Isothermal thickening and thinning processes in low-molecular-weight poly(ethylene oxide) fractions crystallized from the melt. 8. Molecular shape dependence

Three- and four-arm poly(ethylene oxide) (PEOs) fractions were synthesized to investigate the molecular shape dependence on polymer crystallization, melting, and annealing behaviors. Molecular parameters were characterized via vapor pressure osmometry, gel permeation chromatography, light scattering, and Fourier transfer infrared spectroscopy. Results from wide-angle X-ray diffraction indicate the star PEOs to have monoclinic crystal structures identical to those of linear PEOs, implying that the coupling agents are rejected from the crystals. Results from synchroton small-angle X-ray scattering (SAXS) show that during the initial isothermal crystallization at low undercoolings, the long period gradually decreases with increasing time before reaching a thickness corresponding to the summation of the thicknesses of the crystals with an extended arm length and two layers consisting of the coupling agent and the uncrystallized PEO arm. The dependence of the final long period on the crystallization temperature as determined by SAXS, is remarkably similar to the melting temperature response to crystallization temperature obtained via differential scanning calorimetry. A partial melting upon heating and recrystallization during annealing can be identified based on the combined experimental results obtained from the other methods.