Rheological expression of physical gelation in polymers

Polymeric materials at the liquid–solid transition exhibit unusual simplicity and regularity in their relaxation pattern. This expresses itself in a self-similar relaxation modulus G(t)=St–n at long times λ0 < t < ∞, where λ0 is the characteristic time for the crossover to a different relaxation regime (e.g. crossover to glass transition or entanglement region). Rheological features of liquid–solid transitions are very similar for chemical and physical gelation: (1) broadening of the relaxation time spectrum, (2) divergence of the longest relaxation time (with an upper cut-off for physical gels) and (3) self-similar relaxation patterns. We have borrowed terminology from chemical gelation and applied it to an example of physical gelation: the isothermal crystallization of isotactic polypropylene. The transition through the gel point has been investigated by dynamic mechanical experiments. The influence of temperature and crystallization rate have been studied. The degree of crystallinity (estimated by the Avrami equation) at the gel point was very low (6–15% depending on the crystallization temperature).