Session G28: Focus Session: Microphysical Properties of Block Copolymer Aggregates I

Rheo-NMR measurements of shear alignment and banding in non-ionic wormlike micelles

Orientational order produces myriad nonlinear effects in complex fluids such as polymer solutions, liquid crystals, and anisotropic colloids. Wormlike micelles (WLMs) exhibit especially fascinating rheological properties since the component molecules order relative to the tube axis, which itself acts as a supramolecular polymer backbone. Equilibria between wormlike and spherical micelles may be controlled using temperature, concentration, or additives to modulate properties (e. g., viscosity) over several orders of magnitude. We have investigated a new class of WLMs composed of non-ionic triblock copolymers (PEO-PPO-PEO) mixed with alcohol additives to stabilize the core. We have employed in-situ rheo-NMR spectroscopy and microscopy in a Couette shear cell to observe the first shear-induced alignment and banding in a non-ionic block-copolymer system. Deuterium-labeled additives exhibit gap-dependent spectral splittings, assessing the molecular alignment versus shear rate. Proton NMR microscopy probes shear banding across the Couette cell gap with resolutions down to 20 microns.   

Reversible morphological transitions of polystyrene-b-polyisoprene micelles

Morphological transitions of polymer micelles have long been sought after. Until now, the only way to change morphology was through the addition of a co-solvent or homopolymer. We have demonstrated for the first time that reversible morphological transitions can be caused solely through changes in temperature. Two polystyrene-b-polyisoprene samples with the same sized PS block of 20kDa were purposefully synthesized with PI blocks of 4 and 6kDa to be near the spherical-cylindrical and cylindrical-vesicle boundaries. It was found that only small changes in temperature are needed to cause the morphological change. The cylindrical sample adopted a spherical morphology after heating from 25-35\r{ }C and the vesicle sample became cylindrical upon heating from 25-40\r{ }C. While cylindrical and vesicle micelles were once again observed after cooling the samples back to 25\r{ }C, the fraction and size of the micelles were still increasing after several weeks.   

Growth of Nanostructured Bilayers and Vesicles by Self-assembly of ABC Mikto-arm Star Terpolymers

A new class of nanostructured vesicles and bilayers were observed from aqueous self-assembly of ABC mikto-arm star terpolymers with three mutually immiscible blocks. The micellar structures depend on the relative length of each block and can be tuned to form three classical types of micelles, i.e., sphere, cylinder, and vesicles, however with nanostructured multiple core compartments. The nanostructured vesicles grow by a three-stage process: (i) segmented wormlike micelles first self-assemble upon dissolution of terpolymers in water; (ii) then nanostructured bilayer sheets with apparently hexagonal ordered internal structures grow by consumption of already existing segmented worms, and subsequently (iii) large bilayer sheets curve and close off to form vesicles. The micellar structures at various stages were imaged by cryogenic transmission microscopy.   

Self-Assembly of Block Copolymers in an Ionic Liquid

Amphiphilic diblock copolymers poly((1,2-butadiene)-$b$-ethylene oxide) (PB-PEO) were shown to aggregate and form well-defined micelles in an ionic liquid, 1-butyl-3-methyl imidazolium hexafluorophosphate ([BMIM][PF6]). The universal sequence of micellar structures (spherical micelle, wormlike micelle, and bilayered vesicle) were all resolved by varying the block copolymer composition. For the first time, the nanostructures of PB-PEO micelles formed in an ionic liquid were directly visualized by cryogenic transmission electron microscopy (cryo-TEM). The detailed micelle structure information was extracted from cryo-TEM and dynamic light scattering (DLS) measurements, and compared to their aqueous counterparts. The work demonstrates the feasibility of controlling micellar nanostructures of amphiphilic block copolymers in ionic liquids, and also provides important knowledge for further applications of copolymers for forming microemulsions and ion gels.   

Controlling PS-b-PEO Morphologies by Solution Conditions

We have investigated the self-assembly behavior of amphiphilic diblock copolymer polystyrene-b-poly(ethylene oxide) (PS-b-PEO) in DMF/water and DMF/acetonitirile mixtures .The morphology of the block copolymer can be controlled in both these systems by varying copolymer concentration and solvent composition The morphologies were visualized directly by transmission electron microscopy. Increasing the water content in the DMF/water mixture or acetonitrile in the DMF/acetonitrile mixture changes the morphology from spheres to worm-like/rods and then to vesicles. Increasing the copolymer concentration shows a similar effect on the morphology. The block copolymer exhibits distinct phases of both exclusive and mixed morphologies. The morphological transitions were also captured by static light scattering and turbidity measurements. Although the trend in morphological changes is similar, there are remarkable differences in the morphological phase behavior of PS-b-PEO in the two solvent systems and thus the role of the `selective solvent' in such systems is also evidenced.   

Formation of Toroid Micelles: Mechanism and Size Control

The toroid micelle morphology has been produced by the self assembly of poly(acrylic acid)-block-poly(methyl acrylate)-block-polystyrene triblock copolymer via interaction with organic diamines in mixed THF/H2O solution. The formation mechanism has been investigated from three aspects: (i) chain structure of organic counterions, including spacer length, chain hydrophobicity and the number of amine groups; (ii) molecular structure of triblock copolymer, including block length of polystyrene and chain architecture; (iii) variation of solution component, such as different ratios of THF to water and different ratios of amine groups to acid groups. It was found that toroids can be constructed either via a cylinder-toroid transition through elimination of high-energy, cylinder ends, or via a disc-toroid transition by disc perforation from the center. The prevalent mechanism depends on the chain length of polystyrene and the chain structure of diamines. Stability of the toroids was studied by varying the ratio of THF to water, and by applying a perturbation of either heat or sonication. Interestingly, the size of the toroids can be controlled by the amount of added diamines. The toroid morphology was characterized by means of cryo-TEM, SANS and light scattering.   

Kinetics of chain exchange in diblock copolymer micelles

Chain kinetic properties in the core of diblock copolymer micelles were studied using Monte Carlo simulations. In order to escape, a polymer chain first has to assume a compact conformation near the surface of the core. In our simulations, we studied the process of conformational changes of the core-forming block before it is ready to escape from a micelle. The radius of gyration or maximum distance of chain monomers from the core-corona interface were used as criteria to assess the compactness of the chain conformation and therefore its readiness to escape. When the chain exchange kinetics is limited mainly by the rate of conformational changes, the survival fraction of the original chains follows stretched exponential behavior. This is in contrast to theoretical predictions of a single activation process for unimer exchange in diblock copolymer micelles. In our simulations, we recover a single exponential behavior of the survival fraction only for the cases of very low escape probabilities. Increasing the width of the core-corona interface while maintaining the same aggregation number makes it easier for a chain to achieve a ready-to-escape conformation. This accentuates the stretched exponential behavior for the survival fraction which in turn shows a nearly logarithmic decay for a considerable portion of the overall time range.   

Chain Exchange Kinetics in Diblock Copolymer Micelles: Comparison of Experimental and Simulation Results

The exchange of chains between polymeric micelles in equilibrium has been studied experimentally using a novel Time Resolved Small Angle Neutron Scattering technique and modeled using Monte Carlo (MC) simulations. The experiments for PS-PB in alkanes and PEP-PEO micelles in DMF/water solutions revealed a logarithmic time dependence of the exchange kinetics. In the simulations, the time dependence of surviving chains ranges from a single exponential to stretched exponential behavior and is found to be strongly influenced by the chain conformation and escape criteria. In agreement with the experiments, a logarithmic time dependence was found in the MC simulations for an intermediate time range for micelles with a large number of escapable chains from micelle core and low probability of escaping or for micelles with a broader core-corona interface. The MC results suggest that dynamics of conformational rearrangements in a micelle core plays a decisive role in the micelle exchange kinetics.   

Dynamics of Polystyrene-Polyisoprene (PS-PI) Micelles in Selective Solvents.

We have recently shown that dissociation of isotropic-to-micelles phase transition of PS-PI micelles in decane takes place gradually where the micelles dissociated into smaller ones and eventually into a unimolecular micelles where the polystyrene is surrounded by the polyisprene. In here we report the dynamics of the PS-PI block as studied by neutron spin echo experiments as the dissociation takes place. With increasing temperature the entire system becomes more dynamic as expected. However on the length scale of the size of the micelle, the diffusion is constrained up to temperatures where most of the micelles, have dissociated whereas on the segmental level the dynamics is characteristic of that of a free molecule. The dynamics with in the micelle and the nature of the dissociation transition will be discussed.