Variable temperature magic angle spinning 13C NMR experiments on linear alkanes and low molecular weight polyethylene allowed the observation of up to six carbon atoms at the chain ends near the surface of the lamellar crystallites. Conformational order-disorder and intermolecular packing effects are reflected by the spectra. The effects are discussed with regard to the peculiar phase behaviour of the n-alkanes and the fringed micella model for low molecular weight polyethylene. The solid-solid transitions of nonadecane and hexatriacontane appear to be correlated with the occurrence of specific non-liquid-like conformational disordering. Thus, conformational defects were observed for the "rotator phase" which depend on the chain length and which vary along the chain. Whilst the center part of the chain remains all-trans ordered, significant gauche defects were observed for the bonds at the chain ends. For a polydisperse low molecular weight linear polyethylene as well as for monodisperse octahexacontahectane reversible liquid-like conformational disordering of the terminal chain segments could be detected when the melting point was approached. Hence, in this case the NMR spectra indicate pre-melting of the lamella surface, while in case of the shorter n-alkanes the Occurrence of end-gauche defects is correlated with solid-solid transitions.
Cycloalkanes with 12 to 96 methylene groups show solid-solid transitions of high cooperativity. Although the molecules are highly flexible and do not contain mesogenic units as it is typical for liquid crystals, they form a mesomorphic phase. In this mesomorphic state, fast molecular motion with correlation times smaller than s is observed by 2H-NMR. The molecular structure is characterized by dynamic conformational disorder. In addition slow conformational interconversion is evident below the phase transition. Thus, jump type interconversion processes which do not result in disordering occur in the fully ordered crystal. At the phase transition, the increase in the specific volume and the onset of additional molecular mobility results in the formation of a partially disordered, fluid structure.
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