Network stretching, slip processes, and fragmentation of crystallites during uniaxial drawing of polyethylene and related copolymers. A comparative study
Network stretching, slip processes, and fragmentation of crystallites during uniaxial drawing of polyethylene and related copolymers. A comparative study
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DOI：
10.1021/ma981776b
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发表时间：
1999-06-29
期刊：
影响因子：
5.5
通讯作者：
Strobl, G
中科院分区：
文献类型：
--
作者：
Hiss, R;Hobeika, S;Strobl, G
当聚乙烯(PE)变形为大应变时，应力既来自于与微晶滑移和碎裂过程的塑性变形有关的粘性力，也来自于缠绕的非晶区拉伸产生的熵弹性力，不同过程的相对重量随着结晶度的不同而变化。在对一系列样品的综合研究中分析了相关性，这些样品涵盖了广泛的结晶度：PE、低密度聚乙烯(LDPE)和乙烯-乙酸乙烯酯共聚物。这一比较是基于在恒定应变率下测量的真实应力-应变曲线。对于结晶度较高的样品，表现为颈缩，这是通过使用视频控制拉伸试验机实现的。在阶跃循环运行中研究了样品的恢复特性，其中加载是逐步施加的，并在每一步之后由卸载-重新加载循环中断。在机械测试的同时，通过测量WAXS图案来确定相关的纹理变化。尽管从固体到橡胶的总体力学性能有很大的变化，但对于变形行为存在一个共同的一般方案。对于所有样本，我们发现差异符合性发生变化的四个特征点。它们可能与(1)孤立滑移过程的开始，(2)滑移向集体活动的转变，(3)微晶碎裂的开始，以及(4)导致有限的真正不可逆的变形的链解缠有关。随着结晶度的增加，四个转变点处的应力也增大。然而，相关的菌株基本上保持不变。晶体纹理也只是施加应变的函数，这种依赖关系对所有样本都是通用的。实验支持将晶片的颗粒状亚结构作为基本结构特征的新图景。以三个表面为滑移面的块体滑动使系统能够以明确的方式轻松地对每个施加的应变做出反应。
When polyethylene (PE) is deformed to large strains, the stress originates from both the viscous forces associated with the plastic deformation of the crystallites by slip and fragmentation processes and the entropic elastic forces arising from the stretching of the entangled amorphous regions, Relative weights of the different processes change with the crystallinity. The dependencies were analyzed in a comprehensive study on a series of samples encompassing a large range of crystallinities: PE, low-density PE (LDPE), and ethylene-vinylacetate copolymers. The comparison was based on measured true stress-strain curves for constant strain rates. For the samples with higher crystallinity, which show a necking, this was achieved by employing a video-controlled tensile testing machine. Recovery properties of the sample were studied in step-cycle runs, where the load was applied stepwise and interrupted after each step by an unloading-reloading loop. Simultaneous with the mechanical testing, the related texture changes were determined by a measurement of the WAXS patterns. In spite of the large changes in the gross mechanical properties from solid- to rubberlike, there exists a common general scheme for the deformation behavior. For all samples, one finds four characteristic points where the differential compliance changes. They may be associated with (1) the onset of isolated slip processes, (2) a change into a collective activity of the slips, (3) the beginning of crystallite fragmentation, and (4) chain disentanglement resulting in a finite truly irreversible deformation. When the crystallinity is increased, the stresses at the four transition points also increase. The related strains, however, remain essentially constant. Crystal textures are also a function of the imposed strain only, the dependencies being common for all samples. Experiments support the novel picture of a granular substructure of the crystalline lamellae as a basic structural feature. Block slips with the three surfaces as slip planes enable the system to easily react on each imposed strain in a well-defined way.