Study Of Role Of Flow Field On Morphology Development And Properties Of Polypropylene-Based Multiphase Systems

Using the chaotic mixing can avoid the break of the molecular chains and improve the uniformity of the mixing in the polymer processing. Therefore, the chaotic mixing has attracted the attention of both industries and academia during past few years. In this dissertation, polypropylene based multiphase systems were prepared using a single screw extruder. Different screw geometries were used to induce chaotic mixing and shear mixing in the extruder, respectively. The molten multiphase system samples were collected at different locations along the extruder. The morphology, rheological properties, and mechanical properties of the colleted samples were observed and measured. The relationship among mixing type, rheology, phase morphology, and mechanical properties was investigated.The mixing characteristics of the screw elements used in this work are numerically simulated. The results show that the Lyapunov exponents of conventional screw element, pineapple mixing element, chaos screw element, and convective screw element are 0, 0.4, 0.4, and 0.8, respectively. It indicates that the conventional screw element can not induce chaotic mixing, but all the other elements can induce chaotic mixing.PP/poly(ethylene-1-octene) (POE) blends, PP/polyamide 6 (PA6) blends, PP/clay/PA6 blend nanocomposites, and PP/POE/PA6 ternary blends were prepared using the extruder with shear mixing and chaotic mixing, respectively. Online, capillary, and oscillatory rheometers were used to measure the online shear viscosities, shear viscosities and dynamic rheologies of the multiphase system melts, respectively. Stereomicroscope (SM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and X-ray diffraction (XRD) were employed to observe and analyze the morphology and micro stucture of the samples. The size and its distribution of the dispersed particles were quantitatively investigated by analyzing the SEM micrographs. The mechanical properties of the samples were measured.In the shear mixing, the PP pellets are slowly eroded in initial melting stage of the 80/20 PP/POE blends. In mixing stage, the diameter and size distribution coefficient of droplets decrease approximately linearly along the extruder for the 80/20 PP/POE blend, and the dispersed droplet is broken and then coalesced along the extruder for the 60/40 PP/POE blend. For the 57/5/38 PP/clay/PA6 blend nanocomposite, the addition of clay resultes in the decrease of the dispersed domain size.In the chaotic mixing, the melting of solid pellets is obviously accelerated and the distribution of POE in the matrix is improved in initial melting stage for the 80/20 PP/POE blend. In the mixing stage, the laminar layers form at about 2/3 length of the screw, and then they are broken into fibrils and droplets along the extruder when POE content is 20 wt%. When POE content is 40 wt%, the fibrils form initially, and then they are coalesced to a co-continuous structure finally. For the 57/5/38 PP/clay/PA6 blend nanocomposite, the chaotic mixing improves the exfoliation of clay in the PA6 phase.For the 60/40 PP/POE and PP/PA6 blends, four distinct phase morphologies, that is, spherical droplet, short fibril, laminar layer, and co-continuous structure, are obtained along the extruder under both shear mixing and chaotic mixing. For the blends with the droplet morphology, smaller size and more uniform distribution of the droplets results in higher impact strengths. The increase of the length to thickness ratio of fibrils leads to the increase of impact strengths for the blends with the fibril morphology. Comparing the impact strengths of the blends with different phase morphologies shows that the blend with co-continuous structure presents the highest impact strength. In addition, the decrease of dispersed domain size results in the incresase of the low-frequency complex viscosity and storage modulus.The chaotic mixing can create a wide variety of phase morphologies in the multiphase polymer systems, which improves their mechanical properties. So the chaotic mixing is used to prepare the 50/25/25 PP/POE/PA6 ternary blend. It is found that the core-shell structure, that is, PA6 is core, POE is shell, forms with the mixing sequence that POE is blended with PA6 first, and PP is then added. In this phase morphology, the POE phase locates between the PA6 and PP phases and plays the role in connecting the two phases, resulting in higher low-frequency complex viscosity and storage modulus, and impact strength of the ternary blends. Moreover, the PA6 fibers form due to the effect of chaotic mixing, which leads the improvement of tensile strength of the ternary blends. Therefore, the ternary blends can present both good tensile and impact properties.