POLYPROPYLENE CRYSTALLIZATION AS A PHYSICAL GELATION PROCESS

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 degr ees of undercooling, Delta T = 10-26 K, below their nominal melting te mperature. The physical gel point is manifested by slow power law dyna mics, which expresses itself in a shear relaxation modulus G(t) = St(- n) at long times, lambda(0) < t < lambda(pg), where S is the gel stiff ness, n is the relaxation exponent, lambda(0) is the crossover to shor t time dynamics (entanglements, glass modes), and lambda(pg) is the lo ngest relaxation time, which can be considered to be infinite for our experiments due to the long lifetime of the physical bonds. The time t o reach the gel point (gel time t(c)) decreases exponentially with Del ta T, and the critical gel becomes stiffer (smaller n, larger S) with increasing BT. The absolute critical crystallinity at the gel point, X -c, was found to be only about 2% or less. This value was determined f rom published DSC data which, however, needed to be extrapolated to t( c), as measured by mechanical spectroscopy. This very low crystallinit y suggests that only a few junctions are necessary to form a sample sp anning network. The network in this case is ''loosely'' connected, and the critical gel is soft.