Inverse miniemulsion systems

For the formulation of hydrophobic nanoparticles, hydrophobic HPG was dissolved in chloroform and then this solution was miniemulsified in water using SDS as surfactant (see Fig. 6a). The crosslinking reaction was performed by CUSO4 and sodium ascorbate. Hydrophilic nanoparticles of the hyperbranched polymer were obtained in an inverse miniemulsion system by dissolving the polymer in DMF and miniemulsifying the solution in cyclohexane using the block copolymer P(E/B-fc-EO) as surfactant (see Fig. 6b). After the crosslinking reaction performed at 80°C, the obtained particles could be transferred into an aqueous phase. 

Qi and coworkers reported the first study combining RAFT and inverse miniemulsion. Their inverse miniemulsion system comprised cyclohexane as the continuous phase, B246SF as the surfactant, and aqneous solution containing a CTA, acrylamide, and costabilizer MgS04. They found that using a water-soluble initiator, 4,4 -azobis(4-cyanovaleric acid), afforded better eontrol of the polymerization of acrylamide than using a lipophilic one, 2,2 -azobis(2-methylpropionitrile) (AIBN). RAFT control was realized up to 50% monomer conversion and after that significant deviation from RAFT control was observed. More recently, they have extended their RAFT inverse miniemulsion polymerization approach to other hydrophilic (co)polymers. ... 

Utama et al. [46] recently proposed an alternative strategy for the preparation of nanocapsules using RAET polymerization in an inverse miniemulsion system. In this approach, dispersed aqueous droplets (with RAET-based active stabilizers at their interface) simply acted as templates, and chain extension (with hydrophobic monomers and crosslinkers contained in the surrounding continuous phase) yielded the nanocapsules. More specifically, methyl methacrylate (MMA) [46] or styrene [47], crosslinker (EGDMA or DVB, respectively), and initiator (AIBN) were dissolved in a toluene continuous phase. Water droplets containing sodium chloride as lipophobe were formed in these toluene solutions and stabilized with RAET-synthesized poly[M-2-(hydroxypropyl methacrylamide)]-h/oc -poly(methyl methacrylate) (PHPMA-h-PMMA) or PHPMA-h-PS block copolymers, where the PHPMA segment is hydrophilic. The subsequent polymerization was confined to... 

As a new approach to the preparation of water-soluble polymers in inverse miniemulsions, a redox initiation system consisting of ceric ions and carbohydrate-based surfactant Span 60 as a reducing agent has been successfully used for the... 

The polymerization process of two monomers with different polarities was carried out in direct or inverse miniemulsions using the monomer systems AAm/MMA and acrylamide/styrene (AAm/Sty). The monomer, which is insoluble in the continuous phase, is miniemulsified in the continuous phase water or cyclohexane in order to form stable and small droplets with a low amount of surfactant. The monomer with the opposite hydrophilicity dissolves in the continuous phase (and not in the droplets). Starting from those two dispersion situations, the locus of initiation (in one of the two phases or at the interface) was found to have a great influence on the reaction products and on the quality of the obtained copolymers, which can act as hydrogels. 

In the AAm/MMA system, the best copolymer with respect to low content of homopolymers, low blockiness, and good redispersibility in polar and nonpolar solvents was obtained in inverse miniemulsion with initiation in the continuous phase cyclohexane (see Fig. 7). In this case, the MMA chains grow in the continuous phase until they become insoluble and precipitate onto the AAm droplets, which enable the radicals to cross the interface. AAm units can then be added to the polymer chain. 

As a third possibility, nanocapsules in a miniemulsion system could be achieved using different interfacial reactions in inverse miniemulsions. The formation of polyurea, polythiourea, and polyurethane nanocapsules synthesized through the polyaddition reaction has been described in detail [110-112], The size of the nanocapsules could be controlled by the amount of surfactant used and the addition time of the diisocyanate. The wall thickness was adjusted by the amount of employed monomers. dsDNA molecules were successfully encapsulated into poly-butylcyanoacrylate (PBCA) nanocapsules by anionic polymerization, which took place at the interface between the miniemulsion droplets and the continuous phase [113]. 

The crosslinking of starch at the droplet interface in inverse miniemulsion leads to the formation of hydrogels. The formulation process for the preparation of crosslinked starch capsules in inverse miniemulsion is schematically shown in Fig. 10. The influence of different parameters such as the amount of starch, surfactant P(E/B-fe-EO), and crosslinker (2,4-toluene diisocyanate, TDI) on the capsule size and stability of the system were studied. The obtained capsules were in a size range of 320-920 nm. Higher amounts of starch and surfactant result in a smaller capsule size. The TEM images of crosslinked starch capsules prepared with different amount of crosslinker (TDI) are presented in Fig. 11. The nanocapsules can be employed as nanocontainers for the encapsulation of dsDNA molecules with different lengths [114] and for the encapsulation of magnetite nanoparticles. 

The copolymerization in miniemulsion was not limited to systems for which the monomers were in the dispersed phase. Rather, copolymerization could also be carried out with monomers of opposite polarity - that is, with one comonomer in each phase - in both direct and inverse miniemulsion [26]. Water-soluble, surface active, and oil-soluble initiators were employed to start the polymerizations, as shown in Figure 15.2. Oil-soluble initiators were found to produce a higher yield of copolymers of acrylamide and methyl methacrylate with a low degree of blockiness than did water-soluble or surface-active initiators. In contrast, the surface-active polyethylene glycol (PEG) azo-initiator yielded polymers that were almost free from homopolymers, and with a low degree of blockiness, when acrylamide and styrene were copolymerized. At the interface, monomers that only copolymerize alternately [27] as water-soluble poly(hydroxy vinyl ether)s were also successfully polymerized with oil-soluble maleate esters, to yield polymer nanocapsules. 

Since the presence of a low molecular weight surfactant can reduce the properties of films prepared by these latexes, copolymerizable surfactants (known as surfmers = surfactant + monomer) were investigated for different polymerization systems. For some step-growth polymerizations in inverse miniemulsion, the surfactant is incorporated in the particle, owing to the nature of the functional end groups. Because there is usually only one reactive group in the surfactant, the reaction with the surfactant is detrimental to the molecular weight of the... 

In the case of miniemidsion polymerization, aqueous droplets are generated by sonication of the two-phase mixture and become stabilized in organic solvent by oil-soluble surfactants. The kinetically stable emulsion is formed usually at surfaaant concentration below or near its CMC. The inverse miniemulsion approach has been successfully used for the preparation of hoUow PNIPAM miaosphaes or PNIPAM miaogels functionalized by AAc. The synthesis of miaogels based on AAc, AAm, and hydroxy ethyl methacrylate (HEMA) was reported. The size of the miaogels prepared in invase miniemulsion systems typically varies between 150 and 300 nm. 

In a similar manner to the emulsion systems it is possible to perform reverse or inverse miniemulsion polymerizations (Figure 17.12) to afford lipophobic latex particles with a central... 

The anionic polymerization in aqueous dispersed systems concerns mainly the alkyl cyanoacrylate monomers, which can polymerize spontaneously at a very fast rate in the presence of water. (Nano)partides and nanocapsules were synthesized by emulsion, miniemulsion, or inverse miniemulsion polymerization processes. They mainly find applications in the biomedical domains and received for that reason a huge interest which makes it impossible to be exhaustive in this chapter. 

In the case of inverse systems, hydrophilic monomers such as hydroxyethyl acrylate, acrylamide, and acrylic acid were miniemulsified in non-polar media, e.g., cyclohexane or hexadecane [45,46]. Rather small and narrow distributed latexes in a size range between 50 nmsynthesized with nonionic amphiphilic block copolymers. Depending on the system, the surfactant loads can be as low as 1.5 wt% per monomer, which is very low for an inverse heterophase polymerization reaction and clearly underlines the advantages of the miniemulsion technique. 

Interfacial copolymerization of hydrophilic vinylethers with hydrophobic maleates can be conducted in direct [79] and in inverse [80] miniemulsions, leading to encapsulation of organic liquids or water, respectively. The concept is based on two monomers that do not homopolymerize and are located in the organic and aqueous phase, respectively. The polymerization is initiated by an interfacially active azoinitiator. Regarding the system for encapsulation of organic liquids, thermal initiation (60°C) leads to coalescence and destabilization of the miniemulsion, and thus lower reaction temperatures (30°C) are required. UV initiation was also used for the generation of stable capsules. 

Since the solubilization of oil phases other than alkanes in nonbicon-tinuous microemulsions is not very high (say, below 0.5 g of oil per gram of surfactant) unless fancy new molecules are used such as the so-called extended surfactants that are able to solubilize triglycerides [51,52], it is probable that many apparently solubilized systems referred to as O/W microemulsion are really miniemulsions produced by phase inversion [132]. 

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