Session: 03-11-03: NASA Thermoplastics Development for Exploration Applications (TDEA)

Paper Number: 134455

134455 - Multiscale Modeling of Crystallization Kinetics in Thermoplastic Polymers 

Many thermoplastic polymers used in aerospace, automotive and other applications are semi-crystalline in nature.  The relevant features of the microscale span multiple length scales from molecular to continuum.  These include the amorphous and crystalline phases of the polymer which compose lamellae stacks that grow to form semi-crystalline spherulites.  The microstructure is a direct outcome of the processing parameters such as temperature, time, and cooling rate. In addition, crystallization results in effective and local thermomechanical properties that evolve during processing which can significantly influence the residual stresses in the material.   As such, the thermomechanical properties and fracture behavior of the bulk polymer depends on both the overall crystallinity of the material and the morphology of microstructure.  Advanced manufacturing methods for thermoplastic composites, including additive manufacturing, welding, and stamp forming, may result in processing conditions that yield variability in the semi-crystalline microstructure when compared to components manufactured using traditional methods.  Moreover, composite parts with complex geometries and non-uniform thickness may also result in localized variability in the processing conditions. Therefore, there is need for a computational model that can simulate the crystallization kinetics at a local and global scale and predict the thermomechanical properties as a function of both the bulk crystallinity and semi-crystalline morphology.  Here, processing of thermoplastic polymers is simulated by combining a crystallization kinetics model with spherulite nucleation and radial growth models. Resulting in predictions for the semi-crystalline microstructure in 3D space as a function of the processing history. At key time points throughout the processing history the elastic stiffness tensor, coefficients of thermal expansion and thermal conductivities of the polymer are predicted using a previously validated model with the NASA Multiscale Analysis Tool (NASMAT).  The NASMAT model utilizes semi-analytical micromechanics to represent the semi-crystalline structure at each of the relevant length scales in the material (lamellae stacks, spherulite, bulk).  These scales are directly integrated through localization and homogenization for a given time increment within multiscale recursive micromechanics framework.  The inputs to the thermomechanical model are the properties of the amorphous and crystalline phases of the polymers previously obtained from molecular dynamics simulations.  Results are compared to experimental data from the literature when available.

Presenting Author: Evan Pineda National Aeronautics and Space Administration

Presenting Author Biography: Dr. Pineda is in the Multiscale and Multiphysics Modeling Branch at the NASA Glenn Research Center.