Chain end effects and dewetting in thin polymer films

The dewetting of thin films of end-functionalized polymers, ω- and α,ω-barium sulfonato polystyrenes, on a silicon substrate has been investigated as a function of initial film thickness, molecular weight, and functionality of the chains. The lower molecular weight monofunctional chains are found to dewet the substrate analogously to normal polystyrene but display an anomalous flow behavior at the surface. Moreover, after dewetting, the entire silicon surface still remains covered by a monolayer of monofunctional chains. The monolayer consists of a polymer brush of densely packed tethered chains, adsorbed via their ionic end groups. The dense packing and special conformations of the chains in the brush prevent interpenetration with other polymer chains, and the unadsorbed macromolecules dewet the brush. When the molecular weight of the monofunctional chains is increased, entanglements between the adsorbed polymer brush and the unadsorbed chains can occur and the dewetting process is retarded. Thin films of the difunctional chains do not dewet regardless of the molecular weight of the chains. The difference between mono- and difunctional materials is attributed to ionic aggregation, which is responsible for thermoreversible cross-linking and stabilization of thicker films by interaction of aggregates with dangling ends. It is suggested to use high molecular weight end-functionalized chains as polymeric additives to retard thin polymer film dewetting.