Microemulsion polymerization of styrene

Nanoparticles of PS (M =1.0xl0 -3.0xl0 mol ) microlatexes (10-30 nm) have also been successfully prepared from their respective commercial PS for the first time [75]. The dilute PS solutions (cyclohexane, toluene/methanol or cyclohexane/toluene) were induced to form polymer particles at their respective theta temperatures. The cationic CTAB was used to stabihze th microlatexes. The characteristics of these as-formed PS latex particles were quite similar to those obtained from the microemulsion polymerization of styrene as reported in literature. These microlatexes could also be grown to about 50 nm by seeding the polymerization of styrene with a monodisperse size distribution of D /Djj=1.08. This new physical method for preparing polymer nano-sized latexes from commercial polymers may have some potential applications, and therefore warrants further study. 

Comparative mechanistic studies on the microemulsion polymerization of styrene and methyl methacrylate were carried out by several groups [67,73-77,79,81,83,90-93,128]. The results could be coherently interpreted in terms of the relative monomer solubilities in water. In the case of styrene, which has a very low solubility in water (0.031%), it was postulated that initiation takes place in the microemulsion droplets. The polymer particles grow by recruiting monomer and surfactant from uninitiated droplets. Homogeneous nucleation in styrene systems may be relatively insignificant due to the large number of microemulsion droplets, which will capture most of the radicals generated in the aqueous phase before they reach a critical size for precipitation. 

Capek, 1. (1999) Microemulsion polymerization of styrene in the presence of anionic emulsifier. Adv. Colloid Interfac.. 82 (1-3), 253-273. 

The microemulsion polymerization of styrene in o/w system with SDS surfactant and 1-pentanol cosurfactant using water soluble K2S20g or oil-soluble AMBN initiator at 70 °C yielded stable latex of small size (20-30 nm) and high molecular weight (1.2 X 10 ) which implied that each latex particle consisted of 2 or 3... 

Xu XJ, Slow KS, Wong MK, Gan LM (2001) Microemulsion polymerization of styrene using a polymerizable nonionic surfactant and a cationic surfactant. Colloid Polym Sci 279 879-886... 

Microemulsion polymerizations follow a different mechanism from the conventional emulsion polymerizations. The most probable locus of particle nucle-ation was suggested to be the microemulsion monomer droplets [27], although homogeneous nucleation was not completely ruled out. The particle generation rate in microemulsion polymerization is given by an expression similar to Eq. (21), which was used for the miniemulsion polymerization of styrene [28] ... 

Core-shell nanoparticles can also be fabricated using microemulsions. This was performed using a two-stage microemulsion polymerization beginning with a polystyrene seed [62]. Butyl acrylate was then added in a second step to yield a core-shell PS/PBA morphology. The small microlatex led to better mechanical properties than those of similar products produced by emulsion polymerization. Hollow polystyrene particles have also been produced by microemulsion polymerization of MMA in the core with crosslinking of styrene on the shell. After the synthesis of core-shell particles with crosslinked PS shells, the PMMA core was dissolved with methylene chloride [63]. The direct cross-... 

High polymer/surfactant weight ratios (up to about 15 1) of polystyrene microlatexes [73] have been produced in microemulsions stabihzed by polymerizable nonionic surfactant by the semi-continuous process. The copolymerization of styrene with the surfactant ensures the long-term stabihty of the latexes. Nanosized PS microlatexes with polymer content (<25 wt%) were also obtained from an emulsifier-free process [74] by the polymerization of styrene with ionic monomer (sodium styrenesulfonate, NaSS), nonionic comonomer (2-hydroxyethylmethacryalte, HEM A), or both. The surfaces of the latex particles were significantly enriched in NaSS and HEMA, providing better stabilization. 

Rabagliati et al. (14) studied the polymerization of styrene in a three phase system containing an anionic-nonionic surfactant mixture and brine. Both AIBN and potassium persulfate initiators were used. The system was reported to be microemulsion continuous and even multicontinuous. (14). No autoacceleration was observed and the authors concluded that the polymerization exhibits an inverse dependence of the degree of polymerization on initiator concentration, similar to bulk solution polymerization. 

Polymerization of styrene in each of the three types of microemulsions was performed using a water soluble initiator, potassium persulfate (K2S208), as well as an oil-soluble initiator, AIBN. As desired, solid polymeric materials were obtained instead of latex particles. In the anionic system, the cosolvent 2-pentanol or butyl cellosolve separates out during polymerization. Three phases are always obtmned after polymerization. The solid polymer was obtained in the middle with excess phases at the top and bottom. GC analysis of the upper phase indicates more than 80% 2-pentanol, while Karl-Fisher analysis indicated more than 94% water in the lower phase. Some of the initial microemulsion systems have either an excess organic phase on top or an excess water phase as the bottom layer. GC analysis showed the organic phase to be rich in 2-pentanol. However, the volume of the excess phase is much less in the initial system than in the polymerized system. 

Polymerization of styrene in microemulsions has produced porous solid materials with interesting morphology and thermal properties. The morphology, porosity and thermal properties are affected by the type and concentration of surfactant and cosurfactant. The polymers obtained from anionic microemulsions exhibit Tg higher than normal polystyrene, whereas the polymers from nonionic microemulsions exhibit a lower Tg. This is due to the role of electrostatic interactions between the SDS ions and polystyrene. Transport properties of the polymers obtained from microemulsions were also determine. Gas phase permeability and diffusion coefficients of different gases in the polymers are reported. The polymers exhibit some ionic conductivity. 

Further, Wu et al. (2004) exploited radiation chemical technique to synthesize CdS/polystyrene nanocomposite hollow spheres with diameters between 240 and 500 nm under ambient conditions in which the polymerization of styrene and the formation of CdS nanoparticles were initiated by y-irradiation. It was demonstrated that the walls of the hollow spheres were porous and composed of polystyrene containing homogeneously dispersed CdS nanoparticles (Figure 23.14). The quantum-confined effect of the CdS/polystyrene nanocomposite hollow spheres was confirmed by the ultraviolet-visible (UV-vis) and PL spectra. They proposed that the walls of these nanocomposite hollow spheres originated from the simultaneous synthesis of polystyrene and CdS nanoparticles at the interface of microemulsion droplets. 

For both O/W and W/O systems, the amount of monomer is usually restricted to 5-10 wt% with respect to the overall mass, and that of surfactant(s) lies within the same range or even above. Nevertheless, there have been a few studies in which the formulation deviated from these conditions. For instance, surfactant concentrations of 2 wt V() were reported [56-58,69,124,125]. However, in this case the amount of monomer was also very low (< 2 wt%) so that the systems must be considered as micellar solutions rather than true microemulsions. Conversely, a 1994 study of Gan et al. [82] reported the polymerization of styrene up to 15 wt% using only about 1 wt V(> dodecyltrimethylammonium bromide surfactant (DTAB) in a Winsor I-like system. This system consists of a microemulsion (lower) phase topped off with pure styrene. The polymerization takes place in the microemulsion phase, while the styrene phase acts as a monomer reservoir. Such a polymerization process is novel, but it yields latices of large particle size ( 100 nm) that can be more easily obtained by conventional emulsion polymerization. 

Despite these difficulties, most of the earliest studies used an alcohol in the formulation of 0/W microemulsions, and it was only in 1989 that the polymerization of hydrophobic monomers in a three-component O/W cationic system was reported by Ferrick et al. [124]. This spurred new interest, and systematic studies on ternary microemulsions based on cationic surfactants of different alkyl chain lengths ensued, mainly in the group of Gan and those of Puig and Kaler [67,77,81-87,90-93,127,128]. Nonionic surfactants were also used in ternary O/W microemulsions for the polymerizations of styrene and methyl methacrylate [68] and anionic [AOT] surfactants for the polymerization of tetrahydroflirfuryl methacrylate [95,97]. 

Similar studies were conducted for the polymerization of styrene [84]. These led to a single dependence of Rp on surfactant concentration for both emulsion and microemulsion systems (/ p oc [TTAB] ). This confirms that when surfactant concentrations are well above the critical micelle concentration (cmc) ( > 3 wt%) in emulsion or microemulsion systems, the micellar nucleation mechanism prevails in both cases because of the very low solubility of this monomer in water. 

Friberg and coworkers examined the problems of stability in a series of papers related to the polymerization of styrene in water-in-oil microemulsions stabilized by SDS and pentanol [64,132-134]. Their conclusion was that entropic conformational factors were not the only ones of importance in the mechanism of destabilization. A correlation was established between polymer solubility in the cosurfactant and that of monomer by using another cosurfactant, butyl cellosolve. The higher solubility of polystyrene in butyl cellosolve gave better stability than pentanol-containing microemulsions. 

Qutubuddin and coworkers [43,44] were the first to report on the preparation of solid porous materials by polymerization of styrene in Winsor I, II, and III microemulsions stabilized by an anionic surfactant (SDS) and 2-pentanol or by nonionic surfactants. The porosity of materials obtained in the middle phase was greater than that obtained with either oil-continuous or water-continuous microemulsions. This is related to the structure of middle-phase microemulsions, which consist of oily and aqueous bicontinuous interconnected domains. A major difficulty encountered during the thermal polymerization was phase separation. A solid, opaque polymer was obtained in the middle with excess phases at the top (essentially 2-pentanol) and bottom (94% water). The nature of the surfactant had a profound effect on the mechanical properties of polymers. The polymers formed from nonionic microemulsions were ductile and nonconductive and exhibited a glass transition temperature lower than that of normal polystyrene. The polymers formed from anionic microemulsions were brittle and conductive and exhibited a higher Tj,. This was attributed to strong ionic interactions between polystyrene and SDS. 

Thermal polymerization of styrene was also carried out by Rabagliati et al. [45] in a three-phase Winsor III microemulsion-styrene-brine system. The system exhibited features of a solution polymerization process, and no mention was made of the morphology of the final product. 

Colloid Polymer Science 211, No.lO, Oct.1999, p.997-1000 KINETICS OF POLYMERIZATION OF STYRENE-IN-WATER MICROEMULSIONS... 

Mader C, Schndll-Bitai I. Pulsed laser polymerization of styrene in microemulsion determination of band broadening in size exclusion chromatography with multimodal distributions. Macromol Chem Phys 2005 206 649-657. 

Figure 6.2. Number of latex particles per unit volume of water as a function of monomer conversion for (a) 0/W mioroemulsion polymerization of styrene [24] and (b) W/0 microemulsion polymerization of aorylamide [61],...

The synthesis of metal-complexing nanoparticles with an average size ranging from 15 to 25 nm was recently described (Larpent et al, 2003). It consists of a two-step procedure starting first with the microemulsion polymerization of a mixture of styrene and vinylbenzylchloride in the presence of a cationic surfactant, then followed by the postfunctionalization with a selective macrocyclic ligand, tetrazacyclotetradecane (cyclam). The authors showed the ability of these cyclam-functionalized nanoparticles to complex copper and in addition they observed that they exhibited a solution-hke complexation behavior, demonstrating the accessibility of the immobilized ligand. 

---