The reactive extrusion of maleic anhydride grafted polypropylene (PP-g-MAH) with ethylenediamine (EDA) as coupling agent was carried out in a co-rotating twin-screw extruder to produce long chain branched polypropylene (LCBPP). Part of PP-g-MAH was replaced by maleic anhydride grafted high-density polyethylene (HDPE-g-MAH) or linear low-density polyethylene (LLDPE-g-MAH) to obtain hybrid long chain branched (LCB) polymers. Compared with the linear PP, PE and their blends, the LCB polyolefins exhibit excellent dynamic shear and transient extensional rheological characteristics such as increased dynamic modulus, higher low-frequency complex viscosity, significantly enhanced melt strength and strain-hardening behaviors. LCB polymers also have higher tensile strength, tensile modulus, impact strength and lower elongation at break than linear polymer and their blends. Supercritical carbon dioxide (scCO_2) was introduced in the reactive extrusion process. With the presence of scCO_2, the motor current of twin extruder was decreased and LCB polyolefins with lower MFR, higher complex viscosity and increased tensile strength and modulus were obtained. This indicates that the application of scCO_2 can reduce the viscosity of melt in extruder, enhance the diffusion of reactive species, facilitate the reaction between functional groups, and increase the LCB density. The foaming behavior of both linear and LCB polyolefins were studied. The results show that cellular materials produced from the LCB polyolefins have higher weight reduction, smaller cell size and better mechanical properties than those produced from the linear polymers.
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