Polyvinyl alcohol-polyacrylamide interpenetrating polymer network membranes and their pervaporation characteristics for ethanol-water mixtures

Polyvinyl alcohol-polyacrylamide interpenetrating polymer network membranes and their pervaporation characteristics for ethanol-water mixtures

j o u r n a l of MEMBRANE SCIENCE ELSEVIER Journal of Membrane Science 106 (1995) 167-182 Polyvinyl alcohol-polyacrylamide interpenetrating polymer ...

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j o u r n a l of MEMBRANE SCIENCE ELSEVIER

Journal of Membrane Science 106 (1995) 167-182

Polyvinyl alcohol-polyacrylamide interpenetrating polymer network membranes and their pervaporation characteristics for ethanol-water mixtures L. Liang, E. Ruckenstein * Department of Chemical Engineering, State UniversiO' of New York at Buffalo, Buffalo. NY 14260, USA Received 5 January 1995

Abstract Crosslinked polyvinyl alcohol-polyacrylamide interpenetrating polymer networks ( P V A - P A A M IPN) were synthesized by the sequential IPN technique. Glutaraldehyde ( G A L ) and N,N'-methylenebisacrylamide ( B i s A A M ) were used as crosslinking agents for the PVA and PAAM networks, respectively. The P V A - P A A M IPN membranes have been prepared starting from an aqueous solution of PVA to which AAM, BisAAM and potassium persulfate were added. After polymerization, GAL was introduced in the solution and the latter was cast on a glass plate. The effect of the content of each of the crosslinking agents on the uncrosslinked fraction dissolved in water was determined. The swelling of the membranes in water and in the mixtures: water-ethanol and water-acetic acid was also investigated. The mechanical properties of the membranes were studied via tensile testing. The compositions of water-ethanol and water-acetic acid mixtures sorbed in the membranes were determined. The pervaporation of water-ethanol mixtures through the P V A - P A A M IPN membranes was investigated. Keywords: Interpenetrating polymer network: Polyvinyl alcohol; Polyacrylamide; Polymeric membranes; Membrane preparation and characterization; Pervaporation

1. Introduction Pervaporation is recognized as an effective process for separating azeotropic mixtures, close-boiling point compounds, and mixtures consisting of heat-sensitive compounds. In recent years, numerous investigations have been carried out regarding the pervaporation of water-ethanol mixtures through crosslinked poly(vinyl alcohol) (PVA) membranes [ 1-3]. Compared to PVA, the polyacrylamide (PAAM) sorbs a larger amount of water [4], has higher thermostability [5 ] and a higher selectivity for water from * Corresponding author. 0376-7388/95/$09.50 © 1995 Elsevier Science B.V. All rights reserved SSDI0376-7388(95) 00137-9

water-ethanol mixtures [6]. However, PAAM is brittle. In the present paper an attempt is made to prepare an improved material by combining PVA with PAAM. Having this goal in mind, poly(vinyl alcohol)-polyacrylamide interpenetrating networks (PVA-PAAM IPN) were prepared by the sequential IPN technique. Glutaraldehyde (GAL) and N,N'-methylenebisacrylamide (BisAAM) were used as crosslinking agents for PVA and PAAM, respectively. For comparison purposes, crosslinked PVA-GAL membranes were also prepared. The pervaporation characteristics of the PVA-PAAM IPN membranes were investigated.

168

L. Liang, E. Ruckenstein / Journal of Membrane Science 106 (1995) 167-182 D A

C

H

B

G

K

G

I

I

Fig. 1. Schematic diagram of pervaporation apparatus: (A) feed reservoir. (B) thermostat, (C) feed outlet. (D) feed inlet, (E) feed pump, (F) permeation cell (fiat membrane), (G) switch valve, (H) vent to atmosphere, (I) collection trap, (J) safety trap, (K) Pirani gauge, (L) vacuum pump.

2. Experimental 2.1. Materials

The acrylamide (AAM, Aldrich) and potassium persulfate (K2S208, Aldrich) were purified by recrystallization from methanol and water, respectively. Poly(vinyl alcohol) (PVA, Aldrich) of molecular weight 124 000, N,N'-methylenebisacrylamide (BisAAM, Aldrich), glutaraldehyde (GAL, 25 wt% in water, Aldrich), hydrochloric acid (37 wt% in water, Aldrich), sodium hydrogen carbonate (99.7%, Aldrich), ethanol (99%, Aldrich), and acetic acid (99%, Aldrich) were used without purification. Water was deionized and doubly distilled. ~

tt) aO

2.2. Preparation of the crosslinked PVA membranes Crosslinked PVA membranes were prepared by reacting PVA with dialdehydes to generate water inso1-

T V

Table 1 FTIR fingerprints of different polymers

o t

Polymer I

4000

..,t,

3000 wavenumber

i

.

2000 (cm

I

1000 -1)

Fig. 2, FTIR spectra of different polymers: (a) PAAM, (b) PVA, (c) PVA-PAAM IPN; PVA/PAAM (weight ratio)= 5, GAL: 0.5 wt% in the PVA network, BisAAM: 1.0 wt% in the PAAM network.

PAAM PVA

Infrared characteristic absorption bands (cm i )

3500 (NHz) ; 1650 (CONH2) 3400 (OH); 1430(OH) 920 (syndiotactic PVA chain) 850 (isotactic PVA chain) PVA-PAAM IPN 3400 (OH); 1650(CONH2) 920 (syndiotactic PVA chain) 850 (isotactic PVA chain)

L. Liang, E. Ruckenstein / Journal of Membrane Science 106 (1995) 167-182

169

40

30|.

A6 ol o

E 0 r~

"O

..5 =

4

2 0 "G

ol

2

G

0

I

I

I

~

0.5

1

1.5

2.5

GAL

i

° 0

3

(wt. %)

Fig. 3. Effect o f the content o f G A L on the a m o u n t extracted by water and the swelling in water of crosslinked PVA.

uble polyacetals [7]. PVA (9 wt% in water) ( 10 ml), GAL (2.5 wt% in water) (0.2 ml) and HC1 ( 1 N) (0.5 ml), which catalyses the crosslinking, were mixed in a one-neck 100 ml round bottom flask with magnetic stirring for 30 min at room temperature. Further, the solution was cast on a glass plate and dried at ambient temperature for 24 h. The crosslinked membrane was immersed for 30 min in water and then in a dilute solution of sodium hydrogen carbonate (5 wt% in water) to neutralize the residual acid. The membrane was immersed in water three times to remove the salt,

air dried at room temperature, and then transferred to a vacuum oven for completion of the drying.

2.3. Preparation of the crosslinked PAAM Crosslinked PAAM was prepared by the free radical solution polymerization [8], in order to study the swelling of PAAM at various crosslinking agent contents. The reaction was performed in a one-neck 100 ml round bottom flask, with magnetic stirring, in a N2 atmosphere. AAM (0.4 g), BisAAM (0.004 g) and

20

20 A

o n

A15

o

10'o o~ c

_= 5

5

-

I

~

4

8 BisAAM

----tl

i 12 (wt.

=

16

o 20

% )

Fig. 4. Effect of the content o f B i s A A M on the a m o u n t extracted by water a n d swelling in water o f crosslinked P A A M .

170

L. Lian g, E. Ruckenstein / Journal of Membrane Science 106 (1995) 167-182

20

16

~¢J 4

~

~

"O

v

E o

cL 3 A O~

O 8 n o

4

0

I 0

0.5

I

I

1

1.5

log (PVA/ PAAM ) ( weight

tn

i 2

2.5

ratio)

Fig. 5. Effect of PVA/PAAM weight ratio on the amount extracted by water and swelling in water of PVA-PAAM IPN composite. GAL: 0.5 wt% in the PVA network, BisAAM: 1.0 wt% in the PAAM network.

the initiator K 2 S 2 0 8 ( 0 . 0 4 g) were added to 40 ml water. The mixture was heated at 70°C for 8 h, and subsequently cast on a glass plate, dried in air for 48 h, and then transferred to a vacuum oven for completion of the drying.

ml), AAM (0.2 g), BisAAM (0.002 g) and K2S208 (0.02 g) were dissolved in 10 ml water. The mixture was heated at about 70°C for 8 h to generate a PAAM crosslinked network, and cooled to room temperature. GAL (2.5 wt% in water) (0.2 ml) and HC1 ( 1 N) (0.5 ml) were dissolved in the solution; this was followed by mixing for 30 min. The mixture was cast on a fiat glass plate and dried at ambient temperature for 24 h. Transparent PVA-PAAM IPN membranes were thus obtained. The neutralization of the residual acid and the drying of the membrane were carried out as already

2.4. Preparation of the PVA-PAAM IPN membranes The reaction was performed in a one-neck 100 ml round bottom flask, with magnetic stirring, in a N2 atmosphere, as follows: PVA (9 wt% in water) (10 10 I

1

J

I

~

4

4

~

3

o 0

A

I

0.5

1

i

!

1.5

2

2.5

GAL (wt. %)

Fig. 6. The swelling of PVA-PAAM IPN membrane in water at different temperatures: 1, 75°C; 2, 55°C; 3, 30°C; 4, 5°C. PVA-PAAM IPN membrane: PVA/PAAM (weight ratio) = 5, GAL: 0.5 wt% in the PVA network, BisAAM: 1.0 wt% in the PAAM network.

L. Liang, E. Ruckenstein / Journal of Membrane Science 106 (1995) 167-182 80

100

-\ m

171

80

60

60 c



40

j~ "~

40 • I-

20

20

0

~ 0

0.5

t

I

I

I

1.5

2

2.5

,;i,

0 3

GAL (wt. %)

Fig. 7. Relationship between the content of GAL and the tensile strength and elongation at break for crosslinked PVA membranes.

80

30

60

2o ) .Q c

p 40

15 ~ c

_o

_= M

,- 2 0

I=

;

0

),

"

k

k

I

0.5

1

1.5





l

l

2

5

o "'

0

2.5

3

G A L (wt. %)

Fig. 8. Relationship between the content of GAL and the tensile strength and elongation at break for PVA-PAAM IPN membranes. PVA/PAAM (weight ratio) = 5, BisAAM: 1.0 wt% in the PAAM network.

80

30

A ¢l

25

=E

20

~

15

m

=

5

,';i,

0

5 c

p

A

E

60

40

lID

~ 2o @

o

I,,i

]

I

k

i

1

2

3

4

5

BisAAM

(wt.

%)

Fig. 9. Relationship between the content of BisAAM and the tensile strength and elongation at break for PVA-PAAM IPN membranes. PVAPAAM (weight ratio) = 5, GAL: 0.5 wt% in the PVA network.

72

L. Liang, E. Ruckenstein/Journal of Membrane Science 106 (1995) 167-182 5 0) r"

(a)

.O

[]

E

[] v r" O ¢., la. L. O to

o o

•~ 0

40

60

80

100

5

~"

4

E

3

.9 ,,..,

1

0

20

II

i

c

~

.

~

(b)

L-0

0 o

20

40

60

80

100

r-

5 E o

E

4

I

II

~

(c)

[]

"g3 v c O O. t.. O 0~

2 1

0 20

40

60

80

100

weight fraction of ethanol in the mixture (%)

Fig. 10. The swelling of the PVA-PAAMIPN membranein water-ethanol mixtures: (a) 20°C; (b) 40°C; (c) 70°C. [3, crosslinkedPVA membrane;GAL: 0.5 wt%. i , PVA-PAAMIPN membrane:PVA/PAAM (weight ratio) = 5, GAL: 0.5 wt% in the PVA network. BisAAM: 1.0 wt% in the PAAMnetwork.

described. The thickness of the membrane was in the range 3 0 - 1 0 0 / z m . In the pervaporation experiments, the thickness of the membrane was in the range 30--40 /Lm.

It is likely that an IPN membrane was generated, because PVA, which is compatible with PAAM, dissolves together with GAL in the P A A M - B i s A A M network, where they form their own network.

173

L. Liang, E. Ruckenstein/ Journal of Membrane Science 106 (1995) 167-182 14 A

¢1 ,,o

E E ~A ot io °_ D.

g



12

(a)

10 8 6 4 2 0 0

I

I

I

I

20

40

60

80

I

I

I

I

40

60

80

100

20

~

16

J= E

12

e~

8

o

~

4

0

20

weight fraction

100

of acetic acid In the mixture ( % )

Fig. 11. The swelling of PVA-PAAMIPN membrane in water-acetic acid mixtures: (a) 20°C; (b) 40°C. [-1 Crosslinked PVA membrane, l , PVA-PAAMIPN membrane. The membranesare those of Fig. 10. 2.5. FTIR spectra

The FFIR spectra of PVA films, P V A - P A A M IPN films and P A A M powders were recorded using a Mattson Alpha Centauri instrument. 2.6. Extraction experiments o f the crosslinked polymers

Water was employed as solvent to extract the uncrosslinked fraction of the polymer. The percent extracted was calculated using the expression: 100 × ( Wb -- W,) / Wb, where Wa and Wb are the weights of the samples after and before extraction, respectively. 2.7. Swelling o f the crosslinked polymers

The swelling of the crosslinked polymers was investigated by keeping sample strips for 48 h, at different

temperatures, in glass tubes containing the respective liquid. After the strips were taken out from the liquid and carefully wiped with tissue paper to remove the adherent liquid, they were weighed as quickly as possible. The swelling ratio S of the samples was calculated using the expression S = (Ws - Wo)/Wo, where Wo and Ws are the weights of the dried and swollen samples, respectively.

2.8. Tensile testing

The P V A - P A A M IPN and crosslinked PVA membranes were cut to the size required by the ASTM D. 638-58T. The tensile testing was performed using an Instron universal testing instrument (model 1000) at room temperature. The extension speed of the instrument was 20 mm/min.

174

L. Liang, E. Ruckenstein / Journal of Membrane Science 106 (1995) 167-182 m

100 r 80

g o

E

(a)

I

I

2of I

[]

0 i

p

0

20

40

60

80

100

100 7 cu

~-

80

.Q

r

BE

+

eOo~"

60

' C~

40 t



11

(b)

O

f E ]

20 weight

40 fraction

60

of ethanol

In the

80 mixture

100

(%)

Fig. 12. Equilibrium individual sorptions for PVA-PAAM IPN and crosslinked PVA membranes in water-ethanol mixtures at 20°C. (a) PVAPAAM IPN membrane. (b) Crosslinked PVA membrane. II, Water sorption (weight percent in the total amount sorbed). [~, Ethanol sorption (weight percent in the total amount sorbed). The membranes are those of Fig. 10.

2.9. Determination of the composition of the binary liquid mixture sorbed by the membrane The composition of the sorbed liquid mixture at sorption equilibrium was determined as follows: after being weighted, the membrane was placed into a flask connected to a cold trap and a vacuum pump until the weight of the dried membrane was nearly the same as that before sorption. The collected desorbed liquid was analyzed by gas chromatography.

2.10. Pervaporation The schematic pervaporation apparatus is presented in Fig. 1. The feed solution in the reservoir of 3.71 was kept at the selected temperature (which could be varied between - 2 0 to 150°C) by circulating it through a heat exchanger placed in a thermostated water bath. The membrane, located on a porous glass support, had an effective membrane area of 9.6 cm 2. A constant

downstream pressure (3 _ 1 torr) was maintained with a vacuum pump. At the start of the pervaporation process, the apparatus was run at least 2 h at the selected temperature without condensation, to reach the steady state before collecting the permeate. The permeated vapor was collected in a liquid nitrogen trap, weighed and then analyzed with a chromatograph equipped with a 2-m long Porapak Q packed column heated at 160°C; helium was used as carrier gas. The pervaporation performance was characterized in terms of the steady state flux and the separation factor. The permeation rate, J, at steady state was obtained from the expression

J = Q/at where Q is the total amount of permeate during the experimental time interval t at steady state, and A is the effective membrane surface area. The separation factor a was calculated using the expression

L. Liang, E. Ruckenstein / Journal of Membrane Science 106 (1995) 167-182

175

A m

E

v

o

...0/

----.-.---5

~D"---------

[]

p

0

b ~ - - ~

20

0

40

60

0

80

100

3

2sTi '~ E

(b)

°

m

2 r 1.5 ~

o. 0.5

/ D

0

/

i

0

20 weight

40 fraction

60

of ethanol

in the

80 mixture

100

(%)

Fig. 13. Equilibrium total and individual sorptions for PVA-PAAM IPN and crosslinked PVA membranes in water-ethanol mixtures at 20°C. (a) PVA-PAAM IPN membrane. (b) Crosslinked PVA membrane. @, Total sorption. II, Water sorption, rq Ethanol sorption. The membranes are those of Fig. 10.

O[= ( YH20 / YEtOH) / ( X H 2 0 / XEtoH)

where YH2O/YEtOHis the weight ratio of water to ethanol in the permeate and XH:o/XEtoH is the weight ratio of water to ethanol in the feed mixture.

3. R e s u l t s a n d d i s c u s s i o n

3.1. FTIR spectra The FTIR spectra of PVA, PAAM and PVA-PAAM IPN are presented in Fig. 2. The major vibration bands of the chemical groups contained in the respective polymers are listed in Table 1. The spectrum of PAAM (Fig. 2a) exhibits a strong absorption at 1650 c m - 1, which can be assigned to the combined motion of NH2 and CO stretching in the -

CONH2 group [9]. The major vibration bands observed in the spectrum of PVA are at 3400, 1430, 920 and 850 c m - l (Fig. 2b). The strong absorption at 3400 cm-1 is assigned to the hydrogen-bonded OH stretching mode. The peak at 1430 c m - l is assigned to the deformation vibration of OH. The peaks at 920 and 850 c m - ~are assigned to the characteristic absorptions of syndiotactic and isotactic PVA chains, respectively [ 10 ]. All the maj or vibration bands of PAAM and PVA are also present in the FTIR spectrum of the PVAPAAM composite. This observation is compatible with an IPN structure of the composite.

3.2. Effect of the crosslinking agent content on the amount dissolved by water and the swelling in water of the crosslinked PVA and PAAM Fig. 3 and Fig. 4 show that the amount of uncrosslinked PVA and PAAM dissolved by water, expressed

176

L. Lian g, E. Ruckenstein / Journal of Membrane Science 106 (1995) 167-182 =0

100

~

80

~,~"

60

~ o

407

8

20+

t I

O I

0

3 ,

,

,

20

40

60

100

80

100 1 ~.

80 t

(b)

[]

.=

=~

40

.9

~ E

2o 0 0

I

i

~

I

20

40

60

80

weight

fraction

of

acetic

acid

in

the

mixture

100 (%)

Fig. 14. Equilibrium individual sorptions for PVA-PAAM IPN and crosslinked PVA membranes in water-acetic acid mixtures at 20°C. (a) PVA-PAAM IPN membrane. (b) Crosslinked PVA membrane. O, Water sorption (weight percent in the total amount sorbed). C), Acetic acid sorption (weight percent in the total amount sorbed). The membranes are those of Fig. 10.

in wt%, decreases with increasing content of crosslinking agent; the change of the GAL content has a stronger effect on the uncrosslinked PVA fraction than the change of BisAAM on the uncrosslinked PAAM fraction. Fig. 3 and Fig. 4 also provide information regard~ ing the swelling in water. In all cases, the swelling of PAAM is greater than that of PVA, because PAAM has a higher hydrophilicity.

3.3. Effect of the composition of PVA-PAAM IPN composite on the percent dissolved by water and the swelling in water Fig. 5 presents the percent of PVA-PAAM IPN composite dissolved by water and the swelling in water for various weight ratios of PVA to PAAM and for contents of GAL and BisAAM in the corresponding networks of 0.5 and 0.1 wt%, respectively. As expected, the percent dissolved and the swelling of PVA-PAAM IPN

composite increase with increasing content of PAAM in the composite.

3.4. Effect of temperature on the swelling in water of the PVA-PAAM IPN membrane Fig. 6 shows that the swelling in water of PVAP A A M IPN composite increases with increasing temperature. It is clear that this kind of polymer network has a positive thermosensitivity, i.e. higher swelling levels with increasing temperature. The effect of the temperature on the swelling of the composites is weak for temperatures below 20°C, but stronger for temperatures above 20°C.

3.5. Tensile strength and elongation of different crosslinked polymers Fig. 7 shows that, with increasing content of GAL (wt% in the PVA network), the tensile strength passes

L. Liang, E. Ruckenstein / Journal of Membrane Science 106 (1995) 167-182

177

14 A v

12 m e.= 1 0 ,D E 8 A

(a)

*

6

== o

4 2 0 0

i

i

i

i

20

40

60

80

100

~ 5 ~ ~a L (b) .m

.Q E

=oL 0

20 weiaht

40 fraction

of

60 acetic

acid

in

100

80 the

mixture

(%)

Fig. 15. Equilibrium total and individual sorptions for PVA-PAAMIPN and crosslinked PVA membranes in water-acetic acid mixtures at 20°C. (a) PVA-PAAM IPN membrane. (b) Crosslinked PVA membrane. 0, Total sorption. II, Water sorption. 17, Acetic acid sorption. The membranes are those of Fig. 10. through a m a x i m u m and the elongation at break decreases. The elongation decreases because a greater content of G A L generates a larger number of connections among the polymer chains, which opposes the deformation. When the number o f crosslinks is moderate, the strength of the material increases with the G A L content. However, when their number is too large, they restrict the close contact among the chains, thus decreasing the strength of the material. The tensile strength and elongation at break for PVAP A A M IPN membranes are plotted as a function of G A L content in Fig. 8. The plots are similar to those obtained for crosslinked P V A membranes. The tensile strength has a maximum at about G A L = 0.7 wt%. The tensile strengths are somewhat larger than those ofcrosslinked P V A membranes. The increase can be attributed to the IPN structure. Fig. 9 presents the effect of the content of B i s A A M (wt% in the P A A M network) on the tensile strength and elongation at break for P V A - P A A M IPN membrane, for a P V A - P A A M weight ratio of 5 and

G A L content in the P V A network of 0.5 wt%. One can see that the tensile strengths are larger than those in Fig. 7. This is due to the IPN structure. Two factors affect the tensile strength of P V A - P A A M IPN membranes. One is the interpenetration of the two crosslinked networks, and the other is the crosslinking densities in both P V A and P A A M networks.

3.6. Swelling of PVA-PAAM IPN membranes in mixtures of an organic compound and water Fig. 10 presents the swelling of P V A - P A A M IPN membranes as a function of ethanol concentration in ethanol-water mixtures. For comparison, the swelling of the crosslinked P V A membrane in the same mixture is also included. It is clear, that the amounts sorbed by both membranes increase with increasing content of water in the mixture. However, the former membranes sorb larger amounts than the latter ones in the temperature range of 20 to 70°C. The higher the temperature, the higher the amounts sorbed by both membranes.

L. Liang, E. Ruckenstein / Journal of Membrane Science 106(1995) 167-182

178

100000

J::~ 10000 ~ ~1 L _

"-"

O

1000

....

lOO

E lO

1 0

I

I

I

i

20

40

60

80

100

EtOH ( ~ . ~ ) .

Fig. 16. Effect of feed composition on the permeation rate at 75°C for PVA/PAAM wt ratio of 5. 1, PVA-PAAM IPN membrane: GAL: 1.0 wt% in the PVA network; BisAAM: 1.0 wt% in the PAAM network. 2, Crosslinked PVA membrane: GAL: 1.0 wt% in the PVA network. 3, PVA-PAAM |PN membrane: GAL: 2.5 wt% in the PVA network: BisAAM: 1.0 wt% in the PAAM network. 4, PVA-PAAM IPN membrane: GAL: 7.0 wt% in the PVA network: BisAAM: 1.0 wt% in the PAAM network. 5, PVA-PAAM IPN membrane: GAL: 1.0 wt% in the PVA network; BisAAM: 4.0 wt% in the PAAM network.

Both membranes do not swell in pure ethanol at temperatures below 40°C.

The effect of acetic acid concentration in acetic acidwater mixtures on the swelling of both PVA-PAAM

100000

I I J

10000

1000

+

1oo

q q:

8 P ==

10 ~

L 1

I !

0

--

~

-~--'-~--i-"~ I

I

I

20

40

60

80

100

EtOH (wt%)

Fig. 17. Effect of the feed composition on the separation factor at 75°C. The membranes are those of Fig. 16.

L. Liang, E. Ruckenstein / Journal of Membrane Science 106 (1995) 167-182

179

Table 2 Pervaporation performance of different crosslinked PVA Membrane

E t h a n o l i n the feed (wt. %) Temperature (°C)

Crosslinked PVA with GAL and modified by reaction with 90 CICH2COOH Photocrosslinked PVA 90 Crosslinked PVA with amic acid 90 Crosslinked PVA 95 P V A - P A A M IPN ~ 95

J (kg/mZh)

a

Ref.

40

~ 0.05

~ 150 [ 11 ]

40 75 80 75

~0.2 ~ 0.13 0.01 0.06

~23 ~ 280 9500 4100

[12] [ 13] [14] this study

a P V A / P A A M = 5 (weight ratio); GAL: 1.0 wt% in the PVA network BisAAM: 4.0 wt% in the PAAM network.

IPN and crosslinked PVA membranes is presented in Fig. 11. In contrast to the ethanol-water mixture, the swelling exhibits a maximum at about 50 wt% acetic acid at 20°C; the maximum is displaced to about 70 wt% acetic acid at 40°C. The cooperation between the acetic acid molecules, those of water and the hydrophilic membrane is responsible for the maximum in the total sorption.

sorption in different membranes. It is clear that the sorption of water in both membranes is higher than that of ethanol over the entire concentration range in the feed, and that the concentration of water in the membranes is much higher than that in the feed. This is due to the hydrophilicity of the membranes and moderate hydrophilicity of ethanol in comparison with water. The sorption of water-ethanol mixture by crosslinked PVA membranes was also studied by Kang et al [ 11 ], who arrived at similar conclusions. The sorption of water in the PVA-PAAM IPN membrane is higher than in the crosslinked PVA membranes, because PAAM has a higher ability to sorb water than PVA [6]. Fig. 14 presents the equilibrium sorptions of water and acetic acid from water-acetic acid mixtures in PVA-PAAM IPN and crosslinked PVA membranes. The effect of the acetic acid concentration on the total

3. 7. Analysis of the mixtures inside the membranes For completeness, the composition of the water-ethanol and water-acetic acid mixtures inside the membranes was also determined. Fig. 12 presents the equilibrium sorptions of water and ethanol in PVAPAAM IPN and crosslinked PVA membranes. Fig. 13 provides the total equilibrium (water plus ethanol) 800

600 .o

o

~

1

4OO o0

o

0

20

I

I

I

I

t

I

30

40

50

60

70

80

90

temperature (°C) Fig. ] 8. Dependenceof the permeationrate on the temperaturefor 85 wt% ethanol in ethanol-water mixture for PVA/PA,~V[ wt ratio of 5. 1, PVA-PAAM IPN membrane: GAL: 1.0 wt% in the PVA network; BisAAM: 1.0 wt% in the P A A M network. 2, P V A - P A A M IPN membrane: GAL: 1.0 wt% in the PVA network; BisAAM: 4.0 wt% in the P A A M network.

L. Liang. E. Ruckenstein/JournalofMembraneScience106(1995) 167-182

180

100000

10000

1000

-.-..-.......,.. ca Q.

100

== 10

1 20

' I

I

I

I

I

I

30

40

50

60

70

80

temperature

(°¢)

Fig. 19. D e p e n d e n c e o f the s e p a r a t i o n f a c t o r on the t e m p e r a t u r e for 85 w t % ethanol. T h e m e m b r a n e s are those of Fig. 18.

equilibrium ( water plus acetic acid) sorption in various membranes is shown in Fig. 15. While the membranes are enriched in water, the enrichment is much smaller than in the case of ethanol. This occurs because acetic acid is more hydrophilic than ethanol.

3.8. Pervaporation Only the pervaporation of water-ethanol mixtures was carried out, because the previous experiments indicated that the PVA-PAAM IPN membranes have a much higher selectivity for water from water-ethanol mixtures than from water-acetic acid mixtures. Fig. 16 and Fig. 17 show the effect of the composition of the feed mixture on the permeation rate and the separation factor for various PVA-PAAM IPN membranes. For comparison purposes, the pervaporation performance of the crosslinked PVA membrane is also included. As the content of water in the feed mixture increases, the membranes become more swollen. As a result, the permeation rate increases and the selectivity decreases. One can see from Fig. 16 that, under some conditions, the permeation rates through the PVA-PAAM IPN membranes are somewhat larger than those through the crosslinked PVA membranes, particularly at the lower ethanol concentrations. However, the separation factors of the PVA-PAAM IPN membranes can be much higher than those of the crosslinked PVA

membranes (Fig. 17). A separation factor of approximately 4100 was obtained at 75°C for an ethanol concentration of 95 wt% and with the PVA-PAAM IPN membrane that contained 4.0 wt% BisAAM in the PAAM network, 1.0 wt% GAL in the PVA network and a weight ratio PVA/PAAM of 5. The pervaporation performances of different crosslinked PVA membranes used by various groups [ 11-14] for the waterethanol mixture are listed in Table 2. They show that the PVA-PAAM IPN membrane prepared in this study has a high selectivity and an acceptable permeation rate.

3.9. Effect of temperature on pervaporation The effect of temperature on the separation through the PVA-PAAM IPN membranes is presented in Fig. 18 and Fig. 19. As the temperature increases, the permeation rate increases, but the separation factor decreases. This occurs because with increasing temperature, the frequency and amplitude of the polymer chain motion become larger. As a result, the diffusion rates of the permeating molecules and the total permeation rate increase and the separation factor becomes lower. Fig. 20 presents Arrhenius plots for the total permeation rates. The permeation activation energies are listed in Table 3. One can see that the permeation activation energy increases with increasing crosslinking density of the PAAM network in the PVA-PAAM

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L. Liang, E. Ruckenstein/Journal of Membrane Science 106 (1995) 167-182

t'q

e.ex{}

0

_= 0 2.8

~

I

I

I

I

2.9

3

3.1

3.2

3.3

3.4

1000/T (K" 1 ) Fig. 20. Arrheniusplots of total permeationrate vs. temperature. EtOH, 85 wt%. The membranesare those of Fig. 18. Table 3 Activation energies for the permeation of water-ethanol mixture containing85 wt% ethanol Membrane

Activationenergy (kcal/mol)

PVA-PAAMIPNa PVA-PAAMIPNb

8.9 10.2

aPVA/PAAM=5 (weight ratio); GAL: 1.0 wt% in the PVA network; BisAAM: 1.0 wt% in the PAAMnetwork. bpVA/PAAM=5 (weight ratio); GAL: 1.0 wt% in the PVA network; BisAAM:4.0 wt% in the PAAM network. IPN membrane. The permeation activation energies for the two membranes of Table 3 are close to those of the PVA membranes crosslinked with amic acid [ 13].

4. C o n c l u s i o n s Polyvinyl alcohol-polyacrylamide interpenetrating polymer network ( P V A - P A A M IPN) composites have been prepared by means of the sequential IPN technique. Glutaraldehyde ( G A L ) and N,N'-methylenebisacrylamide (BisAAM) were employed as crosslinking agents for the PVA and PAAM networks, respectively. The swelling and mechanical properties of PVA-PAAM IPN composites depend on the P V A / PAAM weight ratio and the contents of the crosslinking agents present in the individual crosslinking networks. The presence of PAAM affects in a positive way the swelling, while the presence of PVA improves the mechanical properties.

The swelling of PVA-PAAM IPN membranes in the water-ethanol mixture monotonously increased with the water content, while a maximum swelling was observed for the acetic acid-water mixture. The determination of the concentrations in the membranes indicated that water was preferentially sorbed from both mixtures, but much less from water-acetic acid mixtures. The PVA-PAAM IPN membranes were found to have pervaporation separation factors ranging from 45 to 4100 and permeation rates of about 0.060.1 k g / m 2 h for 95 wt% ethanol aqueous solution at 75°C. However, for a concentration of 10 wt% ethanol, the permeation rates were as large as 9 k g / m 2 h and the separation factors were about 20.

References [ 1] M. Wesslein,A. Heintzand R.N. Lichtenthaler,Pervaporation of liquid mixtures through poly(vinyl alcohol) (PVA) membranes I. Study of water containingbinary systems with complete and partial miscibility,J. MembraneSci., 51 (1990) 169. [2] H. Karkan,M. Tsuyumoto,Y. Maedaand Z. Honda,Separation of water-ethanol by pervaporationthrough polyion complex composite membrane,J. Appl. Polym. Sci., 42 ( 1991) 3229. [3] H. Ohya, K. Matsumoto,Y. Negishi, T. Hino and H.S. Choi, The separation of water and ethanol by pervaporation with PVA-PAN composite membranes, J. 'Membrane Sci., 68 (1992) 141. [4] H. Kaurand P.R. Chatterji,Interpenetratinghydrogelnetworks 2. Swelling and mechanical properties of the gelatinpolyacrylamide interpenetratingnetworks, Macromolecules, 23 (1990) 4868.

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[5] J. Brandrup and E.H. Immergut, Polymer Handbook, 3rd ed., Wiley, New York, 1989, p. VI/217 and VI/221. [6] E. Ruckenstein and L. Liang, Polyacrylamide-reactive styrene/unsaturated polyester microgel composites, J. Appl. Polym. Sci., 57 (1995) 605. [7] W.R. Sorenson and W.T. Cambell, Preparative Methods in Polymer Chemistry, Interscience, New York, 1961, p. 176. [8] P.R. Chatterji, Interpenetrating hydrogel networks 1. The gelatin-polyacrylamide system, J. Appl. Polym. Sci., 40 (1990) 401. [9] A. Elliott, Infra-red Spectra and Structure of Organic LongChain Polymers, Edward Arnold, London, 1969, p. 86. [ 10] A. Elliott, Infra-red Spectra and Structure of Organic LongChain Polymers, Edward Arnold, London, 1969, p. 100.

[ 11 ] Y.S. Kang, S.W. Lee, U.Y. Kim and J.S. Shim, Pervaporation of water--ethanol mixtures through crosslinked and surfacemodified poly(vinyl alcohol) membrane, J. Membrane Sci., 51 (1990) 215. [12] T. Hirotsu. K. Ichimura, K. Mizoguchi and E. Wakamura, Water-ethanol permseparation by pervaporation through photocrosslinked poly (vinyl alcohol) composite membrane, J. Appl. Polym. Sci., 36 (1988) 1717. [ 13] R.Y.M. Huang and C.K. Yeom, Pervaporation separation of aqueous mixtures using crosslinked poly (vinyl alcohol) (PVA). II. Permeation of ethanol-water mixtures, J. Membrane Sci., 51 (1990) 273. [ 14] Jpn. Pat., JP 59-109 S04 A (1984).