Polyamide-layered silicate nanocomposites by melt processing
Fornes, Timothy Dean
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Polyamide-layered silicate nanocomposites based on nylon-6, 11, and 12 and organically modified montmorillonites (organoclay) were prepared by twin screw extrusion. Carefully designed component structure-nanocomposite morphology and property investigations on these materials were executed to understand why nylon-6 readily exfoliates organoclay. The polyamide structure strongly influences the extent of clay platelet delamination and level of property enhancement, as determined by X-ray, transmission electron microscopy and stress-strain analyses. High molecular weight nylon-6 materials lead to better organoclay exfoliation and greater nanocomposite moduli and yield strengths than lower molecular weight materials; this is attributed to higher levels of shear stress imparted on the clay by the higher viscosity polymer. The ratio of amide to methylene units in the repeat structure of nylon-6 appears to affect the polymer- organoclay affinity since a large increase in aliphatic content, i.e., nylon-6 versus nylon-12, results in less organoclay dispersion and lower reinforcing efficiency. The structure of the organoclay is also critical for producing wellexfoliated nylon-6 nanocomposites. Alkyl ammonium surfactants that cover less montmorillonite surface in the organoclay are more effective at exfoliating clay and generating improved nanocomposite stiffness and strength; such surfactants facilitate more desirable polyamide-silicate interactions, yet maintain sufficient organoclay gallery spacings needed both to overcome the cohesive forces between neighboring platelets and to facilitate polymer intercalation. The source of sodium montmorillonite used to form the organoclay is also important. The superior properties observed in nylon-6 nanocomposites may be explained by conventional ideas of reinforcement as predicted by composite theories like those of Halpin-Tsai or Mori-Tanaka. Based on good agreement between experimental nanocomposite moduli and model predictions it is clear that superior reinforcement stems from the high modulus and aspect ratio of montmorillonite; however, montmorillonite particles clearly affect the proprieties of the polymer phase which may have additional effects on the composite. Differential scanning calorimetry and X-ray analyses show that the clay can alter the nucleation, growth, and type of nylon-6 crystals formed under certain crystallization conditions. Furthermore, exposure of organoclay surfaces during processing can cause considerable polymer degradation and color formation depending upon the type of nylon-6 used and the surfactant structure in the organoclay.