Influence of generation number on the dewetting of hypergraft poly(2-ethyl-2-oxazoline) films

Abstract

We investigate the influence of polymer branching (generation number G) on the dewetting of hypergraft poly(2-ethyl-2-oxazoline) (PEOX) polymer films from model non-polar synthetic polymer and inorganic substrates. The present study contrasts the cases of a zeroth generation G0 hypergraft, which is a comb polymer, and a G2 hypergraft, which resembles a nearly spherical 'microgel' particle. The early stage of dewetting is found to be similar in the G0 and G2 films and is largely independent of substrate (entangled polystyrene film or acid-cleaned silica wafer). However, the late stage of dewetting in the G2 films differs significantly from the GO films because of an inhibition of hole coalescence in the intermediate stage of film dewetting. The holes in the G2 hyperbranched films continue to grow in size until they impinge on each other to form a foam-like structure with a uniform 'cell' size. The boundaries of these cells break up and the vertices of the former cellular network retract to form a relatively uniform droplet configuration. In contrast, hole coalescence in the GO films occurs readily, leading to a polydisperse hole size distribution, and the fluid borders of the holes break up to form the non-uniform 'Voronoi' droplet configuration normally associated with film dewetting. Similar trends have been observed in the dewetting of linear polymer films where entangled polymers behave similarly to the branched (G2) polymers of the present study. This effect is attributed to the viscoelasticity of the G2 hyperbranched polymer fluid.