Determining Zero Shear Viscosity of Polycarbosilane Melts

Abstract

As an important precursor in preparation of silicon carbide ceramic fibers by polymer precursor-derived method, it is very difficult to find the zero shear viscosity through the conventional steady test for the oligomer-like polycarbosilane (PCS) because of the possible oxidation and a prolonged time required to attain a steady state of the samples. In this work, a steady test and a series of transient tests were performed for the PCS melts. The limitation of the steady test is discussed. An averaging method based upon data from the transient tests is developed to determine the zero shear viscosity for the melt. The reliability of the results from the method is discussed and the zero shear viscosity of the pretreated PCS melt was obtained.

Repeatability in Rheological Measurements of Epoxy Resin

“…2(b) for the enlarged curves. The shear stress varied less evidently as the delay time t 1 changed when the measurement time t 2 was 20 s. Obviously, the shear stress became the smallest at the small shear rate when the delay time t 1 and measurement time t 2 were fixed at 20 s+10 s. The phenomenon that the shear stress responses were more sensitive to alterations of shear rate at lower shear rates was similar to those observed with PCS, which might be closely related to the oligomer nature and the time to attain a seady state being much longer than conventional polymers [2][3][4]. The repeataility of the steady test data at low shear rate (<7 s -1 ) was poor.…”

“…Polycarbosilane (PCS) is a typical oligomer having number average molecular weight of 10002000 g/mol. Main diffculities in rheological measurements for PCS melt by ARES included bobbles prodcued due to the gasification of low molecular weight part and the prolonged time to attain a steady state [2][3][4]. Owing to the oligomer nature of PCS, pretreatemnts such as in-situ or the combination of ex-situ and in-situ preheating PCS powder at 315 ºC were employed before the rheological measurements at 295 ºC to eliminate or minimize the air bubbles generated when PCS powder became melt [3].…”

“…Owing to the oligomer nature of PCS, pretreatemnts such as in-situ or the combination of ex-situ and in-situ preheating PCS powder at 315 ºC were employed before the rheological measurements at 295 ºC to eliminate or minimize the air bubbles generated when PCS powder became melt [3]. The averaging method based upon data from the transient tests was also developed to determine the zero shear viscosity of PCS melt [4].…”

Improved Repeatability of Oscillatory Rheological Measurements for Polycarbosilane Melt

“…Low molecule weight part of PCS is gasified and bubbles are evolved during heating, which significantly disturbs the subsequent rheological measurement. To remove the bubbles, high temperature pretreatments in a vacuum and nitrogen atmosphere were employed [3,4], and in-situ pretreatment was also probed [5].The rheological properties of PCS melts at low shear strain rates were investigated previously by carrying out both transient and steady tests [6]. To learn about the zero shear viscosity of PCS, the averaging data was obtained based on the transient test data to compliment the steady test data because a long delay time during the steady test affected the zero shear viscosity [6].Even though the normative rheological processes were modeled, the data of dynamic time sweep test method were shown the dissatisfactory repeatability due to temperature change [7].…”