Processing And Properties Of Polymer And Polymer Composite Under Oscilla- Tory Shear

Polymer processing is an important branch in polymer science. Improving the properties of polymers by controlling the inner structure and morphology of polymers in processing is the goal that researchers pursue. Applying oscillatory shear in the conventional polymer processing can change the inner morphology of polymers and improve the crystallization and mechanical properties. Recently, shear-controlled ori-entation injection molding and dynamic packing injection molding are two concerned oscillatory shear assisted molding technology. After the decade’s research, great im-provement has been achieved. However, the frequency of oscillatory shear in these studies is a very low (-0.3Hz). Moreover, the crystallization and mechanical proper-ties of the polymers processed under oscillatory shear are investigated, but other properties, such as the barrier property and electrical conductivities, are rarely report-ed.In this study, we applied the 1ab-made multi-pass processing device to investigate the effect of oscillatory shear of various frequencies and amplitudes on the crystalline morphology and mechanical properties of polymers and polymer composites. Mean-while, the effect of the oriented and well-dispersed nano-filler induced by oscillatory shear on the barrier property and conductive property are rarely reported of the poly-mers is also investigated in this study. The main results of this study are summarized as follow:1. Effects of oscillatory shear with different frequencies (0-2.5 Hz) and amplitudes (0-20 mm) on the mechanical properties and crystalline morphology of linear low density polyethylene (LLDPE) were investigated. It was found that the mechani-cal properties of LLDPE are improved because of the more perfect crystalline structure when LLDPE crystallizes under low-frequency and small-amplitude (0.2 Hz/4 mm) oscillatory shear. The mechanical properties can be further im-proved by increasing either the frequency or the amplitude of oscillatory shear. The Young’s modulus and tensile strength of LLDPE are improved by 27% and 20%, respectively, when the frequency is increased to 2.5 Hz and the amplitude is maintained at 4 mm; while the Young’s modulus and tensile strength are im-proved by 49% and 47%, respectively, when the amplitude is increased to 20 mm and the frequency is remained as 0.2 Hz. The results of differential scanning cal-orimetry and scanning electron microscopy show that the crystallinization of LLDPE is more perfect when processed under oscillatory shear; the higher fre-quency oscillation shear causes LLDPE has an increased crystallinity while the larger amplitude oscillatory shear orients the LLDPE molecular chains along the flow direction forming shish-kebab superstructure.2. A new oscillation shear form named superimposed oscillatory shear (SOS) is proposed. Compared with the LLDPE samples processed by OS, LLDPE pre-pared under SOS exhibits an enhanced crystallinity of LLDPE (43.5%) and the tensile strength and Young’s modulus are improved by 11% and 12.6%, respec-tively. This is attributed to the continuously changed shear rate in the SOS, which repeatedly compresses and stretchs the LLDPE molecule chains facilitating the formation of a more perfect crystal, therefore, the crystallinity and the mechanical properties are improved. Meanwhile, these results suggest that LLDPE prepared under SOS shows a better self-reinforcement.3. The crystallization behavior and water vapor permeability of a poly (lactic acid) (PLA) nanocomposite containing 5 wt% organic montmorillonite (OMMT) un-der oscillatory shear were investigated. Under the oscillatory shear, OMMT platelets exhibit a better intercalated structure and orient along the flow direction. Some of the OMMT platelets are exfoliated and dispersed in the form of single or few-layer platelets. These well-dispersed OMMT platelets act as more effective nucleating and accelerating agents for the crystallization of PLA, as a result, the cold crystallization enthalpy is significantly decreased, the cold crystallization temperature is much closer to the melting temperature and the crystallinity is dramatically increased. Moreover, the water vapor permeability is decreased by 36% due to the barrier effect of the well-dispersed OMMT and the increased crystallinity of PLA, which increase the tortuous path that water molecules re-quired to permeate.4. The effect of oscillatory shear on the electrical anisotropy of PLA nanocomposite blended with 0-2.1vol% Ketjen black (KB) is investigated. Under the oscillatory shear, the KB particle chains are aligned along the flow direction in the PLA ma-trix, resulting in an oriented conductive network, the percolation threshold along the flow direction is much lower than that perpendicular to the flow direction. When the concentration of KB is in the range of 0.88-1.58 vol%, the volume electrical resistivity along the flow direction (ρ") decreases to ～3×104 Ω·m and that perpendicular to the flow direction (ρ⊥) remains at ～1×1010 Ω·m, showing an extremely large electrical anisotropy, and the ρ"/ρ⊥ value is up to 106, which is also 3 to 4 orders of magnitude higher than that of previously reported carbon nanotube based electrically anisotropic composites.