Poly(propylene)-Layered Silicate Nanocomposites: Gas Permeation Properties and Clay Exfoliation

To monitor the mineral surface coverage and energy, the inorganic cations of two clays (montmorillonite) with different surface area/cation have been exchanged by alkylammonium ions, carrying alkyl chains of different number and length. The prepared OMs were free of unreacted organic ions. With increasing length and number of the alkyl chains, an increase in the basal-plane spacing (d-spacing) of the OM was observed. The d-spacing also increased with increasing CEC of the clay (decreasing available area/cation). The OMs were compounded with PP and their effect on the crystallinity and gas-barrier properties of the polymer was investigated. The OM had no influence on the degree of crystallinity of PP under the processing conditions used. Oxygen permeation through the composites decreased, depending on the cross-sectional area of the exchanged organic cation and the CEC of the clay. These parameters control the mineral surface coverage (consequently the surface energy) as well as the tilt angle of the alkyl chains to the mineral surface, and hence the d-spacing. Increasing the length of the alkyl chains and their number per cation enhanced the d-spacing, clay exfoliation, and the gas-barrier properties of the composites. A mixed morphology, consisting of delaminated aluminosilicate layers and OM tactoids of varying thickness was observed but no intercalation took place. The oxygen permeation coefficient of the nanocomposites was found to be a non-linear function of the volume fraction of the inorganic part of the OM.