D. Martin A. Buzza

Controlling the self-assembly of binary copolymer mixtures in solution through molecular architecture

We present a combined experimental and theoretical study on the role of copolymer architecture in the self-assembly of binary PEO-PCL mixtures in water-THF and show that altering the chain geometry and composition of the copolymers can control the form of the self-assembled structures and lead to the formation of novel aggregates. First, using transmission electron microscopy and turbidity measurements, we study a mixture of sphere-forming and lamella-forming PEO-PCL copolymers and show that increasing the molecular weight of the lamella-former at a constant ratio of its hydrophilic and hydrophobic components leads to the formation of highly curved structures even at low sphere-former concentrations. This result is explained using a simple argument based on the effective volumes of the two sections of the diblock and is reproduced in a coarse-grained mean-field model: self-consistent field theory (SCFT). Using further SCFT calculations, we study the distribution of the two copolymer species within the individual aggregates and discuss how this affects the self-assembled structures. We also investigate a binary mixture of lamella-formers of different molecular weights and find that this system forms vesicles with a wall thickness intermediate to those of the vesicles formed by the two copolymers individually. This result is also reproduced using SCFT. Finally, a mixture of sphere-former and a copolymer with a large hydrophobic block is shown to form a range of structures, including novel elongated vesicles.

Controlling the micellar morphology of binary PEO–PCL block copolymers in water–THF through controlled blending

We study both experimentally and theoretically the self-assembly of binary polycaprolactone–polyethylene oxide (PCL–PEO) block copolymers in dilute solution, where self-assembly is triggered by changing the solvent from the common good solvent THF to the selective solvent water, and where the two species on their own in water form vesicles and spherical micelles respectively. We find that in water the inter-micellar exchange of these block copolymers is extremely slow so that the resultant self-assembled structures are in local but not in global equilibrium (i.e., they are non-ergodic). This opens up the possibility of controlling micelle morphology both thermodynamically and kinetically. Specifically, when the two species are first molecularly dissolved in THF before mixing and self-assembly (‘pre-mixing’) by dilution with water, the morphology of the formed structures is found to depend on the mixing ratio of the two species, going gradually on a route of decreasing surface curvature from vesiclesvia an intermediate regime of micelles in the shape of ‘bulbed’ rods, rings, Y-junctions and finally to spherical micelles as we increase the proportion of the “sphere formers”. On the other hand, if the two species are first partially self-assembled (by partial exchange of the solvent with water) before mixing and further self-assembly (‘intermediate mixing’), novel metastable structures, including nanoscopic ‘pouches’, emerge. These experimental results are corroborated by Self-Consistent Field Theory (SCFT) calculations which reproduce the sequence of morphologies seen in the pre-mixing experiments. SCFT also reveals a clear coupling between polymer composition and aggregate curvature, with regions of positive and negative curvature being stabilized by an enrichment and depletion of sphere formers respectively. Our study demonstrates that both thermodynamic and kinetic blending of block copolymers are effective design parameters to control the resulting structures and allow us to access a much richer range of nano-morphologies than is possible with monomodal block copolymer solutions.

ABC triblock copolymer micelles: Spherical versus worm-like micelles depending on the preparation method

Well-defined ABC triblock copolymers based on two hydrophilic blocks, A and C, and a hydrophobic block B are synthesized and their self-assembly behavior is investigated. Interestingly, at the same solvent, concentration, pH, and temperature, different shape micelles are observed, spherical and worm-like micelles, depending on the preparation method. Specifically, spherical micelles are observed with bulk rehydration while both spherical and worm-like micelles are observed with film rehydration.

Micelle formation in block copolymer/homopolymer blends: Comparison of self-consistent field theory with experiment and scaling theory

We present a self-consistent field theory (SCFT) study of spherical micelle formation in a blend of poly(styrene−butadiene) diblocks and homopolystyrene. The micelle core radii, corona thicknesses, and critical micelle concentrations are calculated as functions of the polymer molecular weights and the composition of the diblocks. We then make a parameter-free comparison of our results with an earlier scaling theory and X-ray scattering data. For the micelle core radii Rc, we find that SCFT reproduces the shape of the variation of Rc with different molecular parameters much more accurately compared to scaling theory, though, like scaling theory, it overestimates Rc by about 20−30%. For the corona thickness Lc, the accuracy of our SCFT results is at least as good as that of scaling theory. For copolymers with lighter core blocks, SCFT predictions for the critical micelle concentration improve over those of scaling theories by an order of magnitude. In the case of heavier core blocks, however, SCFT predicts the critical micelle concentration less well due to inaccuracies in the modeling of the bulk chemical potential. Overall, we find that SCFT gives a good description of spherical micelle formation and is generally more successful than scaling theory.

Surface visco-elastic modes in a spread film of a block copolymer

A linear diblock copolymer of polymethyl methacrylate and poly-4-vinyl pyridine quaternised with ethyl bromide has been spread as a thin film at the air/water interface and the properties of the capillary waves obtained using surface quasi-elastic light scattering. The data have been analysed for surface visco-elastic parameters on the basis of the complete absence of any transverse shear viscosity in the spread film. A resonance between capillary and dilatational modes was observed at a block copolymer surface concentration of 0.8 mg m-2. At this surface concentration the frequency dependence of the surface tension, dilatational modulus and dilatational viscosity exhibited behaviour which suggested that spread film could be represented as a Maxwell fluid with a relaxation time of ca. 3 µs. This Maxwell fluid model also described the dependence of an ‘apparent’ relaxation time of the dilatational mode on the surface concentration for a capillary wave of fixed wavenumber. A comparison of the observed dispersion behaviour (damping as a function of capillary wave frequency) with that predicted by theoretical forms of the dispersion equation, showed that there was no need to include a postulated coupling factor. This observation concurred with the modest dimensions of the surface region occupied by the quaternised vinyl pyridine blocks.

Anisotropic Self-Assembly from Isotropic Colloidal Building Blocks

Spherical colloidal particles generally self-assemble into hexagonal lattices in two dimensions. However, more complex, non-hexagonal phases have been predicted theoretically for isotropic particles with a soft repulsive shoulder but have not been experimentally realized. We study the phase behavior of microspheres in the presence of poly(N-isopropylacrylamide) (PNiPAm) microgels at the air/water interface. We observe a complex phase diagram, including phases with chain and square arrangements, which exclusively form in the presence of the microgels. Our experimental data suggests that the microgels form a corona around the microspheres and induce a soft repulsive shoulder that governs the self-assembly in this system. The observed structures are fully reproduced by both minimum energy calculations and finite temperature Monte Carlo simulations of hard core-soft shoulder particles with experimentally realistic interaction parameters. Our results demonstrate how complex, anisotropic assembly patterns can be realized from entirely isotropic building blocks by control of the interaction potential.