Structure and formation of noninteger and integer folded-chain crystals of linear and branched monodisperse ethylene oligomers

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.