Effect of crosslinking rate on the glass transition temperature of polyimide cross-linked silica aerogels
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ORIGINAL PAPER
Effect of crosslinking rate on the glass transition temperature of polyimide cross-linked silica aerogels Zhongyi Luo 1,2 & Zichun Yang 1,2 & Zhifang Fei 1,2 & Kunfeng Li 1,2 Received: 3 December 2019 / Accepted: 19 March 2020 # The Polymer Society, Taipei 2020
Abstract The polyimide cross-linked silica aerogels (PI -SiO2) was constructed at the atomistic level. The specific volume-temperature curves of the materials at different crosslinking rate were simulated by molecular dynamics. The glass transition temperature of the material was obtained, which was consistent with the results obtained by free volume fraction and mean square displacement. The higher the crosslinking rate, the higher the glass transition temperature. At the same time, the free volume fractions of the material in the rubbery state, transition state and glassy state were analyzed. It was found that the free volume fractions of the material in the rubbery state and glassy state were mainly affected by the degree of crosslinking, while the free volume fraction of the material in the transition state was affected by the crosslinking bond. In this paper, the basic factors affecting the glass transition temperature are explained from the molecular level by studying the changes of the free volume fraction in the material. Combined with the analysis of self-diffusion coefficient, the influence of molecular chain motion on the properties of materials is revealed. Keywords Molecular dynamics . Nano-composite . Glass transition temperature . Free volume fraction
Introduction Silica aerogels have excellent thermal insulation properties and are widely used in various industries. However, its mechanical properties need to be optimized [1–3]. The new materials cross-linked by silica aerogels and organic compounds can enhance the mechanical properties of aerogel materials very well, and still possess excellent properties such as low thermal conductivity and low density [4–6]. However, due to the addition of organics, the glass transition temperature (Tg) of the materials will be affected. As an important index to measure the application range of thermal insulation materials, the study of the glass transition temperature of composite materials will be helpful to expand the application range of composite materials at high temperature.
* Zhongyi Luo [email protected] 1
Institutes of High Temperature Structural Composite Materials for Naval Ship, Naval University of Engineering, Wuhan, China
2
School of Power Engineering, Naval University of Engineering, Wuhan, China
The researches of glass transition temperature include experimental research and theoretical research. The common experimental methods of measuring glass transition temperature are dilatometer method, thermal analysis method (DTA, DSC) and nuclear magnetic resonance method [7–9], The correlation between free volume and thermo-physical properties was studied by positron annihilation lifetime spectroscopy (PALS). At the same time, there are many theoretical studies provide meth
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