The Preparation And Structural Characterization Of Long Chain Branched High Density Polyethylene

High density polyethylene?HDPE?is one of the mostwidely used semi-crystalline polymers in our daily life.It can withstand high temperatures and has good barrier property,mechanical strength,chemicals resistance and electrical insulation,and has been applied widely.However,HDPE is mainly a linear molecular structure and has little branching.It leads to low melt strength in the flow dominated processes,such as thermoforming,blow molding,and foaming etc.According to published reports,introducing long chain branches?LCB?onto HDPE backbone is the most effective way to improve its melt strength.There are three methods have been devised to produce LCBPE: synthesis directly,high energy irradiation,and melt branching.The last one is operate simple and suitable for industrial production.In this paper,LCBPE was prepared by melt branch modification of HDPE in the presence of different vinyl multifunctional monomers.Fourier transformed infrared spectroscopy?FTIR?,rotational rheometer,high temperature gel permeation chromatography coupled with triple detectors,¹³ C NMR,capillary rheometer,Differential scanning calorimetry?DSC?andpolarized optical microscopy?POM?were used to investigate the relationship of structure and properties of LCBPE.Main content includes four aspects:1.LCBPE was prepared by melt branch modification of HDPE in the presence of dicumyl peroxide?DCP?and different vinyl multifunctional monomers.The forming mechanism of LCB was presumed by transient torque curve,and the reaction products were characterized by FTIR,¹³C NMR,GPC and rotational rheometer.Various rheological methods including viscosity curve,storage modulus,Han plot,Cole-Cole plot,and vGP plot were used to investigate the relationship of LCB structure and rheological properties.The results indicated that there are little LCB in the modified samples by DCP and monofunctional monomer.For the modified samples with multifunctional monomers,the content of LCB and melt strength increased with the increasing of functionality of the multifunctional monomers.Furthermore,the introduction of LCB improved the mechanical property of HDPE.2.The topological structures of LCBPE were investigated by weighted relaxation time spectrum,thermorheological behavior,and the molecular mass dependences of zero-shear viscosity??₀-M_w plot?,vGP plot.The results showed that LCBPE is a thermorheological complex fluid and is a mixture of linear PE and branch polyethylene.The flow activation energy of LCBPE increased with the increasing of relaxation time.The ?₀-M_w plotand vGP plot results indicated that LCBPE by melt branching reaction is a star-branched polymer.3.The non-isothermal crystallization behavior and crystal morphology of LCBPE and HDPE were studied by DSC and POM.The results showed that with the increasing of the LCB's contents,the crystallization peak temperature improved,which displayed that LCB molecules have the role of heterogeneous nucleating agent and improved the nucleation density.However,in the crystal growth process,the mobility and reputation ability of HDPE backbones were restrained by the LCB structure,which hindered the crystallization process to increase the time of crystallization.Ozawa,Jeziorny and Mo models were used to analyze the non-isothermal crystallization of LCBPE and HDPE.It was indicated there are secondary crystallizing process in the LCBPE.The introducing of LCB influences the mechanism of nucleation and the growth of HDPE.From the results of POM for the LCBPE,the size of crystals decreased and crystal nucleus increased largely.4.To solve the problem that the modified sample with monofunctional monomer have little LCB,according to the feature of nano-SiO₂ have large quantity of surface silanol group,vinyl monofunctional monomer was in situ grafted onto the nano-SiO₂ surface.LCBPE was prepared with modified nano-SiO₂ as branching pointing by melt branching reaction.Compared with HDPE,the melt strength,thermal stability and mechanical property of LCBPE were all improved obviously.DSC and POM results indicated the size of crystals decreased and the growth rate was lower for LCBPE.