Modeling and Optimizing Temperature-Dependent Elastocaloric Effects in Elastic Olefin Block Copolymer Fibers

Solid-state cooling technology based on elastocaloric effects [1–3] provides a competitive alternative to vapor-compression refrigeration technology by demonstrating higher energy efficiency, compactness, noise-free operation, and environmental friendliness. Well-studied metal alloy-based elastocaloric materials such as Ni-Ti and Cu-Al-Ni fail to reach industrial adoption due to their high cost of fabrication and operation, despite exhibiting high temperature spans (Tspan) of 12~30 K and coefficients of performance (COP) of 6~18 [4]. Polymers, on the other hand, are the largest product synthesized by volume nowadays, and have low tensile stress, chemical inertness, light weight, and low cost. Recent progress in elastocaloric polymer experiments shows performances comparable with those of metal alloys. E.g., Tspan around 15 K and COP over 10 in natural rubber [5] and other elastomers [6] have been demonstrated, opening new opportunities for polymer-based elastocaloric materials.<br/><br/>Different from the well-known cross-linked elastomers like natural rubber, we present an experimental study on an entanglement-enabled-elastic olefin block copolymer(OBC) fibers, whose elastocaloric performances feature a high cooling COP exceeding 10 and stable performance for at least 1000 cyclic loadings. We observed a Tspan over 22 K for the first loading, exceeding all existing polymer-based materials. After over 100 cyclic loadings, the stabilized Tspan at room temperature is 5 K. For stabilized Tspan, we noticed a non-linear temperature dependence on cooling Tspan with peaks at 6 K at -20 ^oC and 60 ^oC, while 2 K at room temperature. We explained such a nonlinear trend quantitatively by modeling the configurational entropy change upon deformations and predicted different temperature dependences for different deformation trajectories. Such temperature and deformation dependencies suggest a new direction for optimizing and designing polymer-based elastocaloric cooling devices. Finally, a conceptual device for heating and cooling is designed and analyzed with practical working cycles.<br/><br/>The entanglement-enabled reversible deformation of flexible polymer segments in the melt-spun OBC fibers makes them elastic at room temperature without cross-linking. We built a twisting-stretching coupled stage with an IR camera and force/torque sensors to measure in-situ temperature responses and force/torque during the fiber adiabatic deformation process. Experiments revealed that our fibers achieve greater than 10 cooling COP under stretching at room temperature and twisting at higher (60 ^oC) and lower (-20 ^oC) temperatures. We also characterized fiber structural changes by X-ray and Raman spectroscopy to understand the mechanisms underlying the elastocaloric fiber performance. Significant drops in both orthorhombic crystalline (6%) and amorphous domains by (13%) were observed upon stretching, while the interfacial oriented amorphous domains grew by 19%, accompanied by Herman's orientation factor increase from 0.02 to 0.17. These observations suggest the entropic change due to chain alignment, rather than strain-induced crystallinity, is revealed to be a dominant factor that explains the elastocaloric effects and its temperature dependence. Finally, the new elastomer fibers are extremely inert and suitable for operation in harsh corrosive environments, as well as fully mechanically recyclable at the end of their lifespan.<br/><br/>Y. L. and B. L. contributed equally. Correspondence should be addressed to Y. Z. and S. V. B.. This work is supported by Centers for Mechanical Engineering Research and Education at MIT and SUSTech (MechERE Centers at MIT and SUSTech). B. L. is supported by the Evergreen Graduate Innovation Fellowship.<br/><br/>References:<br/>1. <i>Appl. Phys. Rev.</i> <b>6,</b> 41316 (2019).<br/>2. <i>Nat. Rev. Mater. 2022 78</i> <b>7,</b> 633–652 (2022).<br/>3. <i>Int. J. Refrig.</i> <b>64,</b> 1–19 (2016).<br/>4. <i>Acta Mater.</i> <b>208,</b> 116741 (2021).<br/>5. <i>Science (80-. ).</i> <b>366,</b> 216 LP – 221 (2019).<br/>6. <i>Nat. Commun. 2022 131</i> <b>13,</b> 1–7 (2022).