Neutron reflectometry investigation of polymer-polymer reactions at the interface between immiscible polymers

Abstract

Primary amine end functionalised deuteropolystyrene has been mixed with unmodified hydrogenous polystyrene and a thin film placed on top of a film of an acrylic polymer that either has carboxylic acid groups located at one end of each molecule or as substituents on each repeat unit. After holding at 453 K for defined times, the interfacial excess layer at the interface between the two polymers has been quantitatively analysed using neutron reflectometry and the extent of grafting of the deuteropolymer at the interface determined. Whilst maintaining the concentration of carboxylic acid units constant (fixed reacting groups) the extent of grafting increases with concentration of the functionalised deuteropolystyrene in the polystyrene layer. On changing the molecular weight of the functionalised deuteropolystyrene but maintaining the molar concentration of reactive end groups constant, the extent of grafting is larger for the lower molecular weight polymer. Although, the qualitative variation of the extent of grafting with time is in agreement with theories for interfacial grafting, exact correspondence cannot be obtained. The initial rate of grafting corresponds to second order rate constants of ∼0.1-0.2 1 mol-1 s-1 but saturation of grafting is evident at far lower values (and hence earlier in the reaction process) than predicted by theory. Moreover, this saturation extent of grafting is at a level much lower than anticipated if brush-like layer formation is encouraged by interfacial grafting.