Effects of clay platelets and natural nanotubes on mechanical properties and gas permeability of Poly (lactic acid) nanocomposites

Environmental concerns have resulted in the increased use of biodegradable polymers and their nanocomposites. We have developed a facile method for measuring the work of adhesion (Wa) between nanoparticles and their matrix, which we showed can be used to determine their degree of intercalation within the matrix. This method was then applied to Poly (lactic acid) (PLA) nanocomposites with either sodium montmorillonite clays (C-Na+), organically modified clays (C-30B), resorcinol di (phenyl phosphate) (RDP) coated C-Na+ (C-RDP), Halloysite nanotubes (HNTs) and RDP coated HNTs (H-RDP). The C-30B showed the highest Wa within the PLA matrix, followed by the C-RDP and the C-Na+. X-ray diffraction and TEM indicated that the C-30B platelets were highly exfoliated, the C-RDP were intercalated, while the C-Na+ were aggregated in the PLA matrix. Oxygen gas permeability measurements showed the largest decrease in permeability for the C-30B nanocomposites, followed by the C-RDP, and the C-Na+ nanocomposites. The values of the particle aspect ratios obtained using Nielsen model were consistent with the TEM images. In contrast to clay platelets, HNTs and H-RDP were not effective in decreasing oxygen gas permeability of PLA matrix, which was in excellent agreement with the model we derived. Despite the higher degree of exfoliation, the C-30B nanocomposites had a significantly lower Izod impact than the C-RDP nanocomposites. SEM images showed numerous micro-voids on the Izod impact fracture surface of C-30B nanocomposites. On the other hand, the nanotubes were much more effective in reinforcing the mechanical properties, with RDP coated tubes having the better performance, consistent with their higher value of Wa with PLA matrix. These results showed that RDP coated silicate nanoparticles can provide environmentally safe alternatives for modifying the mechanical and gas barrier properties of PLA.