The direct and reversible transformation of matter between the solid and liquid phases by light at constant temperature is of great interest because of its potential applications in various manufacturing settings. We report a simple molecular design strategy for the phase transitions: azobenzenes having para-dialkoxy groups with a methyl group at the meta-position. The photolithography processes were demonstrated using the azobenzene as a photoresist in a single process combining development and etching of a copper substrate.

Multiazobenzene compounds, hexakis-O-[4-(phenylazo)phenoxyalkylcarboxyl]-D-mannitols and hexakis-O-[4-(4-hexylphenylazo)phenoxyalkylcarboxyl]-D-mannitols, exhibit photochemically reversible liquefaction and solidification at room temperature. Their photochemical and thermal phase transitions were investigated in detail through thermal analysis, absorption spectroscopy, and dynamic viscoelasticity measurements, and were compared with those of other sugar-alcohol derivatives. Tensile shear strength tests were performed to determine the adhesions of the compounds sandwiched between two glass slides to determine whether the compounds were suitable for application as adhesives. The adhesions were varied by alternately irradiating the compounds with ultraviolet and visible light to photoinduce phase transitions. The azobenzene hexyl tails, lengths of the methylene spacers, and differences in the sugar-alcohol structures affected the photoresponsive properties of the compounds.

Atom transfer radical polymerization was employed to induce the living polymerization of an azobenzene-containing monomer, 10-[4-(4-hexyl­phenylazo)­phenoxy]­decyl acrylate, with the resulting polyacrylate and the corresponding polymethacrylate undergoing a reversible solid–liquid phase transition under isothermal conditions caused by the photoinduced change of the azobenzene moiety shape. Irradiation-induced property changes were investigated by nuclear magnetic resonance, ultraviolet–visible (UV–vis) absorption spectroscopy, and dynamic viscoelasticity measurements, with focus on the effects of the main chain chemical structure and molecular weight. Azobenzene moiety photoisomerization and the concomitant phase transition were faster for the polyacrylate than for the polymethacrylate, which indicated the strong influence of the main chain structure. Finally, phototuning of the adhesion strength using azopolymer-bonded glass substrates was studied by single lap shear tests, with the maximum adhesion strength of >3 MPa being comparable to that of commercial hot-melt adhesives. Irradiation with UV light for only 15 s lowered the adhesion strength to <0.2 MPa, allowing easy debonding upon the application of a small force.

Liquid‐Crystalline behavior over a wide temperature range and reversible phase transitions are observed for the polymer network 1, which is held together by hydrogen bonds between benzoic acid and pyridyl units

Photocurable adhesives based on polymers and resins are an integral part of different production processes because of their fast curing and local area bonding ability. Recently, dismantlable adhesives have attracted a lot of attention for recycling adherends or replacement of adhesion defects. However, adhesives that allow repeatable bonding and debonding solely by light irradiation, i.e., without heat activation, are lacking. Here, ABA-type triblock copolymers consisting of poly(meth)acrylates bearing an azobenzene moiety (A block) and 2-ethylhexyl (B block) side chains were synthesized and utilized as photocurable adhesives. In contrast to the azo homopolymers, the block copolymer structure and incorporation of the soft middle block actualized a low concentration of the azobenzene moiety and consequently, higher flexibility of the resultant copolymers. This enabled film formation of the azobenzene-based adhesives and light-induced bonding for the first time. On the basis of the photoisomerization of the azobenzene moiety, changes in their viscoelastic property, i.e., softening and hardening, were induced by UV irradiation at 365 nm (50-100 mW cm) and green light irradiation at 520 nm (40 mW cm), respectively. In fact, two glass substrates were bonded with the self-standing polymer film, which was sequentially softened and hardened upon UV and green light irradiations. They exhibited shear strengths of 1.5-2.0 MPa, and UV irradiation lowered the adhesion strength to 0.5-0.1 MPa. Interestingly, the repeatable bonding and debonding abilities of the polymers were accomplished without loss of the adhesion strength.

Supramolecular liquid-crystalline networks have been prepared by self-assembly of multifunctional H-bond donor and acceptor molecules through the formation of intermolecular hydrogen bonds. Two tricarboxylic acids, 1,3,5-tris(2-(2-(4-carboxyphenoxy)ethoxy)ethoxy)benzene (1) and 3,4-bis(2-(2-(4-carboxyphenoxy)ethoxy)ethoxy)benzoic acid (2), have been synthesized for the use as trifunctional H-bond donors. These trifunctional H-bond donors have been complexed with bifunctional H-bond acceptors, such as 4,4‘-bipyridine (3), 1,2-bis(4-pyridyl)ethane (4), trans-1,2-bis(4-pyridyl)ethylene (5), and bis(2-(2-(4-(2-(4-pyridyl)ethenyl)phenoxy)ethoxy)ethyl) ether (6), maintaining a 1:1 donor/acceptor group stoichiometry. All individual components are nonmesogenic. Self-assembly of these multifunctional compounds results in the formation of liquid-crystalline network structures. For example, the H-bonded complex 1/5 shows a smectic A phase from 176 to 156 °C, while 2/ 5 exhibits a nematic phase between 200 and 87 °C on cooling. This behavior is attributed to the dynamics of the hydrogen bonds. These results suggest that the trifunctional H-bond donors adopt linear conformations that induce calamitic mesomorphic behavior with bifunctional H-bond acceptors.

A new type of liquid crystalline side-chain polyacrylate has been built through selective intermolecular hydrogen bonding between H-bond donor and acceptor moieties. Polyacrylate P60BA with a 4-oxybenzoic acid pendant group attached to its side chain through a hexamethylene spacer has been prepared for use as an H-bond donor polymer. A series of frans-4-alkoxy-4'-stilbazoles nOSz having the linear alkyl chain C"H2"+i (n = 1-8 and 10) have been used as H-bond acceptors. Self-assembly of the polymer and the stilbazole results in the selective formation of thermotropic side-chain polymeric complexes having well-defined molecular structures. A mesogenic structure that induces a stable mesophase is formed in the polymer side chain through the single hydrogen bond between the benzoic acid pendant group of the polymer and the pyridyl unit of the stilbazole. All polymeric 1:1 complexes exhibit stable and homogeneous smectic A phases. For example, the complex from P60BA and 20Sz shows a smectic A phase between 89 and 200 °C. The isotropization and melting temperatures show odd-even effects for the terminal alkoxy group. Monomeric 1:1 complexes having the same type of H-bonded mesogen as that of the polymeric complexes have been prepared from 4-(hexyloxy)benzoic acid (60BA) and the series of stilbazoles. For example, the monomeric 1:1 complex from 60BA and 20Sz melts at 112 °C and exhibits smectic A and nematic phases. The smectic A-nematic and nematic-isotropic transitions are observed clearly at 128 and 166 °C, respectively. A binary phase diagram of polymeric complex P60BA-20Sz and monomeric complex 60BA-20Sz shows complete miscibility over the entire range of composition.

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