Engineering Mechanocaloric Effects and Tunable Thermal Conductivity in Amorphous Elastic Polymer Fibers

Abstract

Energy-efficient clean technologies for active heating/cooling and passive thermal regulation are in high demand for applications spanning different scales, from city planning and building heating/cooling to wearable and portable devices to miniature electronics. To advance relevant technologies and simultaneously lower their environmental footprint, two key research and engineering questions await to be answered: (1) how to pump/convert energy between thermal and other forms more efficiently, and (2) how to transport thermal energy in a tunable and scalable way for dissipation/insulation applications.

Properly-engineered polymer materials may provide solutions to both challenges in a synergistic way. Among all materials, polymers stand out by several figures of merit, including low cost, chemical inertness, ease of manufacture and scalability, and light weight. They can be engineered by the application of temperature and/or strain, which impose different molecular arrangements within the material, enabling control over the degree of crystallinity, chain entanglement, and the dominant chain orientation. When polymers undergo microscopic structural changes, they may exhibit temperature responses driven by their internal entropy changes, known as mechanocaloric (mC) effects. mC effects offer a venue of conversion between mechanical and thermal forms of energy. Polymer chain alignment, on the other hand, also has a strong effect on the vibration characteristics of polymers, and thus on their thermal conductivity (TC) values. Through a continuous strain-temperature engineering of elastic amorphous polymer fibers, we demonstrate unique opportunities to address both challenges in energy conversion and transfer.

We developed elastic fibers, which are melt spun from an olefin block copolymer (OBC), and exhibit (1) competitive mC performance with the temperature change exceeding 5K and the material coefficient of performance (COP) larger than 10, and (2) reversible thermal conductivity, which is continuously tunable in the range from 1.2 to 2.5 W/mK via uniaxial strain deformations. The entanglement-enabled elasticity of the cross-linker-free block co-polymer chosen for this research allows the fibers to survive thousands of loading-release stretching cycles. In striking contrast with the vulcanized rubber commonly used as an efficient mC material, the OBC is a thermoplastic with a relatively low melting temperature (<120C), which can be easily recycled and molded into different geometries. By optimizing both the fabrication parameters and the operational scenarios, we demonstrated high potential of elastic OBC fibers in advanced thermal applications within a wide temperature window from -20C to 70C. We further analyzed structural changes, thermodynamics, and vibration spectra of OBC fibers under different strains and temperatures, elucidating the mechanisms underlying the observed phenomena. This study provides insights into sustainable engineering and optimization of polymer-based solid-state refrigerators, heat pumps and tunable materials for efficient energy dissipation and passive thermoregulation.