Glass Transition as the Rheological Inverse of Gelation

This is a comparative study in search for common patterns in the relaxation dynamics of amorphous materials in the approach of a liquid-to-solid transition from the liquid side. Observations with two representative materials provide guidance for the study. The first material, a concentrated colloidal suspension, represents the glass transition. The second material is a cross-linking polymer far above its glass transition; it represents gelation. The entire study is founded in Boltzmann's constitutive equation of linear viscoelasticity; the stress is caused by a wide range of relaxation modes where, as argued here, fast modes dominate gelation and slow modes dominate the glass transition. For both classes of amorphous materials, the relaxation time spectrum broadens and adopts powerlaw format, but the powerlaw exponent is positive for the glass transition and negative for gelation, i.e. the relaxation patterns of gelling fluids and glass formers are inverse near the transition. Several examples are shown for each class of materials in order to test the proposed transition behavior for glasses (colloidal and molecular) on the one side and chemical/physical gels on the other. Among several results, this experimental study provides a decisive criterion that distinguishes the glass transition from gelation. It also shows a relation between the zero shear viscosity and the diverging longest relaxation time for both materials.