On the Cooccurrence of Demixing and Thermoreversible Gelation of Polymer Solutions. 3. Overall View

The gelation of sufficiently concentrated solutions of poly(n-butyl methacrylate) (PBMA) in the vicinity of phase separation conditions is attributed to extraordinarily large enthalpies of mixing. These specific thermodynamic conditions render the opening of intersegmental contacts difficult and lead to a pronounced increase of viscometric relaxation times and eventually to gelation. On the basis of the experimentally determined value of -6 kJ for the heat of formation of 1 mol of intersegmental contacts out of 2 mol of solvent/segment contacts and -37 kJ, observed for the heat of gelation per mole of cross-links, one can estimate that a single cross-link may consist of approximately six segments. the characteristics of the thermodynamic mixing functions indicate that the molecular explanation for the cooccurrence of phase separation and thermoreversible gelation of the solutions of PBMA lies in the formation of locally highly ordered and energetically very favorable contacts between the n-butyl side groups of the monomeric units.