Living polymers and polymer solutions: a misleading analogy

 Experimental results from colloidal suspensions of worm-like micelles are currently interpreted in terms of close analogies between this kind of systems and polymeric solutions. In particular, it was hypothesized that the viscoelastic properties of dense systems of giant flexible cylindrical micelles can be rationalized in terms of an entangled network of worm-like aggregates, very similar to a neutral random polymeric network. Such an idea is strongly supported by theoretical results that, in a mean-field appro ximation, suggests for an unlimited growth process of the micellar contour length with concentration. The mean-field theory indicates for an exponentially shaped length distribution function, with mean <L> depending on concentration, φ, in agreement with a scaling law <L>∝φ^α (α=0.5 in the simpler approach). A number of experimental results seem to be successfully interpretable within this framework. Aim of this work is to show that the agreement between theory and experiment is just an accident, being the mean-field approach, in principle, inadequate in describing systems dense enough to show a concen tration dependence of the mean micellar size. It will be unambiguously shown that there is no way to describe semi-diluted micellar solutions through a mean-field approximation and that there does not exist any scaling law of the kind <L>∝φ^α. Furthermore, it will be shown that the shape of the size distribution function is markedly different from the exponential one. The basis for a more realistic approach for the growth process of micellar aggregates is also presented and some pre liminary indications are success fully compared with experimental results. Received: 24 February 1997 Accepted: 22 April 1997     

Percolative phenomena in lecithin reverse micelles: the role of water

 The role played by the solvation water molecules on the macroscopically observed sol–gel transition in lecithin/cyclohexane/water reverse micelles is investigated by quasielastic neutron scattering, dielectric relaxation and conductivity measurements. The experimental results are juxtaposed to those from spherical Aerosol OT reverse micelles. It is shown how the results from lecithin-based system can be interpreted only assuming that, in contrast to Aerosol OT systems, the water molecules are entrapped at the interfaces without coalescing into an inner water pool. It is suggested that, in the case of lecithin, the solvation water can induce a change in the surface curvature, in such a way promoting the formation of branch points. Such a hypothesis is supported by the temperature dependence of the conductivity which agrees with the hypothesis of an intermicellar bond percolation. The investigation of the structures imposed by an external electric field is also studied. The observed electrorheological behaviour seems to confirm the existence of a percolated transient network in the gel phase. Received: 21 March 2001 Accepted: 24 August 2001     

Phase behavior and viscoelastic properties of entangled block copolymer gels

Triblock copolymers in midblock‐selective solvents can form physical gels. However, at low triblock contents (near the percolation threshold), the bridging of chains between micelles can lead to macrophase separation. Adding a styrene–isoprene diblock to a styrene–isoprene–styrene triblock copolymer in squalane can eliminate macrophase separation, yielding a wide range of stable, single‐phase gels with a disordered arrangement of micelles. The plateau modulus of these triblock gels scales with the 2.2 power of polymer content, indicating the importance of entanglements in dictating the modulus. Comparing gels made from the midblock‐saturated derivative of the same polymer [styrene‐(ethylene‐alt‐propylene)‐styrene] in squalane reveals that the modulus differences in the gels are a direct consequence of the difference in the entanglement molecular weight of the midblock homopolymer in bulk. Finally, the broad relaxation spectrum of these triblocks is well‐described by a recent theory for the dynamics of entangled star polymers, with the breadth of the relaxation spectrum dictated by the number of entanglements per midblock in the gel. © 2001 John Wiley & Sons, Inc. J Polym Sci Part B: Polym Phys 39: 2183–2197, 2001     

Unimolecular micelles: Synthesis and characterization of amphiphilic polymer systems

Unimolecular micelles were successfully synthesized from mucic acid, fatty acids, and poly(ethylene glycols) to create biocompatible polymers. These polymers consist of a core‐shell structure that resembles conventional micellar structures but with significant thermodynamic stability in aqueous media. The core of the polymers provide a hydrophobic environment for drug encapsulation via hydrophobic interactions, whereas the shell provides excellent water solubility. The polymers were characterized by nuclear magnetic resonance, infrared and mass spectroscopies, as well as gel permeation chromatography, differential scanning calorimetry, and thermogravimetric and elemental analyses. Encapsulation ability was measured using high‐pressure liquid chromatography to monitor lidocaine, a hydrophobic molecule. Encapsulation capabilities increased as lipophilicity of the core increased. To verify that encapsulation was caused by individual unimolecular micelles, surface tension and dynamic light scattering measurements were performed. The results indicated that these unimolecular micelles have great potential as drug carriers. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 703–711, 1999     

Synthesis of macroporous poly(glycidyl methacrylate) microspheres by surfactant reverse micelles swelling method

Macroporous poly(glycidyl methacrylate-ethylene dimethacrylate) [P(GMA-EDMA)] particles with pore size around 140-200 nm and poly(glycidyl methacrylate-divinylbenzene) [P(GMA-DVB)] particles with pore size of 450 nm were prepared by the surfactant reverse micelles swelling method. This method was similar with the conventional suspension polymerization, and the difference was that higher concentration of surfactant was added in the oil phase. When the oil phase containing surfactant was dispersed in aqueous phase, the surfactant reverse micelles in the oil droplets absorbed water from continuous phase. After polymerization, the large pores were formed by the absorbed water. The effects of the amount and type of surfactants, the cooperation of surfactant and diluents, and the crosslinking agent on the morphology of microspheres were investigated. This study provided a new and simple method to prepare microspheres with the pores of several hundred nanometers, which overcame the disadvantages found in the conventional preparation methods of macroporous microspheres.     

Star-shaped alkylated poly(glycerol methacrylate) reverse micelles: Synthesis and evaluation of their solubilizing properties in dichloromethane

Amphiphilic star-shaped polymers were synthesized by atom transfer radical polymerization (ATRP). Four-, 5-, and 6-arm ATRP initiators were prepared and used to polymerize glycidylmethacrylate. The resulting polymers were hydrolyzed to yield poly(glycerolmethacrylate)s (PG_OHMAs), and then partially esterified with lauroyl or stearoyl chloride (40–60 mol % vs. hydroxyl groups). Alkylated PG_OHMAs were found to assemble into 20–60 nm aggregates in dichloromethane (DCM) above a critical concentration. Particle size essentially depended on the branching degree of the polymer. The micelles were extremely efficient in extracting Congo red from water into DCM, with the highest incorporations (97 mg/g) obtained for the 6-arm polymer modified with stearoyl chloride (60 mol %). The entrapment efficiency for the latter was further improved in alkaline solution. The high level of incorporation achieved for these amphiphilic multiarm polymers suggests that such reverse micelles could be potentially useful as drug delivery systems and nanoreactors in catalytic organic reactions or for water purification. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 2425–2435, 2007     

Small-angle neutron scattering study of sodium cholate and sodium deoxycholate interacting micelles in aqueous medium

Small angle neutron scattering (SANS) measurements of D₂O solutions (0.1 M) of sodium cholate (NaC) and sodium deoxycholate (NaDC) were carried out atT= 298 K. Under compositions very much above the critical micelle concentration (CMC), the bile salt micelle size growths were monitored by adopting Hayter-Penfold type analysis of the scattering data. NaC and NaDC solutions show presence of correlation peaks atQ = 0.12 and 0.1 ?_-1 respectively. Monodisperse ellipsoids of the micelles produce best fits. For NaC and NaDC systems, aggregation number (9.0, 16.0), fraction of the free counterions per micelle (0.79, 0.62), semi-minor (8.0 ?) and semi-major axes (18.4, 31.7 ?) values for the micelles were deduced. Extent of micellar growth was studied using ESR correlation time measurements on a suitable probe incorporating NaC and NaDC micelles. The growth parameter (axial ratio) values were found to be 2.3 and 4.0 for NaC and NaDC systems respectively. The values agree with those of SANS.     

反胶束系统及萃取蛋白质的分子热力学模型1: 反胶束系统的分 子热力学模型

以反胶束系统稳定性的热力学分析为基础,综合分析了反胶束系统的三大效应,即低界面张力效应、界面弯矩效应、混合熵效应,提出了一个分子热力学模型,模型所预言的反胶束水分含量随无机盐种类、浓度、表面活性剂浓度以及助表面活性剂含量的变化与所获实验规律定量相符,还能预言反胶束内表面处电势值、表面活性剂解离度。     

Kinetics of horseradish peroxidase reactions in aqueous and reverse micelles composed of nonionic, polyoxyethylene alkyl ether surfactants

Differences between the rates of oxidation of substrates by horseradish peroxidase in buffer, aqueous micellar solution of Brij 35 and in reverse micelles formed of Brij 30 in several hydrocarbons are discussed. This revised version was published online in August 2006 with corrections to the Cover Date.     

Stability of invertase in reverse micelles

The stability of invertase was studied under various conditions, including at 75°C, in presence of stabilizers (sorbitol and glycerol) at 75°C, and in the presence of denaturants (urea and trichloroacetic acid) at 37°C in reverse micelles. Stability of the invertase in reverse micelles was found to be improved over that of the enzyme in bulk aqueous solution. Sorbitol could enhance enzyme stability as it does in the bulk aqueous system. The stabilizing effect of glycerol was reduced in reverse micelles. The denaturation pattern of urea remains unaltered. However, the denaturation effect of trichloroacetic acid has been reduced in reverse micelles.     

Facile synthesis of size-tunable ZIF-8 nanocrystals using reverse micelles as nanoreactors

This paper describes a robust method for the synthesis of high-quality ZIF-8 nanocrystals using reverse micelles as discrete nanoscale reactors.The precise size control of ZIF-8 nanocrystals is conveniently achieved by tuning the concentration of precursors,reaction temperatures,the amount of water,and the structure of surfactants.The as-synthesized ZIF-8 nanocrystals are of narrow distribution and tunable size.A size-dependent catalytic activity for Knoevenagel condensation reaction is further demonstrated by using ZIF-8 nanocrystals with different sizes as the catalysts.This facile method opens up a new opportunity in the synthesis of various ZIFs nanocrystals.     

Small angle neutron scattering study of alkyl polyoxyethylene sulfate micelles. Sodium dodecyl-(oxyethylene)2 sulfate in D2O-H2O mixtures at 25°C

SANS data have been obtained for C₁₂H₂₅ (OC₂H₄)₂SO₄ Na. Results have been obtained for i) a series of solutions of variable concentration of the surfactant, ii) an approximately 0.07 M surfactant concentration to which variable amounts of NaCl were added, iii) a series of solutions 0.058M in surfactant but in different D₂O-H₂O mixtures. The SANS data can be described in terms of a model of monodispersed hard spheres interacting via a screened Coulombic potential. The micelles seem to be able to tolerate substantial amounts of salt without changing the internal structure.     

SANS study of alkyl polyoxyethylene sulfate micelles. Sodium dodecyl(oxyethylene)4sulfate in D2O at 25°C

n-C ₁₂ H ₂₅ (OC ₂ H ₄ ) ₄ SO ₄ Na micelles have been investigated by small angle neutron scattering technique. Results have been obtained for a series of solutions of variable concentration of surfactant and for an approximately 0.07M solution to which different amounts of NaCl were added. Micellar parameters, aggregation number, and apparent charge have been compared to those previously obtained for C ₁₂ H ₂₅ (OC ₂ H ₄ ) ₄ SO ₄ Na. On changing the surfactant concentration, the growth gradient of micelles of the latter is similar to the growth gradient of micelles of C ₁₂ H ₂₅ (OC ₂ H ₄ ) ₄ SO ₄ Na when no excess salt is present. In the presence of added electrolyte, C ₁₂ H ₂₅ (OC ₂ H ₄ ) ₂ SO ₄ has a higher growth gradient than C ₁₂ H ₂₅ (OC ₂ H ₄ ) ₄ SO ₄ . This result has been interpreted in terms of effect of the oxyethylene group. The monodisperse core+shell spherical model of micellar structure previously proposed for this class of surfactant seems to hold in a wide range of concentrations of surfactant and/or added electrolyte.     

Inhibited phenol ionization in reverse micelles: confinement effect at the nanometer scale

We found that the absorption spectra of 2-acetylphenol (2-HAP), 4-acetylphenol (4-HAP), and p-nitrophenol (p-NPh) in water/sodium 1,4-bis(2-ethylhexyl)sulfosuccinate (AOT)/n-heptane reverse micelles (RMs) at various W(0) (W(0) = [H(2)O]/[surfactant]) values studied changed with time if (-)OH ions were present in the RM water pool. There is an evolution of ionized phenol (phenolate) bands to nonionized phenol absorption bands with time and this process is faster at low W(0) values and with phenols with higher bulk water pK(a) values. That is, in bulk water and at the hydroxide anion concentration used, only phenolate species are observed, whereas in AOT RMs at this fixed hydroxide anion concentration, ionized phenols convert into nonionized phenol species over time. Furthermore, we demonstrate that, independent of the (-)OH concentration used to prepare the AOT RMs, the nonionized phenols are the more stable species in the RM media. We explain our results by considering that strong hydrogen-bonding interactions between phenols and the AOT polar head groups result in the existence of only nonionized phenols at the AOT RM interface. The situation is quite different when the phenols are dissolved in cationic benzyl-n-hexadecyldimethylammonium chloride RMs. Therein, only phenolates species are present at the (-)OH concentrations used. The results clearly demonstrate that the classical definition of pH does not apply in a confined environment, such as in the interior of RMs and challenge the general idea that pH can be determined inside RMs.     

Control of nucleation and growth of gold nanoparticles in AOT/Span80/isooctane mixed reverse micelles

In this study, we demonstrate that mixed reverse micelles are good candidates to be used as nanoreactors for formation of shape-controlled high-quality colloidal nanocrystals and nanowires under mild conditions. Manipulation of the rate of nucleation and subsequent growth of the Au in the mixed reverse micelles induce drastic changes in the particle shape and structure. Here we demonstrate that control of the nucleation and growth kinetics of the Au in the mixed reverse micelles can be used to vary the shapes of the resulting particles from a nearly spherical morphology to cylinders, trigons and cubics. The characterization of the resultant particles, the effects of synthesis conditions (such as concentration of NaCl, addition of glycerol, and reaction temperature) on particle sizes, particle size distribution, and shape of particle formation have been investigated. This study will help us to understand the chemical control synthesis of crystal growth processes at the atomic level.     

Assembly of a self-complementary monomer: formation of supramolecular polymer networks and responsive gels

Self-complementary monomer 1, which combines a macrotricyclic polyether and two dibenzylammonium ions together, was synthesized, and its self-assembly into supramolecular polymer networks by host-guest interactions was studied. For the purpose of comparative study, two model molecules 2 and 3 were also prepared. It was found that model molecule 2 and dibenzylammonium ion 4 form a 1:2 complex in solution and in the solid state, which afforded a model system for the investigation of the assembly behavior of monomer 1. Consequently, the (1)H NMR spectrum of 1 in CD(3)CN showed characteristic proton signals similar to the model system, which suggested that 1 self-assembles into a supramolecular polymer network. Formation of the supramolecular polymer was further evidenced by the MALDI-TOF MS spectrum, viscometry, and dynamic light-scattering (DLS) experiments. Moreover, it was found that the decomposition and re-formation of the supramolecular polymer could be chemically controlled by the use of triethylamine and trifluoroacetic acid. Interestingly, the supramolecular polymer forms an organogel both in CD(3)CN and in 1:1 (v/v) CDCl(3)/CD(3)CN, and reversible thermo- and pH-induced gel-sol transitions were also found. The presented work will provide a new strategy for the construction of supramolecular polymers with specific structures and properties.     

Effect of pH and content of reduction-sensitive copolymer on the guest exchange kinetics of micelles

The micelles formed by reduction-sensitive amphiphilic copolymer have emerged as promising drug nanocarriers due to the controlled drug release and effective anticancer activity triggered by the reducing stimulation. However, the effect of pH on the stability and guest exchange of the micelles formed by reduction-sensitive copolymer have not been systemically investigated. Herein, the micelles formed by a reduction-sensitive copolymer poly(ε-caprolactone)-b-poly[oligo(ethylene glycol) methyl ether methacrylate] (PCL–SS–POEGA) with a single disulfide group at different pH values loaded with dyes 3,3′-dioctadecyloxacarbocyanine perchlorate/1,1′-dioctadecyl-3,3,3′,3′-tetramethylindocarbocyanine perchlorate (DiO/DiI) were prepared through the precipitation-dialysis method. In addition, mixed micelles formed by different contents of reduction-sensitive and reduction-insensitive copolymers encapsulated with DiO/DiI at pH 7.5 were also prepared by the similar approach. The effects of pH and the content of reduction-sensitive copolymer on guest exchange of these micelles were studied by the fluorescence resonance energy transfer method. Results show that the pH value in the environment has great influence on the guest exchange rate of reduction-sensitive micelles in the presence of 10 mM dithiothreitol (DTT) and slight effect on that in the absence of DTT. Under a reducing environment, the guest exchange rate of the micelles containing various contents of disulfide-linked copolymer increases with the increasing content of PCL–SS–POEGA. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 1636–1644     

Thermal stability of aDesulfovibrio gigas periplasmic hydrogenase in reverse micelles

The thermal and operational stabilities ofDesulfovibrio gigas periplasmic hydrogenase were studied at 30 and 50°C in aqueous and micellar media. At the lower temperature the hydrogenase was more stable in reverse micelles of a cationic surfactant. No inactivation was detected over almost 16 d of incubation of the hydrogenase in the reverse micellar solution, during which the enzyme lost around 50% of the initial enzymatic activity in the aqueous solution. At 50°C the hydrogenase was more stable in aqueous medium, because of the changes that occur with the temperature in the organic phase—precipitation of surfactant and solvent evaporation. It was found that only micellar solutions of this enzyme can be repeatedly used, since the hydrogenase is inactivated after the first cycle of oxireduction in aqueous medium. The effect of glycerol and the electron carrier methylviologen as stabilizers of hydrogenase activity was also investigated. The results are interpreted on the grounds of hydrogenase and surfactant electrostatic properties.     

Unimolecular micelles and reverse micelles based on hyperbranched polyethers—Comparative study of AB2 + A‐R and A2 + B3 + A‐R type strategies

Core‐shell type hyperbranched polymers that are capable of forming unimolecular micelles and reverse micelles in aqueous and hydrocarbon medium, respectively, were synthesized via two approaches, namely AB₂ + A‐R and A₂ + B₃ + A‐R type copolymerizations. In case of micelle‐forming polymers, an AB₂ monomer carrying a decamethylene spacer was used along with heptaethylene glycol monomethyl ether (HPEG) as the A‐R type comonomer. One the other hand, for the preparation of reverse micelle‐forming polymers, an AB₂ monomer containing an oligo(oxyethylene) spacer was used along with cetyl alcohol as the A‐R type comonomer. The former was readily soluble in water while the latter was soluble in hydrocarbon solvents like hexane. NMR spectral studies confirmed that both the approaches generated highly branched structures wherein about 65–70% of the terminal B groups were capped by the A‐R comonomer. Dye‐uptake measurements revealed that the polymers prepared via the AB₂ + A‐R approach exhibited a significantly larger uptake compared with those prepared via the A₂ + B₃ + A‐R approach. This suggests that the AB₂ + A‐R approach generates hyperbranched polymers with better defined core‐shell topology when compared with polymers prepared via the A₂ + B₃ + A‐R approach, which is in accordance with previous studies that suggest that A₂ + B₃ approach yields polymers with significantly lower branching levels and consequently less compact structures. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 80–91, 2009