Tensile Mechanical Behaviors Of Polyamide 6 And Polycarbonate And Their K Resin-based Blends

On the basis of a brief introduction of the theory of stress-strainbehavior, yielding characteristics including the unusual double yieldingphenomenon, brittle–ductile transition and fracture behavior of polymers,this thesis reviewed the influencing factors of deformation behavior ofpolymers from the temperature activated molecular mobility, stressactivated molecular mobility, effect of heating history and dependence ofmechanical behavior of polymers, polymer blends on the structures andmorphology and forward the purpose and arrangement of this study.Firstly the dependence of tensile stress-strain behavior of polyamide6 (PA6) /K resin (K) blends and Polycarbonate (PC) /K resin blends atvarious composition on the test conditions was investigated. The resultsshowed that PC/K blends presented the typical stress strain behavior ofnoncrystalline polymers in a wide range of composition, while the PA6/Kblends presented the varied tensile stress-strain behavior at variouscompositions. In a certain range of tensile temperatures, the stress strainbehaviors of PA6/K blends studied showed good similarity. With thetensile temperature increasing, the stress strain curves changed in a sort ofway. While the stress-strain behaviors of PC/K blends studied showed goodsimilarity in all test temperatures. In a wide range of tensile velocities,with the tensile velocity increasing, the basic characteristics of thesestress stain behavior of PA6/K and PC/K blends were unchanged while theoverall stress strain curves were raised and yielding stress depressed.According to the experimental results, the Eyring equation was usedto model the strain rate sensitivity of the stress stain behavior of PA6/Kand PC/K blends. The results showed that PA6 matrix or PC matrix wereeasy to reach the activation volume for yielding with incorporating K resin,which reflected the change of deformation mechanism. Incorporation of thecompatibilizer, maleic anhydride-grafted K resin (K-g-MAH), into PA6/Kor PC/K blends could decrease the stress concentration factors of theseblends. This implied that the compatibility between PA6/K and PC/Ksystems improved by adding K-g-MAH.On the basis of the experimental results, through the use of mathematical methods, the deformation energy of the tensile neckingprocess was discussed and deemed to reflect the compatibility of polymerblends, the results of Dynamic mechanical thermal analysis (DMTA) andscanning electron microscope (SEM) were applied to further confirm ourconsideration.Secondly the unusual double yielding was observed, and the cause offormation and the influencing factors were investigated. In the tensiledeformation of PA6/K blends containing 50wt.% and 70wt.% K resin, astress drop came after the first yield point and then the sample occurrednecking. The stress showed the second maximum with the strain hardeningprocess, then followed by a stress plateau, the second yielding occurred.But the PA6/K blends with other compositions presented usual stress-strainbehavior. No similar double yielding phenomenon was found at the PA6/PSblends with the same compositions under the same conditions. Accordingto the result of differential scanning calorimetry (DSC), wide-angle X-raydiffraction (WAXD) and DMTA analysis, it was superficially suggestedthat the first yielding of PA6/K blends might cause by the deformation ofthe K resin matrix, the second yield point might be correlated to thepermanent plastic deformation of the entanglement of PA6 chains and softphase of K resin (PS-co-PB). The double yielding phenomenon for PA6/Kblends depressed with the increasing of tensile temperature. When thetensile velocity increased to a certain velocity (200mm/min), the stressstrain behavior of PA6/K blends transformed from double yielding to usualstress strain behavior. Too high or too low moisture absorption all madeagainst the occurring of double yielding of PA6/K blends.Thirdly, the brittle–ductile transition (BDT) behaviors of PA6/K andPC/K blends in high speed tensile tests were studied. For the PA6/K blends,BDT occurred with the increase of the K resin content in high speed tensiletests. The more the K resin content was, the higher the BDT tensile speedwas and the lower the BDT tensile temperature was. The effect of tensilespeed on the BDT of polymer blends can be characterized by thecorresponding effect of tensile temperature. For the PC/K blends, BDT didnot occurr with K resin content. The micro deformation process analysis ofthe notched Charpy impact samples in impact test showed that the notchedimpact deformation was really a high speed tensile deformation at the narrow region near notch tips, The BDTs in both Charpy impact and highspeed tensile tests shared the same mechanism.Fourthly, the effect of strain rate on the PA6/ K resin and PC/K resinblends were investigated using the Single Edge Notch Tensile (SENT) test.The similar effect of test speed on the fracture maximum stress was foundin both PA6/K and PC/K blends. The maximum stress of both PA6/K andPC/K blends decreased with increasing K resin content, and increased withthe logarithm of test speed. Test speed seemed to slightly influence theinitiation phase of fracture. However, the influences of test speed onfracture propagation displacement and fracture energy for the PA6/K andPC/K blend presented distinct difference. The PA6/K and PC/K blendcontaining 20wt.% K resin was investigated further. SENT tests were doneat different temperatures over the complete test speed range. Testtemperature apparently affects plastic deformation process of both PA6/Kand PC/K blend. The fracture behaviors of polymer blends at low testspeeds was more easily affected by the morphology, the fracture behaviorsat high test speeds could not be simply expected based on the low speedsresults.Finally, the tensile fracture characteristic of PA6/K and PC/K blendssamples with pre-crack under the low strain rate were studied. The resultsshowed that with a pre-crack, the stress strain differ a lot from conventionaltensile samples: after the elastic deformation and yielding process, theyshowed marked necking and tearing process. The results showed adding Kresin to PA6 could significantly enhance the crack resistance of PA6, thefurther improvement of crack resistance and the energy absorbing abilityfor plastic deformation of PA6/K blends appeared when K-g-MAH wasincorporated. The energy partition results showed the difference of thefracture behavior was mainly from the yielding process. While adding Kresin to PC could not significantly affect the crack propagation or plasticdeformation process of PC, and the energy partition results showed theeffect of K-g-MAH on the fracture toughness for the PAC/K blends wasmainly achieved through its influence on the specific essential of fracture inthe yielding process.