Structure-Property Relationships for Methylsilsesquioxanes

Critical structure-property relationships are delineated for a widely used class of methylsilsesquioxane (MSQ) materials. We obtained a range of MSQ prepolymers with different microstructures by varying the synthetic conditions. The structure of such different oligomers were measured and correlated with the physical properties of final (thermally cured) MSQ films. The microstructural analysis was performed using a combination of graphite plate laser desorption/ionization time-of-flight mass spectrometry (GPLDI-TOF MS), size exclusion chromatography (SEC), and Fourier transform infrared spectroscopy (FT-IR). These measurements show that, as the MSQs grow in size, the propensity to form closed cage-type architectures is higher for molecular masses below 1000 u. When these prepolymers are spin-cast into thin films and thermally cured (vitrified) into Final organosilicate polymers, the cage-type microstructures have detrimental effects on the physical properties of the film. They result in films with poor optical quality and a very large coefficient of thermal expansion. The structural basis for these variations in the physical properties is discussed in detail.