CLOSED-LOOP IMMISCIBILITY IN A TERNARY MIXTURE OF HOMOPOLYMERS

Light scattering and calorimetric techniques were used to investigate the isothermal phase behaviour of a ternary polymer blend consisting o f three pairwise miscible homopolymers. These homopolymers, polystyren e (PS), poly(2-chlorostyrene) (P2ClS) and polycyclohexylacrylate (PCHA ), are apparently the first known examples of homopolymers that form a closed loop in the ternary isothermal phase diagram. The phase diagra m of the ternary blend was determined by light scattering at 433 K, wh ere all three pairs are below their respective lower critical solution temperature (LCST). The miscible compositions exhibit a single glass transition temperature (T(g)) which follow a simple volume additivity relationship. Inside the immiscible closed-loop region, single T(g)s a re also observed, but these are shown to be consistent with the presen ce of two phases. Two binary interaction parameters (for the pairs PS- P2ClS and PS-PCHA) were determined from the locations of LCSTs. The in teraction parameter between P2ClS-PCHA was determined by approximating the observed closed-loop region of immiscibility by the Flory-Huggins lattice model. In analogy with systems comprised of a mixture of two polymers and a solvent, the closed-loop immiscibility gap results from an asymmetry in the interaction parameters between the three pairs (a 'DELTA(chi)' effect). The following ordering was observed: chi(PCHA-P 2ClS) < chi(PCHA-PS) almost-equal-to chi(PS-P2ClS). This trend is cons istent with the curvatures of the composition-dependent glass transiti on T(g)(phi) plots.