Studies on Gelation and Slow Dynamics on Connectivity-Correlated Polymeric Systems

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Polymer gels can be regarded as a connectivity-correlated polymeric system. In a gel, there exist several types of inhomogeneities, which play significant roles in the characterization of gels and gelation thresholds. However, differences among inhomogeneities and their physical implication have not been extensively discussed in the literature so far. In order to elucidate these problems, we carried out a series of investigations on the sol-gel transition of various types of gelling systems. Both dynamic light scattering and small-angle neutron scattering techniques have been employed in order to elucidate the inhomogeneities described above. Particularly, dynamic light scattering has been employed so as to obtain important information, such as dynamic correlation and connectivity correlation of polymer chains. Poly(N-isopropylacrylamide) (NIPA) gels cross-linked with N,N'-methylene-bisacryamide (BIS) was frequently referred to as a chemical-irreversible gelling system with the abbreviations given above. It should be noted here that NIPA gels were employed not only due to their temperature sensitivity but also to their chemical stability against strong bases and acids compared to its sister gel, i.e., polyacrylamide gels. It will be shown that the structure and properties of polymer gels are strongly dependent not only on the observation condition but on the preparation condition. Gelatin and poly(vinyl alcohol)-ion complex systems was introduced as examples of physically gelling systems, undergoing thermoreversible sol-gel transition as well as reentrant sol-gel transition. An irreversible gelation process of β-lactoglobulin was also characterized by dynamic light scattering.