Hexadecyl-functionalized lamellar mesostructured silicates and aluminosilicates designed for polymer-clay nanocomposites. Part I. Clay synthesis and structure

Motivated by a need for synthetic clays that can be dispersed and exfoliated in polymer melts without added compatibilizers, lamellar mesostructured silicates and aluminosilicates with covalently attached hexadecyl functional groups (C16-LMS and C16-LMAS, respectively) were prepared by sol-gel syntheses and their structures were characterized. Based on XRD and TEM data, lamellar products with layer spacings of 4.8-4.9 nm were obtained between room temperature and 60 °C (C16-LMS) or 70 °C (C16-LMAS). The degree of condensation of the aluminosilicate layers increased at the higher synthesis temperatures. Attachment of organic groups to the inorganic sheets was confirmed by 29Si solid state MAS NMR and IR spectroscopy. The sheets of C16-LMS consisted of single or double layers of tetrahedral silicate groups, each attached to a hexadecyl chain. C16-LMAS was composed of pyrophyllite-like layers (Si:Al=2) with an octahedral aluminum layer sandwiched between two tetrahedral silicate layers and hexadecyl surface groups. Tetrahedral aluminum sites were also present. The clay layer spacing could be increased to 5.2 nm by addition of tetraethoxysilanes during the synthesis (C16-SiO2-LMAS). C16-SiO2-LMAS was structurally similar to C16-LMAS; however, the presence of additional silicate groups in this structure increased the inorganic layer thickness and introduced further structural disorder.