Modeling the crystallization kinetics of polymers displaying high levels of secondary crystallization

Catherine A Kelly,Mike J Jenkins

doi:10.1038/s41428-021-00581-0

Abstract

The isothermal crystallization of poly(3-hydroxybutyrate-co-3-hydroxyvalerate) was evaluated using a range of models, namely, Avrami, simplified Hillier, Tobin, Malkin, Urbanovici–Segal, Velisaris–Seferis, and Hay. Two methods of model evaluation were used: determination of the parameters through traditional double log plots and curve fitting via nonlinear, multivariable regression. Visual inspection of the cumulative crystallization curves, calculation of the R2 value and standard error of the regression, and evaluation of the returned parameters were used to assess which model best describes the experimental data. The Hay model was found to generate the best fit, closely followed by the Velisaris–Seferis parallel model, suggesting that primary and secondary crystallization occur concurrently. The Avrami, Malkin, and Tobin models were found to perform well when the data is restricted to the region where primary crystallization dominates; however, they could not be used to successfully model the entire crystallization process. This work highlights the importance of selecting the most appropriate model for analyzing kinetics, especially when high levels of lamellar thickening and infilling occur during crystallization.

Highlights

The kinetics of crystallization are an important consideration in polymer processing because they significantly affect the final thermal and mechanical properties of a material
The Avrami, simplified Hillier, Tobin, and Hay parameters were determined through the traditional method of generating double log plots, and SPSS software was used to determine the fit of these parameters to the experimental data
The results highlight that the models are able to sufficiently describe the primary crystallization, there is an inability to accurately model the region dominated by the secondary crystallization process (Xt > 0.6, as determined by the Hay model)

Summary

Introduction

The kinetics of crystallization are an important consideration in polymer processing because they significantly affect the final thermal and mechanical properties of a material. If primary crystallization occurs slowly, when cooling from the melt phase, only a few nuclei are generated that grow into large spherulites, rendering the material brittle. As the rate of crystallization is dependent on the degree of undercooling from the melt, knowledge of the kinetics allows precise control of material morphology during processing. Polymer crystallization consists of two processes: primary and secondary. Primary crystallization is initiated in the amorphous polymer through the formation of nuclei from small regions of chain entanglements.

Methods

Results

Conclusion