Shear and compression viscoelasticity in polymer monolayers

Abstract

Poly-vinlyacetate (PVAc) forms very stable and reproducible monolayers on the surface ofwater, a model system to understand polymer physics on two dimensions. A recentlyintroduced technique is applied here to to study viscoelasticity of PVAc monolayers. Themethod is based on measurement of surface tension in two orthogonal directions duringanisotropic deformation. Compression and shear moduli are explored over a verylarge concentration range, highlighting a series of four different regimes. At lowconcentration the polymers are in a dilute gas. Above the overlap concentrationΓ* there is a fluid semi-dilute region, where the monolayer propertiesare described by scaling laws. At a threshold concentrationΓ**, a decrease in the gradient of pressure with concentration is observed, and we argue thatthere is still a large fraction of free area on the surface. Compressing further, we thenidentify close packing as the point where the pressure gradient rises sharply and a shearmodulus emerges. This is interpreted as a transition to a soft-solid due to thekinetic arrest of close-packed monomers. The rheological properties of PVAc aboveΓ** have not been studied previously. Discussion includes possible explanations for the observedbehaviour in terms of both equilibrium and non-equilibrium conditions, and therelation to microscopic chain properties. Temperature dependent effects aroundΓ** are also observed and described.