Are metastable, precrystallisation, density-fluctuations a universal phenomena?

Abstract

In-situ observations of crystallisation in minerals and organic polymers have been made by simultaneous, time-resolved small angle X-ray scattering (SAXS) and wide angle X-ray scattering (WAXS) techniques. In isotactic polypropylene slow quiescent crystallisation shows the onset of large scale ordering prior to crystal growth. Rapid crystallisations studied by melt extrusion indicate the development of well resolved oriented SAXS patterns associated with long range order before the development of crystalline peaks in the WAXS region. Block copolymers self-assemble into mesophases in polymer melts above a critical chain length (or above a critical temperature) and this self-assembly process is shown to be susceptible to an incipient crystallisation. Mesophase formation is observed at anomalously high temperatures in ethylene-oxide containing block copolymers below the normal melting point of the polyoxy ethylene chains. Formation of calcium carbonate from aqueous solutions of sodium carbonate and calcium nitrate is observed to be a two-stage process and precipitation proceeds by the production of an amorphous metastable phase. This phase grows until it is volume filling and leads to the formation of the two polymorphs Calcite and Vaterite. These three sets of results suggest pre-nucleation density fluctuations, leading to a metastable phase, play an integral role in all three classes of crystallisation. In due course, this phase undergoes transformation to "normal" crystals.

Crystallization and melting of polyethylene copolymers: Differential scanning calorimetry and atomic force microscopy studies

“…Experimental observations of a SAXS signal intensity increase, before the onset of a WAXS signal (i.e., crystal Bragg peaks), during the very early stages of crystallization were reported for PET, [28][29][30][31] iPP, 32,33 and PE. 34 This was claimed to be due to a precrystalline orientation in the melt and was sometimes explained on the basis of linearized Cahn-Hilliard theory of spinodal decomposition.…”

Distributions of critical flux: modelling, experimental analysis and consequences for cross-flow membrane filtration

“…In polymer phase separation, experimental methods provide evidence of microphase separation caused by chemical polydispersity of the copolymers [20]. In crystallisation, results "suggest prenucleation density fluctuations, leading to a metastable phase, play an integral role in all three classes of crystallisation" [21]. The use of a distribution in critical flux could be a way to account for the existence of a metastable phase preceding the spinodal decomposition when considering the phase transition leading to the formation of a colloidal deposit on a membrane interface;…”

Effect of processing parameters on the morphology development during extrusion of polyethylene tape: An in-line small-angle X-ray scattering (SAXS) study

“…Both observed that increasing the haul-off speed of the fibre increased the orientation and lamellar repeat distances, thus the morphology changed from spherulitic to a sheaf-like structures. Further to this, earlier publications by the authors [29][30][31] have detailed SAXS/WAXS studies on extrusion of polypropylene tapes, which concentrated on observing the onset of large-scale ordering prior to any crystalline growth. The onset was revealed by the development of intensity in the SAXS data before crystalline peaks in the WAXS data are observed.…”