Influence of alkyl groups on the formation of softenable polysilsesquioxanes

Abstract

Polysilsesquioxanes (PSQ) containing phenyl groups as substituents on the silicon atom can form transparent, hard and glassy materials at room temperature, which reversibly soften when heated above the glass transition temperature. Further increase in temperature leads to irreversible curing. With this property, polyphenylsilsesquioxanes can be assigned to the so-called melting gels. In contrast to the aromatic systems, polysilsesquioxanes with alkyl groups belonging to this class of materials are not known. To identify structural differences between aryl and alkyl systems, polyalkylsilsesquioxanes (RPSQs with R = methyl, ethyl, propyl, hexyl, octyl, decyl, dodecyl, hexadecyl, and octadecyl) were synthesised by a solvent-free, acid-catalysed hydrolysis and condensation reaction of alkyltrimethoxysilanes followed by thermal treatment at 300 °C under N2 atmosphere. The influence of the alkyl chain length on the structure built and the thermally initiated further condensation reactions were investigated via NMR, FTIR, TGA, DSC, SEC, and XRD. Depending on the alkyl chain length, the formation of highly crosslinked, insoluble systems (Me-PrPSQ), low molecular weight oligosilsesquioxanes in the form of cages (Hex-DecPSQ) to semicrystalline, lamellar layers (Dodec-OctadecPSQ) were detected. A low degree of condensation, inhibition of self-assembly and preferentially intermolecular condensation reactions were found to be crucial factors in the melting gel formation.Graphical So called melting gels are usually prepared by acid-catalysed hydrolysis and condensation of aromatic trialkoxysilanes. The final materials show a thermoplastic and a thermoset temperature regime based on the stabilisation of silanol groups in the material. In this study we transferred the typical synthetic conditions to alkyltrialkoxysilanes with different alkyl chain lengths and obtained completely changed structures of the synthesised polyalkylsilsesquioxanes compared to their aromatic counterparts. Depending on the chain length, densely cross-linked solids, viscous gels with cage-like structure and waxes with a lamellar composition were formed. Moreover, missing stabilisation of the OH groups and self-assembly phenomena, which favour the cage and semicrystalline layer formation were detected in the resulting materials, which leads to the loss of the typical melting gel properties.