Hole growth in freely standing polystyrene films probed using a differential pressure experiment

We have probed the whole chain mobility of polymer molecules confined to freely standing films by measuring the flow of gas through holes growing in the films at elevated temperatures using a differential pressure experiment. Freely standing polystyrene films were measured for the temperature range 920.2em0ex^∘C<T<1050.2em0ex^∘C for films with two different molecular weights Mw=717×10³ and 2240×10³, with thicknesses 510.3em0exnm<h<970.3em0exnm. This range of film thicknesses is of particular interest because large reductions in the glass transition temperature Tg have been measured previously for freely standing PS films in this thickness range. We find that hole formation and growth, and therefore substantial chain mobility, does not occur until temperatures close to the bulk value of the glass transition temperature Tgᵇᵘˡᵏ. The characteristic growth times τ for the thinnest films, which have reduced values of Tg, are not substantially less than those for thicker films, and we find that these small differences in τ can be understood in terms of the bulk phenomenon of shear thinning. We also show that the viscosity at the edge of the hole inferred from the characteristic growth times obtained in this and previous studies exhibit shear thinning with reduced shear strain rates β that span twelve orders of magnitude.