Formation,Melting And Crystallization Behavior Of TPI Vulcanizate Polymorphs

Trans-1,4-polyisoprene（TPI）has four crystal forms,α,β,γ and δ,but only two crystal forms,α and β,are formed during melt crystallization.Symmetry of molecular structure endows TPI with good crystallinity and high crystallinity.It is widely used in rubber materials（tires,shock absorption/vibration equipment）and medical materials（splints,teeth,etc.）.However,there are also many problems that plague the application development due to insufficient basic research.For example,the Young’s modulus,linear compressibility and bulk compressibility of α-crystal and β-crystal along the chain axis direction are different due to the different chain conformation and chain packing.Secondly,our research group found that the mechanical behavior of pure α-crystalline and pure β-crystalline TPI materials is different: The yield strength of α-crystalline materials is higher than that ofβ-crystalline materials.secondary necks and strain hardening behavior of β-crystal materials during tensile processes that are not in α-crystal materials.Then the ratio of α crystal and β crystal will definitely affect the mesoscopic properties of the material in use.Therefore,elucidating the formation and control of TPI vulcanizate polymorphism and its relationship with mechanical behavior is of great significance for material property control and service stability.Although basic research on TPI polymorphism began in the 1950 s,there has been no report on the formation,crystallization and melting behavior of TPI vulcanizate polymorphs so far.The vulcanization process is an important process that gives TPI the necessary strength,elasticity,and low permanent deformation when it is used as a rubber material.The establishment of different crosslinking networks caused by different accelerators and vulcanization systems and different amounts of sulfur will have a significant impact on the formation,crystallization and melting behavior of the vulcanizate polymorph,and the mechanical behavior associated with it.The elucidation of these issues helps to control the structure and properties of TPI during processing.Based on the above reasons,this paper studies the polymorph formation,melting and crystallization behavior of TPI vulcanizate.The influence mechanism of vulcanizate composition（type of accelerator,amount of sulfur,carbon black）,processing conditions（cooling temperature and cooling rate,etc.）and properties of cross-linking network on formation of αand βcrystals and their kinetics,crystallinity and their ratio,and crystal morphology were studied in detail.Research indicates:Influence of crystallization temperature on polymorph formation and relative proportion of TPI vulcanizate: When isothermal crystallization at 40 °C,TPI vulcanizate only generates α crystals.During isothermal crystallization at 30 °C,two crystals are formed,mainly α crystals.During isothermal crystallization at 20 °C,the two crystals generated by crystallization are mainly β crystals.When isothermal crystallization is performed at 0 °C,almost all of the crystals are formed into beta crystals,and when the sulfur fraction is small,there will be a very small amount of alpha crystals.In isothermal crystallization at 45 °C,TPI vulcanizate only generatesα crystals when the crystallization time is sufficient,and the more sulfur is used,the longer the isothermal crystallization time is.The effect of crystallization cooling rate on TPI vulcanizate polymorph formation and its relative proportions: When TPI vulcanizate is non-isothermally crystallized,when the cooling rate is 1 °C/min,the crystallization of TPI vulcanizate with low cross-linking degree only produces α crystals,and TPI vulcanizate with high cross-linking degree（accelerator M,sulfur 7-10 phr）Crystallization produces both alpha and beta crystals.When the cooling rate is 2 ℃/min,the low cross-linking degree TPI vulcanizate crystallizes into α and β crystals.When cooled at room temperature,TPI vulcanizate crystallizes into two crystal structures.When quenched and cooled by an ice-water mixture,almost all of the TPI vulcanizate crystallized into beta crystals.The faster the cooling rate,the more favorable for the formation of β crystals when the TPI vulcanizate is crystallized.Effects of accelerator type,sulfur content and carbon black on the formation of TPI vulcanizate polymorphs and their relative proportions: The polymorph formation of TPI vulcanizates is affected by the cross-linking effect caused by the type of accelerator and the amount of sulfur.TPI vulcanizate polymorphs and their relative proportions are mainly affected by temperature and its cooling rate.When the crystallization temperature is determined,the greater the degree of cross-linking,the more favorable for the formation of β crystals when the TPI vulcanizate is crystallized.Accelerator NS has greater catalytic activity on TPI vulcanizate,and the crosslinking density of vulcanizate is higher.Compared with accelerator M,the relative content of β crystal is higher under the same sulfur dosage.The higher the amount of sulfur,the higher the relative content of β crystals under the same conditions.From the pure TPI without crosslinking to the vulcanizate with crosslinking,the sudden increase of β crystals is the most obvious.Carbon black is a nucleating agent for α crystals.Under the same conditions,the relative content of αcrystals is more when adding 50 phr of carbon black filled TPI vulcanizate to crystallize.Crystallization behavior of TPI vulcanizates: When the isothermal crystallization temperature of TPI vulcanizate is lower than 10 °C,the crystallization rate is faster,and the crystallization rate is not affected by the amount of sulfur.When the isothermal crystallization temperature is higher than 20 ℃,the crystallization rate becomes slower,and the crystallization rate decreases with the increase of temperature,and decreases with the increase of sulfur dosage.The crystallization rate of NS system is slower than that of M system under the same amount of sulfur.50 phr carbon black filling will reduce the crystallinity of the system and slow down the crystallization rate,which has a significant inhibitory effect on the crystallization process.In the non-isothermal crystallization process of TPI vulcanizate,the Avrami index n does not change with the change of accelerator type and sulfur fraction,which is approximately equal to 3.It shows that the nucleation mode of TPI vulcanizate is homogeneous nucleation and the growth mode is two-dimensional growth.Increasing the amount of sulfur slowed down the crystallization rate and decreased the crystallization rate constant.The crystallization rate constant of the accelerator NS with the same amount of sulfur is smaller,indicating that the accelerator NS has a stronger inhibitory effect on crystallization.Melting behavior of TPI vulcanizates: The melting point of TPI vulcanizate decreases gradually with the increase of sulfur fraction during melting.For crystals with higher relative content of α and β during crystallization,the melting point decreases more obviously with the increase of sulfur content during melting.Carbon black will lower the melting point of TPI vulcanizate by about 2 °C.The WAXD melting point of β crystal is 50-57 °C,the DSC melting point is 47-52 °C,the WAXD melting point of α crystal is 60-69 °C,and the DSC melting point is54-62 °C.β spherulites have a larger refractive index and appear brighter in POM photos,and the brighter β spherulites disappear first during melting.