The End-Linked Star-Shaped Thermoset (ELST) is a unique polymer material based on the A-B type tetra-arm polyethylene glycol (tetra-PEG) system. Unlike typical amorphous polymers, the ELST preserves a uniform chain length distribution with minimal topological defects due to its nearly ideal-network structure. The ELST is synthesized by cross-linking two types of macromers in an equal molar ratio with a well-tuned reaction efficiency, producing nearly ideal tetra-PEG polymer networks free of topological defects.
Unlike previous studies focusing on tetra-PEG hydrogels, this study concentrates on solvent-free tetra-PEG thermosets, which exhibit a shape-memory effect with a distinct melting point and ultra-high stretchability when heated above their melting temperature (Tm). The unique mechanical and physical properties of the tetra-PEG thermoset are attributed to the process of deswelling the end-linked star-shaped PEG macromers.
The ELST can undergo elastic deformation when heated above Tm, maintain its temporary deformation at a fixed stretch ratio upon cooling below Tm, and elastically recover to its original shape when reheated above Tm. To investigate the effect of strain on the material’s thermal conductivity, a series of tetra-PEG thermosets with controlled stretch ratios were prepared and cooled to preserve their stretched states. The thermal conductivity of the ELST was then measured using a home-built steady-state system and a frequency-domain thermoreflectance method (FDTR).
The results showed that the undeformed ELST has a thermal conductivity of 0.24 W m−1 K−1 at T = 30 °C. As the stretch ratio increases, the thermal conductivity of the deformed ELST along the stretch direction significantly increases, reaching 1.42 W m−1 K−1 at T = 30 °C. The ELST exhibits a maximum strain-modulated thermal conductivity tuning ratio up to 11.5 from 0.15 to 2.1 W m−1 K−1 at T = 60 °C. The two-way thermal conductivity tuning of the ELST is primarily driven by strain modulation, with thermal modulation serving as an additional tuning capacity.
The unique thermal conductivity tuning capabilities of the ELST have not been achieved in existing polymeric thermal switches. The response time for the two-way tuning of thermal conductivity in the ELST is estimated to be ~10 s, which is significantly shorter than that of conventional polymers. The short response time for structural relaxation in the ELST is attributed to its unique low topological defects and negligible molecular entanglements.
X-ray scattering characterizations were performed to validate the hypothetic thermal transport mechanism in the ELST. The results showed that at a moderate stretch ratio, the enhanced phonon transport is mainly due to the oriented crystalline domains. As the stretch ratio increases, the crystalline domains are further oriented, and the interstitial amorphous chains are well-aligned along the stretch direction, further enhancing thermal transport. The synergy of oriented crystalline domains, aligned interstitial amorphous chains, and increased crystallinity in the nearly ideal polymer network explains the two-way tuning of thermal conductivity observed in the ELST.
