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Role of Preparation Pathways and Tacticity on the Non-Equilibrium Dynamcis of Polystyrene Films

Rishab Handa. Master Thesis im Studiengang M.Sc. Sustainable Materials, 2016

Under non-equilibrium conditions, polymers undergo conformational rearrangement, causing their behaviour to deviate from bulk. In this thesis, two different non-equilibrium conditions are discussed viz., preparation induced non-equilibrium and flow induced nonequilibrium. In the first part, we discuss about the rapid solvent evaporation induced residual stresses in spin casted (atactic and isotactic) polystyrene films of different thickness. By systematically varying the solution concentration (with respect to the overlap concentration) and rotation speeds used for spin casting, we have shown the possibility of obtaining films of similar thickness, but with significantly varying residual stresses and the corresponding relaxation times. These experiments strongly indicate the importance of preparation conditions and the non-equilibrium nature of as casted polymer films. By relaxing these stresses, we equilibrated the polymer films and used it for the studies in the next part. In the second part, the rapid dewetting dynamics of isotactic polystyrene (iPS) films of two different molecular dispersity (Đ = 1.5 and Đ = 2.8) and various thickness were studied, at temperatures higher than the bulk melting temperature of iPS. It has been earlier shown that the polymers at the dewetting fronts can be extensively sheared. Here we study the response of iPS to this shear. The viscosities obtained from dewetting experiments for all the samples were significantly larger than the zero-shear bulk viscosity of iPS. Further, through Arrhenius temperature dependence of the viscosities, we have obtained activation energy of 170±15 kJ/mol, in contrast to the 50±10 kJ/mol observed for equilibrated melts. These results strongly support the presence of cooperative dynamics in iPS, as shown earlier by Chandran and Reiter.

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