Shear and extensional rheometry of PA6 reinforced with polyacrylic nanoparticles

Shear and extensional rheometry was carried out on polyamide 6 (PA6) reinforced with crosslinked polyacrylic nanoparticles (PNPs) with mean size of 8 nm. The PNPs were dispersed into a commercial, injection grade, PA6 matrix by melt extrusion, at a concentration of 3 wt%. Thermal analysis showed that the PNPs did not influence the melting and decomposition temperature of the polymer matrix. However, grafting of maleic anhydride to the PNPs (denoted PNP-g-MA) increased the decomposition temperature. On the other hand, X-ray scattering and small-angle light scattering showed that the degree of crystallinity and crystal size were reduced, relative to the neat PA6, i.e., the PNPs disrupted the ordering of the polymer chains. The shear rheological properties showed that the PA6/PNP nanocomposites exhibited a linear viscoelastic behavior. Small-strain oscillatory shear showed that PA6 exhibited a predominantly viscous behavior. However, addition of PNPs induced a predominantly elastic behavior, as measured by the mechanical damping tan δ (=G″/G′), and increased the zero-shear viscosity. The increase in melt elasticity and viscosity was greater for the PA6/PNP-g-MA nanocomposite. Extensional rheometry experiments demonstrated that when PNPs were added to PA6, they induced smaller extensional viscosity, ηext, values in the matrix, at low strain rates. However, at higher strain rates the PNPs induced a strain hardening behavior. Whereas the neat polymer melt rapidly broke under extensional flow, the PA6/PNP nanocomposites first exhibited lower ηext than the neat PA6, and then ηext rapidly increased before breaking, i.e. a strain hardening behavior. The higher melt elasticity of the molten PA6 nanocomposites appears to arise from a jamming effect promoted by the PNPs.