The formation of primary crystals and their subsequent structural
rearrangements are
studied by real-time simultaneous small- and wide-angle X-ray
diffraction using linear long-chain alkane
C246H494 and a long alkane with a methyl branch
in the center of the chain
C96H193CH(CH3)C94H189.
The
initial crystals formed below the melting point of the folded chains
are noninteger folded (NIF); this
means that the layer period in the chain direction is
l/p, where l is the extended-chain
length and p is a
noninteger (presently 1 < p < 2). Following their
formation, the NIF crystals transform to the F2
“integer”
form with all chains folded in two in the middle (p = 2).
While this transformation is very rapid in the
branched alkane, it is considerably slower in the linear alkane.
From the electron density profiles across
the lamellar stacks, it is calculated that the NIF form consists of a
crystalline core of the same thickness
as that in the once-folded F2 lamellae, but with an additional thick
interlamellar amorphous layer
containing long uncrystallized chain ends (cilia). In the initial
NIF form, half the molecules are folded
and fully crystallized. The others traverse the crystal layer only
once and are only half-crystallized. It
is suggested that the uncrystallized ends (cilia) of these latter
molecules are generally too short to be
incorported in the crystal and are uneven in length due to the
longitudinally random chain attachment
during crystal growth. The NIF → F2 transformation involves
postcrystallization of the cilia as they
reach a length of l/2 through chain translation. The
rapid transformation in
C96H193CH(CH3)C94H189
is
believed to be due to the initial preference for “correct” chain
attachment, with the branch at the lamellar
surface, thus leaving the cilia of length l/2 suitable for
incorporation into the F2 crystals.
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