Self-assembly of amphiphilic block copolymers into well-ordered structures has attracted significant interest over the past decade. An especially attractive application of block-copolymer self-assembly is the formation of isoporous membranes. A major problem in this process is the lack of sufficient long-range order and the difficulty of up-scaling due to the time-consuming preparation steps. Here, we report an innovative and simple method to prepare isoporous membranes with nanometre-sized pores. The combination of the industrially well-established membrane formation method by non-solvent-induced phase separation with the self-assembly of a block copolymer is demonstrated. The result is the creation of an integral asymmetric membrane of a block copolymer with a highly ordered thin layer on top of a non-ordered sponge-like layer. This straightforward and very fast one-step procedure for membrane formation is reported for the first time. The developed membrane has the potential for highly selective separation.
A novel strategy to synthesize amphiphilic surface-compartmentalized nanoparticles based on linear ABC triblock copolymers is presented. These so-called Janus micelles consist of a cross-linked core and a corona with a "northern" and a "southern" hemisphere. Selectively cross-linking spherical domains of the polybutadiene middle block in a well-ordered bulk morphology of a polystyrene-blockpolybutadiene-block-poly(methyl methacrylate) triblock copolymer (SBM) leads to the conservation of the compartmentalization of the outer blocks after dissolution of the material. Multi-angle laser light scattering gel permeation chromatography, fluorescence correlation spectroscopy, small-angle neutron scattering, and static and dynamic light scattering, as well as scanning force microscopy, indicate the existence of an equilibrium between molecularly dissolved Janus micelles (unimers) and aggregates (multimers), so-called supermicelles.
We describe the synthesis and the solution properties of Janus micelles containing a polybutadiene (PB) core and a compartmentalized corona consisting of a poly(methacrylic acid) (PMAA) and a polystyrene (PS) hemisphere. The Janus micelles were obtained via cross-linking the middle block of a microphase-separated PS-block-PB-block-PMMA triblock copolymer in the bulk state, followed by alkaline hydrolysis of the poly(methyl methacrylate) (PMMA) ester groups. Results of fluorescence correlation spectroscopy, field flow fractionation, light scattering, cryogenic transmission electron microscopy, scanning electron microscopy, and scanning force microscopy indicate that above a critical aggregation concentration of about 0.03 g/L spherical supermicelles are formed from about 30 PS-PMAA micelles in aqueous solution in the presence of NaCl. These supermicelles have radii of 40-60 nm, significantly increasing on ionization (pH >6). In addition, very large spherical objects are observed with radii of 100-250 nm.
The confinement of crystallizable blocks within AB or ABC microphase-separated block copolymers in the nanoscopic scale can be tailored by adequate choice of composition, molecular weight, and chemical structure. In this work we have examined the crystallization behavior of a series of AB and ABC block copolymers incorporating one or two of the following crystallizable blocks: polyethylene, poly(ε-caprolactone), and poly(ethylene oxide). The density of confined microdomain structures (MD) within block copolymers of specific compositions, in cases where the MD are dispersed as spheres, cylinders, or any other isolated morphology, is much higher than the number of heterogeneities available in each crystallizable block. Therefore, fractionated crystallization takes place with one or several crystallization steps at decreasing temperatures. In specific cases, the clear observation of exclusive crystallization from homogeneous nuclei was obtained. The results show that, regardless of the specific morphological features of the MD, it is their vast number as compared to the number of heterogeneities present in the system that determines the fractionated character of the crystallization or in extreme cases homogeneous nucleation. The self-nucleation behavior was also found to depend on the composition of the copolymers. When the crystallizable block is confined into spheres or cylinders and exhibits homogeneous nucleation, the self-nucleation domain disappears. This is a direct consequence of the extremely high density of microdomain structures that need to be self-seeded (on the order of 1015−1016/cm3). Therefore, to increase the density of self-nuclei, the self-nucleation temperature has to be decreased to values so low that extensive partial melting is achieved, and some of the unmelted crystal fragments can be annealed, in some cases even before self-nucleation takes place.
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