Antonis Kelarakis

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.

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.

Aqueous micellar solutions of a triblock copolymer of ethylene oxide and 1,2-butylene oxide, B12E114B12. Scaling the viscoelasticity of fluid and gel

Aqueous solutions of triblock copolymer B12E114B12 have been characterised rheometrically over the concentration range 10–40 wt.% and at several temperatures. Through micellar bridging the micellar interaction is weakly attractive. A wide variety of linear viscoelastic behaviours are observed, and it is shown that they can be superposed onto a master curve of the type developed by Trappe and Weitz (Phys. Rev. Lett., 2000, 85, 449) for conventional colloidal suspensions of weakly-attractive particles.

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).