高聚物拉伸形变中的声发射

对一系列高聚物进行了单轴拉伸过程中声发射的观察,包括玻璃态高聚物、结晶高聚物、共聚物、共混高聚物和一种交联橡胶。非晶态高聚物拉伸时声发射次数很少,伴随银纹和微裂缝的产生而出现。结晶高聚物在屈服成颈时出现强的声发射,在颈部拉伸的初期声发射较少或不出现,拉伸到接近试件断裂前声发射强烈,次数急剧增多。相同高聚物但试件加工成形历史不同会在声发射上得到反映。交联的顺丁橡胶拉伸时声发射很弱,但可以观察到,在拉力-形变曲线开始偏离线性后出现,没有Kaiser效应。共混高聚物拉伸时声发射很多。高耐冲击共混接枝塑料在断裂前不出现强烈的声发射。如试样和试件加工成形条件相同,声发射现象的重演性是相当好的。     

几种高聚物拉伸过程中的声发射现象

研究了４种塑料ＰＳ、ＰＯＭ、ＡＢＳ和ＰＣ拉伸过程中的声发射现象，发现声发射信号主要产生在屈服前和断裂时，说明高聚物的声发射信号主查由于分子链的解缠结，晶体的滑移裂纹的亚临界扩展和分子链的断裂等诸因素产生的，ＡＢＳ拉伸时大面积银纹的生成也可能是信号源，拉伸应变速率，材料的结构和不同的屈服过程等对塑料的拉伸声发射规律有较大影响。     

聚对苯二甲酸乙二酯非晶态膜片单轴热——拉伸中的结晶与松弛

用动态力学损耗温度谱作为测试手段,研究了非晶态PET膜片在78—112℃温度范围内的单轴拉伸。实验结果说明,在较低温度下所得结晶的拉伸试样,完全由于应变诱发结晶,发生在应力-应变曲线的屈服后应力开始上升的阶段。在较高温度下(90℃或更高)拉伸可得非晶态而且光学各向同性的试样,是由于分子链的小尺度取向在拉伸过程中已完全热松弛所致,而分子链的大尺度取向要通过高弹态流动而松弛,其速率较慢,用拉伸后试样两端固定时的应力松弛进行了观察。在较低温度下应力松弛后仍为非晶态,在较高温度下应力松弛到起始应力的1O％下才开始结晶。FTIR研究表明在这种状态下的结晶有一结晶诱导期,其时间尺度与应力松弛阶段相当。     

高聚物从高弹态到流体态的转变

将高聚物由高弹态转变为流体态的转变温度命名为流动温度Tf,该转变温度与高聚物分子量密切相关.在高聚物从高弹态转变为流体态的研究中,由于T1.1的概念忽视对高聚物分子量的依赖性,因此采用Tf的概念更为合理.本文对高聚物的流动温度Tf的讨论涉及高聚物温度-形变曲线、高弹态温区、高聚物熔体剪切粘度.从高聚物凝聚态观点来看,高聚物熔体中凝聚缠结网络中的凝聚结点是分子链的局部向列相互作用使链单元间产生平行凝聚而形成的,而高聚物从高弹态到流体态的转变正是反映了高聚物熔体中凝聚缠结网络的物理交联点,即凝聚结点状态的变化.高聚物熔体可以流动,说明熔体中凝聚网络中的凝聚结点至少是可以在瞬间内打开的,升温使凝聚结点的解凝聚状态存在的时间加长,凝聚状态存在时间减短,当升高到某一温度时,在凝聚结点解凝聚状态的时间内,分子链通过内旋转使质量中心在外加力的方向上可以发生位移,此时高聚物从高弹态转变为流体态,而此时的温度就是流动温度Tf.对高聚物流体弛豫网络的研究,是一个很有前景的研究课题.     

高聚物非晶态结构研究的进展

高聚物非晶态的结构研究,包括玻璃态、橡胶态、粘流态、结晶高聚物中的非晶部分以及结晶熔融态的结构研究。 有关高聚物非晶态的结构,有两种不同的学术观点。Flory(1949)用统计热力学推导出“无规线团模型”(Random coil model,图1(a)),即非晶高聚物中的高分子链无论在θ溶剂中或在本体中均具有相同迴转半径的无规线团构象(服从高斯分布)。而Kargin     

高速纺PET纤维结构第三态的初步探讨

本文应用计算分峰的原理,用系数不同的不对称高斯函数去分别表征PET纤维的中介态和无取向非晶态的X衍射,并在实验上设法把这两种衍射区分开来,从而建立了能够测算纤维中晶态、中介态和无取向非晶态的相对含量的方法。用这方法研究了高速纺PET拉伸丝在各温度下热处理时的结构,结果表明,纤维的结晶度随热处理温度升高而相应增加,但中介态含量、无取向非晶态含量及许多性质在130—160℃之间出现转折。作者从中介态在热理处时可能发生解取向松驰和发生结晶的综合变化加以解释,DSC的研究结果符合上述观点。     

热固性树脂的热-机曲线研究——Ⅰ.不同固化程度环氧树脂热-机曲线的研究

热塑性高聚物的热-机曲线可以确定处在某一定温度范围内的无定形高聚物的三个物理态——即玻璃态、高弹态、粘流态,由此即可了解该种高聚物的使用温度范围,以及它们加工的条件。用热-机曲线的方法研究热塑性高聚物有过不少的报导,B.A.卡尔金,E.B.库夫申斯基,巴尔捷涅夫及其他工作者对在测定过程中的条件,如升温速度、荷重大小、加荷方法等对于 T_c 的影响,以及高分子分子量对于热-机曲线的影响进行了系统的工作。但是对于热固性树脂的热-机曲线,还缺乏较为深入的研究。卡尔金斯洛尼姆斯基     

动态弹簧分析法的改进及其在研究环氧树脂固化过程中的应用

在文献的基础上改进了动态弹簧分析法,提高了测试精度,使此法不但适用于高聚物的高弹态而且也适用于它的玻璃态。 将改进了的动态弹簧分析法试用于环氧树脂的等温固化过程的研究,测定了等温固化过程中环氧树脂动态贮能模量和内耗的变化曲线,并进行了表观动力学分析。     

单边裂纹聚苯乙烯样品拉伸断裂过程的声发射

本文测定了单边裂纹聚苯乙烯(PS)样品在拉伸加载时的声发射以及声发射事件按幅度、脉宽、计数和位置的分布,实验证实,单边裂纹PS样品拉伸时的声发射来源于裂纹端部银纹区的变形、断裂和扩展,大部分总振铃计数由裂纹亚临界和失稳扩展引起,在重复加载时观察到PS声发射的Kaiser效应。     

从溶胀平衡研究玻璃态高聚物的凝聚缠结网络

在13~15℃室温条件下对聚苯乙烯(PS)颗粒在二氧六环/水混合溶剂中的溶胀情况进行目视观察,可以看到,当减小混合溶剂二氧六环/水中的水含量时,PS颗粒从玻璃态到溶胀的玻璃态、溶胀的高弹态、流体态的转变,与PS颗粒升温时从玻璃态到高弹态、流体态的转变相对应.选定混合溶剂二氧六环/水(水6·8wt%)对一个单分散PS试样(Mw=1·68×105)在30℃进行分相平衡和溶胀平衡的测定.分相平衡是先将PS/二氧六环/水(混合溶剂水含量6·8wt%)体系加热到130℃使PS溶解成一均相溶液,然后在冷却过程中分相,在30℃达分相平衡(30天)时,浓相高分子体积浓度p″=0·304,稀相几乎为纯溶剂.从Flory-Huggins相平衡理论得出此体系的高分子-溶剂相互作用参数χ=0·63.本工作使用的单分散PS试样在选定的混合溶剂,即二氧六环/水(水6·8wt%)中,30℃时不能溶解只能溶胀,单分散PS颗粒淬冷试样(密度ρp=1·0451g/mL)到达溶胀平衡(80天)时浓相高分子体积浓度p″=0·308.而此淬冷试样经在80℃热处理100h后的老化试样(ρp=1·0470g/mL)达溶胀平衡(70天)时p″=0·312.从溶胀过程中浓相体积变化曲线可知试样经在80℃热处理过程中凝聚网络趋向于更均一,更接近热处理温度下的平衡态,试样密度增大,网络产生了新的链间凝聚,物理交联密度增大(凝聚点间分子量减小).从溶胀平衡理论并取χ=0·63(分相平衡)可得到淬冷试样的物理交联点间分子量Mc=11·6×104,老化试样的物理交联点间分子量Mc=6·9×104.实验结果说明溶胀过程及溶胀平衡的测定可以反映玻璃态高聚物中的凝聚网络结构的细节,而且非常敏感.     

ACOUSTIC EMISSION DURING STRETCHING OF POLYMERS

Acoustic emission has been studied for a wide range of polymers including amorphous glasses, semi-crystalline polymers, copolymers, polymer blends and a crosslinked rubber during the course of uni-axial stretching at room temperature. For non-crystalline polymers acoustic emission occurred in rather small number of events accompanied by crazing and micro-crack formation. Strong acoustic activity appeared during yielding and necking of crystalline polymers. Rather small number or none of acoustic bursts occurred during the initial stage of neck drawing but numerous strong bursts appeared when drawing proceeded approaching specimen break. Specimens of the same polymer but of different fabrication history may be reflected in their acoustic emission behavior. Acoustic emission during stretching crosslinked polybutadiene rubber was very weak but observable when the force-elongation curve started to deviate from the linear region. No Kaiser effect was observed for the rubber. Very strong and numerous acoustic emission was observed during stretching specimens of polymer blends. High impact resistant polymer modifications showed no sharp increase of acoustic activity before specimen break. So long as the polymer and conditions of specimen fabrication are the same quite reproducible acoustic emission behavior could be observed.     

高聚物的物理老化和链的凝聚缠结

本文基于高聚物在物理老化和拉伸变形过程中构象弛豫行为的实验结果,进一步阐明了高分子链局部凝聚缠结的观点。     

Visualization of deformation-induced structural rearrangements in amorphous polymers

A technique is proposed for decorating amorphous polymers: Before the deformation (shrinkage) of an amorphous polymer, its surface is decorated with a thin metal coating. The subsequent deformation is accompanied by surface structure formation, which makes the processes that occur in the polymer visible. The proposed technique makes it possible to visualize and describe the mechanism of transfer of the polymer from the surface into the bulk and vice versa and to obtain direct information about the direction of the actual local stress. The technique makes it possible to obtain information about the topological heterogeneity of rubber networks, to reveal the features of structural rearrangements that occur during the cold rolling of amorphous polymers, and to describe the phenomenon of self-elongation during annealing of the oriented PET. These microscopic data explain the following features of the structural and mechanical behavior of glassy polymers from a unified viewpoint: stress relaxation in a polymer in the elastic (Hookean) region of the stress-strain curve, an increase in stress in a deformed glassy polymer during its isometric annealing below T _g, the low-temperature shrinkage of a deformed polymer glass in the strain range below its yield point, the storage of internal energy in a deformed glassy polymer in the strain range below the yield point, some anomalies of thermophysical properties, and some other features.     

Toughness Enhancement of Nanostructured Amorphous and Semicrystalline Polymers

An overview is given of different micromechanical deformation processes leading to an enhancement of toughness in heterophase polymers. The well-known mechanism of rubber or particle toughening of semicrystalline polymers was studied in HDPE and PP blends. In particular, the micromechanical processes in the semicrystalline polymer strands between modifier particles were investigated in detail, revealing processes of separation, yielding, breaking and twisting of lamellae. These processes are compared with lamellae forming amorphous SBS block copolymers with alternating soft (polybutadiene) and hard (polystyrene) layers. Depending on the deformation direction, the mechanism of thin layer yielding or chevron formation appears. In both polymeric systems, the initial stage of deformation is characterized by a plastic yielding of the soft phase with a reorganization of the hard (glassy or crystalline) lamellae. The second stage is determined by the alignment of the hard phase towards the deformation direction and the plastic yielding. Detailed comparison of these similar mechanisms in very different polymers with similar nanostructured morphology should help to improve toughening of amorphous as well as semicrystalline polymers.     

The influence of surface phenomena on molecular mobility in glassy polymers

Literature data on molecular mobility in glassy polymers have been analyzed. It has been shown that, in the temperature range corresponding to the glassy state of a polymer, a large-scale (segmental) molecular motion is possible, with this motion being responsible for the physical (thermal) aging of the polymer. Heating of an aged polymer restores its initial state, and the aging process begins again (effect of “rejuvenation”). At the same time, aging processes may be initiated by a mechanical action on a glassy polymer. It is sufficient to subject an aged polymer to a mechanical action to transfer it to a state characteristic of a polymer heated above the glass-transition temperature. It should be noted that deformation of a glassy polymer is nonuniform over its volume and occurs in local zones (shear bands and/or crazes). It is of importance that these zones contain an oriented fibrillized polymer with fibril diameters of a few to tens of nanometers, thereby giving rise to the formation of a developed interfacial surface in the polymer. The analysis of the published data leads to a conclusion that the aging of a mechanically “rejuvenated” polymer is, as a matter of fact, the coalescence of nanosized structural elements (fibrils), which fill the shear bands and/or crazes and have a glasstransition temperature decreased by tens of degrees.     

The “Relaxed” state in a semicrystalline polymer: Experimental characterization and modeling

This work addresses the general issue of the mechanical behavior of the confined amorphous phase in rubbery semicrystalline polymers. Even far above the glassy transition temperature, the amorphous phase in semicrystalline polymers is known to remain constrained by crystals and is less mobile than a purely amorphous polymer close to its equilibrium rubbery state. The aim of this paper, based on Polyamide 11, is to investigate the existence and significance of a relaxed state in the amorphous phase of a semicrystalline polymer far above T _g. A strain-rate independent tensile curve (called the “asymptotic curve”) is evidenced below a critical strainrate, consistently with a fully relaxed state of the rubbery amorphous phase. Nevertheless, a contradictory mechanical phenomenology was observed at the same time (hysteretic unloading, relaxation, and creep involving the same strain-rates as the “asymptotic” loading regime), suggesting joint amorphous and crystalline processes. Modeling of this paradoxical behavior is attempted, based on the experimental results. The first one-dimensional simulations are presented. Published in Russian in Vysokomolekulyarnye Soedineniya, Ser. A, 2008, Vol. 50, No. 5, pp. 797–808. This article was submitted by the authors in English.     

Structural approach to the study of deformation mechanism of amorphous polymers

A new microscopic procedure for the visualization of structural rearrangements in amorphous polymers during their deformation to high strains is described. This approach involves the deposition of thin (several nanometers) metallic coatings onto the surface of the deformed polymer. Subsequent deformation entails the formation of a relief in the deposited coating that can be studied by direct microscopic methods. The above phenomenon of relief formation provides information concerning the deformation mechanism of the polymer support. Experimental data obtained with the use of this procedure are reported, and this evidence allows analysis of the specific features of structural rearrangements during deformation of the amorphous polymer at temperatures above and below its glass transition temperature under the conditions of plane compression and stretching, uniaxial tensile drawing and shrinkage, rolling, and environmental crazing. This direct structural approach originally justified in the works by Academician V.A. Kargin appears to be highly efficient for the study of amorphous polymer systems.     

Nonlinear Viscoelasticity Raised at Low Temperatures by Intermolecular Cooperation of Bulk Amorphous Polymers

We performed dynamic Monte Carlo simulations of stress relaxation in parallel-aligned and uniaxially stretched bulk amorphous polymers at low temperatures.We observed an extra-slowing down in the early stage of stress relaxation,which causes nonlinear viscoelasticity as deviated from Debye relaxation and Arrhenius-fluid behaviors observed previously at high temperatures.Meanwhile,fluctuation analysis of stress relaxation revealed a substantial increase in the stretch fractions of polymers at the transient periods of high-temperature Debye relaxation.Structural analysis of free volume further revealed the scenario that,at low temperatures,the modulus of polymer entropy elasticity decreases with temperature and eventually loses its competition to the imposed modulus (Deborah number becomes larger than one),and hence upon stress relaxation under constant strains,monomers are firstly accumulated nearby two stretching ends of polymers,resulting in tentative global jamming like physical cross-linking there,and thus retarding the coming transient state of stress relaxation.We concluded that intermolecular cooperation raises physical crosslinking for nonlinear viscoelasticity of polymer stress relaxation as well as the rubbery states unique to bulk amorphous polymers.The new microscopic mechanism of the fluid-rubbery transition of polymers may bring insights into the intermolecular cooperation mechanism of glass transition of small molecules,if the fluid-rubbery transition is regarded as an extrapolation of glass transition from low to high molecular weights.