Critical Strains for Lamellae Deformation and Cavitation during Uniaxial Stretching of Annealed Isotactic Polypropylene
Critical Strains for Lamellae Deformation and Cavitation during Uniaxial Stretching of Annealed Isotactic Polypropylene
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DOI：
10.1021/acs.macromol.8b00642
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发表时间：
2018-08-28
期刊：
影响因子：
5.5
通讯作者：
Heinrich, Gert
中科院分区：
文献类型：
--
作者：
Chang, Baobao;Schneider, Konrad;Heinrich, Gert
采用同步加速器小角x射线散射和广角x射线散射技术，研究了退火等规聚丙烯在单轴拉伸过程中的片层变形和空化行为。我们揭示了片层变形是如何在弹性变形、片层内滑移和熔融-再结晶的时间尺度上发生的，它们被三个临界应变分开，这三个临界应变很少受到退火的影响。应变I（0.1）标志着弹性变形的结束和层间滑移的开始，在此之后结晶度逐渐降低。应变II（0.45）标志着再结晶过程的开始，从这里开始，拉伸方向的长周期从最大值开始减小，晶体中的聚合物链开始沿拉伸方向定向。熔化所需的能量来自于薄片之间的摩擦，这是由薄片内部滑动产生的。应变III（0.95）表示再结晶过程结束。超过0.95应变后，长周期和晶粒尺寸基本保持不变。在进一步拉伸过程中，新形成的薄片作为非晶态聚合物链的锚定点。片层之间聚合物链的延伸引发了应变硬化行为。另一方面，退火显著降低了孔洞形成的临界应变，增加了孔洞数，但限制了孔洞尺寸。对于低于90℃退火的样品，在应变II和应变III之间形成空洞，空洞一旦形成就向拉伸方向取向。然而，对于退火温度高于105℃的样品，在应变I和应变II之间形成空隙。在这种情况下，孔洞最初以垂直于拉伸方向的纵轴定向，然后通过孔洞聚结沿拉伸方向转移。此外，孔隙的形成既不影响片层变形的临界应变，也不影响最终的长周期、聚合物链的取向或晶体尺寸。最后的长周期、聚合物链在晶体中的取向和晶体尺寸仅由熔融-再结晶拉伸温度决定。
The lamellae deformation and cavitation behavior of annealed isotactic polypropylene during uniaxial stretching are comprehensively investigated by in situ synchrotron small-angle X-ray scattering and wide-angle X-ray scattering. We reveal how lamellae deformation occurs in the time scales of elastic deformation, intralamellar slip, and melting-recrystallization, separated by three critical strains which are only rarely influenced by annealing. Strain I (0.1) marks the end of elastic deformation and the onset of intralamellar slip, beyond which the crystallinity decreases gradually. Strain II (0.45) signifies the start of the recrystallization process, from where the long period in the stretching direction begins to decrease from its maximum and the polymer chains in the crystal start to orient along the stretching direction. The energy required for melting arises from the friction between the fragmented lamellae produced by intralamellar slip. Strain III (0.95) denotes the end of the recrystallization process. Beyond the strain of 0.95, the long period and the crystal size remain nearly unchanged. During further stretching, the newly formed lamellae serve as the anchoring points for polymer chains in the amorphous phase. The extension of the polymer chains in between lamellae triggers the strain hardening behavior. On the other hand, annealing significantly decreases the critical strain for voids formation and increases the voids number but restricts the void size. For those samples annealed at a temperature lower than 90 degrees C, voids are formed between strain II and strain III, and voids are oriented in the stretching direction once they are formed. However, for those samples annealed at a temperature higher than 105 degrees C, voids are formed between strain I and strain II. In this case, voids are initially oriented with their longitudinal axis perpendicular to the stretching direction and then transferred along stretching direction via void coalescence. Additionally, the formation of voids influences neither the critical strains for lamellae deformation nor the final long period, the orientation of polymer chains, or the crystal size. The final long period, the orientation of polymer chains in the crystal, and the crystal size are determined only by the stretching temperature through melting-recrystallization.