On perpendicular and tilted chains in lamellar crystals

Chain tilt and surface disorder were investigated in end-deuterated long n-alkane C₁₂D₂₅C₁₉₂H₃₈₄C₁₂HD_{2 4} 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 C₁₂D₂₅C₁₉₂H₃₈₄C₁₂HD_{2 4} as-grown from solution have the molecular axis perpendicular to the lamellar surface, but when heated, around 90°C, they start tilting relative to the layer normal. The tilt increases gradually to reach 35° just below the melting point. C₁₆₂H₃₂₆ crystallized from the melt at small supercoolings has chains tilted at 35° 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 ΔT ≥ 10 K. crystals with perpendicular chains form. At still larger ΔT, 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 C₁₂D₂₅C₁₉₂H₃₈₄C₁₂HD_{2 4} 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.