Structural changes of layered alkylsiloxanes during the reversible melting–solidification process

Abstract

Through various in situ analyses, we have revealed the structural changes that occur during the reversible melting–solidification process of layered alkylsiloxanes (C_nLSiloxanes) with carbon numbers (n) of 18 and 16. In situ high-resolution solid-state ¹³C nuclear magnetic resonance (NMR) analysis at controlled temperatures indicates drastic conformational changes of the long alkyl chains during the melting–solidification process. A ¹³C NMR signal at 33 ppm, which shows the highest intensity at room temperature (RT), is assigned to an inner methylene group with an all-trans conformation. As the temperature increases, the 33-ppm signal intensity decreases while the signal intensity at 30.5 ppm simultaneously increases. The 30.5 ppm signal is assigned to an inner methylene group with a trans–gauche conformation. Subsequently, upon cooling, the signal at 33 ppm recovers, even after C_nLSiloxanes have melted. In situ X-ray diffraction measurements at controlled temperatures reveal that the ordered arrangement of the long alkyl chains becomes disordered with elevating temperatures and reordered upon cooling to RT. In situ high-resolution solid-state ²⁹Si NMR analysis shows that the melting–solidification process progresses without any structural change in siloxane sheets of the C_nLSiloxanes. Thus, the in situ analyses show that disordering of the long alkyl chains causes the C_nLSiloxanes to melt. Because the majority of long alkyl chains are packed again in the ordered arrangement with the all-trans conformation upon cooling, the C_nLSiloxanes are reversibly solidified and the C_nLSiloxane structure is recovered.