Photo-triggered large mass transport driven only by a photoresponsive surface skin layer

Issei Kitamura,Mitsuo Hara,Shusaku Nagano,Keisuke Kato,Takashi Nakai,Rafael Benjamin Berk,Takahiro Seki

doi:10.1038/s41598-020-69605-8

Issei Kitamura, Mitsuo Hara + Show 5 more

Open Access

https://doi.org/10.1038/s41598-020-69605-8

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Abstract

Since the discovery 25 years ago, many investigations have reported light-induced macroscopic mass migration of azobenzene-containing polymer films. Various mechanisms have been proposed to account for these motions. This study explores light-inert side chain liquid crystalline polymer (SCLCP) films with a photoresponsive polymer only at the free surface and reports the key effects of the topmost surface to generate surface relief gratings (SRGs) for SCLCP films. The top-coating with an azobenzene-containing SCLCP is achieved by the Langmuir–Schaefer (LS) method or surface segregation. A negligible amount of the photoresponsive skin layer can induce large SRGs upon patterned UV light irradiation. Conversely, the motion of the SRG-forming azobenzene SCLCP is impeded by the existence of a LS monolayer of the octadecyl side chain polymer on the top. These results are well understood by considering the Marangoni flow driven by the surface tension instability. This approach should pave the way toward in-situ inscription of the surface topography for light-inert materials and eliminate the strong light absorption of azobenzene, which is a drawback in optical device applications.

Highlights

Since the discovery 25 years ago, many investigations have reported light-induced macroscopic mass migration of azobenzene-containing polymer films
This study demonstrated the essential contribution of the topmost surface to the macroscopic mass transport process in side chain liquid crystalline polymer (SCLCP) films
The presence of only a molecular level skin layer at the free surface is sufficient to promote or terminate a large-scale surface deformation. These results are well understood by the Marangoni flow driven by the light-triggered surface tension instability

Summary

Introduction

Since the discovery 25 years ago, many investigations have reported light-induced macroscopic mass migration of azobenzene-containing polymer films. The motion of the SRG-forming azobenzene SCLCP is impeded by the existence of a LS monolayer of the octadecyl side chain polymer on the top These results are well understood by considering the Marangoni flow driven by the surface tension instability. The above knowledge inspired us to examine light-driven mass transport for “light-inert” SCLCP films with only a photoresponsive layer at the free surface and a photoresponsive SCLCP films covered with a light-inert molecular layer Such a study should provide decisive evidence to extract the effect of the topmost surface. The existence of 1–2-nm-thick layer of the octadecyl side chain polymer on the top of the film almost fully hinders transport motions (Fig. 1c) These observations highlight the critical role of the topmost surface in the macroscopic mass transfer of polymer films. This work demonstrates the technological importance in the microfabrication of materials without photoreactive units

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