Poly(alkyl methacrylate)-grafted silica nanoparticles in polyethylene nanocomposites

Abstract

Surface-initiated reversible addition-fragmentation chain transfer (SI-RAFT) polymerization has been widely used to synthesize various polymers grafted from nanoparticles (NPs) for incorporation into polymer nanocomposites. It is believed that these grafted polymer brushes, with a similar chemistry as the matrix polymer, can be employed to improve NP dispersion by reducing unfavorable interactions between the inorganic NPs and organic matrices. While controlled radical polymerization methods do not allow the polymerization of polyolefins, a substitute strategy is controllably attaching polyolefin-like polymers onto the NP surface. In the present work, the SI-RAFT polymerization was used to anchor poly(hexyl, lauryl, and stearyl methacrylate) on silica NPs, showing good control of the polymerizations. The long alkyl side chains can create an “olefin-like” interface and improve the compatibility of modified particles with polyolefins. Subsequently, we investigated the dispersion of these poly(alkyl methacrylate)-modified silica NPs in linear low density polyethylene (LLDPE). Poly(stearyl methacrylate)-grafted silica NPs (PSMA-g-SiO2) demonstrated improved dispersion of particles when compared to shorter alkyl side chain methacrylates. TEM images showed that the dispersion of these particles was highly dependent upon the molecular weight and density of the grafted PSMA chains. Differential scanning calorimetry (DSC), wide-angle X-ray scattering (WAXS), small-angle X-ray scattering (SAXS), and dynamic mechanical analysis (DMA) were used to characterize these nanocomposites. SAXS showed that the inter-particle distance (distribution of particle spacings) in the semicrystalline state was broader than in the melt, suggesting that particles spacing was affected by the polyethylene crystallization particularly at lower loadings. Nanocomposites at low loadings, 0.5 wt% core content, showed significant improvement in storage modulus due to the compatible particle-matrix interface. Further increases in particle loadings, however reversed this trend likely due to the increase in soft PSMA content.