Session: 12-07-01: Engineered Materials for Thermal Control

Paper Number: 164951

Tacticity Controlled Thermal Conductivity of Bulk Polymers 

The objective of this work is to determine how tacticity impacts the thermal conductivity of bulk polymers. The tacticity of a polymer chain (i.e., the relative arrangement of the side groups on the backbone) can influence its mechanical, rheological, chemical, and thermal properties. The effect of tacticity on glass transition temperature, self-diffusion coefficient, crystallization, and morphology have been studied. The impact on thermal conductivity, however, remains largely unexplored, despite thermal management being a key factor limiting the application of polymers in electronics packaging, heat exchangers, and thermal interfaces. Previous studies on polymer thermal transport primarily focused on amorphous polymers and processing strategies for increasing thermal conductivity by promoting chain alignment (e.g., straining, drawing, and electrospinning) and composite formation. Efforts to improve thermal conductivity through manipulation of molecular structure remain limited.

Our previous work on single-chain polypropylene and polystyrene revealed a two- to three- fold increase in thermal conductivity for ordered chains, where side groups are arranged on the same side (isotactic) or alternating sides (syndiotactic) of the backbone, compared to atactic chains that have a random arrangement. Building on that foundation, we now investigate the effects of tacticity on the thermal conductivity of bulk polymers, where entangled, coiled chains introduce stress concentration points and phonon scattering centers that are not present in single chains. We leverage the RadonPy package to generate bulk polypropylene and polystyrene structures with specified tacticity via a self-avoiding random walk algorithm. To calculate thermal conductivity, we perform molecular dynamics simulations in LAMMPS and apply the Green-Kubo method. The atomic interactions are modelled using General Amber Force Field Version 2 with a twin-range cutoff set at 8 Å and 12 Å for non-bonded interactions. Long-range Coulomb interactions are treated using a particle-particle particle-mesh method.

Before studying the effect of tacticity, we validated the properties of amorphous polyethylene, atactic polypropylene, and atactic polystyrene. Each system contained 10 polymer chains of equal length, with the degree of polymerization adjusted to maintain approximately 6,400 total atoms. After equilibration at 300 K and atmospheric pressure, the densities of polyethylene, polypropylene, and polystyrene are 0.83 g/cm³, 0.86 g/cm³, and 1.02 g/cm³. These values closely matched the experimental densities of 0.85 g/cm³, 0.87 g/cm³, and 1.04–1.09 g/cm³. The calculated thermal conductivities are 0.41 W/m⋅K, 0.25 W/m⋅K, and 0.22 W/m⋅K, which also align well with the experimental values of 0.32–0.5 W/m⋅K, 0.15–0.21 W/m⋅K, and 0.16 W/m⋅K.

Moving forward, we aim to investigate the effect of tacticity on the thermal conductivity of bulk polypropylene and polystyrene. Our goal is to better understand the mechanisms that influence thermal transport in these materials, with a particular focus on the stiffness and ordering in the polymer chains within the bulk systems. Additionally, we seek to explore the influence of equilibration protocols on the development of semi-crystalline regions that are observed experimentally for ordered isotactic and syndiotactic configurations. Our findings will provide insights on designing polymeric materials with improved thermal properties.

Presenting Author: Shahid Ahmed Carnegie Mellon University

Presenting Author Biography: I am a PhD student in Mechanical Engineering at Carnegie Mellon University. My research focuses on using atomistic simulations to study thermal transport in polymers. As part of this research, I aim to understand how the arrangement of side groups along the polymer backbone affects order and stiffness in polymer chains, and how these factors influence thermal conductivity. This understanding will help guide the design of polymers with improved heat transfer properties for applications in electronics packaging and thermal management.

Authors:

Shahid Ahmed Carnegie Mellon University
Shravan Godse Carnegie Melllon University
Jonathan A. Malen Carnegie Mellon University
Alan J. H. Mcgaughey Carnegie Mellon University