Alignment via face-selective polymer adsorption

Face-selective polymer adsorption to anisotropic nanoparticles for the purpose of aligning the particles along an ordered (LC) polymer phase is an integral part of the biomimetic synthesis concept that was established in this work. Therefore, the principle is briefly presented here as a technique to achieve the formation of highly defined superstructures. An example for face-selective polymer adsorption leading to barium carbonate mesocrystals has already been given in Chapter 1.4.2. Another example is the formation of calcite mesocrystals, which were obtained via a polymer-mediated structure-formation process. The crystallizations were performed by CO2 gas diffusion technique in the presence of polystyrene sulfonate (PSS), leading to a hierarchical micron-sized hybrid structure. PSS was reported to act in different ways during the synthesis. PSS binds to free calcium ions, leading to a shift of the growth mechanism from ionic growth to mesoscale assembly of pre-formed nanoparticle building blocks. The polymer also acts as a nucleating agent and can selectively bind to the (001) face of calcite. The face-selective adsorption of PSS favors the formation of mesostructures composed of truncated triangular units instead of – typical for calcite – rhombohedra. These superstructures were found to be highly porous and to consist of almost perfectly 3D-aligned calcite nanocrystals (Figure 1.24). In case of working at high supersaturations, superstructures with changed symmetry were obtained, suggesting the presence of dipolar interaction potentials between the nanocrystal building units.^103,104

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Figure 1.24: SEM image of a calcite mesocrystal, obtained by gas diffusion technique in presence of polystyrene sulfonate.¹⁰³

Face-selective copolymer adsorption can even lead to self-similar hierarchical 3D superstructures. In the following case, poly(styrene-alt-maleic acid) was used as crystal growth modifier.¹⁰⁵ As in case of working with PSS,^103,104 calcite is not able to expose the (001) faces without a growth modifier. This is due to the fact that these faces are composed of only CO₃^2- and Ca²⁺ ions in a hexagonal orientation and thus possess highly charged faces.

The existence of these faces with their high surface energies in presence of a growth modifier suggests multiple Coulomb binding between the positively charged (001) faces and the negatively charged polymer chains. This interaction results in a surface stabilization and directed aggregation along the c-axis direction.¹⁰⁵

The proposed mechanism for the formation of stacked hierarchical self-similar calcite mesocrystals is illustrated in Figure 1.25A.

Figure 1.25: (A) Mechanism for the formation of hierarchical self-similar calcite mesocrystals, built from triangular calcite building blocks. Nearly spherical CaCO₃ nanoparticles formed in the early reaction stage (a), which then crystallized and aggregated in the presence of poly(styrene-alt-maleic acid) (exposed faces are (001) and (011); b)). Further aggregation, finally forming aggregates with the shape of their subunits, probably along the (011) faces (c). 3D mesocrystal composed of triangular calcite building units formed by mesoscale assembly (d). (B) SEM image of the calcite mesocrystal formed in presence of poly(styrene-alt-maleic acid). The inset illustrates the Sierpinski triangle.¹⁰⁵

Primary nearly spherical CaCO₃ nanoparticles form in the supersaturated solution in the initial reaction stage (Figure 1.25A-a). The above- described copolymer adsorption to the

25 nanoparticles, predominantly to the (001) faces, modifies the growth of the stabilized nanoparticles, leading to a growth by aggregation rather than a coarsening of the individual particles. Aggregation into a hierarchical ordering of triangular-capped building units was observed, which occurs through joint re-assembly of organic and inorganic components and probably via face recognition of the non-stabilized neutral (001) side faces (Figure 1.25A-b and c). The copolymer may function as directing agent for the oriented crystallization of the inorganic compounds. The triangle capped nanoparticle building blocks stacked spontaneously, resulting in a vectorially aligned 3D superstructure (Figure 1.25A-c). The final hierarchical structure with the self-similar structuring (Figure 1.25A-d) is a result of mesoscale assembly of the formed aggregates. The hierarchical structure resembles the typical fractal figure ‘Sierpinski gasket’ (Figure 1.25B; note that the structure is self-similar, but not fractal).¹⁰⁵

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2 Scope of the thesis

Organic–inorganic biomaterials, such as bone and nacre, are composed of stiff and brittle mineral crystals joined by soft and ductile organic materials. Structural key elements of biominerals, leading to the combination of high mechanical stiffness and toughness, are their well-controlled coupling at the interface between organic and inorganic components and their sophisticated hierarchical structuring. Besides superior mechanical properties, biocomposites may also exhibit ingenious optical properties by precisely arranging inorganic crystals with the aid of organic molecules. Despite remarkable achievements of biomimetic lamellar composite structures in recent years, reproducing the hierarchically organized composite structures as found in natural materials in a one-pot synthesis approach with the potential for upscaling still remains a challenging task.

This thesis reports the development of a one-pot scalable synthesis approach to create hierarchically structured composite materials with high inorganic fractions by gluing together anisotropic nanoparticles with functional copolymers and inducing organization on several hierarchical levels through liquid crystal (LC) formation of both organic and inorganic components. Multifunctional materials with well-tuned physical properties can be obtained when using appropriate functional nanoparticles.

The synthesis concept aims at following general principles inspired by a variety of natural biocomposite systems, rather than accurately mimicking one single biological material. In essence, these principles are:

 Hierarchical structuring based on aligned anisotropic, inorganic nanoparticles within a structured organic polymer matrix. The polymeric phase provides deformability to the material.

 Well-defined interfaces with adjustable interaction between organic and inorganic components, controlled by the functional copolymer.

 A high fraction of inorganic nanoparticles, providing the material with strength and hardness.

 Self-assembly of the polymer allowing for guiding the positioning and aligning of the inorganic particles. The anisotropic shape of the inorganic nanoparticle can support the ordering of the inorganic phase by forming an LC phase.

 Face-selective polymer adsorption to orient the anisotropic particles along an ordered polymeric phase.

27 The polymers used for this purpose are functional poly(2-oxazoline)s (LC ‘gluing’

copolymers), carrying pendant cholesteryl units and different charged/polar units. The cholesteryl units allow the polymer to form chiral nematic lyotropic phases, and consequently, an ordering on two hierarchical levels: packing of mesogens (first level, nano-scale) and helical superstructure (second level, meso-scale). The ‘gluing’ units enable the polymer to bind (‘glue’) selectively to nanoparticle faces via Coulomb interactions or hydrogen bridges.

The method for the synthesis of hierarchically structured composites is illustrated in Scheme 2.1. The nanoparticles are dispersed in a (semidilute) solution of the LC ‘gluing’ copolymer, for example in aqueous medium at room temperature under controlled pH to ensure electrostatic binding of the nanoparticles to the gluing units or respectively the formation of hydrogen bonds. If needed, the formed hybrid particles are transferred into an organic medium, in which the polymer is able to form lyotropic LC phases. Evaporation of the solvent and shearing induces the formation of a liquid crystalline ordering in the organic phase with long-range orientation. The anisotropic shape (rods or platelets) of the nanoparticles enables the inorganic phase to form a liquid crystalline phase, too. The composite structure is finally fixed by drying.

Scheme 2.1: Synthesis concept for the fabrication of hierarchically structured composite materials consisting of LC polymers and anisotropic nanoparticles. Face-selective polymer adsorption on the nanoparticles and subsequent liquid crystal formation of both organic and inorganic components induces self-assembly on at least three hierarchical levels.

The concept is established by using commercially available Laponite platelets as a model particle system. These platelets exhibit negatively charged lateral faces and positively charged

28 rims, allowing for face-selective polymer adsorption via electrostatic interaction. The selective attachment to the Laponite faces may allow one to study the influence of different binding modes on the polymer-controlled process of self-assembly. These studies include detailed structural and mechanical investigations. Due to the ability of Laponite particles to function as a gas barrier, the Laponite/LC polymer hybrid system may feature advantageous gas barrier characteristics.

The one-step self-organization concept is transferred to the synthesis of other technologically relevant materials by using ribbon- and rod-shaped functional nanoparticles, namely vanadium pentoxide, gold and calcium sulfate. This further extends the scope of the concept by implementing an electrochromic/-optical functionality (in case of using vanadium pentoxide) and optical functionality (in case of using gold) into the material, beyond mechanical reinforcement.

The interest in calcium sulfate–polymer hybrid materials is based on their potential mechanical properties. These composites are expected to be relevant for applications in the building industry or for biomedical purposes. Advantageous mechanical properties may particularly arise if the composites exhibit a hierarchical structuring. Therefore, the aim of this part of the thesis is to synthesize hierarchically organized calcium sulfate–polymer hybrid materials. This requires a detailed study of the crystallization behavior of the calcium sulfate system in order to establish a method to synthesize anisotropic calcium sulfatenanoparticles of uniform shape, as well as investigations towards the applicability of the respective nanoparticles for the developed fabrication concept based on LC formation.

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3 Liquid crystalline (LC) ‘gluing’ polymers

Part of this chapter has been published:

 Tritschler, U.; Zlotnikov, I.; Zaslansky, P.; Aichmayer, B.; Fratzl, P.; Schlaad, H.;

Cölfen, H. “Hierarchical Structuring of Liquid Crystal Polymer–Laponite Hybrid Materials”. Langmuir 2013, 29, 11093-11101 (including Cover Page).

 Tritschler, U.; Zlotnikov, I.; Zaslansky, P.; Fratzl, P.; Schlaad, H.; Cölfen, H.

“Hierarchically Structured Vanadium Pentoxide–Polymer Hybrid Materials”. ACS Nano 2014, 8, 5089-5104.

 Tritschler, U.; Zlotnikov, I.; Keckeis, P.; Schlaad, H.; Cölfen, H. “Optical Properties of Self-Organized Gold Nanorod–Polymer Hybrid Films”. Langmuir 2014, 30, 13781-13790.