Abstract:
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.
