Thermodynamics of mixing estimated by equation-of-state parameters in miscible blends of polystyrene and tetramethylbisphenol-A polycarbonate

Mixing thermodynamics in miscible blends of polystyrene CPS) and tetramethy lbisphenol-A polycarbonate (TEvIPC) was investigated using liquid state pre ssure-specific volume-temperature (P-V-T) properties of both purl component s and mixtures. The equation-of-state theories used were (1) the lattice fl uid model of Sanchez and Lacombe, (2) the model of Flory, Orwoll, and Vrij, and (3) the modified cell model suggested by Del and Walsh. The compositio n dependence of characteristic pressure was first used to extract the inter action parameter (Delta P*) and Flory interaction parameter expressed in th e second derivative of the free energy of mixing (chi(sc)) It Was found tha t the sign of chi(sc) was negative and the magnitude of it was always signi ficantly larger than the values obtained by small-angle neutron scattering (Yang H, O'Reilly JM. Mater Res Soc Symp Proc 1987;79:129) and diffusion me asurements (Kim E, Kramer EJ, Osby JO, Walsh DJ. J Polym Sci, Part B: Polym Phys 1995;33:467), indicating that the blend P-V-T properties grossly over estimate the attractive interaction. On the other hand, the chi(sc) predict ed from the characteristic temperature was also large but had a positive si gn. These results were similar to what had been found in PS/PVME blends by Ougizawa and coworkers (Ougizawa T, Dee GT, Walsh DJ. Macromolecules 1991;2 4:3834). While the thermal expansion coefficient began to increase as tempe rature is raised above the lower critical solution temperature (LCST), the volume contraction upon mixing was observed above as well as below the LCST . This observation implies that two dissimilar chains are packed together t o form a certain stable stereo structure. We also note that the decreased c hange in core volume rather than the presence of large Delta P* causes the volume contraction upon mixing. (C) 2000 Elsevier Science Ltd. All rights r eserved.