| Abstract
| - Early stages of crystallization of polymers may be viewed as physical gelation. This isshown with four commercial isotactic polypropylenes, which have been studied by dynamic mechanicalexperiments at low degrees of undercooling, ΔT = 10−26 K, below their nominal melting temperature.The physical gel point is manifested by slow power law dynamics, which expresses itself in a shearrelaxation modulus G(t) = St-n at long times, λ0< t < λpg, where S is the gel stiffness, n is the relaxationexponent, λ0 is the crossover to short time dynamics (entanglements, glass modes), and λpg is the longestrelaxation time, which can be considered to be infinite for our experiments due to the long lifetime of thephysical bonds. The time to reach the gel point (gel time tc) decreases exponentially with ΔT, and thecritical gel becomes stiffer (smaller n, larger S) with increasing ΔT. The absolute critical crystallinity atthe gel point, Xc, was found to be only about 2% or less. This value was determined from published DSCdata which, however, needed to be extrapolated to tc, as measured by mechanical spectroscopy. Thisvery low crystallinity suggests that only a few junctions are necessary to form a sample spanning network.The network in this case is “loosely” connected, and the critical gel is soft.
|
