Crystal Transition Behavior During Stretching Of TPI Vulcanizates

Trans-1,4-polyisoprene（TPI）is the trans isomer of natural rubber（NR）,and the symmetrical trans configuration endows it with good crystalline properties and polymorphism,making it widely used as rubber materials（tires,shock absorption/vibration equipment）and medical materials（splints,teeth,etc.）.During the melt crystallization of TPI,only two crystal forms,αandβ,are formed.The Young’s modulus,linear compressibility and bulk compressibility ofαandβcrystals along the chain axis direction are different due to the different chain conformation and chain packing.Our research group found that the mechanical behavior of pureαform and pureβform TPI materials（including neat TPI materials and TPI vulcanizate materials）is different:The yield strength of theαform material is higher than that of theβform material,and theβform material has a secondary neck and its strain hardening behavior that theαform material does not have during the tensile process.Secondly,the study found that the polycrystalline phenomenon of TPI materials will undergo crystal transformation due to changes in temperature and tensile strain during use.Then the ratio ofαform andβform and the transformation of crystal form will definitely affect the mesoscopic properties and material stability of the material in use.Therefore,elucidating the formation and control of TPI vulcanizate polymorphism and its relationship with mechanical behavior is extremely necessary for material property control and service stability tracking.Although the basic research on TPI polymorphism began in the 1950s,there has been no research on the formation,crystallization and melting behavior of TPI vulcanizate polymorphism,as well as temperature and strain on crystal transformation and material performance stability.Furthermore,as a necessary procedure of rubber processing,vulcanization endows TPI with the necessary strength,elasticity and low permanent deformation when used as a rubber material,the establishment of different cross-linking networks caused by different accelerators,vulcanization systems and different sulfur dosages will inevitably affect the formation of vulcanizate polymorphs,crystallization and melting behaviors,crystal transformations,and the associated mechanical behavior and its related mechanical behavior.The clarification of these issues not only helps to control the structure and properties of TPI during processing,but also helps to control the performance stability of TPI during use.As one of a series of studies,wide-angle X-ray diffraction（WAXD）was used to study the crystallization behavior,crystallinity,relative ratio ofαform andβform and orientation degree of TPI vulcanizates under different strains,different sulfur dosages and different strain rates.Small angle X-ray scattering（SAXS）was used to study the crystallization transition process,long period of samples and the change behavior of intermolecular molecules of TPI vulcanizates under different strains,different sulfur dosages and different strain rates,The crystal transformation behavior and its influencing factors of TPI vulcanizate during stretching were analyzed.The specific results are as follows:The effect of strain on the polycrystalline transformation behavior of TPI:（1）As the stretching progresses,the crystallinity of theαform continues to decrease,and theβform is generated when the strain is 0.77,and the crystallinity of the vulcanizate with M accelerator and 2 phr sulfur（M2）system also sharply increased.Since there may be a small-sizedβform that cannot be detected by WAXD at the beginning of stretching,the content of theβform is not 0 when detected.（2）The degree of orientation of theβform just produced is much greater than that of theαform.As the stretching progresses,theβ-crystal skeleton gradually replaces theα-crystal skeleton to bear the stress.At the later stage of stretching,the crystalline framework composed of the residualαcrystal phase and the highly orientedβcrystal phase and the amorphous state in the system are intertwined with each other,forming a structure of two interpenetrating networks.（3）Before the appearance of theβform,the long period（L）keeps decreasing,and after theβform appears,the thickness of the amorphous（L_a）and L increased with the increase of strain,the thickness of the crystal region（L_c）keeps decreasing.By analyzing the end-to-end distances of molecular chains in the M2 system,it was found that the tie molecules and the entangled loops could always exist stably in the M2 system,which led to the continuous increase of L during the stretching process but the increasing amplitude gradually decreased.The effect of the amount of sulfur on the crystal transformation:（1）With the increase of the degree of crosslinking,the size of the lamella gradually decreases,the defects of the crystal region increase,the crystallinity of the vulcanizate gradually decreases,the increase in the number of interlamellar linking molecules leads to the gradual decrease of the strain when theβcrystal form appears with the increase of the sulfur fraction,the crystallinity of the vulcanizate system increased sharply after the appearance of theβform,and the less the sulfur fraction,the greater the increase.The relative content of theαform decreases sharply with the increase of strain at low sulfur fraction,but the degree of crystal transformation gradually decreases with the increase of sulfur fraction.This may be due to the excessive entanglement of molecular chains caused by cross-linking,which slows down and weakens the mobility of molecular chains,increases the content of intermolecular molecules in the system,and decreases the crystallinity and the degree of crystal transformation.The crystallinity ofβform increases with the increase of strain in all vulcanization systems.The more sulfur fraction,the smaller the crystallinity.（2）The degree ofβ-crystal orientation in the vulcanizate is always higher than that of theβ-crystal orientation in the unvulcanized system,while the effect of sulfur on the degree ofα-crystal orientation is small.This may be due to the fact that the thickness of the amorphous region is smaller due to vulcanization,so theα-platelet in the vulcanization system can complete the crystal transformation with less slip and rotation.Therefore,the excess strain acts on the larger orientation of theβform.（3）More entangled loops and tethered molecules can be generated between the amorphous layers in the vulcanized system than in the unvulcanized system,which leads to the fact that the vulcanizate can produceβform under smaller strain and L of the unvulcanized system is always larger than that of the vulcanized system under the same strain.As the stretching progresses,the L and L_a of the unvulcanized system remain basically unchanged,while the L and L_a of the vulcanized system will continue to increase.This may be due to the fact that the cross-linking bond hides the molecular chain length in the amorphous region,resulting in a smaller L_a in the vulcanized system;as the strain increases,the cross-linking bonds are destroyed,and the molecular chain length of the hidden amorphous region is revealed,which leads to the continuous increase of L and L_a in the later stage with the increase of strain.The effect of stretching rate on crystal transformation:（1）A larger stretching rate will lead to a delay in crystal transformation,and when the strain is small,the crystallinity of the vulcanizate gradually increases with the increase of the strain rate.At the end of the stretch,the vulcanizate crystallinity will remain consistent at all stretch rates.This may be because the crystallization rate of TPI mainly depends on the growth rate,which in turn depends on the thermal motion ability of the molecular chain,and the faster stretching rate leads to the stronger motion ability of the TPI molecular chain,therefore,at lower strains,the crystallinity of the system with high stretching rate is higher.And because the different stretching rates lead to different hysteresis,the degree of crystal transformation of TPI vulcanizate is smaller than that under the same conditions at low stretching rates.（2）In neat TPI system,the orientation degree of theα（110）crystal plane gradually decreases with the increase of the stretching rate,while the orientation degree of theβ（120）crystal plane gradually increases with the increase of the stretching rate.The degree of orientation of the two crystal forms does not change with strain in the vulcanization system.This may be due to the fact that the content ofβform is higher at a faster stretching rate,and the system with moreβ-platelet content has a larger anisotropy,which in turn leads to a larger degree of orientation.（3）In TPI and its vulcanizate system,the internal periodic structure（L,L_a and L_c）of the sample is not affected by the stretching rate.The number of intermolecular molecules is independent of the magnitude of the stretching rate.