ISOTHERMAL THICKENING AND THINNING PROCESSES IN LOW-MOLECULAR-WEIGHT POLY(ETHYLENE OXIDE) FRACTIONS .1. FROM NONINTEGRAL-FOLDING TO INTEGRAL-FOLDING CHAIN CRYSTAL TRANSITIONS

On the basis of time-resolved synchrotron small-angle X-ray scattering (SAXS) and differential scanning calorimetry (DSC) experiments, a nonintegral-folding chain (NIF) crystal can be found in low molecular weight poly(ethylene oxide) (PEO) fractions. This crystal grows first as a transient state, and integral-folding chain (IF) crystals form later through an isothermal thickening or thinning process. In a PEO fraction with MW = 4250, it is observed that either isothermal thickening to the IF (n = 0) crystal or thinning to the IF(n = 1) crystal occurs depending upon the thermodynamic stability of the NIF crystal. At low crystallization temperatures, the main process is isothermal thinning; at intermediate temperatures, both thickening and thinning processes are observed; only isothermal thickening can be seen at high temperatures. This observation indicates that the NIF crystal exists as an initial step of the crystal growth over a wide crystallization temperature range. The fold length of this NIF crystal is crystallization temperature dependent, a behavior similar to the observed changes of fold length of polymer lamellar crystals. Systematic study in changes of fold length from the NIF to IF crystals with the time has resulted in a kinetic map of such transitions. This process is related to the chain molecular motion along the crystallographic c-axis in the solid state.