Hybrid Organic-Inorganic Materials Based on Polyhedral Silsesquioxanes Functionalized with OH Groups.

DIANA P. FASCE; SERGIO A. PELLICE; ROBERTO J. J. WILLIAMS

Viña del Mar - Chile.

Workshop; International Workshop “Frontiers in Materials Science”.; 2002

CENTRO PARA LA INVESTIGACION INTERDISCIPLINARIA AVANZADA EN CIENCIAS DE LOS MATERIALES

Polyhedral silsesquioxanes (POSS), (RSiO1.5)n or Tn, where n is an even number and R= H, Cl or a variety of organic groups, are a unique class of materials that have begun to occupy a sizable niche in polymer chemistry. The possible route for the synthesis of POSS is the hydrolysis and condensation of trialkoxy-  or trichlorosilanes. But this path frequently leads to a distribution of random structures, imperfect polyhedra and a small fraction of the desired products, which leads to the need of fractionation and purification steps. During a systematic study of the hydrolytic condensation of different silanes, we found a particular system leading to a very high yield of POSS without the need of fractionation steps. N-(b-aminoethyl)-g-aminopropyltrimethoxysilane was first reacted with a stoichiometric amount of phenylglycidylether (PGE), transforming the amine functionalities into 3 OH groups/mol. Its hydrolytic condensation was performed in tetrahydrofuran (THF), at 50 ºC during 24 h, using a variety of catalysts (HCl, NaOH, HCOOH). Main condensation products were T8, T9(OH) and T10, with traces of T7(OH) and T11(OH), as revealed by mass spectrometry (MALDI-TOF MS). The synthesized POSS contained 3n OH groups/mol (the OH groups are located in the flexible organic arms issuing from the rigid inorganic polyhedron). SiO2-enriched products were also obtained by adding tetraethoxysilane (TEOS) to the initial formulation. This led to an organically-modified silica that was soluble in different solvents. Either POSS, SiO2-enriched products, or their precursors, were used to synthesize different types of hybrid organic-inorganic materials, based on polyepoxides or polyurethanes. Polyepoxides were synthesized by two different ways. a) An epoxy monomer based on diglycidylether of bisphenol A (DGEBA), was added to the formulation containing the modified silane and TEOS in THF. Formic acid was used both as a catalyst of the hydrolytic condensation and an initiator of the epoxy homopolymerization. After a thermal treatment including solvent evaporation, modified-epoxy networks with a glass transition temperature  (Tg) in the range of 50ºC, could be obtained. b) The synthesized SSO was dissolved in the epoxy monomer (DGEBA), including a small amount of benzyldimethylamine (BDMA), to initiate the epoxy homopolymerization. Cure was performed using a thermal cycle with a final step at 120 ºC. Covalent bonds between the SSO and epoxy network were produced by chain transfer steps involving OH groups. The SSO-modified epoxy networks exhibited a decrease of both the glass transition temperature and the rubbery modulus when increasing the SSO content. This behavior was produced by the flexibility of the organic arms attached to the rigid inorganic cores and by the decrease in crosslink density, caused by chain transfer steps (decrease in the length of primary chains). However, both the elastic modulus in the glassy state and the yield strength, increased with the SSO amount in the formulation, revealing the increase in the packing density of the final network. The OH-functionalized POSS were also used in the synthesis of polyurethane networks. Formulations containing linear macrodiols (polycaprolactones) of different molar masses, variable amounts of POSS, and a cycloaliphatic diisocyanate led to rubbery hybrid materials. Depending on the molar mass of the polycaprolactone and in consequence of its ability to crystallize, hybrids  are either transparent or opaque. Thermal and rheological properties of POSS-modified polyurethanes will be reported.