Meltable copolymeric elastomers based on polydimethylsiloxane with multiplets of pendant liquid-crystalline groups as physical crosslinker: A self-healing structural material with a potential for smart applications
Abstract
Elastomers with strong physical crosslinks were prepared, based on alternating polydimethylsiloxane (PDMS) spacer segments and pendant quartets of mesogenic building blocks (LC) of azobenzene type. They are structurally related to the well-studied polymers with pendant-chain LC units (light-sensitive actuators), but are generally highly different: The LC units make up only a small volume fraction in our materials and they do not generate elastic energy upon irradiation, but they act as physical crosslinkers with thermotropic properties.
Our elastomers lack permanent chemical crosslinks - their structure is fully linear (with some dangling units). The aggregation of the relatively rare and spatially separated LC quartets (of small mesogen units) nevertheless proved to be an efficient crosslinking mechanism: The most attractive product displays a rubber plateau extending over 100 degrees C, melts near 70 degrees C and is soluble in organic solvents.
The LC nano-aggregates were also found to be responsible for a continuous temperature region of phase transitions, e.g. two gel points observed by theology. The physical crosslinks are reversibly disconnected by large mechanical strain at room temperature, but they undergo self-healing, also after sample disruption.
The elastomers might be of interest for the development of passive smart materials (e.g. meltable rubbers for 3D-printing, or thermo-reversible visco-elastic mechanical coupling). Our study focuses on the comparison of physical properties and structure-property relationships in two systems, with long and with short PDMS spacer segments.