Gap Dependent Percolation of Spherulites during Crystallization: Rheology, Microscopy and Simulation

We employ simultaneous mechanical rheology and optical microscopy, with augmentation by deterministic reconstruction and simple simulations to develop a geometrical model of spherulitic percolation during polymer crystallization. We observe nucleation of surface and bulk spherulites of isotactic polypropylene which are initially isolated and then impinge on each other to form clusters that eventually span the gap, and correlate this with rheology. There is a strong gap dependence to our observations, which is explained by two distinct mechanisms that both work to enhance kinetics at lower gaps. First, the time required for a cluster to span the gap decreases with decreasing gap width, attributable to finite size effects in percolation theory. Second surface nucleation enhances the overall crystallization rate in a fashion which increases in relative importance as the gap width decreases. The modulus-crystallinity relationship can be described through general effective medium theory which indicates a transformation from percolating behavior at large gap towards a linear behavior at small. We describe our results in terms of dimensionless parameters that indicate when gap dependent effects can be anticipated.