Role of tie molecules in the yielding deformation of isotactic polypropylene

We examined the effects of the tie‐molecule fraction on the yielding behavior of two isotactic polypropylenes, one having little ethylene content and the other as the homopolymer with no ethylene. The tie‐molecule fraction of the samples used in this study was controlled by blending ethylene‐α‐olefin of an α‐olefin content above 50 mol % in the blend of which the copolymers were incorporated into the amorphous regions of polypropylene (PP). An excellent linear relationship was observed between the measured yield stress and the tie‐molecule fraction estimated from the Huang–Brown model, suggesting that the tie‐molecule fraction and lamellar stiffness determine whether the lamellar fragmentation is easily activated or not, depending on the PP composition. Furthermore, an extended Huang–Brown model predicts a lamellar cluster connecting about five lamellae, which has a potential to account for morphological transformation of the spherulitic structure into a fibrillar one. Comparing the immiscible blends showing a phase‐separated morphology with the partially miscible blends mentioned above, the yield stress was lowered by the presence of rubber phase, apparently in a similar manner; but the yielding processes were clearly discriminated between both cases when the yield stress was plotted against the tie‐molecule fraction. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 357–368, 1999     

On the viscoelastic and plastic behavior of semiaromatic polyamides

Different semiaromatic polyamides (SAPA) have been synthesized by step‐growth polymerization of an aliphatic diamine, M (the 2‐methyl 1,5‐pentanediamine), and isophthalic acid, I, or terephthalic acid, T, or mixtures of these two diacids. The influence of the relative amount of randomly distributed MT units on the viscoelastic properties of the materials was investigated. It was shown that the glass transition T_g, as deduced from DSC thermograms, and the relevant mechanical relaxation T_α raise when the content of MT units increases. In contrast, the broad low‐temperature secondary relaxation, called γ, does not markedly depend on the MT content. Samples systematically studied in the absence of any moisture did not exhibit the intermediate‐temperature secondary relaxation, called β, which is characteristic of the wet polyamides. The study of the plastic behavior was focused on the samples MI and I5, which are strictly amorphous, and contain 0% and 50 mol % of MT units, respectively. Mechanical experiments were carried out in both the compression and traction modes, at temperatures ranging from −80°C to T_g. Analysis of the compression data was based on the inspection of the temperature dependence of elastic modulus, E(T), yield stress, ς_y, plastic flow stress, ς_pf, and strain softening ς_y − ς_pf. Whereas the plots of ς_y as a function of temperature, T, reveal some differences between MI and I5 behavior, a unique master curve was obtained by plotting ς_y/E(T) vs. T − T_g, which means that the plastic behavior of these materials is controlled by their chain packing in the glassy state. The strain softening profile of MI and I5 is similar to that already reported in the case of brittle vinyl polymers. This observation is consistent with the traction data, which give evidence for the occurrence of the tensile yielding of MI and I5 at temperatures rather close to T_g. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1131–1139, 1999     

Wear and friction in glassy polymers: Microscratch on blends of polystyrene and poly (2,6-dimethyl-1,4-phenylene oxide)

The microscopic process of abrasive wear and friction in glassy polymers was studied by using a special microscratch technique. A miscible blend of polystyrene (PS) and poly(phenylene oxide) (PPO) was used. It was found that as the composition varies there seems to exist two wear regimes in the blends controlled by different breakdown mechanisms corresponding to the brittle—ductile transition. Detailed study of the contact loads and SEM micrographs indicate that abrasive wear in the glassy polymers is controlled by microcracking under the asperity contacts. The critical load τ_c for initiating microscopic cracks can be linked to the macroscopic wear via a statistical Weibull model where τ_c is taken to be the mean of a strength distribution function. On the other hand, the friction coefficient was found to be independent of the composition but to vary strongly with the contact load. It approaches zero at the extrapolated zero load, but increases rapidly and eventually levels off with contact load. This behavior can be understood by a simple frictional adhesion model in which the polymer deformation during a frictional contact is analyzed by considering the compressive plastic ploughing and shearing yielding around the asperity contact. The shear strength S_o of the polymer/asperity contacts was found to vary with the normal load. The vertical scratch hardness H_v, which characterizes the spontaneous indentation yielding on the polymer surface, was found to be independent of scratch length and depth, and indeed can be regarded as a material constant. Although both S_o and H_v can accurately describe the frictional behavior of the glassy polymers, they bear no correlation to abrasive wear in the same materials. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35 : 1295–1309, 1997     

Challenges and opportunities:a hopeful future of polymer rheology in China

Polymer rheology has a long research tradition and holds an important position in the scientific community.It is supposed to be a valuable subject to the vast polymer industry.This article points out the exciting developments that are transforming our understanding of the world of polymer rheology.We discuss not only the emerging challenges in the area but also how China might wish to seize the moment,pick up the trend and take advantage of the potential economic benefits to the huge petro-chemical industry...     

Rate‐, temperature‐, and structure‐dependent yield kinetics of isotactic polypropylene

The influence of cooling rate on the structure and resulting mechanical performance is explored for a set of isotactic polypropylenes with varying molecular weight, insertion of counits, and addition of a nucleating agent. A continuous variation of crystal type (α–mesomorphic phase competition) and structural features is obtained with cooling rate. These variations are discussed in relation to the strain rate‐ and temperature‐dependent yield stress and time‐to‐failure kinetics. The deformation kinetics, characterized by constant activation volumes and energies in the Ree–Eyring theory, prove to be the same for various structures. Differences in thermal history are solely captured by two rate constants that are function of the structure. © 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012     

双向应力状态下铝硅合金ZL105塑性行为的实验研究

本文对典型的拉、压性能不同的金属材料铝硅合金ZL105的塑性行为,屈服、强化、体积变化和流动特性,在双向应力状态下进行了全面和系统的实验研究,深入地探讨了影响材料塑性行为的主要因素,得出了新的结果和重要的结论。实验结果表明,平均应力直接影响材料的塑性行为,平均压应力使材料的韧性增强,而平均拉应力使材料的脆性增强。平均压应力不利于塑性变形的发生,相应平均压应力增大的应力状态,材料的屈服应力提高。与流行的塑性体积膨胀的观点不同,在平均应力是压应力的应力状态下,塑性体积收缩,因此,经典的正交准则不再适宜描述该材料的流动性质。材料的强化性质明显地依赖于应力状态。     

Effects of lattice incompatibility-induced hardening on the fracture behavior of ductile single crystals

Plane-strain mode-I cracks in a ductile single crystal are studied under conditions of small scale yielding. The specific case of a (0 1 0) crack growing in the [1 0 1] direction for an FCC crystal is considered. Crack initiation and its subsequent growth are computed by specifying a traction-separation relation in the crack plane ahead of the crack tip. The crystal is characterized by a hardening model that incorporates physically motivated gradient effects. Significant traction elevation ahead of the crack tip is obtained by incorporating such effects, allowing a better basis for the explanation of experimentally observed cleavage in the presence of substantial plastic flow in slowly deforming ductile materials. Resistance curves based on parameters characterizing the fracture process and the continuum properties of the crystal are computed. Simulation results indicate that the length-scale of the lattice incompatibility-dominated region has to be comparable or larger than the length of the fracture process zone for gradient effects to have a significant effect on fracture resistance. Both the work of separation and the peak separation stress also play important roles in determining the fracture resistance of the crystal.     

铝合金板材塑性性能的研究

铝合金在汽车工业中的广泛应用对于降低汽车重量、减少燃油消耗和汽车尾气的排放量具有十分重要的意义，但其室温塑性成形性能却受到了锯齿形屈服行为的影响，从而制约了铝合金进一步的推广应用。本文基于合金材料塑性变形过程中位错和溶质原子间相互作用的分析，建立了一个可用于描述锯齿形屈服现象的唯象本构模型。该模型将溶质原子对位错运动的钉扎效应和位错挣脱后的脱钉效应置于一个统一的框架内进行考虑，而这两个效应的相互竞争将决定材料宏观变形行为的发展演化。基于该模型的数值模拟结果和实验测试结果取得了良好的一致性，从而验证了理论和模型的有效性。     

Plastic deformation behavior of polypropylene sheet with transversal orientation

The deformation behavior of a polypropylene (PP) sheet, in which c‐axis of β‐form PP orients perpendicular to the flow direction in the sheet [transverse direction (TD)], is studied focusing on the mechanical anisotropy. The deformation mechanism of this sample is found to be strongly dependent on the stretching direction. Shear yielding is dominant in TD stretching, indicating that reorganization and phase transformation of crystalline form occur easily with low yield stress. On the contrary, in machine direction (MD) stretching, numerous microvoids appear with the transformation of crystalline form as well as the rotation of c‐axis. Moreover, the crazing process, initiated at the orientation defects, occurs locally with remaining undeformed region in which β‐form PP orients to TD. The deformation behavior is applicable to a microporous film. © 2013 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2013, 51, 897–906     

Cyclic nonlinear behavior of a glassy polymer using a contact method

The effects of repeated large strain shear cycles on the dynamics of a glassy acrylate polymer are investigated using an original contact method. It is based on the measurement of the shear properties of thin (about 50 μm) polymer films geometrically confined within contacts between elastic substrates. Under small amplitude (300 nm–10 μm) oscillating lateral displacements, friction at the contact interface can be neglected and the measurement of the contact lateral response thus provides information about the rheology of the sheared polymer film. Using this approach, the complex shear modulus of the polymer film can be measured both in the linear (viscoelastic) and in the nonlinear regimes. The investigations are focused on the changes in mechanical properties induced in a large strain regime where the polymer glass is cyclically sheared up to the yield point. During the application of large strain cycles, the mechanical response of the polymer glass slowly evolves toward a quasi stabilized state which is described from the measurement of an apparent–strain dependent–complex shear modulus. When the applied strain is increased by a tenfold factor, this apparent shear modulus decreases by about one decade. These underlying changes are investigated from a consideration of the time dependent linear viscoelastic properties after the mechanical stimulus. Both mechanical rejuvenation and recovery (ageing) effects are evidenced. © 2011 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2011