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

Abstract

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.

Molecular Engineering of Mesogenic Constituents Within Liquid Crystalline Elastomers to Sharpen Thermotropic Actuation

“…Recent literature reports have documented the reduction in the thermotropic actuation of LCE by adjusting the length of the amine in the chain extended oligomer (in aza‐Michael addition) [ 30 ] or by utilizing mixtures of C6M with other liquid crystalline monomers (such as C3M). [ 25,31 ] These prior works are indirect approaches to disrupt the strength of the mesogen–mesogen interaction in molecular pairs that form in the LCE polymer network. Here, informed by the data in Figure 2, we examine the thermotropic actuation of TM LCE ( Figure a) prepared with C6BAPE (TM‐a), a 1:1 f.g. mixture of C6BAPE:C6M (TM‐b), or C6M (TM‐c).…”

4D Printing of Hygroscopic Liquid Crystal Elastomer Actuators

“…The numberaverage molecular weight (M n ) of the humidity-responsive LC oligomer (h-LCO) after chain extension was 6900 g mol −1 , as determined by 1 H NMR end-group analysis, similar to our previous report (Figure S1, Supporting Information). [27] To prepare a monodomain h-LCE (i.e., a planar-aligned h-LCE), the reactions were performed inside a surface-aligned cell, as shown in Figure 1b. The gel fraction of the h-LCE was 90%, suggesting sufficient crosslinking.…”

Effect of Isomeric Amine Chain Extenders and Crosslink Density on the Properties of Liquid Crystal Elastomers

“…To investigate the effect of isomeric amine chain extenders on the thermal, mechanical, and actuation properties, we prepared a set of LCEs with a similar degree of crosslink density (i.e., a similar molecular weight (MW) of LCOs between the crosslinks). In general, the key to determining the MW of LCOs synthesized by step-growth polymerization is the stoichiometric imbalance between the LC monomer and chain extender [36,40,41]. However, although the amine chain extenders examined in this study were primary amines, they showed different reactivities toward the diacrylate groups in the LC monomer during the aza-Michael addition reaction (i.e., n-butyl > isobutyl > sec-butyl), which is possibly because of the difference in steric hindrance [42].…”

“…The 1 H NMR spectra of LCE-N1, LCE-N2, LCE-I1, LCE-I2, and LCE-S1 and the integration values used to calculate the DP are shown in Figure 2. The M n values of the LCOs were determined by a protocol similar to that described in our previous report [40], and the values are listed in Table 1. Briefly, peaks b (6.40 ppm), c (6.13 ppm), and d (5.82 ppm) correspond to the six protons in the diacrylate end groups in the LCOs, while peak a (8.15 ppm) corresponds to the four aromatic protons in the repeating units.…”

“…Such impressive actuation performance was attributed to extremely strong mechanical properties originating from fully miscible interpenetrating networks that provide both chemical and physical networks, and the ability to preserve chain anisotropy even at high temperatures [39]. As a slightly different approach, White's research group demonstrated that a thick, laminated LCE film with complex director profiles can significantly improve force output and stroke [40].…”