Long‐time relaxations in rubber‐modified polymer systems

Linear and nonlinear viscoelastic properties were measured in the molten state for several model ABS polymers with different rubber particle contents. Linear viscoelastic functions for ABS polymers can be separated in two parts. One is a relaxation associated with the entanglement of matrix SAN chains and the other comes from the particle‐particle interactions of rubber particles. This relaxation depends strongly on the degree of dispersion of rubber particles. The second‐plateau modulus appeared at low frequency with samples in which rubber particles agglomerate. While, the second‐plateau modulus was not observed with samples in which rubber particles are finely dispersed. Matching of AN content between grafted and matrix SAN and optimum graft density form a finely dispersed system. Large deformation relaxation measurements revealed that the damping of ABS polymers having a good dispersion of particles become stronger with an increase in rubber content. This strong damping can be explained by a layered structure. The very long relaxation was found for higher rubber content, when the neighboring grafted SAN chains contact with each other.     

低乙烯含量共聚聚丙烯链结构和相结构与应力应变行为的关系

研究了两种典型的低温抗冲共聚聚丙烯(ICPP)的应力应变行为及温度依赖性,其低温应力应变行为揭示ICPP具有优异的低温抗冲性能和综合力学性能的结构本质.从初始弹性模量、屈服应力及断裂伸长随温度变化所显示的变化规律进一步确认了两种ICPP的序列结构特征和相结构特征.断裂伸长变化显示了乙丙橡胶相的增韧作用,屈服应力变化显示了分散相对基质结合紧密程度的影响,弹性模量则与基质的结晶状况和结晶形态有着比较密切的关系,同时也与乙丙橡胶相和聚丙烯基质的玻璃化转变温度密切相关.     

Cylindrical Approximation of Constrained Chain Model for Rubber Elasticity. I. Constant Volume Deformations

This paper reports a new model for rubber elasticity based on the geometric constraints imposed on a network chain and the chain segments by the crosslinks at each end. Through consideration of these constraints, the number of conformations available to a network chain can be calculated directly assuming the chain is on a lattice. By simplifying the model and by assuming affine deformation, the rigorous equations for the conformational entropy are reduced to an analytic expression for the strain-energy function. From this function, equations describing extension, compression and pure shear are derived. The model is tested against literature data for natural rubber and polybutadiene and is found to reproduce quantitatively all the salient features of the experimental stress-strain curves In the three modes of deformation. It is shown that the decrease in the Gaussian modulus with extension, often characterized by the C₂ term in the Mooney-Rivlin theory, arises naturally from the conformational entropy of a network chain, and special effects such as structure, topology, etc., need not be invoked to explain this phenomenon. The theory also predicts the near-constant modulus of extended highly swollen gels, an effect often considered as verification of Gaussian theory.     

The application of fracture mechanics to the impact behaviour of rubber-toughened polyamides

Rubber-modification of polyamide is a widely-applied method of improving material resistance under high strain rate loading. The processing conditions used for preparing such two-phase blends strongly influence their structure and thus their subsequent impact properties. In the present work the relationships between production parameters, phase structure and impact resistance have been studied, and the rǒle of the rubber phase in promoting energy absorption investigated. It has been found that improved impact resistance, defined as a combination of high resistance to crack initiation and to crack propagation, is achieved by decreasing the polyamide phase viscosity while increasing the extrusion and injection temperatures, the mixing shear rate and the rubber phase volume fraction; an EPR modifier offers superior performance to a polybutadiene modifier; the dominant mechanisms of energy-absorption are shearing and void formation, there being no clear evidence of crazing; the J-integral technique of plastic fracture mechanics can be applied to Charpy impact testing; TEM allied with image analysis techniques provides quantitative morphological information on polymer blends.     

Effects of the processing sequence and critical interparticle distance in PA6-clay/mSEBS nanocomposites

Polyamide 6 (PA6) based nanocomposites were modified by maleinized styrene/ethylene-butylene/styrene (mSEBS) rubber through a modified processing with the aim of ameliorating the microstructure and performance of the nanocomposites, as well as examining the parameters that make the brittle/ductile transition of nanocomposites change. The studied parameters were the rubber and the organoclay contents. After the modified processing, the average dispersed particle size decreased and an overall increase in the impact performance occurred. This is attributed to a more effective reaction between the maleic groups of the rubber and the amine end groups of the PA6 when there is no surfactant present. The analysis of the variation of the critical interparticle distance (τ_c) indicates that it depends on the modulus of elasticity of the matrix (i.e., on the organoclay content) and on the interfacial adhesion, wherein higher modulus of elasticity and higher adhesion leads to smaller τ_c values.     

Relation of elastic modulus to crosslink and entanglement concentrations in rubber networks

The theory of rubber elasticity relates the elastic modulus of unfilled amorphous rubber to the concentration of elastically effective strands. A theoretical relation between this concentration and the concentrations of potential entanglements, random tetrafunctional crosslinks, and chain ends was proposed recently. In the present work, the new relation was combined with the theory of rubber elasticity and verified experimentally. Polydimethylsiloxane samples were cured by ⁶⁰Co irradiation and were extensively extracted to determine gel fraction, which was used to calculate concentrations of crosslinking and scission due to irradiation. Equilibrium modulus values determined from creep tests were in excellent agreement with those calculated using the new relation if the average spacing between potential entanglements is 116 (CH₃)₂SiO units. Thus, in typical commercial silicone rubbers, the contribution to the modulus from trapped entanglements is greater than the direct contribution from crosslinks. The new relation allows the calculation of crosslink concentrations from modulus measurements on other unfilled rubbers once the potential entanglement spacing of the polymer is determined.     

Effects of Epoxidation Content of ENR on Morphology and Mechanical Properties of Natural Rubber Blended PVC

This research work has concerned a study on toughness of PVC/natural rubber (NR) blends compatibilized with epoxidized natural rubber (ENR). The aim of this work was to investigate the effect of degree of epoxidation on morphology and mechanical properties of the blends. Epoxidized natural rubber with a variety of epoxidation contents were prepared by reacting the NR latex with formic acid and hydrogen peroxide at various chemical contents. Chemical structure and epoxidation content of epoxidized natural rubber were evaluated by FTIR and ¹H-NMR techniques. After that, three grades of ENR with epoxidation contents of 15, 25 and 42 % (by mole) were further used for blending with PVC and NR in an internal mixer at 60 rpm and at 170 °C. From tensile and impact tests, it was found that tensile elongation and impact strength of the materials remarkably increased with degree of epoxidation. On the other hand, tensile strength and modulus of the materials rarely changed with the epoxidation content. An increase in toughness of the blends with epoxidation content was related to a better molecular interaction between PVC and ENR as suggested by torque-time curves of the materials.     

Formation of Network by Carbon Black，Silica and Their Mixing Fillers in Isoprene Rubber北大核心CSCD

The network formed by fillers has great influence on the mechanical properties of rubber materials. To understand the formation of network by carbon black,silica,and carbon black/silica mixing fillers in rubber and its influence on the properties of rubber,isoprene rubber/filler composites with different filler loadings are prepared and their micromorphology,rheological and tensile properties are investigated. It is found that the dispersion of fillers is better in rubber after cure than that in rubber before cure for all three rubber systems,and the filler size of silica is smaller than that of carbon black,but the aggregation is more severe in silica filled rubber system. In mixed filler system,the two fillers tend to aggregate separately, leading to the low modulus at small strain than that in single filler system. With the increase of the filler loading,the tensile strength increases first and then decreases,the elongation at break decreases,and the temperature rise in compression flexometer tests increases. Moreover,the temperature rise in mixed filler system is higher than that in single filler system at high filler loading. © 2022, Science Press (China). All rights reserved.     

Influence of structure on the impact behaviour of nylon-rubber blends

Blends of nylon-6 and EPDM rubber were prepared by reaction blending to study the influence of rubber concentration, rubber particle size and interfacial adhesion on the impact strength. Rubber particles induce a sharp brittle-tough transition which is independent of the glass transition of the nylon matrix. Increasing the rubber concentration or decreasing the particle diameter shifts the brittle-tough transition temperature for notched Izod impact tests to lower values. A toughening mechanism is proposed in which the interparticle distance, the rubber modulus and the temperature play crucial roles.     

The physical and mechanical properties and cure characteristics of NBR/silica/MWCNT hybrid composites

The main objective of the present study was to investigate the synergistic effect of simultaneous use of two reinforcing fillers in rubber compounds based on acrylonitrile-butadiene copolymer (NBR). Silica was used as reinforcing filler in all samples and the loading content was 25 phr. 3 and 5 phr of multiwall carbon nanotubes (MWCNT) were used as second reinforcing filler in NBR/silica compounds. Melt mixing method was employed for compound preparation. The effects of carbon nanotube/silica hybrid filler on mechanical and vulcanization characteristics of the rubber compounds were investigated. These results revealed that addition of the reinforcing filler, either carbon nanotube or silica, shortened the optimum cure time (t₉₀) and also scorch time (t_s1) of samples compared to that of pure NBR compound. In hybrid compounds, the reduction in optimum cure time and scorch time was higher than that of for silica-filled NBR or CNT-filled NBR compounds. This can be attributed to the synergistic effect between CNT and silica as two reinforcing agents in NBR compounds. Regardless the composition of the reinforcing filler, an increase of the relaxed storage modulus is observed, while the tan δ value is decreased steadily. The dynamic modulus reinforcement of nanocomposites was examined by the Guth Gold and Modified Guth Gold equations. For hybrid samples, the experimental values show a significant positive deviation from model predictions. According to the Barlow’s formula, hybrid compounds show higher burst strength compared to silica or CNT filled NBR compounds.     

橡胶在动态载荷下的能量损耗分析

以交联密度不同的同类轮胎胎面胶A1和A2为研究对象,通过动态拉伸实验得到储能模量及损耗模量随频率变化的曲线.建立了黏弹性广义Maxwell模型来定量分析不同温度的橡胶在不同频率的动态载荷下的能量损耗.采用非线性规划的方法分别在低频(10~25 Hz)及高频(25~60 Hz)下拟合模量-频率曲线,得到黏弹性广义Maxwell模型的参数值.采用有限元软件Abaqus模拟胎面胶动态拉伸过程并计算胎面胶的损耗角正切,得到不同温度下胎面胶的损耗角正切随激振频率的变化规律,通过和实验结果的比较证明文中所述黏弹性广义Maxwell模型及其参数获取方法可准确应用于胎面胶的动态拉伸性能分析.预测了在不同温度及频率下每一循环载荷周期中胎面胶的应力-应变迟滞回线以及单位体积胶料的能量损耗,阐释了不同温度下的胎面胶的能量损耗随频率的变化规律,同时结合2种胎面胶的交联密度测试数据分析了胶料的构效关系.     

Nanocomposites based on a polyamide 6/maleated styrene-butylene-co-ethylene-styrene blend: Effects of clay loading on morphology and mechanical properties

The addition of up to 6% of an OMMT to a 70/30 polyamide 6 (PA6)/maleated styrene-ethylene/butylene-styrene (mSEBS) blend led to ternary compounds where the rigidifying effect of the clay and the toughening effect of the rubber came together. In fact, in the 70/30 blend with 3% OMMT supertough behaviour was accomplished with a modulus increase of 44% with respect to the pure PA6 matrix. When the changes in morphology of the dispersed rubber phase in presence of OMMT are discussed, the slight decrease in viscosity upon clay addition does not explain the increase in rubber particle size that indicates a decrease in the compatibilization level. Interactions between the surfactant of the OMMT and the maleic anhydride groups of modified rubber are proposed as the reason for the decrease in compatibility. The maximum impact strength attained is rather independent of the clay content and the testing temperature. The increase in modulus of the blend upon clay addition was similar to that observed for the pure PA6 matrix, while maintaining the ductile nature in the ternary PN’s, which is not always present in PA6/OMMT binary materials.     

Energetic Stress in Rubbery Copolymers

Thermoelastic measurements were performed for four rubbery copolymers: Viton A, ethylene-propylene rubber (EPR), Hycar, and styrene-butadiene rubber (SBR). The relative energy contribution, f_e/f, for these elastomers were calculated by means of a recently proposed equation which is based on the temperature coefficient of shear modulus. It was found that the relative energy contribution or the temperature coefficients of the unperturbed dimensions of EPR and SBR are not simply related to those of the parent homopolymers. In addition, the copolymers appear to bear an opposite sign compared with the homopolymers. The thermoelastic behavior of one elastomer, Viton A, was investigated over a 200°C range up to high loads. Linear relations were obeyed within experimental error. Calculations on the basis of statistical theory of rubber elasticity shows that this is to be expected for elastomers that are Gaussian in behavior.     

聚合物共混物脆韧转变性能研究 V.橡胶粒子的分布对聚氯乙烯/丁腈橡胶共混物韧性刚性关系的影响

聚合物共混物脆韧转变性能研究Ｖ．橡胶粒子的分布对聚氯乙烯／丁腈橡胶共混物韧性刚性关系的影响刘浙辉朱晓光张学东漆宗能（中国科学院化学研究所工程塑料国家重点实验室北京１０００８０）蔡忠龙（香港理工大学应用物理系香港九龙）王佛松（中国科学院北京...     

Mechanical relaxation spectroscopy of three triepoxide-based polymers

Three network structure polymers formed by the chemical reactions of a triepoxide with aniline, 3-chloroaniline,and 4-chloroaniline were prepared and their shear modulus relaxation spectra studied over the 10⁻³- to 1-Hz range and temperatures up to their rubber modulus region. The decrease in the unrelaxed modulus with increase in temperature is found to be a reflection of both an increase in volume, and a decrease in the relaxed modulus of the sub-T_g relaxations process. It is quantitatively shown that the increase in the rubber modulus with increase in temperature above T_g is predominantly due to an increase in the entropy and not to a decrease in the number of cross-links density on thermal expansion. The unrelaxed modulus remained unaffected by the change in the overall size of the phenyl groups of the amines and of the steric hindrance to their rotations caused by the proximity of the chlorine atom to the cross-linking N-atom in the network structure, but the rubber modulus was effected. The shear modulus spectra could be fitted to a stretched exponential decay function with a temperature-independent stretch parameter of 0.25 for two polymers and 0.22 for one. The time–temperature superposition of the spectra did not yield a master curve, and a vertical displacement of the data also failed to produce it. This was more clearly demonstrated by the spectra of the mechanical loss tangent. After considering the various contributions to the shear modulus, it was concluded that deviations from the time–temperature superposition of the spectra are intrinsic to these polymers and arise from the change in the viscoelastic functions for segmental dynamics on change in the temperature such that the overall distribution of relaxation times remains unaffected. The mechanical loss tangent of the three polymers is found to be higher than that of polycarbonate at ambient temperature, implying a higher loss of mechanical energy before these polymers may fracture. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 3071–3083, 1999     

A note on the determination of Young's modulus of rubber compounds from the tire cord adhesion test

The cord pull-out force of the tire cord adhesion test (TCAT) is dependent on both cord-rubber adhesion energy and the Young's modulus of the matrix rubber of the TCAT testpiece. In order to determine the adhesion energy from the cord pull-out force, the rubber modulus must also be known. This paper describes a simple procedure for determining the rubber modulus from the load-deflection curve, which is obtained when the cord pull-out force is determined. An extra test operation (for determining modulus) is thereby avoided.     

An experimental and analytical study of the elasticity of model polyurethane networks crosslinked by tri- and quadriisocyanate

Polyurethane networks have been prepared from a mix of tri- and quadriisocyanate and from two types of diols, polyether-based (with molar masses of 1,000, 2,000, and 4,000 g/mol) and polyester-based (1,035 g/mol). The weight fraction of sol has been measured, as well as the elastic shear modulus of the gels. It has been found that the statistical theory of network formation predicts a weight fraction of sol in agreement with the experimental results, but its standard combination with the theory of rubber elasticity disagrees significantly with the elastic modulus measured. This suggests a discrepancy between theory and experiment in terms of elastically active chains. In contrast, the assumption that all nodes in the gel, or even in the system, are elastically active gives much better predictions for the system considered.     

Bubble formation in vulcanized rubbers

A theoretical treatment is given for the formation of visible holes in an elastic solid. It differs from previous treatments of bubble formation in liquids by neglecting the problem of hole nucleation altogether. Instead, a small spherical hole is assumed to be present initially. The inflation of such a hole by a dissolved gas is then considered. A critical internal pressure is deduced, at which the hypothetical hole would become infinitely large. This pressure is given by 5G/2; where G is the shear modulus of the rubber. Some model experiments are described in which swollen rubber vulcanizates were rapidly heated to bring the dissolved liquid into a superheated conditions. The temperatures of rapid bubble formation were determined for a number of vulcanizates having different degrees of crosslinking and hence different values of shear modulus, and for different swelling liquids. The results are shown to be in reasonably good quantitative agreement with the theoretical predictions.     

Gelatin hydrogels cross-linked with bisvinyl sulfonemethyl. 2. The physical and chemical networks

This paper deals with the physical and the chemical gelation of gelatin in the presence of a reactant, bisvinyl sulfonemethyl (BVSM). The strategy of this investigation is to separate the contributions of the two types of cross-links in order to deduce the resultant elasticity of the network. In addition, the question raised by several authors concerning an increase of the thermal stability of the triple helices in the presence of cross-links was examined by using several techniques. In this study, the concentration of gelatin and BVSM were kept constant, while the influence of the thermal protocols was put in evidence. The gel formation was followed by rheological, thermodynamic (microcalorimetry), and optical spectroscopy (optical rotation) measurements. The results demonstrate the large differences which arise on the storage moduli by changing the thermal protocols. Cross-linking of the networks in the presence of the triple helices induce a heterogeneous repartition of the bonds, which can form along the triple helices and at the end of the sequences. Consequently, the rubber like network obtained by denaturation of the triple helices is still reminiscent of the initial twist of the chains, and a large modulus is observed, as if rigid segments were still present (storage modulus 10 times larger than for random cross-linking). The hydrogels have an elastic modulus which is larger that the addition of the physical and chemical contributions. The interpretation of the network elasticity is based on the predominant role of the rigid rods of triple helices, where the BVSM cross-links can either modify the ratio between the apparent length and distance between rods, l/d, and/or increase the rigidity of the interchain connections, which are loose coils for the physical gels. The hydrogels investigated have a network which is still close to the percolation threshold of the physical gel, and therefore, the statistical models known for well developed networks cannot be directly validated in these experimental conditions.     

Thermal,mechanical and rheological properties of biodegradable poly(propylene carbonate) and poly(butylene carbonate) blends

Poly(propylene carbonate)(PPC) was melt blended in a batch mixer with poly(butylene carbonate)(PBC) in an effort to improve the toughness of the PPC without compromising its biodegradability and biocompatibility. DMA results showed that the PPC/PBC blends were an immiscible two-phase system. With the increase in PBC content, the PPC/PBC blends showed decreased tensile strength, however, the elongation at break was increased to 230% for the 50/50 PPC/PBC blend. From the tensile strength experiments, the Pukanszky model gave credit to the modest interfacial adhesion between PPC and PBC, although PPC/PBC was immscible. The impact strength increased significantly which indicated the toughening effects of the PBC on PPC. SEM examination showed that cavitation and shear yielding were the major toughening mechanisms in the blends subjected the impact tests. TGA measurements showed that the thermal stability of PPC decreased with the incorporation of PBC. Rheological investigation demonstrated that the addition of PBC reduced the value of storage modulus, loss modulus and complex viscosity of the PPC/PBC blends to some extent. Moreover, the addition of PBC was found to increase the processability of PPC in extrusion. The introduction of PBC provided an efficient and novel toughened method to extend the application area of PPC.