Changes in Intermolecular Interaction Forces Inducing Glass Transition Determined by Three-Dimensional Aggregated Structural Models of Coal

The three-dimensional aggregated structural models of two types of coals, A and B, were constructed. It is found that the density and the Tg of the models were qualitatively consistent with values obtained experimentally. The T-g of coals A and B calculated using the model structures are 315 degrees C and 328 degrees C, respectively. The effect of temperature on the distribution of cohesive energy was quantitatively elucidated using the three-dimensional aggregated structural models. The cohesive energy density (CED) of coal B was greater than that of coal A at temperatures < T-g. However, at temperatures > T-g, the CED of coal A is comparable to that of coal B. This implies that the molecules are more strongly aggregated in coal B than in coal A at low temperatures due to hydrogen bonding, and the intermolecular interaction is considered to have gradually relaxed above the T-g. It is concluded that differences in the molecular and cohesive structures of the coals led to differences in the distribution of van der Waals energy and electric energy at different temperatures. The van der Waals energy changed from attraction to repulsion at about 450 degrees C and 285 degrees C in coal A and B, respectively. Electric energy remained an attractive force as the temperature increased. The mechanism of the 13 degrees C difference in the calculated T-g of each coal can be explained by the temperature change in the intermolecular force distribution. Therefore, this three-dimensional aggregated structural model can be used to understand the thermal behavior of aggregated molecules, such as coal thermoplasticity.