Transition in tethered layer thickness induced by concentration changes ina spread film of an amphiphilic graft copolymer

Three well-defined graft copolymers and a polynorbornene backbone with a po ly(ethylene oxide) graft on each backbone repeat unit have been synthesized with fully hydrogenous and deuterated poly(ethylene oxide) grafts. The gra ft copolymers have a backbone degree of polymerization of 50 and the degree s of polymerization of the poly(ethylene oxide) grafts in the separate copo lymers have values of 15, 25, and 50, Each of these copolymers has been spr ead at the air-water interface as Langmuir films with surface concentration s from 0.3 to 4.0 mg m(-2). The spatial extent of the regions occupied by t he backbone and the grafts has been determined by neutron reflectometry. Th e polynorbornene backbone is confined to the air phase and contains an appr eciable amount of air, the volume fraction of which increases as the surfac e concentration of graft copolymer increases. The poly(ethylene oxide) graf ts are essentially totally immersed in the aqueous subphase, the distributi on of ethylene oxide segments being described by a combination of a uniform layer with a parabolic tail. This description applies for all surface conc entrations. The total thickness of the poly(ethylene oxide) layer increases with surface concentration in manner that is very similar to that predicte d by single-chain mean-field theory for brushlike layers formed by block co polymers where there is a repulsive interaction between the tethered blocks and the anchor blocks. At the highest surface concentration the layer thic kness is ca. 3-5 times the radius of gyration of the poly(ethylene oxide) g rafts. There is no evidence for a first-order transition in the layer thick ness at a critical surface concentration of the graft copolymers. Individua l thicknesses of the uniform layer and parabolic regions show quite differe nt behavior. The parabolic region thickness approaches an asymptotic value at high surface concentrations. The uniform layer thickness at first shows no change in layer thickness as surface concentration increases but then be gins to increase abruptly at a surface concentration that increases as the degree of polymerization of the poly(ethylene oxide) grafts decreases.