Topology and nanomechanics of polyethylene networks

We show in this paper that three-dimensional networks in bulk polyethylene (mean molecular weight: 350?000?g?mol?1, polydispersity of 2.62) with a defined grafting density of side-chains of 9.6% can be revealed by spin-coating the gel-like swollen polymer onto a flat surface. The polymer contained side-chains with one (33%), two (20%), three (13%) and more (34%) carbon atoms. The polymer structure after spin-coating consists of bundles, droplets at interconnects and globules. The combined tapping mode and contact mode atomic force microscopy (AFM) is able to distinguish topological differences in interconnects, such as entangled junctions from simple crossings of polymer bundles. A solvent treatment of the polymer network on the sample surface reveals that some of the droplets are in fact superpositions of more than one droplet, indicating again that a three-dimensional network is collapsed to two dimensions. We have observed that polymer bundles do branch near such multiple droplets. Finally, the recorded lateral forces while scanning across a polymer bundle in contact mode might be useful to model the mechanical behaviour of polymer bundles. However, without a solid theoretical model it is difficult to translate the experimental data into meaningful materials properties.