Effect of the Miscibility between Grafted and Matrix Chains on the Dispersion and Crystallization of Polyethylene-Grafted Silica Nanocomposites

The properties of polymer nanocomposites (PNCs) greatly depend on the dispersion of nanoparticles within the polymer matrix. In this work, linear high-density polyethylene grafted with SiO2 nanoparticles (HDPE1289-g-SiO2, where 1289 indicates the weight-average molar mass of the grafted polyethylene) was synthesized via the "grafting to" method and added to PE matrices with three different molecular weights and branching degrees (LDPE4k, LDPE34k, and HDPE67k) by a solution blending method. The dispersion and crystallization behaviors of PE/HDPE1289-g-SiO2 PNCs were studied by transmission electron microscopy (TEM), small-angle X-ray scattering (SAXS), differential scanning calorimetry (DSC), and polarized light optical microscopy (PLOM). In a low-density polyethylene (LDPE4k) matrix with a small ratio of matrix-to-grafted chain length (P/N), HDPE1289-g-SiO2 nanoparticles are better dispersed when compared with unmodified SiO2, a result related to the low P/N (entropic effect). With the increase of the molecular weight of the matrix (LDPE34k) with high P/N, both the HDPE1289-g-SiO2 and unmodified SiO2 nanoparticles have a poor dispersion with nanoparticles' aggregates in the matrix and similar nucleation effects on the matrix. In the above two systems, we used the P/N parameter commonly used for linear systems as an approximation because the C-13 NMR results showed that LDPE4k and LDPE34k contain only butyl and longer branches (>= 6C). Interestingly, in the high-density polyethylene (HDPE67k) matrix with high P/N, HDPE1289-g-SiO2 nanoparticles are better dispersed than unmodified SiO2 due to the miscibility between the grafted and matrix polyethylene (enthalpic effect). For the HDPE67k matrix, HDPE1289-g-SiO2 nanoparticles exhibit a supernucleation effect on HDPE, with the nucleation efficiency higher than 100% due to their good dispersion. Our results show that for the PE/HDPE1289-g-SiO2 PNCs, the lower the P/N or the more similar the chain structure of the grafted and matrix polymer is, the better the dispersion of HDPE1289-g-SiO2 nanoparticles, thus improving their nucleation ability.