Time-Resolved Small-Angle X-ray Scattering Studies during the Aqueous Emulsion Polymerization of Methyl Methacrylate

Area of Science:

• Polymer Science and Engineering
• Materials Chemistry
• Physical Chemistry

Background:

• Previous work established time-resolved synchrotron small-angle X-ray scattering (TR-SAXS) for specialty monomer emulsion polymerization.
• High X-ray contrast is crucial for TR-SAXS, previously achieved with fluorinated monomers.
• The current study investigates the applicability of TR-SAXS to methyl methacrylate (MMA) polymerization, a common industrial monomer.

Purpose of the Study:

• To extend TR-SAXS methodology to the emulsion polymerization of methyl methacrylate (MMA).
• To investigate the influence of sodium dodecyl sulfate (SDS) concentration, relative to its critical micelle concentration (CMC), on nucleation and particle growth.
• To correlate TR-SAXS findings with postmortem analyses (NMR, DLS, TEM).

Main Methods:

• Aqueous emulsion polymerization of MMA at 70 °C with varying SDS concentrations (0 mM, 1.0 mM, 20.0 mM).
• Real-time monitoring using TR-SAXS with a stirrable reaction cell.
• Analysis of scattering patterns with a hard-sphere model to determine particle diameter evolution.
• Postmortem characterization including 1H NMR spectroscopy, dynamic light scattering (DLS), and transmission electron microscopy (TEM).

Main Results:

• Sufficient X-ray contrast between water and poly(methyl methacrylate) (PMMA) enabled high-quality TR-SAXS data acquisition.
• Micellar nucleation (SDS > CMC) occurred rapidly (2 min), yielding small particles and high particle concentration.
• Homogeneous nucleation (SDS < CMC or absent) was significantly slower (14-15 min); SDS presence below CMC reduced coagulation, leading to smaller particles than surfactant-free systems.
• TR-SAXS determined nucleation onset and particle size evolution, consistent with DLS and TEM results.

Conclusions:

• TR-SAXS is a viable technique for studying the aqueous emulsion polymerization of MMA, providing insights into nucleation and particle formation dynamics.
• SDS concentration relative to CMC critically influences nucleation pathway (micellar vs. homogeneous) and resulting particle characteristics.
• The methodology shows promise for investigating the emulsion polymerization of other (meth)acrylic monomers and vinyl acetate.