Melt-processed P3HT and PE Polymer Nanofiber Thermal Conductivity
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Melt-processed P3HT and PE Polymer Nanofiber Thermal Conductivity Matthew K. Smith,1,* Thomas L. Bougher,2 Kyriaki Kalaitzidou,1,2 Baratunde A. Cola1,2 1
School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, USA. 2 George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia 30332, USA. *Correspondence to: [email protected] ABSTRACT Thermal management is a growing challenge for electronics packaging because of increased heat fluxes and device miniaturization. Thermal interface materials (TIMs) are used in electronic devices to transfer heat between two adjacent surfaces. TIMs need to exhibit high thermal conductivity and must be soft to minimize thermal contact resistance. Polymers, despite their relative softness, suffer from low thermal conductivity (~0.2 W/m-K). To overcome this challenge, we infiltrate nanoporous anodic aluminum oxide (AAO) templates with molten polymer to fabricate large area arrays of vertically aligned polymer nanofibers. Nanoscale confinement effects and flow induced chain elongation improve polymer chain alignment (measured using polarized Raman spectroscopy) and thermal conductivity (measured using the photoacoustic method) along the fiber’s long axis. Conjugated poly(3-hexylthiophene-2,5-diyl) (P3HT) and nonconjugated polyethylene (PE) of various molecular weights are explored to establish a relationship between polymer structure, nanofiber diameter, and the resulting thermal conductivity. In general, thermal conductivity improves with decreasing fiber diameter and increasing polymer molecular weight. Thermal conductivity of approximately 7 W/m-K was achieved for both the ~200 nm diameter HDPE fibers and the 100 nm diameter P3HT fibers. These results pave the way for optimization of the processing conditions to produce high thermal conductivity fiber arrays using different polymers, which could potentially be used in thermal interface applications. INTRODUCTION Improved chain alignment through mechanical drawing of polyethylene (PE) has been established as a method to enhance polymer thermal conductivity since the late 1970’s [1-3]. More recent efforts have achieved thermal conductivity values as high as ~40 W/m-K in a drawn film [4], and ~100 W/m-K in a drawn nanofiber [5]. Simulations have shown that while the highest thermal conductivity is realized in a one-dimensional (1D) polymer chain, even bulk threedimensional (3D) crystals of PE will have thermal conductivity of ~50 W/m-K in the direction of the polymer backbone (c crystal direction) [6]. In contrast, Choy et al. demonstrated experimentally that the thermal conductivity perpendicular to the polymer chain in a PE crystal is even lower than bulk semi-crystalline PE [4]. Nanoconfinement within nanoporous templates is known to promote polymer chain elongation and alignment in polymer nanofibers, passively enhancing thermal transport anisotropically without the need for mechanical drawing. We find only one nanoporous template based effort to c
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