Crystallization Behavior Of TPI Vulcanizate And Structural Evolution In Tensile Deformation

The rubber-plastic dual characteristics of trans-polyisoprene(TPI)limit its application like ordinary plastics or rubbers,which stems from its good crystallinity and low melting temperature.Its complex polymorphism and crystalline characteristics strongly affect the material properties.So far,the research on TPI has focused on the microstructure to study the crystal structure,cell parameters,formation conditions,thermodynamic analysis,and crystal form transformation of TPI,but it has not been related to the macroscopic properties of the materials corresponding to the structure;On the other hand,the research focuses on the blending of TPI with other rubbers or plastics,which aims at obtaining the blending ratio and filler system to obtain the best performance,but does not reveal the microstructure mechanism under the macroscopic performance.Based on the above reasons,this subject uses two types of accelerator systems,fixed accelerator dosage,and design experiments in the order of increasing sulfur dosage.DSC,polarizing microscope and wide-angle X-ray diffraction(WAXD)/small-angle X-ray scattering(SAXS)to study the non-isothermal crystallization kinetics,spherulite morphology,growth rate,crystal transformation and other microcrystalline properties of TPI,as well as the orientation structure;Macroscopic properties such as vulcanization characteristics and mechanical properties of TPI were studied through a rotorless vulcanizer,a tensile tester,a hardness tester;and a nuclear magnetic crosslink density meter was used to determine the specific crosslink density.The aim is to establish a causal relationship between macroscopic properties(mechanical properties,crystallization properties)and microstructure,to explore the microscopic mechanism under macroscopic properties,and to establish the effect of sulfur system and sulfur dosage on the TPI crystallization properties.The specific results are as follows:For the accelerator NS system,as the amount of sulfur increases:DSC experiment results show that the crystallization rate constant Z_c gradually decreases,the melting enthalpy decreases,and the Avrami index does not change much,indicating that sulfur reduces the crystallization ability of TPI,but it has not fundamentally changed TPI vulcanizate nucleation and growth mode.The results of polarizing microscope show that the TPI crystal area gradually decreases,the growth rate of the spherulites in the crystal area gradually decreases,the size of the spherulites gradually decreases,and two types of crystals are generated successively.The NMR results show that the crosslink density increases rapidly,the relaxation time indicates that the crosslink density gradually increases,and A(T2)and A(Mc)indicate that the three-dimensional space network is becoming more and more perfect.The results of mechanical properties show that the performance of each property decreases significantly,and the effect of the mechanical properties of the rubber compound due to weakened crystallization is stronger than the enhancement effect caused by the establishment of the vulcanization network.When the amount of sulfur is 2 phr,TPI vulcanizate can improve the crystallization performance and have better mechanical properties at normal temperature.For the accelerator M system:the effect of increasing the crosslink density on the TPI crystallization performance and mechanical properties is the same,but the impact trend is slowing down.As the amount of sulfur increases,DSC results show that before4 phr of sulfur,TPI still has two melting peaks and is close to one another.Polarization microscope and nuclear magnetic crosslink density also showed the same results.The mechanical performance results show that TPI vulcanizate can maintain good mechanical properties before the amount of sulfur is 6 phr.When the amount of sulfur is 6 phr,TPI vulcanizate can still crystallize above 0~oC and has good mechanical properties.After adding carbon black reinforcement,the overall trend of the influence of the amount of sulfur on the crystallinity and mechanical properties of the TPI vulcanizate is unchanged for the two vulcanization systems,but there are many differences.DSC data shows that the downward trend of each crystal melting parameter slows down,and the peak melting temperature and peak crystallization temperature are higher than those without carbon black,but the melting enthalpy and crystallization enthalpy are lower than without carbon black;More than the group without carbon black.This may be because carbon black provides a nucleation point,but it suppresses the movement of the TPI molecular segment and the grain growth process is hindered.After adding carbon black,the mechanical properties of TPI vulcanizate were significantly improved.In the experiment,the effect of stretching on the structure evolution of TPI was tested for the accelerator M system.The WAXS and SAXS results showed that the intensity of the characteristic peak of the?crystal form gradually decreased and the diffraction peak gradually changed from sharp symmetry when the amount of sulfur increased Slow down,indicating that the composition of the amorphous region increases and the crystallization ability decreases.During the stretching process,the two crystal forms coexist.For pure TPI,at a strain of 0.4-0.53,the monoclinic?crystal form is transformed into a highly oriented orthogonal?crystal form by stretching.When the amount of sulfur is 4phr,the transformation from?crystal form to?crystal form occurs between strains of 0.8-1.25.