Damien Dupin

pH-responsive liquid marbles stabilized with poly(2-vinylpyridine) particles

Submicron-sized sterically-stabilized lightly cross-linked poly(2-vinylpyridine) latexes were synthesized by non-aqueous dispersion polymerization using a hydrophobic polydimethylsiloxane macromonomer and used to prepare millimetre-sized ‘liquid marbles’. These ‘liquid marbles’ exhibited long-term stability when transferred onto the surface of liquid water provided that the solution pH was pH 4.9 or above. In contrast, the use of more acidic solutions (pH < 2.9) led to immediate ‘liquid marble’ disintegration and spontaneous dispersal of the poly(2-vinylpyridine) particles in their protonated microgel form. Moreover, the liquid marbles on the surface of water immediately disintegrated on addition of acid. Thus the critical minimum pH required for long-term ‘liquid marble’ stability correlates closely with the known pKa value of 4.7 for poly(2-vinylpyridine) chains.

Novel Pickering Emulsifiers Based on pH-Responsive Poly(2-(diethylamino)ethyl methacrylate) Latexes

The emulsion copolymerization of 2-(diethylamino)ethyl methacrylate (DEA) with a divinylbenzene cross-linker in the presence of monomethoxy-capped poly(ethylene glycol) methacrylate (PEGMA) at 70 °C afforded near-monodisperse, sterically stabilized PEGMA-PDEA latexes at 10% solids. Dynamic light scattering studies indicated intensity-average diameters of 190 to 240 nm for these latexes at pH 9. A latex-to-microgel transition occurred on lowering the solution pH to below the latex pKa of 6.9. When dilute HCl/KOH was used to adjust the aqueous pH, a systematic reduction in the cationic microgel hydrodynamic diameter of 80 nm was observed over ten pH cycles as a result of the gradual buildup of background salt. However, no such size reduction was observed when using CO2/N2 gases to regulate the aqueous pH because this protocol does not generate background salt. Thus, the latter approach offers better reversibility, albeit at the cost of slower response times. PEGMA-PDEA microgel does not stabilize Pickering emulsions when homogenized at pH 3 with n-dodecane, sunflower oil, isononyl isononanoate, or isopropyl myristate. In contrast, PEGMA-PDEA latex proved to be a ubiquitous Pickering emulsifier at pH 10, forming stable oil-in-water emulsions with each of these four model oils. Lowering the solution pH from 10 to 3 resulted in demulsification within seconds. This is because these pH-responsive particles undergo a latex-to-microgel transition, which leads to their interfacial desorption. Six successive demulsification/emulsification cycles were performed on these Pickering emulsions using HCl/KOH to adjust the solution pH. Demulsification could also be achieved by purging the emulsion solution with CO2 gas to lower the aqueous pH to 4.8. However, complete phase separation required CO2 purging for 4 h at 20 °C. A subsequent N2 purge raised the aqueous pH sufficiently to induce a microgel-to-latex transition, but rehomogenization did not produce a stable Pickering emulsion. Presumably, a higher pH is required, which cannot be achieved by a N2 purge alone.

Synthesis and characterization of polypyrrole-coated poly(methyl methacrylate) latex particles

A range of uniform, micrometer-sized poly(methyl methacrylate) latexes have been coated with ultrathin overlayers of polypyrrole from aqueous solution. Good control over the targeted conducting polymer overlayer thicknesses is achieved for a 1.19 µm diameter latex and latexes of up to 30 µm diameter can also be efficiently coated with this conducting polymer. Addition of sodium tosylate to the in situpyrrole polymerization leads to a smoother conducting polymer morphology, which in turn enables more uniform surface coverages and higher conductivities to be achieved for the coated particles. Laser diffraction studies of dilute aqueous suspensions indicate that the degree of dispersion achieved for the larger polypyrrole-coated latexes is comparable to that for the corresponding uncoated latex, although some incipient flocculation is observed for the smaller coated latexes. The FT-IR spectrum of 20 µm poly(methyl methacrylate) particles coated with a 20 nm polypyrrole overlayer is dominated by the underlying latex, since this is the major component (>99% by mass). On the other hand, the Raman spectrum of the same coated particles contains relatively strong features arising from the minor conducting polymer component due to a resonance Raman effect, although bands assigned to the underlying latex are also visible. Thermogravimetric analyses confirm that the tosylate-doped polypyrrole bulk powder is significantly more stable than chloride-doped polypyrrole bulk powder and that the polypyrrole-coated latexes are also more thermally stable than the uncoated poly(methyl methacrylate) latex. Since the chemical structure of individual polypyrrole-coated poly(methyl methacrylate) latex particles can be readily assessed using Raman microscopy, our results suggest that such thermally fragile particles will be useful model projectiles for assessing the extent of thermal ablation of organic cosmic dust in aerogel capture experiments such as those deployed during the recent Stardust space mission.

PH-Induced Deswelling Kinetics of Sterically Stabilized Poly(2-vinylpyridine) Microgels Probed by Stopped-Flow Light Scattering

Near-monodisperse, sterically stabilized poly(2-vinylpyridine) (P2VP) microgels were synthesized by emulsion polymerization. These particles exhibited completely reversible pH-responsive swelling/deswelling behavior in aqueous solution. Stopped-flow light scattering was employed to investigate the kinetics of pH-induced deswelling in highly dilute dispersions. Upon a pH jump from 2 to various final solution pH values (>or=5.4), the scattered light intensity of an aqueous dispersion of a 1,960 nm microgel exhibited an abrupt initial increase, followed by a gradual decrease to the final equilibrium value. The whole microgel-to-latex deswelling process occurred over time scales of approximately 0.5-1.0 s, which is much slower than the kinetics for latex-to-microgel swelling. The microgel deswelling kinetics depends on the final pH, with a higher final pH leading to a faster rate of shrinkage. Close inspection of the deswelling kinetics during the early stages (<0.2 s) revealed that initial microgel collapse occurred within approximately 50 ms, with more rapid transitions being observed when higher final pH values were targeted. Addition of external salt significantly accelerates the kinetics of deswelling. Systematic studies of the microgel-to-latex transition for a series of six near-monodisperse P2VP particles (with swollen microgel diameters ranging from 1270 to 4230 nm) has also been investigated. The characteristic deswelling time for initial microgel collapse, tau deswell, correlated fairly well with the initial swollen microgel radius, R, in agreement with the Tanaka equation. Moreover, the collective diffusion coefficient of the gel network, D, calculated from the slope of the tau deswell- R (2) curve, was of the order of 10 (-7) cm (2) s (-1).

First Direct Imaging of Electrolyte-Induced Deswelling Behavior of pH-Responsive Microgels in Aqueous Media Using Scanning Transmission X-ray Microscopy

Lightly cross-linked sterically stabilized poly(2-vinylpyridine) latexes exhibit pH-responsive behavior, undergoing a latex-to-microgel transition below pH 4.1 as a result of protonation of the pyridine pendent groups. We have examined both the latex and microgel states of such particles directly in aqueous solution using scanning transmission X-ray microscopy (STXM). Moreover, near-edge X-ray absorption fine structure (NEXAFS) spectroscopy studies confirm that the nitrogen atoms of the microgel particles are fully protonated at low pH. The addition of salt causes partial deswelling of these microgel particles, but spectroscopic analysis confirms the retention of their cationic character. This is the first direct visualization of the effect of electrolyte screening on microgel dimensions in aqueous solution. In each case, the observed particle dimensions are consistent with dynamic light scattering characterization, especially when polydispersity effects are taken into consideration.

Preparation of stable foams using sterically stabilized pH-responsive latexes synthesized by emulsion polymerization

Three near-monodisperse poly(2-vinylpyridine) (P2VP) latexes of 380 nm, 640 nm and 820 nm diameter were prepared in turn by emulsion copolymerization of 2-vinylpyridine and divinylbenzene using a monomethoxy-capped poly(ethylene glycol) monomethacrylate (PEGMA) macromonomer as a reactive steric stabilizer. Each of these latexes proved to be effective particulate stabilizers for the production of long-lived foams by either hand-shaking or using foam columns. Scanning electron microscopy studies confirmed that the dried foams contained well-defined latex bilayers in each case, suggesting that the original air bubbles were stabilized with latex monolayers. Dried foams prepared using the smallest latex exhibited interesting optical effects when viewed in reflectance mode. This is most likely due to light diffraction by the latex bilayers, since the mean latex diameter of 380 nm is approximately half that of visible light. These PEGMA–P2VP particles undergo a latex-to-microgel transition at low pH. Such swelling causes catastrophic instability within the wet foams, presumably due to microgel desorption from the air–water interface. Thus these latex foams exhibit pH-responsive behavior.

Preparation of stimulus-responsive liquid marbles using a polyacid-stabilised polystyrene latex

Sterically-stabilised polystyrene latex particles prepared with a new polyacid macromonomer are sufficiently hydrophobic at low pH to stabilise ‘liquid marbles’. Such ‘liquid marbles’ remain intact when placed on the surface of liquid water adjusted to pH 4 or below. Moreover, addition of base to this aqueous solution causes immediate destruction of the ‘liquid marble’ since the stabiliser chains become highly hydrophilic above pH 5.5.

Direct Observation of pH-Induced Coalescence of Latex-Stabilized Bubbles Using High-Speed Video Imaging

The coalescence of pairs of 2 mm air bubbles grown in a dilute electrolyte solution containing a lightly cross-linked 380 nm diameter PEGMA-stabilized poly(2-vinylpyridine) (P2VP) latex was monitored using a high-speed video camera. The air bubbles were highly stable at pH 10 when coated with this latex, although coalescence could be induced by increasing the bubble volume when in contact. Conversely, coalescence was rapid when the bubbles were equilibrated at pH 2, since the latex undergoes a latex-to-microgel transition and the swollen microgel particles are no longer adsorbed at the air-water interface. Rapid coalescence was also observed for latex-coated bubbles equilibrated at pH 10 and then abruptly adjusted to pH 2. Time-dependent postrupture oscillations in the projected surface area of coalescing P2VP-coated bubble pairs were studied using a high-speed video camera in order to reinvestigate the rapid acid-induced catastrophic foam collapse previously reported [Dupin, D.; et al. J. Mater. Chem. 2008, 18, 545]. At pH 10, the P2VP latex particles adsorbed at the surface of coalescing bubbles reduce the oscillation frequency significantly. This is attributed to a close-packed latex monolayer, which increases the bubble stiffness and hence restricts surface deformation. The swollen P2VP microgel particles that are formed in acid also affected the coalescence dynamics. It was concluded that there was a high concentration of swollen microgel at the air-water interface, which created a localized, viscous surface gel layer that inhibited at least the first period of the surface area oscillation. Close comparison between latex-coated bubbles at pH 10 and those coated with 66 microm spherical glass beads indicated that the former system exhibits more elastic behavior. This was attributed to the compressibility of the latex monolayer on the bubble surface during coalescence. A comparable elastic response was observed for similar sized titania particles, suggesting that particle size is a significant factor in defining the interfacial elasticity of particle-coated bubbles.

In situ observations of adsorbed microgel particles

The formation and morphological changes of a pH-responsive microgel layer on silica and mica were studied by tapping mode atomic force microscopy. First, lightly cross-linked, sterically-stabilised poly(2-vinylpyridine) (P2VP) particles were adsorbed in their non-solvated latex form at pH 4.8 to produce a structurally-disordered monolayer that covers the entire substrate. Addition of acid to this particulate film induces a latex-to-microgel transition at pH 3.0, causes particle swelling (and also some desorption) and produces a uniform, swollen film with localised hexagonal packing. Returning to pH 4.8 causes partial microgel deswelling to form individual oblate spheroidal P2VP latex particles, which retain the localised order previously induced by swelling. The swelling and collapse of this P2VP film was reversible during subsequent pH cycles, with no further desorption observed. The adsorbed amount of P2VP microgel/latex was quantified at each pH by determining the surface density and dimensions of the adsorbed particles. These measurements allow the microgel surface excess to be calculated for the first time. The initial adsorbed amount is less than that predicted by the standard Random Sequential Adsorption model (RSA) for hard particle adsorption, and is explained by the unexpected deformation of these high Tg particles due to their strong electrostatic attraction to the solid-liquid interface.

Mass spectrometry of hyper-velocity impacts of organic micrograins

The study of hyper-velocity impacts of micrometeoroids is important for the calibration of dust sensors in space applications. For this purpose, submicron-sized synthetic dust grains comprising either polystyrene or poly[bis(4-vinylthiophenyl)sulfide] were coated with an ultrathin overlayer of an electrically conductive organic polymer (either polypyrrole or polyaniline) and were accelerated to speeds between 3 and 35 km s(-1) using the Heidelberg Dust Accelerator facility. Time-of-flight mass spectrometry was applied to analyse the resulting ionic impact plasma using a newly developed Large Area Mass Analyser (LAMA). Depending on the projectile type and the impact speed, both aliphatic and aromatic molecular ions and cluster species were identified in the mass spectra with masses up to 400 u. Clusters resulting from the target material (silver) and mixed clusters of target and projectile species were also observed. Impact velocities of between 10 and 35 km s(-1) are suitable for a principal identification of organic materials in micrometeoroids, whereas impact speeds below approximately 10 km s(-1) allow for an even more detailed analysis. Molecular ions and fragments reflect components of the parent molecule, providing determination of even complex organic molecules embedded in a dust grain. In contrast to previous measurements with the Cosmic Dust Analyser instrument, the employed LAMA instrument has a seven times higher mass resolution--approximately 200--which allowed for a detailed analysis of the complex mass spectra. These fundamental studies are expected to enhance our understanding of cometary, interplanetary and interstellar dust grains, which travel at similar hyper-velocities and are known to contain both aliphatic and aromatic organic compounds.