Water-soluble polymeric surfactants

The conversion of dextran with 1,2-epoxy-3-phenoxypropane, epoxyoctane or epoxydodecane may be exploited for the preparation of amphiphilic dextran derivatives. Polymeric surfactants prepared by hydrophobic modification of polysaccharides have been widely studied, starting with the pioneering work of Landoll [261]. Neutral water-soluble polymeric surfactants can be obtained by reaction of dextran with 1,2-epoxy-3-phenoxypropane in 1 M aqueous NaOH at ambient temperature (Fig. 35, [229,233]). The number n of hydrophobic groups per 100 Glcp units varies between 7 and 22 depending on the reaction conditions. 2-Hydroxy-3-phenoxy propyl dextran ethers (DexP) behave like classical associative polymers in aqueous solution. In dilute solution, the intrinsic viscosity decreases significantly whereas... 

Two PEO-PPO-PEO three-block copolymers of the Synperonic series - F108 and P104 [28, 29] - have been employed. PPO represents the middle hydrophobic block B and both hydrophilic PEO form the two terminal chains A. These commercial, non-ionic, water-soluble polymeric surfactants were used as obtained from BASF. They are pure though not monodisperse. The molecular masses and average EO/PO contents are known from the manufacturer and yield approximate... 

Surfactants used in the polymerization are water-soluble, halogenated surfactants. They are in particular fluorinated surfactant such as ammonium, substituted ammonium, quarternary ammonium, or alkali metal salts of perfluorinated or partially fluorinated alkyl carboxylates, monoalkyl phosphate esters, alkyl ether or polyether carboxylates, alkyl sulfonates, and alkyl sulfates. 

In the steric stabilisation mechanism, the surfactant molecules are water-soluble, polymeric chains that have some conformational mobility (109). When two particles covered with nonionic surfactant approach one another, the adsorbed layer is compressed, thereby limiting the mobility of the stabiliser chains. There is an associated, thermodynamically-undesirable increase in free energy that causes the particles to be repelled from one another. 

Aoki et al. [49] developed a method for making aqueous dispersions of ultra-fine cross-Hnked diaUyl phthalate polymer particles with average diameter 10-300 nm by polymerizing aqueous solutions containing up to 15% diallyl phtha-lates in the presence of 7-30% (on diallyl phthalate) water-soluble polymerization initiators without the presence of surfactants. The dispersions are useful as... 

The preparation of waterborne latices by emulsion polymerization usually employs polymerization in aqueous micellar surfactant solutions. The simplest manifestation of the process involves the presence in an aqueous medium of emulsified monomer drops, micellar surfactant, and a water-soluble polymerization (free-radical) initiator. A combination of monomers is often used, exemplified by combinations of compounds such as methyl methacrylate, butyl acrylate, and styrene. Typically monomers have only slight water solubility. The classic qualitative picture of this process was described by Harkins [2] more than 60 years ago. A schematic of that process is shown in Figure 9.1. The key step is solubilization of the monomers in the micelles where polymerization is initiated. Both the low solubility of the monomer and the relatively low surface area of the monomer emulsion drops means that initiation of polymerization is essentially confined to the micelles. Polymerization now proceeds as more monomer is transported to the swelling micelles from the... 

In 1973, Stallings reportedly ] the polymerization of vinylidene fluoride into a polymer suitable for coatings applications. The polymerization took place in the presence of beta-hydroxyethyl tertiary butyl peroxide (initiator), a lower alkylene oxide and a water-soluble fluorinated surfactant at 10.4 MPa or higher monomer pressure. Alkylene oxide (0.02-0.5% of the monomer weight) played a beneficial role in minimizing polymer build-up on the reactor walls during the run times of 0.5-6 hours. An effective lower alkylene oxide contained 8 carbons or less in its molecule, e.g., ethylene or propylene oxide. 

Early efforts to produce synthetic mbber coupled bulk polymerization with subsequent emulsification (9). Problems controlling the heat generated during bulk polymerization led to the first attempts at emulsion polymerization. In emulsion polymerization hydrophobic monomers are added to water, emulsified by a surfactant into small particles, and polymerized using a water-soluble initiator. The result is a coUoidal suspension of fine particles,... 

Water-soluble initiator is added to the reaction mass, and radicals are generated which enter the micelles. Polymerization starts in the micelle, making it a growing polymer particle. As monomer within the particle converts to polymer, it is replenished by diffusion from the monomer droplets. The concentration of monomer in the particle remains as high as 5—7 molar. The growing polymer particles require more surfactant to remain stable, getting this from the uninitiated micelles. Stage I is complete once the micelles have disappeared, usually at or before 10% monomer conversion. 

The debate as to which mechanism controls particle nucleation continues. There is strong evidence the HUFT and coagulation theories hold tme for the more water-soluble monomers. What remains at issue are the relative rates of micellar entry, homogeneous particle nucleation, and coagulative nucleation when surfactant is present at concentrations above its CMC. It is reasonable to assume each mechanism plays a role, depending on the nature and conditions of the polymerization (26). 

The kinetics of vinyl acetate emulsion polymeriza tion in the presence of alkyl phenyl ethoxylate surfactants of various chain lengths indicate that part of the emulsion polymerization occurs in the aqueous phase and part in the particles (115). A study of the emulsion polymerization of vinyl acetate in the presence of sodium lauryl sulfate reveals that a water-soluble poly(vinyl acetate)—sodium dodecyl sulfate polyelectrolyte complex forms, and that latex stabihty, polymer hydrolysis, and molecular weight are controlled by this phenomenon (116). 

Various initiation strategies and surfactant/cosurfactant systems have been used. Early work involved in situ alkoxyamine formation with either oil soluble (BPO) or water soluble initiators (persulfate) and traditional surfactant and hydrophobic cosurfactants. Later work established that preformed polymer could perform the role of the cosurfactant and surfactant-free systems with persulfate initiation were also developed, l90 222,2i3 Oil soluble (PS capped with TEMPO,221 111,224 PBA capped with 89) and water soluble alkoxyamines (110, sodium salt""4) have also been used as initiators. Addition of ascorbic acid, which reduces the nitroxide which exits the particles to the corresponding hydroxylamine, gave enhanced rates and improved conversions in miniemulsion polymerization with TEMPO.225 Ascorbic acid is localized in the aqueous phase by solubility. 

A novel approach to RAFT emulsion polymerization has recently been reported.461529 In a first step, a water-soluble monomer (AA) was polymerized in the aqueous phase to a low degree of polymerization to form a macro RAFT agent. A hydrophobic monomer (BA) was then added under controlled feed to give amphiphilic oligomers that form micelles. These constitute a RAFT-containing seed. Continued controlled feed of hydrophobic monomer may be used to continue the emulsion polymerization. The process appears directly analogous to the self-stabilizing lattices approach previously used in macromonomer RAFT polymerization (Section 9.5.2). Both processes allow emulsion polymerization without added surfactant. 

As an even more explicit example of this effect Figure 6 shows that EPM is able to reproduce fairly well the experimentally observed dependence of the particle number on surfactant concentration for a different monomer, namely methyl methacrylate (MMA). The polymerization was carried at 80°C at a fixed concentration of ammonium persulfate initiator (0.00635 mol dm 3). Because methyl methacrylate is much more water soluble than styrene, the drop off in particle number is not as steep around the critical micelle concentration (22.) In this instance the experimental data do show a leveling off of the particle number at high and low surfactant concentrations as expected from the theory of particle formation by coagulative nucleation of precursor particles formed by homogeneous nucleation, which has been incorporated into EPM. 

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