Responsive core−shell latex particles as colloidosome microcapsule membranes

Responsive core-shell latex particles are used to prepare colloidosome microcapsules using thermal annealing and internal cross-linking of the shell, allowing production of the microcapsules at high concentrations. The core-shell particles are composed of a polystyrene core and a shell of poly[2-(dimethylamino)ethyl methacrylate]-b-poly[methyl methacrylate] (PDMA-b-PMMA) chains adsorbed onto the core surface, providing steric stabilisation. The PDMA component of adsorbed polymer shell confers the latex particle thermal and pH responsive characteristics, it also provides glass transitions at lower temperatures than that of the core and reactive amine groups. These features facilitate the formation of stable Pickering emulsion droplets and the immobilisation of the latex particle monolayer on these droplets to form colloidosome microcapsules. The immobilisation is achieved through thermal annealing or cross-linking of the shell at mild conditions feasible for large scale economic production. We demonstrate here that it is possible to anneal the particle monolayer on the emulsion drop surface at 75-86 ºC by using the lower glass transition temperature of the shell compared to that of the polystyrene cores (~108 ºC). The colloidosome microcapsules formed have a rigid membrane basically composed of a monolayer of particles. Chemical cross-linking has also been successfully achieved by confining a cross-linker within the disperse droplet. This approach leads to the formation of single-layered stimulus-responsive soft colloidosome membranes and provides the advantage of working at very high emulsion concentrations since inter-droplet cross-linking is thus avoided. The porosity and mechanical strength of microcapsules are also discussed here in terms of the observed structure of the latex particle monolayers forming the capsule membrane.

Scale-up in emulsion polymerisation

The procedure for successful scale-up of batchwise emulsion polymerisation has been studied. The relevant literature on liquid-liquid dispersion on scale-up and on emulsion polymerisation has been crit1cally reviewed. Batchwise emulsion polymerisation of styrene in a specially built 3 litre, unbaffled, reactor confirmed that impeller speed had a direct effect on the latex particle size and on the reaction rate. This was noted to be more significant at low soap concentrations and the phenomenon was related to the depletion of micelle forming soap by soap adsorption onto the monomer emulsion surface. The scale-up procedure necessary to maintain constant monomer emulsion surface area in an unbaffled batch reactor was therefore investigated. Three geometrically similar 'vessels of 152, 229 and 305mm internal diameter, and a range of impeller speeds (190 to 960 r.p.m.) were employed. The droplet sizes were measured either through photomicroscopy or via a Coulter Counter. The power input to the impeller was also measured. A scale-up procedure was proposed based on the governing relationship between droplet diameter, impeller speed and impeller diameter. The relationships between impeller speed soap concentration, latex particle size and reaction rate were investigated in a series of polymerisations employing an amended commercial recipe for polystyrene. The particle size was determined via a light transmission technique. Two computer models, based on the Smith and Ewart approach but taking into account the adsorption/desorption of soap at the monomer surface, were successful 1n predicting the particle size and the progress of the reaction up to the end of stage II, i.e. to the end of the period of constant reaction rate.

The study and characterisation of water-based latices used for can coating interiors

Water-based latices, used in the production of internal liners for beer/beverage cans, were investigated using a number of analytical techniques. The epoxy-graft-acrylic polymers, used to prepare the latices, and films, produced from those latices, were also examined. It was confirmed that acrylic polymer preferentially grafts onto higher molecular weight portions of the epoxy polymer. The amount of epoxy remaining ungrafted was determined to be 80%. This figure is higher than was previously thought. Molecular weight distribution studies were carried out on the epoxy and epoxy-g-acrylic resins. A quantitative method for determining copolymer composition using GPC was evaluated. The GPC method was also used to determine polymer composition as a function of molecular weight. IR spectroscopy was used to determine the total level of acrylic modification of the polymers and NMR was used to determine the level of grafting. Particle size determinations were carried out using transmission electron microscopy and dynamic light scattering. Levels of stabilising amine greatly affected the viscosity of the latex, particle size and amount of soluble polymer but the core particle size, as determined using TEM, was unaffected. NMR spectra of the latices produced spectra only from solvents and amine modifiers. Using solid-state CP/MAS/freezing techniques spectra from the epoxy component could be observed. FT-IR spectra of the latices were obtained after special subtraction of water. The only difference between the spectra of the latices and those of the dry film were due to the presence of the solvents in the former. A distinctive morphology in the films produced from the latices was observed. This suggested that the micelle structure of the latex survives the film forming process. If insufficient acrylic is present, large epoxy domains are produced which gives rise to poor film characteristics. Casting the polymers from organic solutions failed to produce similar morphology.