Thermal analysis of water in p (HEMA) hydrogels (original) (raw)
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Journal of Colloid and Interface Science, 1975
Three classes of water may exist in certain hydrogels. We have previously labeled these as X water (bulk water), Z water (bound water) and Y water (intermediate forms we call interracial water). Bulk gel conductivity data for poly (2-hydroxyethyl methacrylate) (PHEMA) were obtained. The activation energy for specific conduction was calculated. A plot of the activation energy versus wt percent of water in the gel clearly indicated three different zones, showing three possible classes of water in the gels. These results were confirmed by thermal expansion measurements. The high water content gels (50%) demonstrated an extremely sharp volume change at 0°C, indicating the presence of normal bulk water. Lower water content gels (20%) showed no anomalous change in thermal expansion, indicating that the water is bound. The medium water content gels exhibited intermediate behavior. A semiquantitafive analysis of the three classes of water is presented. A further verification of these results was obtained by differential scanning calorimetry (DSC) studies. The low water content gel (20%) consists mainly of bound water, which exhibited no phase transitions over the range -15 to 24°C. The high water content gels showed phase transitions near 0°C. The medium water content gels show gradual shifts of the phase transition temperatures near 0°C.
Thermal analysis of poly(2-hydroxyethyl methacrylate) (pHEMA) hydrogels
2003
Poly(2-hydroxyethyl methacrylate) ( pHEMA) hydrogels have been used, or suggested for use, in a wide range of biomedical applications. In many of these applications, the mechanical properties of the gel are important for its proper functioning. These properties are in¯uenced by a number of factors, including water content. In this study the storage and loss shear moduli were measured as a function of frequency for gels with water contents ranging from 22% to 48% at a temperature of 37 C. At low frequencies and high water contents, deformation frequency had little effect. However, at higher frequencies and lower water contents, both moduli increased markedly with increasing frequency. This can be explained by the gels approaching a glass transition. The curves describing the behavior of each gel were combined to form a master curve, using a method analogous to the time± temperature superposition principle. This master curve can be used to predict the shear moduli for gels with a wide range of water contents and loading frequencies. For example, for a gel with a water content of 47.8% (as a percentage of the mass of gel), the curve provides shear moduli values over a frequency range of 10 À 2 ±10 4 Hz.
Properties of Water Bound in Hydrogels
Gels
In this review, the importance of water in hydrogel (HG) properties and structure is analyzed. A variety of methods such as 1 H NMR (nuclear magnetic resonance), DSC (differential scanning calorimetry), XRD (X-ray powder diffraction), dielectric relaxation spectroscopy, thermally stimulated depolarization current, quasi-elastic neutron scattering, rheometry, diffusion, adsorption, infrared spectroscopy are used to study water in HG. The state of HG water is rather non-uniform. According to thermodynamic features of water in HG, some of it is non-freezing and strongly bound, another fraction is freezing and weakly bound, and the third fraction is non-bound, free water freezing at 0 • C. According to structural features of water in HG, it can be divided into two fractions with strongly associated and weakly associated waters. The properties of the water in HG depend also on the amounts and types of solutes, pH, salinity, structural features of HG functionalities.
Journal of Thermal Analysis and Calorimetry, 2015
Hydrogels are polymeric materials used in many pharmaceutical and biomedical applications due to their ability to form the 3-D hydrophilic polymeric networks, which can absorb large amounts of water. In the present work polyethylene glycols (PEG) were introduced into the hydrogel liquid phase in order to improve the mechanical properties of hydrogels composed of 2-hydroxyethylacrylate and 2hydroxyethylmethacrylate (HEA-HEMA) synthesized with different co-monomer compositions and equilibrated in water or in 20% water-PEG 400 Da and 600 Da solutions. The thermoanalytical techniques (differential scanning calorimetry (DSC) and thermogravimetry (TG)) were used to evaluate the amount and properties of free and bound water in HEA-HEMA hydrogels. Internal structure and the mechanical properties of hydrogels were studied using scanning electron microscopy and friability assay. TG 'loss-on-drying' experiments were applied to study the water-retention properties of hydrogels whereas the combination of TG and DSC allowed estimating the total amount of freezable and non-freezing water in hydrogels. The results show that the addition of the viscous co-solvent (PEG) to the liquid medium results in significant improvement of the mechanical properties of HEA-HEMA hydrogels and also slightly retards the water loss from hydrogels. A redistribution of free and bound water in the hydrogels equilibrated in mixed solutions containing 20 vol% of PEGs takes place.
Contributions to the thermodynamics of polymer hydrogel systems
Polymer, 2004
In this work, an extended version of a quasichemical thermodynamic model is presented. The swelling behavior of crosslinked acrylamide polymer gels and N-substituted derivatives, such as N-isopropylacrylamide and N-tert-butylacrylamide has been compared to predictions from such model which takes into account the specific hydrogen bonding interactions encountered in these systems. The calculated volume transition temperature of the poly(N-isopropylacrylamide) gel is 0.8 8C lower than the experimental value and the predicted solvent volume fraction in the collapsed and swollen gel states are about 2% larger than the corresponding experimental data measured at the transition point. Applying the same energy parameters obtained from regressing poly(N-isopropylacrylamide) gel swelling pressure data, the model has also been capable to correctly represent the major features found in the swelling behavior of linear poly(N-tert-butylacrylamide) and poly(N-tertbutylacrylamide) gels, after the model parameters that characterize the molecular structure were changed in accord to each polymer repetitive unit. q
Biomaterials, 2009
Poly (N-vinyl-2-pyrrolidone-co-2-hydroxyethyl methacrylate) (P(VP-co-HEMA)) hydrogel system with a composition of VP:HEMA ¼ 37:13 was studied using molecular dynamics simulations in order to investigate the effect of the water content on the equilibrium structures and the mechanical properties. The degree of randomness of the monomer sequence for the random and the blocky copolymers, were 1.170 and 0.104, respectively, and the degree of polymerization was fixed at 50. The equilibrated density of the hydrogel was found to be larger for the random sequence than for the blocky sequence at low water contents (<40 wt%), but this density difference decreased with increasing water content. The pair correlation function analysis shows that VP is more hydrophilic than HEMA and that the random sequence hydrogel is solvated more than the blocky sequence hydrogel at low water content, which disappears with increasing water content. Correspondingly, the water structure is more disrupted by the random sequence hydrogel at low water content but eventually develops the expected bulk water-like structure with increasing water content. From mechanical deformation simulations, stress-strain analysis showed that the VP is found to relax more efficiently, especially in the blocky sequence, so that the blocky sequence hydrogel shows less stress levels compared to the random sequence hydrogel. As the water content increases, the stress level becomes identical for both sequences. The elastic moduli of the hydrogels calculated from the constant strain energy minimization show the same trend with the stressstrain analysis.
Thermoporometry and impedance analysis to study dynamics of water and polymer present in hydrogel
International Journal of Biological Macromolecules, 2015
Though various conventional methods are available to explore hydrogels, they have drawbacks such as analysis in solid state and failure to give insights into individual components of hydrogel viz. water (dispersion medium) and hydrophilic polymers (dispersed phase). The combined study of porosity and dielectric nature of hydrogel succeeds, in this context, as it investigates both the components individually. In this study, we have taken well-known hydrogel system gelatin-polyvinyl alcohol (PVA) cross linked with genipin. Thermoporometry has been used to investigate the state of water and porosity whereas Alternative Current (AC) impedance analysis has been used to study about nature of polymers through dielectric properties in hydrogel. The influence of physic-chemical properties was examined with SEM and in vitro drug release using catechin. The study revealed that increasing concentration of PVA to gelatin has retained excessive bound water molecules exhibiting high polarity in each polymeric component. Further, it is shown that reduction in pore size and high reactivity with drug molecules have led to lower initial release and increase total amount of release. We conclude that non-conventional methods such as thermoporometry and AC impedance analysis yield more valuable information about hydrogel, which can aid in designing appropriate biomaterial for intended drug release.
Microstructure of poly(vinyl alcohol) hydrogels investigated with differential scanning calorimetry
1993
The physical state of water in poly(viny1 alcohol) (PVA) hydrogels was studied with differential scanning calorimetry to elucidate their microstructure. PVA hydrogels were prepared after three different methods, viz. chemical crosslinking with glutaraldehyde, annealing of dried film and crystallization at low temperature. The water in PVA hydrogels could be classified into three types: free water, intermediate water and bound water. The concentration of bound water is practically not dependent on the total water content of the hydrogels when prepared by low-temperature crystallization or annealing of the PVA film. The gel prepared by low-temperature crystallization contained less bound water than the gel obtained by annealing of a cast film. Furthermore, the concentration of bound water increases as the degree of polymerization (DP) of PVA increases, whereas the reverse tendency was observed for the intermediate water. From these results it was concluded that the low-temperature crystallization method results in a larger free space between the PVA microcrystallites and a larger size of microcrystallites than the annealing method. It is also likely that PVA of higher DP provides a larger amount of free PVA chains which are not involved in microcrystallites.