Thermal Expansion Behavior of Antiplasticized Polycarbonate

Abstract

Because of its excellent transparency, mechanical toughness and high heat-distortion temperature, polycarbonate (PC) is widely employed in optical applications such as plastic glasses, optical disks, and optical films. A lightweight replacement for an inorganic glass is being developed due to strong demand in the automobile industry to reduce the weight of electric vehicles. In order to serve as a replacement for an inorganic glass, however, rigidity and the dimensional stability under temperature change need to be improved. The role of a conventional plasticizer is, in general, to increase the flexibility in the solid state and flowability in the molten state. A plasticizer weakens the intermolecular topological interaction between neighbor polymer chains, leading to low viscosity in the flow region. Furthermore, the glass-to-rubber transition occurs at low temperature because the relaxation time of the segmental motion is shortened. Even in the glassy state, a plasticizer usually enlarges the free volume fraction, which has been revealed by the positron annihilation lifetime spectroscopy, proton spin-lattice relaxation at nuclear magnetic resonance and pressurevolume-temperature diagram. Because of the enlarged free volume, the modulus decreases and the thermal expansion increases. To counter this normal behavior of plasticization, additives known to enhance the modulus are used, which is called antiplasticization. According to previous studies, the decrease in the free volume is believed to be the origin of the modulus enhancement. Therefore, β -relaxation of an amorphous polymer, i.e., local relaxation mode, is strongly affected by an antiplasticizer, because the mobility in a local mode is suppressed by loss of the free volume. This anomalous but well-known behavior has been reported for various polymers including poly(vinyl chloride), poly(methyl methacrylate), and cellulose esters. PC is also known to show antiplasticization when combined with various materials. The addition of an antiplasticizer enhances the modulus and reduces the β -relaxation mode located around at -100 oC, which is attributed to mechanisms such as ring-flip process of phenyl groups and rotation of the phenylene rings. Although numerous researches have been carried out on antiplasticization, to the best of our knowledge, the thermal expansion behavior of an antiplasticized glass has not yet been reported; this phenomenon should be clarified to improve our understanding of antiplasticization. Considering the mechanism of antiplasticization, it can be predicted that Thermal Expansion Behavior of Antiplasticized Polycarbonate