ISOTHERMAL THICKENING AND THINNING PROCESSES IN LOW-MOLECULAR-WEIGHT POLY(ETHYLENE OXIDE) FRACTIONS CRYSTALLIZED FROM THE MELT .3. MOLECULAR-WEIGHT DEPENDENCE

The existence of nonintegral folding chain (NIF) crystals in a series of poly(ethylene oxide) (PEO) fractions with molecular weight ranging between 3000 and 23 000 has been observed through time-resolved synchrotron small-angle X-ray scattering (SAXS), differential scanning calorimetry (DSC), and transmission electron microscopy (TEM) experiments. It has been found that with increasing molecular weight the isothermal thickening and/or thinning processes which lead to formation of the final integral folding chain (IF) crystals are increasingly hampered. At sufficiently high molecular weights, NIF crystals may be permanently retained. The thermodynamic driving force for these processes is discussed. An explanation of the NIF to IF transformation consists of both a thermodynamic reason (Gibbs free energy change) and a kinetic effect (chain diffusional motion and chain fold number). Of additional interest, the fold length of initial NIF crystals increases with crystallization temperature (or decreasing supercooling) for each fraction as commonly observed in polymer lamellar crystals with a relationship of l = C2 + C1/DELTA-T. However, the slope C1 increases with molecular weight instead of remaining constant as theoretically predicted. This may be caused by a change of the fold surface free energy due to loose folds, chain ends, and hydrogen bonding in the NIF crystals.