Phase Behaviors and Photoresponsive Thin Films of Syndiotactic Side-Chain Liquid Crystalline Polymers with High Densely Substituted Azobenzene Mesogens

Azobenzene-containing polymers (azopolymers) are a kind of fascinating stimuli-responsive materials with broad and versatile applications. In this work, a series of syndiotactic C1 type azopolymers of Pm-Azo-Cn with side-chain azobenzene mesogens of varied length alkoxy tails (n=1, 4, 8, 10) and different length alkyl spacers (m=6, 10) have been prepared via Rh-catalyzed carbene polymerization. The thermal properties and ordered assembly structures of thus synthesized side chain liquid crystalline polymers (SCLCPs) have been systematically investigated with differential scanning calorimetry (DSC), polarized optical microscopy (POM) and variable-temperature small/wide-angle X-ray scattering (SAXS/WAXS) analyses. P10-Azo-C1 and P10-Azo-C4 with shorter alkoxy tails exhibited hierarchical structures SmB/Colob and transformed into SmA/Colob at a higher temperature, while P10-Azo-C8 and P10-Azo-C10 with longer alkoxy tails only displayed side group dominated layered SmB phase and transformed into SmA phase at higher temperatures. For P6-Azo-C4 with a shorter spacer only showed a less ordered SmA phase owing to interference by partly coupling between the side chain azobenzene mesogens and the helical backbone. More importantly, the series high densely substituted syndiotactic C1 azopolymer thin films, exhibited evidently and smoothly reversible photoresponsive properties, which demonstrated promising photoresponsive device applications.

Synthesis and Photoinduced Anisotropy of Polymers Containing Nunchaku-Like Unit with an Azobenzene and a Mesogen

A series of polymers containing nunchaku-like unit with an azo chromophore and a mesogen group was successfully prepared and photoinduced anisotropy of the obtained polymers was minutely investigated. Firstly, monomers containing nunchaku-like unit with an azo chromophore and a mesogen group linked by flexible group were synthesized. The structure of the monomers was confirmed via NMR COSY spectra. Subsequently, the obtained monomers were polymerized into corresponding polymers through RAFT polymerization. The prepared polymer samples were characterized through NMR, FTIR, gel permeation chromatography (GPC), and UV-vis testing while the thermal properties of the samples were investigated through differential scanning calorimeter (DSC) and thermogravimetric analysis (TGA) measurements. The photoinduced isomerization of the polymers, which was researched in situ via measuring UV-vis spectra of the polymer solutions and spin-coated films under irradiation with 450 nm light or putting in darkness, demonstrated the rapid trans-cis-trans isomerization of the polymers. When irradiated with a linearly polarized light, significant photoinduced birefringence and dichroism were observed, suggesting photoinduced isomerization of azobenzene can drive orientation of mesogen in the system. This study blazes a way to design the optical materials with light-controllable birefringence and dichroism.

Macrocycle-based topological azo-polymers: facile synthesis and unusual photoresponsive properties

Macrocycle-based topological azo-polymers with unusual photosensitive properties were synthesized via a selective acyclic diene metathesis polymerization of different monomers using an acrylate-functionalized cyclic azo-polymer as a chain stopper prepared from a linear precursor by “click” cyclization reaction, which will open a new perspective in photoinduced materials.

In Situ Photocontrol of Block Copolymer Morphology During Dip-Coating of Thin Films

We demonstrate a unique combination of simultaneous top-down and bottom-up control of the morphology of block copolymer films by application of in situ optical irradiation during dip-coating. A light-addressable and block-selective small molecule, 4-butyl-4'-hydroxyazobenzene (BHAB), is introduced into a diblock copolymer of polystyrene and poly(4-vinylpyridine) (PS-P4VP) of 28.4 wt % P4VP via supramolecular chemistry, notably by hydrogen bonding to P4VP. We show that the spherical morphology of thin films dip-coated from a THF solution at slow withdrawal rates in the dark convert to cylindrical morphology when dip-coated under illumination. This is attributed to volume expansion of the P4VP/BHAB phase due to trans-cis photoisomerization combined with a light-induced increase in BHAB uptake in the film. The demonstrated photocontrol highlights the potential of dip-coating as a scalable film preparation method that can be easily coupled with external stimuli to direct nanostructured self-assembly in the films as solvent evaporates.