Sol-gel derived inorganic-organic hybrid materials (HYBRIMERs) exhibit photosensitivity, which refers to any permanent refractive index and volume change by Ultra Violet (UV) exposure. This photosensitivity in HYBRIMERs is closely related to the various photo-induced reactions such as photo-polymerization, photodecomposition, and photo-densification. These various photosensitive mechanisms due to the presence of polymer and silica in the HYBRIMER structure are highly dependent on the UV wavelength.
This work focused especially on the change in photosensitivity of HYBRIMER depending on UV wavelength. Photosensitivity in the HYBRRIMER having photopolymerizable organic group was investigated.
For the close examination of photosensitivity mechanisms, we studied the photoinduced behaviors of a germanium-doped methacrylate HYBRIMER depending UV wavelength. IRGACURE 369 was used as a photo-initiator. UV light irradiation (220-260nm) was performed with an Oriel 82511 Hg/Xe lamp and 350-375nm, an Oriel 97345 Hg lamp. Both of the power densities were $0.139J/cm^2$. The changes of refractive index and volume were measured with a prism coupler. The structural change of the hybrid materials upon UV light irradiation were confirmed by FT-IR spectroscopy and the optical absorption, UV-Visible spectrophotometer.
The HYBRIMER exhibited considerable photosensitivity, which is an increase in refractive index accompanying a volume contraction in UV wavelength below 260nm. It was demonstrated that this large change in refractive index and volume was caused by photo-induced polymerization and densification. As for photo-polymerization, two important factors, molecular oxygen and protic solvent, influencing free radical polymerization were studied. The germanium-doped methacrylate HYBRIMER can be polymerized only by UV photon energy, which is similar behavior to commercial polymer, even without adding photo-initiator. Owing to this phenomenon, the detrimental effect of molecular oxygen and protic solvent on free radical polymerization can be insignificant.
On the other hand, in UV wavelength over 350nm, the germanium-doped methacrylate HYBRIMER showed photo-polymerization only by photo-initiator, so that molecular oxygen and protic solvent act as an important inhibitor in photopolymerization behavior of this HYBRIMER. Moreover, the HYBRIMER showed no photo-induced densification and decomposition of methacrylate.
Due to the high photosensitivity in deep UV wavelength, this germanium-doped methacrylate HYBRIMER can be patternable without any developing steps such as wet etching after light exposure. Direct photo-patterning of this HYBRIMER was carried out through a quartz contact mask. An AFM surface scan of the HYBRIMER showed the photo-migration at the beginning of UV irradiation and the direct volume compaction in the UV illuminated regions.
Ge이 첨가된 silica 유리에서의 광 민감성은 1978년 K.O.Hill 등1)에 의해 처음 발견된 이후 많은 연구가 행하여졌다. 최근에는 Ge이 첨가된 실리카 광섬유에 자외선을 조사하여 주기적인 패턴을 가지는 브래그 격자 소자를 제작하는 기술로 발전하였다. 하지만 Ge이 첨가된 무-유기 하이브리드 재료에서의 광 민감성에 대한 연구는 아직 미비한 실정이다. 따라서 본 연구에서는 솔-젤 법을 이용하여 Ge이 첨가된 무-유기 하이브리드 재료를 제조하고자 하였다. 더 나아가 조사한 자외선 파장에 따른 광 민감성의 변화와 그러한 광 민감성에 영향을 주는 다양한 photo-induced 반응들에 대해 알아보고자 하였다.
