Phase Separation Kinetics of Polymer Dispersed Liquid Crystals Confined between Two Parallel Walls

Nonelectric Sustaining Bistable Polymer Framework Liquid Crystal Films with a Novel Semirigid Polymer Matrix

In this work, a bistable polymer framework liquid crystal (PFLC) thin film by thermal curing of epoxy monomers with two different thiols, a traditional flexible-structure thiol and a novel original rigid-structure thiol, has been successfully fabricated, combining a novel mixed morphology of polymer matrix and cholesteric liquid crystals with negative dielectric anisotropy. The polymer framework morphology has been formed by curing two types of epoxy monomers with two types of thiols, and the liquid crystals tend to be focal conic textures with large size domains at the initial state in the PFLC film so that it has a moderate light transmissivity at this state between the transparent state and the opaque state. Thus, the devices based on PFLC films can be switched reversibly between the transparent state and the opaque state by alternative electric field. In addition, the states can be sustained after the electric field is removed. The bistable memory effect comes from the anchoring effects of the polymer frameworks with a novel morphology in the microdomains of the PFLCs. Therefore, the optimized bistable PFLC film keeps its initial state without external electric field and any other energy consumption for a long time after altering the state by applying an instant electric field. The special polymer frameworks in the bistable PFLC films endow the films with excellent electro-optical properties and mechanical properties. The devices are energy-efficient and cost-saving and have great potential applications in energy-efficient reflective displays, electronic papers, writing tablets, and smart windows.

Through-Thickness Vertically Ordered Lamellar Block Copolymer Thin Films on Unmodified Quartz with Cold Zone Annealing

Template-free directed self-assembly of ultrathin (approximately tens of nanometers) lamellar block copolymer (l-BCP) films into vertically oriented nanodomains holds much technological relevance for the fabrication of next-generation devices from nanoelectronics to nanomembranes due to domain interconnectivity and high interfacial area. We report for the first time the formation of full through-thickness vertically oriented lamellar domains in 100 nm thin polystyrene-block-poly(methyl methacrylate) (PS-b-PMMA) films on quartz substrate, achieved without any PMMA-block wetting layer formation, quartz surface modification (templating chemical, topographical) or system modifications (added surfactant, top-layer coat). Vertical ordering of l-BCPs results from the coupling between a molecular and a macroscopic phenomenon. A molecular relaxation induced vertical l-BCP ordering occurs under a transient macroscopic vertical strain field, imposed by a high film thermal expansion rate under sharp thermal gradient cold zone annealing (CZA-S). The parametric window for vertical ordering is quantified via a coupling constant, C (= v∇T), whose range is established in terms of a thermal gradient (∇T) above a threshold value, and an optimal dynamic sample sweep rate (v ∼ d/τ), where τ is the l-BCP's longest molecular relaxation time and d is the T_g,heat - T_g,cool distance. Real-time CZA-S morphology evolution of vertically oriented l-BCP tracked along ∇T using in situ grazing incidence small angle X-ray scattering (GISAXS) exhibited an initial formation phase of vertical lamellae, a polygrain structure formation stage, and a grain coarsening phase to fully vertically ordered l-BCP morphology development. CZA-S is a roll-to-roll manufacturing method, rendering this template-free through-thickness vertical ordering of l-BCP films highly attractive and industrially relevant.

Multiple Bragg Diffractions with Different Wavelengths and Polarizations Composed of Liquid Crystal/Polymer Periodic Phases

We first fabricated holographic polymer-dispersed liquid crystals (HPDLCs) that produce multiple Bragg diffractions with different polarization states for every angle of incidence, through a photopolymerization-induced phase separation by one-time interferential exposure. The polarizations of the Bragg diffractions were well-controlled at individual wavelengths in the fabrication process by the compositional ratio of LCs to monomers. The raw mixtures of extremely low-functionality monomers having very different viscosities were used to reduce the domain size in phase separation and subsequently to form elaborate periodic structures of the LC and polymer phases. A cross-linker (1-vinyl-2-pyrrolidione) and a prepolymer with urethane groups were employed to strengthen the polymer network. Note that the diffractions of our HPDLCs are regarded as not purely but mostly Bragg type, according to the evaluation with the established criteria. The devices, which are monolithic but versatile in diffractive behaviors, have advantages of simple manufacturing and handling.

Liquid crystal size selection of large-size graphene oxide for size-dependent N-doping and oxygen reduction catalysis

Graphene oxide (GO) is aqueous-dispersible oxygenated graphene, which shows colloidal discotic liquid crystallinity. Many properties of GO-based materials, including electrical conductivity and mechanical properties, are limited by the small flake size of GO. Unfortunately, typical sonochemical exfoliation of GO from graphite generally leads to a broad size and shape distribution. Here, we introduce a facile size selection of large-size GO exploiting liquid crystallinity and investigate the size-dependent N-doping and oxygen reduction catalysis. In the biphasic GO dispersion where both isotropic and liquid crystalline phases are equilibrated, large-size GO flakes (>20 μm) are spontaneously concentrated within the liquid crystalline phase. N-Doping and reduction of the size-selected GO exhibit that N-dopant type is highly dependent on GO flake size. Large-size GO demonstrates quaternary dominant N-doping and the lowest onset potential (-0.08 V) for oxygen reduction catalysis, signifying that quaternary N-dopants serve as principal catalytic sites in N-doped graphene.

Phase separation of polymer-dispersed liquid crystals on a chemically patterned substrate

The surface-induced structure formation of polymer-dispersed liquid crystals (PDLCs) on a chemically patterned substrate was studied for the first time. The patterns on the substrate were successfully transferred to the PDLC film, resulting in alternating LC-rich and polymer-rich phases. This simple approach offers a new means of organizing micrometer-sized LC domains into well-ordered structures in a polymer matrix of PDLCs.

Wetting-layer formation mechanisms of surface-directed phase separation under different quench depths with off-critical compositions in polymer binary mixture

Focusing on the off-critical condition, the quench depth dependence of surface-directed phase separation in the polymer binary mixture is numerically investigated by combination of the Cahn-Hilliard-Cook theory and the Flory-Huggins-de Gennes theory. Two distinct situations, i.e., for the wetting, the minority component is preferred by the surface and the majority component is preferred by the surface, are discussed in detail. The simulated results show that the formation mechanism of the wetting layer is affected by both the quench depth and the off-critical extent. Moreover, a diagram, illustrating the formation mechanisms of the wetting layer with various quench depths and compositions, is obtained on the basis of the simulated results. It is found that, when the minority component is preferred by the surface, the growth of the wetting layer can exhibit pure diffusion limited growth law, logarithmic growth law, and Lifshitz-Slyozov growth law. However, when the majority component is preferred by the surface, the wetting layer always grows logarithmically, regardless of the quench depth and the off-critical extent. It is interesting that the surface-induced nucleation can be observed in this case. The simulated results demonstrate that the surface-induced nucleation only occurs below a certain value of the quench depth, and a detailed range about it is calculated and indicated. Furthermore, the formation mechanisms of the wetting layer are theoretically analyzed in depth by the chemical potential gradient.