This paper seeks a clearer understanding of the role of water in hydrated polymers. Pulsed NMR relaxation data for the hydrated copolymer poly(N-vinyl-2-pyrrolidone/methyl methacrylate), P-(NVP/MMA), reveal that a significant part of the water is nonfreezable or bound in the sense that it becomes mobile, much like a glass, at -170 K. Consideration of T2 component intensities allows one to estimate the relative fractions of three distinguishably different types of water in P(NVP/MMA). The bound water can be resolved into a mobile component characterized by a long T2 (type A) and a component of lower mobility that combines with plasticized polymer to form an intermediate T2 (type B). In samples with water content in excess of 276 w t % there is also bulklike water that freezes in the vicinity of 273 K. Parallel DSC measurements predict a lower estimate for the amount of nonfreezable water which correlates rather well with the amount of type A water present. Intercomparison of NMR and DSC data is facilitated by a scheme that envisages five thermal equilibrium states in hydrated P(NVP/MMA). This study highlights the importance of taking the plasticized polymer contribution into account in estimating bound water by the NMR technique. IntroductionThe burgeoning literature on the role of water in natural and synthetic polymers attests to the scientific and commercial importance of these ~ystems.l-~ Substantial progress has been made in unraveling the inherent complexities of water/polymer interactions, with most attention focused upon water in natural polymers, particularly proteins and polypeptides, and in those synthetic polymers such as poly(Zhydroxyethy1 methacrylate) (PHEMA) that

Collated brcad-line NMR and DSC measurements sensitively probe the behavior of water in hydrated poly(hydroxyethy1 methacrylate) (PHEMA). NMR reveals that bound water becomes mobile at -180 K in keeping with observations in many other hydrated polymers; DSC is insensitive to events at these temperatures. In samples with a high water content, a fraction of this mobile water subsequently freezes between 230 and 260 K. The amount of freezable and nonfreezable water in hydrated PHEMA is determined quantitatively. Preliminary cross-relaxation experiments at 253 and 293 K indicate that NMR spin-lattice relaxation rates for water will be overestimated if the effects of cross-relaxation between the polymer and water proton spin systems are neglected. Although PHEMA is less hydrophilic than poly(N-vinyl-2pyrrolidone/methyl methacrylate), studied in part 1, the relative fraction of bound water is significantly higher. Hysteresis effects in hydrated PHEMA are investigated in some detail.

The effect on the adsorbed layer properties of the modification of alpha S1-casein by covalent bonding with an uncharged polysaccharide side chain has been investigated using lattice-based self-consistent field (SCF) theory. Interactions between two hydrophobic planar surfaces coated by a layer of adsorbed modified alpha S1-casein have been studied as a function of pH and ionic strength. While the interactions of the unmodified alpha S1-casein layers become attractive at high ionic strength, it has been shown that the presence of polysaccharide attachment to the alpha S1-casein molecule can confer net repulsive interactions over a wide range of salt concentration. The disordered protein is represented as a linear flexible polyampholyte with a sequence of hydrophobic, polar, and charged units based on the known alpha S1-casein primary structure. The hydrophilic side chain is attached at various fixed positions along the casein backbone. Different lengths and locations of the attached polysaccharide side chain are examined. Interfacial structures and colloidal stability properties of the system are determined, including the surface-surface interaction potential, the extent of protein bridging, and the distribution of protein segments from the surface under different conditions of pH and ionic strength. It has been found that the covalent bonding of short hydrophilic chains may not only enhance but can also worsen the colloidal stabilizing properties of the modified protein, depending on the position of the attachment.

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