Time-resolved Synchrotron Small-angle X-ray Scattering 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.

Shear-Modulated Rates of Phase Transitions in Sphere-Forming Diblock Oligomer Lyotropic Liquid Crystals.

Hydration of the amphiphilic diblock oligomer C16H33(CH2CH2O)20OH (C16E20) leads to concentration-dependent formation of micellar body-centered cubic (BCC) and Frank-Kasper A15 lyotropic liquid crystals (LLCs). Quiescent thermal annealing of aqueous LLCs comprising 56-59 wt % C16E20 at 25 °C after quenching from high temperatures established their ability to form short-lived BCC phases, which transform into long-lived, transient Frank-Kasper σ phases en route to equilibrium A15 morphologies on a time scale of months. Here, the frequency and magnitude of applied oscillatory shear show the potential to either dynamically stabilize the metastable BCC phase at low frequencies or increase the rate of formation of the A15 to minutes at high frequencies. Time-resolved synchrotron small-angle X-ray scattering (TR-SAXS) provides in situ characterization of the structures during shear and thermal processing. This work shows that the LLC morphology and order-order phase transformation rates can be controlled by tuning the shear strain amplitude and frequency.

Kinetics of Photocontrollable Micelles: Light-Induced Self-Assembly and Disassembly of Azobenzene-Based Surfactants Revealed by TR-SAXS.

The kinetics of micelles involving photosensitive surfactants is still not well understood. In this work, we unravel the mechanistic pathways involved in the micelle formation and dissolution of photocontrollable micelles. We focus on the fast self-assembly processes of photosensitive cationic azobenzene-containing surfactants (AzoTMA) that display a change in hydrophobicity induced by a reversible cis-trans conformational transition upon exposure to light. By combining both in situ time-resolved small-angle X-ray scattering (SAXS) and light scattering, we characterized the detailed structure and phase behavior of AzoTMA in mixtures of water and dimethylformamide (DMF). Time-resolved synchrotron SAXS with monochromatic light as a trigger enabled us to observe the nonequilibrium formation and dissolution process of micelles (demicellization) directly on the nanoscale with a time resolution starting from milliseconds. The structural results show that in pure water UV-light illumination leads to a 12% reduction of the aggregation number of the micelles and more than a 50% increase in the critical micelle concentration (CMC). Close to the CMC, adjusted by the addition of DMF, UV light illumination leads to a complete dissolution of the micelles, while shining blue light reverses the process and leads to the reformation of micelles. The UV-triggered dissolution follows a two-step mechanism; the first and rapid (second time scale) release of unimers is followed by a slower decomposition of the micelles (over tens of seconds) as a result of an increase in temperature due to optical absorption. Similarly, the reverse process, i.e., micelle formation, occurs rapidly upon photoconversion to trans conformers under blue light, and micelles are disrupted at long exposure time due to the optical absorption and corresponding increase in temperature. Interestingly, the coexistence of unimers with regular micelles is found at all times, and no other transient assemblies could be detected by SAXS.

Calcium Triggered Lα-H2 Phase Transition Monitored by Combined Rapid Mixing and Time-Resolved Synchrotron SAXS

BackgroundAwad et al. [1] reported on the Ca2+-induced transitions of dioleoyl-phosphatidylglycerol (DOPG)/monoolein (MO) vesicles to bicontinuous cubic phases at equilibrium conditions. In the present study, the combination of rapid mixing and time-resolved synchrotron small-angle X-ray scattering (SAXS) was applied for the in-situ investigations of fast structural transitions of diluted DOPG/MO vesicles into well-ordered nanostructures by the addition of low concentrated Ca2+ solutions.Methodology/Principal FindingsUnder static conditions and the in absence of the divalent cations, the DOPG/MO system forms large vesicles composed of weakly correlated bilayers with a d-spacing of ∼140 Å (Lα-phase). The utilization of a stopped-flow apparatus allowed mixing these DOPG/MO vesicles with a solution of Ca2+ ions within 10 milliseconds (ms). In such a way the dynamics of negatively charged PG to divalent cation interactions, and the kinetics of the induced structural transitions were studied. Ca2+ ions have a very strong impact on the lipidic nanostructures. Intriguingly, already at low salt concentrations (DOPG/Ca2+>2), Ca2+ ions trigger the transformation from bilayers to monolayer nanotubes (inverted hexagonal phase, H2). Our results reveal that a binding ratio of 1 Ca2+ per 8 DOPG is sufficient for the formation of the H2 phase. At 50°C a direct transition from the vesicles to the H2 phase was observed, whereas at ambient temperature (20°C) a short lived intermediate phase (possibly the cubic Pn3m phase) coexisting with the H2 phase was detected.Conclusions/SignificanceThe strong binding of the divalent cations to the negatively charged DOPG molecules enhances the negative spontaneous curvature of the monolayers and causes a rapid collapsing of the vesicles. The rapid loss of the bilayer stability and the reorganization of the lipid molecules within ms support the argument that the transition mechanism is based on a leaky fusion of the vesicles.

Time-resolved SAXS study of the effect of a double hydrophilic block-copolymer on the formation of CaCO3 from a supersaturated salt solution.

The effect of a double hydrophilic block-copolymer additive (made of polyaspartic acid and polyethyleneglycol, pAsp(10)-b-PEG(110)) on the initial formation of calcium carbonate from a supersaturated salt solution has been studied in situ by means of time-resolved synchrotron small-angle X-ray scattering (SAXS). A stopped-flow cell was used for rapidly mixing the 20 mM aqueous reactant solutions of calcium chloride and sodium carbonate. In reference measurements without polymer additive the very rapid formation of primary, overall spherical CaCO(3) particles with a radius of ca. 19 nm and a size polydispersity of ca. 26% was observed within the first 10 ms after mixing. A subsequent, very rapid aggregation of these primary particles was evidenced by a distinct upturn of the SAXS intensity at smallest angles. During the aggregation process the size of the primary particles remained unchanged. From an analysis of the absolute scattering intensity the mass density of these particles was determined to 1.9 g/cm(3). From this rather low density it is concluded that those precursor particles are amorphous, which has been confirmed by simultaneous wide-angle X-ray diffraction measurements. Upon adding 200 pm of the block-copolymer no influence on the size, the size polydispersity and morphology of the primary particles, nor on the kinetics of their formation and growth, was found. On the other hand, the subsequent aggregation and precipitation process is considerably slowed down by the additive and smaller aggregates result. The crystalline morphology of the sediment was studied in situ by WAXS ca. 50 min after mixing the reactants. Several diffraction rings could be detected, which indicate that a transformation of the metastable, amorphous precursor particles to randomly oriented vaterite nanocrystallites has taken place. In addition, a few isolated Bragg spots of high intensity were detected, which are attributed to individual, oriented calcite microcrystals that nucleated at the wall of the capillary.

Kinetics and Mechanism of Morphological Transition from Lamella to Cylinder Microdomain in Polystyrene-block-poly(ethylene-co-but-1-ene)- block-polystyrene Triblock Copolymer

The kinetics and mechanism of morphological transition from nonequilibrium lamella to cylinder microdomain in a polystyrene-block-poly(ethylene-co-but-1-ene)-block-polystyrene (SEBS) triblock copolymer were studied by using time-resolved synchrotron small-angle X-ray scattering (SAXS), transmission electron microscopy (TEM), and rheology. The sample cast from toluene solution formed a nonequilibrium morphology of the alternating lamellae (LAM) of polystyrene (PS) and poly(ethylene-co-but-1-ene) (PEB) blocks because toluene is a good solvent for PS chains but a poor solvent for PEB chains. LAM of PS and PEB blocks was transformed into the hexagonally close packing (HEX) of PS cylinders in PEB matrix when the as-cast sample was annealed above 140 °C, which is above the glass transition temperature of the PS block. From the time-resolved SAXS and TEM, the coexistence of the LAM and HEX microdomains was clearly confirmed during the entire process of the order-to-order transition (OOT) from LAM to HEX, and the...

Formation and Growth of Amorphous Colloidal CaCO3 Precursor Particles as Detected by Time-Resolved SAXS

Time-resolved synchrotron small-angle X-ray scattering (SAXS) studies were performed to investigate the unseeded formation and growth of colloidal calcium carbonate particles. Equimolar aqueous solutions of CaCl2·2H2O and Na2CO3 were rapidly mixed in a stopped-flow apparatus, and SAXS data were recorded using an image-intensified CCD detector. It is shown that SAXS allows studying those processes in situ, with a very good time resolution. It can provide unsurpassed real-time information about the particle size, shape, polydispersity, inner structure, and density. In these studies, well-defined, spherical CaCO3 particles with colloidal dimensions up to ca. 270 nm and a remarkable uniformity in size could be observed. After a short nucleation period, the number density of the growing spheres remains constant. From the evaluation of the absolute scattering intensities, the particle mass density could be determined to be ca. 1.62 g/cm3, which is considerably lower than the density of the crystalline modificat...

Morphological Changes During Crystallization and Melting of Polyoxymethylene Studied by Synchrotron X-Ray Scattering and Modulated Differential Scanning Calorimetry

Temperature scanning techniques, including synchrotron small-angle X-ray scattering (SAXS), wide-angle X-ray diffraction(WAXD), and temperature-modulated differential scanning calorimetry (TMDSC), were used to study melting recrystallization in semicrystalline polyoxymethylene (POM). The isothermal crystallization of POM was also studied by time-resolved SAXS. From SAXS profiles, several morphological variables were calculated, including the long period L, lamellar thickness lc , interlamellar amorphous thickness la , and scattering invariant Q. The lamellar parameters were also obtained using tapping atomic force microscopy (AFM) for two thermal histories, and some lamellar-scale and larger-scale morphological changes were characterized before and after partial melting. These real-space images also provided support to interpretation of SAXS analysis in the interpretation of first- and second-order intensity maxima. During melting at constant heating rates, la from SAXS increased slowly starting at about 100°C, suggesting melting of thin inserted lamellae, and at about 150°C, lc began to increase combined with a more rapid increase in la due to further melting of inserted lamellae and some recrystallization into separate stacks of lamellae. The end of melting was about 182°C. TMDSC data also characterized the level of melting and recrystallization starting at low temperatures for a quenched sample. The DSC data provided the total extent of melting, and this was contrasted with the drop in SAXS and WAXD intensities. SAXS and TMDSC temperature scans on a quenched, but still highly crystalline, POM sample were compared with the data from a high-temperature (145°C) isothermally crystallized POM sample with a higher degree of crystal perfection.

A time-resolved synchrotron SAXS study of crystallization in a miscible blend of poly(vinylidenefluoride) and poly(1,4-butylene adipate)

Time-resolved small-angle x-ray scattering (SAXS) has been used to probe the crystallization dynamics of poly(vinylidenefluoride) (PVF2) in its miscible blend with 30 wt% poly(l,4-butylene adipate) (PBA), as well as that of the pure PVF2 homopolymer. The samples were quenched from 190°C (in the single-phase region for the blend) to 130°C to crystallize the PVF2. The results show that the isothermal crystallization rate of PVF2 in the blend is slightly faster than that of the pure PVF2 homopolymer. Based on the time variation of the invariant, the faster crystallization rate of the blend could be due to a faster growth of the PVF2 lamellar stacks in the blend. The blend has a lower glass transition temperature than that of the homopolymer, hence the PVF2 component in the blend has a higher mobility, which may result in a faster growth of the PVF2 lamellar stacks. During the primary crystallization process, the pure PVF2 homopolymer shows a decrease in the average thickness of the PVF2 crystalline lamellae, suggesting a progressive insertion of thinner PVF2 crystalline lamellae, but an approximately constant long period implies an increase in the average thickness of the interlamellar amorphous region. This increase suggests that the thinner PVF2 crystalline lamellae insertion could occur in the free space left between the existing lamellar stacks, rather than within the existing lamellar stacks. On the other hand, the blend shows a constant average thickness of the PVF2 crystalline lamellae and a constant long period, suggesting that the insertion (if it exits) of PVF2 crystalline lamellae should also be in the free space left between the existing lamellar stacks.