g i v e n e l s e w h e r e

to 25°C. [25].

385

RESULTS AND DISCUSSION Solubility

Studies

The r e s u l t s (Table

of the s o l u b i l i t y

i) i n d i c a t e

polymer

and

pressures.

those

in this

cosolvent

construct

of s i m i l a r (Table

illustrative

used

formate,

investigation combination

purposes

agent

Membrane

preparation

the 25°C,

immersion

and partial

are p r e s e n t e d

information

gradient

is not available,

the c o m p o s i t i o n

I and

The

15-42/58

42 v o l u m e - ~ 18.5-57/43

is near

the edge

procedes

increases,

and

system

were

region

is

system

for further - swelling

is e x t r e m e l y

volatile

still

agent

and the

listed

of the the

film at the time of IPA.

Since

in Table

casting

skin at the

compositions

region

polymer

at two differe

in a cosolvent

EF) is on the e d g e c l o s e s t to F. The

the

concentration

Short

evaporation composition

the s o l u b i l i t y

region

15-% region.

increases,

of the s o l u b i l i t y

parameters

periods

of

to EF a n d

18.5 m a s s - %

from

discuss

2 is i l l u s t r a t e d

solution

solubility

15 m a s s - %

closest

the

of e v a p o r a t i o n

to q u a n t i t a t i v e l y

performance-controlling

the overall within

with

as a f u n c t i o n

the p o l y m e r

increases.

change

cast

significantly

the value

parameters

2 along

medium

film

the 15 m a s s - ~

evaporation

solutions

selected

solubility

initial

F a n d 58 v o l u m e - %

significantly

solubility

a nonsolvent

2.

regions.

The c o s o l v e n t

the

(indicating

does not d i f f e r

cosolvent

F, w a s

i and

or higher

solubility

15 m a s s - %

in Table

information

(not shown)

F/EF

smaller

it is not p o s s i b l e

2. The t w o

are seen to be w i t h i n locations:

in Figures

gelation

within

of the a c t u a l

of gelation.

in Figures

of

25°C results

temperatures

for the overall

into the n o n s o l v e n t

concentration

The

is not.

solutions

corresponding

The

agents

systems.

of a large n u m b e r

in this paper.

the n o n s o l v e n t

The c o m p o s i t i o n s casting

yield

it r e p r e s e n t s

in w h i c h

certain

and s w e l l i n g

as c o s o l v e n t

illustrated

EF, and formamide, because

swelling

time

i) w o u l d

that

d a t a n o t s h o w n h e r e w e r e u s e d to

plots at l o w e r

in all of the figures

ethyl

time

regions

the

p o i n t s and low v a p o u r

indicate

evaluated.

I plus similar

and 35 m a s s - ~

of d i s s o l v i n g

solvents,

can serve

that w e r e

the s o l u b i l i t y

concentrations

also

table are r e p r e s e n t a t i v e

in T a b l e

Construction

at 15 m a s s - ~

are capable

partial

proportions

proportions

presented

For

the results

of nonsolvents,

in the a p p r o p r i a t e results

studies

liquids

that do have high b o i l i n g

However,

combinations

few

that

As

the ratio of the

did not

of the films,

and

the

region w h e n quenched.

In

386

TABLE .

.

.

.

.

.

I Poly(ether .

,iq id

.

.

.

.

.

.

.

.

.

.

.

.

.

sulfone) .

.

.

.

.

.

.

.

.

.

.

solubility .

.

.

.

.

system

tetrahydrofuran 2 - p r o p a n o ] [IPA] m e t h y l e t h y l k e t o n e [MEK] e t h y l f o r m a t e [EF] f o r m a m i d e [F] d i m e t h y l f o r m a m i d e [DMF] 2-pyrrolidone n-methyl -2-pyrrolidone dimethy] sulfoxide n,n-dimethyl acetamide

7-butyrolactone ethylene diamine 4/96-F/EF IO/90-F/EF 25/75-F/EF 35/65-F/EF 40/60-F/EF 50/50-F/EF 65/45-F/EF 77/23-F/EF 86/14-F/EF 13/87-F/MEK 34/66-F/MEK 46/54-F/MEK 89/11-F/MEK 13/87-F/DMF 25/75-F/DMF 42/58-F/DMF

18.5-57/43

.

.

.

at .

.

.

.

15 m a s s - % .

.

.

.

.

.

.

.

.

.

.

35 mass-96

and .

.

.

.

.

.

.

.

.

35 °

NS NS NS NS NS SA S S S S S S

NS NS NS NS NS SA S S S S S S

NS NS NS NS NS SA S S S S S S

NS

NS PS PS PS S S S S S PS PS S S

NS PS PS PS S S S S S PS PS S S

4°

and

35 °

NS NS SA S SA S S

NS NS SA S SA S S

S S

S S

S S

SA=Swelling

solubility

25 °

NS NS SA S SA S S

S PS PS Solvent,

6h 35-~

25 °

S PS PS

Mass-~

F/EF

Time

Polymer

,VO1 .~

S S PS

Agent,

parameters

Evaporation (seconds)

15-42/58

.

4°

PS PS PS S S S S S PS PS S S

PS=Partial

2 Composition

Film

.

15-~

water

S=Solvent,

.

Solubility

(vol./vol. )

TABLE

.

7.6 8.2 7.7 7.8 7.6 84

7.8 28 30 41 41 128

207 39 88 25 41 93

85 95 88 90

67 85 60 80

55 55 35 50

82 93 93 76

56 81 36 45

50 36 20 43

77

50

46

78

63

54

79 79 80

72 76 85

81 82 83 78 80 81 83

98 108 116

59 62 67 75 81 86

53 71

34

81 118

56

85

75

60

85 85

82

65 71

48 85

93

NS=Non-Solvent.

of c a s t i n g

~d

~p

solutions

~h

~t

~v--o~.J

0 5 60 120

15.0 15.2 18.8 21.8

42/58 43/57 57/43 71/29

7.9 7.9 8.1 8.2

7.8 7.8 9.1 10.3

6.2 6.3 7.1 7.4

12.8 12.8 14.0 15.2

O 8 35 65

18.5 19.0 20.9 23.0

57/43 59/41 68/32 79/21

8.1 8.1 8.1 8.2

9.1 9.3 10.0 11.0

7.1 7.2 7.6 8.2

14.0 14.2 15.0 16.0

387 15

120 s.

8p

Figure i. Schematic representation of the 15 mass-% polymer solubility region plus the cosolvent composition initially (15-42/58) and after 5, 60 and 120 seconds evaporation.

15- 42/58

r------------------ 8 h

15

15

15

- . 6 5 s.

8p

8h

d 15

15

Figure 2. Schematic representation of the 15 mass-% polymer solubility region plus the cosolvent composition initially (18.5-57/43) and after 8, 35 and 65 seconds evaporation.

388

other cases

the o v e r a l l

c o m p o s i t i o n of the f i l m had c l e a r l y e x i t e d

from the r e g i o n p r i o r to queching.

These

results

be u s e d b e l o w

will

to e x p l a i n m e m b r a n e p e r f o r m a n c e ,

Membrane Characterization Membrane molar

performance was characterized

flux,

solution mass ppm

flux,

in t e r m s of pure w a t e r

and salt rejection:

salt in feed - p p m salt in p e r m e a t e

rejection = p p m s a l t in f e e d The e x p e r i m e n t a l

data presented below represent average values

found for the two m e m b r a n e s sheet.

On a v e r a g e ,

membranes

a g r e e d w i t h i n 8~ on a r e l a t i v e scale.

10,000 ppm NaCl basis

cut f r o m d i f f e r e n t s e c t i o n s of the s a m e

the flux and r e ~ e c t i o n v a l u e s

feed solution

for the discussion.

rejections,

were

shown

Similar

in F i g u r e s

results,

found for 35,000 p p m

for the t w o The r e s u l t s

3 a n d 4 s e r v e as a

but w i t h

NaCl

lower

feed.

S o l u t i o n flux and s o l u t e r e j e c t i o n by the m e m b r a n e s the

15-42/58

solution

and d e c r e a s e d behaviour

previous

rejection

with

c a s t i n g procedure;

increased evaporation

results using

for the t r a d i t i o n a l

however,

a cosolvent

is in k e e p i n g w i t h

Loeb-

approach cast

[7,8].

from

a trend of d e c r e a s i n g

increasing evaporation that e x p e c t e d

for

the

flux

time. This

the t r a d i t i o n a l

L o e b - S o u r i r a j a n p r o c e d u r e as d i s c u s s e d by K l e i n and These differences

flux

it is in k e e p i n g w i t h

casting system

(Figure 4) f o l l o w

and i n c r e a s i n g r e j e c t i o n w i t h behaviour

from

time. This

S o l u t i o n flux and s o l u t e r e j e c t i o n for the m e m b r a n e s 18.5-57/43 s o l u t i o n

cast

(Figure 3) f o l l o w a trend of i n c r e a s e d

is c o n t r a r y to that e x p e c t e d

Sourirajan

for the

Smith

[9].

can be e x p l a i n e d in t e r m s of the t h e r m o d y n a m i c

n a t u r e of t h e c a s t s o l u t i o n at the t i m e of i m m e r s i o n

in the

gelation medium. A solution near n a m i c a l l y poor;

the e d g e of the s o l u b i l i t y r e g i o n is t h e r m o d y -

that is, p o l y m e r - p o l y m e r

over p o l y m e r - s o l v e n t

interactions.

i n t e r a c t i o n s are

Thus the p o l y m e r

c o i l e d c o n f i g u r a t i o n and tend to a s s o c i a t e molecular

aggregates.

in the f o r m of s u p e r -

The t i g h t n e s s of these coils at e q u i l i b r i u m

d e p e n d s u p o n w h e t h e r the m a j o r c o m p o n e n t of the c o s o l v e n t nonsolvent

or a s w e l l i n g agent.

A solution closer

the e n v e l o p e is t h e r m o d y n a m i c a l l y good, a c t i o n s are

favoured.

favoured

chains assume a

Thus,

to the c e n t r e of

and p o l m e r - s o l v e n t

at e q u i l i b r i u m

is a

the p o l y m e r

a s s u m e an e x t e n d e d c o n f i g u r a t i o n and the s o l u t i o n m o r e

inter-

chains closely

389 0.35, • 8 SIC [VAP TI ur X 60 SiC [YAP TXur A 120 5~C EV/dP TZMr

o 0.30, x

'~ o., ,.~ O. |0

~

0.15

6

x ×

~Zo. to

× A

Figure 3. P e r f o r m a n c e characteristics for the m e m b r a n e s p r e p a r e d from the 15-42/58 casting solution and evaporated 5 ( O ) , 60 ( × ) and 120 ( ~ ) seconds.

x

~0.05 Q

0.00 0.8

G,

G

0.7 ::~ 0.8 ~0.5

0.2 &

A

0,1

~.

~.

A

:~.

k.

&

;.

~.

A

k.

;.

,%.

PRESSURE r..kPa3 x 10 -~

O. 3S O 0.30 -

e X A

$ E C EVkP TIM[ 3S EC (VAP T I l l 8S "mC [ V ~ TI Mr

"~ 0.88.jgu.,~j'm O.' 80 -

Figure 4. P e r f o r m a n c e c h a r a c t e r i s t i c s for the m e m b r a n e s p r e p a r e d from the 18.5-57/43 casting solution and e v a p o r a t e d 8 ( O ) , 35 ( x ) and 65 ( ~ ) seconds.

~ 0.10. ~

. OS' 0.00 0.7, 0.1, :~ o . 5 . ~0.4,

~ 0.3,

4

A

~0.8, 0.1. 0.o

t.

b.

:,.

b.

;.

PRESSURE [ k P o 3

k

k.

x 10 ..3

;.

;o.

390

resembles

a molecular

true s o l u t i o n the e n v e l o p e exists

cannot

exist

state.

because

the initial

solubilty

region

Further

from

interface,

the t h e r m o d y n a m i c

and

casting

quality

extend.

the s o l u t i o n

poor solution.

component

of the cosolvent

membranes

cast

through

this

transition

in s u p e r m o l e c u l e r seconds time

consists

permeable

to being poor;

Gelation

of s l i g h t l y

of this s o l u t i o n

The 1 2 0 - s e c o n d

however,

5-second

membrane.

more

Consequently,

than the 5-second

permeable

than the 6 0 - s e c o n d

g e l l e d as a molecular d i s p e r s i o n solution

of h i g h l y

aggregates extended

cannot

chains,

swollen pack

together

the initial

the s o l u b i l i t y

aggregates. swollen

move

closer

to exit

a more

formed

solution

loss of EF causes

(as the F content together.

the e n v e l o p e

immersion

casting

Continued and phase

in the n o n s o l v e n t

richer

membrane

is m o r e

membrane

is

the latter

is

is g e l l e d

as a

These

as the individual

permeable

is located exsists

near

to b e c o m e

increases)

causes

even

and

to

the c o m p o s i t i o n

to c o m m e n s e

medium.

casting

the edge of

as s w o l l e n

of the c o s o l v e n t evaporation

structure.

the 18.5-57/43

these a g g r e g a t e s

gelation

more

the s o l u t i o n

in the s w e l l i n g

aggregates.

from

separation

at the

a looser,

t h a n t h e y w e r e for the

the former

as c l o s e l y

region and the p o l y m e r

The

more

while

and thus p r o d u c e

of t i g h t c o i l s

returns

because

supermolecular

In the case of the m e m b r a n e s solution,

The m e m b r a n e

The 1 2 0 - s e c o n d

membrane

goes

expanded coils

the 1 2 0 - s e c o n d

membrane.

For the

solution

produces

is n o w

to

gelled after 60

evaporation

the cosolvent

moves

the m a j o r

to poor.

The m e m b r a n e

both

to a t h e r m o -

formamide.

the cast

a g e n t a n d the c o i l s a r e m o r e h i g h l y s w o l l e n

permeable

increase;

dispersion.

agent

to good

form.

of a d i s p e r s i o n

membrane.

region

however,

solution,

poor

the

the c o m p o s i t i o n

is the s w e l l i n g

from

and

concentration

w a s g e l l e d as a p o o r s o l u t i o n

aggregate

of gelation.

improves,

from

the.envelope,

w i l l r e t u r n the s y s t e m

the 15-42/58

gelled after 5 seconds

traverses

and m o l e c u l a r

In this case,

casting

the edge of the

As EF e v a p o r a t e s

the p o l y m e r

until

to

as a nonsolvent.

near

in the interfacial

to c o n t i n u e

Close

a

the p o l y m e r

15-42/58

located

of the c o s o l v e n t

t h e o t h e r s i d e of the e n v e l o p e

from

the

poor.

Simultaneously,

the e v a p o r a t i o n

dynamically

from

solution

restricting coll e x p a n s i o n

factors Allowing

the e n v e l o p e

the c o m p o s i t i o n

viscosity

occurs.

region

agent and the p o l y m e r

is b e h a v i n g

cast

is t h e r m o d y n a m i c a l l y

the s o l u t i o n - a i r

chains

separation

a swelling

the e n v i r o n m e n t

solution,

the s o l u b i l i t y

becomes

In the case of the m e m b r a n e s

polymer

Outside

and phase

the cosolvent

in a s w o l l e n

precipitates

dispersion.

prior to

The p h a s e separation

391

produces

polymer-rich

because

precipitate closely

in the

together.

immersion

4 indicate

in closer

a solution.

packing

60-second

2 indicates

concentrations

gelled

membrane

and s i m i l a r

significantly of p o l y m e r

more

chain

existed

chain

evaporation

unable

gelled

and 65-

of a phase-

period.

second

solution

permeable

and p e r f o r m a n c e

by c o m p a r i n g

was

A comparison

of the

lies

in the k i n e t i c s

18.5-57/43

casting

swollen

solution,

equilibrium

was

insufficient

for

(that

is,

contraction)

and

of tightly

coiled

capable

within

aggregates. chains.

to their m a x i m u m

fully e x t e n d e d

The

60-second

swollen

viscous

possible

of g e l a t i o n

the

The 15-42/58

of s u p p o r t i n g

this s o m e w h a t

solution

size d u r i n g

the

18.5-57/43

and p r o d u c e d

the

state.

period

at the time more

of

The e x p l a n a t i o n

a cosolvent

Thus,

in t e r m s

to be

In the

the chains

membrane.

polymer

film

in a h i g h l y

consisted

is

the

8-second

properties

in the form of s w o l l e n

to e x t e n d

second

the 60 8-

a more

membrane.

T h e p l o t s of f l u x as a f u n c t i o n and 4 a p p e a r resistance.

to be r e l a t i v e l y Kimura

quantifying

of p r e s s u r e s

is first

normalized

through:

in F i g u r e s

good

a means

of

yielding

over a range the pure

in t e r m s of

A is linearly

3

compaction

of a membrane.

determined

for p r e s s u r e

A expressed

H20 / m 2 s kPa. T y p i c a l l y pressure

resistance

experimentally

coefficient

shown

indicating

[26] have p r o p o s e d

or c o m p a c t i o n

permeability

permeability

of p r e s s u r e

straight,

and S o u r i r a j a n

the s t r e n g t h

The pure w a t e r

operating

probably

is, 2 and

the latter

rearrangement

produced

however,

shows

evaporation

was gelled solution

structure

parameters.

permeable.

8-second

substantial

kmoles

8 seconds

18.5-57/43

cosolvent

movement.

molecules

The short

water

that

in F i g u r e s

the 3 5 - s e c o n d

can be made

and the

of the m e m b r a n e s

were

to

and the

in the film; shown

that these t w o films have s i m i l a r

and s o l u b i l i t y

coils;

chains

and pack prior

in the g e l a t i o n

that m e m b r a n e

history

composition

casting

after

result

performance

solution

period

depths

On the o t h e r h a n d

illustration

a f u n c t i o n of s o l u t i o n

polymer

aggregates

of the a g g r e g a t e s

to g r e a t e r

probably

chains s w o l l e n

the p o l y m e r

mixture.

A direct

15-42/58

of p o l y m e r

immersion,

s k i n is f o r m e d . T h e r e s u l t s

evaporations

separated

Upon

the e v a p o r a t i o n

that the m e m b r a n e

still

Table

comprised

of these s w o l l e n

of this effect

a less permeable

second

form

Extending

results

advancement

while

domains

of the high F content.

related

to the

392

in A = in A o - G where

pressure.

The

resistance prepared For

~ is a n

of

the

membrane.

the

15-42/58

comparative

membranes

purposes,

values

comparable

by extrapolating

of A obtained

constant

from

determined of

P

A o is t h e v a l u e

indication

The

G

values

casting also

listed

water

are more

fluxes.

resistant

to zero

the c o m p a c t i o n

for

solution

the m e m b r a n e s

are

listed

in T a b l e

3.

is a r a n g e of e x p e r i m e n t a l l y

[27] for a s y m m e t r i c

pure

of

cellulose

acetate

The sulfonated

to c o m p a c t i o n

membranes

poly(ether

by two orders

sulfone)

of

magnitude. TABLE

3

Compaction

constants Membrane

15-42/58

5-seconds 60-seconds 120-seconds

An

indication

can be obtained various

by studying

electrolytes.

and Na2SO 4 were the

15-42/58

Comparison membrane anion

more of

membrane

a net negative

readily

than

more

cation

strongly

on a basis

(Ca2+).

rejects

of s i m i l a r

material

of

to r e j e c t

of

the

the m e m b r a n e 10,000 ppm

membranes.

The

in F i g u r e

which

monovalent

rejects

anion

a n d CaCl 2 s h o w s

the m o n o v a l e n t trends

molarities

4 C o m p a r i s o n of r e j e c t i o n s m e m b r a n e at 2000 kPa.

NaCl,

and Na2SO 4 indicates charge

the

for N a C l

These

of the m e m b r a n e

are presented

for N a C I

the r e s u l t s

membrane

solutions

to c h a r a c t e r i z e

results

possesses

divalent

TABLE

used

nature

the a b i l i t y

Binary

5-second

of t h e

(SO~)

Comparison

made

of t h e c h e m i c a l

1.9 x 10 -5 2.2 x 10 -5 4.1 x 10 -5

hold

when

as w e l l

CaCl2,

results

that

the d i v a l e n t

that

the

(Na-)

than

the c o m p a r i s o n as shown

Solute

Molarity

Rejection

NaCl NaCl CaCl 2 Na2SO 4

0.171 0.086 0.090 0.084

42.1 59.6 14.4 85.8

the

(CI-).

cation

b y the 1 5 - 4 2 / 5 8

for

5.

in T a b l e

5-second

the

is 4.

393

t.O 8

0.0 0.0

+

S

S

+

+

~0.7 L,. 0.0

o

~

ra

0.5

ra

~0.4 0.3

I MeISO4 10.000 ppI + Heel tO. 000 ppm CoO]t tO. O00 ppm

0.2

0.1

PRE.':;SURE [.kPa3 x 10 -3

Figure

5.

Solute

rejection

15-42/58 Na2so 4

verses

pressure

5 second e v a p o r a t i o n

behaviour

membrane

( + ), and CaCl 2 ( ~ )

( * ), N a C l

for the

with

i0,000 p p m

feed solutions.

CONCLUSIONS Sulfonated a cosolvent

poly(ether

casting

and a s w e l l i n g function

sulfone)

solution

agent.

composition

considerably

cellulose

acetate

prior

to be a

casting

solution,

greater

to gelation. resistance

membranes

the

a n d the

The m e m b r a n e s to c o m p a c t i o n

of s i m i l a r

from

a nonsolvent

was s h o w n

of the c a s t f i l m at the t i m e of g e l a t i o n ,

to have

asymmetric

have been p r e p a r e d

of the polymer,

performance

of the o r i g i n a l

h i s t o r y of the cast s o l u t i o n found

comprised

Membrane

of the c o m p o s i t i o n

membranes

were

than

permeability.

REFERENCES 1. 2. 3. 4. 5.

S.Loeb and S.Sourirajan, Advan. Chem. Set. 38 (1962) 117-132. L . P a g e a u a n d S . S o u r i r a j a n , J. Appl. P o l y m . Sci. 16 (1972) 3185-3206. R.E.Kesting, in " M a t e r i a l s Science of S y n t h e t i c M e m b r a n e s " D . R . L l o y d (Ed.), A m e r i c a n C h e m i c a l Society, W a s h i n g t o n , 1985. H.Strathmann, ibidem. K . K a m i d e and S.Manabe, ibidem.

394

6.

7. 8. 9.

10.

ii. 12. 13. 14. 15. 16.

17. 18.

D.R.Lloyd, L . E . G e r l o w s k i , C.D.Sunderland, J.P.Wightman, J.E.McGrath, M.Iqbal and Y.Kang, in "Synthetic M e m b r a n e s " A . F . T u r b a k (Ed.), Amer. Chem. Soc., Washington, 1981. D.R.Lloyd, K . E . K i n z e r , J . P . W i g h t m a n , a n d J . E . M c G r a t h D e s a l i n a t i o n 46 (1983) 327-334. K . E . K i n z e r , D.R.Lloyd, M.S.Gay, J . P . W i g h t m a n , B . C . J o h n s o n , a n d J.E.McGrath, J. Membr. Scl. 22 (1985) 1-29. E.Klein and J.K.Smith, in "Reverse O s m o s i s M e m b r a n e Research" H.K.Lonsdale and H.E.Podall (Eds.), P l e n u m Press, N e w York, 1972. C.M.Hansen

36

19. 20. 21. 22. 23. 24. 25. 26. 27.

and

K.Skaarup,

J.

Paint

Tech.

39

(1967)

511-514.

C.M.Hansen, J. Paint Tech. 39 (1967) 104-117. C.M.Hansen, J. Paint Tech. 39 (1967) 505-510. C.M.Hansen, I&EC Prod. R&D 8 (1969) 1-11. C.M.Hansen, J. Paint Tech. 42 (1970) 660-664. C.M.Hansen and A.Beerbower, in "Encyclopedia of Chem. Tech. Suppl. Vo]." A . S t a n d e n (Ed.), Interscience, N e w York, 1971. D.R.LIoyd and T.B.Meluch, in " M a t e r i a l s Science of S y n t h e t i c M e m b r a n e s " D.R.Lloyd (Ed.), Amer. Chem. Soc., W a s h i n g t o n , 1985. R.T.Chern, W . J . K o r o s , H . B . H o p f e n b e r g , a n d V . T . S t a n n e t t , ibidem. C.Friedrich, A.Drlancourt, C.Noel and L.Monnerie, D e s a l i n a t i o n

(1981) 3 9 - 6 2 .

T . P . V e n t o z a , S.A.Gouye, a n d D.R.Lloyd, P o l y m . M a t e r . Sci. Eng. 51 (1984) 713-717. S.Krause, in " M a t e r i a l s S c i e n c e of S y n t h e t i c M e m b r a n e s " D . R . L l o y d (Ed.), Amer. Chem. Soc., Washington, 1985. M.H.V.Mulder, F.Kruitz and C.A.Smolders, J. Membr. Sci. 11 (1982) 349-363. J.B,Rose, B r i t i s h Patent G B 2 , 0 9 0 , 8 4 3 July 21, 1982. P.E.Froehling, D.M.Koenhen, A.Bantjes and C.A.Smolders, P o l y m e r 17 (1976) 835-836. Z.Rigbl, P o l y m e r 19 (1978) 1229-1232. J.M.Dickson and D.R.Lloyd, in "Synthetic M e m b r a n e s " A . F . T u r b a k (Ed.), A m e r . Chem. Soc., W a s h i n g t o n , 1981. S . K i m u r a and S.Sourirajan, AIChE J. 13 (1967) 497-503. K.E.Kinzer, M.S. Thesis, The U n i v e r s i t y of Texas, 1983.

ACKNOWLEDGEMENTS The a u t h o r s w ~ s h to g r a t e f u l l y a c k n o w l e d g e the a s s i s t a n c e of Sylvle A. Souye in c o n d u c t i n g the s o l u b i l i t y studies. This r e s e a r c h was f i n a n c e d in part by the S e p a r a t i o n s R e s e a r c h P r o g r a m of The U n i v e r s i t y of Texas.