| Rigidity and toughness are two key parameters that determine whether a polymer can be used as an engineering material.Polypropylene(PP),used widely as a general polymeric material,its application is limited in engineering due to its poor impact resistance,especially at lower temperature.Introducing rubber/elastomer into matrix PP by blending or alloying in reactor can improve the impact resistance to prepare high impact polypropylene(HIPP).However,these methods will lead to a markedly decrease in rigidity,limiting the application of PP in engineering also.Therefore,by combining the brittle-ductile transition theory and the elasticity theory of multi-component polymer in this dissertation,we established a multiphase model to link the rigidity and toughness of polymers,and quantitatively investigated the factors that affected rigidity and toughness.Then,based on the theoretical research,the effects of the structure and properties of PP matrix and modifier on the balanced toughness and rigidity of the PP composites were investigated experimentally.Details of the research contents and the results are summarized as follows:1.Balanced toughness and rigidity of one phase modifier toughened thermoplastic polymer composites:We established a relation between the composite modulus and related parameters including modifier modulus,modifier content,modifier particle size and critical interparticle distance IDc for one phase modifier(such as polyolefin elastomer POE,ethylene-propylene rubber EPR and ethylene-propylene-diene monomer copolymer EPDM)toughened polymer composites.From this relation,the effects of these parameters on the rigidity and toughness of polymer composites were quantitatively investigated.The results indicated that for a typical PP/rubber system(IDc=0.11 μm),26.1%modulus loss was the lowest value for rubber toughened PP composites in theory.Moreover,if the modifier modulus was less than one tenth of the matrix modulus,the modifier modulus had less influence on the composite rigidity,and its content was the dominative parameter that determined the composite rigidity.2.Balanced toughness and rigidity of core-shell rubber modifier toughened thermoplastic polymer composites:We established a relation between the modulus of core-shell modifier toughened polymer composites and particle size,modulus,content of core-shell rubber particles at critical brittle-ductile transition point.Then,the effects of the structure and properties of core-shell rubber particles on the balanced toughness and rigidity of the composites were investigated.The results indicated that higher core modulus and lower shell modulus were more beneficial to the increase of the composite rigidity on the premise that core-shell rubber particles could be used as an effective impact modifier.Furthermore,5.4%and 13.5%modulus loss were the lowest value for the core-shell rubber particles toughened PP composites in theory,corresponding to the core-shell rubber particles with PP core and EPR shell,and core-shell rubber particles with PE core and EPR shell respectively,much smaller than the lowest modulus loss(26.1%)of one phase modifier toughened PP composite.For the PP alloys in reactor,it was difficult to ensure the purity of core and shell phase(i.e.the rubber shell contained homo PP or PE and the PP or PE core contained EPR).So,the purer the EPR shell and the lower the EPR content in PP or PE core,the more beneficial to the balance between toughness and rigidity of PP alloys in reactor.3.Experimental study on the effect of core-shell particles structure on the balanced toughness and rigidity of the impact polypropylene composites:Three kinds of PE with different rigidity were blended with PP and EPR to prepare PP/PE-EPR core-shell composites.The effects of core-shell rubber particles structure on the balance between the toughness and rigidity of the PP composites were investigated.AFM-IR and SEM results indicated that the core-shell structure particles with PE core encapsulated in EPR shell were in situ formed and uniformly dispersed in the PP matrix during blending.It was also found that the PP composites toughened with core-shell rubber particles exhibited a better balance between toughness and rigidity than that with one-phase rubber/elastomers,and harder PE core was more beneficial to the balanced toughness and rigidity of the PP composites.These experimental results were in quantitative agreement with the theory,providing solid experimental evidence in support of the latter.4.Effect of polypropylene molecular weight on the balance between the toughness and rigidity of the impact polypropylene composites:The relation between molecular weight Mn,brittle fracture strength σb and critical interparticle distance IDc and their effects on the brittle-ductile transition of polymer composites were investigated both in theory and experiment.The theoretical and experimental results all indicated that the LDc of the composites was proportional to σb2/3 of the matrix polymer.Moreoverltheσb of the matrix polymer was determined by its molecular weight Mn.So,the matrix polymer with a high molecular weight had higher IDc,and was more beneficial to the brittle-ductile transition of its composite.It was also found that the PP/POE composites with higher molecular weight PP matrix had lower modulus loss.Therefore,matrix PP with a higher molecular weight was more prone to the brittle-ductile transition and had better toughness and rigidity balance for its composite.5.Effect of polypropylene molecular weight distribution on the balance between the toughness and rigidity of the impact polypropylene composites:The effects of matrix PP molecular weight distribution(MWD)on the brittle-ductile transition and the balance between toughness and rigidity for polymer composites were investigated.The experimental results showed that the matrix PP with wider MWD was more prone to brittle-ductile transition of PP/POE composites for the same number average molecular weight Mn.The reason was that the MWD of matrix PP would affect its brittle fracture strength σb.It was also found that the wider the MWD of the matrix PP was,the higherσb of the matrix PP was,and the larger the critical interparticle distance IDc of the composite was.Larger IDc meant that less modifier particles could induce the brittleductile transition of the composite,so the PP/POE composite corresponding to the wider MWD matrix PP had lower modulus loss and eventually exhibited a better balance between toughness and rigidity. |
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