Self-assembly of phosphorous containing oligomers: morphological features and pH-sensitiveness in suspension

Abstract

Methacrylamide-based oligomers bearing phosphonate pending groups at the end of a long alkyl chain and originating from undecylenic acid synthons were subjected to direct oligomer dissolution. Size improvement towards much smaller objects was reached using the nanoprecipitation method: the oligomers were first dissolved in an organic solvent, and then precipitated in water using a syringe pump. Dynamic light scattering (DLS) showed phosphorous containing monomodal and quite narrow-sized self-assemblies in water with hydrodynamic diameters (D_H) ranging from 80 to 280 nm (depending on the oligomer system). Direct visualization using transmission electron microscopy (TEM) and atomic force microscopy (AFM) showed filled and almost individual particles with spherical shape. They were considerably shrunk, suggesting the highly swollen character of the self-assemblies in suspension. Morphological information on the multi-scale self-assembled structures was complementarily obtained using static light scattering (SLS). Thus, at a low length-scale, highly segregated sub-units having sharp boundaries surrounded by water (Porod behaviour) were observed, whereas at a high length-scale random non-compact organization of these sub-units via weak interactions was found, forming a chaplet-like structure (Gaussian behaviour). Furthermore, the pH-sensitiveness of the suspensions obtained after the nanoprecipitation method was studied. Particularly, at pH = 12, the characteristic size drastically increased within few hours from typically ∼280 nm to 2 μm due to electrostatic repulsion between deprotonated hydroxyl groups. At longer times, the observed peculiar behaviour corresponded to the model of diffusion-limited cluster aggregation (DLCA) where the particles stuck easily together upon contact [continuation of the article by C. Bouilhac, C. Travelet, A. Graillot, S. Monge, R. Borsali and J.-J. Robin, Polym. Chem., 2014, 5, 2756–2767].