Single Lamella Crystals of Polyethylene Accessible by Catalytic Emulsion-Polymerization

INTRODUCTION Since many decades, polyethylene (PE) is one of the commodity polymers which has become ubiquitous because of its low price and good mechanical properties. Hence, the number of applications of the material is huge and many millions of tons are produced worldwide annually. However, PE has hardly played any role in the emerging field of nanotechnology so far. This is due to the problem that PE is either produced by free radical polymerization under high pressure and temperature or with metal-organic catalysts working exclusively under strictly water-free conditions. Polymer nanoparticles and their composites with inorganic compounds, however, are very often produced in aqueous systems. Recently, it was demonstrated that ethylene can be polymerized in aqueous systems in a catalytic fashion by Ni(II) complexes. By virtue of this novel synthesis, long chains of polyethylene can be generated in a well-controlled environment and at ambient temperature. Thus, it could be shown that aqueous PE-dispersions can be produced. Up to now, the particles synthesized in this way were semicrystalline and consisted of stacks of many crystalline lamellae. Here we demonstrate that well-defined polyethylene particles consisting of a single lamella can be generated in this way selectively. The analysis presented here employs a combination of cryogenic transmission electron microscopy (cryo-TEM) and small-angle X-ray scattering (SAXS). While cryo-TEM allows us to study the shape of the particles in a shock-frozen solution, the analysis by SAXS leads to the in situ determination of the internal structure of the particles.