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Keywords:

  • Nanocomposites;
  • Polymer nanocomposites;
  • Polysiloxanes;
  • Silicates, layered

Abstract

We report the first multi-system study of a layered-silicate dispersion in polysiloxane/layered-silicate nanocomposites. A variety of layered silicates (montmorillonite, synthetic fluoromica, laponite, and fluorohectorite) and cationic modifiers (single-, twin-, and triple-tailed surfactants with tails of varying lengths and both primary and quaternary head-groups) are combined to form organically modified layered silicates, which are then screened for compatibility with low-molecular-weight silanol-terminated poly(dimethylsiloxane) (PDMS). Promising combinations are then selected and studied in greater depth with respect to both molecular weight and polysiloxane end-group and substituent chemistry. We find that the PDMS backbone is generally incompatible with the layered silicates, regardless of modification type, and that dispersion in PDMS systems results from the presence of polar end-groups, a result unprecedented in the field of polymer nanocomposites. We go on to quantify the substituent effect, not only with respect to end-group chemistry, but taking into account changes in the polysiloxane backbone itself. For instance, in the absence of polar end-groups we observe dispersion in the case of poly(methylphenylsiloxane) but not poly(3,3,3-trifluoropropylmethylsiloxane). Finally, we apply a new epoxy/amine PDMS curing chemistry to PDMS-nanocomposite production and show higher levels of layered-silicate dispersion than observed in comparable silanol-terminated PDMS-based systems. Our findings serve as an indication of what is necessary to achieve a layered-silicate dispersion in polysiloxane/layered-silicate nanocomposites, and may indicate a more general approach for improving dispersion in systems where the polymer backbone is otherwise incompatible with the layered silicate.