Phototriggered Mass Migration Consorted with Surface Dewetting in Thin Films of a Liquid Crystalline Azobenzene-containing Dendrimer

Abstract

A dendritic molecule of poly(propylene imine) whose periphery is modified with photoresponsive mesogenic azobenzene units was synthesized. This compound exhibited highly ordered smectic B or smectic A layer structure orienting parallel to the substrate in the film state. When this film of 90 nm thickness on a hydrophilic substrate was exposed to UV (365 nm) light, the initial flat film morphology started to form holes followed by a drastic transformation to form separated dome structure of micrometer levels. Upon continuous UV light irradiation, the domain height increased and reached to a level of ca. 8-fold higher than that of the initial thickness, which can be correlated with the disordering of layer structures as revealed by grazing angle incidence X-ray diffraction measurements. This phototriggered dewetting behavior depended on the film thickness. The thinner film resulted in local migrations to form the smaller and more homogeneous domes. The dewetting was observed below a threshold thickness of 100 nm. By UV light irradiation through a photomask, phototriggered migration at micrometer distances also occurred. By the combination of phototriggered migration and concerted dewetting, characteristic hierarchical morphologies were formed, depending on the initial film thickness. This work proposes a new possibility to control and design the dewetting processes of thin films using the dendritic material.

Optically driven translational and rotational motions of microrod particles in a nematic liquid crystal

“…We used a commercially available 5CB liquid crystal (Sigma-Aldrich) mixed with 0.1 wt% of poly(propyleneimine)-based azo-dendrimers (32). Glass microrods of a diameter 1.49 ± 0.07 μm and lengths between 10 μm and 25 μm (PF-15S, Nippon Electric Glass) were mixed with the LC.…”

“…Li et al recently synthesized a photosensitive dendrimer with azo linkages (azo-dendrimer) in the mesogenic end chains (Fig. S1) and demonstrated the adsorption of the dendrimer at a glass interface (32). By UV irradiation, an orientational change of the mesogenic moieties occurs associated with the trans−cis photoisomerization, as illustrated in Fig.…”

Meso‐ and Microscopic Motions in Photoresponsive Liquid Crystalline Polymer Films

“…Yonetake et al synthesized and evaluated the LC properties of poly(propylene imine) dendrimers peripherally modified with cyanobiphenyl mesogens. Utilizing this framework, Li et al showed that particular photoinduced surface dewetting takes place in a film of a relevant dendritic azobenzene derivative on a hydrophilic surface upon UV light irradiation (Table , 41 ). Interestingly, when the film was subjected to mass migration with patterned UV light, consorted motions of mass transfer and surface dewetting provided hierarchical relief structures.…”

New strategies and implications for the photoalignment of liquid crystalline polymers

“…Photochromic reactions have frequently been incorporated in LC media because of their repeatability for controlling material properties. This approach has provided various types of smart, light-responsive materials 1 exhibiting surface-mediated photoalignment of LC materials, [2][3][4][5][6] photoinduced phase transitions, [7][8][9][10][11][12] photoorientation/addressing of polymer thin films, [13][14][15][16][17][18] photoinduced mass migrations, [19][20][21][22][23][24][25][26][27][28] phototactic sliding motions, [29][30][31] photo-driven motions and morphology of monolayers, [32][33][34][35] and macroscopic photomechanical deformations. [36][37][38][39][40][41][42][43][44][45][46] Photoalignment research and technology started in 1988 with the discovery of the reversible alignment control of nematic LCs by the photoisomerization of azobenzene on a substra...…”