Polyamide-layered silicate nanocomposites by melt processing
Abstract
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.
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