Organization of well-defined amphiphilic graft copolymers at the air-waterinterface

A well-defined graft copolymer with a polynorbornene backbone and poly(ethy lene oxide) grafts has been spread at the air-water interface and the organ ization of the copolymer investigated over a wide range of surface concentr ation using neutron reflectometry. The data have been analyzed using both o ptical matrix and kinematic approximation methods to provide the dimensions of the regions occupied by the backbone, poly(ethylene oxide) grafts and t he near surface water layer thickness. The backbone region has a constant t hickness of ca. 5 Angstrom over the whole surface concentration range explo red whereas the poly(ethylene oxide) graft layer increases in thickness mon otonically over the same surface concentration range from ca. 15 to 45 Angs trom. A more detailed examination of the reflectivity due to the poly(ethyl ene 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 p arabolic decay of segments. The exponent for the dependence of the poly(eth ylene oxide) total layer thickness on the density of grafting of the water surface (sigma) was 0.66, i.e., much stronger than predicted by theory for a brush like layer. The near surface water layer self-partial structure fac tor could not be fitted with the often used uniform layer model. Attempts t o model this parameter using multiple uniform layer models or a parabolic i ncrease in water number density could not reproduce the features observed e xperimentally, 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).