Studies on the Rheological Behavior of Polycarbosilane Part I: Effect of Time, Temperature and Atmosphere

Silicon carbide based fibers are generally prepared by the melt spinning of ceramic material precursors like polycarbosilane (PCS) and the hetero-metal substituted derivatives (M-PCS). Therefore, the rheological behaviour of PCS in the melt with respect to time, temperature, atmosphere, molecular weight and polydispersity becomes very important for successful processing and for preparing green fibres. In the present study PCSs having different softening points were synthesized via the thermal backbone rearrangement of polydimethylsilane (PDMS) and their melt viscosity variation and thermosetting behavior were studied. Various conditions like, atmosphere (inert & air), temperature, time and shear rate were chosen as variables. In an inert atmosphere, the viscosity of all the PCSs decreases with increasing temperature then under isothermal conditions it remains almost constant for the low softening point samples (high polydispersity) whereas it rises for the high softening point samples (low polydispersity). It has been observed that invariably all of the PCSs crosslinked rapidly in air as compared to in an inert atmosphere of argon. The effect of temperature was found to be prominent with the increase in SiH/SiCH 3 ratio and the processing time reduces with an increase in processing temperature. The high molecular weight PCS thermosets easily due to enhanced crosslinking of the polymer as compared to low molecular weight PCS irrespective of the atmosphere. Thus, for processing of PCS it is essential to have a polydisperse sample and to maintain an inert atmosphere to prevent undesired crosslinking.