Isothermal crystallization of polypropylene/surface modified silica nanocomposites

In the present work, 3-methacryloxypropyltrimethoxy-silane silanized silica (SiO₂-WD70) and 9, 10-dihydro-9-oxa-10-phosphaphenanthrene-10-oxide immobilized silica (SiO₂-WD70-DOPO) nanoparticles were prepared. Silica, SiO₂-WD70 and SiO₂-WD70-DOPO were incorporated into polypropylene (PP) by melt compounding. Differential scanning calorimetry (DSC), X-ray diffraction (XRD) and polarized optical microscopy (POM) were employed to investigate the isothermal crystallization behavior of PP and PP/silica composites. The kinetic constant (k_n), and half crystallization time (t_1/2) were calculated by Avrami equation, while the surface free energy of folding was calculated by Lauritzen-Hoffman theory. The increased k_n, decreased t_1/2 and the surface free energy (σ_e) in the order of PP, PP/SiO₂, PP/SiO₂-WD70 and PP/SiO₂-WD70-DOPO nanocomposites were attributed to the surface modification of silica. XRD indicated that SiO₂-WD70-DOPO addition had no effect on PP crystal structure but accelerated the crystallization rate. POM determined that SiO₂-WD70-DOPO addition promoted the nucleation of PP by inducing a higher nucleation density during isothermal conditions. The surface modified nanoparticle SiO₂-WD70-DOPO might find possible application as a new type of inorganic nano-sized nucleation agent for PP.