The Relationship Between T_g And Composition In Polystyrene/Triphenylmethyl Chloride Blends

The relationship between glass transition temperature and composition in polymer blends,especially polymer solutions,have been studied for many years. Some relationships based on empirical equations, classical thermodynamics, free volume have been developed to explain T_g-composition relation. These equations were fit well for the experiment results in some systems, and could explain the essence of T_g in polymer blends. However, there exists anomaly in T_g-composition curves, i.e."cusp", which means a change of curvature in the central composition region of the T_g-concentration dependence. T_g-composition curve is unmonotonic, and seems to be composed of two monotonically decreasing curve connecting the pure components'T_gs, and the system seems to be composed of two different kinds of blends. We propose a new concept"molecular size asymmetry"to explain the"cusp". We used DSC to study blends of three different kinds of molecular weights'PS and TPCM, and found with PS weight fraction increasing, Tm of TPCM decreased, when the wPS was higher than"cusp", there was no melting peak. At about 20℃, T_g could be found, and was constant when wPS was lower than"cusp", which is consistent with the theory that the critical concentration of the polymer swelling coils is constant. With the Mw of PS increasing, the critical concentration increased, and this is corresponding to the theory. Also, we found that PS/TPCM system is a weak interaction system. Another important phenomenon was that there were melting peaks at 40℃, we think that they are less perfect crystalline which lie in the interfacial between TPCM crystalline and PS chains, and with the annealing temperature increasing, the melting peak moved to higher temperature. The reason is that when annealing temperature increased, the polymer segments gradually stretched, TPCM crystal became more perfect."Molecular size asymmetry"theory can explain the effect of molecular size on T_g in polymer solution, and is consistent with experimental results. The Cp of polymer solutions were also got through Stepscan method, and will provide the theoretical bases for the development of math model in the near future.