The use of epoxy silanes on montmorillonite: an effective way to improve thermal and rheological properties of PLA/MMT nanocomposites obtained via “in situ” polymerization

Polylactic acid (PLA) nanocomposites were prepared via “in situ” ring opening polymerization (ROP) of lactide using a montmorillonite, Cloisite®15A, employed after surface treatment with 3-Glycidoxypropyltrimethoxysilane. The dispersion of the nanoparticles was checked using Wide Angle X-ray Scattering (WAXS) and Transmission Electron Microscopy (TEM); both the effect of different amounts of montmorillonite and silane were measured on molecular weights and on thermal and rheological properties, using Size Exclusion Chromatography (SEC), Differential Scanning Calorimetry (DSC), Thermogravimetric (TGA) and Rheological analyses. It was found that even very low amounts (0.1% w/w) of nanoparticles greatly affect nanocomposites properties. Unmodified montmorillonite tends to decrease molecular weights, deactivating the catalytic system used for ROP of lactide but, when epoxy silane is present, molecular weights increase. Melt crystallization temperatures increase with modified nanoparticles, which enhance crystallization process. TGA analyses show that, when pure montmorillonite is present, nanocomposites have lower thermal stability respect to standard PLA; when silane is used thermal stability can get much higher than standard PLA as silane content increases. The rheological behaviour of nanocomposites show that melt viscosity is far higher than the one of standard PLA at low shear rates and also a marked shear thinning behaviour can be achieved.