Morphological Study on an Azobenzene-Containing Liquid Crystalline Diblock Copolymer

Dual-Stimuli Responsive Single-Chain Polymer Folding via Intrachain Complexation of Tetramethoxyazobenzene and β-Cyclodextrin

We synthesize and characterize a triblock polymer with asymmetric tetramethoxyazobenzene (TMAB) and β-cyclodextrin functionalization, taking advantage of the well-characterized azobenzene derivative-cyclodextrin inclusion complex to promote photoresponsive, self-contained folding of the polymer in an aqueous system. We use ¹H NMR to show the reversibility of (E)-to-(Z) and (Z)-to-(E) TMAB photoisomerization, and evaluate the thermal stability of (Z)-TMAB and the comparatively rapid acid-catalyzed thermal (Z)-to-(E) isomerization. Important for its potential use as a functional material, we show the photoisomerization cyclability of the polymeric TMAB chromophore and calculate isomerization quantum yields by extinction spectroscopy. To verify self-inclusion of the polymeric TMAB and cyclodextrin, we use two-dimensional ¹H NOESY NMR data to show proximity of TMAB and cyclodextrin in the (E)-state only; however, (Z)-TMAB is not locally correlated with cyclodextrin. Finally, the observed decrease in photoisomerization quantum yield for the dual-functionalized polymer compared to the isolated chromophore in an aqueous solution confirms TMAB and β-cyclodextrin not only are in proximity to one another, but also form the inclusion complex.

100th Anniversary of Macromolecular Science Viewpoint: Opportunities for Liquid Crystal Polymers in Nanopatterning and Beyond

Liquid-crystal polymers (LCPs) integrate at a molecular level the characteristics of two important material classes, i.e., liquid crystals (LCs) and polymers. As a result, they exhibit a wide variety of intriguing physical phenomena and have useful properties in various settings. In the nearly 50 years since the discovery of the first melt-processable LCPs, there has been a remarkable expansion in the field encompassing the development of new chain architectures, the incorporation of new classes of mesogens, and the exploration of new properties and applications. As engineering materials, LCPs are historically best known in the context of high strength fibers. In a more contemporary study, the pairing of LC mesophase assembly with block copolymer (BCP) self-assembly in LC BCPs has resulted in a fascinating interplay of ordering phenomena and rich phase behavior, while lightly cross-linked networks, LC elastomers, are extensively investigated as shape memory materials based on their thermomechanical actuation. As this Viewpoint describes, these and other examples are active areas of research in which new, compelling opportunities for LCPs are emerging. We highlight a few selected areas that we view as being potentially significant in the near future, with a particular emphasis on nanopatterning. Here, the ability to readily access small feature sizes, the fluidity of the LC mesophase, and LC-based handles for achieving orientation control present a compelling combination. Opportunities for LCPs are also presented under the broad rubric of "beyond nanopatterning", and we discuss relevant challenges and potential new directions in the field.

Photoreversible Order-Disorder Transition in an Ionic Liquid Solvated Block Polymer

The structure of a solution of poly(methyl methacrylate)-block-poly(benzyl methacrylate-stat-(4-phenylazophenyl methacrylate)) in the ionic liquid 1-ethyl-3-methyl imidazolium bis(trifluoromethanesulfonylimide) can be controlled with light. We explored the phase behavior of this block polymer at a concentration of 30 wt % as a function of temperature. Remarkably, this solution shows ordering into hexagonally closed packed spheres (HCP) upon heating under visible light, as confirmed by small-angle X-ray scattering. Small amplitude oscillatory shear rheology was used to demonstrate that the system could readily be converted between HCP and disorder by switching between visible and UV light wavelengths at 100 °C. This switching was cycled six times, without any noticeable change in the ordered or disordered state. The dependence on illuminating wavelength arises from the changes in polarity of the azobenzene moiety, which modulates its solubility in this ionic liquid to a significant extent. This is the first example of reversible light-triggered ordering of a "coil-coil" block polymer, a phenomenon that has been previously observed only in liquid crystalline systems.

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.

Azobenzenes and catalysis

Azobenzene is the most extensively used class of chromophore in a large variety of applications.