Tuning of the Rheological Properties and Thermal Behavior of Boron-Containing Polysiloxanes

The rheological behavior of boron-containing poly(organosiloxanes) and their thermal decomposition were studied in order to evaluate the spinnability of these preceramic polymers, which might be suitable for the production of high-temperature-resistant ceramic fibers in the SiBCO system. The polymers were synthesized by methanolysis of tris(1-(dichloro(methyl)silyl)ethyl)borane and subsequent hydrolysis of the reaction product. Three different polymers were obtained by varying the amount of water during the hydrolysis step. The degree of cross-linking and hence the viscosity of the resulting polymer increased with the amount of water as analyzed by rheological measurements. The spinnability of polymers is affected by their flow characteristics, especially by their visco-elasticity, expressed by the dynamic moduli G′ (storage modulus, indicating elastic properties) and G′′ (loss modulus, indicating viscous properties), respectively. The thermal behavior of the three samples was analyzed in situ during the pyrolysis process between room temperature and 1400 °C, and ex situ by HREM and EELS after thermal decomposition of the samples. The ceramic yield was determined to be 45 wt % for the polymer with the lowest cross-linking degree and increased up to 75 wt % in case of the highly cross-linked polymer.