Capillary wave properties of a spread film of a polybutadiene-poly(ethylene oxide) block copolymer: 1. Air-water interface

A polybutacliene-poly(ethylene oxide) diblock copolymer has been spread at the air-water interface, and the surface pressure isotherm has been determi ned. Over the same concentration range, a spread film of polybutadiene exhi bits no surface pressure and thus for the copolymer the surface moduli (sur face tension and dilational modulus) are due to the solvated poly(ethylene oxide) block. Surface quasi-elastic light scattering has been used to obtai n surface viscoelastic moduli over a wide range of surface concentration at a surface wavenumber of 395 cm(-1). For copolymer surface concentrations b elow 0.8 mg m(-2), surface tension and dilational modulus obtained in this manner were identical with those derived from the surface pressure isotherm . At higher surface concentrations, the light scattering values exceeded th ose from the isotherm and consequently relaxation processes take place in t he spread film. The capillary wave damping exhibits two resonance points at surface concentrations of ca. 0.7 and 0.9 mg m(-2). The surface wavenumber (q) dependences of the capillary wave frequency, damping, and surface visc oelastic moduli were determined for a copolymer surface concentration of 0. 73 mg m(-2) where there is a resonance between the capillary and dilational modes. The surface moduli follow a Maxwell-like behavior for frequencies g reater than 2 x 10(5) s(-1), and there is a change in the q dependence of t he damping that suggests that dilational behavior dominates at low q and ca pillary mode behavior dominates at high q. This is suggestive of mode mixin g being present.