(73b) Preceramic Polymer Grafted Nanoparticle Composites: Influence of Thermal Curing on Rheology, Microstructure, and Ceramic Yield

Preceramic polymer grafted nanoparticles (PGNPs) have unique benefits over other preceramic materials. They provide routes for increasing ceramic yield, controlling particle arrangement, and reducing volume shrinkage and mass loss. Traditionally, filler particles are introduced to free polymers, but this leads to issues of chemical compatibility, agglomeration, and phase separation when processing techniques are applied. Previous studies have established that thermal curing may be used to improve the efficiency of converting preceramic precursors (polymers and PGNPs) to the final ceramic material. Here, we investigate how the material transformations are affected by exposure to low-temperature thermal treatments prior to ceramic conversion via pyrolysis. Two PGNPs investigated in this study had a silica nanoparticle core, one having the corona poly(1,1-dimethylpropylsilnae) (allyl-GNP) and the other having poly(1,1-dimethylbenzylethylsilane) (styryl-GNP). After thermal treatment, an increase in ceramic yield was observed. This is commonly seen in commercial preceramic polycarbosilanes such as SMP-10 due to present cross-linkable sites (carbon-carbon double bond). Characterizing techniques such as rheology, X-ray photon correlation spectroscopy (XPCS), and differential scanning calorimetry (DSC) were used to elucidate the thermally included material changes in the PGNPs (Figure 1). It was determined that thermally induced changes in the PGNP structure are due to the chemical structure of these grafted nanoparticle chains. It was found that styryl-GNPs experience crosslinking between 180 ËšC and 250 ËšC, which improved the ceramic yield when thermally converted to ceramic at 800 ËšC and induced a transition from diffusive to ballistic behavior at the nanoscale. This was not seen in the allyl-GNPs. Oscillatory rheological measurements were performed during the thermal treatment to 250 ËšC, and a key finding was that thermal jamming occurs in the PGNPs with both styryl and allyl backbones, but a structure is formed faster from the styryl backbone than the allyl backbones. Improving an understanding of cross-linking behavior of these hybrid materials are expected to advance the application of these materials to a turbine engine, advanced friction, and heat-shielding components.