Dilational viscoelastic behaviors of homopolymer monolayers: surface lightscattering analysis

This is to report a new set of viscoelastic analyses of common polymers tha t are surface active at the air/water interface (A/W). The materials are po lyethers 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 a re deduced from the propagation characteristics of spontaneously formed cap illary waves obtained by surface light scattering in conjunction with surfa ce pressure II measurements by the Wilhelmy plate technique. The static fea tures of polymer monolayers at A/W are delineated by the surface pressure d ependence of the static dilational elasticity epsilon(s). 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 stati c features, these polymers fall into two groups, i.e. the good and poor sol vent (not quite theta) groups. In this report, there is a clear corresponde nce between the static features and viscoelastic characteristics of these p olymer monolayers, and each group has its own differentiative static and vi scoelastic profiles. The polymer monolayers belonging to the good solvent c ondition exhibit a viscoelastic profile characterizable as nearly perfectly elastic behavior over a wide range of surface coverage from infinite dilut ion to II approximate to 4 mN m(-1). In contrast, those polymer monolayers under poor solvent conditions, i.e. PMMA and PtBMA, show a viscoelastic pro file of incompressible films with the dynamics approaching the limit of an infinite lateral modulus, epsilon* --> infinity. (C) 2000 Elsevier Science B.V. All rights reserved.