| Polyolefin elastomers are endowed with excellent properties of weatherability and agingresistance due to no C=C double bond or low C=C content in their molecular chains, e.g.ethylene propylene diene terpolymer (EPDM), ethylene propylene monomer (EPM),poly(ethene-co-1-butene)(PEB) and isobutylene isoprene rubber (IIR). Graft copolymerssynthesized by grafting these polyolefin elastomers with vinyl monomers can be used ascompatibilizers and toughening agents in polymer blends to prepare high impact strengthengineering plastics of resistance against oxygen, heat, aging and yellow discoloration. Theseplastics are particularly suited for outdoor applications, such as automotive sealing systemsand roofing for buildings. Therefore, the study of the reaction mechanism of graftcopolymerization of polyolefin elastomers/vinyl monomers can not only help us understandthe reaction process of graft copolymerization and the structure and properties of the graftcopolymers, but also guide us through the synthesis of the graft copolymers that can bettersatisfy the requirement.In this thesis, seven graft copolymerization systems, including EPDM/St-AN, EPDM/St,EPDM/MMA, EPM/St-AN, PEB/MMA-AN and IIR/MMA-AN in suspension andEPDM/MMA in solution, were carried out by free radical polymerization at80℃usingbenzoyl peroxide (BPO) as initiator. For each system, a sequential reaction products ofdifferent reaction times were synthesized. After the synthesis, each product was separated bySoxhlet extraction to obtain purified components, which were subsequently characterized byGPC, FTIR and1H-NMR analyses. The monomer conversion rate (CR), average graft ratio(GRa), graft efficiency (GE), grafted monomer conversion rate (CRg), non-grafted monomerconversion rate (CRf), rubber graft ratio (GRr), true graft ratio (GRt) and gel rate (CPR) werecalculated and plotted against the reaction time. The dependence of these reaction behaviors onreaction time was systematically investigated. The results showed that the chain transfer graftreaction (i.e.“graft from” reaction) occurred at the same time as the free polymerizationreaction at the beginning of the synthesis and stopped after about100min, and the reactionrate of the former was significantly higher than that of the latter in all systems exceptIIR/MMA-AN. A “curled-coil” model of suspension particle formed by aggregates ofmonomers, initiators, free radicals and macromolecules of the polyolefin elastomers/vinylmonomers graft copolymerization system was established to explain the above-mentionedreaction behaviors. Based on the study of the variation of GRawith increasing reaction time and the analysisresult that GRakept rising after the “graft from” reaction stopped, we discovered that therewere still two other potential grafting elementary reactions, which we called the “graft onto”reaction and the “bridge” crosslinking reaction, occurring between the grafted chains of thegraft copolymers and the molecular chains of the free polymers via their reactive side groups.As for EPDM/St-AN and EPDM/MMA in suspension, the “bridge” crosslinking reactionoccurred after the “graft onto” reaction, and both chemical reaction equations of them werederived; however, neither of them but the suspected thermal initiation of St graftpolymerization was found in the EPDM/St suspension system, and in the other systems onlythe “graft onto” reaction was found.GPC analysis showed that there was a bimodal molecular weight distribution of the freepolymers formed in the early stage of the reaction process, due to the co-existence of twodistinct molecular populations of low and high molecular weights, which were respectivelyformed by chain transfer termination and bi-radical termination of the free polymeric radicals.According to this discovery and the mechanism of the chain transfer graft reaction, we deriveda universal formula M g, n=m g M L, n/(m f r)to estimate the number-average molecularweight of the grafted chains of every system. The results showed that the number-averagemolecular weight of the grafted chains was much larger than that of the free polymers formedby chain transfer termination of the free polymeric radicals, and was generally smaller than thatof the free polymers formed by bi-radical termination of the free polymeric radicals.Based on the study of the effect of reaction time on GRrand the molecular weight of thegrafted chains, the free polymers and the non-grafted rubber, we discovered that after the “graftfrom” reaction stopped, the chain scission reaction of grafted and non-grafted rubbers occurredin all systems except EPDM/St, followed by the random radical-radical combination of theresultant chain segments to form soluble “multi-block” copolymers, leading to a furthersignificant improvement of GRr. Both chemical reaction equations of them were derived.The effect of the reaction behaviors on the notched Izod impact strength of the blendsprepared by melt blending of the reaction product and the styrene-acrylonitrile copolymers(SAN resin) was investigated. The results showed that for the same system, GRrplayed adominant role in the toughening effect of the reaction product on the SAN resin; for differentsystems, the reaction product with gel structure formed by “bridge” crosslinking reaction had ahigher toughening effect on the SAN resin. TEM analysis showed that with increasing reactiontime, the domain size of the rubber phase of the blends decreased and the notched Izod impact strength of the blends increased; the dispersed phase of rubber particles which exhibited a“salami” like structure had the highest toughening efficiency on the SAN resin. At the sametime, SEM analysis showed that the toughening mechanism of the blends was high shearyielding with “cold drawing” of the SAN matrix. The co-constitute ratio of the free copolymersand the grafted chains was measured by FTIR and1H-NMR quantitative analyses, respectively,providing experimental proof for analyzing the compatibility of the reaction product and theSAN resin. DMA analysis showed that the graft copolymers in the reaction product couldsignificantly improve the compatibility between the rubber phase and the SAN phase. Meltmass-flow rate (MFR) measurements showed that the MFR of the blends declined withincreasing reaction time.The effect of the recovery method of the reaction product on the structure and properties ofthe reaction product was also studied. The results showed that the method, which ended thereaction using p-benzoquinone (PBQ) as terminator and dried the reaction product at roomtemperature, could effectively avoid the ensuing side reactions induced by the residual BPO inthe reaction product during the recovery process, and thus ensure the accuracy of the reactionbehaviors in the study of the reaction mechanisms of the graft copolymerization system.However, the method, which ended the reaction without using any terminator and dried thereaction product at80℃for8hours, resulted in a profound change in the structure andproperties of the reaction product. |
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