Synthesis Of LLDPE-g-MAH And Its Compatibilization To LLDPE-matrix Composites

Inorganic-organic hybridization is one efficient way to prepare high-performance polymeric materials, and melting compounding technique is widely used. There exists a major challenge to uniformly disperse the inorganic reinforcers in polymer matrix. In the dissertation, maleic-anhydride grafted linear low-density polyethylene copolymers (designated as LLDPE-g-MAH) with high grafting degree and low gel content were synthesized by lower molecular weight linear low-density polyethylene (LLDPE, melt flow index of 49.4g/10min) and solid-phase grafting technology. Then, LLDPE-g-MAH was used as a compatibilizer to prepare LLDPE/LLDPE-g-MAH/talc, LLDPE/LLDPE-g-MAH/SiO2 and LLDPE/LLDPE-g-MAH/TiO2composites. The structure and properties of the above LLDPE-g-MAH and composites were characterized by Fourier transform infrared spectroscopy (FTIR), differential scanning calorimetry (DSC), dynamic mechanical analysis (DMA), wide-angle X-ray diffraction (WAXD), polarized optical microscopy (POM), capillary rheometer and mechanical testers. Based on experiment results, the main conclusions were drawn as following:(1) By means of lower molecular weight linear low-density polyethylene (melt flow index of 49.4g/10min) and solid-phase grafting technology, maleic anhydride is successfully grafted onto LLDPE with the grafting degree ranged from 1.1 to 2.4% and the gel content less than 3.7%, the chain-branching dominates the side reactions for the solid-phase grafting polymerization of MAH onto the lower molecular weight polyethylene. The melting temperature of LLDPE-g-MAH copolymers is higher than that of pure LLDPE due to their increased molecular polarity via the introduction of MAH into LLDPE chains. However, the degree of crystallinity and crystallization rate of LLDPE-g-MAH copolymers decrease due to the branching. And the apparent viscosity of LLDPE-g-MAH copolymers is higher than that of LLDPE, the shear-sensitivity of LLDPE-g-MAH copolymers is weaker than that of LLDPE due to the difficulties in disentanglement under shear force.(2) The compatibilization of LLDPE-g-MAH enhances the exfoliation and dispersion of talc in LLDPE matrix, and improves the interfacial adhesion between talc layers and LLDPE. The addition of LLDPE-g-MAH and talc does not change the crystal structure of LLDPE, but their heterogeneous nucleation increases the crystal size, melting point (Tm) and crystallization temperature (Tc) of LLDPE in composites. As the loading content of talc is up to 30%, Tm and Tc of LLDPE in composites decrease subsequently. Since talc layers and the gels in LLDPE-g-MAH have the prohibition to the diffusion of composite melts, degree of crystallinity and crystallization rate of LLDPE in composites decrease obviously. Additionally, the uniformly-dispersed talc layers have the reinforcement to LLDPE matrix, leading to the increasing in the tensile strength of the composite. And impact strength of the composite with 2.5wt% of LLDPE-g-MAH and 20wt% of talc increases by 36% from 17.7kJ/m2 of LLDPE. It is because the dispersion of talc is multi-scale from nano to micro size. During impact fracture, the layer-by-layer structure of talc can be exfoliated, leading to absorb the fracture energy and toughen the LLDPE matrix.(3) The compatibilization of LLDPE-g-MAH enhances the dispersion of nanosilica in LLDPE matrix, and improves the interfacial adhesion between silica and LLDPE. The addition of LLDPE-g-MAH and nanosilica does not change the crystal structure of LLDPE, but their heterogeneous nucleation increases the crystal size, melting point (Tm) and crystallization temperature (Tc) of LLDPE in composites. Since nanosilica and the gels in LLDPE-g-MAH have the prohibition to the diffusion of composite melts, degree of crystallinity and crystallization rate of LLDPE in composites decrease obviously. Additionally, the nanosilica has no reinforcement to LLDPE matrix. During impact fracture, the uniformly-dispersed nanosilica particles act as stress concentrators leading to the yielding of matrix, debonding at the nanoparticle-polymer interface, leading to toughening the composite; On the other hand, the dispersion of nanosilica particles is multi-scale, the bigger silica aggregates can be exfoliated, leading to absorb the fracture energy and toughen the LLDPE matrix.(4) The compatibilization of LLDPE-g-MAH enhances the dispersion of nanotitania in LLDPE matrix, and improves the interfacial adhesion between titania and LLDPE. The addition of LLDPE-g-MAH increases the crystallization temperature (Tc) and glass transition temperature (Tg) of LLDPE in composites. Since nanotitania and the gels in LLDPE-g-MAH have the prohibition to the diffusion of composite melts, degree of crystallinity and crystallization rate of LLDPE in composites decrease obviously. LLDPE/LLDPE-g-MAH/TiO2 composite melts exhibit non-Newtonian and shear thinning characteristics in the range of applied shear rates, the apparent viscosity of LLDPE/LLDPE-g-MAH/TiO2 composite melts is higher than that of LLDPE, and increases with increasing the content of LLDPE-g-MAH in composite. Under higher shear rate, the apparent viscosity of LLDPE/LLDPE-g-MAH/TiO2 composite melts is closed to that of LLDPE. Additionally, the nanotitania has no reinforcement to LLDPE matrix. During impact fracture, the uniformly-dispersed nanotitania particles act as stress concentrators leading to the yielding of matrix, debonding at the nanoparticle-polymer interface, leading to toughening the composite.