Composition dependence of phase instability and cloud point in solution-blended mixtures of polystyrene with poly(cyclohexyl methacrylate)

Composition dependence of phase behavior in the mixture system of polystyre ne (PS) with poly(cyclohexthyl methacrylate) (PCHMA) was examined in detail s using differential scanning calorimetry, optical and scanning electron mi croscopy, and infrared spectroscopy. This study shows that the 'transparent ' blends of PS/PCHMA with an apparent single T-g, having been thought to be miscible in earlier reports, actually displayed micro-heterogeneity domain s of about 100-300 Angstrom. It is thus not appropriate to assess miscibili ty to the optically clear blends of PS/PCHMA; however, partial miscibility does exist. The PCHMA-rich (PCHMA >70 wt%) compositions are indeed miscible but the PS-rich compositions are only partially miscible with sub-micron d iscrete phase domains beyond detection limit of optical microscopy. The pro ximity of the T(g)s also prevented that the thermal transition of the micro phase domains in the PS-rich blends being resolved. The criterion of singl e T-g commonly employed in in assessing phase behavior should be used with extra precaution in cases where microheterogeneity exists, whose ambiguity is even compounded in cases where the T(g)s of the constituent polymers are close. The FT-ir result indicates that intermolecular interactions are wea k or non-specific for the PCHMA-PS pair. (C) 1999 Elsevier Science Ltd. All rights reserved.