Biomacromolecules 2002, 3, 991-997
1H
991
NMR Study of the States of Water in Equilibrium Poly(HEMA-co-THFMA) Hydrogels Phuong Y. Ghi,† David J. T. Hill,*,† and Andrew K. Whittaker‡
Polymer Materials and Radiation Group, Department of Chemistry and Centre for Magnetic Resonance, The University of Queensland, Brisbane, Queensland 4072, Australia Received March 22, 2002; Revised Manuscript Received May 23, 2002
The spin-spin relaxation times, T2, of hydrated samples of poly(hydroxymethyl methacrylate), PHEMA, poly(tetrahydrofurfuryl methacrylate), PTHFMA, and the corresponding HEMA-THFMA copolymers have been examined to probe the states of the imbibed water in these polymers. The decay in the transverse magnetization of water in fully hydrated samples of PHEMA, PTHFMA, and copolymers of HEMA and THFMA was described by a multiexponential function. The short component of T2 was interpreted as water molecules that were strongly interacting with the polymer chains. The intermediate component of T2 was assigned to water residing in the porous structure of the samples. The long component of T2 was believed to arise from water residing in the remnants of cracks formed in the polymer network during water sorption. Introduction Since water comprises a significant proportion of polymer hydrogels, the interaction of water with the polymer is important in determining many of the properties of the swollen gel. The equilibrium water content of a hydrogel is considered to contribute to the biocompatibility and low thrombogenicity of these materials.1 Additionally, water plays an important role in the function of a polymeric biomaterial, for example, in facilitating the interaction of the polymer with blood2 and in supporting the transport of pharmaceutical agents through the polymeric matrix.3 Nevertheless, the biocompatibility of an hydrogel is not directly related to the amount of water the material contains. For example, poly(hydroxyethyl methacrylate), PHEMA, hydrogels which have an extremely “open” structure, and hence relatively high water contents, were found to be less compatible with tissues than PHEMA gels with a tighter structure and a lower water content.4 While the presence of water is an important factor contributing to the biocompatibility of a hydrogel, the nature or organization of water within the hydrogel may provide a better understanding of the interactions that occur between the hydrogel and the biological system. Interpretation of current calorimetric and spectroscopic results on the structure of water within hydrogels is a subject of controversy.5-10 A three-state model for water in hydrogels was first proposed by Jhon and Andrade2 based on the observation of structured water near the water/solid interfaces and in natural macromolecular gels. They suggested that water can behave dynamically and thermodynamically as part of the polymer chains when the water molecules interact strongly with such specific sites as the hydroxyl or ester * To whom correspondence should be sent. † Department of Chemistry. ‡ Centre for Magnetic Resonance.
groups, and they classified this type of water as “bound” water. When the interaction between the water molecules and the polymeric chains is weaker, or when the water molecules are preferentially structured around the polymer network, “intermediate” water is formed. The third classification of water is “free” or “bulklike” water where the amount of interaction between the water and the polymer chains is insignificant. The properties of the imbibed water in hydrated PHEMA were studied using dilatometry, specific conductivity and differential scanning calorimetry (DSC) by Lee and coworkers.11 The change in specific volume of a series of PHEMA gels with different water content was obtained from dilatometry measurements. In a sample with 20 wt % water, the specific volume change as a function of temperature was linear, whereas for a sample with 50 wt % water, a sharp volume change was observed at 0 °C. For samples with an intermediate amount of water, a hysteresis effect was observed. A plot of the specific conductivity as a function of temperature also showed a sharp decrease in the vicinity of 273 K for samples containing greater than 30 wt % water. Additionally, three distinct regions were observed in the plot of the activation energy for specific conduction as a function of the weight percent of water in PHEMA, which were correlated to the different classes of water. DSC thermograms for PHEMA showed a transition that occurred at higher temperatures with an increase in water content in the PHEMA. The collective data demonstrated that up to 20 wt % of the total water content was comprised of “bound” water, and in samples containing greater than 50 wt % water, the three properties of the imbibed water converged toward that observed for “bulk” water. Differential thermal analysis5 provided two types of results that strongly support the presence of structured water in PHEMA. First, the measured total melting enthalpy is lower
10.1021/bm0200332 CCC: $22.00 © 2002 American Chemical Society Published on Web 06/28/2002
992
Biomacromolecules, Vol. 3, No. 5, 2002
than that of pure water, suggesting significant polymer-water interactions and/or incomplete freezing of the imbibed water. This “nonfreezable” water is equivalent to the abovementioned “bound” water. Second, the shape of the melting curves exhibited two different peaks around 273 K in samples with a water content greater than 20 wt %. The water that melts near 273 K was interpreted as “bulk” water, and the water with a slightly lower melting temperature was termed “intermediate” water. The amounts of bound, intermediate, and bulklike water determined from the DTA thermograms revealed an increase in bulklike water only after the bound and intermediate waters were saturated. Subsequently, Sung et al.5 postulated a three-step imbibition process. The hydrophilic sites within the hydrogels first attract and bind an amount of water, followed by the formation of a secondary hydration shell of water that is preferentially oriented around the bound water and the polymer network structure. Further absorbed water exists as free or “bulklike” water. The terms “free” and “bound” water attest different mobility that can be probed by nuclear magnetic relaxation spectroscopy. Lee et al.6 reported that considerable interactions occurred between the water molecules and the polymer networks by the reduced value of the spin-lattice relaxation time, T1, of the imbibed water (0.132 s for PHEMA containing 50 wt % of water) over that of pure water (4.50 s at 307 K). The results showed that the relative amounts of each class of water varied with the total water content in PHEMA; an increase in the total water content resulted in an increase in the fraction of “bulk” water and a decrease in the fraction of “bound” water. Additionally, when the water content of PHEMA lies below 25% by mass, the values of the spin-spin relaxation time, T2, are constant (ca. 5 ms) and rapidly rise as the water content increases (up to ca. 35 ms at a water content of 50 wt %), supporting the proposed three-step imbibition process.5 McBrierty et al.7,8,12 reported that the trend of T1, T1F, and T2 as a function of temperature for hydrated PHEMA points toward complex water behavior. The minima in the plots of T1F and T1 as a function of temperature at 225 and 200 K, respectively, and the appearance of a long component in T2 at 180 K were interpreted as the onset of motion in a “glasslike” water. In addition, a hysteresis effect was observed in T1 and the long component of T2 between 240 and 280 K. The presence of structured water in PHEMA was refuted by Bouwstra et al.,13 who argue that both the total melting enthalpy and the actual shape of the melting curves are extremely sensitive to the experimental conditions and cannot be reliably interpreted as an indication for different classes of water in hydrogels. First, the low value for the melting enthalpy of frozen PHEMA is due to incomplete freezing of the water in the hydrogel and results in the development of nonequilibrium conditions. Second, the fine structure observed in the melting peaks around 273 K may be interpreted as the result of a devitrification process. The dynamics of water molecules in PHEMA gels were studied using 17O spin-lattice relaxation by Roorda and coworkers.10 The recovery of equilibrium magnetization was
Ghi et al.
found to be monoexponential, and hence, on the time scale of milliseconds or longer, thermodynamically different classes of water are indistinguishable. Despite the controversy surrounding the existence of structured water within PHEMA gels, the practical implications of “bound” and “bulk” water to the efficient application of PHEMA-based membranes in reverse osmosis was demonstrated by Pedley and Tighe.14 They reported that while a high EWC (30 wt %) is desirous to enhance the water flux, a very low “freezing” water content is equally important in retaining and rejecting the salt in saline water. Furthermore, DSC was identified as a valuable technique in the identification of polymers having an EWC of 30%, all of which is “nonfreezable”. In this paper, the spin-spin relaxation times, T2, of hydrated samples of PHEMA, PTHFMA, and copolymers of HEMA and THFMA are presented to probe the states of the imbibed water in these polymers.
Experimental Section Preparation of the Polymers. Cylindrical samples of the copolymers were prepared in Teflon molds. The cylindrical geometry was chosen because the samples then fit neatly into the NMR resonator. The monomers were weighed into a 25 mL Pyrex glass flask in the required mole ratios, and benzoyl peroxide, BPO, was added to yield a 0.05 M solution. The mixture was shaken until the BPO had dissolved, and then it was poured into the mold which had an internal diameter of ≈5 mm and a length of ≈20 mm. The cylindrical mold was closed with a cap containing a small hole, to allow excess mixture to drain out of the cylinder. The polymerization was forced to complete conversion of monomer to polymer in a vacuum oven using the following temperature/time protocol: the samples were held initially at 323 K for 20 h and then at 353 K for 2 h. This polymerization protocol leads to the formation of a polymer cylinder without the generation of excessive heat, which could result in the formation of bubbles or the loss of optical clarity. After polymerization, the polymer cylinders were removed from the molds, the absence of monomer was confirmed by FT-NIR analysis, and the ends were ground to a smooth, flat finish. The cylindrical samples were equilibrated in distilled water at 310 K until they had reached their equilibrium mass uptake of water. Then, just prior to the NMR measurements, they were removed from the water, the surface was dried with a paper tissue, and they were placed in the NMR resonator. 1 H T2 Measurements. The spin-spin relaxation time, T2, is most commonly determined using a spin-echo experiment.15 The Carr-Purcell-Meiboom-Gill,16,17 CPMG, pulse sequence was used to determine these relaxation times for the polymers. Experiments on the fully hydrated polymers were performed on a modified Bruker CXP-200 spectrometer operating at a magnetic field of 4.7 T. The maximum point of every second echo was accumulated using a 90° pulse of 5 µs and a 180° pulse of 10 µs. The delay between the 180° pulses, τ, was 40 µs. The amplitude of every second echo
States of Water in Hydrogels
was measured, a total of 512 points were collected, and 64 scans were coadded using a repetition time, TR, of 3 s. 1 H T2 of PHEMA at Various Hydration Levels. Proton spectra were acquired on a Bruker MSL300 spectrometer using a 90° pulse of 5.75 µs and a recycle delay of 1 s. A total of 4096 data points were collected over a spectral width of 200 kHz and 64 FIDs were coadded. 1H T decays were acquired using the CPMG pulse 2 sequence, using a 90° pulse of 5.75 µs, a 180° pulse of 10.6 µs, and a τ value of 40 µs. A total of 512 points were collected, and the signal-to-noise ratio was enhanced by coadding 128 scans, using a TR of 3 s. 1 H T2 of PTHFMA. The decay of the transverse magnetization of a transparent sample of PTHFMA containing 1 wt % of water was analyzed by collecting a series of Hahn echoes with increasing τ values using a Bruker MSL300 spectrometer operating at a magnetic field of 7 T. A 90° pulse of 3.1 µs and a 180° pulse of 6.2 µs were used, and the echo time, TE, was increased from 10 µs to 6.4 ms and incremented by 100 µs. Each FID contained a total of 2048 data points over a spectral width of 200 kHz, and 64 scans were coadded to increase the signal-to-noise ratio, using a TR of 2 s. 2H T of T20H80. 2H T decays were acquired on a Bruker 2 2 MSL300 spectrometer using a 90° pulse of 4.3 µs and a 180° pulse of 8.6 µs and a recycle delay, TR, of 1 s. A series of Hahn echoes were collected with increasing values of TE from 2 µs to 1 ms in increments of 16 µs. A total of 2048 data points were collected for each FID and 32 FIDs were coadded to increase the signal-to-noise ratio. The solid echo pulse sequence was implemented to detect the short time response of D2O, using a range of TEs from 10 to 74 µs with increments of 2 µs. A total of 1024 data points were collected for each FID, and 128 FIDs were coadded, using a TR of 2 s. Results and Discussion NMR Relaxation and Surface Interactions. The multiexponential characteristic of proton spin-spin and spinlattice relaxation has been interpreted as a consequence of the morphology of the system under investigation. In the study of porous materials, measurements of T1 and T2 are increasingly used to probe surface interactions and can be analyzed to obtain the surface area to pore volume ratio. Zimmerman and Brittin18 provided the theoretical basis to incorporate the effects of diffusion during the relaxationtime measurements and account for diffusion simultaneously with relaxation. A system (e.g., a spin polarization) existing in any one of N discrete phases with probability Pi (where i ) 1, 2, ..., N), can undergo transitions between the phases, although the overall process is considered to be “stationary” since each Pi is time independent. When the system is in phase i, the magnetization recovery, M(t), is characterized by a relaxation time, Ti (either T1 or T2). The average value of M(t), M(t), can be solved given the value of M(0). Two special cases can be considered: First, in the slow exchange limit where transfer between the phases is slow compared to the rate of relaxation, the
Biomacromolecules, Vol. 3, No. 5, 2002 993
system exhibits a multiexponential characteristic given by N
M(t) ) M(0)
∑i Pi exp(-1/Ti)
(1)
Second, in the rapid exchange limit, often referred to as the two-fraction fast-exchange model, the system undergoes rapid transitions between the phases compared with the relaxation times, Ti, and the observed signal is given by N
M(t) ) M(0) exp[-(
∑i Pi/Ti)t]
(2)
The two-fraction fast-exchange model is frequently used to interpret NMR relaxation results in biological samples19-21 where the two “phases” are bound water and free water and the probabilities, Pi, are proportional to the volumes occupied by the bound and free water. The mechanism for the interphase transitions is diffusion. Brownstein and Tarr22 extended the “two-fraction fastexchange” model to treat a continuous system and, hence, provided a more general mathematical foundation for processes governed by a combination of diffusion and relaxation. Solutions to the integral equations were given for planar, cylindrical, and spherical geometries. On the basis of their mathematical formulation and the multiexponential nature of the spin-lattice relaxation times of water in biological cells, Brownstein and Tarr23 were able to obtain estimates of both the shape and size of the cells. The “twophase fast-exchange” model has been used to relate T1 and the pore size (the pore volume to surface area ratio) of porous materials.24 In this analysis, the pore consists of a region bounded by a layer of thickness, λ, at the surface of the pore. Woessner25 reported that for the hectorite-water system, λ is of the order of one to two monolayers. If the pore is large compared to λ, then the proportion of the volume in the surface-affected phase, ps, is given by ps ) λ(s/V)
(3)
where s and V are the surface area and volume of the region, respectively. Therefore, the observed relaxation time within the pore is described by eq 4, given that in the other phase, pb ) 1 - ps, bulk relaxation occurs ps pb 1 ) + T1 T1surface Tb
(4)
where T1surface is the relaxation time that characterizes the surface-affected layer and T1b denotes the relaxation time experienced by molecules within the pore and is characteristic of “free” or “bulk” water. For a pore that is cylindrical in shape, the observed relaxation time is given by
( )
2λ 1 1 1 + ) T1s r T1(r) T1b
(5)
where r is the radius of the cylinder. An analogous expression to eq 3 exists for T2, thereby allowing pore-size determinations from an examination of
994
Biomacromolecules, Vol. 3, No. 5, 2002
Ghi et al.
Table 1. The Value and Fraction of Each Component of T2 (s ) short, i ) intermediate, l ) long) Based on a Triple Exponential Fit of the Experimental Data for the Copolymers Hydrated to Equilibrium at 310 K ID
mole fraction of HEMA
EWC/%
T2s/ms
T2i/ms
T2l/ms
% of T2s
% of T2i
% of T2l
PHEMA T10H90 T20H80 T30H70 T40H60 T50H50 T60H40 T70H30 T80H20 T90H10 PTHFMA
1.0000 0.8944 0.7827 0.6976 0.5956 0.5285 0.4253 0.3345 0.2518 0.0616 0.0000
51.89 38.93 29.26 23.47 18.06 16.04 12.35 9.72 7.66 3.87 2.77
110 100 90 90 80 80 100 130 150 120 100
6 3 2 2 2 2 2 3 4 3 4
44 47 46 74 59 74 43 79 61 175 153
35 35 40 32 38 39 38 38 47 46 55
63 64 58 65 58 56 57 50 45 21 18
1 2 2 3 3 5 5 12 7 34 27
Figure 1. 1H T2 decay for hydrated PHEMA showing fits to single, double, and triple exponential functions. Only every second point is shown in the plot.
T2 data as well as from T1 results. However, since the time scale over which T2 measurements are performed is less than that for the corresponding T1 measurements, the T2 data will represent a pore size distribution averaged over a smaller volume. Proton T2 Measurements. Figure 1 shows the T2 decay curves for PHEMA acquired using the CPMG pulse sequence. The decay of the transverse magnetization was found to be triexponential using a non-negative least-squares data fitting program26 based on the Marquet-Levenberg algorithm to fit the experimental data. The fits based on one, two, and three exponentials are displayed in Figure 1 along with the experimental data. It is clear that a single-exponential decay does not describe the experimental data. Discrimination between the fits using a double and triple exponential function were made by comparing the values of the statistical parameter χ2 for each function. The best representation of the decay of the transverse magnetization was triexponential across the whole composition range for copolymers of HEMA and THFMA, as well as for PTHFMA. Table 1 lists the values of T2 and the relative fraction of each component for the polymers studied, and Figure 2 shows a graphical representation of the T2 data across the composition range. The value of (1) the short component of T2 (T2s) for the polymers ranges from 80 to 160 µs; (2) the intermediate component of T2 (T2i) from 1 to
Figure 2. Spin-spin relaxation times for PHEMA, PTHFMA, and poly(HEMA-co-THFMA).
6 ms, and (3) the long component of T2 (T2l) from 40 to 180 ms. The three components of T2 suggest that water molecules are residing in different environments that can be related to the morphology of the samples as follows: A. The Short Component of T2. The relatively small value of T2 in this range suggests that the water molecules contributing to this component have short diffusive path lengths. This can be correlated to water that is strongly interacting with the polymer chains, the most likely interactions occurring between the water molecules and the polar groups on the chains through hydrogen bonding. The proportion of water molecules relaxing with a time constant of T2s expressed as a weight fraction of the dry polymer for PHEMA is 18 wt %, which is in good agreement with the DSC measurements of the amount of “bound” water in PHEMA, which is 20 wt % as reported by Sung et al.5 B. The Intermediate Component of T2. The relatively longer diffusive path length of this type of water suggests that the water molecules are residing in porous regions within the polymers. A porous structure containing “bulk” water has been observed in PHEMA where the pore sizes range from 0.5 to 5 nm depending on the cross-linking density.27 On the basis of the “two-phase fast-exchange” model, the pore sizes in which the water molecules are residing may be calculated using eq 5. Assuming that the surface relaxation time, T2surface, is around 100 µs28 and the spin-spin relaxation time of water, T2b, at 307 K is 1.0 s, and if the thickness of
Biomacromolecules, Vol. 3, No. 5, 2002 995
States of Water in Hydrogels
Table 2. The Value and Fraction of Each Component of T2 (s ) short, i ) intermediate, l ) long) Based on a Triple Exponential Fit of the Experimental Data for PHEMA Hydrated to Various Water Contents at 310 K sorption time1/2/min1/2
water content/%
T2s/µs
T2i/ms
T2l/ms
% of T2s
% of T2i
% of T2l
0 5.4772 7.7460 12.2474 17.3205 22.0454 37.9473 53.8516 64.6529
0.0 7.65 9.91 15.40 19.09 26.07 39.62 45.26 46.10
80 100 100 60 100 90 100 100
0.5 8 10 6 1.2 2.1 2.7 3.2
0.9 1.5 1.9 1.7 2.5 4.9 8.9 12
28 36 29 32 27 27 25 21
29 47 46 24 48 68 72 72
43 16 24 44 25 5 3 7
the surface-affected layer, λ, is 4 nm,25 the pore size corresponding to the observed T2i lies in the range of 80480 nm. The variation in T2i (Figure 2) may also be interpreted as a change in the value of T2surface across the composition range. C. The Long Component of T2. The long component of T2, T2l, corresponds to water that is highly mobile and exists in large defects within the matrixes, such as larger pores or remnants of cracks.29 Applying the above analysis to determine these pore sizes yields values lying between 3 and 18 µm. The water sorption curve for PTHFMA30 shows an initial sorption curve characterized by Fickian diffusion while at longer times, a pronounced increase in the sorption of water was observed. The time at which the sorption rate increased significantly corresponds to the onset of opacity in the sample arising from the formation of “cracks”.29 To determine the validity of the assignment of T2l to water molecules residing in cracks, the decay in the transverse magnetization of water in a transparent sample of PTHFMA (before the development of visible “cracks”) was analyzed. A T2 decay obtained using the progression method based on the Hahn spin-echo sequence revealed the presence of only the intermediate T2 component (1.36 ms). The experimental conditions precluded the observation of a short component of T2. Consequently, in the absence of a network of cracks in the structure of PTHFMA, a long T2 component was not obtained, supporting the assignment of the long component of T2 to water residing in large pores or “cracks”. Figure 2 shows that the value of T2l increases significantly with increasing THFMA content in the copolymer: from 47 to 175 ms for mole fraction THFMA ) 0.11 and 0.94, respectively. While Figure 3 shows a trend in the fraction of each type of water across the composition range, Figure 4 clearly shows that beyond a THFMA content, f THFMA > 0.8, the morphology of the samples changed from a porous structure to a network of large pores. The amount of water residing in small pores decreases with a concomitant increase in the amount of “bound” water beyond this critical THFMA content. 1H T as a Function of Hydration State of PHEMA. A 2 dry sample of PHEMA was immersed in distilled water at 310 K and removed from the water at various times to record the proton spectrum and to accumulate a T2 decay using the CPMG sequence. The water content at these times was also calculated by measuring the increase in mass of the polymer. The decay of the transverse magnetization was found to be
Figure 3. The amount of water, expressed as a weight fraction of the dry polymer, for the short, intermediate, and long T2 components.
Figure 4. The fraction of water contributing to each of the T2 components as a function of copolymer composition.
triexponential at each stage of the diffusion. The results from the fits to the experimental data are tabulated in Table 2. The amount of water contributing to each of the T2 components, expressed as a mass ratio normalized to the dry mass of the sample, may be calculated using the value of the water content.30 The value of T2s remained constant at ca. 100 ms throughout the diffusion process, while the values for T2i and T2l gradually increased with increasing water content (Figures 5 and 6). The sharpest increase in T2i and T2l occurred at a water content corresponding to a value of Mt/
996
Biomacromolecules, Vol. 3, No. 5, 2002
Ghi et al. Table 3. Values of Each Component of the Deuterium T2 Decay and the Amount of D2O Contributing to Each Component for Poly(HEMA-co-THFMA), fHEMA ) 0.8, Hydrated in D2O at 310 K short intermediate long
Figure 5. Variation in 1H T2 for the three components for PHEMA with increasing sorption time.
Figure 6. The amount of water contributing to the three components for PHEMA with increasing sorption time.
M∞ ≈ 0.5. This water sorption corresponds approximately to the complete disappearance of the glassy core in the polymer cylinders.30 During the early stages of diffusion, the polymer contains a glassy core that is surrounded by a swollen region as a result of the incorporation of water. The forces of expansion and contraction exerted by these two layers are responsible for the development of stress and, hence, the formation of defects such as cracks in the sample at the interface between the rubbery and glassy regions.29 Consequently, at the early stages of the diffusion process, a contribution from water molecules with a long diffusive path length, T2l, was also observed, along with T2s and T2i. The increase in the values of T2i beyond a water content corresponding to a value of Mt/M∞ ≈ 0.5 may be interpreted as an increase in the mobility of the water molecules resulting from the aggregation of the molecules in the porous network. This observation supports the postulate that the porous structure in PHEMA is a consequence of the imbibed water occupying the free volume and expanding the polymer network.31 The increase in the values of T2l beyond a water content corresponding to a value of Mt/M∞ ≈ 0.5 suggests that the
T2
water content (wt %)
50.0 µs 315 µs 1.60 ms
4.4 2.0 22.9
larger amount of water molecules serves to increase the radii of the cracks and, hence, the diffusive path length of the water molecules. The increased values of T2l may also arise from the formation of fewer and larger cracks as the strain on the glassy core increases as the radius of the swollen rubbery region increases.32 These two effects are consistent with the occurrence of a “healing” process behind the diffusion front at long times, since the proportion of T2l remains unchanged from the early stages of sorption. Deuterium T2 of T20H80. A sample of T20H80 was hydrated in fully deuterated water and subsequently dried in a vacuum oven at 353 K. The sorption/drying process was repeated five times. Deuterium T2 decays of the fully-D2Osaturated sample were examined for the presence of three types of structures within the network. The decay of the transverse magnetization of D2O was investigated by collecting a series of Hahn echoes at increasing echo times, TE. Three components of T2 were obtainable and are tabulated in Table 3 along with the fraction of D2O contributing to each of these components expressed as a weight fraction normalized to the dry mass of the polymer. The greatly reduced values of the deuterium T2 compared to the proton T2 is a consequence of the greater quadrupolar coupling constant of 2H compared to the dipolar coupling among the 1H nuclei. Additionally, the five cycles of absorption and desorption of D2O imposed on the sample may account for the differences observed in the fractions of each of the T2 components. While the 1H T2s was ascribed to water that is strongly interacting with the polymeric chains, it is equally plausible that this component may arise from the greatly increased mobility of the polymeric chains in the hydrated state of the polymers. However, the observation of the short component of the 2H T2 strongly supports the assignment of the short 1H T to “bound” water, since the hydroxyl proton is rapidly 2 exchanging with the imbibed water and hence is indistinguishable on the time scale of the experiment. Finally, Barbieri et al.33 have examined the relaxation behavior of water in three copolymer hydrogels of HEMA and dihydroxypropyl methacrylate, DHPMA. They were able to explain their results in terms of a single-exponential decay with a relaxation time equivalent to the intermediate component T2i reported herein. However, the pulse spacing used by these workers in their relaxation time measurements was 1 ms, which means that the short component T2s reported herein, even if present, would not have been observed. In addition, the long component, T2l, found for the HEMA/ THFMA hydrogels may not be easily delineated in the HEMA/DHPMA gels, because the percentage contribution from this relaxation component was small for PHEMA and
States of Water in Hydrogels
found to decrease with increasing hydroxyl group content of the HEMA/THFMA gels. Conclusions The decay in the transverse magnetization of water in fully hydrated samples of PHEMA, PTHFMA, and copolymers of HEMA and THFMA was described by a multiexponential function. The short component of T2, T2s, was interpreted as arising from water molecules that were strongly interacting with the polymer chains. While T2s may also be construed as a contribution from the plasticized polymer segments, the observation of an equivalent rapidly relaxing component in the deuterium T2 decay contradicts this possibility. The amount of water contributing to T2s, for PHEMA, expressed as a weight fraction of the dry mass of the polymer (18%), agrees well with the value (20%) reported by Sung et al.5 for “bound” water. The intermediate component of T2, T2i, was assigned to water residing in the porous structure of the samples. The value of T2i was observed to increase with increasing water content in a sample of PHEMA, supporting the model of a porous structure within PHEMA created by the free volume of the polymer. The increased amount of water in the PHEMA network aggregates in the free volume and effectively expands the polymer network to create a greater diffusive path length, hence increasing the value of T2i. The long component of T2, T2l, was shown to arise from water residing in cracks in the polymer network. Samples of PTHFMA and T90H10 became opaque when immersed in water at long times and, hence, clearly contained a network of cracks, and they showed a significantly increased value of T2l compared to the other samples. In a transparent sample of PTHFMA at the early stages of the diffusion process and prior to the formation of the cracks, the decay in the transverse magnetization did not show a slowly relaxing component. Acknowledgment. The authors wish to acknowledge financial support from the Australian Research Council for this work. References and Notes (1) Bruck, S. D. J. Biomed. Mater. Res. 1973, 7, 387-404. (2) Jhon, M. S.; Andrade, J. D. J. Biomed. Mater. Res. 1973, 7, 509522.
Biomacromolecules, Vol. 3, No. 5, 2002 997 (3) Andrade, J. D.; Lee, H. B.; Jhon, M. S.; Kim, S. W.; Hibbs, J. B. J. Trans. Am. Soc. Artif. Intern. Organs 1973, 14, 1-7. (4) Sprincl, L.; Vacik, J.; Kopecek, J. J. Biomed. Mater. Res. 1973, 7, 123-136. (5) Sung, Y. K.; Gregonis, D. E.; Jhon, M. S.; Andrade, J. D. J. Appl. Polym. Sci. 1981, 26, 3719-3728. (6) Lee, H. B.; Andrade, J. D.; Jhon, M. S. Polym. Prepr. 1974, 15, 706-711. (7) Smyth, G.; Quinn, F. X.; McBrierty, V. J. Macromolecules 1988, 21, 3196-3204. (8) Coyle, F. M.; Martin, S. J.; McBrierty, V. J. J. Mol. Liquids 1996, 69, 95-116. (9) Roorda, W. E. J. Biomater. Sci. Polym. Ed. 1994, 5 (5), 383-395. (10) Roorda, W. E.; de Bleyser, J.; Juninger, H. E.; Leyte, J. C. Biomaterials 1990, 11, 17-23. (11) Lee, H. B.; Jhon, M. S.; Andrade, J. D. J. Colloid Interface Sci. 1975, 51 (2), 225-231. (12) Quinn, F. X.; McBrierty, V. J.; Wilson, A. C.; Friends, G. D. Macromolecules 1990, 23, 4576-4581. (13) Bouwstra, J. A.; van Miltenburg, J. C.; Roorda, W. E.; Juninger, H. E. Polym. Bull. 1987, 18, 337-341. (14) Pedley, D. G.; Tighe, B. J. Br. Polym. J. 1979, 11, 130-136. (15) Hahn, E. L. Phys. ReV. 1950, 80, 590-594. (16) Carr, H. Y.; Purcell, E. M. Phys. ReV. 1954, 94, 630-638. (17) Meiboom, S.; Gill, D. ReV. Sci. Instrum. 1958, 29, 688-691. (18) Zimmerman, J. R.; Brittin, W. E. J. Phys. Chem. 1957, 61, 13281333. (19) Hansen, E. W.; Schmidt, R.; Stocker, M.; Akporiaye, D. Microporous Mater. 1995, 5, 143-150. (20) Latour, L. L.; Kleinberg, R. L.; Mitra, P. P.; Sotak, C. H. J. Magn. Reson., Ser. A 1995, 112, 83-91. (21) Davis, P. J.; Gallegos, D. P.; Smith, D. M. Powder Technol. 1987, 53, 39-47. (22) Brownstein, K. R.; Tarr, C. E. J. Magn. Reson. 1977, 26, 17-24. (23) Brownstein, K. R.; Tarr, C. E. Phys. ReV. 1979, A19 (6), 24462453. (24) Wisniewski, S.; Kim, S. W. J. Membr. Sci. 1980, 6, 299. (25) Woessner, D. E. J. Magn. Reson. 1980, 39, 297-308. (26) Lawson, C. L.; Hanson, R. J. SolVing Least Squares Problems; Prentice Hall: Englewood Cliffs, NJ, 1974. (27) Reinhart, C. T.; Korsmeyer, R. W.; Peppas, N. A. Int. J. Pharm. Technol. 1981, 2 (2), 9. (28) Araujo, C. D.; MacKay, A. L.; Whittal, K. P.; Hailey, J. R. T. J. Magn. Reson., Ser. B 1993, 101, 248-261. (29) Ghi, P. Y.; Hill, D. J. T.; Whittaker, A. K. Biomacromolecules 2001, 2, 504-510. (30) Ghi, P. Y.; Hill, D. J. T.; Whittaker, A. K. J. Polym. Sci. 2000, 38, 1939-1946. (31) Yasuda, H., Lamaze, C. E.; Ikenberry, L. D. Die Makrom. Chem. 1968, 118, 19-35. (32) Alfrey, T. J.; Gurnee, E. F.; Lloyd, W. G. J. Polym. Sci.: Part C 1966, 12, 249-61. (33) Barbieri, R.; Quaglia, M.; Delfini, M.; Brosio E. Polymer 1998, 39, 1059-1066.
BM0200332
