Rheological properties of silicon polymer networks: The influence of the crosslink density

Abstract

This study was focused on the influence of the microstructural properties of a silicon polymer network on its rheological properties. Two commercial silicon oils were mixed in different ratios to form, by hydrosilylation, networks with different crosslink densities. The chemical compositions of the oils were determined by NMR, whereas their molecular weights and viscosities were studied with gel permeation chromatography and viscosimetry, respectively. The different networks were characterized through their crosslink densities. Afterward, the rheological properties were studied. The formulation notably influenced some characteristic values of the rheological behavior: the ␣-transition temperature and the onset temperature of the caoutchoutic plateau shifted toward higher temperatures as the crosslink density increased, the storage modulus at the onset temperature of the caoutchoutic plateau increased with the crosslink density, and the amplitude of the peak associated with the ␣-transition temperature decreased. These behaviors were explained as follows: as the crosslink density increased, a drastic decrease in the amount of free chains in the network occurred, and both phenomena induced a large decrease in the chain mobility, which might explain the aforementioned behavior.

Preparation, Characterization, and Rheological Behaviors of Polysiloxanes Presenting Densely Simple and Well‐Defined Short‐Branched Chains

“…A mixture of desired composition of D 4 , D 4 Vi , disiloxane end-capping agent MM Vi listed in Table 1, and 0.5‰ wt% of the transient catalyst Me 4 NOH·5H 2 O were charged into a dried three-necked flask equipped with a magnetic stirrer and a condenser. While nitrogen (N 2 ) gas of high purity was bubbled for 1 h inside the mixture, the reaction was initiated by heating the reaction mixture with a silicon oil bath to 90 °C for about 4 h until the reaction reached equilibrium as monitored by 29 Si nuclear magnetic resonance (NMR) spectroscopy. Then, the reaction temperature was brought up to 150 °C and maintained for 4 h until decomposition of the transient catalyst Me 4 NOH.…”

“…Hydrosilylation of P 1 -P 5 with MD H M (Scheme 2) was carried out at 90 °C in a dried three-necked flask equipped with a magnetic stirrer, a condenser. The material mass ratio of P 1 -P 5 over MD H M was calculated from 29 Si NMR. Highly pure N 2 was bubbled for 24 h in the presence of 1‰ wt% Karstedt's catalyst to give G 1 -G 5 .…”

“…The temperature was then increased to 180 °C to remove the low-boiling components under a reduced atmosphere ranging from 1.0 to 1.5 kPa. The G 1 -G 5 products were obtained colorless liquids and were characterized by 1 H NMR, 29 Si NMR, Fourier transform-infrared (FTIR) spectroscopy, and Abbe refractometry. The results revealed that the products presented densely simple and well-defined short-branched chains.…”

“…1 H and 29 Si NMR spectra were acquired on a 400 MHz Bruker Avance 400 and a 300 MHz Bruker Avance 300 spectrometers, respectively. Deuterated chloroform (CDCl 3 ) was used as the solvent, tetramethylsilane was the external reference, and the chemical shifts (δ) were expressed in ppm.…”

“…Deuterated chloroform (CDCl 3 ) was used as the solvent, tetramethylsilane was the external reference, and the chemical shifts (δ) were expressed in ppm. The nonpolar paramagnetic relaxation agent for 29 Si NMR was chromium acetylacetonate.…”