Rheology of crystallizing polymers: The role of spherulitic superstructures, gap height, and nucleation densities

Published

November 1, 2019

Author(s)

Debjani Roy, Debra Audus, Kalman D. Migler

Abstract

A longstanding goal in polymer rheology is to develop a physical picture that relates the growth of mechanical moduli during polymer crystallization to that of structure. Here we utilize simultaneous mechanical rheology and optical microscopy, with augmentation by deterministic reconstruction and stochastic simulations, to study isothermal crystallization in isotactic polypropylene (iPP). We observe the nucleation and growth of surface and bulk spherulites which are initially isolated and then impinge to form clusters and superstructures that eventually span the gap. We find that spherulitic superstructures play a critical role in the rheology, especially in the characteristic sharp upturn in moduli. Both the rheology and the spherulitic superstructures show pronounced gap dependencies, which we explain via finite- size effects in percolation phenomena and via surface-induced nucleation. The modulus- crystallinity relationship can be described through general effective medium theory. It indicates that for thicker gaps, the visco-elastic liquid to solid transition can be described via percolation whereas for our thinnest gap, it is best described by the linear mixing rule. We describe our results in terms of dimensionless nucleation rates and spherulite size, which enables estimation of when gap-dependent superstructure effects can be anticipated.

Citation

Journal of Rheology

Volume

63

Issue

6

Pub Type

Journals

Download Paper

https://doi.org/10.1122/1.5111358

Local Download

Keywords

polymer, rheology, crystallization, percolation

Citation

Roy, D. , Audus, D. and Migler, K. (2019), Rheology of crystallizing polymers: The role of spherulitic superstructures, gap height, and nucleation densities, Journal of Rheology, [online], https://doi.org/10.1122/1.5111358, https://tsapps.nist.gov/publication/get_pdf.cfm?pub_id=927436 (Accessed December 13, 2024)