The Tg of a polymer is a common phenomenon exhibited under the varying temperatures. As for the explanation of the glass transition phenomenon, there is no perfect theory so far [27]. At present, the most common theory is the free volume theory put forward by Fox and Flory [28]. Naa11mely, the chain segment motion was frozen initially due to the low energy of molecular motion under the Tg. Expectedly, the change of the free volume or the void volume of the polymer is very small with the increasing of temperature. But above the Tg, the free volume increases with the increasing of temperature since the molecular motion has enough energy. Correspondingly, the chain segment also begins to enter motion state. Therefore, the Tg is the certain temperature when the free volume reaches a critical value. One common approach to determine the Tg is to investigate the specific volume dependence of temperature. Then, the inflection point should be the Tg, originating from the change of the thermal expansion coefficient when passing from a glassy to a rubbery system.