Organisation and dynamics of well-defined graft copolymers at the air-water interface

Abstract

Novel amphiphilic graft copolymers with a backbone of poly(norbornene) (PNB) with poly(ethylene oxide) (PEO) grafts have been synthesised by a combination of ring opening metathesis and anionic polymerisation methods. The polymer has been prepared with hydrogenous and deuterated grafts and with grafts of different degrees of polymerisation. These graft copolymers spread at the air-water and air-PEO solution interface forming thin films and their organisation and dynamic behaviour is discussed. Monolayer behaviour was characterised from surface pressure isotherms and it was demonstrated that the shape of the isotherm is dependent on graft length and on the concentration of PEO in the subphase. Using neutron reflectometry the organisation of such spread films at the air-water interface have been obtained over a range of surface concentrations for each length of PEO graft. Data were analysed by both exact calculation methods and the partial kinematic approximation and the models adopted were verified by applying the model independent Bayesian analysis. All yield the same description i.e. the hydrophobic backbone remains at the uppermost surface while the PEO grafts penetrate the subphase. The PEO layer increases in thickness with increased surface concentration and graft length. In each case the rate of increase with surface concentration was initially rapid but above a critical concentration, a slower rate was observed. In this latter regime the variation of the tethered layer height scales with surface density (ơ) and degree of polymerisation of the graft (N) as, r(_s) = N(^1.06)ơ(^0.33),which agrees well with scaling and self consistent field theory of polymer brushes. The dynamic behaviour of each copolymer film spread on water has been studied using surface quasi-elastic light scattering. A resonance between the capillary and dilational waves is observed at identical surface concentrations for each copolymer film. The viscoelastic behaviour of the dilational mode is reminiscent of Kramers-Kronig relations. The phenomenon of resistive mode mixing was observed in frequency dependency studies.