Solids-stabilised emulsions and assembly of supracolloidal arrays recently gained considerable commercial interest in important applications such as the food industry, cosmetics, and coatings. Over the last 10 years, Armeset al.have investigated several systems by inserting alcoholic silica sols in dispersion polymerisation processes.1-5 Notably, they studied the case of polystyrene latexes and used different types of initiators (oil soluble, cationic) and reported the preparation of lightly cross-linked poly(4-vinyl pyridine)/silica nanocomposite particles as an interesting new class of pH-responsive particulate emulsifiers for oils of varying polarity at around neutral pH.6 Lewis and co- workers described that a stable colloidal system of negligible charged silica spheres could be obtained upon addition of a critical volume fraction of highly charged nanoparticles of hydrous zirconia.7 These nanoparticles did not adsorb onto the silica spheres, and the stabilisation mechanism was referred to as “haloing”. The mechanisms of different types of colloidal interaction and pioneering works on their driven assemblies at liquid-liquid or solid-liquid interfaces are described in Chapter I, see section I.4.
In 1996, Velev et al. described the synthesis of hollow supracolloidal structures via assembly of polystyrene latex at the liquid-liquid interface of emulsion droplets.8-9 The synthesis of selectively permeable capsules composed of colloidal particles was reported by Dinsmore et al., who tentatively named these supracolloidal structures “colloidosomes”.10 Recently they also published a temperature – responsive system in which the colloidosomes composed of poly(N-isopropylacrylamide-co-acrylic acid) microgel particles were formed by self-assembly.11Paunovet al.have shown the preparation of colloidosomes with
a gelled aqueous core stabilised by a shell of agarose.12-13 Magnetic Pickering emulsifiers were studied by Fuller et al..14 They demonstrated the ability to induce triggered macroscopic phase separation of solid-stabilised emulsions using paramagnetic particles at an oil/water interface upon application of external magnetic fields. Russel et al. reported on supracolloidal capsules of quantum dots using both radical and ring-opening metathesis polymerisation techniques to cross-link vinyl benzyl and norbornene functional quantum dots, respectively.15-16 Most of those studies use particles with diameters in the range of submicron to few microns. The use of smaller solids-stabilisers, such as cadmium selenide nanoparticles, can direct nanoparticle assembly at liquid- liquid interfaces, the small particle size giving rise to an interfacial adsorption energy comparable to a few times the thermal energy.17
Our interest lies in the formation of supracolloidal structures by polymerising submicron monomer droplets which have solid particles adhered to their interface via a miniemulsion polymerisation process. Early pioneering works were undertaken by Hohenstein et al. in 1946 describing the polymerisation of olefins and diolefins notably in suspension polymerisation in which some experiments used solid particles as stabiliser.18-19 Our group demonstrated the utility of the building blocks assembly at liquid-liquid interfaces for the formation of reactor vessels and their buckling behaviour of giant droplets upon evaporation of its content.20 We further developed this strategy to prepare for the first time polymer latexes with a Pickering miniemulsion polymerisation process.21 Armoured latex particles of polystyrene and various hydrophobic (meth)acrylates were made upon solidification by polymerisation of the monomer droplets stabilised with Laponite clay platelets.
Laponite RD is a synthetic trioctahedric hectorite clay, composed of two tetrahedral silica sheets and a central octahedral magnesia sheet with a density of 2.570 g cm-3and a general chemical formula which can be written as follows: [Si8(Mg5.45Li0.4)O20(OH)4]Na0.7. The overall negative charge accompanied with
the positive charges located between the silica sheets, and therefore at the rim of the disc, confer to those particles unique and very complex colloidal properties. Stable dispersions at high pH showed different types of gel structures by X-ray scattering measurements22 and the mechanisms of swelling and dispersion of several clays into individual particles has been described.23-24 The addition of salt reduces the critical flocculation concentration at which gels are formed which is opposite to the behaviour of spherical particles acting as isotropic liquids upon compression of the double layer. At low ionic strength, an unconnected face-face alignment of clay discs is suggested responding to dominating electrostatic repulsions, whereas at higher ionic strength, the so- called “house-of-card”25 structure was observed. Although, recent simulations suggested the configuration of “overlapping-coins” to be the most favourable.26 Amongst various investigations, atomic force microscopy experiments lately addressed the dispersity of the particle size and revealed its disc-liked shape to be about 25 to 35 nm in diameter with thickness of ca. 1 nm.27 Those dimensions are of suitable order for the stabilisation of droplets of submicron sizes as confirmed by previous group work on Pickering miniemulsion polymerisations using nanosized Laponite clay discs as stabiliser.21
Ashby and Binks reported a systematic study on the stability of oil-in-water emulsion droplets using Laponite RD as Pickering stabiliser for different concentrations in salt.28They only observed the formation of stable emulsions of
toluene, water and Laponite RD clay particles under the conditions where the clay are flocculated, for concentrations in sodium chloride not lower than 0.1 M, and at intermediate concentrations in particles, that is to say 1.0 to 3.5 wt%. At higher clay content, shearing difficulties were encountered caused by the high viscosity of the dispersion. No phase inversion was observed for variety of oil and polar oils and even at high oil volume fraction due to the hydrophilic nature of Laponite RD clay.21 The average droplet size was found to be strongly affected by the oil volume fraction but did not depend on the clay concentration. Binks also observed a limited Ostwald-ripening effect and suggested a mechanism for which the droplet interface is quickly fully-covered by the clay, hence inhibiting further oil diffusion. Stability of the system is provided not only from the suggested Pickering stabilisation of the liquid-liquid interface but also from the network or moderately viscous gel formed by the Laponite discs in the aqueous medium. Those clay particles were already integrated in polymerisation processes in late 50’s, as published by Wiley in two patents. One patent describes the use of clay as thickener for suspension polymerisation and the other one is on quiescent suspension polymerisation including the use of colloidal particles as stabiliser.29-30 Combination of two types of clay in an emulsion polymerisation process also stabilised by anionic surfactant showed enhancement of the colloidal stability of the final latexes for which Choiet al. suggested an ensemble of potential factors.31-32 Stability would be achieved not only from the electrostatic repulsive charges between the clay particles and the anionic surfactant or from their adsorption onto the latex spheres acting like steric barriers, but also by creating barriers when forming layers within the continuous aqueous phase and increasing the viscosity thereby reducing the
latex particles mobility and lowering the impact and the probability of collisions between them.
Solids-stabilisers were employed successfully in (mini)emulsion polymerisation processes, but always in presence of other surfactants and/or co- monomers.32-33 The use of nanosized clay particles in absence of molecular surfactants allowed successful stabilisation of styrene miniemulsion droplets.21 A mechanistic approach described the Pickering miniemulsion polymerisation kinetics of styrene, as well as the extension to a series of hydrophobic monomers.34 The average particle size distributions of the latexes could be tailored with the concentrations in monomer and synthetic clay used. A semi- empirical model based on the hypothetical equilibrium of Laponite clay discs distributed between the aqueous phase (excess) and onto the latex surfaces (Pickering stabiliser) allowed prediction of the latex particle sizes. Recent work on the preparation of ‘‘soft–hard’’ poly(lauryl acrylate) – Laponite hybrid particles made via Pickering miniemulsion polymerisation led to marked mechanical property enhancements when these nanocomposite supracolloidal particles were blended at various low concentrations with a standard poly(butyl acrylate-co-acrylic acid) latex for application as a waterborne pressure-sensitive adhesive.35
In this chapter, the development of Laponite clay-stabilised poly(vinyl esters) latexes of submicron sizes via miniemulsion polymerisation was investigated. Limitations of this process are addressed to meet up with industrial requirements, i.e. latexes with high overall solids contents and “pH-friendly”, giving special consideration to the sensitivity of the monomer/polymer to undergo hydrolysis.