SYNTHESIS AND PROPERTIES OF SELF-ASSEMBLED BRIDGED SILSESQUIOXANES

R. J. J. WILLIAMS

Portoroz, Eslovenia

Congreso; European Polymer Congress; 2007

Bridged silsesquioxanes are a family of organic-inorganic hybrid materials synthesized by the hydrolysis and condensation of monomers containing an organic bridging group joining two (or eventually more) trialkoxysilyl or trichlorosilyl groups.1-3 The organic group, covalently bonded to the trialkoxysilyl groups, can be varied in composition, length, rigidity and functionalization. It can exhibit self-assembling properties that may direct the formation of a tridimensional hybrid network by a self-organization process in the course of the hydrolysis and polycondensation of the trifunctional silyl groups. This nanostructuring may be used to tune the properties of the hybrid material. Three examples will be analyzed where the self-assembly of organic bridges was produced by a different kind of chemical interactions. In the first example nanostructuration was produced by incorporation of a pendant dodecyl chain in the organic bridge. The precursor of this hybrid was obtained by the reaction of glycidoxypropyl(trimethoxysilane) (GPMS) (2 moles) with dodecylamine (1 mol). Polycondensation was produced with formic acid, either in mass or using tetrahydrofuran or isopropanol as solvents. The resulting bridged silsesquioxane was characterized by the presence of both ordered and disordered domains. Experimental evidence obtained from SAXS, WAXS, 29Si NMR, FTIR, HRTEM and SAED techniques suggested that the basic structure of ordered domains consisted of hybrid organic-inorganic multilayers separated by hydrophobic regions with a thickness equal to the length of a tail-to-tail association of dodecylamine chains in all-trans conformations. To our knowledge this is the first example of the presence of this kind of structure in a crosslinked hybrid material. A hierarchical organization of ordered domains into semicylindrical shells was observed in a microscopic scale. Due to the presence of pendant hydrophobic chains, the precursor of this hybrid material may be used for the dispersion of hydrophobic molecules or of nanoparticles stabilized by hydrophobic chains.                                   Figure 1.  Tail-to-tail association of two dodecyl chains.   In the second example a bridged silsesquioxane was obtained from a precursor synthesized by the reaction of glycidoxypropyl(trimethoxysilane) (2 moles) with cyclohexylamine (1 mol). The polycondensation in the presence of formic acid produced a short-range order based on elongated organic channels accommodating the pendant cyclohexyl fragments, bounded by inorganic ladders. The presence of functional groups in the organic channels (tertiary amine, ether, hydroxyl), can be used to retain small organic molecules capable of forming hydrogen bonds. Aspirin was used as a probe to illustrate this possibility. The addition of two aspirin molecules per organic bridge produced its dispersion at a molecular level inside the self-assembled structure. It was suggested that aspirin was mainly incorporated inside the organic channels as inferred from the increase of the separation between inorganic domains revealed by SAXS (decrease in the height of the channels), increase in the mass density and no perturbation of the lamellar structure observed in SEM micrographs. Therefore, the material may be used as a host of small organic molecules capable of forming H-bonds with the functional groups present in the organic bridges.                                        Figure 2.  Scheme of the self-assembled structure.   In the third example the precursors of the bridged silsesquioxanes were two different aromatic diamines: 4,4’-[1,3 phenylenebis-(1-methylethylidene)]bis(aniline) (BSA) and Safranin-O (Saf-O), end-capped with 3-isocyanatopropyltriethoxysilane. The molar ratio of Saf-O/BSA was close to 1/1000. The inorganic polycondensation was produced using either formic or acetic acid leading to nanostructured films with an intense pink colour. In this case nanostructuration was produced by the self-assembly of organic bridges promoted by the H-bonds of neighbouring urea groups. The covalent bonding of Saf-O in the bridged silsesquioxane was proved by placing the films in contact with ethanol (a good solvent for Saf-O) during 1 week, removal of solvent and drying. Films kept their intense pink colour indicating that the colorant was covalently bonded in the crosslinked structure. The films exhibited a strong photoluminescence peak characteristic of Saf-O centered at about 610 nm when using formic acid and 650 nm when employing acetic acid. The excitation spectra were very broad extending over the wavelengths of the whole visible region. The highest intensity of the photoluminescence peak was observed when exciting at 550 nm. The method used to synthesize this particular bridged silsesquioxane may be extended to incorporate other colorants bearing amine groups in their structures.   References 1.        D.A. Loy, K.J. Shea, Chem. Rev. 1995, 95, 1431. 2.        R.J.P. Corriu, Angew. Chem., Int. Ed. 2000, 39, 1376. 3.        K.J. Shea, D.A. Loy, Chem. Mater. 2001, 13, 3306.