Hierarchically structured composite materials

For the preparation of hierarchically structured organic-inorganic composite materials, dispersions in DMF consisting of Laponite/P4 1:1 w/w and 2:1 w/w were sheared by either lateral shearing between two glass slides in one direction or by slow and controlled rotation by means of a motor using a defined distance, inducing long-range orientation of the polymeric LC phase. Laponite/P4 composite materials were transparent in the dry state compared to Laponite/PBOx-Chol-DEA composites, which appeared much duller.

The dried Laponite/P4 composites were analyzed on the micro- to millimeter scale by POM and a quantitative birefringence imaging microscopy (Abrio), on the micrometer scale by X-ray microtomography, and on the micro- to nanometer scale by scanning electron microscopy (SEM), SAXS, and transmission electron microscopy (TEM). As for the polymeric lyotropic phases, the Abrio images of the Laponite/P4 composites (Figure 4.12) reveal the presence of lyotropic regions with same colors indicating the same structural orientations on the length scale of several hundred micrometers (the prerequisite of a constant film thickness on the submillimeter scale was checked by means of a profilometer, see Figure S22 in the Appendix). Such oriented domains were observed independent of the ratio of organics to inorganics and independent of the shearing method or shearing forces applied (manual lateral shearing or rotational shearing by means of a shear cell, see the Experimental Section 4.7.2 for more details). No considerable differences in the size of lyotropic regions, i.e. structural orientation on the micrometer length scale, were found between manually sheared samples or when controlled changing the shearing rate (see Abrio images in Figure S23 obtained with

61 different rotational shearing rates). The lyotropic regions of the composites are considerably larger than the ones found just for the polymer alone (Figure 3.5).

Laponite/polymer 1:1 and 2:1 w/w composites exhibiting lyotropic regions with same colors and consequently same structural orientations were also obtained by applying a strong magnetic field of 5.9 T parallel to the composite surface (i.e. parallel to the expected Laponite platelet orientation; see Figure S24 in the Appendix) instead of applying mechanical shearing forces.

Figure 4.12: Quantitative birefringence optical micrographs (Abrio). Laponite/P4 composite materials with Laponite:P4 = 1:1 w/w (upper row) and 2:1 (lower row). Composites were sheared by rotation (left column) or by lateral shearing (right column) and investigated in the dry state.

A phase-contrast-enhanced monochromatic (10 KeV) radiograph and a slice in a tomographic reconstruction of a Laponite/P4 1:1 w/w composite, obtained after shearing by rotation, is shown in Figure 4.13. The radiograph (Figure 4.13a) shows a projection through the sample, where the phase-contrast enhancement is seen mainly at the edges, suggesting that no significant variations in density exist internally, as already indicated by SAXS analysis using a beam size of 400 µm. A typical cross sectional virtual slice through the reconstructed volume is shown in Figure 4.13b, where the rotationally sheared Laponite/P4 1:1 w/w composite surface is on the top and the sample was fixed below. A movie rendering of this

62 data (see enclosed DVD) shows top and side views of the sample and reveals the topography and irregular surface texture.

Figure 4.13: Phase-contrast-enhanced monochromatic (10 KeV) radiograph (a) (gray level intensities correspond to the extent of transmission where 1.00 corresponds to complete transmission and values higher than 1.00 are due to interference fringes localizing at interfaces) and a tomographic reconstruction slice (b) of a dried composite sample consisting of Laponite/P4 1:1 w/w, obtained after shearing by rotation.

SEM analysis of the Laponite/P4 composites (images of the 2:1 w/w sample shown in Figure 4.14) indicates that specimens are composed of layers possessing a thickness of only ~50 nm, which is below the resolution revealed by micro-tomography.

Figure 4.14:SEM images of a dried Laponite/P4 2:1 w/w composite obtained after shearing by rotation. Images of the cross section (fracture surface) of the composite reveal the layer structure on the length scale of ~50 nm, which is illustrated by increasing the magnification from left to right.

SAXS measurements were performed to obtain structural information from the specimens with the incident X-ray beam perpendicular and parallel to the expected orientation of Laponite platelets (shearing direction). Figure 4.15 presents representative 2D SAXS patterns from a Laponite/P4 1:1 w/w sample produced by rotation. With the beam perpendicular to the rotational direction, an isotropic pattern was obtained (Figure 4.15a), suggesting that the Laponite platelets are facing the beam. With the beam parallel to the rotational direction, the 2D pattern is anisotropic (Figure 4.15b), indicating the orientation of the platelets as portrayed in Figure 4.15c. Similarly, anisotropic patterns were observed for all specimen preparations

63 (lateral and rotational shearing) regardless of the Laponite content. The SAXS intensity from Figure 4.15 was radially integrated in order to obtain information on the packing of the Laponite platelets.

Figure 4.15: Representative SAXS 2D patterns from dried Laponite/P4 1:1 w/w composites prepared by rotation, obtained with the incident beam perpendicular (a) and parallel (b) to the shearing direction.

Figure 4.16: Representative SAXS plots of Laponite reference and Laponite/P4 1:1 w/w and 2:1 w/w composites prepared by rotation. Solid and dashed lines represent integrated data obtained parallel and perpendicular to the rotational direction, respectively. Insert: Kratky SAXS plot of the same data.

Parallel to the shearing direction (solid lines in Figure 4.16), a shoulder is visible in the region Q = 3-5 nm^-1, which cannot be explained by the form factor of the Laponite platelets computed on the basis of their known shape. From the Q region where the shoulder appears, it was found that the stacking distance between platelets must be in the order of 1 nm and therefore in the direction perpendicular to the Laponite platelet surface. To a first approximation, the radially averaged SAXS intensity of a stack of cardlike Laponite platelets

64 can be expressed as I(Q) = (2π/Q²)I₁(Q), where I₁(Q) is the squared Fourier transform of the electron density variation in the direction perpendicular to the platelets.¹⁶⁴ Therefore, a Kratky plot, Q²I(Q), was included in the inset of Figure 4.16. This graph shows clearly visible maxima that can be directly interpreted as 2π/d, where d is the mean spacing between stacked Laponite platelets. For pure Laponite, a d spacing of ~1.3 nm was found. With the addition of polymer, this peak shifts to smaller Q values corresponding to d spacings of ~1.5 nm (Laponite/P4 2:1 w/w) and ~1.8 nm (1:1 w/w).

When comparing integrated intensities for the sample Laponite/P4 1:1 w/w of SAXS data collected with the beam perpendicular and parallel to the shearing direction (dashed blue line and solid blue line in Figure 4.16, respectively), a shoulder at Q of approximately 0.25 nm^-1 appears. This value corresponds to a d spacing in the size range of the known diameter of the platelets (~25 nm), suggesting edge-to-edge packing. At the same time, the shoulder corresponding to the stacking of the platelets (d) disappears.

Therefore, it was concluded that the Laponite platelets form a closely packed columnar LC phase that is embedded in a polymer matrix (sketch in Figure 4.16). In every column, the thickness d of the polymeric layer between the platelets depends on the polymer to Laponite ratio. Indeed, the greater the amount of polymer used, the larger the thickness of the polymeric layer between Laponite platelets.

Additionally, the shear-induced hierarchically organized structuring of this composite material was visualized via TEM (Figure 4.17a).

Figure 4.17: TEM image (a) and corresponding electron diffraction (b, ED pattern assigned according to Neumann et al.¹⁶⁵) of a cross section of the composite material Laponite/P4 2:1 w/w prepared via shearing by rotation. The inset of the TEM image shows columnar structuring of Laponite nanoparticles as observed via SAXS (see inset scheme in Figure 4.16).

65 into arcs, indicating a slight misalignment (angle of ~13°) of the superstructure. However, it cannot be excluded that the misorientation observed is an artifact due to mechanical stress induced during TEM sample preparation. By combining the layered characteristics of stacks of the nanoplatelets in the samples observed by SAXS and the spot-like TEM pattern, it is concluded that a mesocrystalline arrangement of Laponite nanoparticles was formed.^61,62 µCT and POM indicate homogenous distributions of nanoparticles and polymer as well as long-range orientation over several hundreds of micrometers.

In addition to structural characterization, mechanical characterization using nanoindentation confirmed the existence of anisotropy. The values of reduced elastic modulus and hardness (~8.5 GPa and ~0.45 GPa, respectively) measured in the direction parallel to the columns were almost identical for both Laponite to polymer ratios. The properties increased when measuring in the direction perpendicular to the columns, to a reduced modulus and hardness of ~11 and ~0.5 GPa in the case of Laponite-P4 1:1 w/w and ~14.5 and ~0.6 GPa in the case of Laponite-P4 2:1 w/w, respectively. The obtained results are comparable to previously reported nanoindentation measurements on oriented polymer/Laponite composites prepared by layer-by-layer technique. For example, Patro et al.¹⁶⁶ reported for PVA/Laponite composites reduced modulus and hardness of 10.3 and 0.48 GPa, respectively. Vertlib et al.¹⁶⁷ reported for PDDA/Laponite a reduced modulus and hardness of 6.7 and 0.38 GPa, respectively. Note that in both cases nanoindentation was performed only in one direction, perpendicular to the surface of the platelets.