Monika Basiura

Evidence for a mesomorphic phase in precisely branched polyethylenes

Recently several authors suggested the presence of a transient mesomorphic phase to explain the crystallization-induced morphological changes in polyethylenes [1-2]. Up to now the assumed layer-like mesomorphic phase could not be observed by scattering techniques, probably because the amount is too low or its lifetime prior to conversion into crystalline material too short. In the group of K. Wagener precisely branched polyethylenes were synthesized using Acyclic Diene Metathesis (ADMET) polycondensation chemistry [3]. Here, the focus is on the temperature dependent morphology of two ADMET polyethylenes with precisely positioned hexyl branches every 21st and 15th carbon along the polyethylene backbone. During its crystallization the polyethylene with 1 hexyl branch at every 21st carbon atom organizes into paracrystalline stacks of monodisperse crystalline layers, separated by amorphous matter. Small and Wide angle synchrotron X-ray and small angle light scattering experiments reveal that the ethylene sequences in the layers are tilted approximately 45° with respect to the layer normal and that they are arranged in a monoclinic fashion after a rapid transition from amorphous (mesomorphic) precursor layers in which the ethylene sequences are not tilted. The thickness of these precursor layers closely agrees with the value predicted by the Strobl crystallization line for polyethylene [4]. A polyethylene with 1 hexyl branch every 15th carbon atom crystallizes just before vitrification without passing a mesomorphic layer phase. The formed, unidentified presumably crystalline structures melt closely after devitrification. However, upon further heating, amorphous layers are formed with a density deviating from that of the matrix. The fraction of the layers increases with annealing time and the layers melt at the stability limit of mesomorphic layers with a thickness identical to the stretched ethylene sequences and predicted by the Strobl crystallization line [4]. For this particular sample, X-ray scattering experiments support the presence of non-crystalline, mesomorphic layers in a rather wide temperature range.