Strain Softening Behavior Of Isoprene Rubber Vulcanizates And Their Nanocomposites

Payne effect and Mullins effect are common viscoelastic behaviors related to strain softening of rubber martials,and they significantly affect hysteresis,heat generation and damping under dynamic mechanical actions.The crosslinking network and viscoelastic behaviors of rubber materials are strongly influenced by the defect chains formed in the nonideal crosslinking network of cured rubber,and by the absorption of additives to fillers as well as strain amplification effect induced by the filler inclusions.However,it is still controversial on the research of strain softening of rubber nanocomposites,and few studies focus on the methods for tuning crosslinking network,and the relationship between strain softening behavior and crosslinking network.In this dissertation,the strain softening behavior of isoprene rubber(IR)and their nanocomposites was investigated,and the methods for regulating crosslinking network were also discussed.First,the influence of the crosslinking network cured by different vulcanization system and changed during thermo-oxidation on the nonlinear rheology behavior was comparatively investigated.Then,based on the relationship between crosslinking network and nonlinear rheology behavior,two methods including the covulcanization of IR and liquid rubber(LR)with low molecular weight and the blending of IR with other rubbers were used for regulating the crosslinking network.This work gives a new insight for regulating the reinforcement and mechanical hysteresis behaviors of rubber nanocomposites,and provides theoretical basis for developing high strength and low hysteresis rubber materials.The detailed research works and results are as following.IR vulcanizates cured by conventional vulcanization(CV)and efficient vulcanization(EV)systems respectively were thermo-oxidatively aged.The influence of changed crosslinking network during thermo-oxidation on the nonlinear rheological behavior was investigated.The results indicate that the nonlinear rheological behavior of EV vulcanizates with slight chain scission during thermo-oxidative aging largely dependences on crosslinking density,whereas such behavior of CV vulcanizates is determined by not only crosslinking density but defective chains owning to serious chain scission during thermo-oxidative aging.Enlighted by the role of defect chains on the nonlinear rheology behavior,IR was co-vulcanized with liquid isoprene rubber(LIR)with low molecular weight for tuning the topological structure of the crosslinking network in IR vulcanizates and their carbon black(CB)filled vulcanizate nanocomposites.The addition of LIR increases the tensile strength under large deformation with only slightly affecting the elongation at break when LIR content being less than 40 phr.Meanwhile,adding LIR also lowers storage modulus and loss factor at low strain amplitude during dynamic shear deformation,as well as mechanical hysteresis in large deformation during cyclic tension runs which is attributed to the delayed onset strain of strain induced crystallization(SIC)and weakened ability in SIC.For CB filled IR vulcanizate nanocomposites,the reinforcement effect of CB on matrix is higher than that predicted by hydrodynamic effect,and the addition of LIR slightly influences the distribution of CB in the matrix,the content of bound rubber and glass transition temperature of rubber matrix.In addition,their Payne effect can’t be described by Kraus model which attributes Payne effect to the breakdown of filler network.Moreover,the co-vulcanization of IR with LIR reduces mechanical hysteresis without impairing the mechanical properties,which is assigned to the change in crosslinking density and LIR content.This result provides a new strategy for balancing the contradiction between the nanoparticle reinforcement effect and energy dissipation in rubber nanocomposites.During thermo-oxidation aging,the crosslinking density and sol content of IR/LIR vulcanizates and their nanocomposites are higher and less than those of IR vulcanizates and their nanocomposites,respectively.Thus,LIR introduced is beneficial to stabilize the mechanical strength during thermo-oxidation aging.To choose a LR with appropriated molecular weight and molecular weight distribution for effectively regulating the crosslinking structure,liquid butadiene rubber and LIR were used to prepare a binary LR(wide distribution in molecular weight)with similar weight-average molecular weight with that of LIR(narrow distribution in molecular weight),and the effect of molecular weight distribution of LR on the strain softening behavior of IR vulcanizates and their silica filled vulcanizate nanocomposites was investigated.For silica filled vulcanizate nanocomposites,the distribution of silica has weakly influenced by the addition of LR and its molecular weight distribution,and their Payne effect can’t be fitted by Kraus model.In comparison with the vulcanizates containing LR with narrow molecular weight distribution,the sol content of the vulcanizates containing LR with broad molecular weight distribution is higher with decreased crosslinking density.Thus,the LR in the latter vulcanizates has a lower ability to influence the mechanical hysteresis behavior.However,if the sol contents in both vulcanizates are close to each other,the rheology and cyclic deformation behaviors will be weakly affected by the molecular weight distribution of LR.To weaken the contribution of SIC to the mechanical hysteresis of IR vulcanizates,IR and NBR were blended to prepare CB filled vulcanizate nanocomposites,and different mixing sequences were used to change the selective location of CB in two rubber phases.The influences of rubber composition and micromorphology on the SIC of IR components and strain softening behavior of nanocomposites were investigated.The results indicate that filling does not alter the compatibility between IR and NBR components,and NBR could weaken the strain hardening behavior of the IR vulcanizates.The location of CB in the blends slightly influences the Payne effect.In a fixed strain during cyclic deformation,SIC of the IR phase has more prominent influence on the mechanical hysteresis in the second and subsequent cyclic deformations than that in the first cycle.On the other hand,the selective distribution of CB can influence the mechanical hysteresis of the nanocomposites via affecting the SIC behavior of the IR phase.In comparison with the rubbers without a strong SIC propensity,those demonstrating strong SIC behaviors are more sensitive to the CB location.Finally,LIR was added to the IR/NBR vulcanizate nanocomposites for further reducing the mechanical hysteresis.