Controlling Entanglement Formation Of Polyethylene Chains By Modifying Structure Of Silica

Ultrahigh molecular weight polyethylene（UHMWPE）,with some excellent properties（i.e.high modulus,high toughness and abrasion resistance）,is widely used in aviation,navigation,and medical field.Generally,commercial UHMWPE synthesized in a high polymerization temperature with Ziegler-Natta catalyst owns a great number of chain entanglement,which increases the difficulties in the products process.What’s more,the entanglements can influence the crystallization of polyethylene chains and cause the crystalline defect.It will lead to a low modulus and tensile strength of the polyethylene fiber,which restricts the application of UHMWPE materials.Thus,to solve this problem,the method to get a weakly entangled UHMWPE is badly in need.Based on the effect of spatial spacers,this study firstly aims to build a polymeric environment for ethylene polymerization by a heterogeneous catalyst which consists of a modified-silica and a homogeneous catalyst.Under a controlled condition,the silica modified by POSSs was used to support FI catalyst to synthesize UHMWPE.The POSSs,serving as physical spacers between the active sites,can distribute uniformLy by MAO on the silica surface.In the early polymerization stage,polymerization activity is low because of inevitable mass transfer resistance of ethylene monomer produced by carrier’s pore channels.Besides,the POSSs can effectively prevent the overlapping of the growing chains.Thus,a weakly entangled UHMWPE chains will be synthesized with a big crystallinity in a short polymerization period.However,this hindrance of POSSs will be disturbed due to the higher polymerization rate caused by the carrier fragmentation.Furthermore,when the fragmentation of carrier is finished,the hindrance of POSSs will disappear.In a long polymerization period,a more entangled chains will be formed.To decrease the influence of catalyst fragmentation on the polymerization,it prepared a controlled-polymeric environment,a inorganic/organic composite carrier（mSiO₂/c-PS）,to support Cp₂TiCl₂ to synthesize polyethylene.Firstly,the silica with different pore size is used to synthesize SiO₂/PS composite carriers.It suggests that955 Silica/PS owns a spherical morphology and a good pore structure.Then,mSiO₂/c-PS/Cp₂TiCl₂ is prepared to synthesize polyethylene under different solvent,which aims to study the effect of the swelling behavior of c-PS chains in a confined space on the ethylene polymerization and the entanglement evolution.It reveals that the swelled c-PS chains can provide a ideal environment for the crystallization of polyethylene chains and a huge space for the better distribution of active sites.Thus,the product shows a great crystallinity and a weakly entangled polyethylene chain.