Session: 03-08-01: Micromechanics and Multiscale Modeling I

Paper Number: 100935

100935 - Multiscale Modeling of Semi-Crystalline Thermoplastics Using a Combination of Micromechanics and Molecular Dynamics 

Thermoplastic materials, including Polyether ether ketone (PEEK) and Polyether ketone ketone (PEKK) are high-performance semi-crystalline polymers ideal for aerospace applications because of their excellent properties, toughness, resistance to aging and manufacturability. To design improved composite matrix materials incorporating thermoplastics, a thorough understanding of the molecular structure, continuum-level microstructure, and the resulting bulk response of the thermoplastics is imperative.

      Computational atomistic simulation techniques can be used to establish structure-property relationships for thermoplastic materials. Specifically, molecular dynamics (MD) simulation techniques are well-suited for predicting bulk-level properties of single-phase polymers based on molecular structure. However, MD simulation techniques alone cannot predict the bulk level properties of semi-crystalline materials because the characteristic length scale of these materials (microns) is orders of magnitude larger than those that can be efficiently simulated with fully atomistic MD models (nanometers). To effectively simulate semi-crystalline polymer materials, a multi-scale approach can be used in which the amorphous and crystalline phases are modeled separately using MD, and micromechanics is subsequently used to obtain the bulk properties using repeating unit cells (RUCs) that represent the spatial arrangement of these phases at larger length scales.

      It has been demonstrated previously that MD modeling coupled hierarchically with the multiscale generalized method of cells (MsGMC) can be used to accurately predict the mechanical properties of semi-crystalline PEEK [1]. A methodology fwas esatblished for well-equilibrated amorphous and crystalline PEEK MD models with the Reactive Force Field (ReaxFF), as well as the procedure for predicting the mechanical response of each phase at the molecular level. This study also presented  methodology for modeling the semi-crystalline structure of PEEK with the MsGMC using the simulated molecular response as input. Following this, the predicted mechanical properties are compared to experiment for model validation. The developed multiscale modeling methodology can be easily extended to other semi-crystalline polymer systems. 

      Recently, the fidelity of the multiscale continuum model for the semi-crystalline spherulite has been improved.  The cubic model of the spherulite at the highest scale has been replaced with a better representation of a sphere, utilizing the high fidelity generalized method of cells (HFGMC) at the highest scale in a multiscale recursive micromechanics (MsRM) framework.  HFGMC is employed at the highest scale to capture the local fields arising from the presence of a spherical inclusion more accurately, while the generalized method of cells (GMC) is retained for all additional subscales. The MsRM framework is implemented within the NASA Multiscale Analysis Tool (NASMAT) software package.

      The focus of the current work is to validate the new MsRM model against experimental data, and other computational results,  for the elastic mechanical properties.  The model will also be used to predict the mechanical properties of PEKK.  Furthermore, the model will be used to predict other bulk properties such as coefficient of thermal expansion (CTE), thermal conductivity, and/or electrical conductivity of PEKK. As with the mechanical properties, the thermal and/or electric properties of the crystalline and amorphous phases of the polymer will be predicted using atomistic modeling.  A generalized HFGMC solution for physics governed by vector-based constitutive laws has been implemented with NASMAT, and outputs from the atomistic modeling will be used as inputs to predict the bulk properties of PEKK.

[1] Pisani, W. A., Radue, M. S., Chinkanjanarot, S., Bednarcyk, B. A., Pineda, E. J., Waters, K., Pandey, R., King, J. A., and Odegard, G. M. (2019). “Multiscale Modelling of PEEK using Reactive Molecular Dynamics Modeling and Micromechanics,” Polymer, Vol. 163, pp. 96-105.

Presenting Author: Evan Pineda National Aeronautics and Space Administration

Presenting Author Biography: I am a Research Aerospace Engineer in the Multiscale and Multiphysics Branch, Materials and Structures Division at the NASA Glenn Research Center in Cleveland, Ohio.

Authors:

Evan Pineda National Aeronautics and Space Administration
Jamal Husseini University of Massachusetts, Lowell
Joshua Kemppainen Michigan Technological University
Gregory Odegard Michigan Technological University