Mulama Austine Amukayia
Mulama Austine Amukayia
School of Physical and Biological Sciences
RESEARCH TOPIC:
Rheological Properties of Non-equilibrium Thin Polymer Films via Dewetting for Advanced Nanophotonic Systems.
ABSTRACT:
The quest for increasing complex film architecture and multiphase systems and the continuous demands for enhanced performance, require a reliable assessment of stress on a submicron scale from spatially resolved techniques. Despite enormous efforts on production of well-controlled polymer nanostructures using direct focused methods such as multistage e-beam, no clear relationship among preparation parameters, rheological and morphological 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. The study 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. Based on its simplicity and effectiveness, dewetting of a thin polymer film has gained attention as a feasible process for improving the scalability and productivity. Further characterization on the spin-coated films was done by optical microscopy (OM), atomic force microscopy (AFM), X-ray diffractometer (XRD) and differential scanning calorimeter (DSC). No characteristic peaks were observed on the XRD spectroscope, implying that iPpMS films were dominated by amorphous properties. However, a weak peak was noted at ~23 ° as an indication of one-dimensional chain – chain processes taking place in iPpMS films. Thinner films (≤140 nm) exhibited high residual stresses accompanied by shorter relaxation times compared to thicker films (>140 nm). The amount of residual stress was not affected by T_dew 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. An activation energy ranging from 60 ± 10 kJ/mol (pure iPpMS) to 90 ± 10 kJ/mol (pure iPS) was obtained. 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, iPpMS exhibited reduced energy barrier for flow indicating that presence of transient clusters of monomers have a short lifetime in iPpMS. The experiments suggest thatpreparation-induced residual stresses affect material properties such as elastic modulus and viscosity of iPpMS as a function of temperature. The results of this study will be useful in production of advanced nanophotonics such as biosensor chips and nanoantennas.