Poly(urethane-urea) nanocapsules prepared by interfacial step polymerisation in miniemulsion. The droplet size: A key-factor for the molecular and thermal characteristics of the polymeric membrane of the nanocapsules?

International audienceWe present some original results as regards the influence of the size reduction of oily core polyurethane nanocapsule and their chemical structure. Nanocapsules with mean diameter of 70 nm were obtained by interfacial step polymerisation in direct miniemulsion. The chemical microstructure of the nanocapsule membrane was investigated by 13C NMR, MALDI-TOF mass spectrometry and FTIR spectroscopy. 13C NMR investigation allowed to point out the different reactivity of the primary and secondary-NCO groups of IPDI toward water and 1,6-hexanediol. The morphology of the nanocapsules was studied by Scanning and Transmission Electron Microscopy. The thermal properties of the nanocapsule membrane were also studied. The results were compared to the characteristics of previously studied nanocapsules with mean diameter of 200 nm, obtained from the same monomers. Although interfacial polymerisation is well known to depend on the characteristics of the interface between the dispersed and the continuous phases, it appeared that the chemical structure of the membrane and the final properties of the nanocapsules were not influenced by the size of the nanodroplets of the miniemulsion in the range 70 - 200 nm. Neither isocyanate hydrolysis nor hydrophilic monomer diffusion to the organic internal phase was modified by the change of the interfacial area. Furthermore the results definitely attested for the low diffusion of the hydrophilic monomer toward the internal phase compared to hydrolysis of IPDI with cyclohexane as dispersed phase. This work presents an accurate study of nanocapsules prepared by interfacial step polymerisation, especially concerning the chemical microstructure analysed by MALDI-TOF mass spectrometry and the different reactivity of the isocyanate groups of IPDI investigated by 13C NMR