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X-ray and neutron scattering studies of the structure of water in a hydrogel
Volume 188, number I,2
CHEMICAL PHYSICS LETTERS
3 January 1992
X-ray and neutron scattering studies of the structure of water in a hydrogel L. Bosio ‘, G.P. Johari b, M. Oumezzine w and J. Teixeira d a LaboratoirePhysique des Liquides et Electrochimie (CNRS). ESPCI IO rue Vauquelin, 75231 Paris Cedex OS, France b Department of Materials and Engineering, MC Master University, Hamilton. Onrario, Canada US 4L 7 c DPpartement de Physique, FacultP des Sciences, Monastir 5000, Tunisia ’ Labora toire LPon Brillouin (CEA-CNRS), CEN-Saclay, 91 I91 Gif-sur-Yvette Cedex, France
Received 10 September 199 I; in final form 8 October 1991
The structureof waterabsorbedin polyHEMA is studied by X-ray diffraction in the vitreous stateat 77 K as well as in the liquid state at room temperature. Molecular packing at the level of the first neighbors ISfound similar to that in bulk water but a distortion is observed at the level of the second neighbors which is attributed to the bending of the H bonds. From the widths of the Bragg peaks of the crystalline ices from the small-angle neutron scattering results, we deduce that the pore size and the interpore distance in the swollen polyHEMA are smaller than 30 A.
1. Introduction
A variety of natural or synthetic macromolecules can imbibe a substantial amount of water. Among them, poly(2-hydroxyethylmethacrylate) YHj t-y-CH,--), / 1 dC OCH,OH referred to as polyHEMA has been the subject of a variety of studies, mainly because of its application for optical and medicinal purposes. At room temperature, this polymer absorbs 30 to 43 wt% water in its microchannels or pores whose sizes have been estimated to be smaller than 50A [1,2]. The structure of water confined in such small cavities of an Hbonded network is though to be complex, and it has been suggested that this structure varies depending upon the state of the water, namely interfacial bonded and bulk [3]. This assertion has been experimentally verified by calorimetric [ 21 and dielectric [ 41 measurements: Water in polyHEMA vitrifies at a slow cooling rate
in comparison with the hyperquenching needed to prepare vitreous bulk water [ 5,6]. On heating, a glass transition is observed at Tg= 162 K; crystallization to cubic I, ice occurs in the temperature range 210265 K and the transition to the hexagonal I,, phase slightly below 273 K. The activation enthalpy near 162 K is twice that of hyperquenched glassy water and the T, is higher than that observed in the bulk samples for which Tgbulk= 136 K whereas the crystallization to I, ice begins at 150 K [ 61. To explain these properties, Hofer et al. [ 21 suggested that water in polyHEMA can be affected by a loss of tetrahedral bonding. Moreover, the heat of melting of ice indicates that the total amount of water contained in hydrogels does not crystallize and it is established that this unfreezable water, when annealed above 130 K, exhibits a glass-liquid transition around T,, = 132 K [21. In view of these thermodynamic observations it seems necessary to study the structure of water confined in the pores of polyHEMA, while keeping in mind that the contribution of the polymer structure to the scattered radiation would be important. In this paper, we show that structural information can be extracted from X-ray diffraction: measurements were carried out at room temperature, at 77 K in the vit-
0009-2614/92/$ 05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.
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reous state and at temperatures above those of the crystallization to cubic and hexagonal phases of the freezable water. Because the patterns related to swollen polyHEMA exhibit unusual features, it was important to carry out small-angle neutron scattering measurements at room temperature on both dry and DzOcontaining polyHEMA.
well as the X-ray absorption coefficients in both dry and swollen states. The small-angle neutron scattering study was performed on the PAXE spectrometer at Laboratoire Leon Brillouin (Saclay) using flat, 1 mm thick, disc samples. For these studies, polyHEMA was swollen in 99.95% pure DzO at room temperature for several days. Measurements were done with a wavelength of 6 A and with the detector set 2.84 m from the sample.
2. Experimental details
3. Structure determination
PolyHEMA samples (ref. S38) were obtained from Smith & Nephew, London. Studies on the dry samples were performed after the samples had been systematically evacuated for one week at room temperature to remove residual water. The hydrogels were prepared by keeping the previously dried samples immersed in deionized water for several days at 310 K. The amount of water in the swollen samples ranged from 29 to 38 v&O/o which was determined by weighing. The X-ray diffraction studies were carried out on a standard 19-20 diffractometer operating in the transmission mode. MO Ku radiation (J~O.7093 A), monochromated by a bent asymmetric quartz crystal was used. Scattered intensities were measured in steps ofO.l”(28) inthe l-40’(20) range,and0.25”(2@ at higher angles, using a scintillation counter in conjunction with a pulse-height analyser. Two sets of experiments were performed on the as-received samples of about 15 mm diameter and 8 mm thickness with the radiation either perpendicular or parallel to the axis of the disc. A third set of measurements were made on a flat disc-shaped sample of 4 mm thickness. These measurements were essentially used in the data reduction. The samples were usually set inside an evacuated cryostat, equipped with mylar windows. The vacuum was released during the experiments performed at room temperature. Hydrogels were quenched to 77 K by dropping the samples into liquid nitrogen before mounting inside the previously cooled cryostat. A platinum resistor, located in the copper sample holder, was used as a temperature sensing element. In all cases, the diffraction from dry polyHEMA was measured with a great accuracy as
3. I. Wide-angle X-ray scattering
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The diffraction from the dry samples was measured with a statistical error lower than 1%up to the momentum transfer q equal to 15 A- ’ (q= 4n sin 6/ Iz,where 26 is the diffraction angle and h the wavelength) under the same experimental conditions and temperatures as from the hydrogels. In the q range where the comparison can be made, the X-ray scattering curves are identical to those already obtained for an unoriented glassy sample of polyHEMA by Mitchel et al. [ 71. A variety of interactions exist between water and the polymer network via the OH...0 bonds which makes it difficult to extract the true signal related only to the water structure. Nevertheless, we have deduced the effective scattering from water in polyHEMA by subtracting the contribution of dry polymer and taking into account the absorption caused by water. This is justified in view of the observations that for the wavelength used in studies of both dry and swollen states, polyHEMA has almost the same absorption coefficient as pure water, thus making the data processing easier. All the usual corrections were subsequentially made to obtain the structure factors S(q) for which a water molecule was treated as a spherical entity [ 8 1. The continuous lines in fig. 1 show the structure factors as determined from our experiments at 77 K (curve (a) ) and 293 IS (curve (b) ). For comparison, the structure factors of low-density amorphous water at 77 K [ 9 ] and of liquid water at 293 K [ 8 ] are also drawn (dotted lines). Except in the low q range (this will be discussed in section 3.2) the S(q) curves attributed to water in polyHEMA are similar
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9 //? Fig. 1,The structure factors of water in the polyHEMA hydrogel shown by the continuous lines: (a) vitreous state at 77 K; (b) liquid at room temperature. For comparison, the structure factors of the bulk (refs. [8,9] ) at the same temperatures are also shown (dotted lines).
to those obtained for bulk samples and although the magnitude of the structure factors differs its oscillations are in phase with those in the bulk. The damping at high q is either due to a lower definition in the near molecular neighboring or to a blur of the water signal when scattered from within the matrix. The pair correlation function, g(r), of the molecular centers can be calculated by Fourier transformation of S(q) according to the relation g(r) = 1+
1
q[S(q) - 1I sin(v) 2n-pr J‘
7
dq,
(1)
only if the number density p of water within polyHEMA is known. We have used the bulk values for p and the results obtained justify this choice. Fig. 2 shows the g(r) functions measured at 77 K (curve (a) ) and at 293 K (curve (b)). The flatness of the curves for intermolecular distances less than 2.2 A, i.e. within a region where it is known that g(r) is featureless, best justifies the procedure we have used and the assumptions made in the data treatment. Here, although the S(q) contribution in the low q range is very small, since S(q) is weighted by q in the calculation of g(r), we have artifically put S(q) equal to the bulk value for q-c 1 A-‘. The most important feature of the structure of
Fig. 2. The pair correlation functions g(v) of water in polyHEMA at (a) 77 K and at (h) 293 K. Note the hump around r= 3.6 A. water contained in polyHEMA is the hump seen in g(r) around 3.6 A, which is barely perceptible at 77 K but clearly resolved at room temperature. As pointed out by Gorbaty and Demaniets [ lo], the origin of this peak in g(r) at rz 3.6 A has been discussed by several workers. But, this effect here cannot be attributed to a truncation error, because S(q) is totally flat at high 4. It is now known chat a shoulder or a maximum located on the right-hand side of the main peak of g( r) does appear in both water under hydrostatic pressure [IO] and in the high-density amorphous water [ 91, in which the H bonds are thought to be distorted. Therefore, we conclude that this feature in the g(r) of water in hydrogels seen in fig. 2 is an indication of the existence of bent hydrogen bonds leading to a disorder at the level of the second molecular neighbors and that the first nearneighbor H bonds are not significantly affected. At low temperature, the first shell located at the mean vaIue of r-2.8 8, is well resolved. Thus the coordination number, inferred from the radial distribution function, 41tr*pg( r), and assuming that our approximation for p is correct, is equal to about 3.9. Although this value is slightly lower than that in the bulk water, the result shows that no substantial loss of tetrahedral bonding occurs in the structure. Measurements performed at 170 K, i.e. above the glass transition, did not show significant differences with
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the results obtained for the vitreous state after quenching and after annealing. The results of water in hydrogel in fig. 2 are of course a combination of the freezable and unfreezable water structures. On heating the sample to 258 and 270 K, partial crystallization of the water contained in polyHEMA occurs into ice Ic and I,,, respectively. Fig. 3 (curve (a) ) shows the uncorrected intensity for cubic ice as determined after subsequent cooling to 77 K and after subtraction of the dry polymer contribution. Here, we observe under the Bragg’s peaks features attributable to the uncrystallized water. Assuming that the scattered intensity of this structure of water is identical with that of the vitrified water previously studied, we have adjusted the amplitude of the signal and subtracted it from the measured signal to obtain a flat curve for the crystallized phases. Curves (b) and (c) (fig. 3) have been obtained after the subtraction of an amount equal to 45 wt% of vitreous water. Even though the proportion of unfreezable water varies from one sample to another, this result is in good
Fig. 3. [a) Scattered intensity recorded at 77 K from both cubic ice and unfreezable water after the subtraction of the dry polymer contribution. (b), (c) Patterns of cubic and hexagonal ice obtained after the subtraction of an amount of 45 wt% ofvitreous water. Inset: The Bra@ peaks of ice in the polymer, here the (1 I I ) reflection of I, ice, are considerably broadened. The diffractometer resolution depicted by a solid line is given for comparison.
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agreement with the value deduced from the heat of melting [ 21. Other information can be drawn from the diffraction patterns of ice. As already pointed out by Hofer et al. [ 21, either in I, or in Ih ice, the Bragg reflections are considerably broadened in comparison with those of bulk samples. From the widths of these reflections (see inset fig. 3), we deduce the size of the crystallites by using the Scherrer relation. The dimension of these crystallites is around 28 A, which is in good agreement with the conclusion of Refojo and Leong [ 11 who, from studies of penetration of large molecules through polyHEMA membranes, determined that the pore size in the hydrogel can be as large as 30 A. 3.2. Small-angle scattering In the equations relating the g(r) to S( 4), i.e. eq. ( 1) , the Dirac function located at q =O is generally
neglected since its contribution is experimentally undetectable because of the large size of the sample usually studied. In our studies of the hydrogel, the water contained in small pores gives rise to a smallangle scattering. Fig. 4 clearly shows this effect on the scattered intensity which is plotted against q dur-
0
1
2
3
4
5
6
Fig. 4. Scattered intensity recorded during the process of swelling of polyHEMA; the duration of each scan is about twelve hours. Note that the dry polymer (curve 1) exhibits no small-angle scattering.
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ing the process of swelling. The spectra here were obtained at room temperature for about twelve hours and correspond to different states of swelling, equally spaced in time. As seen in the lower curve of fig. 4, dry polyHEMA exhibits no small-angle scattering but a signal appears and increases during the swelling. Because the q resolution in our X-ray experiments was insufficient to allow an unambiguous conclusion from these results, measurements using small-angle neutron scattering were performed. Small-angle scattering gives information about the size and the spatial arrangement of the pores filled with water. When in the X-ray scattering the contrast is due to the difference between the electronic densities of the matrix and water, in the neutron scattering it is due to the difference between the coherent scattering length densities of the polymer and water. Therefore, we used deuterated water in order to increase the signal. Fig. 5 shows the curves for the dry polymer (curve (a) ) and the polymer swollen with heavy water (curve (b) ) after subtraction of an estimated amount of incoherent scattering. The qdependence of the scattered intensity I from the swollen sample, shown in the inset of fig. 5, indicates a correlation length < of about 19 A which is compat-
0
0.04
0.08, , q /A-
0.12
0.16
Fig. 5. Small-angle scattering as determined by neutron measurements at room temperature. (a) Dry polyHEMA; (b) heavy water-imbibed polyHEMA. Inset: plot of the inverse of the difference between the scattered intensity by the hydrogel and the dry polymer versus the squared momentum transfer 4. The straight line fitting the data indicates that the correlation length
3 January 1992
ible with the characteristic distances present in the porous sample, i.e. the pore size and the interpore distance. The statistical distribution of these two quantities does not, of course, allow their separation and only an Omstein-Zemike-type correlation length (can be obtained from the relation I=K/( 1+<‘q2), A remarkable observation in the X-ray studies remains unexplained, namely, that when crystallization into cubic or hexagonal ice occurs, the small-angle scattering intensity greatly decreases or even disappears, but reappears on melting. Further experiments by neutron scattering at low temperatures are necessary to clarify the X-ray observations, particularly because the contrast in this case is almost insensitive to density effects.
4. Conclusion The X-ray diffraction studies of hydrogels provide valuable information on the structure of the confined water even when the contribution of the matrix to the scattered intensity is significant. (i ) From the width of the Bragg reflections in the crystalline forms and from the small-angle scattering, we deduce that water is contained inside pores or microchannels whose size is smaller than 30 A. In these cavities, water keeps the bulk properties in the sense that the phase transitions can be normally observed. The temperatures at which these transitions occur are raised in comparison with those in bulk water, probably because correlations with the polymer reinforce or disturb the H-bonded network. In some other cavities, water remains unfreezable because the eventual nuclei cannot grow as the interaction with the polymer matrix prevents establishment of a three-dimensional order. (ii) The molecular packing of water in swollen polyHEMA is not significantly affected at the level of the first neighbors and the tetrahedrally bonded structure persists. On the contrary, a distortion at the second neighbor level is observed in a manner identical to that induced by a pressure increase. In its confined state in polyHEMA, the structure of water contains bent or broken H bonds, leading to a complexity in the packing of the tetrahedral entities. In this respect, precise determination of the number 117
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density of water confined in the polyHEMA pores and microchannels would be useful.
[ 1] M.F. Refojo and F.L. Leong, J. Polym. Sci. Polym. Symp. 66 (1979) 227. [2] K. Hofer, E. Mayer and G.P. Johari, J. Phys. Chem. 44 ( 1990) 2689. [ 31 G. Smyth, F.X. Quinn and V.J. MC &ierty, Macromolecules 21 (1988) 3198.
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[4] K. Pathmanathan and G.P. Johari, J. Polym. Sci. Polym. Phys. 28 (1990) 675. [5] J. Dubochet and L. Lepault, I. Phys. (Paris) 79 (1984) C785. [ 61 P. Briiggeller and E. Mayer, Nature 298 ( 1982) 7 15. [7]G.R. Mitchel, D. Brown and A.H. Windle, Makromol. Chem. 184 (1983) 1937. [ 81 A. Narten and H. Levy, J. Chem. Phys. 55 ( 197I ) 2263. [ 91 A. Bizid, L. Bosio, A. Defrain and M. Oumezzine, J. Chem. Phys. 87 (1987) 2225. [IO] E. Gorbaty and Yu. Demaniets, Mol.Phys. 55 (1985) 571.
CHEMICAL PHYSICS LETTERS
3 January 1992
X-ray and neutron scattering studies of the structure of water in a hydrogel L. Bosio ‘, G.P. Johari b, M. Oumezzine w and J. Teixeira d a LaboratoirePhysique des Liquides et Electrochimie (CNRS). ESPCI IO rue Vauquelin, 75231 Paris Cedex OS, France b Department of Materials and Engineering, MC Master University, Hamilton. Onrario, Canada US 4L 7 c DPpartement de Physique, FacultP des Sciences, Monastir 5000, Tunisia ’ Labora toire LPon Brillouin (CEA-CNRS), CEN-Saclay, 91 I91 Gif-sur-Yvette Cedex, France
Received 10 September 199 I; in final form 8 October 1991
The structureof waterabsorbedin polyHEMA is studied by X-ray diffraction in the vitreous stateat 77 K as well as in the liquid state at room temperature. Molecular packing at the level of the first neighbors ISfound similar to that in bulk water but a distortion is observed at the level of the second neighbors which is attributed to the bending of the H bonds. From the widths of the Bragg peaks of the crystalline ices from the small-angle neutron scattering results, we deduce that the pore size and the interpore distance in the swollen polyHEMA are smaller than 30 A.
1. Introduction
A variety of natural or synthetic macromolecules can imbibe a substantial amount of water. Among them, poly(2-hydroxyethylmethacrylate) YHj t-y-CH,--), / 1 dC OCH,OH referred to as polyHEMA has been the subject of a variety of studies, mainly because of its application for optical and medicinal purposes. At room temperature, this polymer absorbs 30 to 43 wt% water in its microchannels or pores whose sizes have been estimated to be smaller than 50A [1,2]. The structure of water confined in such small cavities of an Hbonded network is though to be complex, and it has been suggested that this structure varies depending upon the state of the water, namely interfacial bonded and bulk [3]. This assertion has been experimentally verified by calorimetric [ 21 and dielectric [ 41 measurements: Water in polyHEMA vitrifies at a slow cooling rate
in comparison with the hyperquenching needed to prepare vitreous bulk water [ 5,6]. On heating, a glass transition is observed at Tg= 162 K; crystallization to cubic I, ice occurs in the temperature range 210265 K and the transition to the hexagonal I,, phase slightly below 273 K. The activation enthalpy near 162 K is twice that of hyperquenched glassy water and the T, is higher than that observed in the bulk samples for which Tgbulk= 136 K whereas the crystallization to I, ice begins at 150 K [ 61. To explain these properties, Hofer et al. [ 21 suggested that water in polyHEMA can be affected by a loss of tetrahedral bonding. Moreover, the heat of melting of ice indicates that the total amount of water contained in hydrogels does not crystallize and it is established that this unfreezable water, when annealed above 130 K, exhibits a glass-liquid transition around T,, = 132 K [21. In view of these thermodynamic observations it seems necessary to study the structure of water confined in the pores of polyHEMA, while keeping in mind that the contribution of the polymer structure to the scattered radiation would be important. In this paper, we show that structural information can be extracted from X-ray diffraction: measurements were carried out at room temperature, at 77 K in the vit-
0009-2614/92/$ 05.00 0 1992 Elsevier Science Publishers B.V. All rights reserved.
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3 January 1992
reous state and at temperatures above those of the crystallization to cubic and hexagonal phases of the freezable water. Because the patterns related to swollen polyHEMA exhibit unusual features, it was important to carry out small-angle neutron scattering measurements at room temperature on both dry and DzOcontaining polyHEMA.
well as the X-ray absorption coefficients in both dry and swollen states. The small-angle neutron scattering study was performed on the PAXE spectrometer at Laboratoire Leon Brillouin (Saclay) using flat, 1 mm thick, disc samples. For these studies, polyHEMA was swollen in 99.95% pure DzO at room temperature for several days. Measurements were done with a wavelength of 6 A and with the detector set 2.84 m from the sample.
2. Experimental details
3. Structure determination
PolyHEMA samples (ref. S38) were obtained from Smith & Nephew, London. Studies on the dry samples were performed after the samples had been systematically evacuated for one week at room temperature to remove residual water. The hydrogels were prepared by keeping the previously dried samples immersed in deionized water for several days at 310 K. The amount of water in the swollen samples ranged from 29 to 38 v&O/o which was determined by weighing. The X-ray diffraction studies were carried out on a standard 19-20 diffractometer operating in the transmission mode. MO Ku radiation (J~O.7093 A), monochromated by a bent asymmetric quartz crystal was used. Scattered intensities were measured in steps ofO.l”(28) inthe l-40’(20) range,and0.25”(2@ at higher angles, using a scintillation counter in conjunction with a pulse-height analyser. Two sets of experiments were performed on the as-received samples of about 15 mm diameter and 8 mm thickness with the radiation either perpendicular or parallel to the axis of the disc. A third set of measurements were made on a flat disc-shaped sample of 4 mm thickness. These measurements were essentially used in the data reduction. The samples were usually set inside an evacuated cryostat, equipped with mylar windows. The vacuum was released during the experiments performed at room temperature. Hydrogels were quenched to 77 K by dropping the samples into liquid nitrogen before mounting inside the previously cooled cryostat. A platinum resistor, located in the copper sample holder, was used as a temperature sensing element. In all cases, the diffraction from dry polyHEMA was measured with a great accuracy as
3. I. Wide-angle X-ray scattering
114
The diffraction from the dry samples was measured with a statistical error lower than 1%up to the momentum transfer q equal to 15 A- ’ (q= 4n sin 6/ Iz,where 26 is the diffraction angle and h the wavelength) under the same experimental conditions and temperatures as from the hydrogels. In the q range where the comparison can be made, the X-ray scattering curves are identical to those already obtained for an unoriented glassy sample of polyHEMA by Mitchel et al. [ 71. A variety of interactions exist between water and the polymer network via the OH...0 bonds which makes it difficult to extract the true signal related only to the water structure. Nevertheless, we have deduced the effective scattering from water in polyHEMA by subtracting the contribution of dry polymer and taking into account the absorption caused by water. This is justified in view of the observations that for the wavelength used in studies of both dry and swollen states, polyHEMA has almost the same absorption coefficient as pure water, thus making the data processing easier. All the usual corrections were subsequentially made to obtain the structure factors S(q) for which a water molecule was treated as a spherical entity [ 8 1. The continuous lines in fig. 1 show the structure factors as determined from our experiments at 77 K (curve (a) ) and 293 IS (curve (b) ). For comparison, the structure factors of low-density amorphous water at 77 K [ 9 ] and of liquid water at 293 K [ 8 ] are also drawn (dotted lines). Except in the low q range (this will be discussed in section 3.2) the S(q) curves attributed to water in polyHEMA are similar
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to those obtained for bulk samples and although the magnitude of the structure factors differs its oscillations are in phase with those in the bulk. The damping at high q is either due to a lower definition in the near molecular neighboring or to a blur of the water signal when scattered from within the matrix. The pair correlation function, g(r), of the molecular centers can be calculated by Fourier transformation of S(q) according to the relation g(r) = 1+
1
q[S(q) - 1I sin(v) 2n-pr J‘
7
dq,
(1)
only if the number density p of water within polyHEMA is known. We have used the bulk values for p and the results obtained justify this choice. Fig. 2 shows the g(r) functions measured at 77 K (curve (a) ) and at 293 K (curve (b)). The flatness of the curves for intermolecular distances less than 2.2 A, i.e. within a region where it is known that g(r) is featureless, best justifies the procedure we have used and the assumptions made in the data treatment. Here, although the S(q) contribution in the low q range is very small, since S(q) is weighted by q in the calculation of g(r), we have artifically put S(q) equal to the bulk value for q-c 1 A-‘. The most important feature of the structure of
Fig. 2. The pair correlation functions g(v) of water in polyHEMA at (a) 77 K and at (h) 293 K. Note the hump around r= 3.6 A. water contained in polyHEMA is the hump seen in g(r) around 3.6 A, which is barely perceptible at 77 K but clearly resolved at room temperature. As pointed out by Gorbaty and Demaniets [ lo], the origin of this peak in g(r) at rz 3.6 A has been discussed by several workers. But, this effect here cannot be attributed to a truncation error, because S(q) is totally flat at high 4. It is now known chat a shoulder or a maximum located on the right-hand side of the main peak of g( r) does appear in both water under hydrostatic pressure [IO] and in the high-density amorphous water [ 91, in which the H bonds are thought to be distorted. Therefore, we conclude that this feature in the g(r) of water in hydrogels seen in fig. 2 is an indication of the existence of bent hydrogen bonds leading to a disorder at the level of the second molecular neighbors and that the first nearneighbor H bonds are not significantly affected. At low temperature, the first shell located at the mean vaIue of r-2.8 8, is well resolved. Thus the coordination number, inferred from the radial distribution function, 41tr*pg( r), and assuming that our approximation for p is correct, is equal to about 3.9. Although this value is slightly lower than that in the bulk water, the result shows that no substantial loss of tetrahedral bonding occurs in the structure. Measurements performed at 170 K, i.e. above the glass transition, did not show significant differences with
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the results obtained for the vitreous state after quenching and after annealing. The results of water in hydrogel in fig. 2 are of course a combination of the freezable and unfreezable water structures. On heating the sample to 258 and 270 K, partial crystallization of the water contained in polyHEMA occurs into ice Ic and I,,, respectively. Fig. 3 (curve (a) ) shows the uncorrected intensity for cubic ice as determined after subsequent cooling to 77 K and after subtraction of the dry polymer contribution. Here, we observe under the Bragg’s peaks features attributable to the uncrystallized water. Assuming that the scattered intensity of this structure of water is identical with that of the vitrified water previously studied, we have adjusted the amplitude of the signal and subtracted it from the measured signal to obtain a flat curve for the crystallized phases. Curves (b) and (c) (fig. 3) have been obtained after the subtraction of an amount equal to 45 wt% of vitreous water. Even though the proportion of unfreezable water varies from one sample to another, this result is in good
Fig. 3. [a) Scattered intensity recorded at 77 K from both cubic ice and unfreezable water after the subtraction of the dry polymer contribution. (b), (c) Patterns of cubic and hexagonal ice obtained after the subtraction of an amount of 45 wt% ofvitreous water. Inset: The Bra@ peaks of ice in the polymer, here the (1 I I ) reflection of I, ice, are considerably broadened. The diffractometer resolution depicted by a solid line is given for comparison.
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agreement with the value deduced from the heat of melting [ 21. Other information can be drawn from the diffraction patterns of ice. As already pointed out by Hofer et al. [ 21, either in I, or in Ih ice, the Bragg reflections are considerably broadened in comparison with those of bulk samples. From the widths of these reflections (see inset fig. 3), we deduce the size of the crystallites by using the Scherrer relation. The dimension of these crystallites is around 28 A, which is in good agreement with the conclusion of Refojo and Leong [ 11 who, from studies of penetration of large molecules through polyHEMA membranes, determined that the pore size in the hydrogel can be as large as 30 A. 3.2. Small-angle scattering In the equations relating the g(r) to S( 4), i.e. eq. ( 1) , the Dirac function located at q =O is generally
neglected since its contribution is experimentally undetectable because of the large size of the sample usually studied. In our studies of the hydrogel, the water contained in small pores gives rise to a smallangle scattering. Fig. 4 clearly shows this effect on the scattered intensity which is plotted against q dur-
0
1
2
3
4
5
6
Fig. 4. Scattered intensity recorded during the process of swelling of polyHEMA; the duration of each scan is about twelve hours. Note that the dry polymer (curve 1) exhibits no small-angle scattering.
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ing the process of swelling. The spectra here were obtained at room temperature for about twelve hours and correspond to different states of swelling, equally spaced in time. As seen in the lower curve of fig. 4, dry polyHEMA exhibits no small-angle scattering but a signal appears and increases during the swelling. Because the q resolution in our X-ray experiments was insufficient to allow an unambiguous conclusion from these results, measurements using small-angle neutron scattering were performed. Small-angle scattering gives information about the size and the spatial arrangement of the pores filled with water. When in the X-ray scattering the contrast is due to the difference between the electronic densities of the matrix and water, in the neutron scattering it is due to the difference between the coherent scattering length densities of the polymer and water. Therefore, we used deuterated water in order to increase the signal. Fig. 5 shows the curves for the dry polymer (curve (a) ) and the polymer swollen with heavy water (curve (b) ) after subtraction of an estimated amount of incoherent scattering. The qdependence of the scattered intensity I from the swollen sample, shown in the inset of fig. 5, indicates a correlation length < of about 19 A which is compat-
0
0.04
0.08, , q /A-
0.12
0.16
Fig. 5. Small-angle scattering as determined by neutron measurements at room temperature. (a) Dry polyHEMA; (b) heavy water-imbibed polyHEMA. Inset: plot of the inverse of the difference between the scattered intensity by the hydrogel and the dry polymer versus the squared momentum transfer 4. The straight line fitting the data indicates that the correlation length
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ible with the characteristic distances present in the porous sample, i.e. the pore size and the interpore distance. The statistical distribution of these two quantities does not, of course, allow their separation and only an Omstein-Zemike-type correlation length (can be obtained from the relation I=K/( 1+<‘q2), A remarkable observation in the X-ray studies remains unexplained, namely, that when crystallization into cubic or hexagonal ice occurs, the small-angle scattering intensity greatly decreases or even disappears, but reappears on melting. Further experiments by neutron scattering at low temperatures are necessary to clarify the X-ray observations, particularly because the contrast in this case is almost insensitive to density effects.
4. Conclusion The X-ray diffraction studies of hydrogels provide valuable information on the structure of the confined water even when the contribution of the matrix to the scattered intensity is significant. (i ) From the width of the Bragg reflections in the crystalline forms and from the small-angle scattering, we deduce that water is contained inside pores or microchannels whose size is smaller than 30 A. In these cavities, water keeps the bulk properties in the sense that the phase transitions can be normally observed. The temperatures at which these transitions occur are raised in comparison with those in bulk water, probably because correlations with the polymer reinforce or disturb the H-bonded network. In some other cavities, water remains unfreezable because the eventual nuclei cannot grow as the interaction with the polymer matrix prevents establishment of a three-dimensional order. (ii) The molecular packing of water in swollen polyHEMA is not significantly affected at the level of the first neighbors and the tetrahedrally bonded structure persists. On the contrary, a distortion at the second neighbor level is observed in a manner identical to that induced by a pressure increase. In its confined state in polyHEMA, the structure of water contains bent or broken H bonds, leading to a complexity in the packing of the tetrahedral entities. In this respect, precise determination of the number 117
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density of water confined in the polyHEMA pores and microchannels would be useful.
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[4] K. Pathmanathan and G.P. Johari, J. Polym. Sci. Polym. Phys. 28 (1990) 675. [5] J. Dubochet and L. Lepault, I. Phys. (Paris) 79 (1984) C785. [ 61 P. Briiggeller and E. Mayer, Nature 298 ( 1982) 7 15. [7]G.R. Mitchel, D. Brown and A.H. Windle, Makromol. Chem. 184 (1983) 1937. [ 81 A. Narten and H. Levy, J. Chem. Phys. 55 ( 197I ) 2263. [ 91 A. Bizid, L. Bosio, A. Defrain and M. Oumezzine, J. Chem. Phys. 87 (1987) 2225. [IO] E. Gorbaty and Yu. Demaniets, Mol.Phys. 55 (1985) 571.