Synthesis And Properties Of Imidazolium-based Polyethylene Ionomer

Polyethylene(PE)is widely applied into numerous fields due to its good mechanical properties,excellent processing properties and chemical stability.However,PE acts as a typical non-polar polymer material characterized by poor dyeability,hydrophilicity,adhesion and compatibility with polar polymers.One of the best ways to improved its performance is introducing polar groups on PE chain.Polyethylene ionomer(PEI)represents a common type of functionalized polyolefin which contains some ionic groups in side chain.In non-polar polyolefin matrix,these ionic groups tend to form ionic aggregations to enhance intermolecular force,thereby improving the wear resistance,toughness and strength of PE.The novel ethylene/5-iodomethyl-2-norbornene(IMNB)copolymers were firstly synthesized by successfully copolymerizing ethylene and IMNB with a high-activity titanium complex bearing two?-ketiminato ligands.As reactive sites,the iodine atoms distributed randomly along the copolymer chains were subsequently converted into ionic groups via nucleophilic substitution reaction with N-methylimidazole,followed by exchanging the I^-counteranions with methane-sulfonate(CH₃SO₃^-),trifluoromethanesulfonate(CF₃SO₃^-),and bis(trifluoromethane)-sulfonimide(Tf₂N^-).Thus,a series of imidazolium-based PEI bearing different counteranions were readily obtained.As expected,copolymer crystallization was hindered to some extent by ionic groups and the degree of crystallinity for ionomers decreased with increasing the ion content.It was noticeable that PEIs possessed higher glass transition temperature(T_g)than that of ethylene/IMNB copolymer.Furthermore,for polyethylene ionomers with the same counteranion,the increase of ion content contributed to the increase of T_g.After the ionization of the copolymers,the mechanical properties improved significantly.With the same ion content,the stress-at-break values of the ionomers followed the order:CH₃SO₃^->CF₃SO₃^->Tf₂N^-,whereas the strain-at-break values were in the opposite order.Based on the copolymerization of ethylene and IMNB,hexene(HE)was introduced to conduct terpolymerization.And various imidazolium-based ionomers bearing different counteranions were obtained via post-ionization of the ethylene/HE/IMNB terpolymers which served as reactive polyolefin intermediates.Small angle X-ray scattering(SAXS)results proved the existence of ion aggregates in polyethylene ionomers containing CH₃SO₃^-?CF₃SO₃^-and Tf₂N^-.Wide-angle X-ray diffraction(WAXD)results suggested that PEIs were still capable of crystallization when the ion content was in a certain range.According to uniaxial tensile tests,the introduced ionic groups could effectively enhance the tensile strength of ionomers.The elastic properties of ionomers were investigated under cyclic tensile loadings.As observed,the ionomer paired with CH₃SO₃^-counterions had the best elastic recovery during the whole cycle of tensile deformation,which showed elastic recovery up to 96% at strain of 600%.