Steady-state Rheological Properties Of Polycarbosilane Melt At Different Temperatures

Polycarbosilane(PCS)is an important precursor in preparation of continuous silicon carbide ceramic fibers by polymer-derived method.The rheological properties of PCS melt directly determine the success of melt spinning,which is critical to structure and property of continuous silicon carbide ceramic fibers.As a typical organic silicon oligomer,rheological properties of PCS are difficult to be studied due to unclear complex microstructure and lack of oligomer theory.Therefore,it is of great scientific significance to investigate the steady-state rheological properties of PCS materials by developing the test and analysis methods particularly suitable for PCS in an effort to provide a fundamental basis for oligomer rheologic theory.In this work,a series of PCS samples with various softening points,molecular weights and polydispersity coefficients,as well as with different heat treatments were examined.The steady-state rheological tests were carried out at different temperatures.The cause that the first Newtonian platform was not observed under low shear rates in the viscosity-shear rate curve was studied.The method for analyzing the rheological data at small shear rates was established and the zero shear viscosity was determined.Furthermore,the stress-shear rate curve was analyzed and fitted,and the effects of temperature,heat treatment time,softening point,molecular weight and polydispersity coefficient on the fitting functions were discussed.The relationship between stress and strain in the cone-plate flow field was deduced,and the viscosity-shear rate curves calculated under the same strain were compared with those measured from the steady-state tests.The results showed that the time required for PCS melt to reach equilibrium under low shear rates was more than 200 s,and therefore,the first Newtonian platform was not observed in the viscosity-shear rate curve.To overcome the problems caused by long delay time and easy oxidation,the rheological data under low shear rates near equilibrium state were obtained by averaging method and the zero shear viscosity of PCS melt was evaluated to be 4332?9424 Pa s at 280 0C(i.e.,the empirical spinning temperature)by linear fitting.In addition,the stress-shear rate curve exhibited a non-monotonic change(a peak-valley phenomenon),and the peak-valley phenomenon gradually weakened and finally disappeared from 290 ? to 250 ?,demonstrating strong temperature dependence.The stress-shear rate curves at the shear rates less than 0.002 s-1 could be fitted by linear function,while more than 0.3 s-1 fitted by quadratic function.The increase of temperature led to sudden changes for both slope and intercept of linear function at 280 ?.The constant term(C)in quadratic function at 290 ??270 ?,as well as the first-order coefficient(A)and second-order coefficient(B)in quadratic function at the same temperature,increased first and then stabilized with the extension of heat treatment time,and increased with the increase of softening point temperature.When the heat treatment time was shorter than 15 h,C values decreased with the increase of temperature,but when it was longer than 15 h,C values increased first with the increase of temperature,and then decreased at 270 ?.For different samples,both A and B values decreased with the increase of temperature.The stress under any strain could be determined by interpolation method and fitting method,respectively.The stress response of PCS melt was affected by shear rate and also depended on strain.When the shear rates were 0.0005?10s-1,the viscosities under the same strain decreased with the increases of shear rate,and the first Newtonian platform,which appeared in the viscosity-shear rate curve obtained from the steady-state test,could not be observed.