SURFMER

Surficial uranium deposits, 17 521 Surfmers, 14 719 Surfynol 104, 8 244t, 8 241t Surgery... 

The colloidal stable polymer dispersions, the monodisperse polymer particles, and high conversions (85-100%) can be obtained with most of the other macromonomers (MAL,VB, and MA) of PEO (MW>PEO=2000)) [76]. Also, when macromonomers are used (3.1 wt% based on styrene), there is practically no coagulum produced. This is not the case in the presence of polymerizable PEO surfactants (surfmer I R1=CH3(CH2)11-, R2=H, n=34 and surfmer II R =CH3 (CH2)n-, R2=H, n=42) despite the higher amounts of stabilizer used (up to 60 wt% of coagulum). Furthermore, the particles are more monodisperse with PEO macromonomer (Dw/Dn=1.025 for PEO-MA and 1.13 for PVPo) compared to those with surfmer. Comparatively poorer results were obtained with conventional surfactants such as ethoxylated nonylphenol, even when used in large amounts. 

The incorporation yields of the stabilizers are always very limited, but the highest value is obtained for the PEO-MA macromonomer, and the lowest for the amphiphilic compound (surfmer) with the longest hydrophobic sequence ca. 0.5% ... 

Inisurfs, Transurfs and Surfmers may be used to reduce/avoid the use of conventional surfactants in emulsion polymerization. However, when Inisurfs and Transurfs are used, the stability of the system cannot be adjusted without affecting either the polymerization rate (Inisurfs) or the molecular weight distribution (Transurfs). Furthermore, the efficiency rate of Inisurfs is low due to the cage effect. It is therefore not obvious yet that these classes will become commercially significant. 

In common with conventional surfactants, Inisurfs and Transurfs, Surfmers form micelles in aqueous solutions above the CMC. The organized monomer aggregates of colloidal dimension are microscopically heterogeneous and may affect polymerization kinetics and polymer structure and properties. 

One important requirement in replacing a conventional, nonreactive surfactant with a reactive one is that neither the molecular weight nor the particle size distribution of the latex may significantly change. Also, the Surfmer reactivity is important if the Surfmer is too reactive compared to the other monomers in the recipe, it will become partially buried inside the growing polymer particles. This will cause poor stability during polymerization and broadening of the particle size distribution. 

Most of the reactive surfactants used for emulsion polymerization have the reactive group at the end of the hydrophobic moiety of the molecule, on the assumption that the polymerization process takes place in the latex particle. Work of Ferguson et al. [14] shows indeed a lower stability of lattices produced with Surfmers with an acrylate group attached to the end of the hydrophilic chain than those produced with the equivalent terminated with an ethyl ester group. 

Ionic Surfmers. Ionic Surfmers were extensively considered in the early developments of polymerizable surfactants. Examples of products with anionic, cationic and amphoteric moieties are given in Figures 6.40-6.48. 

The work of Guyot [7] reviews the use and effects of ionic Surfmers in different polymerization processes. 

In the last 10-15 years a number of considerations including performance, cost effectiveness, synthetic complexity and raw material availability have favored the emergence of other classes of Surfmers, and these are discussed below. 

Maleate Surfmers. Surfmers with allylic, acrylic and vinylic moieties tend to homopoly-merize and produce water-soluble polyelectrolytes if used above their CMC. This has shifted researchers attention to maleic derivatives that do not homopolymerize at normal temperatures because their ceiling temperature is too low. Tauer and co-workers have pioneered the synthetic work [4,15] which led originally to compounds like those given in Figure 6.49. An example of maleic-derived Surfmer used in emulsion polymerization lattices is reported in [16] and the advantages provided in commercial paint formulations are discussed later. 

Maleic-derived Surfmers have been shown to be quantitatively bound to latex particles. For example the surface tension of the latex serum from the emulsion polymerization of styrene remains above 70mNm-1 after polymerization even if amounts in excess of 100 times the CMC are used (15). 

Maleate Surfmers were found to outperform methacrylic and crotonic compounds in the copolymerization of styrene, butyl acrylate and acrylic acid in seeded and nonseeded semicontinuous processes [17]. The maleate Surfmer achieved high conversion without homopolymerization in the aqueous phase which can result in emulsion instability. The methacrylate Surfmer was too reactive as opposed to the crotonate which was not sufficiently reactive. The reported dependence of the maleate Surfmer conversion on the particle diameter is consistent with a reaction at the particle surface. 

The simple maleate Surfmer (i.e. the neutralized hemi ester of a fatty alcohol) was used to prepare seeds of polystyrene latex which were grown with a shell of film-forming polymers. The reported incorporation yield was of the order of 75% [18]. The reported latex stability could be further improved by Surfmers in which the ester moiety was substituted for an amide moiety by reaction with a fatty amine. An overall improved stability and a reduced hydrolysis at high temperature were observed [19]. 

Performance enhancement ofmaleate Surfmers. Several options have been proposed to enhance the performance of maleate Surfmers. In particular the modulation of the reactivity has been considered, to achieve a controlled and moderate reactivity during most of the polymerization and a high conversion at the end of the process. These requirements limit the useful range of values of the reactivity ratios of the Surfmer/monomer systems [22]. 

One way to achieve this result relies on the change in the relative monomer reactivity following composition drifts. Thus, in a combination ofhigh and low reactivity monomers, the former will preferentially react first, leaving a considerable proportion of the latter for copolymerization when the supply of the high reactive monomer is depleted. This has been confirmed in the terpolymerization of methyl methacrylate/butyl acrylate/vinyl acetate in the presence of the maleate Surfmer reported in Figure 6.49. 

Another suggested approach is to provide for a larger surface area for the Surfmers in the late stage of the polymerization process, for example by introducing a new seed of particles with a small amount of monomer. It is reported that this approach increased the conversion of a Surfmer from about 50% to nearly 100% [22]. 

Cationic and amphoteric Surfmers were synthesized from the hemi ester or hemi amide with a C12-C20 alkyl chain and diethyl (chloroethyl) amine followed by quaternization with conventional agents or by reaction with propanesultone [23]. 

The cationic Surfmers produced much smaller particle sizes in the emulsion polymerization of styrene and styrene/butyl acrylate than the amphoterics (20-50 nm versus 100-300 nm). Some of the latter, however, conferred to the copolymer lattices stability to electrolytes and freeze-thaw [24]. Similar, but nonreactive surfactants produced from succinic anhydride gave similar stability but had much inferior water resistance [25]. 

The acylation of alcohol-containing monomers, e.g. hydroxyethyl acrylates or vinyl benzyl alcohol with maleic, succinic or sulfosuccinic anhydride leads to bifunctional polymerizable surfactants. A range of such products has been synthesized and tested in batch polymerization and core-shell polymerization of styrene and butyl acrylate [26]. In both cases good stability, high conversion and little burying of the Surfmers were observed. Water rebound was also limited. These advantageous features were however offset by an unacceptable resistance to electrolytes and to freeze-thaw. 

Non ionic Surfmers. The alkoxylation of polymerizable substrates has been for many years a source of building blocks for innovative surfactant species as well as for the synthesis ofhigh performance Surfmers. Some of the early experimental prototypes have evolved into commercial products and the growing understanding of structure-performance relationships... 

However one constraint of alkoxylated Surfmers is their cloud point versus the polymerization temperature. If the former is lower than the latter, salting-out of the Surfmer occurs, with loss of surface activity and reactivity. The cloud point of nonionic alkoxylates can be adjusted to a certain extent by the choice of the alkoxylation initiator, the relative percentage of hydrophilic and hydrophobic alkoxylation moieties and their order of addition. Also, introducing some ionic character in the molecule (e.g. by weak polar groups that do not substantially affect the nonionic behavior of the molecule) may prove useful. Nevertheless there have been and there can be instances where nonionic Surfmers cannot be used. 

Reverse ethylene oxide/propylene oxide block copolymers (in which a hydrophilic core of PEO is terminated at both ends with hydrophobic PO moieties) are used in industrial applications. This is because of the different and unique performance properties compared to the conventional block copolymers, where a hydrophobic PO core is block copolymerized with EO. Dufour and Guyot [30] have built on this observation and synthesized Surfmers in which a PEG core (about 37 EO units) was tipped with about 10 PO units to further react with a chlorine-carrying polymerizable group or with maleic anhydride to produce reactive Surfmers. 

The maleic Surfmers were tested in core-shell emulsion polymerization of styrene/butyl acrylate in comparison with a standard nonreactive surfactant (nonyl phenol reacted with 30 mol of EO - NP30). While the methacrylic-derived Surfmer was completely incorporated during the polymerization (although about one-third of it was buried inside the particles) the NP30, the maleic Surfmer and the allylic and vinyl Surfmers were not incorporated and could be extracted with acetone (for the last two probably because of the formation of acetone-extractable oligomers due to a chain transfer behavior) [31]. 

Recently Uniqema has introduced commercially a Surfmer under the trade name of Maxemul 5011. Maxemul is produced by esterification of an unsaturated fatty anhydride with a methoxy PEG such that the reactive group is close to the hydrophilic moiety [ 34 ]. Stable latexes with a solid content of 52% were produced in the seeded emulsion polymerization of film-forming methyl methacrylate/butyl acrylate/acrylic acid (3% Surfmer on monomers, constant monomer feeding rate over 4 h, potassium persulfate/sodium metabisulfate redox initiator). The latexes were stable to electrolytes but not to freeze-thaw. 

It was estimated that, if all the Surfmers contributed to stabilization, the surface coverage would be close to 20% at the end of the process. When Surfmer burial is considered, the minimum surface coverage is in the region of 14.7-15.0 % [35]. The authors have also studied the influence of the addition procedure on the evolution of the Surfmer conversion and concluded that, despite the low reactivity due to the presence of the alkenyl double bond, the incorporation could be increased to 72% from the original 58% obtained with a constant feeding rate. A mathematical model able to describe Surfmer polymerization was used in the optimization process [36]. 

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