Preparation Of Polymer Microspheres Through Precipitation Polymerization And Their Self-assembly

Monodisperse polymer microspheres have been largely applied in many fields.Compared with the traditional methods for their preparation,precipitation polymerization does not need any additives and post-treatment.Wax droplets and microspheres have been widely used in leather,paper,textiles,automobile and cosmetic industries.In these applications,wax is commonly used under the form of emulsions or dispersions.Wax microspheres of large size with several hundreds of microns in diameter are also useful in tissue engineering and functional Janus particles.To keep the emulsion stable,surfactants or stabilizers with co-surfactants and other additives are usually needed.The abundant use of the associated additives,mainly the surfactants/dispersants,increases not only the prime cost of the product;they are also unpleasant in many applications,in particular in biological and medical applications.Fine solid particles of nano-or micro-scales,namely Pickering stabilizer,can also function in similar ways as surfactants and stabilizers to form stable emulsion.Compared with the emulsions stabilized using traditional emulsifiers,Pickering emulsion has many advantages or features,such as good stability and low toxicity.In this paper,polymer particles were prepared through precipitation polymerization of trihydroxymethyl propane triacrylate（TMPTA）with 1-dodecene（DC）,in binary solvent of water-ethanol（H₂O-EtOH）,without any surfactant and stabilizer.The experimental results showed that crosslinked microspheres were obtained when the amount of H₂O was between0²0 wt%.Increased of H₂O amount in the continue phase led to a higher monomer conversion and a higher production for the monodisperse polymer microspheres.With the other experimental conditions unchanged,the yield of the polymer microspheres and monomer conversion can also improve when increased the amount of the initiator.Prolonging reaction time from 0.5 to 8 h,P（DC-TMPTA）polymer microspheres with size from 1.0 to 1.7μm were prepared.Pickering emulsion of wax in H₂O-EtOH was prepared by self-assembly of P（DC-TMPTA）microspheres on wax droplets in H₂O-EtOH with simple shaking at 70°C,above the melting point of the wax.By rapidly cooling the system down to a temperature far below the melting point of the wax,wax microspheres with narrow size distribution were obtained.The process was optimized with regard to the composition of H₂O-EtOH,shaking rate,the amount and the size of P（DC-TMPTA）particles.Results showed that the addition of water in the continuous phase has great effect on the morphology and particle size in wax spheres preparation.With water content of 12 wt%in the continues phase,wax-H₂O/EtOH-polymer contact angle（θ）near 90o,stable paraffin emulsion with small size and quite good monodispersity was obtained.As shaking rate increased from 200 to 360osc/min,the size of the wax microspheres obtained decresed from 279.3μm to 101.4μm.Wax microspheres size increased with the increase in the size of P（DC-TMPTA）polymer microspheres,and decreased with the increase in the amount of the polymer microspheres.Morphology of the wax microspheres was examined by SEM,showing the wax spheres were covered by a layer of P（DC-TMPTA）particles.The wax microspheres were crushed after soaking in liquid nitrogen to see the interior structure under SEM.It was well confirmed that the P（DC-TMPTA）particles were not present inside but solely concentrated on the surface of the wax microspheres.The formation mechanism of the wax microspheres was also discussed on the basis of the adsorption energy of P（DC-TMPTA）particles on wax microspheres,estimated from the corresponding contact angle of the solvent toward the P（DC-TMPTA）particles and the wax.