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.Item Perpendicular and tilted chains in lamellar crystals(Journal of Macromolecular Science-Physics, 2003) de Silva, D.S.M.; Zeng, X.B.; Ungar, G.; Spells, S.J.Chain tilt and surface disorder were investigated in end-deuterated long n-alkane C12D25C192H384C12HD24 crystallized from solution and in n-alkane C162H326 crystallized from melt. Small-angle X-ray scattering and infrared spectroscopy were employed. Extended-chain crystals of C12D25C192H384C12HD24 as-grown from solution have the molecular axis perpendicular to the lamellar surface, but when heated, around 908C, they start tilting relative to the layer normal. The tilt increases gradually to reach 358 just below the melting point. C162H326 crystallized from the melt at small supercoolings has chains tilted at 358 at the outset, as found previously for all melt-crystallized long alkanes and polyethylene. However, for the first time in long alkanes, it is found that when molten C162H326 is supercooled to DT $ 10K; crystals with perpendicular chains form. At still larger DT, the chains are once-folded, with a mixed population of tilted and perpendicular chain crystals. The use of Davydov splitting of the CH2 and CD2 bending vibration of the end-labelled alkane C12D25C192H384C12HD24 allows independent IR probing of molecular disorder at the deuterated surface and in the hydrogenous crystal interior. The initially small CD2 splitting and the presence of an additional singlet component indicate a rough surface in as-grown crystals, with considerable longitudinal interchain disorder. It is estimated that about 10% of chains are displaced by up to a dozen C-atoms. The increase in splitting and decrease in absorbance of the singlet, which occur on annealing at progressively higher temperatures, are evidence of steady improvement in translational surface order, occurring simultaneously with increasing chain tilt angle. From the above evidence, it is concluded that the absence of tilt in as-grown crystals is not the result of high surface order, as in the case of shorter odd n-alkanes, but rather of a high frozen-in longitudinal disorder with chain ends or folds protruding out of or sunken beneath the crystal surface. It is also concluded that chain tilt only becomes necessary as the crystal surface becomes translationally more ordered and the crystal–amorphous interface sharpens. The effect of chain tilt on the Davydov splitting is addressed briefly.Item Chain tilt and surface disorder in lamellar crystals. A FTIR and SAXS study of labeled long alkanes(Macromolecules, 2002) de Silva, D.S.M.; Zeng, X.; Ungar, G.; Spells, S.J.Small-angle X-ray scattering (SAXS) and infrared spectroscopy (IR) are employed in a study of chain tilt and disorder in solution-crystallized long alkanes C198H398 and C12D25C192H384C12HD24 in extended and once-folded conformations. The as-grown crystals have chains perpendicular to the lamellar surface, but around 90 °C they start tilting relative to the layer normal. The tilt increases gradually to reach 35° just below melting point. The end-labeled alkane allows independent IR probing of molecular disorder at the deuterated surface layer and in the hydrogenous interior of the crystals. The initially small splitting of the CD2 bending mode doublet and the presence of a singlet component indicated a rough surface in as-grown crystals, with considerable translational disorder. The increase in splitting and decrease in absorbance of the singlet which occur on annealing at progressively higher temperatures showed a steady improvement in translational surface order, concomitant with an increase in chain tilt angle. Thus, it is concluded that the absence of tilt in as-grown crystals is not due to high surface order, as in the case of shorter odd n-alkanes, but rather to high nonequilibrium surface disorder with chain ends or folds protruding out of or sunk beneath the surface. It is also concluded that chain tilt only becomes necessary as the crystal surface becomes translationally more ordered and the crystal-amorphous interface sharpens. IR also demonstrated the reversible increase in conformational disorder in the surface layer with increasing temperature and an almost negligible increase in the crystal interior. The gradual change in tilt angle and the existence of noncrystallographic basal planes is interpreted in terms of translational molecular disorder at the surface. The increased central SAXS scatter during the tilting process indicates the creation of voids associated with ridge formation and corrugation of the lamellae.Item Dilution wave and negative-order crystallization kinetics of chain molecules(Physical Review Letters, 2000) Ungar, G.; Mandal, P.K.; Higgs, P.G.; de Silva, D.S.M.; Boda, E.; Chen, C.M.We show that the crystal growth rate of a very long-chain n-alkane C198H398 from solution can decrease with increasing supersaturation and follow strongly negative order kinetics. The experimental behavior can be well represented by a theoretical model which allows the molecule to attach and detach as either extended or folded in two. The obstruction of extended-chain growth by unstable folded depositions increases disproportionately with increasing concentration. As a consequence of this abnormal kinetics, a “dilution wave” can propagate and trigger a folded-to-extended-chain transformation on its way.