Evaluations of Mesogen Orientation in Thin Films of Polyacrylate with Cyanobiphenyl Side Chain

Soft and tough cellulose nanocrystals interlocked with polyacrylate-bearing cyanobiphenyl ionogels through a double network strategy

There are several examples of double network gels prepared from cellulose nanocrystals (CNC) conjugated with water-soluble polymers. However, soft but tough gels composed of mostly CNCs have been challenging to design due to aggregation or precipitation of CNCs at higher concentrations in solvated and gelled systems. We report a general strategy to introduce covalent and non-covalent interactions within cellulose nanocrystals (CNCs) to develop tough, processable, liquid crystalline (LC) gels. A liquid crystalline 12-methylene spacer cyanobiphenyl (CB12OH), is grafted onto acrylic acid via Steglich esterification and subsequently copolymerized with acrylic acid via free radical polymerization to afford an amphiphilic liquid crystalline polymer (LCP), PAACB12-r-PAA. The free carboxyl (COOH) groups from polyacrylic acid (PAA) in the copolymer are randomly interlocked with the hydroxyl (OH) groups of CNC through covalent crosslinking and host-guest supramolecular hydrogen bonding in DMSO and 1-butyl-3-methylimidazolium acetate ionic liquid. The dominant cyanobiphenyl-cyanobiphenyl (CB12-CB12) interactions yield hierarchically structured, tough, free-standing liquid crystalline nanocomposite ionogels. Rheological studies of the interlocked free-standing ionogels display a typical crosslinked gel behavior with the storage moduli (G') ranging 103-104 Pa across the entire angular frequency range. Mechanical properties are tailored by varying the concentration of CNCs in the nanocomposite gel. This facile design provides a general strategy to prepare soft and tough liquid crystalline gels from renewable resources.

Sol-Gel Transition of a Thermo-Responsive Polymer at the Closest Solid Interface

Thermo-responsive polymers are applied as surface modifications for the temperature switching of hydrophilic and hydrophobic properties through adsorption and grafting on solid substrates. The current understanding of the influence of polymer chains bound to the solid surface on the transition behavior of thermo-responsive polymers is rather restricted. In this study, we aim to elucidate the effect of the bound polymer chains at the interface on the thermo-responsive sol-gel transition behavior of aqueous methylcellulose (MC) solutions by employing a quartz crystal microbalance (QCM) to evaluate the shear modulus near the solid interface. When the sample thickness was thinner on the order of the millimeter scale, the sol-gel transition temperature evaluated by the cloud point decreased because the condensation of MC near the solid interface promoted the sol-gel transition. On the other hand, focusing on the closest solid interface on the nanometer scale by QCM, the sol-gel transition temperature increased when approaching the solid interface. Adsorption and interfacial interactions reduced the chain mobility and restrained the sol-gel transition by preventing MC chain aggregation. We demonstrated the physical properties evaluation at the closest interface between the thermo-responsive polymer and solid substrate by combining a simple analytical model of QCM and controlling the analytical depth of the QCM sensors. In conclusion, the mobility change of the bound polymer chains at the solid interface caused by adsorption and interfacial interactions must be considered when a thermo-responsive polymer is applied as in adsorbed or thin films on solid substrates for the functionalization of biomaterials.

Random Planar Orientation in Liquid-Crystalline Block Copolymers with Azobenzene Side Chains by Surface Segregation

Rodlike liquid-crystalline (LC) mesogens preferentially adopt a homeotropic orientation by excluded volume effects at the free surface in side-chain LC (SCLC) polymer films. The homeotropic orientation is not advantageous for in-plane LC alignment processes. Surface segregation of polymers is the phenomenon in which one component with a low surface free energy covers the surface in a mixture of two or more polymers or a block copolymer film. In SCLC block copolymer films, the surface segregation structure induces a random planar orientation due to the formation of a microphase-separated interface parallel to the substrate via the covering of one of the segregated polymer blocks. This feature article focuses on the unique, random planar orientation induced by the surface segregation of SCLC block copolymer films with the photoresponsive azobenzene (Az) mesogenic group. A transition moment of the Az mesogens is parallel to the molecular long axis, and light irradiation is conducted perpendicular to the film surface in general photoreaction processes. Therefore, the homeotropic molecular orientation in the SCLC polymer systems with Az mesogenic units inhibits efficient photoreaction reorientations in thin films. The random planar orientations by the surface segregation of a coil block in SCLC block polymers provide efficient in-plane photoreorientation and photoswitching with LC hierarchical mesostructures, such as microphase-separated structures of SCLC block copolymers and laminated LC polymer films. On the other hand, surface-segregated SCLC blocks form a high-density polymer LC brush layer with a random planar orientation by self-assembly, which exhibits efficient angular selective photoreactions. These approaches using the surface segregation of SCLC block copolymers are expected to offer new concepts for the LC photoalignment process for LC polymer devices.

Orientation and Morphology Control of the Liquid Crystalline Block Copolymer Thin Film by Liquid Crystalline Solvent

The critical challenge to engineer the morphological structures in the strongly phase-segregated block copolymer thin film is to overcome the preferential wetting of the blocks at the interface and direct the self-assembly process. Herein, we utilize surface activity and selective solvation of a nematic (N) liquid crystalline (LC) solvent, 5CB, to facilely alter the LC anchoring and the orientation of the nanophase separated structures of the smectic-nematic (S-N) LC block copolymer thin film. For the neat S-N diblock copolymer thin film, the nanostructures are parallel aligned. In contrast, with continuous introduction of 5CB into the system, the orientations of the characteristic nanostructures and the morphologies of the LC thin film can be consequently changed, yielding the perpendicularly oriented lamellar or cylindrical structures with the feature size below 10 nm. The homeotropic alignment of the 5CB nematics near the air interface plays a critical role to induce this unique behavior in the S-N/5CB systems, which offers an opportunity to fine-tune the interfacial structures and the morphological patterning in the block copolymer thin film.