Time-resolved Small-angle X-ray Scattering Studies Research Articles

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

Recently, we reported time-resolved synchrotron small-angle X-ray scattering (TR-SAXS) studies during aqueous emulsion polymerization using a bespoke stirrable reaction cell (J. Am. Chem. Soc. 2021, 143, 1474-1484). This proof-of-concept study utilized a semifluorinated specialty monomer (2,2,2-trifluoroethyl methacrylate) to ensure high X-ray contrast relative to water. Herein, we extend this approach to emulsion polymerization of methyl methacrylate (MMA) in the presence or absence of sodium dodecyl sulfate (SDS) at 70 °C. Solution conductivity measurements for this anionic surfactant indicated a critical micelle concentration (CMC) of 10.9 mM at this temperature. Thus, SDS was employed at either 1.0 or 20.0 mM, which corresponds to well below or well above its CMC. Postmortem analysis by 1H NMR spectroscopy indicated MMA conversions of 93-95% for these three formulations. We demonstrate that the X-ray contrast between water and PMMA is sufficiently large to produce high-quality scattering patterns during TR-SAXS experiments. Such patterns were fitted using a hard-sphere scattering model to monitor the evolution in particle diameter. This enabled (i) determination of the time point for the onset of nucleation and (ii) the evolution in particle size to be monitored during the MMA polymerization. The final particle diameters obtained from such TR-SAXS studies were consistent with postmortem DLS analyses, while TEM studies confirmed that near-monodisperse latex particles were formed. Micellar nucleation occurs within just 2 min when the SDS concentration is well above its CMC, resulting in a high particle number concentration and relatively small latex particles. In contrast, when SDS is either absent or present below its CMC, particle nuclei are formed by homogeneous nucleation over significantly longer time scales (14-15 min). In the latter case, adsorption of SDS onto nascent particles reduces their coagulation, giving rise to a larger number of smaller particles compared to the surfactant-free polymerization. However, the characteristic time required for the onset of nucleation is barely affected because this is mainly controlled by the kinetics of homogeneous polymerization of the relatively water-soluble MMA monomer within the aqueous phase. These results suggest that the aqueous emulsion polymerization of several other (meth)acrylic monomers, and perhaps also vinyl acetate, may be amenable to TR-SAXS studies.

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.

Ferritin Assembly Revisited: A Time-Resolved Small-Angle X-ray Scattering Study.

The assembly reaction of Escherichia coli ferritin A (EcFtnA) was studied using time-resolved small-angle X-ray scattering (TR-SAXS). EcFtnA forms a cagelike structure that consists of 24 identical subunits and dissociates into dimers at acidic pH. The dimer maintains nativelike secondary and tertiary structures and is able to reassemble into a 24-mer when the pH is increased. The reassembly reaction was induced by pH jump, and reassembly was followed by TR-SAXS. Time-dependent changes in the forward scattering intensity and in the gyration radius suggested the existence of a significant population of intermediate oligomers during the assembly reaction. The initial reaction was a mixture of second- and third-order reactions (formation of tetramers and hexamers) from the protein concentration dependence of the initial velocity. The time-dependent change in the SAXS profile was roughly explained by a simple model in which only tetramers, hexamers, and dodecamers were considered as intermediates.

Time-Resolved Small-Angle X-ray Scattering Study on the Growth Behavior of Silver Nanoparticles

Silver nanoparticles were prepared with a chemical reduction method in the presence of polyvinylpyrrolidone (PVP) used as a stabilizing agent. During the synthesis of Ag nanoparticles, aqueous silver nitrate solutions were used as the precursors and sodium borohydride was used as the reducing agent. Four Ag-nanoparticle suspensions were prepared with the initial Ag+ concentrations of 2, 4, 6, and 8 mM in the corresponding precursors. The in-situ time-resolved small-angle X-ray-scattering (SAXS) technique was used to monitor the nucleation and growth processes of Ag nanoparticles. The particle-size change with time was obtained by analyzing the SAXS data with a tangent-by-tangent method. The SAXS results demonstrate that the Ag nanoparticle growth behaves as a linear relation in the initial growth stage (<1 s), which was used to evaluate the critical particle size. The growth of Ag nanoparticles experienced a fast stage and then a slow stage in their whole formation process. A diffusion–coalescence model has been proposed to describe the growth behavior. The particle size change with growth time can be fitted well by this model. The effect of initial Ag+ concentration on the final particle size and the growth mechanism of Ag nanoparticles are discussed in this paper.

Time-Resolved Small-Angle X-Ray Scattering Study of the Early Formation of Amyloid Protofibrils on a Apomyoglobin Mutant

Early prefibrillar intermediates are a key issue in amyloid researches, as they show the highest cytotoxicity with respect to mature fibrils, which are less toxic or even harmless. However, the early aggregation process is largely unknown, since nucleation events are rare, and elongation and reorganization processes are very fast. Time-resolved synchrotron Small Angle X-ray Scattering (SAXS), coupled to extended model fitting analysis, has been used to provide in a millisecond temporal range the structural and aggregational properties of the early and soluble protofibrils formed by a model protein after a pH jump. The amyloidogenic apomyoglobin mutant W7FW14F has been investigated as fibril former: this mutant, which is in a monomeric, partly folded state at acidic pH, undergoes at neutral pH a nucleation-dependent polymerization reaction, resulting in the formation of cytotoxic amyloid fibrils similar to those detected for proteins involved in amyloid diseases. SAXS evidenced that oligomerization of W7FW14F in solution happens in less than 100 ms after the pH jump from 4.0 to 7.0., while the resulting pattern of protein prefibrillation reveals the simultaneous presence of worm-like species and of cylindrically-shaped aggregates, whose structural features mainly change after 20 ms from the solution pH jump. Model fitting analysis gives the composition of the different oligomers and their relative concentration as a function of time, suggesting that protofibril formation occurs through the presence of aggregation and rearrangement competing processes and through the contribution of multiple coexisting elongated and worm-like protein species. The possibility to use SAXS in monitoring the effects of cosolvents and/or pharmaceutical agents in modifying or preventing the early amyloid aggregation patterns is then demonstrated.

Time-Resolved Small-Angle X-ray Scattering Studies of Polymer−Silica Nanocomposite Particles: Initial Formation and Subsequent Silica Redistribution

Small angle X-ray scattering (SAXS) is a powerful characterization technique for the analysis of polymer-silica nanocomposite particles due to their relatively narrow particle size distributions and high electron density contrast between the polymer core and the silica shell. Time-resolved SAXS is used to follow the kinetics of both nanocomposite particle formation (via silica nanoparticle adsorption onto sterically stabilized poly(2-vinylpyridine) (P2VP) latex in dilute aqueous solution) and also the spontaneous redistribution of silica that occurs when such P2VP-silica nanocomposite particles are challenged by the addition of sterically stabilized P2VP latex. Silica adsorption is complete within a few seconds at 20 °C and the rate of adsorption strongly dependent on the extent of silica surface coverage. Similar very short time scales for silica redistribution are consistent with facile silica exchange occurring as a result of rapid interparticle collisions due to Brownian motion; this interpretation is consistent with a zeroth-order Smoluchowski-type calculation.

Time-Resolved Small-Angle X-ray Scattering Study of the Folding Dynamics of Barnase

Structural changes of barnase during folding were investigated using time-resolved small-angle X-ray scattering (SAXS). The folding of barnase involves a burst-phase intermediate, sometimes designated as the denatured state under physiological conditions, D phys, and a second hidden intermediate. Equilibrium SAXS measurements showed that the radius of gyration ( R g) of the guanidine unfolded state (U) is 26.9 ± 0.7 Å, which remains largely constant over a wide denaturant concentration range. Time-resolved SAXS measurements showed that the R g value extrapolated from kinetic R g data to time zero, R g,0, is 24.3 ± 0.1 Å, which is smaller than that of U but which is expanded from that of folding intermediates of other proteins with similar chain lengths (19 Å). After the burst-phase change, a single-exponential reduction in R g 2 was observed, which corresponds to the formation of the native state for the major component containing the native trans proline isomer. We estimated R g of the minor component of D phys containing the non-native cis proline isomer (D phys,cis) to be 25.7 ± 0.6 Å. Moreover, R g of the major component of D phys containing the native proline isomer (D phys,tra) was estimated as 23.9 ± 0.2 Å based on R g,0. Consequently, both components of the burst-phase intermediate of barnase (D phys,tra and D phys,cis) are still largely expanded. It was inferred that D phys possesses the N-terminal helix and the center of the β-sheet formed independently and that the formation of the remainder of the protein occurs in the slower phase.

A time-resolved SAXS study on the crystalline morphology of long-term stored isotactic polypropylene

The crystalline morphology of a long-term stored metallocene isotactic polypropylene (mPP) has been studied with time-resolved small-angle X-ray scattering (SAXS), differential scanning calorimetry (DSC) and wide-angle X-ray diffraction (WAXD), and compared with that of the freshly crystallized sample. It was found that both α and γ crystal forms occur in the long-term stored mPP, whereas freshly crystallized mPP contains only the α crystal form. Double SAXS peaks were observed for the long-term stored mPP, which can be assigned to the α and γ crystals. Based on the SAXS result, it is inferred that the crystal stacks of the α and γ forms in the long-term stored mPP are arranged according to the block mode.

Time-Resolved Small-Angle X-ray Scattering Study of the Kinetics of Disorder−Order Transition in a Triblock Copolymer in a Selective Solvent for the Middle Block

The kinetics of the disorder−order transition (ODT) in a triblock of polystyrene (PS) and poly(ethylene-co-butylene) (PEB), in mineral oil, a selective solvent for the middle PEB block, was followed by time-resolved SAXS using both temperature ramp and rapid temperature quench techniques. Data analysis, using the Percus−Yevick interacting hard-sphere model plus Gaussians for the Bragg peaks in the ordered body-centered-cubic (bcc) state, shows that the core radius Rc, the hard-sphere volume fraction φ, and their interaction radius Rhs increased with decreasing temperature in the disordered state. At the ODT, φ reaches a maximum and Rc and φ change on ordering. The kinetics exhibits two stages corresponding to temperature equilibration and supercooling of the micellar fluid followed by nucleation and growth of the ordered state. The induction time for the onset of the second stage shows a minimum around the glass transition of the polystyrene cores.

Time-Resolved Small-Angle X-ray Scattering Studies of Nanocrystal Superlattice Self-Assembly

Time-Resolved Small-Angle X-ray Scattering Studies of Nanocrystal Superlattice Self-Assembly

Time-Resolved Small-Angle X-ray Scattering Studies of Spinodal Decomposition Kinetics in a Semidilute Polystyrene−Dioctyl Phthalate Solution

Synchrotron-based time-resolved small-angle X-ray scattering was used to measure the early-stage spinodal decomposition kinetics in a highly viscous semidilute solution of polystyrene in dioctyl phthalate. Although strong nonlinearities in the kinetics were observed, a linear theory analysis could fit the evolution of the structure factor at short times and low wavenumbers. Nonlinearities become significant earlier at higher wavenumbers, which is consistent with recent computer simulation studies. From the linear theory analysis, the exponential relaxation rate of the structure factor R(q) was found to be linear in q2 in the low-wavenumber region of the experiments, in agreement with mean-field theory for spinodal decomposition in polymer solutions. The wavenumber-dependent Onsager transport coefficient Λ(q), which is the Fourier transform of the nonlocal mobility in a polymer system, was determined to scale as q-4. This q dependence, which is stronger than that predicted by existing theory, may be related to entanglement effects.