Poly(ethylene oxide) Crystallization and Gelation in Butanol Studied by In Situ SAXS/WAXD

Poly(ethylene oxide) in butanol reveals two exotherms in differential scanning calorimetry (DSC) scans during cooling and two endotherms in DSC scans during heating. Using small-angle X-ray scattering (SAXS) and wide-angle X-ray diffraction (WAXD) to characterize structural evolutions during cooling, annealing, and heating, we found that the thermal behavior is related to poly(ethylene oxide) (PEO) crystallization, PEO gelation, and butanol complexation. The low exotherm (T_exo,L) and endotherm (T_endo,L) correspond to heat flows resulting from PEO crystallization and melting, respectively. During cooling or annealing from a molten state, PEO crystallized at T_exo,L to form interconnected networks at the macroscale, stacks of lamellae at the mesoscale, and monoclinic crystals at the molecular scale. The microscale networks tightly freeze the motion of butanol molecules. Thus, gels formed and turbidity increased due to PEO crystallization. By heating, PEO crystallites melted and collapsed at T_endo,L, beyond which the gels dissolved to form transparent solutions. In contrast, butanol molecules were complexed with PEO at T_exo,H. We believe that butanol complexes with PEO should be due to hydrogen bonds between ether groups in PEO and hydroxyl groups in butanol. The butanol complexes dissolved at T_endo,H. Because the scattering length density of butanol complexes is similar to that of free butanol molecules, the formation and dissolution of butanol complexes have no effect on the optical property of solutions of PEO in butanol.