Reversible Thermal Conductivity Modulation in Elastic Polyolefin Fibers Through Phase Change Fillers and Strain Engineering

Thermal switches capable of reversible and rapid thermal conductivity changes are essential for various modern technologies, including electronics, automotive, aerospace, and medicine [1]. These devices play a crucial role in managing heat, which is vital for improving the efficiency and longevity of many applications. However, the performance of current thermal switches is often hindered by limited tuning ratios, slow tuning speeds, stepwise tuning mechanisms, and high material and fabrication costs. This limitation creates a significant demand for new materials and designs that can overcome these challenges.<br/><br/>Among potential material systems for thermal switches, polymers stand out due to their suitability for low-cost mass production, chemical inertness, and the ability to be engineered for high mechanical strength and elasticity. Polymers can be tailored to meet specific requirements, making them versatile for a wide range of applications. Nevertheless, the thermal conductivity of commonly used industrial polymers is restricted to a narrow range of 0.1 to 0.5 W/mK, which limits their effectiveness in thermal management.<br/><br/>In this study, we introduce a thermal switch based on an elastic, mostly amorphous polyolefin-based fiber. This switch exhibits a fast and continuously tunable thermal conductivity change with a switching ratio of 6.5 in the first cycle and 2.312 over 1000 cycles, starting from a base value of 0.426 W/mK through uniaxial tensile stretching, providing a dynamic way to manage heat in various applications. Furthermore, by incorporating a phase-change hydrocarbon material as fillers, the thermal conductivity can be reversibly tuned at its melting temperature of 35°C with a switching ratio of 5.56, which has the potential to enhance the switch’s thermal tuning capability further. The employed polymer matrix is thermoplastic, enabling large-scale fabrication via standard industrial low-cost melt-spinning techniques and mechanical recycling at the end of its lifespan, making the production process cost-effective and environmentally friendly.<br/><br/>The thermal conductivity of the polyolefin fibers was measured in situ using the Angstrom method [2], enabling quantitave estimates of the effect of both mechanical deformation of the fiber and a first-order phase transition of a PCM filler. Structural changes in the fibers due to mechanical deformation were characterized using wide-angle X-ray scattering (WAXS) and Raman spectroscopy, revealing that the thermal conductivity change does not result from strain-induced crystallization but from an alignment effect. The change is hypothesized to be due to alignment-induced vibration delocalization, where the population of higher-frequency localized vibration modes decreases. This hypothesis is supported by Raman spectroscopy, which shows a decrease in the relative intensity of localized C-H stretching modes upon fiber stretching. The delocalized modes contribute more to thermal transport in polymers [3], leading to the observed increase in thermal conductivity.<br/><br/>Correspondence should be addressed to Y. Z. and S. V. B. This work is supported by the U.S. Department of Energy grant DE-FG02-02ER45977 (for studies of thermal conductivity in polymers) and the Centers for Mechanical Engineering Research and Education at MIT and SUSTech, MechERE Centers at MIT and SUSTech (for engineering polyolefin-based elastic fibers for thermal energy harnessing applications). D.X. is supported by the MathWorks MechE Graduate Fellowship.<br/><br/><b>References:</b><br/>[1] <i>Appl. Phys. Rev.</i> <b>4,</b> (2017).<br/>[2] <i>Int. J. Heat Mass Transf.</i> <b>169,</b> 120938 (2021).<br/>[3]<i> Commun. Phys. 2022 51</i> <b>5,</b> 1–10 (2022).