Effects of salts on the micellization and gelation of a triblock copolymer studied by rheology and light scattering

The phase behavior and aggregation properties of a triblock copolymer of ethylene oxide (EO) and propylene oxide (PO), with a measured composition (EO)29(PO)40(EO)29, in aqueous solutions containing salt, have been examined using dynamic light scattering, rheological techniques, and sedimentation and viscosity measurements. The copolymer is dissolved as a unimer at low temperatures and forms spherical micelles with increasing temperature. At higher temperatures, a sphere-to-rod transition is seen for the micelles. Two types of gel are formed at higher concentrations of the copolymer. With different inorganic salts, the micellization and gelation properties of the copolymer follow the same type of transitions as the salt-free system, but all transition temperatures are shifted. The spherical micelles thus transform into rod-like micelles at around 38 °C in 1 M KF, which is approximately 36 deg below the transition temperature in the salt-free system. Rod lengths in 1 M KF are between 1000 and 1800 Å, at 40 °C. The higher-temperature gel phase is seen at all concentrations down to 0.5 −1 wt %. The elasticity of this gel is due to hindered rotation of rods. Its relaxation time decreases with increasing concentration, indicating that the gel relaxes due to a partial breakdown or dissolution of the rods at the cross points. The strain dependence of this gel suggests that ordered structures of rods are formed at concentrations above 27 wt %

A calorimetry and light scattering study of the formation and shape transition of mixed micelles of EO20PO68EO20 triblock copolymer (P123) and nonionic surfactant (C12EO6)

The interaction between the nonionic surfactant C12EO6 and the poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) triblock copolymer EO20PO68EO20 (P123) has been investigated by means of isothermal titration and differential scanning calorimetry (DSC) as well as static and dynamic light scattering (SLS and DLS). P123 self-assembles in water into spherical micelles at ambient temperatures. At raised temperatures, the DSC data revealed a sphere-to-rod transition of the P123 micelles around 60 degrees C. C12EO6 interacts strongly with P123 micelles in aqueous solution to give mixed micelles with a critical micelle concentration (cmc) well below the cmc for pure C12EO6. The presence of C12EO6 also lowers the critical micelle temperature of P123 so aggregation starts at significantly lower temperatures. A new phenomenon was observed in the P123-C12EO6 system, namely, a well-defined sphere-to-rod transition of the mixed micelles. A visual phase study of mixtures containing 1.00 wt % P123 showed that in a narrow concentration range of C12EO6 both the sphere-to-rod transition and the liquid-liquid phase separation temperature are strongly depressed compared to the pure P123-water system. The hydrodynamic radius of spherical mixed micelles at a C12EO6/P123 molar ratio of 2.2 was estimated from DLS to be 9.1 nm, whereas it is 24.1 nm for the rodlike micelles. Furthermore, the hydrodynamic length of the rods at a molar ratio of 2.2 is in the range of 100 nm. The retarded kinetics of the shape transition was detected in titration calorimetric experiments at 40 degrees C and further studied by using time-resolved DLS and SLS. The rate of growth, which was slow (> 2000 s), was found to increase with the total concentration.111215911592

Influence of ionic surfactants on the aggregation of poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide) block copolymers studied by differential scanning and isothermal titration calorimetry

The interaction between three triblock copolymers of poly(ethylene oxide) and poly(propylene oxide), EOnPOmEOn, and the ionic surfactants sodium dodecyl sulfate, SDS, and hexadecyltrimethylammonium chloride, CTAC, has been studied in dilute aqueous solution using differential scanning calorimetry, DSC, and isothermal titration calorimetry. The length of the PPO block was the same in all three copolymers (68-69 PO units), and the lengths of the PEO groups varied from 5, 20 and 97 EO units. The copolymers are denoted L121, P123, and F127 in order of increasing PEO block size. In dilute aqueous solution P123 and F127 aggregate to form micelles, while the most hydrophobic polymer, L121, forms aggregates which, eventually, separate to give a liquid crystalline phase. Differential scanning calorimetry was used to follow the effect on the copolymer aggregates upon addition of ionic surfactants. Addition of SDS to P123 and L121 increased the temperature for aggregation, but polymer aggregates still formed in 6.2 mmol/L SDS. The effect is different on F 127 where after an initial decrease in the aggregation temperature, the peak in the DSC curve flattens out and disappears at low SDS concentration, as has been observed previously. The addition of CTAC to solutions of the three polymers does not change significantly the aggregation temperature, but the transition peak decreases and eventually disappears in 2-3 mmol/L CTAC. The prominent feature of calorimetric titration curves at 40 degreesC from consecutive additions of surfactant to polymer solution is a well-defined exothermic peak that stems from the disruption of the polymer micelles/aggregates and accompanying hydration of the PPO block. The beginning of the peak indicates the start of binding of the surfactant to the polymer aggregates, and after the end of the peak, the titration curves indicate binding to polymer unimers. At 40 degreesC, about 20 SDS molecules per polymer chain are required to disarrange the P 123 micelles and L121 aggregates, while about 10 suffice to disrupt the F127 micelles. The same amount of CTAC, about 10 molecules per polymer chain, destroys the aggregates of all three polymers.10661239124

Interaction Between Peo-ppo-peo Triblock Copolymers And Ionic Surfactants In Aqueous Solution Studied Using Light Scattering And Calorimetry

Properties of nonionic triblock copolymers of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) (EOnPOmEOn) in aqueous solution and their interaction with the ionic surfactants sodium dodecyl sulfate and hexadecyltrimethylammonium chloride have been investigated by static and dynamic light scattering, high sensitivity differential scanning, and isothermal titration calorimetry. The studied copolymers (denoted P123 and F127) have the same hydrophobic PPO central block (m = 68), but different length of the endblocks, n = 20 and 97. At 40°C, the copolymers are associated into micelles with hydrodynamic radius of 9.8 nm (P123) and 12.5 nm (F127) composed of a hydrophobic PPO core and a water-swollen PEO corona. The different copolymer/surfactant systems have been investigated at a constant copolymer concentration of 1 wt% and with varying surfactant concentration up to about 120 mM. 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Micellization of water-soluble complex salts of an ionic surfactant with hairy polymeric counterions

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)For ionic surfactants in general, a change from simple to polymeric counterions leads to increasing attraction between micelles, condensing them in a concentrated phase. In the present study, two novel "complex salts" were prepared in which the cationic surfactant hexadecyltrimethylammonium was neutralized by two different copolyions, both having poly(methacrylate) main chains randomly decorated with oligo(ethylene oxide) side chains. The presence of hydrophilic side chains in the polyion backbone is proposed as a strategy to stabilize the complex salt aggregates in aqueous solutions and prevent them from separating out in a concentrated phase. Surface tension experiments reveal that the complex salts form soluble nano-aggregates by surfactant ion self-assembly at a distinct critical micellization concentration (cmc), similar to the micellization of a conventional ionic surfactant. This is the first time that cmc values have been determined for complex salts in the absence of all other ions. The physicochemical nature of the aggregates formed was investigated by dynamic light scattering, nuclear magnetic resonance self-diffusion measurements and steady-state fluorescence spectroscopy. Much larger aggregates are formed when the temperature is increased, but the small aggregates reform at room temperature, suggesting that the soluble aggregates are equilibrium structures, much like the micelles of conventional surfactants.92515526Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Swedish Research Council (VR)Linnaeus grant Organizing Molecular Matter (OMM) through VR [239-2009-6749]Knut and Alice Wallenberg FoundationCrafoord FoundationFundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq)Linnaeus grant Organizing Molecular Matter (OMM) through VR [239-2009-6749