Session: 11-05-02: Thermophysical Properties: Characterization and Modeling Across Scales

Paper Number: 149927

149927 - Effect of Chain Tacticity on the Thermal Conductivity of Polymers 

The use of polymers in everyday life has increased exponentially, primarily due to their comparatively lower manufacturing costs and a wide chemical design space which allows us to tailor their properties according to our needs. However, the use of polymers in technologies such as electronics cooling, heat exchangers, are lacking due to their intrinsically low thermal conductivities (0.1-0.3 W/m-K). As highlighted by Roy et al.[1], the key factors in raising polymer thermal conductivity are enhancing polymer crystallinity, increasing polymer chain alignment and reinforcing inter-chain interactions. Tacticity, which is related to the stereochemistry of side groups along the carbon backbone, can affect crystallinity of polymers. Since isotactic and syndiotactic polymer chains have a well-defined order, they tend to show higher crystallinity than their atactic counterparts. By using specific organometallic catalysts, polymer tacticity can be controlled giving rise to commercial products such as isotactic polypropylene (PP) and polystyrene (PS). Several works have reported the effect of tacticity on the physico-thermal properties of PP and PS[2-5], such as viscoelastic modulii, glass transition temperatures, thermal stability, diffusion coefficients, etc. However, the role of tacticity in altering thermal conductivity is still unclear. Furthermore, tacticity control in polymers other than PP and PS is not well established. In this work, we use the laser-based methodology of Frequency Domain Thermoreflectance (FDTR) to examine thermal transport in poly (dimethyl acrylamide) (PDMA) thin films with varying tacticities. Tacticity control is achieved using a Lewis acid mediated photoiniferter method and is characterized using NMR spectroscopy. The Lewis acid aids in tacticity control and the photoiniferter technique allows us to obtain ultrahigh molecular weight polymers which further promote crystallinity. Differential Scanning Calorimetry (DSC) is used to examine crystallinity and heat capacity. We find a distinct crystalline peak in the DSC profile upon increasing tacticity from 53% to 86%. Polymer films are spun coat on glass and Silicon substrates and their thicknesses are characterized using Atomic Force Microscopy. Our preliminary FDTR results indicate that upon increasing tacticity from 69% to 84%, thermal conductivity shows a two-fold jump from 0.14 W/m-K to about 0.27 W/m-K. We also employ equilibrium molecular dynamics simulations using the Universal Force Field to elucidate the role of tacticity in tailoring thermal conductivity. Simulations are carried on single chain polymers of PP, PS and PDMA, as well bulk structures, and the role of tacticity across different scales is investigated. Mechanisms underpinning the increased thermal conductivity and directions for future work towards this goal are presented.

References: 

1] Roy, R., Stevens, K., Treaster, K., Sumerlin, B. S., McGaughey, A., Malen, J. A., and Evans, A. M., 2024, “Intrinsically Thermally Conductive Polymers,” Materials Horizons.

2] Jones, T. D., Chaffin, K. A., Bates, F. S., Annis, B. K., Hagaman, E. W., Kim, M.-H., Wignall, G. D., Fan, W., and Waymouth, R., 2002, “Effect of Tacticity on Coil Dimensions and Thermodynamic Properties of Polypropylene,” Macromolecules, 35(13), pp. 5061–5068.

3] Huang, C.-L., Chen, Y.-C., Hsiao, T.-J., Tsai, J.-C., and Wang, C., 2011, “Effect of Tacticity on Viscoelastic Properties of Polystyrene,” Macromolecules, 44(15), pp. 6155–6161.

4] Chen, K., Harris, K., and Vyazovkin, S., 2007, “Tacticity as a Factor Contributing to the Thermal Stability of Polystyrene,” Macromolecular Chemistry and Physics, 208(23), pp. 2525–2532.

5] Von Meerwall, E., Waheed, N., and Mattice, W. L., 2009, “Effect of Stereochemistry on Diffusion of Polypropylene Melts: Comparison of Simulation and Experiment,” Macromolecules, 42(22), pp. 8864–8869.

Presenting Author: Shravan Godse Carnegie Mellon University

Presenting Author Biography: Shravan Godse is a PhD candidate in The Malen Laboratory at Carnegie Mellon University, with a deep interest in thermal transport phenomena in materials. His PhD research focuses on understanding and characterizing thermal transport in polymers using a combination of experimental techniques such as Frequency Domain Thermoreflectance and Transient Hot Wire, alongside computational methods including lattice dynamics and molecular dynamics simulations.
Prior to joining CMU, Shravan completed his bachelor's in mechanical engineering from the Indian Institute of Technology Bombay in 2022. His bachelor's thesis focused on the first principles simulation of nanoscale thermal transport in type-I inorganic clathrates.
In his PhD, Shravan aims to integrate his computational and experimental expertise to uncover design principles that enable the development of the next generation of thermally conductive polymeric materials, potentially revolutionizing applications in electronics, energy, and advanced manufacturing.

Authors:

Shravan Godse Carnegie Mellon University
Kaden Stevens University of Florida
Manoj Settipalli Carnegie Mellon University
Brent Sumerlin University of Florida
Alan Mcgaughey Carnegie Mellon University
Jonathan Malen Carnegie Mellon University