【作者】 朱姝；

【导师】 余木火； Guenter Reiter；

【作者基本信息】 东华大学 ， 材料学， 2013， 博士

【摘要】 轻质高强是材料发展的永恒主题之一。高分子材料比重小,已经成为人类生活、经济建设、国家安全和科技进步不可或缺的重要材料。但它与金属材料和天然复合材料(贝壳、骨头等)比较,其强度和韧性较差。自从上世纪50年代首次提出高分子的增韧理论以来,人们已经成功发展了多种有效的增韧方法(最成功的就是以橡胶增韧的ABS材料),但这些方法多以牺牲强度和刚性为代价。与之相比,天然材料中有很多可以同时提高韧性、强度和刚性的多级有序微观结构(如贝壳中多级层状结构)。因此研究可以同时提高高分子材料韧性、强度和刚性的强韧化方法,具有理论意义和应用价值。我们受到天然贝壳多级有序层状结构的启发,发展了一种半结晶高分子材料强韧化的新概念及其方法——压力诱导流动成型方法(pressure-induced flow processing, PIF)。半结晶高分子材料存在晶区和无定形区,晶区由纳米厚度的折叠链片晶组成,片晶以放射状形成球晶,是一种多级结构。我们采用压力诱导流动成型法将近似球体的不规则多面体球晶转变为取向的扁平状,使球晶内部的片晶束发生重组与重排,形成多级有序层状微观结构,从而同时提高其韧性、强度和刚性。论文系统研究了典型高分子材料多级结构在PIF成型过程中的演变规律,提出了PIF成型半结晶高分子材料的多级结构模型及强韧化机理,并通过对多种半结晶高分子材料的印证、比较,证实了这一方法的有效性。主要研究结果如下：1、通过系统研究半结晶高分子在PIF成型过程中多级微观结构演变规律,发展了塑性变形的分子机制。从70年代Peterlin等人建立塑性变形理论以来,在塑性变形过程中,从球晶到取向分子链,球晶如何变形破坏、片晶如何运动和破坏、晶区和非晶区的分子链如何运动等科学问题,至今没有明确答案。本论文建立了半结晶高分子本体微观结构的无损表征和分析方法(包括“低温超薄切片、化学刻蚀和原子力显微镜照片拼接技术结合”,同步辐射小角X光散射、广角X光衍射及相关计算方法等),以典型的半结晶高分子为研究对象,包括等规聚丙烯(i-PP)、左旋聚乳酸(PLLA)、聚酰胺6(PA6)等,深入系统地研究了PIF过程中的晶区和非晶区等多级结构演变规律,包括晶区中多级微观结构(球晶、片晶、折叠链)演变和非晶区分子链运动规律。研究结果表明,在PIF成型过程中,大部分片晶在开始阶段首先发生偏转、然后破脆成为细小晶粒(数纳米),这些细小晶粒由系带分子(tie molecules)连接成为层状取向的富晶区；在压力诱导流动场中,这些细小晶粒仍然保持原有的片晶折叠链,但重新排列形成有序层状结构,片晶折叠链的方向与流动方向平行,这是塑性变形分子机制的新发展。2、提出了PIF成型i-PP材料的强韧化机理和结构模型。我们用光学显微镜(optical microscope, OM)、扫描电子显微镜(scanning electron microscopy, SEM)、原子力显微镜(atomic force microscopy, AFM)等方法对i-PP材料断裂方式及断面形貌与结构进行了系统分析,结合贝壳珍珠层的砖泥模型,我们提出了PIF成型在i-PP材料中的增韧增强机制,即通过片晶运动变化及球晶变形,在材料内部形成取向的多级薄弱界面,构成有序排列的多级层状结构,使材料在断裂过程中自动引发更多的裂纹扩展而增加能量耗散；同时由于“受限无定形”分子链段的作用,提高了片晶束的有效应力。两者协同作用,达到宏观力学的增韧与增强。3、建立了一种可以同时提高韧性、强度和刚性的半结晶高分子材料高性能化的新方法——压力诱导流动成型方法。对多种半结晶高分子材料进行的压力诱导流动成型的系统研究,为这些材料的高性能化提供了一种新途径及理论支撑。论文分别针对i-PP、PLLA、聚酰胺66(PA66)、PA6、PA6/PP、聚酰胺6/蒙脱土(PA6/MMT)、PP/MMT等7种半结晶高分子及其共混、杂化材料,研究了PIF成型过程中的各种加工参数(温度、压力、压缩比、保压时间等)对材料结构和性能的影响规律。所选择的半结晶高分子材料,在实验室条件下,韧性可提高2-22倍,强度提高2-3.5倍,刚性提高2-3倍。由此我们证实了,用PIF成型方法可以在半结晶高分子材料中构筑多级有序层状微观结构,从而大幅度提高材料的冲击韧性,并同时提高强度和刚性。更多还原

【Abstract】 Combination of low density and high strength is the eternal objective for advanced materials, especially the structural materials. Due to their much lower densities than traditional metallic and inorganic materials, polymeric materials are becoming more and more important for social and economic life, defense security, and scientific progresses. However,, these materials exhibit inferior strength and toughness compared to metals and some natural composites (e.g., nacre, bones, etc.).Since the first proposal of toughening theory for polymeric materials in1950s, people have built various effective methods for toughening polymers (among which the rubber toughened ABS material is the most successful one). However, most of these methods take the cost of strength and stiffness. On the contrary, many natural materials could entitle themselves with simultaneously improved toughness, strength and stiffness by the formation of hierarchical ordered microstructures (such as the oriented layered structures of multi-level in nacre). Therefore the exploration of an effective method to simultaneously boost the toughness, strength and stiffness for polymeric materials is not only meaningful in the theoretical aspects, but also very important for potential applications in industry.Inspired by the hierarchical ordered structures in seashell nacre, we came up with a novel method, pressure-induced flow processing (PIF-processing) to obtain high performances for commodity semicrystalline polymeric materials. It is well-known that semicrystalline polymers exhibit hierarchical structures, because they consist of crystalline regions and amorphous regions, the former being composed of lamellae of folded chains with thickness of several nanometers, and then form spherulites. With PIF-processing, the isotropic crystalline spherulites can be deformed into oriented ellipsoids, in which the stacks of lamellae undergo re-construction and re-arrangement, so as to form oriented hierarchical structures in semicrystalline polymers. These typical structures would introduce improvements on the toughness, strength and stiffness for the materials at the same time. In this thesis, we systematically investigated the evolvement of hierarchical microstructures during PIF-processing for typical polymeric materials, and suggested the structural evolution model and related toughening and strengthening mechanisms for semicrystalline polymers with PIF-processing, and confirmed the validity of this method by comparative studies on many semicrystalline polymeric materials. The main results are as follows.1. We discovered the principles in the evolvement of the hierarchical microstructures for semicrystalline polymers during PIF-processing, and revealed the molecular mechanisms for plastic deformation.In spite of the long-established plastic deformation theories since1970s, many issues during the plastic deformation from the spherulites to oriented molecular chains (e.g., how are the spherulites deformed and destroyed, how are the lamellae moving and destroyed, how are molecular chains moving in the crystalline regions and amorphous regions) remain controversial. With the newly developed methods for characterizing the hierarchical structures of semicrystalline polymers (such as synchrotron radiation small angle X-ray scattering, wide angle X-ray diffraction, and the combination of low-temperature microtoming, chemical etching and full splicing of AFM images), we carefully studied the structural evolutions in crystalline (including spherulite, lamellae and their stacks, and fold chains) and amorphous regions during PIF-processing, and confirmed that most of lamellae are broken into nano-sized crystalline fragments, which are then inter-connected by tie molecules to form crystal-rich areas of aligned layers. These crystalline fragments remain the long periods of original lamellae, however, the fold chains of them exhibit a preferred orientation along the flow direction. These new findings enriched the content of molecular mechanism for plastic deformation.2. We proposed a comprehensive structural model to illustrate the toughening and strengthening mechanisms of PIF-processing for semicrystalline polymeric materials.We made a systematic analysis on the fracture type as well as the morphologies and microstructures for the fractured surfaces, by means of optical microscope (OM), scanning electron microscopy (SEM) and atomic force microscopy (AFM) observations. Based on the motor-brick model for seashell nacre, we proposed the molecular mechanisms of PIF-processing on toughening and strengthening the semicrystalline polymeric materials. Specifically, the motion of lamellae and deformation of spherulites result in well aligned hierarchical stratified structures with oriented multi-leveled weak boundaries between those layers. These weak boundaries would initiate abundant cracks and promote their propagations along flow direction, and thus leading to a greatly increased sum distance of energy-dissipating paths in the materials. On the other hand, the formation of "confined amorphous regions" consisting of tie-molecules increases the effective stress within lamellae stacks. The above two factors cooperatively contribute to the much improvement of both impact and tensile strength for the PIF-processed samples.3. We set up a novel method for simultaneously enhancing the toughness, strength and stiffness for semicrystalline polymeric materials.The new processing method of PIF-processing offers a novel route to entitle semicrystalline polymeric materials with high performances. For seven commodity polymeric materials including some semicrystalline polymers and their blending or hybrid materials (i-PP, PLLA, PA66, PA6, PA6/PP, PA6/MMT, PP/MMT), in details we discussed the effects of various processing parameters (such as temperature, pressure, compression ratio, and pressure holding time) on the microstructures and resultant properties, and found that the toughness could be improved by2-22folds, while the strength could be improved by2-3.5folds and stiffness by2-3folds. We confirmed that hierarchical oriented layered microstructures could be formed in semicrystalline polymeric materials, hence greatly boosting the impact strength without hurting the tensile strength and stiffness.更多还原