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 (-OH, -OCH[sub 3],-OC(CH[sub 3])[sub 3], and -OC(CH[sub 3])[sub 3], and -OC[sub 6]H[sub 5]) have been systematically studied. The end-group effect on diffusional motion in the melt of these PEO fractions was characterized by self-diffusion coefficient measurements through nuclear magnetic resonance. Wide-angle X-ray diffraction experiments indicated that the crystal structures of the PEO fractions with different end groups were identical during and after crystallization. The existence of nonintegral folding chain (NIF) crystals in these PEO fractions in a wide undercooling region was observed by time-resolved synchrotron small-angle X-ray scattering, differential scanning calorimetry, and transmission electron microscopy experiments. The integral folding chain (IF) crystals were found to be formed through both thickening 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 increasingly hampered. Of additional interest, the fold length of initial NIF crystals not only increases with crystallization temperature (or decreasing undercooling) for each PEO fraction as commonly observed in polymer lamellar crystals but also changes with the size of the end group. The kinetics of transformation from NIF to IF crystals is explained through the chain sliding diffusional motion along the direction perpendicular to the lamellar surface, which is end-group size dependent. Linear crystal growth rate data measured via polarized light microscopy confirmed that the existence of bulky end groups reduces NIF crystal growth rates for PEO (MW = 3000) fractions. With increasing molecular weight, the end-group dependence gradually vanishes due to the introduction of chain entanglement.