Kinetics of polymer crystallization under processing conditions: transformation of dormant nuclei by the action of flow

In previous papers [Janeschitz-Kriegl H, Ratajski E, Stadlbauer M. Flow as an effective promotor of nucleation in polymer melts: a quantitative evaluation. Rheol Acta 42 (2003) 355–364; Stadlbauer M, Janeschitz-Kriegl H, Eder G, Ratajski E. New extensional rheometer for creep flow at high tensile stress. Part II. Flow induced nucleation for the crystallization of iPP, J Rheol 48 (2004) 631–639. [1,2]] two types of experiments were carried out: (a) rapid quenches of quiescent melts of i-PP from their state of equilibrium to a series of rather low temperatures and (b) short term shearing or extension of melts at only mild degrees of undercooling. Strong undercooling as well as high mechanical loads cause similar tremendous increases in the number densities of nuclei by many decades. Unexpectedly, an extremely non-linear dependence of the said number densities on the loading times has been found. In the present paper an explanation for this non-linear relation is tried. The assumption is made that in a quiescent melt there is a huge reservoir of badly organized aggregates (local alignments) of chain molecules, which as such can become effective nuclei only at rather low temperatures. These aggregates are assumed to grow by the action of flow after being oriented in the flow direction. In this way a large amount of low quality dormant nuclei can be transformed into nuclei of a better quality, which are active at higher temperatures. Starting at individual aggregates this growth can lead to thread-like precursors initiating shish-kebab structures. Instead of the flow time the specific mechanical work has been found to be a useful universal parameter. Some measurements of the optical retardation are quoted, which strongly support the basic assumption.