Synthesis and hierarchical structure of cross-linked Laponite/polymer

4.5.2 Synthesis and hierarchical structure of cross-linked Laponite/polymer composites

For polymer cross-linking, poly[2-(3-butenyl)-2-oxazoline] (number-average molecular weight of M_n ~ 9 500 g mol^-1, i.e. average number of repeat units, n ~ 75, polydispersity of 1.3) was functionalized with 3-MPA (10% of double bonds) and Chol-SH (5% of double bonds) (Scheme 4.5). The intention of using a polymer with a lower degree of functionalization was to preserve more double bonds for the later cross-linking. The formation of polymeric lyotropic phases on the length scale of several hundreds of micrometers upon lateral shearing of this LC polymer in one direction was observed by Abrio (see Figure S19 in the Appendix).

Scheme 4.5: LC ‘gluing‘ statistical copolymer PBOx-Chol-MPA obtained by simultaneous modification of poly[2-(3-butenyl)-2-oxazoline] with thiocholesterol and 3-MPA ([C=C]/[Chol-SH]/[MPA] = a/b/c = 0.85/0.05/0.10). The precursor polymer exhibited an average number of repeat units of n ~ 75.

A viscous solution of LC polymer, cross-linking agent 2-hydroxyethyl methacrylate (HEMA) and photoinitiator 2-hydroxy-4’-(2-hydroxyethoxy)-2-methylpropiophenone (Irgacure 2959) was sheared and exposed to UV light, initiating the photo-crosslinking reaction. The fraction of LC polymer chains incorporated into the formed cross-linked organic matrix vs. the fraction of free LC polymer chains was determined by GPC analysis. To this end, the dried cross-linked LC polymer film was mechanically crushed. After swelling the polymeric material in DMF, followed by exhaustive ultrasonication, the suspension was centrifuged and the supernatant dried in vacuum. The small amount of residue isolated from the supernatant

70 was dissolved in NMP and submitted to GPC analysis, revealing that the residue consists of low-molecular weight products of ca. 2 100 g mol^-1 (PDI of 2.6), probably HEMA oligomers (for comparison, the LC polymer exhibits a molecular weight of 18 900 g mol^-1 and a dispersity of 1.3 according to GPC). This result indicates that the LC polymer chains were quantitatively cross-linked, forming an organic matrix that exhibits lyotropic phases over several hundreds of micrometers (see Figure S28 in the Appendix), as observed for the non-cross-linked polymer films.

The reaction conditions were applied for cross-linking the organic matrix of Laponite/polymer composites. Cross-linked Laponite/polymer 1:1 and 2:1 w/w composites were prepared analogously to the corresponding non-cross-linked Laponite/polymer composites. The only difference was the addition of HEMA and Irgacure 2959 during sample preparation. Drying of the sheared composites was carried out under UV light to initiate the photo-crosslinking reaction. Including HEMA, the cross-linked composites consist of final inorganic/organic ratios of 0.6:1 w/w (Laponite/LC polymer 1:1 w/w) and 1.2:1 w/w (Laponite/LC polymer 2:1 w/w). Analogous to the pure polymer film, the fraction of LC polymer chains incorporated into the cross-linked organic matrix vs. the fraction of free LC polymer chains was determined by GPC analysis. As for the pure polymer, only very small amounts of low-molecular weight products, probably HEMA oligomers, were isolated after swelling in DMF (M_n = 1 200 g mol^-1 and PDI = 1.8 for the Laponite/polymer 1:1 w/w composite, and M_n = 1 100 g mol^-1 and PDI = 1.8 for 2:1 w/w composite), suggesting quantitative cross-linking of the LC polymer chains.

The cross-linked composites were structurally investigated in the dry state from the millimeter to the micrometer down to the nanometer scale by Abrio, SEM and SAXS analysis. As for the corresponding non-linked composites (cf. Chapter 4.4), the Abrio images of the cross-linked composites reveal the presence of lyotropic regions with the same color, suggesting the same structural orientation, on the length scale of several hundreds of micrometers, independent of the ratio of organics to inorganics used (Figure 4.18).

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Figure 4.18: Quantitative birefringence optical micrographs (Abrio). Cross-linked Laponite/PBOx-Chol-MPA composite materials with Laponite:LC polymer = 1:1 w/w (left) and 2:1 (right). Composites were sheared by rotation, followed by crosslinking under UV irradiation until being dried.

Figure 4.19: SEM images of cross-linked Laponite/PBOx-Chol-MPA 2:1 w/w composite obtained after shearing, followed by cross-linking with UV light until being dried. Images of the cross section (fracture surface) of the composite reveal a layered structuring on the length scale of ~50 nm (magnification increases from left to right).

Figure 4.20: Representative SAXS 2D patterns from cross-linked Laponite/LC polymer 2:1 w/w composites, obtained with the incident beam perpendicular (a) and parallel (b) to the shearing direction.

72 SEM analysis of the cross section of the cross-linked composites suggests that the samples consist of layers possessing a thickness of ~50 nm (see Figure 4.19, Laponite/LC polymer 2:1 w/w, and Figure S29 in the Appendix, Laponite/LC polymer 1:1 w/w). A similar structuring was observed for the corresponding non-cross-linked composites (cf. Chapter 4.4).

Representative 2D SAXS patterns from a Laponite/LC polymer 2:1 w/w sample are shown in Figure 4.20. As for the corresponding non-cross-linked composites, an isotropic 2D pattern was obtained when measuring with the incident X-ray beam perpendicular to the rotational direction (Figure 4.20a), while an anisotropic pattern was obtained when measuring with the beam parallel to the shearing direction (Figure 4.20b). These observations indicate that the Laponite platelets are arranged as schematically shown in Figure 4.20c. In order to compare the packing of the Laponite platelets within the cross-linked organic matrix with the packing of Laponite within the non-cross-linked polymer matrix, the SAXS intensity obtained from both cross-linked composites (Laponite/polymer = 1:1 and 2:1 w/w) was radially integrated.

As observed for the non-cross-linked samples, a shoulder appears in the region Q = 3-5 nm^-1 parallel to the shearing direction for the cross-linked samples (Figure 4.21).

Figure 4.21: Representative SAXS plots of Laponite reference and cross-linked Laponite/PBOx-Chol-MPA 1:1 and 2:1 w/w composites. Solid and dashed lines represent integrated data obtained parallel and perpendicular to the shearing direction, respectively. The SAXS measurement was performed by using a distance between detector and sample of 260 mm (main graph) and 1050 mm (graph lower right). Graph upper right: Kratky SAXS plot of the same data.

Consequently, the stacking distance between the platelets must also be in the order of 1 nm and therefore in the direction perpendicular to the Laponite platelet surface. A Kratky plot, Q²I(Q), obtained from the data of the cross-linked samples, reveals a d spacing between

73 stacked Laponite platelets of ~1.9 nm (Laponite/polymer 2:1 w/w) and ~2.1 nm (Laponite/polymer 1:1 w/w). These d spacings are considerable larger than the d spacings of the corresponding non-cross-linked samples (~1.5 and ~1.8 nm for Laponite/polymer 2:1 and 1:1 w/w, respectively), probably due to the higher organic content deriving from the added crosslinking agent HEMA (see entries 4 and 5 in Table 4.1, Section 4.6). Notably, the observed stacking when measuring parallel to the shearing direction and the lacking of the corresponding shoulder in the respective Q range when measuring perpendicular to the shearing direction is even more pronounced for the cross-linked composites compared to the non-cross-linked samples. The higher degree of orientation in the cross-linked samples might be due to fixing the alignment of nanoparticle when cross-linking the organic matrix during the drying process. When comparing the integrated intensities obtained with the beam perpendicular and parallel to the shearing direction, exemplary for the cross-linked Laponite/LC polymer 2:1 w/w composite (dashed red line and solid red line, respectively, in the graph lower right, Figure 4.21), a shoulder at Q of ~0.25 nm^-1 is only visible when measuring perpendicular to the shearing direction. This Q value correlates with a d spacing in the size range of the diameter of the Laponite platelets (~25 nm) and consequently indicates an edge-to-edge packing of the platelets, which was also observed for the corresponding non-cross-linked composites.