Haiyan Peng

Spatial and Temporal Control of Thiol-Michael Addition via Photocaged Superbase in Photopatterning and Two-Stage Polymer Networks Formation.

Photochemical processes enable spatial and temporal control of reactions, which can be implemented as an accurate external control approach in both polymer synthesis and materials applications. “Click” reactions have also been employed as efficient tools in the same field. Herein, we combined photochemical processes and thiol-Michael “click” reactions to achieve a “photo-click” reaction that can be used in surface patterning and controlled polymer network formation, owing to the ease of spatial and temporal control through use of photolabile amines as appropriate catalysts.

High modulus and low-voltage driving nematic liquid-crystalline physical gels for light-scattering displays

Liquid-crystalline (LC) physical gels with a high modulus and low driving voltage were prepared through the self-assembly of sorbitol derivatives as gelators in a nematic liquid crystal, 4-pentyl-4′-cyanobiphenyl (5CB). The structural difference among the used gelators, i.e. 1,3:2,4-di-O-benzylidene-D-sorbitol (DBS), 1,3:2,4-di-O-p-methylbenzylidene-D-sorbitol (MDBS) and 1,3:2,4-di-O-m,p-dimethylbenzylidene-D-sorbitol (DMDBS), is only the number of methyl groups on their phenyl rings. The phase transition temperature, mechanical and electro-optical properties of three LC gels were systematically investigated. Compared with DBS, MDBS and DMDBS with methyl groups on phenyl rings have higher gelation ability in 5CB. The three LC gels exhibit good self-supporting ability with storage moduli higher than 104 Pa when the gelator content is increased to 1.5 wt%. At 3.0 wt% and a gelator content less than 1.0 wt%, both moduli of MDBS and DMDBS gels are obviously higher than that of DBS gel due to the enhanced reinforcement of the more rigid, thicker nano-fibrils and the formed nano-fibrillar network texture in MDBS and DMDBS gels. Also, the driving voltages of LC gels decrease in the order of DBS, MDBS and DMDBS gels with increase of LC domain size and nano-fibril diameter. For DMDBS gel with 3.0 wt% gelators, the threshold voltage and saturation voltage are only 0.5 and 3.5 V μm−1, showing its potential application in self-supporting light-scattering electro-optical displays.

Highly diffractive, reversibly fast responsive gratings formulated through holography

Versatile and reversibly rapid responsive one-dimensional photonic crystals with a diffraction efficiency of 97%, which consisted of uniformly 273 ± 48 nm wide liquid crystal belts within transmission holographic polymer dispersed liquid crystal (HPDLC) gratings, were formulated by a facile single step holography based on a hyperbranched monomer. The effect of the monomer average functionality on the photopolymerization kinetics and the electro-optical performances as well as the grating morphologies was investigated. The results show that the low intrinsic viscosity of the hyperbranched monomer accounts for a well-structured morphology in terms of providing a prolonged gelation time for the liquid crystals to diffuse from the light illumination region during the holographic polymerization induced phase separation. Another intriguing observation is that with an increase in the hyperbranched monomer loading, the diffraction efficiency of the HPDLC gratings gradually increases from zero to an average of 94% and then levels off. This is quite different from previous results that gave less than a 50% diffraction efficiency when the monomer average functionality was larger than 4.

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 1 to dopant 4. The β value of chiral dopants in nematic LCs decreases from dopant 1 to dopant 4 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.

Deep eutectic solvents for green and efficient iron-mediated ligand-free atom transfer radical polymerization

A variety of deep eutectic solvents (DESs) were used as novel, green, and intriguing additives for atom transfer radical polymerization (ATRP) of methyl methacrylate with FeBr2 in the absence of any additional ligands. The polymerization processes had a well controlled capability for producing well-defined polymers with predictable molecular weights as well as narrow molecular weight distributions, and the living feature was further confirmed by chain extension experiments. Relative research on species, dosage, and hydrogen bonding interaction between the components of the DESs together with the solubility of the catalyst have been discussed in detail with the help of NMR spectra and cyclic voltammograms. This DESs-tuned ATRP provides us with the unique opportunity to utilize environmentally friendly media and drastically reduce the high costs of common ligands.

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 (AG1 and AG2) 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 AG2 in P0616A, the P0616A/AG1 gels showed better mechanical property. When the gelator content was above 3 wt %, the P0616A/AG1 gels possessed good self-supporting ability with a storage modulus more than 104 Pa. Thus, the photoresponsive electro-optical properties and structures of P0616A/AG1 gels were focused in detail. It was surprising that the electro-optical response speed of the P0616A/AG1 gels could be promoted upon UV irradiation. In particular, the decay time (τoff) 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.

Photomodulated Morphologies in Halogen Bond-Driven Assembly during Gel-Sol Transition

Photoresponsive supramolecular gels with various applications are being constantly pursued; however, achieving well-defined morphology changes of gels via light irradiation remains a formidable challenge. In this study, a gel is prepared through halogen bond between azopyridine-containing Azopy-C10 and 1,4-tetrafluorodiiodobenzene. The gel exhibits gel-sol transition due to trans-cis isomerization of the azopyridine moiety upon UV irradiation. During this transition, the morphologies vary from flake to fluffy bobble-like and finally to peony-like with increasing exposure time, which is difficult to achieve in traditional assembly systems. The microstructure change is attributed to the variations of cis-isomer content and halogen-bonding strength. The supramolecular gel provides a novel method to achieve photomodulated morphologies and broadens the applications of such kind of materials, ranging from information storage to high-tech anticounterfeit.

Liquid crystals under confinement in submicrometer capsules

Liquid crystal (LC) ordering and phase transition behavior under confined conditions have attracted extensive attention and enabled many applications. However, the ordering and phase transition behavior of LCs in submicrometer capsules have seldom been studied, primarily due to the lack of proper capsulizing and visualization approaches to such small LC microcapsules. Herein, we achieve submicrometer LC capsules with the sizes down to 100 nm by using emulsion-based interfacial sol-gel reaction. The behavior of LCs under the submicrometer confinement conditions is investigated while the sizes and chemical composition of the microcapsule shell surface are tuned in a controllable way. The phase transition temperatures of LCs in the submicrometer capsules shift from those of bulk LCs due to the surface-induced ordering of LCs under the strong confinement conditions, which causes formation of topological defects and alters the order parameter. Using nonlinear optical imaging technology, we explore the structures of director field of LCs that arise as a result of the competition between the surface boundary conditions and LC elasticity. The results show that the nanoscale encapsulation can significantly influence the structural configurations of the director and phase transitions of LCs under various confinement conditions.

Low-voltage-driven and highly-diffractive holographic polymer dispersed liquid crystals with spherical morphology

It is a constant pursuit to form highly-diffractive and low-voltage-driven holographic polymer dispersed liquid crystals (HPDLCs) for meeting the requirements of practical applications. Nevertheless, the high-voltage-driven characteristic is usually given while improving the diffraction efficiency of HPDLCs, and it remains a challenge to form HPDLCs with concurrent features of high diffraction and low driving voltage via a simple method. In this work, we synthesize a non-room-temperature LC, 4-butyloxy-4′-cyanobiphenyl (4OCB), and mix it with a room-temperature nematic LC mixture named P0616A. These new LC mixtures are then homogeneously mixed with monomers and a photoinitibitor composed of 3,3′-carbonylbis(7-diethylaminocoumarin) (KCD) and N-phenylglycine (NPG), followed by patterning via laser interference, generating well-structured HPDLCs. The introduction of 4OCB into the standard formulation is found to be able to optimize the morphology and electro-optical properties of the resulting HPDLC transmission gratings. By doping 5 wt% of 4OCB into the HPDLCs, a high diffraction efficiency of 92 ± 3% is obtained; meanwhile, the threshold and saturated voltages significantly decrease by 80.8% (i.e., from 12.0 ± 0.8 to 2.3 ± 0.9 V μm−1) and 73.2% (i.e., from 19.0 ± 0.6 to 5.1 ± 0.7 V μm−1), respectively, in comparison with the pristine. The enhanced performance is believed to be ascribed to the formed larger LC droplets (70 ± 20 nm) and lower interface anchoring strength (0.7 μN m−1) of the polymer network on LCs.

Holographic recording in two-stage networks

We demonstrate holography in a traditional two-component holographic photopolymer in which the solid polymer host matrix has three distinct sets of material properties: 1) an initially liquid state appropriate for formulation and casting into the desired final shape, 2) a rubbery state with low glass transition temperature appropriate for holographic recording, and 3) a final higher modulus state with improved mechanical robustness. The general chemical scheme is to form the second stage rubbery polymer network via a thiol-acrylate Michael addition with an excess of one functional group. Holographic recording then takes place via radically initiated photopolymerization of a mobile high refractive index monomer, per the common two-chemistry process. During final flood illumination of the material, the remaining monomer and excess functional groups are polymerized to increase crosslink density and improve the mechanical properties of the matrix. We described three such material schemes and report general trends. We demonstrate high (96%) efficiency holographic recording, low (1.1%) shrinkage, no oxygen sensitivity and stage 2 glass transition temperatures at or above room temperature, sufficient to enable self-supporting films.