NONINTEGRAL AND INTEGRAL FOLDING CRYSTAL-GROWTH IN LOW-MOLECULAR MASS POLY(ETHYLENE OXIDE) FRACTIONS .1. ISOTHERMAL LAMELLAR THICKENING AND THINNING

The overall crystallization and crystal melting of one low-molecular mass poly (ethylene oxide) (PEO) fraction (MW 3000) have been investigated by differential scanning calorimetry (DSC) and in situ small-angle x-ray scattering (SAXS). The salient new results indicate that initial transient crystals with nonintegral folding (NIF) chain lengths form over a wide range of crystallization temperatures. This NIF structure subsequently transforms into crystal forms with integral folding (IF). The PEO IF crystals consist of the extended chain (n = 0) crystal and the once-folded chain (n = 1) crystal, while the NIF has an intermediate fold length. The NIF --> IF transformation occurs either by lamellar thickening or thinning. The NIF crystal is less stable than the IF (n = 1) crystal, but its growth is more rapid. Crystallization of the PEO (MW 3000) fraction is thus recognized as a compromise between the direction of the thermodynamic driving force and the kinetic pathway. Some potential consequences of these observations are also addressed.