Abstract
The aim of this study is to prepare a series of fluorinated polymethylsilsesquioxanes (FPMSQ) grafted with different quantities of Fluoro silane and TEOS, which have low refractive index (RI), together with the appropriate properties of wettability and adhesion. Two major parts are included in this study; which are (1) Preparation of silane-grafted polymethylsilsesquioxane(SGPSQ) with a tunable refractive index (RI) at low temperature (80 °C or below), (2) Preparation of phase separated fluorinated polymethylsilsesquioxane (FPMSQ) for using in antireflection coatings. The first part of this study presents the preparation and fabrication of a series of silane-grafted polymethylsilsesquioxane (SGPMSQ) with a controllable structure and molecular weight. The synthesized polymethylsilsesquioxane (PMSQ) with methyl- triethoxysilane (MTES) formed a regular structure such as the cage or ladder structure. The molecular structure started to change from regular to an irregular random network structure after grafting with TEOS and PFDTS on the PMSQ. The changes in the molecular structure of SGPMSQ generated by grafting silane are similar to the changes in the molecular structure of PMSQ film heated from 150 to over 250 °C, reducing the refractive index (RI). The RI of PMSQ at 583 nm decreased from 1.50 to 1.42 merely by heating it to 80 °C, by introducing silane and controlling the molecular structure. The decomposition temperature (Td) of SGPMSQ was 340 °C, which is much higher than that of pure PMSQ (Td = 267 °C). Although the glass transition temperature (Tg) of SGPMSQ was 20 °C lower than that of pure PMSQ, its decomposition temperature was 73 °C higher. This result shows that SGPMSQ is more flexible and possesses greater thermal stability than that of pure PMSQ. In the second part of this study, the phase separated fluorinated polymethylsilsesquioxane (FPMSQ) films were prepared and used in antireflection coatings. The FPMSQ was synthesized from methyltrimethoxysilane (MTMS) and grafting TEOS and 1H,1H,2H,2H-perfluorooctyltriethoxysilane (FTES) by means of a sol gel process. The FPMSQ possesses both hydrophilic (such as hydroxyl groups) and hydrophobic (such as fluoro groups) characteristics. There are hydroxyl groups on the glass substrate. When PMSQ was spin coated on the glass substrate the hydroxyl groups would attach to the surface of the glass via hydrogen bonding which will enhance the adhesion. The fluoro groups migrate away from the substrate towards the surface of the film during heating, and phase segregation thus takes place. The phase segregation of the PMSQ film forms two layers, the first being the layer of the main structure of the PMSQ, and the second being a layer rich in fluorine atoms that cause the desired low reflectivity (less than 1 %).