Integral Folding Research Articles

Nonintegral and integral folding crystal growth in low‐molecular mass poly(ethylene oxide) fractions. I. Isothermal lamellar thickening and thinning

AbstractThe 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.

Nonintegral and integral folding crystal growth in low‐molecular mass poly (ethylene oxide) fractions. II. End‐group effect: α,ω‐methoxy‐poly (ethylene oxide)

AbstractDifferential scanning calorimetry (DSC) and in situ small‐angle x‐ray scattering (SAXS) indicate that in an α ω‐methoxy‐poly(ethylene oxide) (MPEO) fraction (MW 3000) a transient nonintegral folding (NIF) crystal initially forms during crystallization throughout a wide range of crystallization temperatures. Subsequent transformations of the NIF to IF (integral folding) crystals at low temperatures occur mainly through isothermal thickening or thinning via perfection processes or, at higher temperatures, through primary crystal formation. The NIF crystal is thermodynamically the least stable state among the crystal forms, but its growth is the most rapid. The overall crystallization and crystal melting of this MPEO fraction reveal that the NIF crystal and the NIF → IF crystal transformations are common to low‐molecular mass PEO fractions without regard to the end group. Nevertheless, diffusion coefficient and viscosity measurements provide clear evidence of an end‐group effect in PEO and MPEO fractions. The difference in the overall crystallization and isothermal thickening and thinning kinetics of low‐molecular mass PEO and MPEO fractions can lead to further understanding of end‐group effects.