Rheological properties of non-equilibrium thin polymer films via dewetting for advanced nanophotonic systems

Abstract

Advanced nano-devices require the tuning of the size and shape of nanostructures with nanometre-scale precision over a large area. These rigid requirements are beyond the reach of conventional lithographic techniques or self-assembly approaches which are complex, with multiple processes that are difficult to scale to large-area and non-rigid substrates. Despite enormous efforts to uncover non-equilibrium dynamics in polymer films over the last few decades, no clear relationship between preparation parameters and rheological properties has been found. Further, there is inadequate information on the rheological properties of polystyrene (PS) when a nonpolar methyl group is introduced to the phenyl ring of styrene monomers to give rise to poly(para-methylstyrene) (PpMS) yet these monomers have advantages of strength and stability in comparison to styrene. In this study, we investigated the influence of preparation parameters, dewetting temperature (T_dew) and isotactic PS (iPS) blending dynamics on rheological properties of isotactic PpMS (iPpMS) films spin-coated on slippery silicon substrates via dewetting. Further characterization on the spin-coated films was done by optical microscopy, atomic force microscopy, X-ray diffractometer and differential scanning calorimeter. At a given time, the dewetting patterns were found to depend on the film thickness, spin-coating speed, with thinner films exhibiting more patterns than thicker films possibly due to high amount of residual stresses(σ_res) and shorter relaxation times. Intriguingly, dewetting dynamics in iPpMS was characterized by a transition at T_dew≥220 ℃ where the initial dewetting velocity increased slowly with T_dew. A particular focus was on the amount of residual stresses σres induced by film preparation, the variation with temperature of the corresponding relaxation time τ, obtained via three independent dewetting parameters, the shear modulus G, obtained from σres and the maximum height of the dewetting rim, and the polymer viscosity η, deduced via an assumed Maxwell-type model as the product of G•τ. As residual stresses were related to non-equilibrated chain conformations frozen-in in the course of rapid solvent evaporation during film preparation, its amount was not affected by Tdew at which dewetting was performed. Correspondingly, the shear modulus of iPpMS films was found to decrease monotonically with increasing temperature which might be related to the reduction of the activation energy for molecular relaxations as deduced from the observed temperature-dependence of various relaxation processes in spin-coated polymer films. Using a Maxwell-type model, the corresponding viscosity of the film showed the expected decrease with increasing temperature. Within error, all three values of τ were identical and followed an Arrhenius behavior yielding an activation energy ranging from 60 ± 10 kJ/mol (pure iPpMS) to 78 ± 10 kJ/mol (blended iPpMS). It is clear that the activation energies characterizing the relaxation of preparation-induced residual stresses seem not to be affected by the size of the side groups. However, in comparison to iPS dopant, iPpMS exhibit reduced energy barrier for flow indicating that presence of transient clusters of monomers have a short lifetime in iPpMS. This is an indication that it is possible to tune the rheological properties of iPpMS system through blending. Our experiments suggest thatpreparation-induced residual stresses affect material properties such as elastic modulus or viscosity of iPpMS as a function of temperature. Thus, the here presented study demonstrates clearly that systematic dewetting experiments provide a convenient and simple approach for a quantitative determination of rheological properties of thin polymer films. We expect the result of the study to provide a basis for the use of thin-film dewetting to realise the industrial-level fabrication of various practical advanced photonic systems.