Study On Branched Structure And Rheological Properties Of Polypropylene

The properties of polymers are closely related to their molecular structure.Defining the corresponding relationship between molecular structure and processing rheological properties of polypropylene has a key guiding significance for the development of high-performance products.The introduction of branched chain structure can greatly improve the impact resistance and melt instability of polypropylene.The molecular structure of branched polypropylene is often very complex,therefore,the accurate characterization of polymer branching structure has always been the focus and difficulty of industry and academia.At present,there are many people who choose polyethylene to study the relationship between branched structure and properties,but there are relatively few studies on long chain branched polypropylene(LCBPP)with complex preparation process and poor controllability of product structure.In particular,there is still little research on the quantification of branched structure of polypropylene?In order to establish the relationship between branching structure and macroscopic rheological behavior of polypropylene,this paper is based on the idea of two-way verification between macro and micro,experiment and theoretical simulation.Firstly,the molecular structures of commercial linear random copolymerized polypropylene(PPR)and high melt strength polypropylene(HMSPP)were positively studied by means of gel permeation chromatography coupled with multi-detector(GPC-IR-IV-TALLS),nuclear magnetic resonance carbon spectroscopy(¹³C-NMR)and rheological methods.Secondly,the static properties of different branched polymers with ideal structure in dilute solution were studied by molecular dynamics simulation based on coarse granulation model,and the effect of branched structure on molecular conformation was explored from the microscopic point of view.To provide a theory for in-depth understanding of the structure-activity relationship of this system.The results showed that the branched structures in HMSPP were detected by gel permeation chromatography combined with multi-detector(GPC-IR-IV-TALLS),nuclear magnetic resonance carbon spectroscopy(¹³C-NMR)and rheology.Among them,the molecular weight distribution of HMSPP samples was wider and more branched structures were found in high molecular weight parts by gel permeation chromatography combined with multi-detector(GPC-IR-IV-TALLS).The number of branching points in each chain of HMSPP is 0.11,and the relationship between intrinsic viscosity,mean square radius of gyration and molecular weight are[?]?M^0.68?R_g?M^0.56respectively.The average number of branching points on the main chain of the HMSPP was 0.14 by ¹³C-NMR method,which was close to the result of GPC.The rheological results show that PPR-4220 has higher viscoelasticity,higher entanglement density and longer relaxation time than HMSPP,which is the result of the interaction of molecular weight,molecular weight distribution and branching structure of the two kinds of polypropylene.The scaling law of the mean square radius of gyration and the degree of polymerization of the coarse grained model PE with different branched structures at different temperatures was simulated by molecular dynamics method.It is found that increasing the length of branched chains can strongly affect the static properties of comb molecules in good solvents,but the effect of increasing the number of branching points is not as obvious as increasing the length of branched chains.