3.6 Experimental
3.6.4 Polymer modification: Synthesis of statistical LC ‘gluing’ copolymer
Polymer modification with Chol-SH and Boc-Cys. A mixture of poly[2-(3-butenyl)-2-oxazoline] (PBOx) (0.50 g, 4 mmol C=C), Chol-SH (0.48 g, 1.2 mmol), and Boc-Cys (0.27 g,
43 1.2 mmol) in tetrahydrofuran (THF, 10 mL) was degassed twice, put under argon atmosphere and irradiated with UV light (mercury medium pressure UV lamp, Heraeus TQ 150, glass filter) for 24 h at room temperature. The reaction mixture was dialyzed against methanol, evaporated to dryness, and the residual solid freeze-dried from benzene. Yield: 0.7 g
1H NMR (400 MHz, CDCl3) 0.62.7, 1.43 (CH3 Boc-Cys), 2.37 (br, NC(=O)CH2-CH2), 2.57 (br, CH2S), 3.03 (br, SCH2 Boc-Cys), 3.23.7 (br, NCH2 backbone), 4.50 (br, SCH2C H Boc-Cys), 4.95.1 (m, =CH2 BOx), 5.32 (s, =CH Chol), 5.5-5.7 (br, NH Boc-Cys), 5.755.9 (CH= BOx) (see 1H NMR spectrum, Figure 3.3).
Polymer modification with Chol-SH and 3-MPA. Chol-SH (49.2 mg, 0.13 mmol) and 3-MPA (13.5 mg, 0.13 mol) were added to a solution of PBOx (51 mg, 0.4 mmol C=C) in dry THF (1.5 mL). The reaction mixture was degassed three times, put under an argon atmosphere, and exposed to UV light (source: Heraeus TQ 150) at room temperature for 24 h. After exhaustive dialysis of the reaction mixture against THF and ethanol (MWCO: 1 kDa), the solvent was evaporated to dryness, and the residual solid was freeze-dried from benzene (gravimetric yield: 104.5 mg, > 95%). 1H NMR (400 MHz, CDCl3) 0.62.7, 2.75 (br, SCH2 MPA), 3.33.6 (br, NCH2 backbone), 4.95.1 (m, =CH2 BOx), 5.3 (s, =CH Chol), 5.85.9 (CH=
BOx) (see 1H NMR spectrum, Figure S15 in the Appendix).
Polymer modification with Chol-SH and 2-DEA. A mixture of Chol-SH (0.24 g, 0.59 mmol), 2-DEA (86 mg, 0.51 mmol) and PBOx (0.21 g, 1.7 mmol C=C) and DMPA (0.02 mmol, [DMPA]0/[C=C]0 = 1%) in 6.2 mL 1-BuOH was degassed twice, put under argon atmosphere and irradiated with UV light (mercury medium pressure UV lamp, Heraeus TQ 150, glass filter) for 24 h at room temperature. After dialysis against ethanol and THF (MWCO: 1 kDa), the solvent was evaporated to dryness, and the residual solid freeze-dried from benzene (gravimetric yield: 0.42 g, > 95%). 1H NMR (400 MHz, CDCl3) 0.62.7, 3.02 (br, SCH2
DEA), 3.22 (br, SCH2-CH2 DEA, NCH2 DEA), 3.33.8 (br, NCH2 backbone), 4.95.1 (m,
=CH2 BOx), 5.3 (s, =CH Chol), 5.75.9 (CH= BOx) (see 1H NMR spectrum, Figure S16 in the Appendix).
44
4 Liquid crystal polymer–Laponite hybrid materials
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).
4.1 Abstract
A model system for the synthesis of biomimetic, hierarchically structured organic-inorganic composites via a one-step self-organization process based on liquid crystal (LC) formation was established. The organic component was a polyoxazoline with pendent cholesteryl and carboxyl or cholesteryl and amine side chains, respectively, which is able to form a chiral nematic lyotropic phase and to selectively bind to charged Laponite faces via electrostatic interaction. While polymer adsorption on the negatively charged exposed faces of Laponite did not lead to Laponite/polymer composites with a defined structuring via this synthesis strategy, face-selective binding of the LC polymer to Laponite edges afforded composite structures organized on three hierarchical levels: a columnar LC arrangement of Laponite platelets (one level) within the chiral nematic polymeric lyotropic phase (two levels).
Consequently, this fast and efficient fabrication method allows mimicking the structures of columnar nacre, even when reducing the polymer fraction to ca. 10 wt%. Crosslinking of the polymeric LC phase supported the formation of even more pronounced composite structures on the nanometer scale exhibiting enhanced mechanical stability, compared to the corresponding non-cross-linked counterparts. The one-step self-organization synthesis approach is transferrable to other anisotropic particles like vanadia or gold rods (cf. Chapters 5 and 6).
45
4.2 Introduction and aim
Inspired by the structure and outstanding properties of biominerals, many research groups have undertaken the major challenge of developing biomimetic lamellar composite structures.
One of these interesting topics is the development of artificial nacre. Its structure and mechanical properties can be mimicked or even be outperformed by using clay minerals, montmorillonite (a natural aluminosilicate133) and Laponite clay platelets (see next page).
Clay minerals are layered silicates often appearing as anisotropic plate-shaped nanoparticles that can act as a mesogen. Consequently, clay minerals were found to form lyotropic colloidal liquid crystals (LCs).134,135 Laponite is a synthetic smectite clay exhibiting a layered structure similar to the structure of natural clays such as bentonite and hectorite. Laponite is composed of SiO2 66.2, MgO 30.2, Na2O 2.9 and Li2O 0.7 (%w/w), given in the molecular formula Si8[Mg5.5Li0.3O20(OH)4]0.7– [Na0.7]0.7+.136,137 In diluted, colloidally stable dispersions, Laponite platelets have dimensions of ca. 25 nm in diameter and ca. 1 nm in thickness (Figure 4.1a).137
Figure 4.1: (a) Single Laponite crystal. (b) Idealized structural formula: 6 octahedral magnesium ions are situated between two layers consisting each of 4 tetrahedral silicon atoms. The unit cell is completed by 20 oxygen atoms and 4 hydroxyl groups.137
The net negative charge resulting from the isomorphous substitution of magnesium ions by lithium ions is compensated by sodium ions, which are adsorbed onto the surfaces of the crystals (Figure 4.1b).137,138 When the sodium ions dissociate in aqueous solution, the faces of the Laponite particles become negatively charged. The edges of the platelet-shaped particles are comparable to hydrous oxides. Their charges are dependent on the pH value, by which ionization and protonation of hydroxyl groups located at the particle edges are controlled.137,138 When dispersing Laponite particles in water, the hydroxyl groups dissociate from the edges, giving rise to an increased pH (depending on temperature and particle concentration). The positive charge at the edges decreases when the pH is increased (given a constant temperature). This is due to deprotonation of the edges at higher pH value. The positive rim charge is neutralized at a pH of ~11.2.138
46 Investigations of aqueous suspensions of Laponite nanoplatelets, revealed the formation of a nematic LC phase,139 which was demonstrated by small angle X-ray studies.140 Large-scale ordering of an aqueous Laponite suspension near the air–Laponite suspension interface was reported by Joshi and co-workers.141 The behavior of suspensions of Laponite mixed with mesogenic hosts such as thermotropic liquid crystal K15, showing large-scale structures of aggregates, was investigated. The aggregates consist of mixtures of single and stacked self-organized platelets142 or cholesteric liquid crystals, revealing a stabilizing effect on cholesteric planar textures.143 Because of this self-organization into arrays of parallelly ordered clay platelets and the morphological similarity to the micrometer-sized aragonite platelets in nacre, bio-inspired nacre composites were fabricated by using nanoplatelets, in particular, clays. The advantage of the nanosized clay particles compared to the microsized aragonite platelets in nacre is that materials become insensitive to flaws on the nanoscale.43
Bonnet et al. reported the preparation of artificial nacre via the self-assembly of Na/Ca montmorillonite platelets, which was obtained by the evaporation of dilute aqueous dispersions of delaminated platelets.144 Another approach to mimicking nacre was the layering of montmorillonite clay platelets and polyelectrolytes, as described by Ozturk et al.36 and Kotov et al.145, silica and polyelectrolytes, as described by Char et al.,146 and CaCO3 and polymers147,148 via layer-by-layer assembly. Investigations of the influence of the size and surface area of different clays are reported by Stefanescu and Negulescu et al.,149 who prepared multilayered films consisting of poly(ethylene oxide) (PEO) and montmorillonite clay platelets or PEO and Laponite clay platelets. Polymer–clay nanocomposites were also synthesized by mixing poly(vinyl alcohol) (PVA) and montmorillonite platelets and subsequent doctor-blading.150 Similar paper-making technology could be used to produce multilayered clay–polyelectrolyte papers with fire-retardant properties151 in addition to advantageous mechanical properties. Shigehara et al.152 observed the formation of a disco-nematic liquid crystalline structure of Laponite nanoparticles within an amorphous PEG matrix in nanocomposites obtained by solution casting. Bonderer et al.153 reported the preparation of multilayered hybrid films, combining both high tensile strength154 and ductile behavior154 via a bottom-up colloidal assembly of alumina platelets within a chitosan matrix.
Ceramic sintering155 and ice-templating156 approaches were also used to produce layered nacre mimic materials. Nacre mimic materials with a fracture toughness outperforming that of natural nacre and the highest fracture toughness reported to date were reported by Munch et al.155 Another approach using microelectromechanical systems technology was applied to produce microcomposites consisting of silicon particles and polymeric photoresist, mimicking
47 the crossed-lamellar microstructure of mollusk shells. This shows significant strength and work of fracture as well as some energy-dissipating cracking patterns like bridged cracks.157 Very recently, Erb et al.158 succeeded in synthesizing hierarchically reinforced, wear-resistant materials by using reinforcing particles of micrometer size coated with superparamagnetic nanoparticles, thus controlling the orientation and distribution of the reinforcing particles by weak magnetic fields. In a similar approach, materials with remarkable mechanical properties were obtained when composites from polyurethane-based thermoplastic polymers, Laponite nanoplatelets, and alumina microplatelets were synthesized.159 Reproducing the hierarchically organized composite structures as found in natural materials in a ‘one-pot’ scalable synthesis approach still remains a challenging task.
In this chapter, an alternative ‘one-pot’ scalable synthesis concept was aimed for, where hierarchically organized composite structures were created inspired by findings in natural materials. Laponite clay platelets were used as a model system to establish this fabrication concept via a one-step self-organization based on LC formation (see Scheme 4.1).
Scheme 4.1: Fabrication concept for hierarchically structured Laponite/LC polymer composites. Face-selective polymer adsorption on the nanoplatelets and subsequent liquid crystal formation of both organic and inorganic components induces self-assembly on three hierarchical levels.
Laponite platelets, possessing positively charged rims and negatively charged exposed faces, allow for face-selective polymer adsorption on the rims or the exposed faces and, consequently, for studying the influence of the nature of polymer adsorption on the final composite structure. To this end, statistical copolymers were synthesized carrying LC side chains (cholesteryl) and ‘gluing’ side chains (carboxyl, amine). Depending on the pH, the
48 carboxyl or amine groups can be negatively or positively charged, respectively, enabling the polymer to selectively bind to the Laponite nanoplatelets via the positively charged rims or negatively charged exposed faces, involving Coulomb interactions. The cholesterol side chains form two levels of the hierarchical structuring through the formation of a chiral nematic LC phase. The Laponite nanoplatelets, i.e. the inorganic phase, can also form an LC phase, building up the third level of the hierarchical structuring. Shearing of the hybrid particles induces long-range orientation of the polymeric LC phase (cf. Chapter 3.4). The composite structure gets fixed by drying (see Scheme 4.1).
By cross-linking the polymeric matrix of the Laponite/polymer composites, an additional fixing of the composite structure is possible, which may lead to composites with increased mechanical properties.