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 rear rangements 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)C 94H189. The initial crystals formed below the melting point of the fol ded chains are noninteger folded (NIF); this means that the layer peri od 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 formatio n, the NIF crystals transform to the F2 ''integer'' form with all chai ns folded in two in the middle (p = 2). While this transformation is v ery rapid in the branched alkane, it is considerably slower in the lin ear alkane. From the electron density profiles across the lamellar sta cks, 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 wit h an additional thick interlamellar amorphous layer containing long un crystallized chain ends (cilia). In the initial NIF form, half the mol ecules,are folded and fully crystallized. The others traverse the crys tal layer only once and are only half-crystallized. It is suggested th at the uncrystallized ends (cilia) of these latter molecules are gener ally too short to be incorporated in the crystal and are uneven in len gth due to the longitudinally random chain attachment during crystal g rowth. The NIF --> F2 transformation involves postcrystallization of t he 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 bra nch at the lamellar surface, thus leaving the cilia of length l/2 suit able for incorporation into the F2 crystals.