Surface Glass Transition Temperature Region of Diarylethene Films Determined by Nano-Marangoni Effect

For the last two decades, research has addressed whether the glass transition temperature and the molecular motions on the surface of organic films are significantly different from those inside the bulk glasses. It is reported that the surface of the photochromic diarylethene film prepared by vacuum deposition has fluidity and the vacuum deposition of small amount of rubrene molecules induces surface tension fluctuations, generating dents due to the Marangoni flow in nanoscale. The depth of the dents increases in proportion to these radii for the colorless diarylethene film with a bulk glass transition temperature (Tg) close to room temperature. On the other hand, in the colored diarylethene obtained by UV irradiation to the colorless film, the depth becomes constant at a certain level. The Tg distribution in the depth direction is clarified based on an analysis of the dent depth. By approximating the obtained Tg depth distribution with an exponential function, the outermost surface Tg is about 100 K lower than the bulk Tg in the case of photoisomerized diarylethene.

Automated summary

Research has investigated the differences between the glass transition temperature and molecular motions on the surface of organic films compared to the bulk glasses. It was found that vacuum deposition of photochromic diarylethene film results in fluidity on the surface, and the deposition of small amounts of rubrene molecules causes fluctuations in surface tension, generating nanoscale dents due to Marangoni flow. The depth of the dents increases proportionally to the radii for colorless diarylethene films with a bulk glass transition temperature close to room temperature. However, the depth becomes constant for colored diarylethene films obtained through UV irradiation. The distribution of Tg in the depth direction is determined through the analysis of dent depth, revealing that the outermost surface Tg is about 100 K lower than the bulk Tg for photoisomerized diarylethene.