Gas transport in polyurethane-polystyrene interpenetrating polymer network membranes. II. Effect of crosslinked state and annealing

Journal of Membrane Science, 75 (1992) 15-27

15

Elsevier Science Publishers B.V., Amsterdam

Gas transport in polyurethane-polystyrene interpenetrating polymer network membranes. II. Effect of crosslinked state and annealing Doo Sung Leea, Won Kil Kangb, Jeong Ho An” and Sung Chul Kimd “Department of Polymer Science and Engineering, Sung Kyun Kwan University, Suwon, Kyungki 440-746 (S. Korea) ‘Department of Textile Engineering, Sung Kyun Kwan University, Suwon, Kyungki 440-746 (S. Korea) “Department of Chemical Engineering, Pohang Institute of Science and Technology, P.O. Box 125, Pohang, Kyungbuk 790-600 (S. Korea) dDepartment of Chemical Engineering, Korea Advanced Institute of Science and Technology, P.O. Box 131, Cheongryangri, Seoul 130-650 (S. Korea)

(Received September 19,1991; accepted in revised form June 30,1992)

Abstract

A series of polyurethane (PU) -polystyrene (PS) interpenetrating polymer network (IPN), semi-IPN (only PU component crosslinked) and linear blend membranes were prepared with varying synthesis temperature and composition. PU was first thermally polymerized, and then PS was polymerized by photolytic methods at two different temperatures, 0’ C and 40 oC. The permeability coefficient decreased and the separation factor increased when preparation took place at the lower temperature due to the increase in homogeneity of PU and PS as shown in the previous papers. The minimum permeability coefficient and the maximum separation factor were observed at a composition with ca. 25 wt.% PU. The permeability coefficient increased in the following sequence; IPN, semi-IPN and linear blends. Thermal annealing was adopted to study the effect of homogeneity of the two polymer components in the membrane on the gas permeation characteristics. Annealing was found to increase the permeability coefficients, while it caused a decrease in selectivity and the extent of synergistic behavior. The annealing effect on the permeation characteristics depended on the nature of crosslinking of the membrane (IPN < semi-IPN < linear blends). It was shown that a change in homogeneity, modulated by change of composition, state of crosslinking, synthesis temperature and annealing temperature etc., could play an important role in controlling the gas transport characteristics through the multicomponent polymeric membranes. The observed permeability characteristics were also substantiated by considering the densities and the degree of relative intermixing (by means of Ts variations). Keywords: IPN related membrane;

gas permselectivity;

crosslinking;

phase separation by annealing;

ho-

mogeneity dependence

Correspondence to: D.S. Lee, Department

of Polymer Science and Engineering, Sung Kyun Kwan University, Suwon,

Kyungki 440-746 (S. Korea).

0376-7388/92/$05.00

0 1992 Elsevier Science Publishers B.V. All rights reserved.

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Introduction

Recently many studies have been reported on gas transport phenomena in heterogeneous and homogeneous polymer blends [l-8], in which the gas transport characteristics were correlated with the chemical structure and the morphology of the polymer blends. In a number of the previous studies, it was shown that the degree of intermixing of the two components can be controlled by controlling the relative rate of phase separation and network formation during the IPN synthesis. It was possible to increase the degree of interpenetration and the homogeneity between the constituents by applying high pressure during the simultaneous polymerization process of polyurethane-poly (methyl methacrylate ) and PU-PS IPN’s [g-12]. These IPN’s synthesized under high pressure appeared transparent which is indicative of a very small domain size and/or formation of near molecular level mixture. Similar homogeneity enhancement could be accomplished by performing the polymerization at low temperature, thus reducing the phase separation rate by increasing the viscosity of the medium [ 13-161. These results suggest to us the possibility to change the degree of intermixing and morphology with only little change in the chemical structure. Therefore, IPN’s prepared under various synthesis conditions (pressure and temperature) provide us with a useful system to study the effects of homogeneity and phase morphology on the permeability and selectivity of multicomponent polymeric membranes [ 16-181. In the previous papers [ 16,191, IPN membranes of polyurethane (PU) and polystyrene (PS) were prepared at low temperature and the effects of synthesis temperature, composition, molecular weight of polyol and the kind of diisocyanate (MDI, TDI and HDI) upon the gas

Doo Sung Lee et al./J.Membrane

Sci. 75 (1992) 15-27

permeability characteristics of the membranes were illustrated. In this paper, IPN, semi-IPN and linear blend membranes composed of PU-PS were synthesized at low temperature and the gas permeability characteristics through the membranes were investigated. The present article deals with the evaluation of the effect of the state of crosslinking and annealing upon the gas (oxygen and nitrogen) permeability and selectivity. We were able to obtain a highly miscible state via low temperature synthesis, and the homogeneity of the membrane could be gradually changed by annealing at an appropriate temperature. Since our experimental scheme can be considered to affect only the morphology without causing any change in the chemical structure of the constituents, the effect of homogeneity could be investigated exclusively.

Experimental Membrane preparation Poly (tetramethylene ether) glycol (PTMG, molecular weight of 1000) and 1:4 ratio of 1,4butanediol (1,4-BD) and trimethylol propane (TMP) were used for the PU component. Styrene monomer and divinylbenzene (DVB ) were used for the other component. The simultaneous polymerization method was adapted for the preparation of IPN membranes. The styrene was photo-polymerized by UV light. The reaction was carried out at 0°C and 40” C. The wavelength of the UV light was 3,500 A. The detailed synthesis method of the PU-PS IPN membranes was reported in a previous paper [ 191. The resulting calculated theoretical molecular weight between crosslinks (A&) of the IPN membrane was 2,000. Semi-IPN and linear blend membranes were also prepared at varying reaction temperature

Doo Sung Lee et al./J.Membrane

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Sci. 75 (1992) 15-27

and composition. In semi-IPN, the PU component was crosslinked while a linear PS component was employed. Semi-IPN and linear blends (both components are linear) were formed by excluding the appropriate crosslinking agents (TMP and DVB ) in both component formulations. Samples were dried under vacuum at room temperature for 3 days before testing. Annealing experiments of the membranes at 80’ C and 110’ C for 12 hr were also carried out to study the effect of homogeneity of the components on the permeation characteristics. Measurement The gas permeability characteristics, dynamic mechanical behavior (with Rheovibron ) and density (with a density gradient column) were measured. The detailed testing conditions and procedures were described in the previous paper [ 191. Results and discussion Gaspermeation and selectivity characteristics The gas permeability can be expressed by means of diffusivity and solubility. Diffusivities are regarded as kinetic in nature, associated with the gas activity through the membranes, being dependent on the size of the gas molecules, the extent of the free volume, mobility and chain length of the polymers, etc.. Solubilities can also be regarded as thermodynamic in nature, being determined by the interactions between the polymers and the gases. We can also define a separation factor, cy, as a measure of the membrane selectivity for one component of a mixture over another. This separation factor is the ratio of the permeability coefficients of the two components in the mixture. The separation factor can be resolved into two factors, the so called “mobility selec-

tivity”

and “solubility

cr=P*IPs

= (DA/&)

selectivity”

terms:

(S*/S,)

where (DA/&) and (SJS, ) are the mobility and solubility terms of the permeability and separation factor, respectively. For rubbery polymers, since they have a better overall chain mobility, the variation in the diffusion coefficients of the various penetrant gas molecules is small regardless of their size. The activation energy of diffusion for rubbery polymers is higher than for glassy polymers. Consequently, the permeation of rubbery polymers depends more on the difference of the solubility of the penetrant gases than on the difference in diffusivity, so they retain a high permeability coefficient and a low separation factor. On the other hand, glassy polymers have a very stiff chain structure caused by the restriction of the molecular motion in the polymers. Therefore, the penetrant gas can only diffuse through the voids in the stiff polymer, resulting in a lower activation energy for diffusion in the glassy polymer than in the rubbery polymer. Consequently, the diffusion rates vary remarkably depending on the sizes of the penetrant gases. As indicated above, the permeability and selectivity in the glassy polymers depend more on the distance between and the length of the polymer chains than on the gas-polymer interactions. The glassy polymer has a relatively higher selectivity instead of having reduced gas permeability relative to the rubbery one. Effect of synthesis temperature and crosslinked state The mean permeability coefficient, P, and separation factor, a!, for the oxygen and nitrogen as a function of the composition in the IPN, semi-IPN and linear blend membranes are dis-

Doo Sung Lee et al.jJ.Membrane Sci. 75 (1992) 15-27

18

played in Figs. 1,2 and 3, respectively. Each sample was synthesized at two different temperatures, 0’ C and 40” C, to investigate the effect of synthesis temperature. The three different types of samples, IPN, semi-IPN and linear blend, show a similar overall behavior, having permeability coefficients for oxygen and nitro-

169

I(a)

0, N,

.OPEN

SYMBOL

CLOSED

SYMBI

10-g 0,

OPEN

N,:CLOSED

SYMBOL

I

SYMBC IL

lo-"

I

0

0.25

0.5

PS MASS

0.75

1

FRACTION

(b)

0

025

05

PS MASS

075

1

FRACTION

0

025

R

0.5

MASS

0.75

1

FRACTION

Fig. 2. Permeability coefficient (a) and separation factor (b) vs. PS content of semi-IPN membranes synthesized at 0°C (solid line) and 40°C (dashedline).

I

0

1

I

0.25

I

0.5

Ps MASS

075

J 1

FRACTION

Fig. 1. Permeability coefficient (a) and separation factor (b) vs. PS content of IPN membranes synthesized at 0°C (solid line) and 40°C (dashed line).

gen in the order of lo-’ and 10-l’, respectively. However, a lower permeability was observed for samples prepared at lower temperature at every experimental point. In trying to explain why low temperature synthesis causes a reduced permeability, it is relevant to discuss the IPN formation process.

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Sci. 75 (1992) 15-27

1cP

,o-‘l

I-_!

0

0.25

0.5

0.75

1

FS MASS FRACTION

L 0

I

025

I

I

J

0.5

0.75

1

FS MASS FRbCTION

Fig. 3. Permeability coefficient (a) and separation factor (b) vs. PS content of linear blend membranes synthesized at 0°C (solid line) and 40°C (dashed line).

When the membranes were synthesized at lower temperature, 0’ C, the rate of phase separation is reduced due to the higher viscosity of the medium of the mixture at low temperature, whereas the relative rate of network formation is only marginally affected by the change in synthesis temperature. Such reduced rate of phase separation at low temperature results in

19

a decrease in free volume as well as in an improved homogeneity. These features are substantiated by the experimental data, e.g. density measurement, as reported in the previous papers [ 16,191. Therefore, the reduced permeability in the samples synthesized at low temperature could be considered to be a consequence of the reduced free volume. The permeability coefficient becomes lower as the content of the PS component increases. Similar trends have recently been reported in a poly (phenylene oxide)-PS miscible system [ 201 and a PU-polyepoxide IPN system [ 211. Additionally, the permeability coefficient shows a minimum value at a composition with ca. 75 wt.% PS. The separation factor of the specimens increases as the permeability coefficient decreases. The separation factor shows a maximum value indicating a synergistic effect at a composition of ca. 75 wt.% PS where the permeability coefficient showed a minimum value. This is probably due to the fact that the degree of mixing of the IPN membrane is highest and the free volume is lowest at this point. The morphology and the density behavior also supported this result, as was illustrated in the previous paper [ 191. Thus, the difference in selectivity between IPN’s prepared at 0°C and 40°C is much larger at this specific composition than in any other composition range. Especially, specimens synthesized at 0°C show a more improved synergistic effect than those prepared at high temperature. Such synergistic effect can not be generally expected in the case of a single component polymeric membrane. We now turn our attention to the comparison of the permeability characteristics of IPN, semi-IPN and linear blend. It is generally believed that the introduction of crosslinks into both polymers or either one of them restricts the phase separation due to physical interlocking (interpenetration), thus resulting in en-

20

hanced compatibility (homogeneity) between the constituents [ 22,231. Such interpenetration effects are most significant when both components are crosslinked (IPN) and the effect is less in semi-IPN and linear blends (in that order). Therefore, the influence of homogeneity variation caused by a change in the synthesis temperature on the permeability characteristics could be expected to be most significant in IPN and to be least significant in linear blends. This expectation could be confirmed by comparison of the selectivity and permeability data of each class of sample as summarized in Fig. 4. First of all, the maximum values of the separation factor, which are observed at a PS content of 75 wt.% for each sample, are decreasing in the order of IPN, semi-IPN and linear blend. The selectivity difference between 0’ C and 40 ’ C samples at this maximum point is also decreasing in the same order (Figs. 1,2 and 3). At the same time, the maximum gradually disappears in the case of semi-IPN and linear blend, while a distinct maximum could be observed in IPN samples. These differences among the three types of samples are also supported by the density data given in the next discussion. The permeability increases in the order of IPN
Doo Sung Lee et al./J.Membrane

0:

0

025

Sci. 75 (1992) 15-27

LIWRBLFN

0.5

0.75

1

PS MASS FRACTION

ON

f’S MASS FRACTION

Fig. 4. Comparison of permeability coefficient (a) and separation factor (b) vs. PS content with varying crosslinked state of the membranes synthesized at 0°C. Closed symbol: NZ, open symbol : Oz. Dashed line: calculated value from a parallel flux model [ 19,241.

cant morphological change might occur. Transmission electron micrographs of IPN’s with varying PS content show a morphological change [ 16,191. In those papers, the homogeneity of the IPN membrane was nearly equal at a PS content of 25 and 50 wt.%, but the mor-

Doo Sung Lee et al./J.Membrane

Sci. 75 (1992) 15-27

a

action parameters with varying PS content. This homogeneity increase reduces the free volume of the membrane and the permeability coefficient, while it increases the separation factor. When we compare the deviation of the experimental density values from those obtained with the simple rule of mixing in the three types of samples, the difference is decreasing in the order of IPN, semi-IPN and linear blends. The absolute densities are following the sequence IPN > semi-IPN > linear blend and this density behavior supports the earlier results.

1

Effect of thermal annealing

1.09

1

Is

1.08

z

E i7l

1.07

1.06

I

0

I

0.25 R

I

05

MASS

075

FRACTION

Fig. 5. Density vs. PS content with varying crosslinked state of the membranes synthesized at 0 ’ C.

l.lC )

i

It-

t 1.06

1.05 I

0

025

PS MASS

21

I

I

0.5

0.75

1

FRACTION

Fig. 6. Density vs. PS content with varying crosslinked state of the membranes synthesized at 40” C.

phology of the IPN membrane having a PS content of 75 wt.% was more homogeneous than that of the other two PS compositions. This means that there is some difference in inter-

In order to conform the effect of degree of mixing on the permeability coefficient and selectivity, annealing experiments were carried out. Figures 7, 8 and 9 show the permeability coefficient and the separation factor of specimens synthesized at O”C, and their changes after 12 hr annealing at 80” C (below Tg of PS ) and 110°C (above T,of PS). After annealing of the membranes, the permeability increases and the separation factor decreases because the free volume increases due to the further phase separation caused by annealing. It is also found that a positive deviation from the simple additive rule could be observed depending on composition and state of crosslinking. Annealing effects become more significant when annealing was performed at 110’ C relative to 80’ C. Comparing the effect of annealing in the three types of samples, the linear blends experience the most significant change in both permeability and selectivity, while IPN and semi-IPN are relatively less sensitive to annealing. This is probably due to the fact that the polymer chains in linear blends are relatively easier to relax due to the lack of physical interlocking.

Doo Sung Lee et al./J.Membrane Sci. 75 (1992) 15-27

0

0.25 Ps MASS

075

0.5

1

to-11 -II___ 0

025

FRACTION

0.5 PS MASS

~~ I 0.75

1

FRACTION

1(b)

1 0

,

,

0.25 I=5 MASS

0

110 OC lRNNEALING

0.5

075

1 1

FRACTION

0

025

0.5 PS MASS

075

1

FRACTION

Fig. ‘7. Permeability coefficient (a) and separation factor (b) vs. PS content of IPN membranes synthesized at 0°C and of their samples annealed at 80°C and 110°C for 12 hr. Closed symbol: Nz, open symbol: Oz. Dashed line: calculated values from a parallel flux model [ 19,241.

Fig. 8. Permeability coefficient (a) and separation factor (b) vs. PS content of semi-IPN membranes synthesized at 0 ’ C and of their samples annealed at 80 ’ C and 110 oC for 12 hr. Closed symbol: Nz, open symbol: Oz. Dashed line: calculated values from a parallel flux model [ 19,241.

The above observation can be explained on the basis of the free volume change estimated from the density behavior shown in Figs. 10 and 11. In a certain range in composition of semiIPN and linear blend, a positive volume change of mixing was observed after annealing, ex-

plaining the permeability increases above the value calculated from the additivity rule. A large positive deviation from the additive rule was observed in annealed samples (see Figs. 8 and 9). Such positive deviation is more readilly accomplished by annealing in linear blends than

23

Doo Sung Lee et al./J.Membrane Sci. 75 (1992) 15-27

o ,,,, -c

ANNEALING

16

1.06

lo-"

I

0

025

05

075

J 1

0

0.25

I

I

0.5

0.75

Ps t%SSFRKTIChl

PS MASS kRACTlON

Fig. 10. Density vs. PS content in IPN membranes synthesized at 0°C and their samples annealed at 80°C and 110°C for 12 hr. Dashed line: calculated values based on volume additivity rule.

M 6-

I 0 0 DCSYNTHESlS A 80 OC ANNEALING 0

0

1

025

0.5

110

cc

n

80

0

110 “C ANNEALING

“C ANNEALING

ANNEALING

0.75

1

FS MASS FRACTION

Fig. 9. Permeability coefficient (a) and separation factor (b) vs. PS content of linear blend membranes synthesized at 0” C and of their samples annealed at 80” C and 110°C for 12 hr. Closed symbol: Ns, open symbol: Oz. Dashed line: calculated values from a parallel flux model [ 19,241.

in semi-IPN. It is worthwhile to note that the difference in permeability between pure PU and PS tends to diminish after annealing. This seems to result from a post reaction or the reaction of few unreacted parts within the pure PU component, which may still be present after low temperature synthesis.

1.06 -

1.05 : i-1__0

025

05

-1~ 0.75

~~ J 1

PS MASS FRACTION

Fig. 11. Density vs. PS content in linear blend membranes synthesized at 0” C and their samples annealed at 80 ’ C and 110°C for 12 hr. Dashed line: calculated values based on volume additivity rule.

24

Doo Sung Lee et al./J.Membrane

In the case of selectivity, the maximum selectivity at 75 wt.% PS content decreases gradually with annealing in all three types of samples, but the selectivity of IPN remains the highest, even after annealing. From the above annealing experiment, it could be concluded that, for gas separation blend membranes which show a negative volume change on mixing, the homogeneity of the membranes is a very important parameter in determining the gas permeation characteristics. From Figs. 7-9, the effect of annealing on the gas permeation characteristics of the three kinds of samples can be summarized as follows. IPN membranes maintain a high separation factor at high PS composition even after annealing at 110°C for 12 hr. But linear blend membranes show a low separation factor after annealing due to phase separation. IPN formationpresents a good method to maintain long term stable membrane properties (permeability, selectivity and mechanical characteristics [ 191, etc.), owing to its crosslinked structure and the enhanced homogeneity during membrane formation.

1.50

Dynamic mechanical behavior The dynamic mechanical properties of PU25/PS75 IPN membranes with varying synthesis and annealing temperatures are shown in Fig. 12. The tanspeak of the PS phase of an IPN membrane prepared at 0’ C is shifted to low temperature, which indicates that the homogeneity is improved compared with the IPN prepared at 40’ C. After annealing at 80 oC and llO”C, the tans peaks is shifted to high temperature, suggesting that further phase separation takes place by annealing. From the peak height, we can observe that the membrane annealed at 110” C becomes more heterogeneous than that annealed at 80” C. This indicates that the extent of phase separation increases with higher annealing temperature, and

Sci. 75 (1992) 15-27 1

r^----

1.00 42

z

2 0.50

0.00 0

20

40

60

80

100

120

140

TEMPERATURE (“C) Fig. 12. Tan6 vs. temperature of PU25PS75 IPN synthesized at O”C, their samples annealed at 80°C and 110°C and IPN synthesized at 40” C. TABLE 1 Z’, of the PU25PS75 IPN” Synthesis temp (“C)

Annealing temp (“C)

0

_.

0 0 40

80 110 _.

T, (K) low T,

high Tg

304 307

383 405 405 395

* Tp’s of the crosslinked homopolymer: PU: 295 K, PS: 405 K.

that the degree of intermixing in the specimens annealed at 110” C is reduced due to higher phase separation. These results agree well with the above-mentioned permeation characteristics. The T,s from the peak temperatures are listed in Table 1. Morphology Transmission electron micrographs of IPN’s with varying PS content are shown in Fig. 13. The homogeneities of the IPN membranes with 25 and 50 wt.% PS content are almost the same, but the IPN membrane with 75 wt.% PS content shows a more homogeneous morphological state than the other two PS composition. This

Doo Sung Lee et al./J.Membrane

Sci. 75 (1992) 15-27

(a) Fig. 13. Transmission (c) PU25PS75.

25

(cl

(b) electron

micrographs

(4 Fig. 14. Transmission electron micrographs andannealedat (c) 8O”C, (d) 110°C.

of the IPN membranes

(b)

synthesized

at 0°C. (a) PU75PS25,

(d)

(cl

of the PU5OPS50 linear blend membranes

(b) PU5OPS50,

synthesized

at (a) O”C, (b) 40°C

26

means that the miscibility depends somewhat on the composition with varying PS content. This homogeneity increase reduces the free volume of the membrane and it reduces the permeability coefficient and increases the separation factor. Figure 14 shows the annealing effect for the PU5OPS50 linear blend at 80 and 110’ C. The domain size of the linear blend synthesized at 0” C is about hundreds of angstroms. But when it is treated thermally at 80 oC, followed by 110’ C, the phase domain size is somewhat increased, gradually up to about one thousand of angstroms. The fluctuations in the concentrations of the component polymers in each phase domain are coarsened by high temperature annealing. This implies that the phase separation induced by thermal treatment decreases the homogeneity and increases the free volume. These results also agree well with the permeability characteristics and the density behavior discussed previously. Conclusion

The permeability coefficient decreased and the separation factor increased when the membranes were prepared at lower temperature compared with high temperature, due to an increase in the homogeneity of PU and PS. The permeability coefficient varied with varying state of crosslinking and it increased in the following order; IPN, semi-IPN and linear blends. Annealing was found to increase the permeability coefficients due to further phase separation, while it caused a decrease in selectivity and the extent of synergistic behavior. The effect of annealing on the permeation characteristics was dependent on the nature of crosslinking of the membranes (IPN < semi-IPN
Doo Sung Lee et al. JJ.Membrane Sci. 75 (1992) 15-27

characteristics. The crosslinked nature of the IPN can make the membrane highly homogeneous during the synthesis process and can preserve the morphological structure and the membrane properties over a long period of time. In conclusion, the IPN is a novel route for the preparation of gas separation membranes. Acknowledgement

This research was supported by The Korean Science and Engineering Foundation, through Grant 90-03-00-29. References B.G. Ranby, Two-component polymer systems: Physical properties as related to compatibility and interaction, J. Polym. Sci., Polym. Symp. Ed., 51 (1975) 89. Y.J. Shur and B. Ranby, Gas permeation of polymer blends. I. PVC/ethylene-vinyl acetate copolymer (EVA) and II. PVC/acrylonitrile-butadiene copolymer (NBR), J, Appl. Polym. Sci., 19 (1975) 1337, 2143. Y.J. Shur and B. Ranby, Gas permeation of polymer blends. III. PVC/ethylene-chlorinated polyethylene (CPE), J. Appl. Polym. Sci., 20 (1976) 3105. Y.J. Shur and B. Ranby, Gas permeation of polymer blends. IV. PVC/acrylonitrile-butadiene-styrene (ABS) terpolymer, J. Appl. Polym. Sci., 20 (1976) 3121. J.A. Barrie and K. Munday, Gas transport in heterogeneous polymer blends. I. Polydimethylsiloxane-gpolystyrene and polydimethylsiloxane-b-polystyrene, J. Membrane Sci., 13 (1983) 175. J.A. Barrie and M.J.L. Williams, Gas transport in heterogeneous polymer blends. III. Alternating block copolymers of poly(bisphenol-A carbonate) and polydimethylsiloxane, J. Membrane Sci., 21 (1984) 185. J.S. Chiou and D.R. Paul, Sorption and transport of inert gases in PVFZ/PMMA blends, J. Appl. Polym. Sci., 32 (1986) 4793. D.R. Paul, Gas transport in homogeneous multicomponent polymers, J. Membrane Sci., 18 (1984) 75. D.S. Lee and S.C. Kim, Polyurethane interpenetrating polymer networks (IPN’s) synthesized under high pressure. 1. Morphology and ‘I’, behavior of polyurethane-poly(methy1 methacrylate) IPN’s, Macromolecules, 17 (1984) 268.

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D.S. Lee and S.C. Kim, Polyurethane interpenetrating polymer networks (IPN’s) synthesized under high pressure. 2. Morphology and T, behavior of polyurethane-polystyrene IPN’s, Macromolecules, 17 (1984) 2193. D.S. Lee and SC. Kim, Polyurethane interpenetrating polymer networks (IPN’s) synthesized under high pressure. 3. Morphology and Z’, behavior of polyurethane-polystyrene semi-IPN’s and linear blends, Macromolecules, 17 (1984) 2222. D.S. Lee and S.C. Kim, Polyurethane interpenetrating polymer networks (IPN’s) synthesized under high pressure. 4. Compositional variation of polyurethanepolystyrene IPN’s and linear blends, Macromolecules, 18 (1985) 2173. B.S. Kim, D.S. Lee and S.C. Kim, Polyurethane-polystyrene interpenetrating polymer networks: effect of photopolymerization temperature, Macromolecules, 19 (1984) 2589. D.S. Lee and T.S. Park, Polyurethane-polystyrene interpenetration polymer networks synthesized at low temperature: effect of temperature change during synthesis, Polym. J., 23 (1991) 241. D.S. Lee and T.S. Park, Polyurethane-polystyrene interpenetration polymer networks synthesized at low temperature: effect of crosslinking level, J. Appl. Polym. Sci., 43 (1991) 481. D.S. Lee, T.M. Tak, G.S. Kim and SC. Kim, Gas transport in polyurethane-polystyrene interpenetrating polymer network membranes, Polym. Adv. Tech., 1 (1990) 231.

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J.H. Lee and S.C. Kim, Hydrophilic-hydrophobic interpenetrating polymer networks (IPN’s) synthesized under high pressure. 1. Morphology, dynamic mechanical properties, and swelling behavior of polyurethane-polystyrene IPN’s, Macromolecules, 19 (1986) 644. W.M. Lee, Selection of barrier materials from molecular structure, Polym. Eng. Sci., 20 (1980) 65. D.S. Lee, D.S. Jung, T.H. Kim and S.C. Kim, Gas transport in polyurethane-polystyrene interpenetrating polymer network membranes. I. Effect of synthesis temperature and molecular structure variation, J. Membrane Sci., 60 (1991) 233. R.L. Stalling, H.B. Hopfenberg and V. Stannett, Transport of fixed gases in blends of polystyrene and poly (phenylene oxide), J. Polym. Sci., Polym. Symp. Ed., 41 (1973) 23. S.A. Chen and H.L. Ju, Permeation of oxygen through polyurethane-polyepoxide interpenetrating polymer networks, J. Appl. Polym. Sci., 25 (1980) 1105. S.C. Kim, D. Klempner, K.C. Frisch, H.L. Frisch and H. Ghiradella, Polyurethane-polystyrene interpenetrating polymer networks, Polym. Eng. Sci., 15 (1975) 339. L.H. Sperling, Interpenetrating Polymer Networks and Related Materials, Plenum Press, New York, NY, 1981. H.B. Hopfenberg and D.R. Paul, Transport phenomena in polymer Blends, in: D.R. Paul and S. Newman (Eds.), Polymer blends, Academic press, New York, NY, 1978, p.445.