Discrepancy in determination of χ parameters by melting point depression versus small-angle neutron scattering in blends of deuterated polycarbonate and isotactic poly(methyl methacrylate)

Abstract

Discrepancy in determination of v parameters by melting point depression versus small-angle neutron scattering in blends of deuterated polycarbonate and isotactic poly(methyl methacrylate) Thein Kyu,* Rushikesh A. Matkar, Dong Soo Lim and Chechian Ko Department of Polymer Engineering, University of Akron, Akron, OH-44325, USA. Correspondence e-mail: tkyu@uakron.eduThe discrepancy in the interaction parameters of deuterated polycarbonate/ isotactic poly(methyl methacrylate) blends as determined by the melting point depression approach and the small-angle scattering technique is reported. We have modified the Flory diluent theory by removing the inherent assumption of complete rejection of the solvent from the crystal solid by taking into consideration the crystal-amorphous, amorphous-crystal, and crystal-crystal interactions. The discrepancy in values obtained by the two methods is discussed. IntroductionThe tacticity of poly(methyl methacrylate) (PMMA) isomers has been recognized to exert profound effects on its miscibility with other polymers (Schurer et al., 1975;Silvestre et al., 1987). In our laboratory, blends of polycarbonate (PC)/PMMA isomers have been explored as a means of controlling the transparency of their blend films and the refractive index gradient (Lim & Kyu, 1991). It was found that PC/ syndiotactic poly(methyl methacrylate) (sPMMA) and PC/atactic poly(methyl methacrylate) (aPMMA) can be characterized as partially miscible with a cloud point phase diagram reminiscent of a lower critical solution temperature (LCST). The proximity of the aforementioned LCST to the glass transition temperatures of the constituents of the PC/PMMA blends, coupled with slow mutual diffusion of the polymer chains, impeded the exploration of the miscibility of PC/aPMMA and PC/sPMMA blends near their glass transition temperatures (T g ) by conventional techniques such as light scattering or optical microcopy. However, thermal reversibility of the LCST phase behavior could not be established for either of these blends. Hence, we shall focus on small-angle neutron scattering (SANS) experiments of deuterated polycarbonate (dPC)/isotactic poly(methyl methacrylate) (iPMMA) blends.In the present paper, SANS has been employed for the determination of the interaction parameter in the vicinity of the coexistence lines of the dPC/iPMMA blend using the Ornstein-Zernike approach (Kirste et al., 1975). The uniqueness of the present PC/ iPMMA blend is that both constituents can crystallize upon annealing above their glass transition temperatures. The melting point depression of PC was analyzed in the framework of the polymer diluent theory (Flory, 1953;Nishi & Wang, 1975). The value of the PC/ iPMMA blend obtained by the Flory diluent analysis was subsequently compared with that obtained by SANS. Materials and methodsThe deuterated PC (dPC) was obtained from Dr Steve Smith of the Procter & Gamble Company. The as-received dPC was purified by dissolving in tetrahydrofuran (thf) at a concentration of 2 wt% and filtered twice using a mi...