Chemistry
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Item The effects of self-poisoning on crystal morphology and growth rates(Advances in Polymer Science, 2005) Ungar, G.; Putra, E.G.R.; de Silva, D.S.M.; Shcherbina, M.A.Recent extensive experimental work and the limited theoretical studies of the phenomenon of self-poisoning of the crystal growth face are reviewed. The effect arises from incorrect but nearly stable stem attachments which obstruct productive growth. Experimental data on the temperature and concentration dependence of growth rates and the morphology of long-chain monodisperse n-alkanes from C162H326 to C390H782 are surveyed and compared to some previously established data on poly(ethylene oxide) fractions, as well as on polyethylene. The anomalous growth rate minima in both temperature and concentration dependence of growth rates are accompanied by profound changes in crystal habits, which have been analysed in terms of growth rates on different crystallographic faces, and in terms of separate rates of step nucleation and propagation. In some cases non-nucleated rough-surface growth is approached. The phenomena covered include “poisoning” minima induced by guest species, the “dilution wave” effect, autocatalytic crystallization, pre-ordering in solution, two-dimensional nucleation, and the kinetic roughening and tilt of basal surfaces.Item Events at the growth face as revealed by concentration dependence of crystallization rate of long alkanes(American Chemical Society, 2004) Ungar, G.; Putra, E.G.R.; de Silva, D.S.M.It is by now well known that monodisoerse n-alkanes, long enough to exhibit chain folding, show a minimum in crystal growth rate with increasing supercooling ΔT. The minimum occurs near the transition from extended-chain (E) and once-folded chain growth (F),1 or from once-folded to twice-folded growth.2,3 The anomaly is attributed to the self-poisoning or pinning effect whereby, as transition temperature is approached from above, the unstable once-folded overgrowth diverges and virtually stops the growth of extended-chain crystals.4 Preliminary solution-crystallization experiments have shown a similar kinetic anomaly as a function of increasing concentration.5 Here we present a more detailed study of concentration dependence of crystal growth rate, resolved into step initiation and step propagation rates. The study confirms the strongly negative reaction order in the range of strong poisoning, and an order of 2 to 3 for folded-chain secondary nucleation in the range of weak poisoning, giving support to the idea of a 2-dimensional secondary nucleus.