Isothermal thickening and thinning processes in low molecular weight poly(ethylene oxide) fractions crystallized from the melt: Effects of molecular configurational defects on crystallization, melting, and annealing

Three two-arm poly(ethylene oxide) fractions (PEOs) with molecular weights of 2220 g/mol for each arm have been prepared by a coupling reaction using 1,4-, 1,3-, and 1,2-benzene dicarbonyl dichloride. The two arms at the coupling agents thus form angles of 180°, 120°, and 60°, respectively. Self-diffusion coefficients of these two-arm PEOs in the melt are surprisingly different. Wide angle X-ray diffraction patterns reveal that these PEOs possess the identical crystal structure to that of linear PEO. Observations of time-resolved synchrotron small angle X-ray scattering (SAXS) indicate that the samples crystallized below 38°C forming crystals with non-integrally folded (NIF) overall molecular conformations (OMCs). Crystals with integrally folded (IF) OMCs form when the crystallization tempearture (T_c) is above 44°C,. Two different crystal populations with extended and once-folded OMCs are observed in both the 1,4- and 1,3-two-arm PEOs. Only one crystal population with mixed IF OMCs is found for the 1,2-two-arm PEO. The dependence of long period as a function of T_c for these two-arm PEOs is remarkably similar to the melting temperature response to T_c. The annealing effect is examined for samples crystallized at 32°C, subsequently heated to 50°C, and isothermally annealed for various periods of time. A partial melting upon heating and recrystallization during annealing can be identified.