On thermoreversible gels: Their classification, relation to phase transitions and vitrification, their morphology and properties
Authors: Callister S. , Keller A. , Hikmet R. M. 
Published: March 2011

Consequences for classification, structure and properties are assessed in the simplest case of an uncrystallizable chemically unspecific polymer, atactic polystyrene (a‐PS), for these studies, in closely monodisperse form. The mechanism first proposed by Berghmans and collaborators, namely liquid‐liquid phase separation as intercepted by glass transition was reaffirmed, followed up by the establishment of the character of the connectivity required for the still disperse phase‐segregated and vitrified system to become a gel. These connectivity types, leading to three distinctly different gel species were identified according to whether connectivity is through (i) individual solvated molecular chains, (ii) continuous glassy phase, (iii) adhesive contact of the glassy phase. (i) corresponds to the familiar gel consistency of swollen rubber, (ii) to porous glass or foam (closed or open), or to glassy network, (iii) to the colloidal gel of loosely packed spheres forming ramifying aggregates. The above generic definition widens the convential concept of a gel creating a unifying umbrella with predictive potential for material design.

The existence of the above three connectivity types was confirmed morphologically. The origin of their existence and their delineation could be attributed to the combined concentration and molecular weight dependence of the solution which was to undergo phase separation. In particular, for the higher molecular weights (M), type (i) arises in the most dilute solutions, with overlap conditions following the c* criterion, but for a somewhat larger radius of gyration than usual for c*(c* = critical concentration).

For the highest M‐s, the limiting solution concentration increasingly departs from the above: the corresponding gel type now becomes as for type (iii).

For the highest M‐s the change‐over from gel type (i) to (ii) occurs at highly asymmetric phase ratios, corresponding to very small amounts of concentrated phase giving rise to interpenetrating networks (IPN). It was shown by combination of light scattering and morphological tests that this corresponds to passing from a nucleated to spinodal phase segregation mechanism, revealing (by morphological means) a very sharply defined boundary between the two.

The different gel types had widely differing consistencies: (i) rubbery, (ii) glassy, fragile for IPN, robust for foams and porous glass, (iii) paste like. (i) and (ii) were characterised in terms of G'(storeage) modulus extrapolated to zero strains and frequency, confirming the consistency tests, with a dramatic transition between the two.

Beyond the issue of gelation itself that of treating two‐fluid separation, as interrupted by glass transition, in terms of equilibrium thermodynamics has been raised. For this a conceptual framework has been laid down together with some preliminary experiments on delayed, incomplete phase separation in the glass. We believe the above to be of wider relevance to the glassy state and phase studies in general.