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 have been synthes ized to investigate the molecular shape dependence on polymer crystallizati on, melting, and annealing behaviors. Each arm in these star PEOs has a mol ecular weight (MW) of 2220 (M-a = 2220) and has been coupled using 1,3,5-be nzene tricarbonyl trichloride and 1,2,4,5-benzene tetracarboxylic acid. Mol ecular parameters have been characterized via vapor pressure osmometry, gel permeation chromatography, light scattering, and Fourier transfer infrared spectroscopy. The coupling agents containing phenylene groups act as chemi cal defects at the center of the molecule. Diffusion in the melt of these s tar PEOs as measured by nuclear magnetic resonance shows that at constant t emperature the self-diffusion slows down with an increase in the number of arms. Wide-angle X-ray diffraction (WAXD) experiments indicate that these s tar PEOs possess monoclinic crystal structures identical to those of linear PEOs, implying that the coupling agents are rejected from the crystals. Th e overall crystallization rate of the star PEOs decreases with increasing c rystallization temperature (T-c) and number of arms. On the basis of synchr otron small-angle X-ray scattering (SAXS) experiments, it has been found th at during the initial isothermal crystallization at low undercoolings (Delt a T degrees s) the long period gradually decreases with increasing time bef ore reaching a thickness corresponding to the summation of the thicknesses of the crystals with an extended arm length and two layers consisting of th e coupling agent and the uncrystallized PEO arm. The final crystals possess the regular overall molecular conformations. At high Delta T degrees s, th e long period remains constant both during and after crystallization with n o observed thinning process. Only irregular overall molecular conformations in these crystals can be expected. The dependence of the final long period on T-c, as determined by SAXS, is remarkably similar to the melting temper ature response to T-c obtained via differential scanning calorimetry (DSC). The annealing effect has been examined for samples crystallized at 32 degr ees C, subsequently heated to 50 degrees C, and isothermally annealed for v arious periods of time. A partial melting upon heating and recrystallizatio n during annealing can be identified based on the combined experimental res ults obtained from DSC, WAXD, and SAXS methods.