Mechanocaloric Effects Characterization of Low-Crystalline Thermoplastic Polyurethanes Fiber

Abstract

Mechanocaloric cooling/heat pumping with zero carbon emission and high efficiency shows great potential for replacing traditional refrigeration with vapor compression. Mechanocaloric prototypes that are developed using shape memory alloys (SMAs) face the problems of a large driving force and high cost. In this work, we report a low-crystalline thermoplastic polyetherurethane (TPU) elastomer fiber with a low actuation force and good mechanocaloric performance. We fabricate the TPU fiber and develop a multifunctional mechanical tester to measure both the elastocaloric and twistocaloric effects. In the experiments, the applied stress required to induce mechanocaloric effects of the TPU fiber is only 10~30 MPa, which is much lower than that of widely used NiTi elastocaloric SMAs (600~1200 MPa). The TPU fiber produces a maximum twistocaloric adiabatic temperature change of 10.2 K, which is 78.9% larger than its elastocaloric effect of 5.7 K. The wide-angle X-ray scattering (WAXS) results show that the strain-induced amorphous chain alignment and associated configurational entropy change are the main causes of the good mechanocaloric effects of the TPU fiber, rather than the strain-induced crystallization. This work demonstrates the potential of achieving low-force heat-efficient mechanocaloric cooling using thermoplastic elastomer fibers.

Keywords: elastocaloric; mechanocaloric; solid-state cooling; thermoplastic elastomer; twistocaloric.

Figure A1

Variation in the maximum twist density allowed for the fabricated TPU fiber with an increase in the pre-strain.

Figure 1

Fabrication of the TPU fiber. (A) Fabrication process of the TPU fiber through a micro twin-screw extruder. (B) Pictures of the TPU pellets and the as-spun TPU fiber with a diameter of 1.7 mm.

Figure 2

Material properties of the TPU pellets. (A) Heat flow of the TPU in the heating process. (B) Specific heat capacity of the TPU in the heating process.

Figure 3

Experimental setup for mechanocaloric effect characterization. (A) Design of a multifunctional experimental tester for stretching and twisting the TPU fiber. (B) Pictures of the experimental tester and the assembly of the polymer fiber with the loading fixtures.

Figure 4

Elastocaloric effect of the TPU fiber. (A) Stress–strain responses of the TPU fiber under uniaxial tensile loading with a maximum strain of 400% for different numbers of loading cycles. (B) Elastocaloric adiabatic temperature changes of the TPU fiber at varied applied strains. (C) Thermal images of the TPU fiber in the loading and unloading process. (D) Elastocaloric temperature changes of the TPU fiber at varied applied strains. (E) Variation of the material COP and hysteresis loop area (net input work) of the TPU fiber with the number of loading cycles.

Figure 5

Twistocaloric effect of the TPU fiber. (A) Pictures of the TPU fiber during the twisting. (B) Definition of the twist density. (C) IR camera images of the TPU fiber at varied twist densities. (D–F) Twistocaloric adiabatic temperature change of the TPU fiber versus the twist density at a pre-strain of 0%, 50%, and 100%, respectively. (G–I) Variation in the twistocaloric adiabatic temperature change of the TPU fiber with the number of loading cycles at a pre-strain of 0%, 50%, and 100%, respectively.

Figure 6

Microstructure characterization of the TPU fiber. (A) WAXS patterns of pristine, stretched, and twisted fibers under various strains/pre-strains. (B) WAXS spectra of the pristine, stretched, and twisted fibers. (C) Variation in the adiabatic temperature change and HOF of the fiber with the strain. (D) Polarized Raman spectra of the twisted TPU fiber with pre-strain of 100%. Top line: polarization perpendicular to the fiber direction. Bottom line: polarization parallel to the fiber direction. (E) Intensity of the 1187 cm⁻¹ peak as a function of the polarization angle for pristine, stretched, and twisted fibers from the polarized Raman spectra. (F) HOF of pristine, stretched, and twisted fibers under various strains/pre-strains.