Physical aging and mechanical performance of poly(L-lactide)/ZnO nanocomposites

In this work nanocomposites based on poly (L-lactide) (PLLA) and zinc oxide (ZnO) nanoparticles with a concentration up to 5 wt % have been prepared by solvent-precipitation followed by compression moulding at 200 degrees C. Structural evolution of nanocomposites as a function of time and nanoparticle concentration has been monitored by differential scanning calorimetry (DSC). Results reveal a marked reduction of the enthalpy relaxation rate beta(H) from 3.273 J/g for neat polymer to 0.912 J/g for its 0.25 wt % reinforced counterpart, revealing slower aging dynamics induced by zinc oxide. It is shown by field emission scanning electron microscopy (FE-SEM) that concentrations larger than 1 wt % yield nanoparticle agglomeration. These large aggregates decrease the amount of nanoparticle surfaces exposed to PLLA chains, notably reducing the efficiency of ZnO nanoparticles to delay the physical aging of its hosting matrix. Mechanical tests show an increased stiffness upon ZnO loading as denoted by the increase in modulus from 2310 MPa to 2780 MPa for the 1 wt % nanocomposite. Obtained findings through this work lead the way for the development of nanocomposites based on renewable polymers and natural fillers to be used in packaging applications, where the use of nonbiodegradable materials for short-term applications is extended. (C) 2016 Wiley Periodicals, Inc.