MOLEC PROCESSES IN CROSSLINKED POLYMER HYDROGEL VOLUMETRIC EXPANSION CONTRACTION

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We have found that sudden volumetric contractions of hydrogels of 1) poly(N-isopropylacrylan-~ide), 2) poly (N-n-propylacrylamide), 3) poly(N-isopropylmethacrylamide) and 4) poly(N-n-propylmethacrylamide) have occured at 33.7, 23.0, 41.0, and 27.5' C, respectively. For brevity, let these temperatures be called "transition temperatures". Proton NMR of the above polymer networks below the transition temperatures exhibited well-resolved spectra corresponding to all chemical groups (condition A). Just above the respective transition temperatures, these peaks began to broaden with increasing temperature (condition B) and eventually coalesced to one broad peak (condition Q in higher temperature ranges (10 to 20' C above the respective transition temperatures). Intensity of these broad peaks gradually decreased with lowering temperature. At the same time, broadened peaks corresponding to all chemical groups gradually appeared (condition D). This trend continued nearly 10' C beyond the transition temperatures. These thermal processes represented constitute a cyclic loop that is unique to each system. It consists of 2 reversible (conditions A to B and conditions C to D) and 2 irreversible (B to C and D to A) processes. The gel states corresponding at conditions A and C are thermodynamically stable states and those under conditions B and D seem to be some sort of meta-stable states with rather long life-time. The above finding reveals how the polymer component behaves in the volumetric contraction-expansion of gels with varying temperature at the molecular level. In addition to the polymer component, proton spin-lattice relaxation times (T,) of HOD molecule in gels were also determined in the above thermo-cycling. T, significantly changed with varying temperature, around the transition temperature, in particular. This behavior is compatible with changes in the density of both'polymer and water in gels. In the state corresponding to conditions C and D, the T, of HOD consists of two components with a small frequency separation. One component can be identified as HOD strongly coupled to the polymer. By using the data for density, we can carry out the theoretical analysis of temperature dependence of T, of HOD and thus can characterize dynamics of HOD molecules within gels.