Complex interfaces in "phase-change" contrast agents

In this paper we report on the study of the interface of hybrid shell droplets encapsulating decafluoropentane (DFP), which exhibit interesting potentialities for ultrasound (US) imaging. The fabrication of the droplets is based on the deposition of a dextran methacrylate layer onto the surface of surfactants. The droplets have been stabilized against coalescence by UV curing, introducing crosslinks in the polymer layer and transforming the shell into an elastomeric membrane with a thickness of about 300 nm with viscoelastic behaviour. US irradiation induces the evaporation of the DFP core of the droplets transforming the particles into microbubbles (MBs). The presence of a robust crosslinked polymer shell introduces an unusual stability of the droplets also during the core phase transition and allows the recovery of the initial droplet state after a few minutes from switching off US. The interfacial tension of the droplets has been investigated by two approaches, the pendant drop method and an indirect method, based on the determination of the liquid ↔ gas transition point of DFP confined in the droplet core. The re-condensation process has been followed by capturing images of single MBs by confocal microscopy. The time evolution of MB relaxation to droplets was analysed in terms of a modified Church model to account for the structural complexity of the MB shell, i.e. a crosslinked polymer layer over a layer of surfactants. In this way the microrheology parameters of the shell were determined. In a previous paper (Chem. Commun., 2013, 49, 5763-5765) we showed that these systems could be used as ultrasound contrast agents (UCAs). In this work we substantiate this view assessing some key features offered by the viscoelastic nature of the droplet shell.

Macromolecular surfactants for miniemulsion polymerization

The use of polymeric surfactants as stabilizers in miniemulsion polymerization was reviewed. The structural characteristics of reported polymeric surfactants were detailed and compared. The concept of multi-functional polymeric surfactants was evidenced. The specificities brought by polymeric surfactants in the process of miniemulsion polymerization in comparison to molecular surfactants were analysed for the stability of the initial monomer emulsion, polymerization kinetics and characteristics of the obtained latexes. The contribution of polymeric surfactants to the control of the characteristics of the obtained nanoparticles was detailed with regard to the nature of the core material and to the surface coverage. Polymeric surfactants can be seen as powerful tools for the design of original nanoparticles. On the basis of the available data, possible research topics are suggested.

Poly(n-butyl cyanoacrylate) nanoparticles via miniemulsion polymerization (1): dextran-based surfactants

This study aims at synthesizing polysaccharide-coated poly(n-butyl cyanoacrylate) nanoparticles by miniemulsion polymerization. Because of the high reactivity of n-butyl cyanoacrylate, drastic conditions are required in order to emulsify the monomer in water while limiting its anionic polymerization. However, nanoparticles were successfully obtained by miniemulsion polymerization of butyl cyanoacrylate-in-water emulsions stabilized by amphiphilic dextran derivatives. Their physico-chemical properties were thoroughly investigated as a function of amphiphilic dextran structure and concentration. The substitution degree of the amphiphilic dextran used as stabilizer had little influence on the final properties of the obtained nanoparticles. Particle size decreased with the concentration of amphiphilic dextran in the aqueous phase whereas the hydrophilic layer thickness and the amount of adsorbed polysaccharide were nearly constant in the entire range of concentrations studied.

Surfactive water-soluble copolymers for the preparation of controlled surface nanoparticles by double emulsion/solvent evaporation

We have already shown that polylactide (PLA) nanoparticles covered with a hydrophilic polymeric layer can be prepared by simple emulsion/solvent evaporation by using amphiphilic copolymers as surfactants during the procedure. The external layer is then constituted by the hydrophilic part of the macromolecular surfactant. This kind of nanospheres is useful for the encapsulation of lipohilic molecules. The use of amphiphilic copolymers as surfactants in the preparation of PLA nanospheres with controlled surface properties, was then applied to the double emulsion/solvent evaporation procedure. The aim was to allow the encapsulation of water-soluble bioactive molecules in PLA particles with controlled surface properties. In this paper, we describe the results obtained with three different water-soluble monomethoxypolyethylene oxide (MPEO)-b-PLA diblock copolymers used as surfactants in the preparation of nanoparticles by double emulsion/solvent evaporation. After organic solvent evaporation, the obtained nanospheres were proved to be really covered by a MPEO layer whose characteristics were determined. It was firstly shown that the MPEO-covered particles did not flocculate at 25 degrees C, even in 4 M NaCl while suspensions of bare nanospheres were destabilized for a NaCl concentration as low as 0.04 M. On the other hand, the suspensions of MPEO-covered nanoparticles in 0.3 M Na2SO4 were found to be very sensitive to temperature as they flocculated at a temperature lying between 45 and 55 degrees C depending on the MPEO-b-PLA composition. This property was attributed to the fact that MPEO is a polymer with a low critical solution temperature. The concentration of MPEO at the nanoparticle surface was then calculated for the three kinds of particles, from the initial flocculation temperature, and was found to be comparable to the value determined directly.

Aqueous Solutions of Native and Hydrophobically Modified Polysaccharides: Temperature Effect

Amphiphilic polysaccharides, obtained by the attachment of various hydrocarbon groups onto dextran, are studied in aqueous solutions. The viscosity of their aqueous solutions is examined as a function of concentration and temperature in the range 25-65 degrees C. Varying polymer concentration, viscosity follows a polynomial development of Huggins equation in which the coefficients can be calculated from the Huggins constant determined in the dilute domain (Matsuoka-Cowman equation). For all polymers, the solution viscosity follows an Arrhenius-like variation with temperature. The activation energy of the aqueous solutions is determined as a function of polymer concentration and structural characteristics (nature and amount of grafted hydrocarbon groups). The variation of activation energy with polymer concentration is shown to be consistent with predictions based on the Matsuoka-Cowman equation combined with the equation of Andrade. This conclusion is extended to other polysaccharides using data from the literature.

Amphiphilic polysaccharides: useful tools for the preparation of nanoparticles with controlled surface characteristics

Polymeric surfactants obtained by hydrophobic modification of dextran are used as stabilizers for oil-in-water emulsions. The kinetics of interfacial tension decrease is studied as a function of polymer structural characteristics (degree of hydrophobic substitution) and at various polymer concentrations. Several hydrocarbon oils, either aliphatic (octane, decane, dodecane, and hexadecane) or aromatic (styrene), are tested. Kinetics exhibits the same general trends no matter which oil or polymer is considered. The emulsifying properties of the polymeric surfactants are illustrated by the preparation of oil-in-water emulsions. The droplet size at the preparation is correlated to the amount of oil and to the polymer concentration in the aqueous phase. For low polymer/oil ratios, it is shown that the droplet size is limited by the initial amount of polymer. On the contrary, for high polymer/oil ratios, the droplet size seems to level down, indicating that other parameters become predominant. Emulsion aging occurs by Ostwald ripening, and it is demonstrated that the theoretical equation of Lifshitz, Slyozov, and Wagner (LSW) correctly describes the experimental results. The nature of the oil has important effects on emulsion aging, as described by the LSW equation. The aging of emulsions containing oil mixtures is quantitatively described on the basis of the results with pure oils. The influence of polymer chemical structure can be conveniently correlated to interfacial tension results through the LSW equation. On the contrary, the influence of oil volume fraction seems to be overestimated by the usual correction factor, k(phi). The effect of temperature on emulsion aging is finally examined. Miniemulsions stabilized with dextran derivatives are used for the radical polymerization of styrene. Following this procedure, polysaccharide-covered polystyrene nanoparticles are prepared and characterized (size and surface coverage). The size of the particles is directly correlated to that of the initial droplets for styrene volume fractions around 10%. On the contrary, for initial styrene volume fractions around 20%, particles exhibit a larger size than the initial droplets, indicating that coalescence processes take place during polymerization. The amount of dextran at the surface of the particles is determined and compared to the adsorbed amounts resulting from emulsion preparation.