Ionic nanoporous membranes from self-assembled liquid crystalline brush-like imidazolium triblock copolymers.

There is a need to generate mechanically and thermally robust ionic nanoporous membranes for separation and fuel cell applications. Herein, we report a general approach to the preparation of ionic nanoporous membranes through custom synthesis, self-assembly, and subsequent chemical manipulations of ionic brush block copolymers. We synthesized polynorbornene-based triblock copolymers containing imidazolium cations balanced by counter anions in the central block, side-chain liquid crystalline units, and sidechain polylactide end blocks. This unique platform comprises: (1) imidazolium/bis(trifluoromethanesulfonyl)imide (TFSI) as the middle block, which has an excellent ion-exchange ability, (2) cyanobiphenyl liquid crystalline end block, a sterically hindered hydrophobic segment, which is chemically stable and immune to hydroxide attack, (3) polylactide brush-like units on the other end block that is easily etched under mild alkaline conditions and (4) a polynorbornene backbone, a lightly crosslinked system that offers mechanical robustness. These membranes retain their morphology before and after backbone crosslinking as well as etching of polylactide sidechains. The ion exchange performance and dimensional stability of these membranes were investigated by water uptake capability and swelling ratio. Moreover, the length of the carbon spacer in the imidazolium/TFSI central block moiety endowed the membrane with improved ionic conductivity. The ionic nanoporous materials are unusual due to their singular thermal, mechanical, alkaline stability and ion transport properties. Applications of these materials include electrochemical actuators, solid-state ionic nanochannel biosensors, and ion-conducting membranes.

Self-assembled materials from cellulose nanocrystals conjugated with a thermotropic liquid crystalline moiety.

Manipulating molecular and supramolecular interactions within cellulose nanocrystals (CNCs) to introduce different levels of assemblies combined with multiple functionalities is required for the development of degradable smart materials from renewable resources. To attain hierarchical structures and stimuli-responsive properties, a new class of liquid crystalline cellulosic hybrid materials is synthesized. Herein, main-chain rigid-rod-like oxidized cellulose (CNC-COOH) is prepared from a Cellulose Whatman filter paper (Cellulose W.P.) by acid hydrolysis and oxidized using 2,2,6,6-tetramethyl-1-piperidinyloxy (TEMPO). Thermotropic LC molecule, 4-cyano-4'-hydroxybiphenyl with a 12-methylene spacer (CB12-OH) is grafted onto the carboxylic acid group of CNC-COOH via Steglich esterification. The liquid crystalline functionalized CNCs cellulose nanocrystals (CNC-COO-CB12) are readily soluble in DMSO and ionic liquids. The extent of functionalization and structure of CNC-COO-CB12 are confirmed by solution-state 1H NMR and supported by other characterization techniques. We investigate the interplay of liquid crystalline orientational order of CNCs and cyanobiphenyl (CB12), and the supramolecular hydrogen bonding of CNCs within CNC-COO-CB12 and compare it with CNC-COOH. The introduction of thermotropic CB12 side chains onto rigid-rod CNCs shows the exclusive formation of smectic mesophases from the assemblies of CB12 with the absence of the cholesteric mesophase typically observed from CNC-COOH as verified by temperature-controlled SAXS (T-SAXS). This is further verified by UV-visible and SEM studies that show CNC-COO-CB12 forms smectic domains while CNC-COOH forms a visible light reflecting cholesteric mesophase in dried films. Thus, the interplay of liquid crystalline order of CNCs and CB12 and supramolecular hydrogen bonding of CNCs results in ordered, smectic-mesostructured CNCs for use in stimuli-responsive functional materials.

Dynamic magnetic field alignment and polarized emission of semiconductor nanoplatelets in a liquid crystal polymer.

Reconfigurable arrays of 2D nanomaterials are essential for the realization of switchable and intelligent material systems. Using liquid crystals (LCs) as a medium represents a promising approach, in principle, to enable such control. In practice, however, this approach is hampered by the difficulty of achieving stable dispersions of nanomaterials. Here, we report on good dispersions of pristine CdSe nanoplatelets (NPLs) in LCs, and reversible, rapid control of their alignment and associated anisotropic photoluminescence, using a magnetic field. We reveal that dispersion stability is greatly enhanced using polymeric, rather than small molecule, LCs and is considerably greater in the smectic phases of the resulting systems relative to the nematic phases. Aligned composites exhibit highly polarized emission that is readily manipulated by field-realignment. Such dynamic alignment of optically-active 2D nanomaterials may enable the development of programmable materials for photonic applications and the methodology can guide designs for anisotropic nanomaterial composites for a broad set of related nanomaterials.