Balancing Compatibility and Gelability for High-Performance Cholesteric Liquid Crystalline Physical Gels

Liquid crystalline physical gels (LCPGs) have attracted increasing interest because of their mechanical properties and stimulus–response behaviors. However, due to their gelator properties such as thermal stability, gelation capability, and compatibility in liquid crystals, development of LCPGs with high performances still remains a huge challenging task. Herein, four novel gelators ((l)-PH, (d)-PH, (l)-P2H, and (d)-P2H) based on 1,4-benzenedicarboxamide phenylalanine derivatives containing one or two ethylene glycol groups have been designed and synthesized. It is found that the ethylene glycol group plays a significant role in improving the compatibility between the gelator and the liquid crystal. All of the prepared compounds can form stable LCPGs in P0616A. In particular, the storage modulus of LCPG with 9.0 wt % of (l)-PH with one ethylene glycol unit is higher than 106 Pa, which is similar to SmC gels and advantageous over previously reported nematic LCPGs. Furthermore, the prepared gels display a strong Cotton effect with hand-preferred twisted fiber networks and the self-assembled aggregates of (l)-PH can induce P0616A to form a cholesteric fingerprint structure. Thus, these low molecular weight gelators provide a strategy to construct high-performance cholesteric LCPGs for the realization of LC device applications

3D Image Storage in Photopolymer/ZnS Nanocomposites Tailored by “Photoinitibitor”

We synthesize zinc sulfide (ZnS) nanoparticles with a diameter of ∼5 nm and formulate novel photopolymer/ZnS nanocomposites for holographic recording. By taking advantage of the photoinitibitor, composed of 3,3′-carbonylbis­(7-diethylaminocoumarin) (KCD) and <i>N</i>-phenylglycine (NPG), with a capability of spatiotemporally tailoring the grating formation process, we successfully achieve high performance holographic photopolymer/ZnS nanocomposites with as high as 93.6% of diffraction efficiency (η), 26.6 × 10^–3 of refractive index modulation (<i>n</i>₁), 8.4 per 200 μm of dynamic range, and 9.8 cm/mJ of photosensitivity. In addition, for an aim of roughly describing the grating formation process, we establish a novel exponential correlation between the ZnS nanoparticles segregation degree (SD) and the ratio of photopolymerization gelation time (<i>t</i>_gel) to holographic mixture viscosity (<i>v</i>). Finally, we reconstruct and display 3D images that are clearly identifiable to the naked eye through a master technique, opening a versatile class of potential applications in high capacity data storage, stereoadvertisements, and anticounterfeiting

Photomechanically Controlled Encapsulation and Release from pH-Responsive and Photoresponsive Microcapsules

Poly­(acrylic acid)/azobenzene microcapsules were obtained through distillation precipitation polymerization and the selective removal of silica templates by hydrofluoric acid etching. The uniform, robust, and monodisperse microcapsules, confirmed by transmission electron microscopy and scanning electron microscopy, had reversible photoisomerization under ultraviolet (UV) and visible light. Under UV irradiation, azobenzene cross-linking sites in the main chain transformed from the trans to cis isomer, which induced the shrinkage of microcapsules. These photomechanical effects of azobenzene moieties were applied to the encapsulation and release of model molecules. After loading with rhodamine B (RhB), the release behaviors were completely distinct. Under steady UV irradiation, the shrinkage adjusted the permeability of the capsule, providing a novel way to encapsulate RhB molecules. Under alternate UV/visible light irradiation, a maximal release amount was reached due to the continual movement of shell networks by cyclic trans–cis photoisomerization. Also, microcapsules had absolute pH responsiveness. The diffusion rate and the final release percentage of RhB both increased with pH. The release behaviors under different irradiation modes and pH values were in excellent agreement with the Baker–Lonsdale model, indicating a diffusion-controlled release behavior. Important applications are expected in the development of photocontrolled encapsulation and release systems as well as in pH-sensitive materials and membranes

Photomodulated Electro-optical Response in Self-Supporting Liquid Crystalline Physical Gels

Photoresponsive liquid crystal (LC) physical gels have attracted more and more attention because of the nature of strong response via light stimulus. Although many efforts on the breaking and recovering of physical gels through photoisomerization have been focused, fast electro-optical response and high mechanical properties even upon light irradiations are difficult to achieve at the same time. In this work, two kinds of azobenzene-containing gelators (<b>AG1</b> and <b>AG2</b>) with different terminal groups were designed and synthesized. Both gelators could induce the nematic LC P0616A self-assemble into anisotropic phase-separated LC physical gels at low contents. Their phase-transition behavior, thermal stability, microstructure, and mechanical strength were systematically studied. Compared with <b>AG2</b> in P0616A, the P0616A/<b>AG1</b> gels showed better mechanical property. When the gelator content was above 3 wt %, the P0616A/<b>AG1</b> gels possessed good self-supporting ability with a storage modulus more than 10⁴ Pa. Thus, the photoresponsive electro-optical properties and structures of P0616A/<b>AG1</b> gels were focused in detail. It was surprising that the electro-optical response speed of the P0616A/<b>AG1</b> gels could be promoted upon UV irradiation. In particular, the decay time (τ_off) was only about half when compared with the initial state, whereas the gels still exhibited good self-supporting ability; also the network of the LC physical gels had no change at macro- and microstructural levels. These exciting results would open a door for the application of this material in electro-optical devices

Ultralow-Carbon Nanotube-Toughened Epoxy: The Critical Role of a Double-Layer Interface

Understanding the chemistry and structure of interfaces within epoxy resins is important for studying the mechanical properties of nanofiller-filled nanocomposites as well as for developing high-performance polymer nanocomposites. Despite the intensive efforts to construct nanofiller/matrix interfaces, few studies have demonstrated an enhanced stress-transferring efficiency while avoiding unfavorable deformation due to undesirable interface fractures. Here, we report an optimized method to prepare epoxy-based nanocomposites whose interfaces are chemically modulated by poly­(glycidyl methacrylate)-<i>block</i>-poly­(hexyl methacrylate) (PGMA-<i>b</i>-PHMA)-functionalized multiwalled carbon nanotubes (bc@fMWNTs) and also offer a fundamental explanation of crack growth behavior and the toughening mechanism of the resulting nanocomposites. The presence of block copolymers on the surface of the MWNT results in a promising double-layered interface, in which (1) the outer-layered PGMA segment provides good dispersion in and strong interface bonding with the epoxy matrix, which enhances load transfer efficiency and debonding stress, and (2) the interlayered rubbery PHMA segment around the MWNT provides the maximum removable space for nanotubes as well as triggering cavitation while promoting local plastic matrix deformation, for example, shear banding to dissipate fracture energy. An outstanding toughening effect is achieved with only a 0.05 wt % carbon nanotube loading with the bc@fMWNT, that is, needing only a 20-times lower loading to obtain improvements in fracture toughness comparable to epoxy-based nanocomposites. The enhancements of their corresponding ultimate mode-I fracture toughnesses and fracture energies are 4 times higher than those of pristine MWNT-filled epoxy. These results demonstrate that a MWNT/epoxy interface could be optimized by changing the component structure of grafted modifiers, thereby facilitating the transfer of both mechanical load and energy dissipation across the nanofiller/matrix interface. This work provides a new route for the rational design and development of polymer nanocomposites with exceptional mechanical performance

Hierarchical Hybrids of Carbon Nanotubes in Amphiphilic Poly(ethylene oxide)-<i>block</i>-polyaniline through a Facile Method: From Smooth to Thorny

A facile approach was developed to synthesize conjugated block copolymer (BCP) poly­(ethylene oxide)-<i>b</i>-polyaniline (PEO–PANI). Aldehyde group-terminated PEO was prepared by an esterification reaction of <i>p</i>-formylbenzoic acid and PEO and then reacted with PANI from chemical oxidative polymerization. FT-IR, ¹H NMR, and GPC results indicated that BCPs with different PEO block lengths were successfully synthesized. Moreover, the BCPs were employed to noncovalently modify multiwalled carbon nanotubes (MWNTs) through either the direct or indirect method. In the former method, transmission electron microscopy images showed that a core–shell MWNT@BCP hybrid with a shell thickness of gyration diameter of PEO block (2<i>R</i>_g,PEO) was obtained in 1-methyl-2-pyrrolidone (NMP). These hybrids can be well dispersed in many common solvents and poly­(vinyl alcohol) matrix. With the increase of PEO block length, the stability of the MWNT dispersion would be highly improved. Interestingly, in the indirect method where deionized water was added to the NMP solution of BCP/MWNT mixture, the surface of the hybrid micelles encapsulated with MWNTs changed from smooth into hierarchically thorny with the increase of BCP/MWNT weight ratio. In this case, the water contact angle had a minimum value of ∼70° at the ratio of 1/8, indicating that the hierarchical thorns followed a Cassie–Baxter regime rather than a Wenzel one. A possible formation mechanism of the unique structure was also proposed

Photoinitiation and Inhibition under Monochromatic Green Light for Storage of Colored 3D Images in Holographic Polymer-Dispersed Liquid Crystals

Holographic photopolymer composites have garnered a great deal of interest in recent decades, not only because of their advantageous light sensitivity but also due to their attractive capabilities of realizing high capacity three-dimensional (3D) data storage that is long-term stable within two-dimensional (2D) thin films. For achieving high performance holographic photopolymer composites, it is of critical importance to implement precisely spatiotemporal control over the photopolymerization kinetics and gelation during holographic recording. Though a monochromatic blue light photoinitibitor has been demonstrated to be useful for improving the holographic performance, it is impractical to be employed for constructing holograms under green light due to the severe restriction of the First Law of Photochemistry, while holography under green light is highly desirable considering the relatively low cost of laser source and high tolerance to ambient vibration for image reconstruction. Herein, we disclose the concurrent photoinitiation and inhibition functions of the rose bengal (RB)/<i>N</i>-phenylglycine (NPG) system upon green light illumination, which result in significant enhancement of the diffraction efficiency of holographic polymer-dispersed liquid crystal (HPDLC) gratings from zero up to 87.6 ± 1.3%, with an augmentation of the RB concentration from 0.06 × 10^–3 to 9.41 × 10^–3 mol L^–1. Interestingly, no detectable variation of the ϕ^1/2<i>k</i>_p/<i>k</i>_t^1/2, which reflects the initiation efficiency and kinetic constants, is given when increasing the RB concentration. The radical inhibition by RBH^• is believed to account for the greatly improved phase separation and enhanced diffraction efficiency, through shortening the weight-average polymer chain length and subsequently delaying the photopolymerization gelation. The reconstructed colored 3D images that are easily identifiable to the naked eye under white light demonstrate great potential to be applied for advanced anticounterfeiting

Precisely Tuning Helical Twisting Power via Photoisomerization Kinetics of Dopants in Chiral Nematic Liquid Crystals

It has been paid much attention to improve the helical twisting power (β) of dopants in chiral nematic liquid crystals (CLCs); however, the correlations between the β value and the molecular structures as well as the interaction with nematic LCs are far from clear. In this work, a series of reversibly photo-switchable axially chiral dopants with different lengths of alkyl or alkoxyl substituent groups have been successfully synthesized through nucleophilic substitution and the thiol–ene click reaction. Then, the effect of miscibility between these dopants and nematic LCs on the β values, as well as the time-dependent decay/growth of the β values upon irradiations, has been investigated. The theoretical Teas solubility parameter shows that the miscibility between dopants and nematic LCs decreases with increasing of the length of substituent groups from dopant <b>1</b> to dopant <b>4</b>. The β value of chiral dopants in nematic LCs decreases from dopant <b>1</b> to dopant <b>4</b> both at the visible light photostationary state (PSS) and at the UV PSS after UV irradiation. With increasing of the length of substituent groups, the photoisomerization rate constant of dopants increases for trans–cis transformation upon UV irradiation and decreases for the reverse process upon visible light irradiation either in isotropic ethyl acetate or in anisotropic LCs, although the constant in ethyl acetate is several times larger than the corresponding value in LCs. Also, the color of the CLCs could be tuned upon light irradiations. These results enable the precise tuning of the pitch and selective reflection wavelength/color of CLCs, which paves the way to the applications in electro-optic devices, information storage, high-tech anticounterfeit, and so forth

Monochromatic Visible Light “Photoinitibitor”: Janus-Faced Initiation and Inhibition for Storage of Colored 3D Images

Controlling the kinetics and gelation of photopolymerization is a significant challenge in the fabrication of complex three-dimensional (3D) objects as is critical in numerous imaging, lithography, and additive manufacturing techniques. We propose a novel, visible light sensitive “photoinitibitor” which simultaneously generates two distinct radicals, each with their own unique purpose–one radical each for initiation and inhibition. The Janus-faced functions of this photoinitibitor delay gelation and dramatically amplify the gelation time difference between the constructive and destructive interference regions of the exposed holographic pattern. This approach enhances the photopolymerization induced phase separation of liquid crystal/acrylate resins and the formation of fine holographic polymer dispersed liquid crystal (HPDLC) gratings. Moreover, we construct colored 3D holographic images that are visually recognizable to the naked eye under white light

Monochromatic Visible Light “Photoinitibitor”: Janus-Faced Initiation and Inhibition for Storage of Colored 3D Images

Controlling the kinetics and gelation of photopolymerization is a significant challenge in the fabrication of complex three-dimensional (3D) objects as is critical in numerous imaging, lithography, and additive manufacturing techniques. We propose a novel, visible light sensitive “photoinitibitor” which simultaneously generates two distinct radicals, each with their own unique purpose–one radical each for initiation and inhibition. The Janus-faced functions of this photoinitibitor delay gelation and dramatically amplify the gelation time difference between the constructive and destructive interference regions of the exposed holographic pattern. This approach enhances the photopolymerization induced phase separation of liquid crystal/acrylate resins and the formation of fine holographic polymer dispersed liquid crystal (HPDLC) gratings. Moreover, we construct colored 3D holographic images that are visually recognizable to the naked eye under white light