Bottom-up synthesis of hierarchical structures via self-assembly of

considered to play a crucial role in achieving sophisticated bio-inspired structures.

Supramolecular architectures possessing defined shapes and functions have been obtained via self-assembly processes involving non-covalent interactions, such as hydrogen bonding, hydrophobic and hydrophilic effects, electrostatic interactions, microphase segregation, and shape effects. Copolymers carrying anisotropic mesogens (rods or disks; rod-like macromolecules) are known to form lyotropic liquid crystalline (LC) phases in concentrated solution, i.e. nematic, smectic, and chiral nematic (cholesteric) phases. Polymer liquid crystals (LCs) carrying the mesogens in the main chain or in the side chains are called ‘main-chain polymer LCs’ and ‘side-chain polymer LCs’, respectively.

LCs are generally defined as anisotropic fluids of supramolecular assemblies, which form an

‘intermediate’ phase between an isotropic fluid phase and a solid crystal phase. An LC is called thermotropic if it shows a phase transition into the LC phase as a function of the temperature. In contrast, an LC is termed lyotropic if it exhibits a phase transition depending on temperature and its concentration in a solvent. Nematic LCs exhibit a mesophase, in which the LC molecules do not possess a positional ordering, but a directional ordering by aligning along one axis (see Figure 1.16a). There are various smectic phases, differing in the degrees of positional and orientational order. The alignment of molecules in a smectic A phase, in which the molecules are oriented along the layer normal, and in a smectic C phase, in which the molecules are tilted away from the layer normal, are schematically illustrated in Figure 1.16b.^80-82

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Figure 1.16: Arrangement of mesogens in a nematic LC phase (a), a smectic A (b, left) and a smectic C LC phase (b, right), and a chiral nematic LC phase (c). The molecules in the chiral nematic phase are twisted perpendicular to the director, with the molecule axis parallel to the director; p/2 refers to the half-helical pitch (d).⁸¹

The chiral nematic phase, also called cholesteric phase (as it was first observed for cholesterol derivatives), exhibits chirality. Similar to the nematic LC phase, the chiral nematic phase possesses an orientational order, but the molecules of each domain rotate perpendicular to the director, with the molecule axis parallel to the director (leading to a clockwise or counter-clockwise twisted structure; Figure 1.16c and d). The helical pitch p is defined as the distance along the helical axis, in which the director rotates by 360°, corresponding to one full helix turn. The half helix turn, after which the chiral nematic structure repeats itself, is called half-helical pitch p/2 (Figure 1.16d). An example for a lyotropic chiral nematic LC phase, observed via polarized optical microcopy, is illustrated in Figure 1.17.^80-82

Figure 1.17: (a) Polarized optical microcopy of a 10 wt% solution of the polymer shown in (b), revealing the formation of a lyotropic chiral nematic LC phase with a characteristic double-spiraled texture characteristic. The inset shows a fingerprint texture of the lyotropic phase with a half-helical pitch of 1.5 µm. (b) Scheme of the aligned polymer chains forming the chiral nematic LC phase.⁸²

The polymers (polyacetylenes), in which the main chains act as LC mesogens, form a helically twisted structure with a double-spiraled texture characteristic. The inset of Figure 1.17a shows the fingerprint texture exhibiting a distance of 1.5 µm between striae. This distance corresponds to the half-helical pitch of the chiral nematic LC phase.⁸² Examples for polymers that are able to form LC phases include cellulose, (helical) polypeptides,⁸³ and polymers carrying pendant oligoester or cholesterol mesogen side chains.⁸⁴ Ordering of

18 polymers occurs on the length scale of few nanometers (packing of mesogens) and on the few tens to hundreds nanometer length scale (microphase separated structure of block copolymers or helical superstructure of chiral mesogens).

Historically, the term ‘lyotropic liquid crystal’ described the phase behavior of amphiphilic molecules (surfactants) consisting of a hydrophilic head-group and a hydrophobic chain in a solvent, mostly water. These molecules, ionic or non-ionic, self-assemble into aggregates.

This self-assembly is driven by the hydrophobic effect, which describes the tendency of nonpolar molecules to form aggregates in aqueous media, segregating water molecules.

Aggregation of amphiphilic molecules takes place above the so-called ‘critical micelle concentration’, forming a structure in which the hydrophobic chains are shielded by the hydrophilic head-groups from contact with water. The structure of the micelle can be predicted by the ‘packing parameter’, defined as the quotient of the volume of the hydrophobic chain and the product of both surface area of the hydrophilic head-group and chain length of the hydrophobic part. The shape of the aggregate depends on temperature, pressure, and concentration of the species as well as on characteristics of the medium, such as ionic strength and pH. Lyotropic liquid crystalline phases (e.g. micellar cubic phases, hexagonal phases, and lamellar phases) form when the concentration of the surfactant in water is increased beyond the critical micelle concentration. Phase transitions can be induced by changing the concentration or temperature.^80,85

A hierarchical structuring of polymers was achieved, for example, by bottom-up synthesis of hierarchical structures via self-assembly of peptide-based block copolymers⁸⁶ or rod-coil block copolymers⁸⁷ from solution. Peptide-based block copolymers are block copolymers with synthetic segments and amino acid sequences, also called ‘molecular chimeras’.⁸³ Rod-coil block copolymers combine the stiff rod-like liquid crystalline characteristic and the flexible coil-like characteristic in one single polymer, both influencing the molecular packing of the macromolecules.⁸⁷ In contrast, coil-coil block molecules consist of two different immiscible segments, allowing to form micro-phase separated supramolecular structures.⁸⁷ Rod-coil copolymer systems can be based on mesogenic rods and combine the effect of microphase separation and molecular anisotropy of the rod block. As an example, the rod-coil copolymers prepared by Stupp et al. consisting of an elongated mesogen (rod length of 6 nm) and a monodisperse polyisoprene are shown in Figure 1.18.^87-90

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Figure 1.18: Rod-coil copolymer consisting of a mesogenic rod (length of 6 nm) and a monodisperse polyisoprene.⁸⁷

Depending on the rod volume fraction, the copolymers organize into a strip morphology (rod volume fraction of 0.36, Figure 1.19a) or self-assemble into a hexagonal superlattice (rod volume fraction of 0.25, Figure 1.19b). Consequently, the supramolecular structure of this polymer system can be controlled by varying the rod/coil volume ratio.^87,88

Figure 1.19: Scheme of the stacking arrangement of rod-coil copolymer with f_rod = 0.36 (a) and the close-packed hexagonal superlattices of rod-coil copolymer with f_rod = 0.25 (b).⁸⁸

Hierarchical structuring of polymers via phase separation and subsequent crystallization was shown for poly(2-isopropyl-2-oxzaoline) in hot aqueous medium, which is described in more detail (together with other methods allowing to introduce a hierarchical structure by using polyoxazolines) in Chapter 3.2.

A structuring of nanoparticles was obtained by self-assembled hierarchical block copolymer phases, finally leading to mesoporous metals.⁹¹ Ligand-stabilized platinum nanoparticles (core diameter of 1.8 nm; see Figure 1.20A and B) co-assembled with block copolymers poly(isoprene-block-dimethylaminoethyl methacrylate) (Figure 1.20C) during solvent evaporation. Important criteria included the dispersibility of the nanoparticles in only one block of the block copolymer (controlled polymer-nanoparticle interaction) and a nanoparticle size below a critical limit (to promote efficient mixing of the nanoparticles with the relevant block of the copolymer). The obtained mesostructured hybrids (Figure 1.20D) were pyrolized by heating under inert gas atmosphere, giving rise to ordered mesoporous Pt-C composites (Figure 1.20E). Depending on the nanoparticle volume fraction, lamellar and inverse

20 hexagonal hybrid mesostructures were produced. For applications like fuel cells, the metal surface is needed. However, in case of heating the hybrids under air to remove C, the mesostructure collapsed. Removal of the polymer phase by applying Ar-O plasma or acid etch finally led to the formation of ordered porous metal mesostructures (Figure 1.20F).⁹¹

Figure 1.20: (A) N,N-di-2-propoxyethyl-N-3-mercaptopropyl-N-methylammonium chloride was used as ligand to obtain moderately hydrophilic platinum nanoparticles with high solubility. (B) Platinum nanoparticle with a core diameter of 1.8 nm and a ligand shell of 1.4 nm. (C) Poly(isoprene-block-dimethylaminoethyl methacrylate) (molecular weight of 28 000 - 31 000 g mol^-1 and polydispersity of 1.04 - 1.05). (D) Self-assembly of platinum nanoparticles with block copolymer, leading to a mesostructured hybrid. (E) Mesoporous Pt-C composite obtained by pyrolysis under inert atmosphere. (F) Ordered mesoporous platinum structure after removal of C via Ar-O plasma treatment or acid etch.⁹¹

The organization of anisotropic nanoparticles forming a liquid crystalline phase can be almost perfect, resulting in mesocrystals. Liquid crystals are known to consist of anisotropic nanoparticle or molecular building units, i.e. rigid and anisotropic units (mesogens). However, in contrast to mesocrystals described in Chapter 1.4, building units in liquid crystals are interspaced by a liquid, thus allowing fluidity. The liquid crystalline analogues to mesocrystal microparticles are called tactoids.^61,62 One such example is V₂O₅, which is described in more detail in Chapter 5.

Self-assembly via liquid crystal formation can be found in many biological systems.⁹² Recently, the optical and structural properties of golden beetle Chrysina resplendens were mimicked using self-organization and self-alignment of LC polymers.⁹³ The beetle Chrysina resplendens exhibits a metallic gold color (Figure 1.21a), due to two helical layers, which are separated by an untwisted birefringent layer. The pitch between the helical layers is varied, leading to a widening of the reflection band.⁹⁴ The structural concept of the beetle was mimicked by using reactive mesogens, which are cross-linkable monomers that exhibit LC phases. Upon photopolymerization of the liquid crystals, the conformation of the LC phase

21 was fixed. In case the reactive monomer is in a nematic phase, a birefringent film is obtained.

The properties of the film can be tuned via the film thickness, leading to a quarter of half-wave retarder. In case the reactive monomer is in a cholesteric phase, the resulting film acts as wavelength-selective reflector.⁹³

The self-organizing layers of photopolymerized LCs usually require an aligning layer.

However, in the approach described by Matranga et al. each layer self-organizes on top of the previous layer. The biomimetic film fabrication was initiated with a nematic alignment layer, followed by the deposition of two cholesteric layers with different reflection peaks (the first layer exhibits a reflection peak at 639 nm, the second one at 562 nm). On top of these layers, an untwisted layer was deposited, functioning as a half-wave retarder at a wavelength between the above-mentioned peaks (595 nm). Coating this layer with two further cholesteric layers (562 and 639 nm) led to the formation of a metallic golden film (see Figure 1.21a and b).⁹³

Figure 1.21: (a) Comparison of a composite cholesteric film made of reactive monomer structures (left) and a Chrysina resplendens beetle (right). (b) SEM image of the composite cholesteric film consisting of different layers: 1) cholesteric, 2) untwisted retarder layer, 3) cholesteric and 4) untwisted alignment layer.⁹³

Inorganic materials exhibiting a defined porous structure were obtained by self-assembled lyotropic liquid crystals.^95-99 Materials combining mesoporosity and chiral organization were fabricated by using chiral surfactants,^100,101 or by using nanocrystalline cellulose,¹⁰² leading to chiral long-range orientation. The materials obtained by Shopsowitz et al.¹⁰² will serve as an example. Aqueous suspensions of nanocrystalline cellulose exhibit chiral nematic phases, as shown in Figure 1.22a. These phases were preserved upon air-drying, finally leading to iridescent films. When mixing aqueous suspensions of nanocrystalline cellulose with a silica precursor like Si(OEt)4, a homogenous mixture of the hydrolyzed precursor and nanocrystalline cellulose was formed. The chiral nematic phase of nanocrystalline cellulose was still observed in the presence of silica (Figure 1.22b). Free-standing composite films were

22 obtained after drying (Figure 1.22c), which possess left-handed helical structures, as observed for pure nanocrystalline cellulose films (demonstrated by circular dichroism). The cellulose template was removed by calcination (540°C under air). The free-standing mesoporous silica films (Figure 1.22d) exhibit a similar texture compared to the composite films.¹⁰²

Figure 1.22: (a) Scheme of chiral nematic ordering of nanocrystalline cellulose crystallites, showing the half-helical pitch P/2 (ca. 150-650 nm). (b) POM image of a nanocrystalline cellulose/Si(OEt)4 suspension, acquired during solvent evaporation at room temperature. The fingerprint texture characteristic indicates a chiral nematic ordering. (c) POM image of a nanocrystalline cellulose/silicate composite film, revealing regions with different orientations. (d) POM image of a mesoporous silica film, which was obtained by calcination of the nanocrystalline cellulose/silicate composite film shown in (c). (b–d) Scale bar, 100 µm.¹⁰²

Both the chiral nematic organization and the high surface area of nanocrystalline cellulose were replicated in the inorganic phase. The helical structure led to a chiral reflectance of the mesoporous films, tunable over the visible spectrum by changing the initial ratio of silica and nanocrystalline cellulose (Figure 1.23).¹⁰²

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Figure 1.23: Optical characterization of mesoporous silica films, which were obtained by calcination of nanocrystalline cellulose/silicate composite films. The ratio of silica:nanocrystalline cellulose was increased from S1 to S4. (a) The transmission spectra of the mesoporous silica films reveal a blue-shift of the reflectance peaks of ~300 nm, leading to films that reflect light across the whole visible spectrum. (b) Photographs of the mesoporous silica films S1 to S4. The colors result only from the chiral nematic pore structure of the films. The coin (diameter of 1.8 cm) represents the scale bar.¹⁰²