Chain configuration influences phase behavior of blends of poly(methyl methacrylate) (PMMA) of different tactic configurations (syndiotacticity, isotacticity, or atacticity) with poly(L-lactic acid) (PLLA). Blends system of sPMMA/PLLA is immiscible with an asymmetry-shaped UCST at $250 C. The phase behavior of the sPMMA/PLLA blend is similar to the aPMMA/PLLA blend that has been already proven in the previous work to exhibit similar UCST temperatures (230-250 C) and asymmetry shapes in the UCST diagrams. On the other hand, the iPMMA/PLLA blend remains immiscible up to thermal degradation without showing any transition to UCST upon heating. The blend system with UCST, that is, sPMMA/PLLA, can be frozen in a state of miscibility by quenching to rapidly solidify from the homogeneous liquid at UCST, where the T g -composition relationship for the sPMMA/PLLA blend fits well with the Gordon-Taylor T g model with k ¼ 0.15 and the blend's T 0 m leads to v 12 ¼ À0.26 for the UCST-quenched sPMMA/PLLA blend. Both parameters (k and v) as characterized for the frozen miscible blend suggest a relatively weak interaction between the two constituents (sPMMA and PLLA) in the blends. The interaction strength is likely not strong enough to maintain a thermodynamic miscibility when the blend is at ambient temperature or any lower temperatures below UCST.

Summary: Miscibility in amorphous phase and behavior in a crystalline phase of blends of two semicrystalline and isomeric polymers, isotactic polystyrene (iPS) and syndiotactic polystyrene (sPS), was probed. Optical and scanning electron microscopy results indicate no discernible heterogeneity in iPS/sPS blends in either melt state or rapidly quenched amorphous super‐cooled state, while the Tg behavior of the quenched amorphous blends shows an intimately mixed state of two polymer chains. The crystal forms of the blends were further analyzed to provide additional evidence of miscibility in the amorphous domain. The sPS in the iPS/sPS blends upon melt crystallization was found to predominantly exist as the more stable β‐form (rather than mixed β‐form and α‐form in neat sPS), which also suggests evidence of miscibility in the iPS/sPS blends. The melting behavior of semicrystalline sPS in the iPS/sPS mixtures was analyzed using the Flory‐Huggins approach for estimation of interactions. By measuring the equilibrium melting point of the higher‐melting sPS species in the sPS/iPS blends, a small negative value, for the interaction parameter (χ ≈ −0.11) was found. Further, by introducing a third polymer, poly(2,6‐dimethyl‐p‐phenylene oxide) (PPO), a ternary iPS/sPS/PPO blend system was also proven miscible, which constituted a further test for stable phase miscibility in the iPS/sPS blend. General nature of miscibility in blends composed of two crystalline isomeric polymers is discussed. Issues in dealing with blends of polymers of the same chemical repeat unit but different tacticities were addressed.X‐ray diffractograms for neat sPS and iPS/sPS blends, each having been isothermally crystallized at 245 °C for 4 h.magnified imageX‐ray diffractograms for neat sPS and iPS/sPS blends, each having been isothermally crystallized at 245 °C for 4 h.

Fourier transform infrared (FTIR) spectroscopy, optical microscopy (OM), and differential scanning calorimetry (DSC) techniques were used to probe phase behavior and interactions in blends of poly(vinylidene fluoride) (PVDF) and polyesters [poly(trimethylene adipate) (PTA) and poly(pentamethylene adipate) (PPA)] of relatively low crystallizability. DSC thermal analysis and OM characterization proved that PVDF was miscible with PTA and PPA with a low lower critical solution temperature. Small negative values of the interaction parameters (w 12 5 20.13 for a PVDF/PPA blend) were obtained with the meltingpoint depression method. FTIR spectroscopy results revealed that interactions between À ÀCF 2 of PVDF and the À ÀC¼ ¼O group of the polyester were weak, in agreement with the thermal analysis results. An increase in the coarseness and/or ring-band spacing further provided supportive evidence that miscibility did exist between the polyester and PVDF constituents in the blends. Pattern changes in ring-band spherulites of the miscible blends further substantiated the favorable, though weak, interactions between the PVDF and polyester constituents.