Effect of crosslinking rate on the glass transition temperature of polyimide cross-linked silica aerogels

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.