Experimental observations of time‐dependent behaviors for polycarbosilane melt

Abstract

ABSTRACT:The distinct characteristics of thixotropic and viscoelastic phenomena of polycarbosilane (PCS) melt were reported first time based on experimental observations. The PCS was synthesized according to the Yajima process with the number of average molecular weight being $ 1170, followed by an appropriate preshear treatment to remove air bubbles encountered during the heating or pressing process of PCS specimens. A series of shear thixotropic loop, start-up, and step-down rate measurements were carried out at 280 C to systematically examine the time-dependent behaviors of PCS melt. It was found that the PCS melt was nonlinear complex fluid with a strong time-dependent in viscoelasticity and apparent thixotropy. Compared with the conventional high-density polyethylene, the shear stress-shear time responses of PCS melt were more susceptible to the changes of shear rate, shear time, and standing time because of the nature of lowmolecular-weight polymer.

Determining Zero Shear Viscosity of Polycarbosilane Melts

“…Materials and pretreatments. The homemade PCS raw samples with the softening point temperature of 205°C were pretreated by being heated in a homemade device from room temperature to 280°C and then held at the temperature for 1 h under a nitrogen atmosphere as described previously [4,5]. The softening point temperature of the pretreated PCS samples slightly increased to 219°C.…”

“…Under the melt spinning temperature, bubbles are produced by gasification of low molecular weight part of PCS, which disturb rheological measurements and require prolonged time to attain a steady state, ultimately, lead to unreliable data. To reduce the influence of the bubbles, high-temperature-distillation and heating-in-vacuum methods were established to heat-treating PCS samples by Cheng et al [4,5]. The zero shear viscosity is normally approximated from the apparent viscosities obtained from the steady test, which measures the apparent viscosity associated with an equilibrated state of the samples at a given shear strain rate.…”

“…In this work, a series of transient tests and a steady test were performed. The PCS samples were pretreated by employing the same methods as described previously [4,5]. The limitation of the steady test will be discussed.…”

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].…”

Rheological Measurement of <i>In Situ</i> Preheating Polycarbosilane Melt

“…Unforturnaterly, a large amount of air bubbles are continuously encountered during the pretreating PCS raw materials due mainly to the decomposition of small molecule, which adversely influences the subsequent rheological measurements. To reduce the air bubbles generated during the heat treatments of PCS, the specially designed devices for vacuum heating [5] or high distillation pretreatment [6] have been previously employed. However, those heat pretreatments of PCS are carried out ex-situ, and the air bubbles will be regenerated after the temperatures resume to room temperatue, at which the specimens are pressed into the desired shapes.…”