Structure, Microscopic Ordering, and Viscous Properties of Amorphous Poly( n ‐alkylsilsesquioxane) Liquids and Solids Synthesized by Cosolvent‐Free Hydrolytic Polycondensation of n ‐Alkyltrimethoxysilanes

Poly( n ‐alkylsilsesquioxane) [poly(R‐SQ)] liquids with R = Et, Pr, or Bu are prepared from acid‐catalyzed n ‐alkyltrimethoxysilane – water binary systems through hydrolytic polycondensation followed by aging. The resultant liquids contain a lot of SiOH groups whereas the viscosities remain almost constant during storage at room temperature over a month. Heat treatment for 1 day at 200 °C in N 2 yields poly(Et‐SQ) and poly(Pr‐SQ) solids whereas poly(Bu‐SQ) remains viscous. High‐energy X‐ray diffraction measurements confirm that all samples are amorphous and nonporous. The total structure factor S ( Q ) of poly(R‐SQ)s and amorphous silica are similar, apart from the prominent prepeak observed for poly(R‐SQ)s at Q  ≈  0.5 − 0.7 Å −1 . This prepeak undergoes a distinct shift to lower Q with an increase in the alkyl chain length. These observations indicate the presence of microscopic inhomogeneity consisting of silica‐rich and alkyl‐rich domains and their ordering with a periodicity of ≈9 − 12 Å both in liquid and solid forms of poly(R‐SQ). Thermal curing modifies S ( Q ) of poly(Et‐SQ) only slightly. The overall shapes of the total correlation function T ( r ) are similar between poly(R‐SQ)s and a ‐SiO 2 , suggesting the similarity of short‐range order of Si−O network between these materials.