Segmental Mobility in Crystalline Poly(3-hydroxybutyrate) Studied by EPR Probe Technique

Abstract

The molecular mobility of poly(3-hydroxybutyrate) (PHB) was studied in the temperature interval from 20 to 90 °C by EPR using stable nitroxyl radicals 2,2,6,6-tetramethyl-1-piperidinyloxy (Tempo) and 4-hydroxy-2,2,6,6-tetramethyl-1piperidinyloxy (Tempol) as spin probes. Two series of PHB samples, prepared by different methods, possessed isotropic and textured morphologies. The noncrystalline phase of PHB contain regions of two types with markedly different molecular mobilities. It is suggested that “dense” regions, characterized by a comparatively low mobility of polymer chains, are located near the surface of crystalline grains, while the “loose” (amorphous) regions with a higher mobility of chains are more distant from the surface of grains. Molecular mobility in the dense regions was virtually the same for both isotropic and textured PHB samples, whereas the mobility in the loose regions was lower in the isotropic samples than in the textured ones. Saturation of the polymer with water vapor affected both the mobility of polymer chains and the relative content of loose and dense regions in the samples.