RHEOLOGICAL EXPRESSION OF PHYSICAL GELATION IN POLYMERS

Polymeric materials at the liquid-solid transition exhibit unusual sim plicity and regularity in their relaxation pattern. This expresses its elf in a self-similar relaxation modulus G(t) = St(-n) at long times l ambda(0) < t < infinity, where lambda(0) is the characteristic time fo r the crossover to a different relaxation regime (e.g. crossover to gl ass 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 th e longest relaxation time (with an upper cut-off for physical gels) an d (3) self-similar relaxation patterns. We have borrowed terminology f rom chemical gelation and applied it to an example of physical gelatio n: the isothermal crystallization of isotactic polypropylene. The tran sition through the gel point has been investigated by dynamic mechanic al experiments. The influence of temperature and crystallization rate have been studied. The degree of crystallinity (estimated by the Avram i equation) at the gel point was very low (6-15% depending on the crys tallization temperature).