Kathleen A. Barnes

Influence of nanoparticles and polymer branching on the dewetting of polymer films

Abstract Previous studies have shown that spun-cast films of unentangled synthetic polymers commonly dewet inorganic or organic substrates, leading to technologically detrimental results for many applications. We illustrate two strategies for influencing polymer film dewetting on inorganic and organic substrates. First, the addition of small amounts of C 60 fullerene nanoparticles to the spin-casting polymer solutions of model synthetic polymers [polystyrene (PS) and polybutadiene (PB)] leads to a significant inhibition of film dewetting on Si. This effect is associated with the formation of a diffuse fullerene layer near the solid substrate that frustrates the dewetting hole growth process. Next, we consider polymer branching effects on the dewetting of various generations of hypergraft polymer poly(2-ethyl-2-oxazoline) (PEOX) films cast on high molecular weight polystyrene substrates. The early stage of dewetting is found to be similar in a zeroth generation G0 hypergraft (a comb polymer) and a G2 hyper-graft (resembling a spherical ‘micro-gel’ particle). The late stage of dewetting in the G2 films, however, differs significantly from the low generation films because of an inhibition of hole coalescence in the intermediate stage of film dewetting. This behavior resembles previous observations of dewetting in ‘entangled’ polystyrene films. Thus, the viscoelasticity of the polymer film can have an inhibitory effect on film dewetting, leading to changes in the dewetted film morphology rather than a suppression of film dewetting.

Microstructure analysis using temperature-induced contrast variation in small-angle scattering

We propose to introduce Temperature-Induced Contrast Variation (TICV) as a technique for improving the specificity of microstructure analyses obtained using small-angle scattering methods. TICV exploits the fact that, at temperatures well removed from melting or annealing regimes, the scattering contrast in many multicomponent systems exhibits a significant temperature dependence while the form factors for the scattering processes change only slightly. Difference scattering patterns formed by subtracting the pattern measured at a suitable reference temperature from the pattern obtained at another temperature emphasize those components of the microstructure that vary most strongly with temperature over the chosen range. This can facilitate an effective separation of the observed scattering patterns into separate components for differing scattering mechanisms. Examples of distinct scattering mechanisms commonly found in polymers include liquid-like scattering from amorphous components, contributions from the intramolecular amorphous halo, Guinier scattering from inclusions, and the scattering from ordered stacks of crystalline lamellae. The experimental data presented here provide qualitative demonstrations of general features of the method. Further work is needed to obtain improved descriptors to characterize microstructure in these systems.