JOURNAL OF MATERIALS SCIENCE LETTERS 5 (1986) 1193 1194 Miscibility study of low molecular weight polystyrene/styrene-methyl methacrylate random copolymer blend using DSC JOSI ~ L. FEIJOO, ALEJANDRO J. MOLLER, JESOS R. ACOSTA Departamento de Ciencia de los Materiales, Universidad Sim6n Bolfvar, Apartado 80659, Caracas, Venezuela It is well known that in the DSC analysis of polymer binary mixtures, detection of a single glass transition (Tg), whose temperature is intermediate between those corresponding to the two component polymers, is the most unambiguous criterion of polymer miscibility. Two-phase polymer blends, on the other hand, exhibit Tgs identical in value and width to those of the pure components. However, certain intermediate cases exist where there is partial mixing between com- ponents and this can be observed as a shift in the Tg values of the individual components to intermediate temperatures [1]. In this work, the state of mixing of a PS/COP (S-MMA) blend was investigated by means of dif- ferential scanning calorimetry (DSC). Physical ageing proved to be indispensable for elucidating partial compatibility of the components. The polymers used were low molecular weight poly- styrene (PS) and a styrene-methyl methacrylate ran- dom copolymer (COP (S-MMA)), both supplied by Polysciences Inc. The monomeric composition of the random copolymer was 61.5% methyl methacrylate, determined by elementary analysis. The glass tran- sition temperatures of the low molecular weight PS and COP (S-MMA) were 45 and 103° C, respectively, determined by DSC. The intrinsic viscosity of PS at 25°C in toluene is 0.17dig l and that of COP (MMA-S) in butanone at 25°C is 0.31 dlg -l. Thin films were obtained at room temperature by casting on to glass from a high viscosity solution of the two polymers in chloroform, evaporating to dry- ness, heating for 2 h in vacuum at about 150° C, and cooling. A Perkin-Elmer DSC-2C calorimeter, model 2600 was used to record thermograms. Fig. 1 shows DSC thermograms of several com- positions (wt %) of the PS/COP (S-MMA) blend. Two curves were drawn for each composition; the upper curve was obtained after annealing at room temperature (~ 26° C) during a three month period. The lower curve is a second heating run of the aged sample after a rapid quenching. All the samples analysed were translucent to opaque. Second heating runs show in all cases a single broad glass transition (Tg), whose value is between those of the pure polymers. This behaviour is somewhat sur- prising, because two Tgs were expected in view of the non-transparency of all the solvent cast films, and the well separated Tg values of the homopolymers. Such apparent "single" transition can be well resolved into two separate Tg s when running physically aged sam- pies, as seen in the first heating runs of Fig. 1. The endotherm just above the Tg inflection in the DSC thermogram of a glassy sample submitted to anneal- ing below its Tg is a well-known phenomenon in organic glasses, associated with the relaxation of excess thermodynamic properties, e.g. volume or enthalpy relaxations due to the non-equilibrium nature of the glassy states [2]. PS and COP (S-MMA) both exhibit a single physi- cal ageing endotherm in their first run, as expected for single-phase materials. For the five blend com- positions presented in Fig. 1, two physical ageing endotherms are clearly seen and each endotherm must be associated with each phase. This is strong evidence of the presence of the two phases which cannot be determined without the aid of physical ageing. Fig. 1 also shows that Tg values of the individual components are shifted to intermediate values that depend on the blend composition. This shifting indi- cates partial mixing between components [1, 3]. In a similar work, Massa [4] has reported immiscibility for certain PS/COP (S-MMA) blends (with a different random copolymer composition) without reporting the degree of molecular mixing of the immiscible pairs, although he used DSC and optical clarity criteria to evaluate them. It is important to note that blend composition determines the shifting values of Tg. As the percentage of COP (S-MMA) increases, the two Tg values of the blend are shifted to values closer to the COP (S-MMA) Tg and vice versa; this result is in agree- ment with the general trend reported by Massa [4]: as the COP (S-MMA) content increases the miscibility of the blend also increases. The PS/COP (S-MMA) blend prepared in this work is immiscible in the range of composition reported here but exhibits a certain degree of partial mixing as evidenced by the shifting of the TgS of the individual component to intermediate values. These values are determined by the composition of the blend. In this work, physical ageing was an important tool for ensuring the presence of two phases and how they mutually interact. Physical ageing may provide an easy way to discern between a single or double glass transition for systems where miscibility is uncertain. Acknowledgements The authors thank Ms Maria Eugenia Alvarez for her valuable collaboration in this work and Dr Giuseppe Di Filippo for his valuable comments. 0261-8028/86 $03.00 + .12 © 1986 Chapman and Hall Ltd. 1 ] 93