Exfoliation of Layered Magnesium Aluminum Silicate Platelets in Polymer Hosts Enabled by Cation Chemistry and Temperature

摘要

Montmorillonite-smectite clay consists of anisotropic clay platelets, generally a nanometer in thickness by hundreds of nanometers laterally, bound together by charge balancing cations. Clay dispersion in polymer hosts has been shown to impart a number of improvements including increased barrier properties, fire resistance, reduced coefficient of thermal expansion, and superior mechanical properties all in relation to the near host polymer. The dispersion of clay in a host material occurs first by intercalation, where the host medium penetrates the galleries causing an increase in inter-plate distance, followed by exfoliation, where the clay platelets migrate away from each other disrupting long range order. A high degree of exfoliation can be difficult to achieve in many polymer hosts due to the polarity of the clay platelet and the inorganic nature of naturally occurring cations such as sodium. Dispersion may be facilitated by imparting organic character to the clay through a cation exchange with an organic ammonium salt, and also by introducing polar nature to the intended polymer host. Recently, it has been shown that clay with certain organic cations exhibits spontaneous room temperature and temperature-initiated exfoliation in hydroxyl- and carboxyl-terminated liquid polybutadiene rubbers (CTPB and HTPB, respectively) in the absence of shear. These observations represent a simple process to achieve sought after barrier properties in polymer materials in a relatively simple and reproducible manner. This work represents a more comprehensive examination of the attempted dispersion of a number of different grades of clay in HTPB using polarized optical microscopy and rheology as investigative techniques.