Solid state 13C n.m.r. detection of molecular mixing in polymer blends that exhibit multiple eutectic phase transformations

Abstract

Blends of poly(ethylene oxide) with various isomers and derivatives of dihydroxybenzene generate rich macroscopic phase behaviour characterized by solid-solid-liquid (eutectic) transformations. These phase transitions, which occur above ambient temperature, are measured directly by d.s.c., and a thermal analysis summary is contained in the temperature-composition projections. The work described here focuses on site-specific interactions at the molecular level that are responsible for this unique phase behaviour. Solid state n.m.r. spectroscopy is useful as a diagnostic probe of strong interactions and crystal structure modifications. The isotropic 13C chemical shift detects phase coexistence at temperatures well below the eutectic and liquidus transitions where the d.s.c. thermograms are featureless. I.r.-detectable hydrogen bonds between the ether oxygen of poly(ethylene oxide) and the hydroxyl protons in either hydroquinone, 2-methylresorcinol or 5-methylresorcinol distort electron density within the π-orbitals of the aromatic ring. Consequently, the aromatic carbon n.m.r. signals of the resorcinol-like small molecules are sensitive to hydrogen bonding and the formation of molecular complexes. Multiple signals are observed for chemically equivalent carbon sites in the small molecules when phase coexistence is favoured. Crystal structure considerations and molecular packing within the unit cell also generate 13C chemical shift multiplicities, illustrated here for undiluted hydroquinone. The correlation between phase behaviour and 13C n.m.r. diagnostics provides a phenomenological interpretation of the spectroscopic results, particularly for trieutectic blends of poly(ethylene oxide) with 2-methylresorcinol. The combination of n.m.r. and thermal analysis allows high-temperature d.s.c.-measured phase boundaries to be extrapolated to lower temperatures dictated by the n.m.r. experiment.