The Contribution of Oligomerization Reaction Chemistry to the Thermomechanical Properties of Surface-Aligned Liquid Crystalline Elastomers

Abstract

Liquid crystalline elastomers (LCEs) are stimulus-responsive materials that, when macroscopically aligned, undergo large, directional deformation amplified by the disruption of order within a polymer network. Recently, surface-enforced alignment has been used to prepare LCEs with spatial variation in the director orientation that enables the realization of complex 3-D shape transformation. Surface-enforced alignment is advantageous because it allows for high-resolution, nonlinear patterning of the nematic director within an LCE. When using surface alignment to enforce orientation within LCEs, multiple two-step reaction schemes have been commonly used, primarily aza-Michael oligomerization or thiol-Michael oligomerization with each addition reaction followed by photopolymerization of the residual acrylate-terminated oligomers. Here, LCEs were prepared by each polymerization technique with varied amine or thiol concentrations. The mechanical properties, thermomechanical response, reaction kinetics, and alignment were characterized and assessed for LCEs. This examination emphasizes the importance of the molecular weight between crosslinks over the reaction used to form the oligomer in determining the magnitude of the stimulus response of LCEs.

Liquid Crystal Elastomers Prepared by Thiol–Ene Photopolymerization Amenable to Surface-Enforced Alignment

“…The orientation parameter was calculated from the azimuthal intensity profile (via eqs and ) and ranged from 0.51 to 0.56 (see Table S3). These values confirm strong nematic alignment similar to values reported in other LCEs prepared by surface alignment. ,,, …”

“…These values confirm strong nematic alignment similar to values reported in other LCEs prepared by surface alignment. 16,24,27,28 The microstructure is evident in the polarized micrographs with regular striation normal to the direction of alignment (Figures 3C and S4). Further, the wide-angle X-ray scattering (WAXS) pattern in Figure 3B contains multiple faint, diffuse arcs at smaller angles, indicating additional liquid crystalline ordering.…”

“…However, the polymer network formation is heterogeneous, which is evident in distinctive mechanical properties . We have recently demonstrated that adjustment to the kinetics (with comparatively slow base catalysts) of the thiol-Michael addition reaction to prepare oligomeric species can produce aligned LCEs via surface anchoring. , However, as with LCEs prepared by aza-Michael addition reactions, the oligomerization reaction can occur over hours and limits the throughput of the LCE sample preparation.…”

Enhanced Electromechanical Output in Liquid Crystal Elastomers Prepared by Thiol‐ene Photopolymerization

“…As a starting point, we canvased the literature for Young's modulus values of LCEs aligned by surfaceanchoring. [48][49][50][51][52] In particular, we isolate Young's modulus perpendicular to the direction of alignment (i.e., the direction of electromechanical deformation) as well as Young's modulus contrast (i.e., the ratio of the parallel and perpendicular Young's modulus) as representations of mechanical anisotropy. This analysis is summarized in Figure 2.…”

Enhanced Viscosity Control in Thermosets Derived from Epoxy and Acrylate Monomers Based on Thermoreversible Aza-Michael Chemistry

“…It should be noted that a primary amine group can react with two acrylate functionalities, while a secondary amine can only react with one acrylate moiety, with primary amines reacting faster than secondary amines for this reaction, as demonstrated earlier. 49,50 An index of 100 refers to a 1:1 ratio where every primary and secondary amine can in principle react with an acrylate functionality (2 mol of acrylates per 1 mol of amines), which constitutes a perfect network structure. Deviation of this index to either side will lead to a decrease in the cross-link density.…”