Chemistry

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Study of disorder in different phases of tetratriacontane and binary alkane mixture, using vibrational spectroscopy
(Journal of Physical Chemistry, 2007) Wickremarachchi, P.A.S.R.; Spells, S.J.; de Silva, D.S.M.
Raman spectroscopy has been used to investigate the monoclinic crystal f rotator f melt phase transitions in n-C34H70, for both real-time heating and cooling runs. Changes in band intensity and frequency in the CH2 bending, CH2 twisting, skeletal C-C stretching, and CH3 rocking regions revealed both transitions, particularly when using band components related to gauche bonds. In the room temperature infrared spectrum, the CH2 rocking-twisting and CH2 wagging progressions were observed and indexed for n-C34H70 and a 2:1 (w/w) mixture of C34H70 and C36D74. This led to best estimates for the all-trans crystal core in both cases of 33 to 34 carbon atoms, indicating that the core corresponds to almost the whole of the C34H70 molecule.
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.
FTIR studies of molecular disorder in long chain n-alkanes: Chain tilt and chain end disorder
(Macromolecular Symposia, 2002) de Silva, D.S.M.; Gorce, J.P.; Wickremarachchi, P.A.S.R.; Spells, S.J.
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.

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