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BP: Fachverband Biologische Physik
BP 21: Focus: Stress Relaxation in Polymers - From single molecules to biological cells (with CPP)
BP 21.2: Vortrag
Donnerstag, 29. März 2012, 10:00–10:15, C 243
Direct stress measurements in nonequilibrium thin polymer films. — •Katherine Thomas1 and Ullrich Steiner2 — 1Max Planck Insitute for Dynamics and Self-Organisation, Goettingen, Germany — 2Department of Physics, University of Cambridge, UK
Residual stresses in polymer films often arise from the solution deposition protocol. The origin of stresses in polymer films is easily justified: film formation by solvent evaporation steadily increases the polymer concentration, raising the glass transition temperature of the solution. When Tg reaches the ambient temperature the polymer vitrifies, suppressing further equilibration of the chains. Due to the entangled nature of the polymer network, evaporation of the remaining solvent induces substantial biaxial tensile stresses. Stresses in thin films are not, however, easy to measure and can often only be deduced indirectly.
Here stresses were quantitatively determined by measuring the deflection of cantilevers cut from film-covered SiN membranes using a focused ion beam. Spin-cast films showed notably high residual stresses, far greater than the bulk polymer tensile strength. Stress relaxation via thermal annealing suggests two relaxation mechanisms, both much faster than the reptation time. The fast relaxation indicates that the stress arises from segmental deformations of the chains, rather than entropic effects. Comparison of these data with EHD film stability experiments, suggests the same relaxation mechanisms, providing further evidence for the existence of a thin, highly stressed layer at the surface of the film. These experiments show the utility of our method for the systematic examination of non-equilibrium thin film properties.