A Fundamental Study on Gelation of Polymer Solutions

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Introduction of chemical or physical crosslinks to a linear polymer in solution often gels the whole solution. Fundamental problems regarding this phenomenon include those related to the mechanisms of gelation and the critical number of crosslinks to be introduced to a polymer solution with givencharacteristics. This project has dealt with these problems with three typesof systems : (I) dihydroxypropylcellulose (DHPC) /borax/aqueous solutions, (II) acylated cellobiose (ACB) /n-alkane solutions, and (III) poly (methyl methacrylate) with pendant anthryl groups (A-PMMA) /tetrahydrofuran solutions.Spectroscopic studies on systems (I) and (II) , which are physically gelling systems, have made clear the mechanisms of gelation of those solutions. In system (I) , for example, the gelation takes place by a 2 : 1 complexation of the pendant diol sites and a borate ion, and the number of these crosslinking sites could be accurately determined by ^<11>B-NMR,showing that the sol-gel diagrams of this system can be scaled into a single master curve in terms of the overlap concentration c^*, that the critical number of crosslinks per chain is a decreasing function of c, and that the classic gelation theory approximately holds when c is sufficiently large. System (II) is a chemically gelling system induced by the photodimerization of the anthryl groups. The number of such dimers could also be accurately determined by UV spectroscopy, which along with light scattering data of the molecular weight and radius of the crosslinked chains has shown that the classic theory holds also in this system at high concentrations. System (II) has been found to be a new type of gelling system in which ACB molecules associate into linear chains, which locally coagulate into liquid crystallites working as crosslinks.