Studies of structures of poly-ε-caprolactam/montmorillonite nanocomposite: Part 2. Tests of annealed specimens

Using the novel version of thermomechanical analysis (TMA) method, a poly-ε-caprolactam (PA6) and its nanocomposite (PNC) containing 1.6 wt.% of montmorillonite (MMT) were examined. Several disc specimens of those as investigated in the first part of this article were then melted, annealed, and sheared at a rate of 0.5 s⁻¹ engendering ca. 140% strain at 240 °C in a rotational rheometer, and next solidified in ambient air. In PNC as in PA6 specimens prepared in the same way, an amorphous isotropic structure in the surface layer up to 0.5 mm thick was identified, with topological regions differing in thermal expansion properties and related a state of order. This finding differs from other tests, which detect a high level of crystallinity. Probably, it is related with small thickness of the tested layer of material and kinetics of solidification in ambient air.An increase in the free volume fraction, V_f^TMA, evaluated as equal to 3ΔαT_g (Δα is the difference in linear thermal expansion coefficients below and above the glass transition temperature T_g) and resulted from melting, annealing, shearing, and next solidification in ambient air was observed. It suggests that these operations introduce into the specimen some amount of gases (e.g., evaporated water absorbed when cooling) what increases thermal expansion over high-temperature transition of the high-temperature topological region. It means that V_f^TMA is a sum of all voids within a specimen tested (not only a real free volume) independently on their origin. Because of this, it is better to term it as micro or nanoporosity.     

Studies of structures of poly-ε-caprolactam/montmorillonite nanocomposite: Part 1. Tests of compression moulded specimens

A poly-ε-caprolactam (PA6) taken as a reference and its nanocomposite (PNC) containing 1.6 wt.% of montmorillonite were examined. The specimens as discs were prepared by compression moulding at 235 °C and 70 MPa. Using the novel version of TMA, in the PA6 and PNC specimens, within the temperature range from −100 to 250 °C, a semi-crystalline structure with anisotropy of distribution of the more ordered (crystalline?) portion was found in the surface layer up to 0.5 mm thick. The amorphous regions have differed in a state of order (different transition temperatures) and related compactness. The free volume fraction in amorphous regions determined in machine direction (normal to the surface) is increased when in transverse direction is reduced; simultaneously molecular weights of polymer chains between junctions were increased by incorporation of the nanofiller.