Journal of Membrane Science, 42 (1989) 303-314 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands

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A DIFFERENTIAL SCANNING CALORIMETRY STUDY OF THE STATES OF WATER IN SWOLLEN POLY(VINYL ALCOHOL) MEMBRANES CONTAINING NONVOLATILE ADDITIVES

WEI-ZHONG ZHANG, MITSURU SATOH and JIRO KOMIYAMA

Department of Polymer Science, Tokyo Institute of Technology, Ookayama, Meguro-ku, Tokyo 152 (Japan)

(Received December 1,1987; accepted in revised form June 10,1988)

Summary

To find a possible reason for the enhanced solubility and hence the permselectivity of 0, over N2 in various membranes, the states of water in poly (vinyl alcohol) membranes swollen to swelling ratios (H) ranging from 0.3 to 0.7 (water/swollen membrane, g/g) with water or aqueous solutions of five nonvolatile substances were investigated by differential scanning calorimetry (DSC) over the temperature range from 170 to 320 K. The enthalpy of the phase transition of the water confirmed that at least three states of water exist in the membranes, namely free, freezing bound and nonfreezing water, as already postulated. The addition of substances with high affinities to the polymer was found to result in a large increase in the amount of both freezing bound water and nonfreezing water. The extra increase of the latter type of water, with the hydration of the polymer and the additives being taken into account, is attributed to interactions in the polymer chain/surrounding water region where the added substances are concentrated.

Introduction

An understanding of the properties of water in gel membranes is important for interpreting the transport behavior of solutes, including gases, in separa- tion processes, and for elucidating the mechanism of membrane permeability. It has been found that water attached directly to hydrophilic groups of a mem- brane-constituting polymer through hydrogen bonding shows no phase tran- sition from 320 to 170 K [ 11, such as melting or crystallization. This kind of water has been categorized as nonfreezing water. Water having a phase tran- sition temperature lower than that of bulk water due to a weak interaction of the water with the polymer chain [2] and/or to capillary condensation [3,4] is defined as freezing bound water.

The presence of such states of water in the membranes would have a specific effect on the permeability behavior of solute, since the molecular arrangements and motion of these water states should be more or less different from those of bulk water [ 51. In fact, such water states have often been invoked to account

0376-7388/89/$03.50 0 1989 Elsevier Science Publishers B.V.

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for the permeation behavior of solutes in water-swollen membranes [ 6-91. Higuchi et al. [ lo,11 ] proposed a parallel permeation model to interpret the permeabilities of various gases in water-swollen gel cellophane and poly (vinyl alcohol-co-itaconic acid) membranes. It was deduced that the diffusion coef- ficients of the gases in the nonfreezing water are lower than those in free water, and the solubilities in the nonfreezing water are higher than in free water. Recently [ 12,131, we dealt with the permeabilities to O2 and N2 of poly(viny1 alcohol) (PVA) membranes swollen with aqueous solutions of various non- volatile substances, and found that the addition of certain substances with high affinities to the polymer enhances significantly the permselectivity to 0, over Nz. Such behavior was inferred to be attributable to the presence of a polymer- perturbed solution phase in the membranes, in which the gas solubilities were markedly increased.

Differential scanning calorimetry (DSC) analysis is often used to estimate the amounts of free, freezing bound and nonfreezing water in water-swollen membranes. In the case of hydrophilic polymers having ionic groups, it is known that water molecules are strongly associated with the ionic group and form the hydration shell [ 141. Recently, Hatakeyama et al. [ 151 investigated water- sodium cellulose sulphate systems with various water contents by DSC. It was indicated quantitatively from the enthalpy of the phase transition of water that each -OSO,Na group is associated with about three water molecules in the nonfreezing state. The interaction between water molecules and ionic groups was further studied by Nakamura et al. [ 161 for systems of water-carboxy- methylcellulose with various counter-ions and a wide range of water content. It was reported that the amount of nonfreezing water increases almost linearly with the decrease of the ionic radius of the counter-ions through their hydration.

Thus, in order to account for the enhanced permselectivity to 0, over N, or more specifically to ascertain the origin of the markedly enhanced solubility of O2 in the polymer perturbed solution phase, the present DSC study aims to investigate the effects of various nonvolatile additives on the water states in water- and solution-swollen poly(viny1 alcohol) membranes with a swelling ratio range of 0.3-0.7. The results will be discussed by reference to the partition coefficients of the additives between the solution phase within the membrane and the aqueous solution.

Experimental

Samples Poly (vinyl alcohol) supplied by Nakarai Co. Ltd. (polymerization degree 2

x 103, degree of hydrolysis 99-100% ) was used after extracting impurities with methanol. Other chemicals used were described elsewhere [ 12,131. As de- scribed there, PVA membranes having swelling ratios of 0.3-0.7 were prepared by immersing the cast PVA membranes in a crosslinkage solution of 0.1% glu-

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taraldehyde at 25’ C for 0.5-48 hours. After being immersed for 3 days in 5-10 wt% aqueous solutions of urea, thiourea, cesium chloride, lithium chloride and lithium iodide, the membranes were employed for measurements by DSC. The swelling ratio, H, of the membranes swollen with the solutions is defined by:

H = (Wl--W,)/Wl (1)

where IV, represents the weight of the membrane after careful blotting, and W, that of the membrane dried in uucuo at 100” C for 24 hours.

DSC measurements External surface water was removed from the membranes by carefully blot-

ting with filter paper. Samples of about 25 mg were weighed, and sealed in aluminum pans that had previously been treated with boiling water for 2 hours to avoid reaction of the aluminum with water during the measurements.

A Seiko differential scanning calorimeter, model DSC-10, equipped with a cooling apparatus was used. DSC curves were obtained in the temperature range 170-320 K at a scanning rate of 2.5 K/min, to avoid the time lag of response caused by a faster scanning rate. Temperatures and enthalpies of crystalliza- tion and melting of the samples were calibrated using pure water as the stan- dard; their standard deviations were found to be within 2 0.5 K and ? 5%, respectively. The amount of free (IV,) and freezing bound (IV,) water was estimated by the following relation:

VV, or IV, (mg) = Y,AZ/dH(T)B (2)

where Y1 (mJ/min) is the range sensitivity, A (cm2) the peak area, AH(T) (mJ/mg ) the transition enthalpy of water at the transition temperature T, B (cm/min) the chart speed of the recorder, and 2 the calibration constant (in the present study, 1.21 cm-l). For supercooled free water, the transition en- thalpy , AH ( T) , is known to be lower than that of free water at 273 K, as ex- pressed by [ 17,181.

273

AH(T) = B-1(273)- j,4C,dT T

(3)

where AC, is the difference in the heat capacities of liquid water and super- cooled water. The same relationship may be applied to the freezing bound water. In the present study, the value LIH (273) = 333.34 mJ/mg was used for ap- proximately calculating the amount of the relevant water from the heating curves. For temperatures above 243 K, this approximation gave values within a deviation of + 10% [ 181. The amount of nonfreezing water ( W,,) was esti- mated by subtracting the amounts of W, and W, from the total water content ( W,) of the membranes:

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Results and discussion

Figure 1 shows DSC heating curves of water-swollen PVA membranes. With the decrease in the swelling ratio of membranes, it was found that the peak around 273 K due to the melting of bulk water is separated into two peaks; one remains close to 273 K and the other shifts toward a lower temperature, about 253 K. For the membrane with a swelling ratio of 0.340, no endothermic peak is observed in the DSC heating curve in the range 170-320 K. These results indicate that there are at least three states of water in the membrane, i.e., free water having a melting temperature (peak lh) comparable with that of bulk water, freezing bound water showing a lower temperature peak (peak ah), pre- sumably by weak interaction of the water with the polymer chain, and non- freezing water, without phase transition, due to a direct association with hy- droxyl groups of the polymer, as already suggested [ 191.

DSC cooling curves of the corresponding water-swollen PVA membranes are shown in Fig. 2. In contrast to the heating curves, an exothermic peak (peak 1”) due to the crystallization of free water was observed near 263 K, and an-

-___.. _--- H 3 0.340

s

I, f- H=l

f I

233 253 273 293

Temperature / K

Fig. 1. DSC heating curves of water and water-swollen PVA membranes. Heating rate = 2.5 K/ min.

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213 233 253 273

Temperature/K

Fig. 2. DSC cooling curves of water and water-swollen PVA membranes. Cooling rate = 2.5 K/ min.

other broad peak (peak 2’) appeared at about 233 K. The depression of about lo-20 K in the crystallization temperature of water, compared with the melting temperature, is explained by supercooling [ 201. Similarly, no crystallization peak appeared for the membrane with H= 0.340. It is thus assumed that, up to this swelling ratio, only nonfreezing water is present in the membrane.

To examine the effect of nonvolatile additives on the water states, DSC anal- yses of PVA membranes swollen with various aqueous solutions were per- formed. Figures 3 and 4 show the heating and cooling results for the mem- branes swollen with 5 wt% aqueous LiCl solution, as typical examples. It is found, compared with the results on water-swollen membranes shown in Figs. 1 and 2, that not only peak 1 but also peak 2 are shifted toward a lower tem- perature range, both for melting and crystallization, and peak 3” appears at a temperature between those for peaks 1” and 2” during the cooling process when LiCl is present in the membranes. Moreover, the area of peak 2h relative to lh, or of peak 2” relative to lc, should be noted, this definitely increases with the addition of LiCl. This suggests that water within the LiCl-containing mem- branes is more strongly perturbed by the presence of the salt. Such character- istic behavior, however, differs to varying extents for the membranes swollen with different aqueous solutions. Therefore, the amounts of free, freezing bound

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Hz0599

Hz0497

H:O 402

_ A-__ ALL.. APL. __~~ ~_. --_ .~.

233 253 273 293 213 233 253 273

Temperature/K Temperature /K

Fig. 3. DSC heating curves of 5 wt% LiCl aqueous solution and PVA membranes swollen with this solution. Heating rate = 2.5 K/min.

Fig. 4. DSC cooling curves of 5 wt% LiCl aqueous solution and PVA membranes swollen with this solution. Cooling rate = 2.5 K/min.

and nonfreezing water, as well as their phase transition temperatures, were estimated quantitatively for a more detailed discussion.

Figure 5 shows phase transition temperatures (which were read at the in- tercepts of the extended base-lines and the maximum oblique lines of peaks) of water within the membranes swollen with various aqueous solutions as a function of the swelling ratio. It is clear that the temperatures are dependent on the nature of the additive. As shown in Fig. 5a, peak lh, corresponding to the melting of free water, and peak 2h, due to that of freezing bound water, are shifted least for the membranes swollen with 5 wt% aqueous CsCl. For the membranes swollen, with 5 wt% aqueous LiCl and 10 wt% aqueous LiI, how- ever, large depressions, of about 10 K, are found in both peak lh and peak 2h. On the other hand, for the membranes swollen with 5 wt% aqueous urea, the temperature of peak lh shows a slight increase to about 278 K, and that of peak 2h decreases with decreasing swelling ratio to the values for water-swollen membranes at H= 0.40-0.50. For the membranes swollen with 5 wt% aqueous thiourea, the temperature of peak lh remains unchanged but that of peak 2h decreases in the same way as in the case of urea. Similar behaviour of the

273 -.-.-.y 273

(a) (b)

173 L 173

0 0.2 O-4 0.6 0.8 1.0 0 0.2 0.4 0.6 0.8 1.0

H H

Fig. 5. Relationship between phase transition temperatures of water in PVA membranes swollen with various aqueous solutions and water content. (a) melting temperature from heating curves; (b) temperature of crystallization from cooling curves. Filled and open symbols represent the transition temperatures estimated for the higher and lower temperature peaks, respectively. (m, 0 ) water-swollen membranes. (0, 0 ) membranes swollen with 5 wt% aqueous LiCl solution. ( A, A ) membranes swollen with 10 wt% aqueous LiI solution. ( l , 0) membranes swollen with 5 wt% aqueous CsCl solution. (+, 0) membranes swollen with 5 wt% aqueous thiourea solution. (v , V ) membranes swollen with 5 wt% aqueous urea solution.

changes in the crystallization temperatures of water in the corresponding membranes can be found in Fig. 5b.

The amounts of free ( IV,) freezing bound ( IV,) and nonfreezing ( IV,,) water in the membranes are shown in Table 1, as a function of the sum of the weights of polymer and added substance, ( W, + IV,). Using eqns. (2 ) - (4)) the values were calculated from enthalpies given by the areas of peaks lh and 2h and the total water content.

The changes of ( Wr+ W,) / ( W, + IV,) and W,J ( Wp+ IV,) with the swell- ing ratio in the membranes swollen with water or 5 wt% aqueous LiCl are illustrated in Fig. 6 for comparison purposes. For the water-swollen mem- branes, it can be seen that free and freezing bound water begin to appear at a swelling ratio of about 0.36, where the (IV,+ IV,) line is extrapolated to the horizontal axis, and the sum of these types of water increases linearly with increasing swelling ratio of the membrane. Unfortunately, the respective amounts of free and freezing bound water could not be estimated because of the uncertainty in dividing their peak areas exactly. The amount of nonfreez- ing water increases slightly from H= 0.40 to 0.50, and stays roughly constant at 0.47 above H = 0.50. By addition of LiCl to the corresponding water-swollen membranes, however, free and freezing bound water seem to appear at a higher swelling ratio of about 0.40, and the amount of nonfreezing water increases to a constant value of about 0.60. Such increases in nonfreezing water were also observed, as shown in Table 1, in the membranes swollen with the other aqueous

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TABLE 1

Dependence of ( W,+ W,) and W,,, on water content in PVA membranes swollen with various aqueous solutions

Membrane Swelling ratio (H)

wr+ wflJ W”,

w,+ w, w,+ws

Water-swollen 0.677 1.626 0.470 0.487 0.508 0.442 0.380 0.234 0.379

LiCl-containing 0.599 0.881 0.613 0.497 0.436 0.551 0.402 0.056 0.541

LiI-containing 0.563 0.630 0.658 0.507 0.363 0.665 0.384 0.046 0.577

CsCl-containing 0.541 0.686 0.493

0.470 0.430 0.457 0.392 0.226 0.418

Thiourea-containing 0.605 0.992 0.540 0.547 0.701 0.507 0.449 0.242 0.573

Urea-containing 0.629 1.097 0.598 0.532 0.532 0.604 0.455 0.314 0.520

0 O-2 O-4 0.6 0.8 1-O

H

Fig. 6. Changes of ( W,+ W,) and W,,, in PVA membranes swollen with water or 5 wt% aqueous LiCl solution. (w, 0 ) water-swollen membranes. (0,O ) membranes swollen with 5 wt% aqueous LiCl solution

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solutions investigated here. Furthermore, the degree of the increases depends on the nature of the added substance. As examples, the value of W,J ( W, + IV,) is estimated to be 0.65,0.48,0.54 and 0.59 for the membranes swollen with 10% aqueous LiI, 5 wt% aqueous CsCl, 5 wt% aqueous thiourea and 5 wt% aqueous urea, respectively.

The differences in the increased amounts of nonfreezing water can be ex- plained primarily in terms of the water of hydration of the partitioned addi- tives in the membranes. In our previous paper [ 131 the partition coefficients, K, of these substances were investigated in membranes with different swelling ratios. It was reported that K for CsCl decreases monotonically with a decrease in swelling ratio, and reaches a value at 0.60 at H = 0.30. This implies that CsCl is largely excluded from the bound water, probably because the affinity of the substance for this region is low. However, the K values of urea, thiourea, LiCl and LiI were found to be 0.95-1.9 for the membranes at H< 0.4 [Zl]. These high partition coefficients were explained by strong interactions be- tween the additives and the polymer chain and/or the water perturbed by the polymer chain. Referring to these results, the observed large increases in non- freezing water may be attributed to the presence of these substances in the vicinity of the polymer chains in the membranes.

To enable semiquantitative discussion, the amounts of nonfreezing water in aqueous solutions of CsCl, LiCl, LiI, urea and thiourea [14] were obtained from the DSC analysis by reference to the enthalpy of the melting of water. The values are listed in the fourth column of Table 2. It is understandable that the ability of the additives to bind water should differ for different species. Thus, CsCl and urea give nonfreezing hydrations of about 0.40 and - 0.03 mg of water per 1 milligram, respectively, while LiCl, LiI and thiourea have much stronger abilities to interact with water molecules. The particular behavior of

TABLE 2

Comparison of experimental and calculated amounts of nonfreezing water in solution-swollen membranes

Additive H Wnfa W"P K W"fC wp+ws WS wp+ws*

CsCl, 5 wt% 0.392 0.418 0.40 0.65 0.39 LiCl, 5 wt% 0.402 0.541 2.39 0.95 0.46 LiI, 10 wt% 0.384 0.517 1.93 1.8 9.52 Thiourea, 5 wt% 0.449 0.573 2.03 1.2 0.50 Urea, 5 wt% 0.455 0.520 -0.03 1.1 0.41

“Experimentally observed values. bValues observed for aqueous solutions. ‘Values calculated based on the assumption of additivity of nonfreezing water of the polymer and additives.

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urea in water has been reported to result from its breaking effect on the water structure [ 221.

The values of W,,/ ( W, + W,) *, also shown in Table 2, were calculated from the assumed additivity of the amounts of nonfreezing water due to the polymer chain and to the additives. The former values were estimated for water-swollen membranes with swelling ratios corresponding to the respective solution-swol- len membranes, The latter values were estimated for the additive concentra- tions in the membranes, with their partition coefficient, K, in the correspond- ing membranes being taken into account. It can be seen from the table that, compared with other additives, the amount of nonfreezing water for the CsCl system thus calculated is the closest ( N 7% less) to that obtained experimen- tally for the solution-swollen membrane with H = 0.392. This, as expected from the fact that CsCl is largely excluded from the polymer-perturbed water phase, suggests that the nonfreezing water in the membrane swollen with aqueous CsCl comprises the respectively hydrations of the polymer chain and CsCl. However, for the cases of LiCl, LiI, urea and thiourea, the values of I+‘,,/ ( I+‘,+ IV,)* in Table 2 are 10 to 20% less than for the corresponding solution- swollen membranes. Remarkably, urea has no ability for nonfreezing hydra- tion in its aqueous solution, giving a calculated value of 0.41 for W,,/ ( W, + W,) * for the membrane with H = 0.455, while the value measured from the urea solution-swollen membrane is about 0.52. For the cases of LiCl, LiI and thi- ourea, the differences between the calculated and measured values are found to be 0.06-0.08 for the respective membranes. The above arguments have led to the assumption that such an excess in the increased nonfreezing water in the membranes swollen with aqueous LiCl, LiI, urea and thiourea solutions arises from specific interactions between the polymer chain and/or the sur- rounding water with the added substances, in addition to the purely additive contributions of the polymer and additives.

In conclusion, the DSC data revealed the presence of three different states of water in water- or solution-swollen PVA membranes. By subtracting the hydrations of the additives in water, the presence of an increased amount of nonfreezing water in the solution-swollen membranes was confirmed, and was suggested to be caused by specific interactions between the polymer chain and the surrounding water and additives having high affinities to the polymer chain.

Now, returning to the problem of the enhanced solubility of O2 in the poly- mer-perturbed solution phase in the various membranes, it may be speculated that this extra nonfreezing water is likely to accomodate a large amount of 0,. To understand how such water contributes to the solubility of 0, is at present beyond our reasoning ability. However, the well-known versatile hydrogen bonding structure of water, which provides microscopic voids of varying sizes, may be invoked here for preferential solubilization of 0, in the solution phase [ 23,241. The water-swollen PVA membrane contains ca. 0.61 g of nonfreezing water per gram of polymer, which corresponds to ca. 1.5 mol of such water per

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PVA residue. By the addition of the specified additives, the nonfreezing water is increased by 10 to 20%. This amount may seem trivial compared with the total amount of nonfreezing water in the membrane; however, this may not be so if this extra amount of nonfreezing water serves to modify the nearby water structure so as to facilitate O2 dissolution. In support of this argument, atten- tion should be drawn to the observation that the addition of such simple salts as LiCl and LiI to the membrane leads to preferential solubilization of O2 [ 131. In water, 0, solubility decreases with the concentration of these salts [ 251. This argument may also provide a basis for speculation that the structural modification of the water may occur in the region where polymer, water and added substance are undergoing interaction.

Acknowledgements

The authors gratefully acknowledge the support for this study provided by a Grant-in-Aid for scientific research (No. 61550653) from the Ministry of Education, and also by General Sekiyu Research & Development Encourage- ment & Assistance Foundation. We wish to thank Dr. T. Seo for his valuable advice and discussion.

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