ISOTHERMAL THICKENING AND THINNING PROCESSES IN LOW-MOLECULAR-WEIGHT POLY(ETHYLENE OXIDE) FRACTIONS CRYSTALLIZED FROM THE MELT .4. END-GROUP DEPENDENCE

A series of low molecular weight poly(ethylene oxide) (PEO) fractions with different molecular weights (MW=3000 and 7100) and end groups (-O H, -OCH3, -OC(CH3)3, and -OC6H5) have been systematically studied. The end-group effect on diffusional motion in the melt of these PEO fract ions was characterized by self-diffusion coefficient measurements thro ugh nuclear magnetic resonance. Wide-angle X-ray diffraction experimen ts indicated that the crystal structures of the PEO fractions with dif ferent end groups were identical during and after crystallization. The existence of nonintegral folding chain (NIF) crystals in these PEO fr actions in a wide undercooling region was observed by time-resolved sy nchrotron small-angle X-ray scattering, differential scanning calorime try, and transmission electron microscopy experiments. The integral fo lding chain (IF) crystals were found to be formed through both thicken ing and thinning processes during and/or after the NIF crystallization . It was also found that, with increasing molecular weight and size of the end group, the thickening and thinning processes were increasingl y hampered. Of additional interest, the fold length of initial NIF cry stals not only increases with crystallization temperature (or decreasi ng undercooling) for each PEO fraction as commonly observed in polymer lamellar crystals but also changes with the size of the end group. Th e kinetics of transformation from NIF to IF crystals is explained thro ugh the chain sliding diffusional motion along the direction perpendic ular to the lamellar surface, which is end-group size dependent. Linea r crystal growth rate data measured via polarized light microscopy con firmed that the existence of bulky end groups reduces NIF crystal grow th rates for PEO (MW = 3000) fractions. With increasing molecular weig ht, the end-group dependence gradually vanishes due to the introductio n of chain entanglement.