The morphology of ABC triblock copolymers based on poly styrene-5/oc/s-polybutadiene-6/oc&polyfmethyl methacrylate) [PS-i-PB-6-PMMA] (SBM series) and their hydrogenated analogues polystyrene-6Zoc&-poly(ethylene-co-butylene)-6Zoc£-poly(methyl methacrylate) [PS-6-PEB-6-PMMA] (SEBM series) block copolymers is governed by the relatively weak incompatibility of the end blocks PS and PMMA in comparison to the strong incompatibility of the polybutadiene or poly(ethylene-co-butylene) midblock. The paper describes the morphologies of high molecular weight (Mn ~200 000) block copolymers, which are symmetrical with respect to the PS and the PMMA blocks with a varying elastomeric center block (0.06 < wei < 0.38). Besides an ABC lamellar morphology (11) (38 wt % PB or PEB), two other lamellar morphologies are observed for shorter elastomer chains: At 17 wt % of the elastomeric center block a "cylinder at the wall" morphology is observed where PB or PEB cylinders are located at the lamellar PS/PMMA interface (lc). At 6 wt % of elastomer, the polybutadiene forms spheres at the PS/PMMA interface ("ball at the wall") (Is). In this case hydrogenation of the butadiene block, which is associated with a further increase in the immiscibility to the end blocks, induces a change of the overall lamellar structure into a cylindrical morphology in which PS cylinders surrounded by PEB rings are dispersed in a PMMA matrix (cr). The transition from the lamellar (11) morphology to the (lc) and (Is) morphologies is described by a simple extension of the Meier/Alexander/de Gennes/Semenov theories of AB block copolymers in the strong segration limit to ABC block copolymers. The theoretical description predicts that morphological transitions can be achieved in ABC triblock copolymers at constant composition even in the limit of strong segregation.
In asymmetric ABC triblock copolymers with C being the matrix-forming majority component, the formation of cylindrical morphologies is governed by the sum of the volume fractions of the components A and B (@A + t & )as well as by their ratio (@B/@A). The paper describes the morphologies of various polystyrene-block-polybutadiene-blockpoly(methy1 methacrylate) (SBM) and polystyrene-block-poly(ethy1ene-co-buty1ene)-block-poly(methy1 methacrylate) block copolymers (SEBM) in dependence on these parameters. In addition to previously reported cylinder in cylinder (tic, core shell) and helical (hel) morphologies, new microphase separated structures as cylinders at cylinder (c,c), undulatedperforated cylinder in cylinder (ucic) and spheres on cylinder (s,c) are discussed. Based on a simple theoretical approach the transitions between these various morphologies are predicted, especially focusing on the transition between the cic and c,c morphologies.
Materials with a macroscopic electric polarization display a variety of useful properties, such as piezo-and pyroelectricity and second-order nonlinear optical activity 1 . Macroscopic polarization results when dipolar molecules are orientated in the same direction, or when ions are organized in a non-centrosymmetric crystal structure 2 . Centrosymmetric molecules have no dipole moment and so cannot generate a macroscopic polarization. Non-centrosymmetry in amorphous materials can be engineered by depositing particular sequences of layers on top of each other, or by applying external ®elds (generally electric) to orientate the molecules 3 . Here we report the formation of a non-centrosymmetric structure in an amorphous material through spontaneous selfassembly. Block copolymers are known to form ordered structures at the microscale owing to segregation of the different blocks 4,5 . We show that a mixture of a ternary triblock copolymer and a binary diblock copolymer will organize itself into a noncentrosymmetric layered structure in which the layers are occupied by different blocks. The structure is periodic with a length scale of around 60 nm.The formation of liquid-crystalline non-centrosymmetric materials from molecules which are themselves non-centrosymmetric but not chiral has been much debated in the past two decades 6,7 . In particular, molecules consisting of three or four sub-groups, a±b±g and a±b±g±a have been suggested as good candidates to obtain longitudinal ferroelectric smectics 8±10 (that is, materials where noncentrosymmetric molecules self-assemble into stacks of liquid-like layers with all the molecules being orientated in the same direction). Such materials have been synthesized and studied 11,12 . Achiral banana-shaped molecules forming antiferroelectric chiral smectics have been reported, which can be transformed into ferroelectric materials by application of an electric ®eld 13,14 . In the case of lowmolecular-weight self-assembling compounds, smectic periodicity lies in the range of a few nanometres. To achieve smectic order with larger layer thickness (and thus larger periodicity) through selfassembling, it is natural to use linear block copolymers: these molecules consist of long sequences of chemically different monomers linked together by a covalent bond. Thanks to the improvement of synthetic techniques, it is now possible to make A±B±C triblock copolymers with well controlled molecular architecture, that is, with well controlled block lengths. Depending on the chemical structure and the length of blocks, these molecules form very interesting mesophases 15±20 . In most cases, centrosymmetric structures have been observed. The only exception reported so far was a helical morphology, where the occurrence of right-and lefthanded helices suppressed the formation of a well developed longrange-ordered structure 21 . One of us (R.S.) has proposed that by mixing suitably chosen A±B±C triblock copolymers with A±C diblock copolymers it should be possible to obtain self-assembled non-centrosymme...
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