Article pubs.acs.org/JPCB
Neutron Diffraction of Ice in Hydrogels Yurina Sekine,*,† Tomoko Ikeda-Fukazawa,‡ Mamoru Aizawa,‡ Riki Kobayashi,§,|| Songxue Chi,|| Jaime A. Fernandez-Baca,|| Hiroki Yamauchi,† and Hiroshi Fukazawa† †
Quantum Beam Science Center, Japan Atomic Energy Agency, 2-4 Shirakata-Shirane, Naka-gun, Tokai 319-1195, Japan Department of Applied Chemistry, Meiji University, 1-1-1 Higashi-Mita, Tama-ku, Kawasaki 214-8571, Japan § Institute for Solid State Physics, University of Tokyo, 5-1-5 Kashiwanoha, Kashiwa, Chiba 277-8581, Japan || Quantum Condensed Matter Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831-6393, United States ‡
ABSTRACT: Neutron diffraction patterns for deuterated poly-N,N,-dimethylacrylamide (PDMAA) hydrogels were measured from 10 to 300 K to investigate the structure and properties of water in the gels. Diffraction peaks observed below 250 K indicate the existence of ice in the hydrogels. Some diffraction peaks from the ice are at lower diffraction angles than those in ordinary hexagonal ice (Ih). These shifts in peaks indicate that the lattice constants of the a and c axes in the ice are about 0.29 and 0.3% higher than those in ice Ih, respectively. The results show that bulk low-density ice can exist in PDMAA hydrogels. The distortions in the lattice structure of ice imply significant interactions between water molecules and the surrounding polymer chains, which play an important role in the chemical and mechanical properties of the hydrogel.
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INTRODUCTION Water and ice are the most common liquid and solid on Earth, and they constitute an important medium for life. Because of the flexibility of hydrogen bonds, water and ice exhibit several different structures and properties in different situations. Therefore, microscopic understanding of the structure and properties of water under different conditions is crucial in various fields, including physics, chemistry, and biology.1−3 Hydrogels are unique materials because large amounts of water can be contained among their 3D cross-linked polymer chains.4−6 Water content varies from a dry condition to nearly 98 wt %.7−12 The scanning electron microscope (SEM) image in Figure 1 shows that hydrogels have empty spaces (dark areas in the figure) that can accommodate water molecules, However, the structure and dynamics of water molecules in these spaces are not fully understood at the atomic level. On the basis of macroscopic aspects, such as thermal properties, water in hydrogels has been classified into three types, bound water, intermediate water, and free water.7−9,12,13 Bound water forms hydrogen bonds with polar polymers and/ or interacts strongly with ionic groups in the polymer chains. Free water comprises molecules that have structures similar to those in ordinary liquid water. Intermediate water interacts weakly with the polymer chains and therefore exists in states between bound and free water. From a Raman scattering experiment on a poly-N,N-dimethylacrylamide (PDMAA) hydrogel, Sekine and Ikeda-Fukazawa7,8 observed the O−H stretching vibrations of bound water in the gel. The analysis of the Raman spectra showed that bound water strongly interacts with polar groups on the polymer chains. Neutron diffraction is useful to investigate structural changes in water because of the large neutron scattering cross sections of deuterium and oxygen.14,15 Neutrons are also especially © XXXX American Chemical Society
suited for studying D2O water in hydrogels because hydrogen atoms (H) on the polymer chains are not detectable.16 Therefore, we have used neutron diffraction to study deuterated hydrogels at one of the highest flux research reactors, the high-flux isotope reactor (HFIR). We report the diffraction profiles of the deuterated PDMAA hydrogel, which contains bound, intermediate, and free water, in a temperature range of 10−300 K.
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MATERIALS AND METHODS
Two hydrogel samples containing D2O and a N-substituted acrylamide polymer, PDMAA, were prepared to investigate the structures of the water molecules; one sample had high and the other had low water content. The PDMAA hydrogel was synthesized by mixing D2O, N,N-dimethylacrylamide (DMAA), N,N′-methylenebis(acrylamide) (BIS), potassium persulfate (KPS), and N,N,N′,N′-tetramethylenediamine (TEMED). BIS, KPS, and TEMED were used as the cross-linker, initiator, and catalyst, respectively. Before mixing, the DMAA monomer was purified by filtering through activated alumina. An aqueous solution of D2O (33.3 g), DMAA (2.97 g), BIS (0.37 g), KPS (0.03 g), and TEMED (24 μL) was poured into a sample cup with a diameter of 12 mm and a depth of 23 mm. The crosslinking density was 8 mol % (mole ratio of cross-linker to monomer). The D2O content W (weight ratio of water to hydrogel) of each sample is defined as Special Issue: Physics and Chemistry of Ice 2014 Received: August 15, 2014
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data for dried PDMAA (W = 0) as background data. The background data were subtracted from all diffraction profiles. We have measured SEM images of hydrogels.9 For the SEM observations, we have dehydrated the hydrogels because the samples should be kept in vacuum. Figure 1a shows a SEM image of the dehydrated PDMAA hydrogel, which is the sample with W = 0.90 for the neutron diffraction measurements. We have dehydrated our sample by using freeze-drying method as follows. The gel sample was immersed in liquid nitrogen. Then, the frozen gel was lyophilized in a freeze-dryer for 2 days to obtain the dried sample. Specimens were cut from the freezedried gel. Before SEM observation, the specimens of the dried gel were fixed on a stage and coated with platinum for 60 s. SEM images were obtained using a SEM instrument (JEOL, JMS-6390LA) at an accelerating voltage of 5 kV.
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RESULTS AND DISCUSSION Neutron Diffraction of PDMAA Hydrogel. Neutron diffraction profiles were obtained at temperatures of 10−300 K. Figure 2a shows the neutron diffraction profile of the high D2O content PDMAA hydrogel (W = 0.90) at temperatures of T = 10, 250, and 300 K. As shown in this figure, the profile at 300 K has a broad liquid-like diffraction feature in the 2θ range from 10 to 65°. This broad feature disappears at 250 K, and the profile at T ≤ 250 K has strong sharp peaks. The sharp peaks in
Figure 1. (a) SEM image of a dried chemically cross-linked hydrogel. The white structures indicate the cross-linked polymers, which form empty spaces in the micron range. Empty spaces can accommodate water molecules. (b) Chemical structure of the PDMAA hydrogel, which is cross-linked by BIS.
W=
(Mgel − Md) Mgel
(1)
where Mgel is the weight of the gel sample and Md is the weight of the completely dehydrated sample. The D2O content W of samples was controlled to 0.90 and 0.65. Figure 1a shows a SEM image of the dried PDMAA hydrogel. Figure 1b shows the chemical structure of the polymer network of the PDMAA hydrogel. Many pieces of about 1 g in weight were cut from the bulk of gel and were fully put into the vanadium cell (60 mm long, 10 mm diameter, 0.2 mm thick) with helium gas and vapor of D2O. The sample can of vanadium was sealed by indium. The equilibrium was thus kept during the experiments. We confirmed that water contents in each sample are the same values before and after experiments. Neutron diffraction measurements were performed using the wide-angle neutron diffractometer (WAND)14,15,17 at the HFIR at Oak Ridge National Laboratory (ORNL), U.S.A. The diffraction patterns were collected using a wavelength of 1.48 Å with a step angle of 0.2°. The hydrogel samples at W = 0.90 and 0.65 were attached to a top-loading He cryostat at 10 K. The neutron diffraction patterns of the gel at W = 0.90 were collected at 10, 50, 100, 150, 200, 250, 260, 270, 273, and 300 ± 4 K. For the gel at W = 0.65, the neutron diffraction patterns were collected at 10, 50, 100, 150, 200, 250, 260, 270, 280, 290, and 300 ± 4 K. We conducted the experiment twice for each of the gel samples. In addition, we measured neutron diffraction
Figure 2. (a, left) Neutron diffraction profiles of the high D2O content PDMAA hydrogel (W = 0.9) at 10 (bottom), 250 (center), and 300 K (top). (b, right) Neutron diffraction profiles of the low D2O content PDMAA hydrogel (W = 0.65) at 10 (bottom), 250 (center), and 300 K (top). The dots mark the measured intensities. The solid line shows the calculated diffraction pattern using the best-fit parameters from Rietveld analysis. The peak positions calculated from the structure of ice Ih (P63/mmc) are shown by ticks below the diffraction patterns. The broken lines are the deviation between the measured and calculated intensities. B
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analysis of the diffraction profiles. The data were fit to polynomial expression
the diffraction profiles at temperatures of 10−250 K are similar to diffraction patterns of hexagonal ice Ih. For example, the profile at 10 K has five sharp peaks at 21.43, 23.02, 24.43, 31.82, and 38.23°, which are observed in the profile of ordinary ice Ih, although some peak positions appear at lower scattering angles. The sharp peaks are attributed to Bragg diffraction of the (100), (002), (101), (102), and (110) planes in ice Ih. The figure indicates that the hydrogel at W = 0.9 contained liquid water at 300 K, and it transformed to crystalline ice Ih below 250 K. Figure 2b shows neutron diffraction profiles for the low D2O content PDMAA hydrogels (W = 0.65) at 10, 250, and 300 K. The diffraction profiles at 10 and 250 K have sharp peaks that are similar to the diffraction patterns of the high D2O content PDMAA hydrogel (W = 0.90) and ice Ih. At 300 K, where ice is not expected, no strong peaks appear in the diffraction profile. The profile at 300 K appears to have a liquid-like broad band in the 2θ range of 15−40°; however, the signal is weak because of the small amount of water in the gel at W = 0.65. Ice in the Hydrogel. To investigate the structure of ice in the PDMAA hydrogels, we performed Rietveld analysis18 for the diffraction profiles of the PDMAA hydrogels at W = 0.90 and 0.65 under the assumption that the ice Ih (P63/mmc) exists. We used the computer program RIETAN-FP developed by Izumi et al.19 As shown in Figure 2, the fits were adequate for all diffraction profiles. Figure3a and b shows the temperature dependence of the lattice constants a and c for ice in the gels at W = 0.90 and 0.65, respectively. These values were obtained from the Rietveld
a(T ), c(T )(nm) = A 0 + A1T + A 2T 2 + A3T 3 + ... + A8T 8 (2)
where T is the temperature in K and A0, A1, ..., A8 are coefficients. The fits are shown by the broken and dotted lines in Figure 3a and b. Values for the coefficients for the gels at W = 0.9 and 0.65 are given in Table 1. The lattice constants of the a and c axes for ice in the gels (open triangles for W = 0.90 and solid circles for W = 0.65) are higher than those for ordinary D2O ice Ih (open circles).20 At each temperature, values of the lattice constants a and c for the gel at W = 0.90 are approximately 0.15 and 0.16% higher than those of ordinary ice Ih. For the gel at W = 0.65, the lattice constants a and c are 0.29 and 0.31% higher than those of ordinary ice Ih. These higher values of the lattice constants show that the a and c axes of ice in the gel increase because of interactions with the surrounding polymer. These results are consistent with previous Raman results.7,8 Those Raman results showed peak positions for O−H stretching vibrations in liquid water at W = 0.90 and 0.65 to be 3 and 13 cm−1 higher than those for ordinary water. The high-frequency shifts of O−H stretching modes show an increase of molecular distance.21 Thus, the results indicate the existence of low-density water as intermediate and bound water in the gels. The lattice constants of the a and c axes for ice in the gels increase as D2O content decreases. At each temperature, values of the lattice constants a and c for the gel at W = 0.65 are approximately 0.14 and 0.15% higher than those at W = 0.90. These increases in the lattice constants indicate that the ratio of low-density water increases as the amount of water in the gel is reduced. According to estimates by Ikeda-Fukazawa et al.,9 the PDMAA hydrogel at W = 0.65 contains 39% bound water, 55% intermediate water, and 6% free water, although most of the water in the hydrogel at W = 0.90 is free water (70% free water, 15% intermediate water, and 15% bound water). The volume of the gel at W = 0.65 is about 76% smaller than that of W = 0.90. The decrease of the pore size might be one of the causes for the increase of the lattice constant of the ice. Figure 3c shows unit cell volumes of ice in the gels. The data were also fit to a polynomial expression Vunit cell(T ) = A 0 + A1T + A 2 T 2 + A3T 3 + ... + A8T 8
(3)
The unit cell volumes of ice in the gel at W = 0.9 and 0.65 were 0.46 and 0.9% higher than those of ordinary water, respectively. These results indicate that ice in the gels is expanded compared to ordinary ice. Because the compressibility of ice Ih is about 0.12 GPa−1,22,23 the increase in the volume of ice at W = 0.90 and 0.65 is consistent with that caused by the negative pressures of 38 and 54 MPa, respectively. Thus, strong interactions with the polymer chains affect the structure of water in the hydrogel. Anomalous features of ice on the hydrophobic surface24 and in mesoporous silica25 have already been reported. We compare the coefficients for hydrogel with those for ice Ih.20 As shown in Table 1, the values of A3, ..., A8 for the gels are different from those for ice Ih. In fact, the shapes of the dotted and broken curves in Figure 3a−c are different from those of the solid curve. For example, the unit cell volume of ice Ih decreases with increasing temperature in the temperature range from 10 to 70 K. However, the unit cell volume of the gel at W = 0.65 is almost constant in the temperature range from
Figure 3. Temperature dependence of lattice constants a and c and unit cell volumes of ice in the gels at W = 0.9 (open triangles) and 0.65 (solid circles). Open circles are reported values for ordinary ice Ih.20 Broken, dotted, and solid curves are fits to polynomial expression in absolute temperature with coefficients given in Table 1 for the gel at W = 0.65 and 0.90 and ordinary ice Ih, respectively. C
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10−9 10−11 10−13 10−15 10−18 10−21
10 to 100 K. Therefore, the temperature dependences of the lattice constant and the unit cell volume of the gels are also different from those of ice Ih.
0.129 − − 1.60 × −5.24 × 6.40 × −3.54 × 9.23 × −9.23 ×
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10−10 10−12 10−13 10−15 10−18 10−21
CONCLUSIONS We measured neutron diffraction profiles of poly-N,Ndimethylacrylamide hydrogels at temperatures of 10−300 K. The profiles below 250 K show diffraction peaks from water molecules that indicate the existence of low-density ice Ih in the hydrogels. On the basis of these diffraction profiles, the lattice structures of ice in the hydrogels are expanded compared to those in ordinary ice. The water molecules in the hydrogels are affected by strong interactions with the surrounding polymer chains.
0.129 − − −2.76 × −9.07 × 2.68 × −2.06 × 6.49 × −7.42 ×
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10−9 10−11 10−13 10−15 10−18 10−4
W = 0.9 ice Ih
Article
0.128 − − −2.26 × 5.16 × −4.58 × 2.09 × −4.86 × 4.57 ×
20
unit cell volume of ice in the gel
W = 0.65
The Journal of Physical Chemistry B
AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected].
REFERENCES
10−9 10−11 10−14 10−17 10−20 10−9 10−10 10−12 10−15 10−18
0.734 − − −1.17 × 1.68 × −5.69 × 4.25 × 3.65 × − 10−9 10−11 10−13 10−15 10−18 10−10 10−12 10−4 10−16 10−19 10−9 10−11 10−13 10−16 10−19
W = 0.9
0.733 − − −6.72 × 1.47 × −1.13 × 3.75 × −4.59 × −
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0.732 − − −3.06 × 5.90 × −3.92 × 1.15 × −1.27 ×
ice Ih
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0.451 − − 1.03 × −5.19 × 8.19 × −3.79 × 5.53 × −
W = 0.65
ACKNOWLEDGMENTS This work was performed under the auspices of the U.S.−Japan Cooperative Program on Neutron Scattering. This work (authors Y.S. and H.F.) was also partially supported by a Grant-in-Aid for the research activity from the JSPS. Research conducted at ORNL’s High-Flux Isotope Reactor was sponsored by the Scientific User Facilities Division, Office of Basic Energy Sciences, U.S. Department of Energy. We thank Katie Andrews for her experimental assistance.
0.450 − − −1.75 × 3.48 × −2.32 × 6.79 × −7.47 × −
W = 0.9 ice Ih
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0.450 − − −3.91 × 10−9 9.79 × 10−11 −9.74 × 10−13 4.93 × 10−15 −1.26 × 10−17 1.28× 10−20
lattice constant a of ice in the gel
The authors declare no competing financial interest.
A0 A1 A2 A3 A4 A5 A6 A7 A8
lattice constant c of ice in the gel
20 20
Table 1. Coefficients in Polynomial Fits to Lattice Constants a and c and the Unit Cell Volume
W = 0.65
Notes
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