Decoding Liquid Crystal Oligomer Phase Transitions: Toward Molecularly Engineered Shape Changing Materials

YH Guo and J Lee and J Son and SK Ahn and JMY Carrillo and BG Sumpter, MACROMOLECULES, 52, 6878-6888 (2019).

DOI: 10.1021/acs.macromol.9b01218

This work details an integrated investigation of liquid crystal (LC) oligomers that combines experiments and molecular dynamics simulations to obtain a detailed understanding of the molecular structure of LC oligomers and the mechanism underlying their phase transition temperatures. We synthesized and characterized a series of LC oligomers prepared from different lengths of methylene spacers in the reactive LC monomers and n-alkylamine chain extenders via the aza-Michael addition reaction. In parallel, we performed isothermal-isobaric (NPT) ensemble coarse-grained molecular dynamics (CG-MD) simulation of analogue mesogens that are connected to flexible spacers and extenders at varying temperatures, spacer lengths, and extender lengths. This approach allowed the effect of length in the flexible spacer as well as in the chain extender on the nematic-isotropic transition temperature (T-ni) to be determined. The results showed that increasing the length of the extender decreases T-ni for LC oligomers and amplifies the decrease of T-ni in LC oligomers when the spacer length is short. We infer that the combination of spacer and extender changes the shape anisotropy of LC oligomers, changing the packing behavior of constituent mesogens, thus affecting their ability to transition from the isotropic to the nematic phase. The detailed molecular structure-property relationships formulated enable prescribing design rules for LC oligomers geared toward molecularly engineered shape changing materials.

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