No abstract

Self-assembly of block copolymers into interesting and useful nanostructures, in both solution and bulk, is a vibrant research arena. While much attention has been paid to characterization and prediction of equilibrium phases, the associated dynamic processes are far from fully understood. Here, we explore what is known and not known about the equilibration of particle phases in the bulk, and spherical micelles in solution. The presumed primary equilibration mechanisms are chain exchange, fusion, and fragmentation. These processes have been extensively studied in surfactants and lipids, where they occur on subsecond time scales. In contrast, increased chain lengths in block copolymers create much larger barriers, and time scales can become prohibitively slow. In practice, equilibration of block copolymers is achievable only in proximity to the critical micelle temperature (in solution) or the order–disorder transition (in the bulk). Detailed theories for these processes in block copolymers are few. In the bulk, the rate of chain exchange can be quantified by tracer diffusion measurements. Often the rate of equilibration, in terms of number density and aggregation number of particles, is much slower than chain exchange, and consequently observed particle phases are often metastable. This is particularly true in regions of the phase diagram where Frank–Kasper phases occur. Chain exchange in solution has been explored quantitatively by time-resolved SANS, but the results are not well captured by theory. Computer simulations, particularly via dissipative particle dynamics, are beginning to shed light on the chain escape mechanism at the molecular level. The rate of fragmentation has been quantified in a few experimental systems, and TEM images support a mechanism akin to the anaphase stage of mitosis in cells, via a thin neck that pinches off to produce two smaller micelles. Direct measurements of micelle fusion are quite rare. Suggestions for future theoretical, computational, and experimental efforts are offered.

No abstract

No abstract

Self-assembly behavior of polymer grafted nanoparticles in ordered phases of geometrically confined diblock copolymers is studied using self-consistent field theory. Entropy loss and structural frustration introduced by physical confinement significantly alter the morphology of ordered phases from the bulk behavior. In particular, a rich variety of three-dimensional microstructures, for example, helical structures, are obtained under confinement. In the present study, we demonstrate that ordered microstructures of diblock copolymers can be employed as promising structural scaffolds to host and self-assemble nanoparticles within the selective domain. Templated self-assembly of nanoparticles offers a potential route to fabricate advanced nanomaterials with superior properties. Analysis reveals various stable equilibrium phases of block copolymers embedded with nanoparticles with a high degree of nanoscale ordering. The arrangement of nanoparticles is controlled by tuning various parameters such as block fraction in diblock copolymers, particle loading, size and number of grafted chains, and degree of confinement. At a low volume fraction, nanoparticles self-organize into chiral microstructures, such as single and double helices, even though the system contains only achiral species. Upon enhancing particle loading, the helical structure becomes less favorable and various other three-dimensional phases such as ring and disk morphologies are obtained. The regions of helical, ring, disk, and concentric lamellar phases are identified in terms of parameters related to grafted particles. Understanding the factors affecting localization of nanoparticles enables us to control the particulate self-assembly behavior of nanoparticles to design novel and advanced nanocomposites with desirable properties.