Vasiliki Havredaki

Micellisation and gelation of diblock copolymers of ethylene oxide and propylene oxide in aqueous solution, the effect of P-block length

The aqueous solution properties of five diblock copolymers prepared by sequential anionic copolymerisation (i.e. E102P37, E104P52, E92P55, E104P60 and E98P73 where E denotes oxyethylene and P denotes oxypropylene) were studied across a wide range of concentration. The techniques used to study micellisation and micellar properties in dilute solution were static and dynamic light scattering, surface tension, and eluent gel-permeation chromatography. The gelation of concentrated solutions was also investigated. As expected, the critical micelle concentration (CMC) was lowered and the association number of the micelles was increased by an increase in P-block length. In contrast, the critical gel concentration was unchanged, consistent with the constant E-block length leading to micelles with essentially identical E-block fringes. Comparison of the CMCs of the diblock copolymers with those of triblock EmPnEm copolymers with the same P-block length shows the diblock copolymers to micellise more efficiently. A similar comparison of the CMCs of the diblock copolymers with those of EmBn copolymer (B denotes oxybutylene) shows the hydrophobicity of a P unit to be one-sixth that of a B unit. The possibility is explored of correlating the limiting association number of a spherical micelle with the hydrophobe block length of its constituent copolymer. Of the five copolymers, only dilute solutions of E98P73 were predominantly micellar at both room temperature and body temperature, and this copolymer must be a prime candidate in any consideration of the potential application of EmPn copolymers in the solubilisation and controlled release of drugs.

Interactions of an anionic surfactant with poly(oxyalkylene) copolymers in aqueous solution.

The interactions of sodium dodecyl sulfate (SDS) with poly(ethylene oxide)/poly(alkylene oxide) (E/A) block copolymers are explored in this study. With respect to the specific compositional characteristics of the copolymer, introduction of SDS can induce fundamentally different effects to the self-assembly behavior of E/A copolymer solutions. In the case of the E(18)B(10)-SDS system (E = poly(ethylene oxide) and B = poly(butylene oxide)) development of large surfactant-polymer aggregates was observed. In the case of B(20)E(610)-SDS, B(12)E(227)B(12)-SDS, E(40)B(10)E(40)-SDS, E(19)P(43)E(19)-SDS (P = poly(propylene oxide)), the formation of smaller particles compared to pure polymeric micelles points to micellar suppression induced by the ionic surfactant. This effect can be ascribed to a physical binding between the hydrophobic block of unassociated macromolecules and the non-polar tail of the surfactant. Analysis of critical micelle concentrations (cmc(*)) of polymer-surfactant aqueous solutions within the framework of regular solution theory for binary surfactants revealed negative deviations from ideal behavior for E(40)B(10)E(40)-SDS and E(19)P(43)E(19)-SDS, but positive deviations for E(18)B(10)-SDS. Ultrasonic studies performed for the E(19)P(43)E(19)-SDS system enabled the identification of three distinct regions, corresponding to three main steps of the complexation; SDS absorption to the hydrophobic backbone of polymer, development of polymer-surfactant complexes and gradual breakdown of the mixed aggregates.

Association properties of a diblock copolymer of ethylene oxide and styrene oxide in aqueous solution studied by light scattering and rheometry

Copolymer S13E60 (E = oxyethylene unit, S = oxyphenylethylene unit) was synthesised and characterised by gel permeation chromatography (for distribution width) and 13C NMR spectroscopy (for absolute molar mass and composition). Dynamic and static light scattering were used to determine micellar properties in dilute aqueous solution at three temperatures (20, 30 and 40°C): i. e. association number, hydrodynamic and thermodynamic radii. Comparison with reported results for related copolymers allowed exploration of the dependence of these properties on hydrophobe block length. The phase behaviour of the copolymer in aqueous solution was defined using tube inversion and rheometry (for yield stress and dynamic modulus). The hard-gel boundary was detected by both methods in satisfactory agreement. Discussion is focused on effects of micelle stability on the shape and extent of the hard-gel region of the phase diagram. A region of soft gel was detected at low concentrations by rheometry, and assigned to a percolation mechanism.

Rheology and structures of aqueous gels of diblock(oxyethylene–oxybutylene) copolymers with lengthy oxyethylene blocks

Aqueous solutions of diblock copolymers E96B18, E184B18, E315B17 and E398B19 (E=oxyethylene unit, B=oxybutylene unit) were investigated by rheometry and small-angle X-ray scattering (SAXS). Storage (G′) and loss (G″) modulus and yield strength (σy) were used to define hard- and soft-gel phases in experiments which covered the concentration range 2–14 wt.% copolymer. Values of G′ correlated with those of yield strength, the ratio G′/σy being ca. 0.03. SAXS was used to explore hard-gel structures, and to confirm hard-gel/soft-gel boundaries. The sol/soft-gel boundary was identified as a percolation threshold. The effect of an increase in E-block length was to move the gel phases to lower concentrations without changing the pattern of their behaviour. In this respect, the critical conditions for hard-gel formation (c*, T*) served as parameters for a ‘universal ’ phase diagram.

Interactions of Bovine Serum Albumin with Ethylene Oxide/Butylene Oxide Copolymers in Aqueous Solution

The interactions of bovine serum albumin (BSA) with three ethylene oxide/butylene oxide (E/B) copolymers having different block lengths and varying molecular architectures is examined in this study in aqueous solutions. Dynamic light scattering (DLS) indicates the absence of BSA-polymer binding in micellar systems of copolymers with lengthy hydrophilic blocks. On the contrary, stable protein-polymer aggregates were observed in the case of E 18B 10 block copolymer. Results from DLS and SAXS suggest the dissociation of E/B copolymer micelles in the presence of protein and the absorption of polymer chains to BSA surface. At high protein loadings, bound BSA adopts a more compact conformation in solution. The secondary structure of the protein remains essentially unaffected even at high polymer concentrations. Raman spectroscopy was used to give insight to the configurations of the bound molecules in concentrated solutions. In the vicinity of the critical gel concentration of E 18B 10 introduction of BSA can dramatically modify the phase diagram, inducing a gel-sol-gel transition. The overall picture of the interaction diagram of the E 18B 10-BSA reflects the shrinkage of the suspended particles due to destabilization of micelles induced by BSA and the gelator nature of the globular protein. SAXS and rheology were used to further characterize the structure and flow behavior of the polymer-protein hybrid gels and sols.

Experimental studies in relation to a new theoretical description of the permeation of dilute adsorbable gases through porous membranes

Abstract Precise data on the permeability of porous silica and alumina membranes to dilute gases are reported as a function of the nature of the gas and of temperature. It is shown that the unusual permeability behaviour previously observed only in “Vycor” porous glass at high temperatures [8-10] is a more general phenomenon. These results cannot be accounted for by conventional “surface diffusion” theory [1, 2] even qualitatively, but can be understood on the basis of recent, more advanced, theoretical treatments [3, 4, 7]. The present data provide an experimental test (not possible on the basis of previous data) of the general correlation between permeability and extent of sorption (including both the nature of the gas and temperature) predicted by the new theoretical approach, which is shown to be remarkably successful. Differences in the detailed permeability behaviour noted here, and in the previous porous glass study [8-10], are also satisfactorily accounted for in terms of differences in the mean effective pore size of the respective membranes.

Mixed micelles of block copolymers of ethylene oxide and 1,2-butylene oxide. Solutions and gels of triblock BEB plus diblock EB copolymers studied by light scattering and rheology

Because of the hydrophobic end blocks, micelles of triblock BEB copolymers (E denotes an oxyethylene unit, B an oxybutylene unit) formed in aqueous solution show effects attributable to intermicellar bridging. The formation of mixed micelles with a diblock EB copolymer reduces but does not eliminate the effect. Light scattering methods applied to dilute solutions have been used to show that this is the case for mixed micelles of B12E114B12 and E43B11 (the subscripts denote block lengths). This work focuses on 20 wt% aqueous solutions and on the effect of mixing on properties relating to the gel state, i.e. dynamic storage modulus (G′) and yield strength (σy). There are two contributory factors determining the gel properties: micelle bridging and micelle packing, with just the latter operative for a diblock gel. Increasing the proportion of diblock copolymer in the mixture reduces the contribution from micelle bridging. Considering 20 wt% solutions at 5 °C, the diblock copolymer solution does not gel at this temperature and values of G′ and σy fall away monotonically to zero. For 20 wt% solutions at 25 °C, the diblock copolymer solution does gel at this temperature and values of G′ and σy pass through minima as the system is changed from 100% triblock to 100% diblock copolymer.

Polymer-surfactant vesicular complexes in aqueous medium.

The introduction of ionic single-tailed surfactants to aqueous solutions of EO(18)BO(10) [EO = poly(ethylene oxide), BO = poly(1,2-butylene oxide), subscripts denote the number of repeating units] leads to the formation of vesicles, as probed by laser scanning confocal microscopy. Dynamic light scattering showed that the dimensions of these aggregates at early stages of development do not depend on the sign of the surfactant head group charge. Small-angle X-ray scattering (SAXS) analysis indicated the coexistence of smaller micelles of different sizes and varying polymer content in solution. In strong contrast to the dramatic increase of size of dispersed particles induced by surfactants in dilute solution, the d-spacing of corresponding mesophases reduces monotonically upon increasing surfactant loading. This effect points to the suppression of vesicles as a consequence of increasing ionic strength in concentrated solutions. Maximum enhancements of storage modulus and thermal stability of hybrid gels take place at different compositions, indicating a delicate balance between the number and size of polymer-poor aggregates (population increases with surfactant loading) and the number and size of polymer-surfactant complexes (number and size decrease in high surfactant concentrations).

Effect of end group on the micelle properties of diblock copolymers of ethylene oxide and 1,2-butylene oxide

Copolymers of ethylene oxide and 1,2-butylene oxide (E18B10, E43B9, E40B10, E90B10 , E96B18 and E184B18 , E = oxyethylene unit, B = oxybutylene unit, subscripts denote number-average chain lengths) with B blocks terminated by hydroxy groups (denoted EmBnH) were methylated to provide copolymers having the same chain length and composition but with B blocks terminated by methoxy groups (denoted EmBnM). Micelle properties of the M copolymers were determined by dynamic and static light scattering (hydrodynamic radius, association number, thermodynamic radius) and the values obtained compared with those for the H copolymers, most of which had been published previously. The results for copolymer E18B10M in solution at 40°C were consistent with the formation of worm-like micelles, the micelles of the other copolymers being spherical, including E18B10H in solution at 40°C and E18B10M in solution at 30°C. For micelles of the B18 copolymers, methylation reduced the values of all properties by ca. 10%. For micelles of the B9–B10 copolymers, the effect of methylation was to reduce the hydrodynamic radius by ca. 10%, but to increase the association number by ca. 25% and thermodynamic radius by ca. 10%. The explanation of these effects takes into account the increased hydrophobicity of the methylated B blocks, the highly stretched state of the B18 blocks in their micelles, and the probability that water will concentrate at the centre of the cores of micelles of copolymers with hydroxy-ended B blocks. For copolymers forming spherical micelles, the effect of methylation on association number is equivalent to raising the temperature of the solution by ca. 10°C. For micelles of copolymer E18B10, the effect of methylation is to lower the temperature of the sphere-to-worm transition from 40–50°C (E18B10H) to 30–40°C (E18B10M).

Micellar and surface properties of a poly(methyl methacrylate)–block–poly(N-isopropylacrylamide) copolymer in aqueous solution

Critical micelle concentrations (cmc) of aqueous solutions of poly(methyl methacrylate)-block-poly(N-isopropylacrylamide) were determined at several temperatures by surface tensiometry. Below the lower critical solution temperature (LCST), the low Delta mic H 0 determined can be assigned to the PMMA block being tightly coiled in the dispersed molecular state, so that the unfavorable interactions of hydrophobic entities with water are minimized. Above the LCST the cmc value was found to increase; an anomalous behavior that can be directly related to the micelle-globule transition of the hydrophilic block. Interestingly, above the LCST the surface tension of relatively concentrated solutions was found to depend weakly on temperature not following the usual strong decrease with temperature expected for aqueous solutions.