Behavior of Absorbed Water in and Oxygen Permeability of

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Chapter 24

Behavior of Absorbed Water in and Oxygen Permeability of Hydrophilic Membranes with Bulky Hydrophobic Side Chain Groups Shuichi Takahashi, Akiko Saito, and Tsutomu Nakagawa Department of Industrial Chemistry, Meiji University, 1-1-1 Higashimita, Tama-ku, Kawasaki 214-8571, Japan

Polymeric membranes exhibiting high water permeability and high water-dissolved oxygen permeability are required for materials such as contact lenses (CL) or artificial lungs, which must be swollen with water to fit into the human body. In this study, the effect of composition of hydrophilic groups in copolymer membranes on water content or water-dissolved oxygen permeability was investigated in detail. In copolymers containing 4-vinylpyridine (4VP), we attempted to modify the copolymer membranes by quaternization to improve the wettability and permeability. We also investigated the effect of the type of quaternization ammonium salt on copolymer membrane properties. The water content and water-dissolved oxygen permeability of each copolymer membrane increased with increasing quaternization ratio.

352

© 2004 American Chemical Society

Pinnau and Freeman; Advanced Materials for Membrane Separations ACS Symposium Series; American Chemical Society: Washington, DC, 2004.

353 In the polymer medical field, polymeric membranes exhibiting high permeability for water and water-dissolved oxygen are required for materials such as contact lenses (CL) or artificial lungs, since membranes in these applications are swollen with water inside the human body. It is commonly known that the wettability of a C L has an effect on its comfort. Therefore, research to improve C L s should focus on the development of polymers having both high gas permeability and high water content. In general, there are two approaches for preparing hydrophilic membranes with high oxygen permeability. The first involves the development of higher water content materials. High water content C L materials increase the supply of oxygen to the cornea. The second approach involves copolymerization of methacrylate monomers containing a bulky pendant group with hydrophilic monomers (1-4). In this study, the effect of composition of the hydrophilic group in the copolymer membranes on water content and water-dissolved oxygen permeability was investigated in detail. The copolymers were synthesized by bulk radical copolymerization of a monomer having a bulky hydrophobic group with the hydrophilic monomers 4-vinylpyridine (4VP) and N-vinyl-2-pyrolidone (NVP). Furthermore, the effect of the type of quaternary ammonium salt on the copolymer membranes was also investigated.

Experimental Material and Synthesis of Copolymer The monomer structures used for membrane preparation and the polymerization steps for the copolymers used are shown in Figure 1. 3methacryloxypropyl tris(trimethylsiloxy)silane (SiMA), 4 V P and N V P were purified by distillation under vacuum prior to use. A l l copolymers were synthesized by a radical polymerization technique with A I B N as the initiator. These copolymers, copoly(SiMA-NVP) and copoly(SiMA-4VP), were finally cast on a horizontal Teflon plate. The mole fraction of 4VP and N V P in the copolymer was determined by elemental analysis of nitrogen using a combustion method.

Modification of Copolymer Membranes The copolymers were modified with quaternary ammonium salts using methyl iodide in 5 wt% EtOH solution for 24 hours. The chemistry of this modification of the copolymer membranes is also shown in Figure 1. In this modification reaction, some copoly(SiMA-4VP) membranes were utilized. The modified copolymers were cast onto a hot plate at 333 K . The quaternization ratio was calculated by determining the amount of iodide in the final membrane using a combustion method.


354 CHa

c= o=c o

CH2

CH=CHj

2

(CH ) (CH ) Si-0-Si-0-Si(CH ) 2

3

3

6

3

CH=CH, I NVO

3

3

4VP

Si(CH3)3

NVP

— hydrophilic monomer -

SiMA 55°C , 24hr in 0~30wt%THF

0.05mol% AIBN

6

®

m

(CH ) (CH ) ShOSi-0-Si(CHj) 2

3

3

6

3

3

N

Si(CH )3 3

Copoly(SiMA-4VP) , Copoly(SiMA-NVP) > 60°C , 24hr in 5wt% EtOH solution

5~20mol% CH I 3

CH-CH

(CH ) N (CH ) StO-Si-0-Si(CHj) O Si(CH3)3 2

3

3

3

3

I CH

3

Copoly(SiMA-4VIP) Figure 1. Synthesis and structure of copolymer and modified copolymer.


355 Characterization of Used Polymer Membranes The water content was calculated as follows: Water content [%] = [(W - W ) / W ] x 100 t

0

(1)

0

where W ! is the weight of a water-swollen membrane, and W is that of a completely dry membrane. Water-swollen membranes were blotted to remove excess water, and the measurement of weight (Wi) was repeated until a constant value was obtained. The glass transition temperature (Tg) was determined using a Perkin-Elmer DSC-7 at a heating rate of 20 K/min. Additionally, the phase transition peaks of water absorbed in the membrane were evaluated using the Perkin-Elmer DSC-7. These peaks were estimated to be in the temperature range from 153 to 293 K at heating and cooling rates of 5 K/min. D S C measurements were repeated at least 3 times. The ^-spacing was measured in the dried and hydrated states by wide angle X-ray diffraction ( W A X D : R I G A K U RINT-1200) using Bragg's equation; X= 2d sin0 (Cu - K a = 1.54 A). In the analysis of W A X D , die two broad peaks obtained were classified as the intermolecular distance and the intramolecular distance according to the paper reported by Nakamae et al (5). Intermolecular and intramolecular distances show the relative distance between two main chains and between two side chains, respectively. In the copolymer membranes modified by quaternization, the contact angle, d, of water on the polymer membrane surface was also measured to track changes in the membrane surface due to modification. 0

Oxygen Permeability Measurement in the Dry and Wet States The oxygen permeability in the dry state was determined using a vacuumpressure method at 308 K (6). The upstream pressure was up to 1 atmosphere. The coefficient of variation in the gas permeability coefficient was less than ± 3 % . The water-dissolved oxygen permeability in the wet state was measured using an oxygen electrode according to the experimental method developed by Minoura et al (7). This apparent water-dissolved oxygen permeability coefficient includes the effect of the boundary layer. Therefore, the true permeability coefficient, which eliminated the effect of the boundary layer, is calculated using five data points measured at different membrane thicknesses. The water-dissolved oxygen permeability coefficients were calculated using the following equation and measurements from five membranes ranging in thickness from 5 0 - 2 5 0 pm(7-5). l/P =l/P g

t r a e

+ (l/R)(l/d)


(2)

356 The above equation yields the true water-dissolved oxygen permeability coefficient (Ptme) at infinite thickness from the apparent one (Pg) of a membrane of thickness d. The term (1/R) is the sum of the resistances of the boundary layers.

Results and Discussions Characterization of Copolymer Membranes Basic characterization data for copoly(SiMA-NVP) and copoly(SiMA-4VP) membranes are recorded in Table I and Table II, respectively. In Table I, the NVP content in the copoly(SiMA-NVP) membranes is different from that measured in the preparation before polymerization because the reactivity of SiMA was much faster than that of NVP. The T and water content increased with increases in the NVP content of the copolymer. In contrast, the intramolecular distance decreased as NVP content increased. In Table II, the water content and the intramolecular distance of copoly(SiMA-4VP) membranes showed the same tendency with hydrophilic comonomer concentration as the copoly(SiMA-NVP) membranes. The T also increased as 4VP content increased. In copoly(4VP 70, 80 and 90), however, two transition peaks were detected by DSC because these copolymers formed a microphase separated structure. All of the synthesized copolymer membranes were colorless and transparent in the dry state. Also, the water content of all copolymer membranes was low due to the presence of bulky hydrophobic groups on the side chains. g

g

Oxygen Permeation Properties in the Dry and Wet States The relationship between sorbed water into the copolymer membranes and the hydrophilic monomer molefractionin the copolymer membranes is shown in Figure 2-a, b. The vertical axis and horizontal axis represent the amount of water per gram of polymer and the mole fraction of hydrophilic monomer, respectively. In this study, water in the copolymers was classified into 3 states (9-12). The state of the water in the water-swollen membrane was determined by DSC at a scanning rate of 5 K/min. The weight offreezingwater (W ) was calculated using the following equation: bu[kand


bomd

357

Table I Characterization of Copoly(SiMA-NVP) membranes. Intermolecular Intramolecular NVP Water distance distance content content Sample d-spacing d-spacing [A] [mol%] [wt%] [A] [°C] PSiMA Copoly(NVPlO) Copoly(NVP20) Copoly(NVP30) Copoly(NVP40)

0.00 7.10 17.1 29.1 41.4

Copoly(NVPSO) PNVP

49.8 100

4.70 6.20 14.8 28.2 31.3 44.6 180

0.70 0.12 1.3 3.2 13 18 soluble

7.45 7.29 7.27 7.08 7.28 7.32 -

5.05 4.95 4.88 4.88 4.80 4.74 -

Table II Characterization of Copoly(SiMA-4VP) membranes. Intermolecular Intramolecular NVP Water distance distance T content content Sample d~$pacing d-spacing [A] [mol%] [A] [wt%] t°C] g

PSiMA Copoly(4VP40) Copoly(4VP50) Copoly(4VP70) Copoly(4VP80) Copoly(4VP90) P4VP

0.00 37.6 48.0 69.5 80.1 88.9 100

32.6,151 31.6,153

0.70 2.1 2.8 7.9 11 22

7.45 7.32 7.32 7.32 7.37 7.49

153

35

-

4.70 17.8 32.3 26.2,140

5.05 4.89 4.87 4.62 4.50 4.48 4.39


358

0.4 (a) O Bulk water O Bound water • Non-freezing water

0.3 0.2 0.1

o

o»

0.03 0.02 1 1

0.01 0.00 Quaternization ratio of Copoly(4VIP40) [moi%]

0

& 60

1 00

0.00^=, Quaternization ratio of Copoly(4VIP50) [moi%] 0.15

—,

1

!

1

1

(c) Copoly (4VIP70) o

0.10

00

0.05 w 00

0.00 Quaternization ratio of Copoly(4VIP70) [mol%] Figure 4. Relationship between classified water in the copolymer and quaternization ratio for (a) copoly(4 VIP40), (b) copoly(4VIP50) and (c) copoly(4 VIP70) : 0 is bulk water, m is bound water, • is non-freezing water, respectively.


363 Table III Characterization of Copoly(SiMA-4VP) membranes. Quaternization ratio

T8

Water content

Contact angle

[mol%]

[°C]

[mol%]

[0]

Copoly(4VP70)

0.00

26.5,141

7.9

96±1

Copoly(4VIP70-A)

2.02

21.5,152

9.1

95±1

Copoly(4VIP70-B)

6.44

39.7, 150

19

73±1

Sample

1

Copoly(4VP50)

0.00

22.1

2.8

93±1

Copoly(4VIP50-A)

1.79

16.7

7.2

96±1

Copoly(4VIP50-B)

1.97

24.7

8.6

94±1

Copoly(4VP40)

0.00

11.5

2.1

95±1

Copoiy(4VIP40-A)

0.95

13.7

2.8

95±1

Copoly(4VIP40-B)

1.60

15.7

3.4

94±1

Copoly(4VIP40-C)

1.91

29.1

5.2

97±1

10-7 r

o3

"

9

10"

I 0

l 2

l 4

t 6

8

Quaternization ratio of copoly(SiMA-4VIP) membranes [mol%] Figure 5. The effect of the quaternization ratio on the permeability coefficients for water-dissolved oxygen in copoly(SiMA-4 VIP) membranes at 35°C: • is copoly(4VIP70) © is copoly(4VIP50) • is copoly(4VIP40), respectively. t

t


364 the 6.44 mol% sample of copoly(4VIP70), the amount of bound water sharply increased. Figure 5 shows die effect of the quaternization ratio on the waterdissolved oxygen permeability coefficients at 308 K. The water-dissolved oxygen permeability increased with modification by quaternization. We believe that this increase in the water-dissolved oxygen permeability depends on the increase in the amount of non-freezing water that accompanies quarternization. In the 6.44 mol% sample of quaternized copoly(4VDP70), however, the waterdissolved oxygen permeability decreased. We consider that an increase in the amount of bound water with increasing quarternization, as shown in Figure 4-c, interfered with the diffusion of oxygen.

Conclusions Water entering the membrane breaks the hydrogen bonds and facilitates chain mobility in the copolymer membrane. For water-dissolved oxygen permeability in the wet state, the non-freezing water and bound water increase diffusion by increasing polymer chain mobility. In the polymers which have a bulky hydrophobic side-chain, however, the interference with oxygen diffusion due to bound water counteracts the increase in oxygen solubility with increasing water content. The water content of each copolymer membrane increased with an increase in the quaternization ratio. Moreover, the water-dissolved oxygen permeability also increased as the extent of quaternization increased. This increase in waterdissolved oxygen permeability appears to be strongly related to the increase in non-freezing water that accompanies quaternization. In the 6.44 mol% sample of quaternized copoly(4VIP70), however, the water-dissolved oxygen permeability decreased because the amount of bound water was increased by quaternization.

References 1. 2. 3. 4. 5. 6. 7.

Lai, Y. C.; Valint, P. L. Jr. J. Appl. Polym. Sci. 1996, 61, 2051-2058. Lai, Y. C. J. Appl. Polym. Sci. 1995, 56, 317-324. Lai, Y. C. J. Appl. Polym. Sci. 1996, 60, 1193-1199. Kunzler, J.; Ozark, R. J. Appl. Polym. Sci. 1995, 55, 611-619. Minoura, N.; Fujiwara, Y.; Nakagawa, T. J. Appl. Polym. Sci. 1979, 24, 965-973. Nakagawa, T.; Hopfenberg, H. B.; Stannett, V. J. Appl. Polym. Sci. 1971, 15, 231-245. Refojo, M. F.; Leong, F. L. J. Membr. Sci. 1979, 4, 415-426.


365 8. 9. 10. 11. 12. 13.

Nakagawa, T. J. Jpn. CL. Soc. 1988, 30, 1-12. Hatakeyama, T.; Hatakeyama, H.; Nakamura, K. Thermochimica Acta. 1995, 253, 137-148. Hatakeyama, T.; Yoshida, H.; Hatakeyama, H. Polymer 1987, 28, 12821286. Hatakeyama, T.; Yoshida, H.; Hatakeyama, H. Thermochimica Acta. 1995, 266, 343-354. Takahashi, S.; Nakagawa, T.; Yoshida, M.; Asano, M. J. Membr. Sci. 2002, 206, 165-177. Nakamura, K.; Hatakeyama, T.; Hatakeyama, H. Tex. Res. J. 1981, 51, 607613.


Behavior of Absorbed Water in and Oxygen Permeability of

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