Reversible Destabilization of UV-Responsive Polymer Particles (Latex) using a Photoresponsive Surfactant.

Production of aqueous dispersions of polymeric nanoparticles via heterogeneous radical polymerization in emulsion-type systems is of enormous commercial importance. The ability to reversibly destabilize such a latex is highly desirable, for example, to save transportation costs. Herein, a method for synthesis of photo-responsive polymer latexes that can be destabilized (leading to sedimentation) by only using UV irradiation (no addition of chemicals or change in the experimental conditions) and subsequently redispersed by stirring under visible light irradiation is described. The destabilization/redispersion mechanism relies on photoinduced trans-cis isomerization of the cationic diazene surfactant 2-(4-(4-butylphenyl)diazenylphenoxy)ethyltrimethylammonium bromide (C4AzoTAB) used in conjunction with the anionic surfactant sodium dodecyl sulfate. It is demonstrated that reversible destabilization can be achieved very rapidly (90 s residence time) employing continuous flow technology.

Estimation of Copolymer/Water Interfacial Tensions Using Pendant Drop Tensiometry.

Copolymer/water interfacial tensions of statistical copolymers of styrene/ n-butyl acrylate were estimated by pendant drop tensiometry using an "inverse" configuration according to which a drop of water was formed in toluene/copolymer solutions. The study first involved the precise measurement of copolymer solutions density using pycnometry. Subsequently, interfacial tensions of copolymer solutions against water were plotted as a function of copolymer concentration in toluene. Several methods were explored to fit the experimental data and obtain estimates of copolymer/water interfacial tensions at 100% copolymer concentration in toluene by extrapolation. The Belton-Evans extrapolation resulted in the best fit with the experimental data. When plotted as a function of the styrene composition of the copolymer, the interfacial tensions estimates followed an additivity relationship. This enabled estimation of the copolymer/water interfacial tensions directly from their respective homopolymer/water interfacial tensions values. These results are particularly useful for the prediction of composite particle morphology involving copolymerization of multiple monomers.

The use of chemical actinometry for the evaluation of the light absorption efficiency in scattering photopolymerizable miniemulsions.

Oil-in-water miniemulsions containing a mixture of monomers as the dispersed organic phase have been shown recently to be promising media for the development of photoinitiated polymerization processes. Albeit a crucial factor for a successful application, the efficiency of light absorption by the photoinitiator in these highly scattering systems is difficult to evaluate. In this work, a well-characterized water insoluble chemical actinometer (DFIS) replaced the oil-soluble photoinitiator, and was used as a probe and a model for UV light absorption in miniemulsions of variable droplet sizes and organic phase compositions (i.e. at different levels of scattered light). In the first step, the photon flux absorbed by the actinometer was determined in model miniemulsions based on an inert solvent (ethyl acetate), at a low oil phase content (3.0-6.0 wt%). For these low to moderately scattering systems, the photon flux absorbed by the actinometer in the miniemulsions was comparable to that in a homogeneous solution of ethyl acetate. In the second step, the absorbed photon flux was investigated in photopolymerizable miniemulsions (a mixture of acrylate monomers as oil phase). Surprisingly, in spite of much higher scattering coefficients than those found for ethyl acetate based miniemulsions of otherwise the same composition, the photon flux absorbed by the actinometer in photopolymerizable miniemulsions showed only a small decreasing trend. Such a result may be considered favorable for the further development of applications of photopolymerizations in miniemulsions.

Electrically conductive polymer/rGO nanocomposite films at ambient temperature via miniemulsion polymerization using GO as surfactant.

We have developed a facile and industrially scalable method to synthesize colloidally stable polymer nanoparticles decorated with graphene oxide (GO) sheets via miniemulsion polymerization, which in turn enables the preparation of electrically conductive films using a simple dropcasting method at ambient temperature. The resulting nanocomposite films exhibited high electrical conductivity with a wide range of potential applications as conductive coatings.

Ambient-Temperature Waterborne Polymer/rGO Nanocomposite Films: Effect of rGO Distribution on Electrical Conductivity.

Electrically conductive polymer/rGO (reduced graphene oxide) films based on styrene and n-butyl acrylate are prepared by a variety of aqueous latex based routes involving ambient temperature film formation. Techniques based on miniemulsion polymerization using GO as surfactant and "physical mixing" approaches (i.e., mixing an aqueous polymer latex with an aqueous GO dispersion) are employed, followed by heat treatment of the films to convert GO to rGO. The distribution of GO sheets and the electrical conductivity depend strongly on the preparation method, with electrical conductivities in the range 9 × 10-4 to 3.4 × 102 S/m. Higher electrical conductivities are obtained using physical mixing compared to miniemulsion polymerization, which is attributed to the former providing a higher level of self-alignment of rGO into larger linear domains. The present results illustrate how the distribution of GO sheets within these hybrid materials can to some extent be controlled by judicious choice of preparation method, thereby providing an attractive means of nanoengineering for specific potential applications.

Light-Mediated Thiol-Ene Polymerization in Miniemulsion: A Fast Route to Semicrystalline Polysulfide Nanoparticles.

Historically, the synthesis of aqueous polymer dispersions has focused on radical chain-growth polymerization of low-cost acrylate or styrene emulsions. Herein, we demonstrate the potential of UV-initiated thiol-ene step-growth radical polymerization, departing from a nontransparent difunctional monomer miniemulsion based on ethylene glycol dithiol and diallyl adipate. Performed without solvent and at ambient conditions, the photopolymerization process is energy-effective, environmentally friendly, and ultrafast, leading to full monomer consumption in 2 s, upon irradiating a miniemulsion contained in a 1 mm thick quartz cell microreactor. The resultant linear poly(thioether ester) particles have an average diameter of 130 nm. After water evaporation, they yield a clear elastomeric film combining chemical resistance and high degree of crystallinity (55%).