2024 AIChE Annual Meeting

Ascertaining the Influence of Molecular Weight on Phase Transition Temperature: Applications in Liquid Crystal Elastomer Synthesis

Liquid crystal elastomers (LCE) have garnered substantial attention in recent years due to their role as a stimuli-responsive soft materials with various applications in optical polymers, soft robotics, and artificial muscles. When external stimuli such as light or temperature interact with LCEs, they undergo a transition from a high order material where all liquid crystal molecules (mesogens) are aligned along an imposed nematic director, to a material with low alignment. This change in alignment initiates a transformation in shape of the material resulting in actuation or optical impedance. The merit of liquid crystal elastomers (LCEs) as programmable materials depends on the alignment of these mesogens. Greater initial alignment leads to more pronounced responses. Moreover, the temperature at which these transitions occur is crucial, as it directly determines the material's programmability. Such materials have traditionally been difficult to synthesize due to extended preparation times and elevated temperatures. Thiol-ene chemistry offers fine-tunability of synthesizing LCEs, thereby minimizing the time required to produce a monodomain LCE. Thiol-ene chemistry has also been thoroughly studied for application in polymer synthesis. In the context of liquid crystals, thiol-ene photopolymerization facilitates effective mechanical alignment of liquid crystal elastomers (LCEs) while ensuring control over the resulting material properties. Adjustments in the LCE formulation and variations in alignment levels directly influence the response's strain and temperature.

This work elucidates the influence that molecular weight of linear thiol-ene oligomers has on the temperature phase space of LCE polymers. Determining the temperature and the characteristics of how LCEs transition is critical to tuning the temperature where actuation occurs in the material. To investigate this, the monomer ThE4b4 was polymerized at varying times to increase the degree of polymerization and thus the molar mass. ThE4b4 is unique in this regard due to its thiol-ene functionalization. Possessing both a thiol and an alkene functional group enables photopolymerization in the presence of an initiator without requiring the addition of a thiol which can disguise the temperature of transition. Idealized step growth of ThE4B4 entails the production of a linear polymer. Analysis of linear polymers is desirable to decouple effects of crosslinking with the effects of increasing molecular weight as both can influence the phase transition temperature. IR spectroscopy and NMR will determine the percent conversion of thiols and alkenes and the degree of polymerization, respectively. Once the phase space is adequately defined for a linear polymer, crosslinkers will then be introduced to produce LCE films whose actuation and strain properties will then be determined utilizing dynamic mechanical analysis.

Preliminary results indicate sharp monomer peaks, as molecular weight increases these peaks begin to broaden with transition temperature increasing as well. An interesting observation is that the energy required to initiate a phase transition is concave down before reaching an inflection point where the trend reverses itself. A greater understanding of actuation and the nature of phase space allows for greater tunability and versatility when synthesizing LCEs.