Organization of Well-Defined Amphiphilic Graft Copolymers at the Air−Water Interface

Abstract

A well-defined graft copolymer with a polynorbornene backbone and poly(ethylene oxide) grafts has been spread at the air-water interface and the organization of the copolymer investigated over a wide range of surface concentration using neutron reflectometry. The data have been analyzed using both optical matrix and kinematic approximation methods to provide the dimensions of the regions occupied by the backbone, poly(ethylene oxide) grafts and the near surface water layer thickness. The backbone region has a constant thickness of ca. 5 A over the whole surface concentration range explored whereas the poly(ethylene oxide) graft layer increases in thickness monotonically over the same surface concentration range from ca. 15 to 45 A. A more detailed examination of the reflectivity due to the poly(ethylene oxide) layers suggests that it is composed of two layers, one of which has a uniform density of ethylene oxide segments with the second having a parabolic decay of segments. The exponent for the dependence of the poly(ethylene oxide) total layer thickness on the density of grafting of the water surface (σ) was 0.66, i.e., much stronger than predicted by theory for a brush like layer. The near surface water layer self-partial structure factor could not be fitted with the often used uniform layer model. Attempts to model this parameter using multiple uniform layer models or a parabolic increase in water number density could not reproduce the features observed experimentally, suggesting that the near surface water layer is organized in a more complex manner than hitherto suspected when in the presence of poly(ethylene oxide).