Polymerization of Wormlike Micelles Induced by Hydrotropic Salt

Water-based synthesis and processing of novel biodegradable elastomers for medical applications

Novel biodegradable nanoelastomers are synthesized. They can self-assemble and generate morphologies in nanometric, micrometric, or bulk scale with tunable properties. They are smart biodegradable materials with potential applications.

Structure – Property Relationships of “Gemini” Surfactants and Synergism with Hydrotropes

Two series of cationic dimeric ((gemini)) surfactants are presented with respect to key surfactant properties, such as surface activity and micellization. The influence of the dimerization and of the spacer group are examined, in order to establish structure-property relationships. Importantly, the addition of organic anions strongly modifies the solution properties and aggregate morphology. The surface tension and critical micellar concentration can be markedly reduced compared to the dimeric surfactants in the pure state. Moreover, dimeric surfactants with an appropriate spacer group can produce viscoelastic solutions after addition of certain organic anions even when bearing only dodecyl chains, whereas no effect is observed for such mixtures involving the analogous (monomeric) surfactant. Also, first results on the use of polymerizable counterions and their polymerization in such mixtures are presented.

Structure of polymerizable surfactant micelles: insights from neutron scattering

Although polymerization of reactive surfactants (surfmers) in micelles and other self-assembled phases has been studied for at least 30 years, the last decade or so has seen substantial advances in understanding both the structure and dynamics of these systems. In this review we highlight the new insights yielded primarily by small-angle neutron scattering (SANS) using high-flux sources, the perspective this provides for realizing topochemical polymerization in micellar systems, and the prospects and new developments for further exploiting SANS in this field. We present some new neutron contrast variation results exemplifying these elements.

Micellization of monomeric and poly-ω-methacryloyloxyundecyltrimethylammonium surfactants

We have used small-angle neutron scattering to study how micelle morphology of the tail-polymerizable surfactants MUTAB and MUTAC (ω-methacryloyloxyundecyltrimethylammonium bromide and chloride) is affected by classic self-assembly modifiers such as temperature changes, salt addition, and counterion exchange, as a function of their conversion from monomer into polymer amphiphile in aqueous solution. Contrary to common assumptions about polymerized surfactants, these systems remain in dynamic equilibrium under all conditions examined and at all conversions (except for a small amount of high-molecular-weight precipitation by MUTAC). Counterintuitively, the polymerized methacrylate backbone has little influence on aggregate morphology, except for the formation of rod-like mixed micelles of polymerized and unpolymerized surfactant at intermediate conversions. The addition of salt produces a transition to rod-like micelles at all conversions except in the unpolymerized surfactant, which has some characteristics of an asymmetric bolaform surfactant and retains its spheroidal geometry under almost all conditions.

Formation of hybrid wormlike micelles upon mixing cetyl trimethylammonium bromide with poly(methyl methacrylate-co-sodium styrene sulfonate) copolymers in aqueous solution

The association of cetyltrimethylammonium bromide, CTAB, with a series of P(MMAx-co-SSNa) random copolymers of sodium styrene sulfonate (SSNa) with methyl methacrylate (MMA) was explored in aqueous solution as a function of the MMA molar content, x, of the copolymers. The polyelectrolyte/surfactant complexation in aqueous solution was verified through pyrene fluorescence probing. In addition, turbidimetry studies in dilute or more concentrated aqueous solutions elucidated the phase separation behavior of the P(MMAx-co-SSNa)/CTAB systems as a function of the copolymer composition x and the surfactant to polyelectrolyte mixing charge ratio. It is found that practically phase separation is completely suppressed within the studied mixing range when the MMA content of the copolymers is ∼30-40 mol%. For lower MMA contents the polyelectrolyte/surfactant complex separates out from water, while for higher x values the solubility limits of the copolymers in water are attained. For the intermediate MMA contents, viscoelastic systems are obtained in more concentrated polymer/surfactant solutions provided that the polyelectrolyte is fully complexed with the cationic surfactant ((1)H NMR results). Moreover, the (1)H NMR studies indicate that hybrid P(MMAx-co-SSNa)/CTAB wormlike micelles are formed in water under these conditions. Finally, it is shown that addition of salt prevents syneresis problems and facilitates the rheological investigation.

Polymerizable cationic micelles form cylinders at intermediate conversions

The structural evolution of micelles of the polymerizable surfactant omega-methacryloyloxyundecyltrimethylammonium bromide (MUTAB) during UV-initiated polymerization in aqueous micellar solution has been followed by small-angle neutron scattering. Although the micelles are short spheroids both before and after polymerization, a significant, distinct population of rodlike micelles develops during the reaction, which accounts for as much as 40 vol % of the micellized surfactant and coexists with the spheroids and dissolved monomer. These coexisting micelle populations are shown to remain in dynamic equilibrium throughout the reaction and can be understood by treating it as a ternary mixture of surfactant, amphiphilic polyelectrolyte, and water.

Structure changes in micelles and adsorbed layers during surfactant polymerization

We have studied the self-assembled structures formed by the cationic surfactant 11-(methacryloyloxy)undecyltrimethylammonium bromide (MUTAB) using small angle neutron scattering as it undergoes UV-initiated polymerization in bulk solution, and the subsequent adsorbed structures at the mica/solution interface using atomic force microscopy. MUTAB forms spheroidal aggregates in aqueous solution with an axial ratio of 2-3 both before and after polymerization, as previously reported in numerous studies. However, at intermediate conversions the micelles surprisingly form elongated structures up to 200 A long. Except for the unpolymerized MUTAB, which forms a featureless adsorbed layer, the micelle structures are largely retained after adsorption: structures are elongated at 50% conversion and globular at 100% conversion. These results demonstrate that the structure of the unpolymerized micelle is not simply kinetically-trapped during polymerization, but undergoes extensive reorganization as the reaction proceeds. This has implications for attempts to capture micellar structure by polymerization and use it to template nanostructured materials or for encapsulation.

Wormlike micelles: where do we stand? Recent developments, linear rheology and scattering techniques

Wormlike micelles are elongated flexible self-assembly structures formed by the aggregation of amphiphiles. Above a threshold concentration, they entangle into a dynamic network, reminiscent of polymer solutions, and display remarkable visco-elastic properties, which have been exploited in numerous industrial and technological fields. Relating the microstructure of these intricate structures with their bulk properties is still an ongoing quest. In this review, we present a classification of wormlike micelles, with a focus on novel systems and applications. We describe the current state of understanding of their linear rheology and give a detailed account of recent progress in small-angle neutron scattering, a particularly powerful technique to elucidate their microstructure on a wide range of length-scales.

Solution Behavior of Rod-Like Polyelectrolyte-Surfactant Aggregates Polymerized from Wormlike Micelles

The behavior of a rod-like, water-soluble, polyelectrolyte-surfactant aggregate system (pC16TVB) in aqueous solution is characterized to determine the partitioning of surfactant in these systems and the impact on aggregate structure. These aggregates are generated by in situ polymerization of a cationic surfactant-hydrotrope wormlike micelle system. This system differs from most other polyelectrolyte-surfactant systems in that the monomer groups and the surfactant are present in ion pairs in the absence of added salts or counterions, so the stoichiometry (with respect to charge) is 1:1 for the system. Therefore, after polymerization the surfactant acts as the counterion for the polyelectrolyte chains as other counterions (salts) are not available. Despite being present in a 1:1 molar ratio, the aggregates are surprisingly stable in water (concentrations >600 mg/mL have been achieved). The conformation of the polyelectrolyte in the aggregate is analogous to the case of a polymer chain in tight confinement in a "tube" or cylindrical pore in which the pore walls are attractive--the tube is formed by the surfactant which is free to dissociate from the aggregates. A simple model for the structure and partitioning is presented and the ability to manipulate the aggregate structure is demonstrated.

Properties, main applications and perspectives of worm micelles

This tutorial review deals with one of the most remarkable forms of surfactant aggregates, described as having a flexible, elongated cylindrical shape. Three structural scale lengths are pertinent to the flexibility and mobility of worm micelles: the cross-sectional radius, r(cs), the overall (contour) length, L, and the persistence length, l(p). The diversity of l(p) values in amphiphilic systems is demonstrated as well as the relation between L and l(p). The review also discusses the viscoelasticity of worm micelles and the relaxation mechanisms underlying this dominant property. Many aspects of viscoelasticity--such as non-linearity, shear banding, flow-induced phase transition, rheochaos--are only shortly described. The prevailing application of worm micelles, namely as fracture fluids and drag reducing agents are discussed in detail, stressing the effect of variations in the surfactant molecular structure on the efficacy of worm micelles. The vague possibility of using "smart" worm micelles in the foreseeable future is tersely outlined.

Anionic Hybrid Threadlike Micelle Formation in an Aqueous Solution

Novel anionic hybrid threadlike micelle formation was found in an aqueous solution of an anionic surfactant, sodium tetradecylsulfate (NaC(14)S), and partially quarternized polyelectrolyte at ca. 0.55, poly(N,N-diallyl-N-methylamine-ran-N,N-diallyl-N-ethyl-N-methylammonium bromide) (P(DAM/DAEMBr)). The system precipitated insoluble complexes at a composition of iso-electric points, forming long and stable hybrid threadlike micelles at a composition close to an iso-molar point between NaC(14)S and P(DAM/DAEMBr) in monomer units. Then, the system turned into transparent liquids and showed remarkable viscoelasticity due to entanglements between the formed anionic hybrid threadlike micelles.

Polymerization of anionic wormlike micelles

Polymerizable anionic wormlike micelles are obtained upon mixing the hydrotropic salt p-toluidine hydrochloride (PTHC) with the reactive anionic surfactant sodium 4-(8-methacryloyloxyoctyl)oxybenzene sulfonate (MOBS). Polymerization captures the cross-sectional radius of the micelles (approximately 2 nm), induces micellar growth, and leads to the formation of a stable single-phase dispersion of wormlike micellar polymers. The unpolymerized and polymerized micelles were characterized using static and dynamic laser light scattering, small-angle neutron scattering, 1H NMR, and stopped-flow light scattering. Stopped-flow light scattering was also used to measure the average lifetime of the unpolymerized wormlike micelles. A comparison of the average lifetime of unpolymerized wormlike micelles with the surfactant monomer propagation rate was used to elucidate the mechanism of polymerization. There is a significant correlation between the ratio of the average lifetime to the monomer propagation rate and the average aggregation number of the polymerized wormlike micelles.