Design,Synthesis And Properties Of Ethylene/Propylene Rubbers Crosslinked With Dynamic Covalent Bonds

Rubber is an essential material for modern human life,and the increasing use requirements and emerging application needs are constantly driving the renewal and development of high value-added rubber materials.Ethylene/propylene copolymer(ethylene/propylene rubber,EPR),as an important variety of synthetic rubber,suffers from insufficient functionalization and crosslinking modification methods and poor compatibility with polar materials due to the lack of reactive groups.Crosslinking is an indispensable process to provide EPR with excellent properties,however,the traditional sulfur vulcanization and peroxide crosslinking methods produce irreversible chemical crosslinking points,which make the rubber material unable to be reprocessed,shorten the life cycle,cause great waste of resources and serious environmental pollution,and do not meet the strategic requirements of circular economy development.In this thesis,anthracene,furan and aldehyde groups were introduced onto the polymer chain of EPR by polymerization method under the initiation of metallocene compound respectively to prepare light/thermal-reversible crosslinked EPR based on[4+4]cycloaddition reaction,thermalreversible crosslinked EPR based on Diels-Alder(D-A)reaction,associative dynamic covalently bonded crosslinked EPR based on imine bond exchange reaction and EPR/poly(ethylene glycol)(PEG)elastomer hydrogel based on D-A reaction,and the relationships between their structures and properties were investigated and summarized as follows:(1)A functional olefin monomer containing anthracene group,9-octenylanthracene(AO)was designed and implemented in ethylene/propylene/AO copolymerization catalyzed by racEt(Ind)2ZrCl2/MAO to obtain EPR with anthracene group on the side group.The copolymerization reaction exhibited high activity(>106 gEPR molZr-1 h-1),and effective adjustment of anthracene group content(AO unit content 0?11.0 mol%)was achieved by controlling the polymerization reaction conditions;the AO unit content had a great influence on the Tg of the copolymers,and as the AO unit content varied from 0 to 11.0 mol%,Tg of the copolymers increased from-52.0? to-17.8?.The reversible[4+4]cycloaddition reaction based on anthracene groups realized the crosslinking/de-crosslinking and self-healing functions of EPR under light/heat stimulation.The crosslinking reaction conversion yield was maintained at a high level of?90%,with the degree of reversibility around 70%;the tensile test results showed that the mechanical properties of EPR were greatly improved after crosslinking,and the self-healing efficiency of crosslinked EPR with anthracene group content of 5.7 mol%could reach 82.9%.(2)A functional olefin monomer containing furan group,8-furyl-1-octene(FO)was designed and implemented in ethylene/propylene/FO copolymerization catalyzed by racEt(Ind)2ZrCl2/MAO to obtain EPR with furan group on the side group.The copolymerization reaction exhibited high activity(>106 gEPR molZr-1 h-1),and effective adjustment of furan group content(FO unit content 0?10.7 mol%)was achieved by controlling the polymerization reaction conditions;The flexible side group of FO endowed the copolymers with low Tg(around-50?)and the Tg decreased with increasing FO content.Based on the dissociative DA reaction between furan and maleimide groups,bismaleimide small molecules,BMC6 and BMPh2,were used as crosslinkers respectively to achieve thermal-reversible crosslinking of EPR with high efficiency.With the increase of the crosslinking degree of EPR,the stress at break and modulus of the material increased and the elongation at break decreased;the crosslinked EPR could realize de-crosslinking by rD-A reaction as the temperature was increased to 120?,which gave the EPR reprocessability;the study showed that BMC6 system had better reproducibility of mechanical properties than BMPh2 system after repeated processing(the recovery ratio of stress at break and Young's modulus of the material could reach 93%and 97%,respectively).(3)The aldehyde group was introduced into the side group of EPR by a postfunctionalization method,and EPR with aldehyde group content of 1.4 mol%,3.3 mol%and 4.6 mol%was synthesized;then,the aldehyde group-functionalized EPR was reacted with small molecule crosslinker,tris(2-aminoethyl)amine(TREA),to form associative covalent crosslinked network with dynamic imine bond.The resulting crosslinked network could maintain the integrity even in xylene at 120?;with the increase of crosslinking degree,EPR was strengthened:for EPR with aldehyde group of 4.6 mol%,as-NH2/-CHO was increased from 0.3 to 1.0,the stress at break increased from 0.83 MPa to 1.81 MPa and the modulus increased from 0.99 MPa to 5.41 MPa while the elongation at break decreased from 135%to 42%.The crosslinked material exhibited obvious stress relaxation behavior at high temperature.and its activation energy(Ea)was calculated to be 57.2 kJ mol-1,indicating that the imine bond exchange reaction could be well conducted within EPR and the crosslinked network has solidstate plasticity,and the higher the crosslinking degree,the faster the stress relaxation rate of the network.The crosslinked EPR could be remolded after hot pressing at 160?,and the mechanical properties of the recycled samples were comparable to those of the original samples.(4)A new elastomer hydrogel was obtained for the first time by combining lipophilic EPR with hydrophilic PEG through reversible D-A reaction,using furan group-functionalized EPR as the main chain and PEG as the crosslinker,and the crosslinked copolymer could realize decrosslinking above 120? and re-crosslinking at room temperature.By varying the maleimide/furan feeding ratio(0.1?2.0)and the molecular weight of PEG(2000?8000 g mol1),the crosslinking degree and microstructure of the copolymer could be controlled,which in turn enabled the regulation of the water absorption and mechanical properties of the material.Increasing the molecular weight of PEG could increase the aggregated phase size and crystallinity of PEG within the crosslinked system,which endowed the material with good water absorption,high strength and high toughness:for the crosslinked copolymer prepared from PEG with molecular weight of 8000 g mol-1,S=148%,?b=4.82 MPa,E=23.27 MPa,?b=295%;the strength of the crosslinked copolymer decreased significantly after absorbing water,while the toughness was further improved;in vitro cytotoxicity experiment showed that the elastomer hydrogel had low cytotoxicity.