Towards Subcritical Phase Transitions in Liquid Crystalline Elastomers

Liquid crystalline elastomers (LCEs) are functional materials widely studied for their ability to produce large deformations. This deformation is a result of order disruption between mesogenic moieties incorporated in the polymer network. Often, reports detail that LCEs undergo phase transitions analogous to low molar mass liquid crystalline systems. However, in practice, LCEs deviate from this behavior. Prior investigation has associated the deviation from first order behavior in LCEs to the restraint on the mesogens that is imposed by crosslinks. This talk will highlight some of our recent efforts in working toward achieving first order, subcritical phase transitions in LCEs.



One approach that we have taken toward realizing phase transitions in LCEs is molecular engineering of the mesogenic core in liquid crystalline monomers. Historically, many LCEs utilize a mesogen with three aromatic rings in the rigid core. Here, we demonstrate that using a mesogen with two aromatic rings reduces the energy input required for order disruption by reducing the strength of intermolecular interactions between mesogens. We show that the reduced magnitude of mesogen-mesogen interaction strength enables thermomechanical actuation at lower temperatures and faster rates as well as photomechanical actuation with higher magnitude strain generation and faster response times.



A second route we have taken toward achieving first order phase transitions is the incorporation of supramolecular bonds in LCEs. The work presented in this talk will highlight the use of mesogenic moieties that contain hydrogen-bonding carboxylic acid groups within the mesogenic monomer core. When incorporated in a crosslinked LCE, these supramolecular bonds dissociate at elevated temperatures. We couple this supramolecular bond dissociation with thermomechanical actuation to achieve faster, more efficient actuation as a result of reduced crosslink density during order disruption.