(693j) Crosslinking Post Self-Assembly of Liquid Crystalline Oligomers Enables the Emergence of Toughening and Shape-Memory Mechanisms

Rational design of soft yet tough materials is an active area of research with polymer networks being typical candidates. This study shows that ‘small’ liquid-crystalline (LC) oligomers assembled into lamellar phase and crosslinked post assembly can not only strengthen the material at low strains but also exhibit toughening behavior at large strains. Specifically, a coarse-grained (CG) model was developed to study a triblock oligomer comprised of a ‘rod-like’ quarterthiophene backbone and flexible ethylene oxide units capped with crosslinkable acrylate groups at both ends. Following self-assembly, the crosslinked configurations were prepared using a distance-based bonding reaction protocol aimed at mimicking the UV-induced crosslinking reactions associated with the acrylate groups. This oligomer (without any acrylate functionalization) has been shown to self-assemble into the smectic phase at room temperature [Liu., Z. et al., Adv. Funct. Mater. 2019, 29 , 1805220] and characterized for its mixed ion-electron conduction properties. [Dong, B.X. et. al., ACS Nano 2019, 13, 7665]. For the uncrosslinked system, our results show that upon application of uniaxial strain parallel to the direction of lamellar stacking, there is a significant drop in system stress post yield point due to the buckling instability, commonly observed in layered materials with ‘hard’ and ‘soft’ layers, eventually resulting in lamellar breakage. However, for systems with intermediate extent of crosslinking, beyond the elastic regime, there is a small drop in stress due to buckling followed by a sharp increase in stress back to similar values as the yield stress, a recurrent pattern that yields a ‘sawtooth-like’ stress-strain trend. This toughening behavior is primarily attributed to the ‘recrystallization’ of the thiophene cores to obtain a lamellar packing commensurate to the simulation box dimensions at a given strain. The onset of this phenomena is postulated to result from inter-layer bonds formed during crosslinking, which prevent tilting of the thiophene cores upon deformation (as observed for the uncrosslinked case), helping maintain the layering parallel to the deformation direction. This recrystallization behavior can also be seen as a mechanism for shape-memory imprinting and for preventing bond breaking and hence allowing for self-healing upon high strain deformations. For highly crosslinked systems, a stress plateau (for a short strain window) followed by monotonous increase in stress is observed beyond the elastic regime, a behavior common for LC elastomers. Although these simulations pertain a chemistry-specific CG model, the molecular-level understanding of the toughening behavior should be applicable to other crosslinkable LC oligomers that can self-assemble into the lamellar phase. This study demonstrates a viable strategy to design materials with enhanced toughness and self-healing properties.