Synthesis And Characterization Of Long Chain Branched Polypropylene

Conventional linear isotactic PP (iPP) has poor melt strength and cannot be easily used in process where elongational flows are dominant, such as foaming, thermoforming, extrusion coating or blow molding. Linear chains do not show strain hardening in extensional flow. Long chain branching strucuture, which is generally known to enhance the melt properties of a polymer, can solve these problems. Long chain branched polypropylene (LCBPP) become more and more important in many industrial applications.In this work, LCBPPs were firstly prepared by the macromolecular reaction between maleic anhydride grafted polypropylene (PP-g-MA) and amine grafted polypropylene (PP-g-NH2) in xylene solution. The branch frequency and branch length were controlled by varying molar ratios of anhydride to amine and the molecular weight of given both PP-g-MA and PP-g-NH2. Long chain branch frequency from 0.5 to 1.5 branches per chain was achieved, while the branch length was varied from 75.4 to 128.2 kg/mol. Compared to the linear iPP with the same backbone, LCBPP exhibit increased dynamic modulus, enhanced low-frequency complex viscosity, reduced phase angle, and enhanced flow activation energy. Moreover, changes in these rheological properties were related to the branch frequency and branch length, and became more significant with the increased branch frequency and branch length.Secondly, LCBPP have also been prepared by the direct melting reaction between PP-g-MA and ethylene diamine (EDA) in a two-screw extruder. To avoid degradation of iPP, extrusion temperature was decreased from 200℃ to 160 ℃ by injecting supercritical CO₂(scCO₂). The presence of SCCO₂ in iPP promoted the mobility of polymer chain segment and diffusibility of small molecules. As a result, the branch density of LCBPP increased significantly. Compared to the LCBPP prepared by solution reaction, the scLCBPP which would be synthesized by supercritical reactive extrusion exhibit well-defined appearance, adjusted melt index (MI, 15-25 g/min), excellent dynamic rheological characteristics and mechanical properties. The notched impact strength of scLCBPP, which reached to 7 kJ/m², increased 200% compared to that of the linear iPP as raw material. The tensile strength of scLCBPP was more than 30MPa and increased about 10% over linear iPP. The tensile moduluswas close to 1350N/mm , which was an 20% increased over linear iPP. The elongation at-break exceeded 900%. Furthermore, the association between scCCh and the polar group in scLCBPP has considerable effects on the crystallization behavior of scLCBPP and leads to the increase in the melt temperature and crystallization temperature of scLCBPP.Finally, the structure of obtained LCBPP was confirmed by Tube Theory and Tsenoglou CJ & Gotsis AD Model. The model results showed that topological structure of the obtained LCBPP was star-like. Isothermal crystallization kinetics of obtained LCBPP, linear iPP with the same backbone or the same molecular weight, were also studied by differential scanning calorimetry (DSC) and polarized optical microscopy (POM). Compared with linear iPP having the same backbone, LCBPP had the same crystallization temperature but much narrower crystallization peaks, and the crystallization activity energy was increased from 249.5kJ/mol to 327.3kJ/mol. Moreover, the crystallization activity energy of linear propylene with the same molecular weight was 365.7kJ/mol. It was obvious that the LCBPP had lower crystallization activity energy but higher crystallization temperature and similar crystallization peaks. The works of chain folding of above three samples were also calculated as 3.5 X 10"20, 7.8 X 10'20 and 4.2 X10"20 (J/fold) , respectively. The results indicated that the work of chain folding decreased because of long chain branches, but it increased significantly by the enhancement of grafting functional group.