Polypropylene Crystallization as a Physical Gelation Process

Abstract

Early stages of crystallization of polymers may be viewed as physical gelation. This is shown with four commercial isotactic polypropylenes, which have been studied by dynamic mechanical experiments at low degrees of undercooling, ¢T ) 10-26 K, below their nominal melting temperature. The physical gel point is manifested by slow power law dynamics, which expresses itself in a shear relaxation modulus G(t) ) St -n at long times, I0 < t < Ipg, where S is the gel stiffness, n is the relaxation exponent, I0 is the crossover to short time dynamics (entanglements, glass modes), and Ipg is the longest relaxation time, which can be considered to be infinite for our experiments due to the long lifetime of the physical bonds. The time to reach the gel point (gel time tc) decreases exponentially with ¢T, and the critical gel becomes stiffer (smaller n, larger S) with increasing ¢T. The absolute critical crystallinity at the gel point, Xc, was found to be only about 2% or less. This value was determined from published DSC data which, however, needed to be extrapolated to tc, as measured by mechanical spectroscopy. This very 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.