Molecular-level free volume as a crucial complementary factor affecting miscibility and nanoscopic homogeneity of polyarylate/poly(vinyl chloride) blends

Abstract

Molecular-level miscibility and scale of mixing were characterized and evaluated for three model blends of polyarylates having structural variants with PVC through cross polarization/magic-angle spinning (CP/MAS) 13C nuclear magnetic resonance (NMR) spectroscopy. From this study, we found that the polyarylate with sulfonyl central connector and tetramethyl substitutions on bisphenol rings (shortly, TMBPS-PAr) was nanoscopically miscible with PVC as homogeneously mixed down to a scale of ∼2.3 nm. In contrast, the structurally similar polyarylate, just in absence of tetramethyl substitutions from TMBPS-PAr, (BPS-PAr) was partially mixed with PVC at a homogenization scale of at least ∼27–32 nm, regardless of almost identical specific intermolecular interaction relative to TMBPS-PAr. In the case of polyarylate with isopropyl central connector other than the sulfonyl (TMBPA-PAr) was found to be completely immiscible with PVC, despite the presence of tetramethyl substitutions, due to the lack of polar sulfonyl groups that is known to promote mixing. These observations, in conjunction with the structural combinations of polyarylates, suggest the need to seek another complementary key parameter to promote miscibility and nanoscopic homogeneity in addition to the specific interaction. From the results of the positron annihilation analysis (PALS) on the intermolecular free volume through the entire composition ratio, the TMBPS-PAr/PVC blend resulted in a larger negative deviation of the ortho-positronium ( o-Ps) pick-off lifetimes ( τ 3) and the free cavity sizes ( R) from linear additivity of those parameters of component polymers compared to the BPS-PAr/PVC blend. The larger negative deviation was interpreted to be a higher/greater contraction of free space and more tightly packing of molecular chains in TMBPS-PAr/PVC upon mixing irrespective of similar specific intermolecular interaction between two blend systems, which shows good agreement with the results of densities with blend compositions. Recognizing that the only structural variation between TMBPS-PAr and BPS-PAr was tetramethyl substitutions and that TMBPA-PAr/PVC was completely immiscible despite the tetramethyl substitutions, the combined results obtained from NMR and PALS lead to rationalize additional effects of the molecular free volume space created by the tetramethyl groups on the good miscibility of the blends under the circumstance when enough specific intermolecular interaction existed upon mixing.