Dilational viscoelastic behaviors of homopolymer monolayers: surface light scattering analysis

This is to report a new set of viscoelastic analyses of common polymers that are surface active at the air/water interface (A/W). The materials are polyethers such as poly(ethyleneoxide) (PEO) and poly(tetrahydrofuran) (PTHF), and vinyl polymers with pendant ester groups such as poly(vinylacetate) (PVAc), poly(methylacrylate) (PMA), poly(methylmethacrylate) (PMMA) and poly(t-butylmethacrylate) (PtBMA). Experimentally the viscoelastic parameters are deduced from the propagation characteristics of spontaneously formed capillary waves obtained by surface light scattering in conjunction with surface pressure Π measurements by the Wilhelmy plate technique. The static features of polymer monolayers at A/W are delineated by the surface pressure dependence of the static dilational elasticity εs. Two static limits are expected for the dependence, one being the good solvent condition and the other the theta condition. By sorting polymers according to their static features, these polymers fall into two groups, i.e. the good and poor solvent (not quite theta) groups. In this report, there is a clear correspondence between the static features and viscoelastic characteristics of these polymer monolayers, and each group has its own differentiative static and viscoelastic profiles. The polymer monolayers belonging to the good solvent condition exhibit a viscoelastic profile characterizable as nearly perfectly elastic behavior over a wide range of surface coverage from infinite dilution to Π≈4 mN m−1. In contrast, those polymer monolayers under poor solvent conditions, i.e. PMMA and PtBMA, show a viscoelastic profile of incompressible films with the dynamics approaching the limit of an infinite lateral modulus, ε*→∞.