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.

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.

Fast Photoswitchable Order-Disorder Transitions in Liquid-Crystalline Block Co-oligomers.

Optically driven ordering transitions are rarely observed in macromolecular systems, often because of kinetic limitations. Here, we report a series of block co-oligomers (BCOs) that rapidly order and disorder at room temperature in response to optical illumination, and the absence thereof. The system is a triblock where rigid azobenzene (Azo) mesogens are attached to each end of a flexible siloxane chain. UV-induced trans-to-cis Azo isomerization, and vice versa in the absence of UV light, drive disordering and ordering of lamellar superstructures and smectic mesophases, as manifested by liquefaction and solidification of the material, respectively. The impacts of chemical structure on BCO self-assembly and photoswitching kinetics are explored by in situ microscopy and X-ray measurements for different mesogen end groups (NO2 or CN), and different carbon chain lengths (0C or 12C) between the siloxane and the mesogen. The presence of the 12C spacer leads to hierarchical ordering with smectic layers of mesogens existing alongside larger length-scale lamellae, versus only smectic ordering without the spacer. These hierarchically ordered BCOs display highly persistent lamellar sheets that contrast with the tortuous, low-persistence "fingerprint"-type structures seen in conventional block copolymers. The reordering kinetics upon removal of UV illumination are extremely rapid (<5 s). This fast response is due to the electron-withdrawing NO2 and CN, which facilitate cis-to-trans isomerization via thermal relaxation at room temperature without additional stimuli. This work elucidates structure-property relationships in photoswitching BCOs and advances the possibility of developing systems in which ordered nanostructures can be easily optically written and erased.

Stabilization and X-ray Attenuation of PEGylated Cholesterol/Polycaprolactone-Based Perfluorooctyl Bromide Nanocapsules for CT Imaging.

Contrast-enhanced X-ray computed tomography plays an important role in cancer imaging and disease progression monitoring. Imaging using radiopaque nanoparticle platforms can provide insights on the likelihood of nanoparticle accumulation and can enable image-guided therapies. Perfluorooctyl bromide (PFOB)-loaded nanocapsules designed for this purpose were stabilized using an in-house synthesized PEGylated polycaprolactone-based copolymer (PEG-b-PCL(Ch)) and compared with commercial polycaprolactone employing a Quality-by-Design approach. PFOB is a dense liquid, weakly polarizable, and immiscible in organic and aqueous solvents; thus, carefully designed formulations for optimal colloidal stabilization to overcome settling-associated instability are required. PFOB-loaded nanocapsules exhibited high PFOB loading due to the intrinsic properties of PEG-b-PCL(Ch). Settling and caking are major sources of instability for PFOB formulations. However, the PEG-b-PCL(Ch) copolymer conferred the nanocapsules enough steric impediment and polymer shell elasticity to settle without significant caking, increasing the overall colloidal stability of the formulation. Furthermore, a clear relationship between nanocapsule physical properties and X-ray attenuation was established. Nanocapsules were able to enhance the X-ray contrast in vitro as a function of PFOB loading. This nanocapsule-based platform is promising for future translational studies and image-guided tumor therapy due to its enhanced contrastability and optimal colloidal stability.

Cholesteric mesophase based 1D photonic materials from self-assembly of liquid crystalline block and random terpolymers containing chromonic molecules.

We describe the influence of competing self-organizing phenomena on the formation of cholesteric mesophase in liquid crystalline brush block terpolymers (LCBBTs) and liquid crystalline random brush terpolymers (LCRBTs) containing chromonic molecules. A library of LCBBTs and LCRBTs are synthesized using ring-opening metathesis polymerization (ROMP) of norbornene side-chain functionalized monomers comprising cholesteryl mesogen (NBCh9), chromonic xanthenone (NBXan), and poly(ethylene glycol) (NBMPEG). Compression molded films of LCRBTs containing chromonic molecules display multilevel hierarchical structure in which cholesteric mesophase co-exists with π-π stacking of the chromonic mesophase along with PEG microphase segregated domains. This is unexpected as conventional LCBCPs and LCBBCs that lack chromonic molecules do not form cholesteric mesophases. The presence of π-π interactions modifies the interface at the IMDS so that both chromonic and cholesteric mesophases coexist leading to the manifestation of cholesteric phase for the first time within block architecture and is very reminiscent of previously published LCRBCs without chromonic molecules. The key to the observed hierarchical assembly in these LCBBTs containing chromonic molecules lies in the interplay of LC order, chromonic π-π stacking, PEG side chain microphase segregation, and their supramolecular cooperative motion. This unique "single component" polymer scaffold transforms our capacity to attain nanoscale hierarchies and optical properties from block architecture similar to nanoscale mesophases resulting in random architecture.