Curving and Frustrating Flatland

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Science  01 Oct 2004:
Vol. 306, Issue 5693, pp. 76
DOI: 10.1126/science.1100090


Two polymer chains that occupy equal volumes when covalently linked together at one end self-assemble into an alternating lamellar morphology that has a characteristic period dictated by the molecular weight. When such copolymers are confined within alumina membranes that have cylindrical pores with diameters comparable to the repeat period, the interaction of the blocks with the confining walls and the imposed curvature induces a morphological transformation to relieve the constraints. Here, we show a lamella-to-toroid transition, captured through the dissolution of the surrounding membrane.

Molecular conformations and assemblies are strongly influenced by their environment. Constraints imposed by bounding surfaces can break the symmetry of a structure, causing marked departures from classical equilibrium behavior. This opens new pathways for materials with unique structures. Diblock copolymers, formed by two chemically different polymers joined together at one end, self-organize into periodic domains that are receiving much attention as nanostructured materials for device applications (1, 2). In the bulk, i.e., without external constraints, the morphology of a block copolymer is dictated by the interactions between segments comprising the copolymer, the volume fraction of the blocks, and the area occupied by each block at the interface between the domains. Placing the copolymer between hard, flat surfaces spatially confines the morphology. If the copolymer period and the wall-separation distance are incommensurate, stretching or compression of the copolymer molecules will occur, altering the fundamental repeat period (3). If the confining geometry is nonplanar, as with a cylindrical pore geometry, then both commensurability and imposed curvature influence the morphology (4). When the diameter of the confining cylinder is large in comparison to the copolymer period, morphologies identical to those observed under planar confinement are found (5). When the cylinder diameter is small and incommensurate with the copolymer period, the imposed curvature produces unusual morphologies, expanding the repertoire of nanoscopic structures attainable with these versatile molecules.

Melts of symmetric diblock copolymers of styrene (PS) and butadiene (PBD) with molecular weights of 28,900 [PS-b-PBD (I)] and 18,400 [PS-b-PBD (II)] with polydispersities of 1.02 and 1.03, respectively, were drawn into nanoporous alumina membranes by capillary action (5, 6). We used a weak base to dissolve the alumina and produce freestanding nanoscopic rods of copolymers. PS and PBD are highly immiscible, and PBD, having a low surface energy, strongly segregates to the high-surface-energy alumina interface. The morphologies of these copolymers in the bulk are lamellar with repeat periods L0 of 23.5 and 17.6 nm, respectively. Figure 1A is a transmission electron microscopy (TEM) image of PS-b-PBD (I) confined to pores with diameters d of ∼190 nm (d/L0 ∼ 8.1), commensurate with L0. As expected from simulations and theory (79) and as shown elsewhere (5), eight concentric alternating cylinders of PS and PBD can be seen. The lower surface-energy PBD domain (dark regions) is located at the cylinder walls. Figure 1B is a TEM image of PS-b-PBD (I) in 45-nm pores (d/L0 ∼ 1.9), and only a central core of PS, surrounded by a layer of PBD, appears. This structure requires a substantial deformation of the block copolymer chains, but because of the strong immiscibility of PS and PBD and the low interfacial energy of PBD, a lamellar morphology persists.

Fig. 1.

(A to D) TEM images of PS-b-PBD nanorods from a nanoporous alumina membrane. The pore diameters d and lamellar repeating periods L0 are (A) 190 and 23.5 nm, (B) 45 and 23.5 nm, and both (C) and (D), 45 and 17.6 nm, respectively. (A), (B), and (C) are cross-sections cut normal to the rod axis, and (D) is a cross-section parallel to the rod axis. Scale bars, 50 nm.

TEM images of PS-b-PBD (II) in 45-nm diameter pores (d/L0 ∼ 2.6, highly incommensurate) are shown normal to and along the pore axes (Fig. 1, C and D, respectively). Here, d and L0 are incommensurate. With planar surfaces, a compressed lamellar morphology would be seen (3). However, in a cylindrical geometry, the high degree of curvature imposed on the planar lamellar morphology causes a frustration of chain packing at the interface, resulting in a fundamental change in the morphology. Normal to the rod axis, concentric layers are observed with PBD located in the centers and at the outer walls of the rods. Along the rods, a stacked PS lamellar structure is seen, with a central spine and outer edges of PBD. Thus, a transition from a lamellar to a stacked-disc or toroidal-type structure occurs. This morphology, forced on the block copolymer by curvature and incommensurability, is not accessible by other means.

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Materials and Methods

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