Contribution of amorphous soft segments orientation and their strain induced crystallization to the elastocaloric effect of thermoplastic polyurethaneurea elastomers

Abstract

This study investigates the mechanisms underlying the elastocaloric (eC) effect in thermoplastic polyurethaneurea elastomers, focusing on the contributions of amorphous chain orientation and strain-induced crystallization (SIC) of the soft segments. Two types of TPU systems-strain induced crystallizable (PTMO-based polyurethaneurea (TPUU)) and non-crystallizable (PEO-based polyurea (TPU))-were synthesized with different amounts of HMDI hard segments (20 or 30 wt%) and subjected to mechanical loading under in situ wide-angle X-ray scattering (WAXS) and infrared thermography. The amorphous chain orientation is found to evolve much faster with strain in the TPUUs and SIC is found to initiate in these materials above a critical value of the Hermann's orientation factor of 0.7-0.75. SIC in these systems is found to enhance the eC effect via latent heat, but amorphous orientation is also found to significantly contribute through thermoelastic entropy changes, as attested by high heat sources of 15 MW m-3 for crystallizable TPUU before SIC (at 100 % of deformation) as compared to 5 MW m-3 for non-crystallizable TPUs. Under continuous cycles, TPUU samples demonstrate stronger eC responses and greater reversibility than TPU, resulting in higher coefficients of performance (Best COP = 13 for the TPUU20), particularly after cyclic pre-conditioning. This interpretation of molecular orientation and crystallization in relation with eC properties opens new design strategies for polymer-based solid-state cooling, where microstructural tailoring-beyond relying solely on crystallization-can optimize elastocaloric performance in practical applications.