Small-angle X-ray Scattering Studies of Reaction Kinetics During Synthesis of Colloidal Polymer Particles

This work is focused on the development and application of small-angle x-ray scattering (SAXS) techniques for measuring reaction kinetics during the in situ synthesis of polymer nanoparticles. A combination of scattering equations, derived for the various structural morphologies formed during such reactions, and equations that account for mass balance of the reaction components are used to analyse time-resolved SAXS patterns to obtain detailed mechanistic information. Exchange of both reagent and solvent molecules between the self-assembled particles and the surrounding medium occurs during the polymerisation. This causes a continuous change in scattering length density for the system components and enables determination of both the local concentration of the reaction components and various structural parameters for the growing self-assembled particles. The new SAXS approach developed in this work is applied to three different systems: (i) synthesis of poly(stearyl methacrylate)-poly(benzyl methacrylate) (PSMA-PBzMA) diblock copolymer nano-objects via reversible addition-fragmentation chain transfer (RAFT) dispersion polymerisation in a non-polar medium (mineral oil), (ii) synthesis of poly[2-(dimethylamino)ethyl methacrylate]-poly(benzyl methacrylate) (PDMA-PBzMA) diblock copolymer nano-objects via RAFT emulsion polymerisation in a polar medium (80/20 ethanol/water mixture) and (iii) synthesis of poly(2,2,2-trifluoroethyl methacrylate) (PTFEMA)/silica nanocomposite particles via conventional free radical emulsion polymerisation in aqueous media.
For RAFT dispersion polymerisation in mineral oil, SAXS analysis revealed that after micellar nucleation, the rate of polymerisation was proportional to the local monomer concentration within the micelle cores which is consistent with theoretical predictions. It was also found that SAXS could be used to determine Flory-Huggins parameters for pairs of the system components. Moreover, SAXS indicated that the nascent nuclei comprise spherical particles containing a mixture of mineral oil and BzMA monomer within the particle cores stabilised by short PSMA chains.
For RAFT emulsion polymerisation in an 80/20 ethanol/water mixture, SAXS provided detailed information about morphological development in the system from initially dissolved PDMA stabiliser blocks into PDMA-PBzMA deblock copolymers forming spherical micelles at a later stage of the synthesis. The binary solvent composition complicated the SAXS analysis, so two scenarios were considered. According to the most physically realistic scenario, the monomer concentration within the micelle cores was close to zero throughout the reaction, suggesting that the rate of polymerisation was controlled mainly by diffusion of the BzMA monomer through the solvent medium to the spherical micelles and not by the rate of reaction like in the RAFT dispersion polymerisation.
For the free radical aqueous emulsion polymerisation to produce PTFEMA-silica (core-shell) nanocomposite particles, SAXS analysis, apart from measurements of the monomer diffusion coefficient and parameters describing the reaction kinetics, provided information regarding formation of the silica shell and also the silica nanoparticle packing density within this shell.
Overall, in situ SAXS measurements combined with a new approach for scattering analysis counting mass balance of reaction components are shown to be more informative than traditional post-mortem characterisation techniques such as electron microscopy or dynamic light scattering.