Reversibly meltable layered alkylsiloxanes with melting points controllable by alkyl chain lengths

Abstract

Meltable layered alkylsiloxanes (C_nLSiloxanes) were synthesized from tetraethoxysilane and alkyltrialkoxysilanes with carbon numbers (n) of 12, 16, and 18 via hydrolysis and polycondensation at 100 and 150 °C under basic conditions. Differential scanning calorimetric (DSC) measurements revealed that C_nLSiloxanes melted reversibly at −0.8 to 51.3 °C, the melting points being dependent on n. Scanning electron microscope (SEM) images showed that thin plates were stacked. X-ray diffraction (XRD) peaks were observed at angles corresponding to distances (d) of 2.1, 2.5, and 2.8 nm for C₁₂-, C₁₆-, and C₁₈LSiloxanes, respectively. High-resolution solid-state ¹³C nuclear magnetic resonance (NMR) measurements showed that the organic moieties were alkylsilyl groups with long alkyl chains and that an all-trans conformation was dominant. This was supported by the XRD peak corresponding to a d value of 0.41 nm. High-resolution solid-state ²⁹Si NMR measurements demonstrated the presence of SiO₄ and CSiO₃ units. A structural model has been proposed for C_nLSiloxanes, where siloxane sheets consisting of the SiO₄ and CSiO₃ units are stacked with the ordered interdigitated monolayer of the alkyl chains in between and are bonded covalently with the alkyl chains.