【作者】 刘艳萍；

【导师】 李良彬；

【作者基本信息】 中国科学技术大学 ， 同步辐射及应用， 2014， 博士

【摘要】 结构与性能的关系长期以来都是都是高分子材料科学领域的重要课题之一近半个世纪以来一直是材料学家关注的重点。材料的任何物理性能都是由其内部结构决定的,因此建立性能与结构的对应关系对于材料性能的优化具有非常重要的意义。由于高分子材料结构的复杂性以及检测手段的单一性,使得该课题的进步困难重重。解决此问题需要从两方面入手：一方面发展特殊的研究手段,另一方便选取合适的研究对象。本课题采用的研究手段主要基于单轴双向拉伸装置与同步辐射X射线散射装置联用,可以同时获得材料形变过程中结构变化与力学响应信号。研究对象主要选取具有多相态结构的高分子材料——聚(1-丁烯)(PB-1)和等规聚丙烯(iPP)。在受到外界刺激即拉伸外场时,材料的相态会发生一定的转变,例如PB-1发生晶型Ⅱ到晶型Ⅰ的转变,以及iPP的α晶转变为中间相(mesophase).通过原位检测装置追踪这种显著地结构变化,为建立微观结构与宏观性能之间的关系创造可能性。基于此,本论文主要对以下内容进行了探究：(一)利用同步辐射广角X射线散射(WAXS)原位跟踪了温度对拉伸诱导PB-1相转变的影响,证实了拉伸变形对晶型Ⅱ到晶型Ⅰ的转变有显著地促进作用。不同实验温度下拉伸诱导的相转变表现出两种路径,低温拉伸为直接转变,而高温拉伸时晶型Ⅱ先转变为无定型态然后进一步生成稳定的晶型Ⅰ,即熔融再结晶过程。在该工作中还发现了工程应力应变曲线的三个区域：弹性区、平台区以及应变硬化区正好与PB-1相转变过程中晶型Ⅰ的孕育、成核和凝胶网络形成三个阶段吻合,由此建立了相转变过程与力学性能的对应关系。(二)利用同步辐射X射线的时间分辨优势,检测了应变速率对拉伸诱导晶型Ⅱ到晶型Ⅰ相转变的影响。拉伸速率的增加可以加速相转变的发生。发现相转变的起点由应变控制而相转变程度由外场功的多少决定。高温高速拉伸结束后还发现了晶型Ⅱ的重结晶现象,说明高温转变符合拉伸熔融再结晶机理而低温拉伸则遵循Young提出的位错理论。(三)PB-1中晶型Ⅱ到晶型Ⅰ相转变的发生会导致片晶各方向尺寸不同程度的收缩,因此可以利用同步辐射小角X射线散射(SAXS)跟踪拉伸过程中片晶周期的变化。实验结果还表明在力学曲线上宏观屈服应变之前还存在一个微屈服点,这可能是晶体Ⅱ的破坏开始。(四)为了研究拉伸诱导α-iPP晶体向中间相的转变过程,在该部分工作中利用iPP的特征结构——交叉投影(cross-hatched structure)在流动场中会沿着不同方向取向的性质,提出了能够在原位环境中分辨子母晶的预取向方法。结合同步辐射广角x射线衍射原位跟踪了从不同角度拉伸子母晶时的结构演化过程。不论拉伸方向如何母晶都早于子晶被破坏,并且中间相出现在较小的应变,恰好紧接着子母晶的破坏。由此推测拉伸诱导的中间相可能是由子母晶演化来的小晶粒组成。更多还原

【Abstract】 All of the material properties are determined by the intrinsic structure, so the relationship between structure and properties is very important for optimizing and tuning material properties. In the field of polymer science, the relationship between structure and mechanical properties occupies very important status, which has been drawing attentions of scientists since half a century before. Because of the complexity of polymer structure and oneness of detecting method, the development in this issue is very slow. To solve this problem we can strive from two aspects, which are developing suitable detecting techniques and choosing appropriate research objects.In our study, the uniaxial tensile apparatus was adopted in combination with synchrotron radiation X-ray scattering, which can provide information both of structural evolution and mechanical response. Polybutene-1(PB-1) and isotactic polypropylene (iPP) was studied due to their multiphase structures. Under external simulation which is drawing deformation in this research, there are phase transitions such as the phase transition from forms Ⅱ to Ⅰ in PB-1and from a crystal to mesophase in iPP. By means of in-situ testing method, the structural evolution can be tracked easily, which affords the possibilities to establish the relationship between micro-structure and mechanical properties. The research contents of this work are as follows:i. The effect of temperature on the deformation induced phase transition of PB-1is studied with in situ synchrotron radiation wide angle X-ray scattering (WAXS), which verify that tensile deformation can accelerate the transition from forms Ⅱ to Ⅰ. The phase transition at different temperatures is interpreted based on either a direct crystal-crystal transition at lower temperature or an indirect approach via an intermediate stage of melt at higher temperature, namely a melting recrystallization process. A three-stage mechanical deformation including linear deformation, stress plateau and strain hardening is observed in the engineering stress strain curves, which corresponds to a process of incubation, nucleation and gelation of form Ⅰ crystals. It establishes a nice correlation between phase transition and mechanical behavior in this study.ii. Taking advantage of the time resolution of synchrotron radiation X-ray scattering, the effect of strain rates on deformation induced phase transition from forms Ⅱ to Ⅰ is studied. The phase transition is faster under larger strain rates. In this work, the beginning of phase transition is found to be strain-controlled, while the transformation degree is related to the mechanical work. After deformation under higher strain rates at80℃, the recrystallization of form Ⅱ occurs, which verifies the transition at higher temperature accords with the mechanics of deformation induced melting-recrystallization, while the phase transition at lower temperature follows Young’s dislocation model.iii. As we know, the phase transition from forms Ⅱ to Ⅰ will lead to shrinkage in different directions of lamellae, so in this work synchrotron radiation small angle X-ray is adopted to follow the change of long spacing. A micro-yield point on the engineering stress strain curve is found before macroscopic yielding, which may be the real beginning of form Ⅱ crystal’s destruction.iv. To study deformation induced transition from α-iPP crystal to mesophase, the cross-hatched structure that is the characteristic feature of iPP can orient along with two directions in flow field which is utilized in this work. A method to distinguish the parent-daughter lamellae with in situ environment is developed. Combining with synchrotron radiation WAXS, the stretching induced structural evolution of parent and daughter lamellae is studied from different directions. No matter what the tensile direction is, parent lamellae are destroyed before daughter ones. Mesophase is observed at very small strain, immediately after the damage of parent lamellae. Deformation induced mesophase is proved to be small crystal cluster which is transformed from parent-daughter lamellae.更多还原