Water in hydrogels. 1. A study of water in poly(N-vinyl-2-pyrrolidone/methyl methacrylate) copolymer

Abstract

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

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