ScienceWeek

CHEMISTRY: ON ORGANIZED POLYMERS

The following points are made by D.G. Bucknall and H.L. Anderson (Science 2003 302:1904):

1) Polymers often behave like tangled spaghetti, but sometimes the sheer size of polymer molecules can make them organize into regular nanometer-scale structures. Chemists are increasingly looking for ways to understand and manipulate such self-organization, with potential applications from solar cells to drug delivery.

2) Three key factors affect the macroscopic behavior of polymers: chemical composition, molecular architecture (the way in which the monomer building blocks are bound together in the polymer chain), and supramolecular architecture (the organization of the chains relative to each other). The combination of these factors leads to a hierarchy of architectures over a range of length scales, similar to the primary, secondary, and tertiary structures found in DNA and proteins.

3) The simplest polymers are isolated linear chains, which tend to form random coils. More complex polymers include branched polymers, block copolymers, polymer networks, cyclic polymers, and tree-like structures (such as dendrimers). Recent advances in polymer synthesis have extended the range of molecular architectures dramatically (1).

4) The weak, noncovalent interactions of the monomer units along a polymer chain tend to add up cooperatively, amplifying supramolecular effects. Thus, polymers are naturally predisposed to self-organize. Perhaps the simplest manifestation of this effect is the tendency of most polymer blends to phase-separate. This tendency is exploited in the case of block copolymers, which consist of two or more linear polymers of different chemical composition joined end-to-end. Block copolymers frequently form phase-separated structures such as micelles and vesicles with nanometer-scale dimensions. They have already found widespread industrial applications. For instance, in toughened plastics such as high-impact polystyrene, they act as interface modifiers between two normally immiscible homopolymers.

5) The phase behavior of diblock copolymers is driven by the immiscibility of the two blocks. A variety of architectures can be produced by manipulating the ratio of the volumes of the two blocks and the degree of immiscibility between them(2). Structures produced in this way include spheres, cylinders, and lamellae, which can form cubic, hexagonal, and layered architectures, respectively. These three-dimensional (3D) architectures have feature sizes of 5 to 50 nm, a range that is difficult to achieve by conventional lithography.

6) The transition from a disordered to highly ordered phase in simple diblock copolymer melts has been used to produce nanometer-scale templates for patterning planar substrates for very high density devices (3). Semiconducting block copolymers with hole- and electron-transporting blocks that self-organize into interpenetrating networks are being developed for use in solar cells and light-emitting diodes (4). The dimensions of these structures can be expanded by adding a component that swells one of the polymer domains, leading to materials with periodicities comparable to the wavelengths of visible light. These materials have applications as photonic crystals (5).

References (abridged):

1. J. Pyun, X.-Z. Zhou, E. Drockenmuller, C. J. Hawker, J. Mater. Chem. 13, 2653 (2003)

2. S. Forster, T. Plantenberg, Angew. Chem. Int. Ed. 41, 689 (2002)

3. M. Park et al., Science 276, [1401] (1997). Y. Heischkel, H. W. Schmidt, Macromol. Chem. Phys. 199, 869 (1998)

4. A. C. Edrington et al., Adv. Mater. 13, 421 (2001)

5. D. Bendejacq et al., Macromolecules 35, 6645 (2002)

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GIANT SUPRAMOLECULAR LIQUID CRYSTAL LATTICE

The following points are made by G. Ungar et al (Science 2003 299:1208):

1) Molecular self-assembly into a variety of bulk phases with two-dimensional (2D) and 3D nanoscale periodicity, such as cubic, cylindrical, or mesh phases, has been researched intensely in lyotropic (e.g., surfactant-water) liquid crystals (LCs) (1), block copolymers (2-4), and thermotropic (solvent-free) LCs (5). Lyotropics can provide templates for porous inorganic materials with well-defined structures, and LC complexes of DNA with cationic and neutral lipids are potential carriers for gene delivery. Complex organic nanostructures may serve as scaffolds for photonic materials and other nanoarrays and for surface nanopatterning.

2) In the case of bicontinuous cubic phases (1,2,5), the same structures have been observed in lyotropics, thermotropics, and block copolymers. The recent inclusion of self-assembling dendrons into the category of nanostructured soft matter has shown their tapered shape to be responsible for creating similar bulk phases and enabling "self-processing" of electronics components. It has also become apparent that manipulating the size and distribution of "micelles" aggregated from self-assembling dendrons can be used in controlling polymerization. Regarding creation of structural diversity, dendron shape can be fine-tuned in ways unavailable in lyotropic LCs or block copolymers, and hence have the potential of forming hitherto unobserved phases.

3) In summary: Self-organized supramolecular organic nanostructures have potential applications that include molecular electronics, photonics, and precursors for nanoporous catalysts. Accordingly, understanding how self-assembly is controlled by molecular architecture will enable the design of increasingly complex structures. The authors report a liquid crystal (LC) phase with a tetragonal three-dimensional unit cell containing 30 globular supramolecular dendrimers, each of which is self-assembled from 12 dendron (tree-like) molecules. The present structure is one of the most complex LC phases yet discovered. A model explaining how spatial arrangement of self-assembled dendritic aggregates depends on molecular architecture and temperature is proposed.

References (abridged):

1. J. M. Seddon, Ber. Bunsenges. Phys. Chem. 100, 380 (1996)

2. E. L. Thomas, D. M. Anderson, C. S. Henkee, D. Hoffman, Nature 334, 598 (1988)

3. I. W. Hamley, The Physics of Block Copolymers (Oxford Univ. Press, Oxford, 1998)

4. M. A. Hillmyer, et al., Science 271, 976 (1996)

5. A. M. Levelut and M. Clerc, Liq. Cryst. 24, 105 (1998)

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SELF-ASSEMBLY OF PHASE-SEGREGATED LIQUID CRYSTAL STRUCTURES

The following points are made by Takashi Kato (Science 2002 295:2414):

1) Molecules in the liquid crystalline (LC) state, although still mobile, have an orientational ordering that depends on external conditions, such as temperature or electric fields (1,2). The best known example would be the thermotropic LC materials, which exhibit different phases as a function of temperature, that are used in displays (3). In contrast to these high-tech materials, biological self-assembled systems consist of a variety of discrete molecules forming heterogeneous and hierarchical structures (structures within structures) on various scales of length ranging from the nanometer to the micrometer scale. For example, lipids exhibit lyotropic LC states in which phase formation depends on solution concentration (4). Biological cell membranes are formed through self-assembly of lipids with other biomolecules such as proteins and steroids.

2) Although the intrinsic ordering within a LC material can be controlled by external parameters, the ordering can also be controlled internally through weak interactions to create more complex structures that add functionality to the material. One approach is to obtain self-assembled structures from a variety of different molecules (2,5). Another is to form phase-segregated structures of "block" molecules of longer polymers and smaller molecules on the nanometer and micrometer scale (2). The control of intermolecular interactions (noncovalent interactions) within these structures is critical because several competing effects are at work.

3) In the formation of the self-assembled LC structures, specific intermolecular interactions such as hydrogen bonding and ionic interactions play key roles. A notable example is the formation of supramolecular liquid crystals (5). In the case of these materials, well-defined structures such as rods and disks are made through self-assembly of two or more complementary molecular components by hydrogen bonding. For example, benzoic acids and pyridines form supramolecular rod-like complexes exhibiting thermally stable nematic and smectic (layered) LC phases (5). Moreover, identical molecules self-assemble into unconventional LC complexes. Studies of these materials have initiated and stimulated the development of supramolecular polymers.

4) In summary: Additional functionality can be incorporated into liquid crystalline materials by using phase segregation and self-assembly. Intermolecular interactions such as hydrogen bonding and ionic interactions play key roles in the formation of these complex structures. One-, two-, and three-dimensional phase-segregated structures on various scales of length are formed by self-assembly of a variety of partially incompatible molecules. Such structures can enhance anisotropic properties such as ionic conductivity.

References (abridged):

1. D. Demus, J. W. Goodby, G. W. Gray, H.-W. Spiess, V. Vill, Eds., Handbook of Liquid Crystals (Wiley-VCH, Weinheim, Germany, 1998)

2. J. W. Goodby, Curr. Opin. Solid State Mater. Sci. 4, 361 (1999)

3. J. H. Haaren and D. Broer, Chem. Ind. 1998, 1017 (1998)

4. C. E. Fairhurst, S. Fuller, J. Gray, M. C. Holmes, G. J. T. Tiddy, in (1), vol. 3, chap. 7.

5. T. Kato, Struct. Bonding (Berlin) 96, 95 (2000)

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