Effect of the synthetic methodology on molecular architecture: From statistical to gradient copolymers
Résumé
Styrene-butyl acrylate (S-BuA) copolymers were synthesized by nitroxide-mediated controlled radical polymerization using an alkoxyamine as an initiator. Using different synthetic methodologies, statistical copolymers were be obtained by batch nitroxide mediated polymerization while the gradient composition was a forced gradient by continuous addition of S during BuA polymerization (semi-batch process). These gradient copolymers have been studied by 1H NMR and size exclusion chromatography to characterize the gradient composition molecular structure. The evolution of the composition was correlated with the glass transition temperature (Tg) of the copolymers. The gradient copolymers exhibit one Tg with a value in between the Tg of polystyrene and poly(butyl acrylate), indicating that the materials did not present well defined microphase separation. Specific organization at the air-polymer interface of such copolymers has also been demonstrated by comparison between classical and attenuated total reflection (ATR) Fourier transform infra-red spectra. This bulk soft matter assembly was confirmed by AFM analysis, which showed a different morphology at the surface and in the bulk following removal of the top layer. Moreover, for the most well defined gradient composition, a specific nano-structuring was demonstrated by small angle neutron scattering. The preliminary rheological properties of these gradient copolymers were studied and are discussed in relation with their molecular structure.