Time-Resolved Small-Angle X-ray Scattering Studies during Aqueous Emulsion Polymerization.

Adam Czajka,Steven P Armes

doi:10.1021/jacs.0c11183

Abstract

The persulfate-initiated aqueous emulsion polymerization of 2,2,2-trifluoroethyl methacrylate (TFEMA) is studied by time-resolved small-angle X-ray scattering (SAXS) at 60 °C using a stirrable reaction cell. TFEMA was preferred to styrene because it offers much greater X-ray scattering contrast relative to water, which is essential for sufficient temporal resolution. The evolution in particle size is monitored by both in situ SAXS and ex situ DLS in the absence or presence of an anionic surfactant (sodium dodecyl sulfate, SDS). Post-mortem SAXS studies confirmed the formation of well-defined spherical latexes, with volume-average diameters of 353 ± 9 nm and 68 ± 4 nm being obtained for the surfactant-free and SDS formulations, respectively. 1H NMR spectroscopy studies of the equivalent laboratory-scale formulations indicated TFEMA conversions of 99% within 80 min and 93% within 60 min for the surfactant-free and SDS formulations, respectively. Comparable polymerization kinetics are observed for the in situ SAXS experiments and the laboratory-scale syntheses, with nucleation occurring after approximately 6 min in each case. After nucleation, scattering patterns are fitted using a hard sphere scattering model to determine the evolution in particle growth for both formulations. Moreover, in situ SAXS enables identification of the three main intervals (I, II, and III) that are observed during aqueous emulsion polymerization in the presence of surfactant. These intervals are consistent with those indicated by solution conductivity and optical microscopy studies. Significant differences between the surfactant-free and SDS formulations are observed, providing useful insights into the mechanism of emulsion polymerization.

Highlights

Various in situ techniques have been utilized to monitor the kinetics of aqueous emulsion polymerization, including 1H NMR spectroscopy combined with a flow cell,[11] Raman spectroscopy,[12] and near-IR spectroscopy.[13,14]
The persulfate-initiated aqueous emulsion polymerization of trifluoroethyl methacrylate (TFEMA) at 60 °C leads to the formation of well-defined spherical latex particles when performed either under surfactant-free conditions or in the presence of SDS surfactant
This cell has a reaction solution volume of approximately 2.0 mL, which is sufficient to allow post-mortem analysis of the final latex particles by 1H NMR spectroscopy, dynamic light scattering (DLS), and TEM. For both formulations, the rate of polymerization appears to be unaffected when subjected to synchrotron X-ray irradiation. This is in marked contrast to our prior in situ small-angle X-ray scattering (SAXS) study of the synthesis of diblock copolymer nanoparticles via reversible addition−fragmentation chain transfer (RAFT) dispersion polymerization in mineral oil, whereby the enhanced rate of polymerization was attributed to an additional radical flux generated by the high-energy X-ray beam.[36,38]

Summary

■ INTRODUCTION

Aqueous emulsion polymerization is an environmentally friendly process that is widely used on an industrial scale to polymerize many water-immiscible vinyl monomers, including styrene, methacrylates, acrylates, vinyl acetate, vinyl chloride, etc.[1,2] Such heterophase polymerizations account for approximately 25% of synthetic polymers produced globally,[3] with tens of millions of tons of copolymer latexes being prepared each year.[3,4] Importantly, microcompartmentalization enables the efficient production of high molecular weight polymer chains in convenient low-viscosity form while offering good control over heat dissipation.[2,5−9] The resulting latex particles are used for many applications, including architectural paints, anticorrosion coatings, adhesives, varnishes, cement and concrete additives, rheology modifiers; they can serve as the mobile phase for immunodiagnostic assays.[10]. Such formulations usually require vigorous stirring to generate micrometer-sized monomer droplets Such droplets act as reservoirs and provide a sufficiently high interfacial area to ensure efficient mass transport of the water-immiscible monomer to the growing particles during polymerization. Polymerization continues primarily within monomer-swollen particles with monomer droplets serving as reservoirs to supply the growing particles with further monomer (and surfactant) This particle growth stage (Interval II, Figure 1b) is complete when there are no remaining monomer droplets. Article decomposition of an ionic initiator (e.g., persulfate) generates charged water-soluble radicals that react with dissolved monomer within the aqueous phase This generates a growing polymer radical with a terminal anionic sulfate group that becomes insoluble at some critical chain length to form a primary particle. We examined two formulations: one was conducted in the presence of an anionic surfactant (sodium dodecyl sulfate, SDS) while the other was performed under surfactant-free conditions,[23] (see Figure 2)

■ RESULTS AND DISCUSSION

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES