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

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.


Laboratory-based synthesis of PTFEMA latex particles via aqueous emulsion polymerization
The experimental protocol employed for the synthesis of surfactant-stabilized PTFEMA latex particles was as follows. KPS initiator (24 mg), SDS surfactant (32 mg, approx. 2.0 mol % based on TFEMA) and deionized water (20 mL) were weighed into a 10 mL round-bottom flask containing a magnetic stirrer bar. The reaction mixture was adjusted to pH 7.5 by addition of 200 μL of a 0.1 M NaOH solution and then degassed with N 2 gas for approximately 30 min. After removing its MEHQ inhibitor, cold TFEMA was degassed separately using N 2 gas for 30 min with the aid of an ice bath. Degassed TFEMA (1.00 g) was then added to the reaction mixture and a 0.10 mL aliquot was immediately extracted for 1 H NMR spectroscopy analysis. The heterogeneous reaction mixture was degassed for a further 5 min prior to immersion in a 60 o C oil bath with magnetic stirring at 800 rpm. The 'zero time' (t = 0 min) for this polymerization was arbitrarily taken to be the point when the degassed reaction solution was first immersed in the oil bath, rather than the time at which the reaction solution had reached this temperature. Aliquots were subsequently removed under N 2 via syringe at various time intervals for 1 H NMR, DLS and TEM analysis. Each 0.10 mL aliquot was quenched by cooling using an ice bath with concomitant exposure to air. For 1 H NMR analysis, 80 μL of each aliquot was diluted using CDCl 3 (500 μL). A small quantity of anhydrous MgSO 4 was added to remove water and each deuterated solution was then passed through a cotton wool-plugged pipet to remove any solids. For both TEM and DLS analysis, each aliquot was diluted fifty-fold using deionized water at 20 o C to produce 0.20% w/w dispersions.
For the synthesis of the surfactant-free charge-stabilized PTFEMA latex, the above protocol was employed at 60 °C and pH 10 using a stirring speed of 800 rpm except that the SDS surfactant was omitted. The following reagent quantities were used: TFEMA (1.00 g), KPS (147 mg) and deionized water (21.8 mL).

In situ SAXS studies of PTFEMA latex particles using the stirrable reaction cell
The experimental protocol employed for the synthesis of surfactant-stabilized PTFEMA latex particles was as follows. KPS initiator (2.40 mg), SDS (3.20 mg) and deionized water (2.0 mL) were weighed into a 14 mL sample vial. The reaction mixture was adjusted to pH 7.5 by addition of 10.0 μL of a 0.1 M NaOH solution and then degassed with N 2 gas for approximately 30 min. After removing its MEHQ inhibitor, cold TFEMA was degassed separately using N 2 gas for 30 min with the aid of an ice bath. Degassed TFEMA (0.10 g) was added to the reaction mixture, which was then transferred via degassed syringe to the stirrable reaction cell (containing a magnetic flea and equipped with a magnetic stirrer unit) which had been separately purged with N 2 gas for 20 min. This cell was then attached to the sample stage in station I22 at Diamond Light Source and aligned relative to its synchrotron SAXS beam. Then the TFEMA polymerization was initiated by using a water-circulating jacket to heat the cell up to 60 o C as the X-ray beam shutter was opened. This polymerization was monitored until no further evolution in the 1D SAXS pattern was observed, at which point it was assumed that the reaction was complete. For in situ SAXS studies of surfactant-free charge-stabilized PTFEMA latex particles, the synthesis protocol involved the following masses and conditions: TFEMA (0.10 g), KPS (14.7 mg), deionized water (2.18 mL), 60 °C and pH 10.

In situ conductivity studies during the aqueous emulsion polymerization of TFEMA in the presence or absence of SDS surfactant
The experimental protocol employed for determining the solution conductivity during the synthesis of SDS-stabilized PTFEMA latex particles was as follows. A 100 mL two-neck roundbottom flask was fitted with a Primo 5 conductivity probe. SDS (0.143 g) and deionized water (86.23 mL) were weighed into the 100 mL round-bottom flask. The reaction mixture was adjusted to pH 7.5 by adding 0.1 M NaOH solution (100 μL) via micropipette and then degassing with N 2 gas at 60 °C for approximately 30 min. KPS initiator (0.107 g) and deionized water (3.97 mL) were weighed into a 14 mL sample vial and degassed with N 2 gas for approximately 30 min. After removing its MEHQ inhibitor, cold TFEMA was degassed separately using N 2 gas for 30 min with the aid of an ice bath. Degassed TFEMA (4.5 g) was added to the reaction mixture and allowed to stir at 500 rpm for 3 min. The degassed aqueous KPS solution was then added to the reaction mixture. The 'zero time' (t = 0 min) for this polymerization was arbitrarily taken to be the point when the degassed initiator solution was added to the reaction mixture. Conductivity data from the Primo 5 were recorded using the camera facility of a mobile phone. 1 H NMR analysis indicated that a final TFEMA monomer conversion of 98% was achieved during this synthesis. Essentially the same protocol (with the omission of SDS) was used to measure the solution conductivity during the surfactant-free aqueous emulsion polymerization formulation. S4

H NMR Spectroscopy
All NMR spectra were recorded in CDCl 3 at 298 K using a 400 MHz Bruker Avance-400 spectrometer (64 scans averaged per spectrum).

Dynamic Light Scattering (DLS)
DLS studies were conducted on 0.20% w/w aqueous dispersions at 25 °C in disposable plastic cuvettes using a Malvern Zetasizer NanoZS instrument that detects back-scattered light at an angle of 173°. Intensity-average hydrodynamic diameters were calculated via the Stokes−Einstein equation using a non-negative least squares (NNLS) algorithm. All data were averaged over three consecutive runs. Intensity-average hydrodynamic diameters were converted into volume-average hydrodynamic diameters using Malvern Zetasizer Software (Version 7.01).

Transmission Electron Microscopy (TEM)
Aliquots extracted from the polymerizing reaction mixture were diluted fifty-fold at 20 °C to generate 0.20% w/w dispersions. Copper/palladium TEM grids (Agar Scientific, UK) were surface-coated in-house to yield a thin film of amorphous carbon. The grids were then plasma glow-discharged for 30 s to create a hydrophilic surface. Individual samples (0.20% w/w, 5.0 μL) were adsorbed onto the freshly glow-discharged grids for 1 min and then blotted with filter paper to remove excess solution. To stain the aggregates, uranyl formate solution (0.75% w/v, 5.0 μL) was soaked on the sample-loaded grid for 20 s and carefully blotted to remove excess stain. The grids were then dried using a vacuum hose. Imaging was performed using a Technai T12 Spirit microscope at 120 kV equipped with a Gatan 1 k CCD camera.

Optical microscopy
Aliquots were extracted from the reaction mixture at 60 °C and the TFEMA polymerization was quenched by cooling to 20 °C with concomitant exposure to air. Optical microscopy images were recorded immediately at 20 °C using a Motic DMBA300 digital biological microscope equipped with a built-in camera and Motic Images Plus 2.0 ML software.

Aqueous Electrophoresis
Measurements were conducted in the presence of 1 mM KCl using a Malvern Zetasizer Nano ZS instrument. The zeta potential was calculated from the electrophoretic mobility using the Smoluchowski relationship. The solution pH was adjusted by adding small volumes of either HCl or NaOH. All data were averaged over three consecutive runs.

Gel Permeation Chromatography (GPC)
Copolymer molecular weights and dispersities were determined using a THF GPC set-up operating at 60 °C and comprising two Polymer Laboratories PL gel 5 μm Mixed-C columns connected in series to a Varian 390-LC multidetector suite (refractive index detector only) and a Varian 290-LC pump injection module. The GPC eluent was HPLC-grade THF containing 10 mM LiBr at a flow rate of 1.0 mL min −1 and DMSO was used as a flow-rate marker. Calibration was conducted using a series of ten near-monodisperse polystyrene standards (M n = 625 to 618,000 g mol −1 ). Chromatograms were analyzed using Varian Cirrus GPC software (version 3.3).

Small-Angle X-ray Scattering (SAXS)
SAXS patterns were recorded at a synchrotron facility (station I22 at Diamond, Didcot, Oxfordshire, UK). A monochromatic X-ray beam (λ = 0.124 nm), a 2D Pilatus 2M pixel detector (Dectris, Switzerland) and a q range of 0.02−2.00 nm −1 were used for these experiments, where q = (4π sin θ)/λ corresponds to the modulus of the scattering vector and θ is half of the scattering angle. For these time-resolved measurements, a custom-designed SAXS cell was used as the sample holder, see Figure 2 in the main manuscript. SAXS patterns were recorded every 10 seconds for 10 min, every 30 seconds for the next 30 min and every 60 seconds thereafter until no further change in the SAXS patterns was observed. X-ray scattering data were reduced (integrated, normalized, and background-subtracted) using Dawn software supplied by Diamond Light Source. The X-ray scattering intensity for water was used for absolute scale calibration of the scattering patterns. Irena SAS macros for Igor Pro were utilized for modeling and further SAXS analysis. Initial in situ scattering experiments were conducted during the aqueous emulsion polymerization of styrene at 80 °C targeting 5.0% w/w solids. First, the polymerization kinetics were determined for a laboratory-based synthesis. 1 H NMR studies determined that the styrene polymerization was complete within 180 min.
The scattering length densities (SLD) of water and polystyrene are 9.42 x10 10 cm -2 and 9.41 x10 10 cm -2 , respectively. This gives an SLD difference of just 0.01 x 10 10 cm -2 between solvent and scattering object, which does not provide sufficient contrast (see Figure  S1). Figure S1a shows the scattering profile for water at 80 °C. Figure S1b shows this water background and the scattering profile recorded after 73 min (~ 40% polystyrene conversion) during the in situ synthesis of polystyrene at 80 °C targeting 5.0% w/w solids. The two profiles overlap substantially, confirming very poor X-ray contrast between the polystyrene and the water under such conditions.

Determining volume-average particle sizes by DLS (Mie theory)
Dynamic light scattering studies were conducted during the laboratory-based polymerization of TFEMA under surfactant-free conditions and also in the presence of SDS surfactant. Intensity-average particle diameters were converted into volume-average particle diameters using the instrument software (Malvern Zetasizer software version 7.11). Both the volume and number distributions are calculated from the intensity distribution using Mie theory. A brief overview of how this calculation is performed within the software is provided below. For further information, see the Malvern website. 1 According to Mie theory, 2 the scattering intensity measured by the photomultiplier detector can be described using the expression given below.
F(θ,ϕ) = |S 1 ( )| 2 sin 2 ϕ + |S 2 ( )| 2 cos 2 ϕ where and correspond to the angles used to identify the location of the detector relative θ ϕ to the polarization plane of the incident light. and correspond to the scattering functions S 1 S 2 that embody the phase and amplitude of the scattered light. The Mie coefficients are contained within the scattering functions S 1 and S 2 . Given the refractive index of the particle and the scattering angle, Mie theory can be used to calculate the scattering intensity as a function of particle diameter.
The number distribution can be calculated using the equation shown below, where I(x) corresponds to the DLS intensity distribution for a particle of diameter x, N(x) is the number distribution, and M(x, n D , n P ) is the Mie scattering formula where n D and n P refer to the dispersant and particle respectively.
The volume distribution, V(x), can then be determined using the equation shown below, which assumes a spherical morphology.    Figure S4. Zeta potential versus pH plots obtained in the presence of 1 mM KCl background salt for chargestabilized PTFEMA latexes synthesized in either the presence or absence of SDS surfactant (each latex was prepared at 60 °C targeting 5% w/w solids). Anionic surface groups arising from the SDS surfactant (sulfonate) and/or persulfate initiator (sulfate) ensure that both latexes remain highly anionic over the whole pH range studied. S11 Figure S5. Solution conductivity measurements recorded in situ during the aqueous emulsion polymerization of TFEMA under surfactant-free conditions at 60 °C targeting 5.0% w/w solids. S12   Figure S9. Residual analysis for data fits to the final scattering patterns recorded for each formulation (as indicated by the red and blue arrows). The oscillations observed in the low q region of the residual pattern (red data set) for the larger particles obtained for the surfactant-free formulation occur as a result of a smearing effect owing to the finite size of the X-ray beam and the detector pixels. In principle, this instrument effect can be accounted for either by desmearing the scattering pattern or by smearing the model fit. In practice, this is beyond the scope of the present study.

Dynamic Light Scattering Studies
Table S1. Volume-average particle diameters and corresponding polydispersities obtained from dynamic light scattering studies during the kinetic analysis (see Figure 4 in main manuscript) for both the surfactant-free and surfactant formulations.

Scattering Model
The SAXS patterns produced by PTFEMA latex particles formed either (a) in the presence of SDS surfactant or (b) under surfactant-free conditions were each fitted to a homogeneous sphere model. The Irena SAS macros for Igor Pro 3 were used for modeling and fitting such SAXS patterns (see Figure S4 for typical fits).
The scattered intensity I(q) is measured as a function of the modulus of X-ray photon momentum transfer vector , where is the scattering angle. The normalized = ( 4 )sin θ 2θ scattered intensity (after subtraction of the solvent background) for a suspension of particles can be represented as (S1) where is the number of different populations of particles in the suspension, is the i ( ) structure factor arising from interparticle interactions, is the number density of scattering i particles of the population, is the form factor that describes the particle th | pop i ( , )| morphology (including contrast and volume parameters of the particles), and is the size Ψ i ( ) distribution function of scattering particles corresponding to the population. The PTFEMA th latex particles can be represented by a single population of particles (i.e. n = 1 in eq S1) with the following functions and parameters describing the model: (S3) S18 is the form factor of a spherical particle, 4 is the volume of the ( ) = 4 3 ( + ΔR) 3 /3 particles, and is the scattering contrast between scattering length density of the latex ∆ξ (ξ PTFEMA ) and the solvent (ξ water ); is the Gaussian (normal) particle size distribution, where R c is the mean PTFEMA particle radius and is the standard deviation of the size polydispersity for the particle radius; where is the relative volume fraction of the PTFEMA particles in the aqueous dispersion.
As a result of the highly charged nature of these latexes (see Figure S3), a structure factor S(q) that incorporates a hard-sphere structure factor solved with the Percus-Yevick closure relation is required to account for the interactions between the PTFEMA particles despite the relatively low solids content of 5% w/w that is targeted for their synthesis.