Intrinsic Stresses in Thin Glassy Polymer Films Revealed by Crack Formation

Both spin-coating and thermal annealing of thin supported  glassy polymer films generally cause stresses arising from rapid solvent  evaporation and from a mismatch in expansivities of film and substrate,  respectively. While films on adhesive surfaces are typically stable, on slippery  substrates film stability is sensitively determined by the thermal protocol,  revealing the presence of these stresses. There, contraction of the film upon  cooling causes in-plane tensile stresses that can lead to film fracture. While this  is a general effect, the details of the film’s response to thermal cycling allow to  disentangle the origin of stresses within the film. For high molecular weight  polymers, we found that preparation-induced stresses cause substantially  reduced thermal expansivities. This demonstrates that intrinsic out-of-equilibrium states in spin-coated glassy films of long polymers are long-lived  and thus difficult to equilibrate.