SOME CONSIDERATIONS CONCERNING THE TEMPERATURE-DEPENDENCE OF THE BULKCRYSTALLIZATION RATE CONSTANTS OF POLYMERIC MATERIALS

The experimentally estimated 'normalized' rate constants of the bulk c rystallization of poly(L-lactic acid) and natural rubber at different temperatures have been used in order to test different equations descr ibing the temperature dependence of the normalized crystallization rat e constants. Ten of these expressions are variants of the well-known T urnbull-Fisher relationship k = k(0) exp[-U/R(T - T-infinity)] exp[K- g/T(Delta T)f] which differ essentially by the assumed number of fixed parameters [one or two Williams, Landel, and Ferry (WLF) constants i. e., U = C-1 and/or C-2]. Two additionally used equations are expressi ons derived on the basis of the above mentioned general relation, whic h introduce T-g instead of T-infinity = T-g - C-2 as the low temperatu re limit of crystallization and two energies of activation, for nuclea tion and crystal growth, respectively. It is shown that the temperatur e dependence of the normalized bulk crystallization rate constants is more accurately described if three parameters (beside k(0), U, and K- g or the respective activation energies) are assumed to be adjustable and when instead of the widely accepted expression for the low tempera ture limit, T-infinity = T-g - C-2(K) with C-2 = 51.6 or 30 degrees C, respectively, T-g(i.e., C-2 = 0 degrees C) is considered. A qualitati ve explanation for this finding is suggested in terms of the increasin g mobility of the crystallizing segments at temperatures above T-g, ta king into account that generally the glass transition is considered to be a 'freezing in' process. (C) 1997 John Wiley & Sons, Inc.