SOLID-STATE C-13-NMR DETECTION OF THE ISOTROPIC CARBONYL LINE-SHAPE IN BLENDS OF POLY(VINYLPHENOL) WITH MAIN-CHAIN POLYESTERS

The solid-state NMR isotropic line shape of the carbonyl C-13 resonance is useful as a qualitative diagnostic probe of the polyester component's morphology and molecular mobility in partially miscible blends with poly(vinylphenol), PVPh. The main-chain polyesters chosen for investigation in this study are poly(ethylene succinate), poly(ethylene adipate), poly (butylene adipate), and poly (caprolactone). A crystalline phase exists for polyester-rich mixtures in all cases. Verification of this claim is provided by DSC endothermic transitions that map out melting point depression in the temperature-composition phase diagrams. The carbonyl C-13-NMR signal in the crystalline domains exhibits a full width at half height of 1-2 ppm when the glass transition temperature of the blends is below the temperature of the NMR experiment. In all cases, a single concentration-dependent glass-transition temperature is measured by DSC, which increases monotonically from below ambient for polyester-rich blends to well above ambient for blends that are rich in poly (vinylphenol). When the concentration of the amorphous proton donor PVPh is sufficient to thwart crystallization of the polyester and increase the glass transition temperature of the blends above the temperature of the NMR experiment, the line width of the carbonyl resonance increases three- to fourfold to ca. 5-6 ppm. When the blends are completely amorphous and T(g) is above ambient, the polyester carbonyl C-13 line shape reveals at least two morphologically inequivalent microenvironments. A partially resolved carbonyl signal in rigid amorphous blends is (a) identified at higher chemical shift relative to the crystalline component, and (b) attributed to hydrogen bonding in the amorphous phase. This interaction-sensitive hydrogen-bonded carbonyl signal accounts for an increasing fraction of the overall NMR absorption envelope of the carbonyl carbon site when the polyester is saturated with PVPh. The main-chain polyesters were chosen to probe the effect of chemical structure of the proton acceptor on the potential for hydrogen-bond formation. Aliphatic CH2 spacers between the carbonyl groups dilute the concentration of interacting sites, and the dependence of the carbonyl C-13-NMR line shape on blend concentration reveals unique spectroscopic behavior in each of the four blend systems investigated.