【作者】 何厚康；

【导师】 张瑜；

【作者基本信息】 东华大学 ， 材料科学与工程， 2014， 博士

【摘要】 经过亿万年进化,生物体在有限的资源和残酷的自然选择中,为生存而进化出许多超乎人类想象的结构,引起科学家的普遍兴趣。远在三千年前梯度钢结构首次被生产出来之前,大自然就已经把梯度结构这个概念引入许多生物体组织中。自然界中大多数生命体所形成的天然复合材料中,不仅形成层次化结构,而且分散相或增强相多为非均匀分布,从而使材料具有良好的力学性能并完成复杂的生理功能。受这些启示,材料科学家们研究和开发了许多具有优异功能的材料,其中最具代表性的是梯度功能材料(Functionally graded materials)。聚合物共混是一种公认的制备满足复杂性能要求的多相聚合物材料的最通用和最经济的方法。多相聚合物材料的性能强烈地依赖于材料的形貌。具有基质-微纤形貌(Matrix-fibril morphology)的多相聚合物材料是近年来该领域研究热点之一。通过控制加工流场、聚合物组分间的界面相互作用,可以制备具有基质-微纤形貌的共混物。通过制备具有基质-微纤形貌的共混纤维,可以改善传统合成纤维的性能,如力学性能、可染色性,还可以制备超细纤维和具有其它特种性能的纤维,为基质-微纤形貌研究提供了广阔的应用舞台。前人对基质-微纤型共混纤维的研究多集中于加工条件对形貌的影响,对形貌形成和发展机理的理论研究和形貌模拟的关注较少。深入开展这方面的理论研究,一方面可以指导基质-微纤型共混纤维的制备,另一方也是聚合物共混理论和熔融纺丝动力学理论的补充和拓展。课题组前期研究发现,在聚丙烯／聚苯乙烯(PP/PS)和低密度聚乙烯／聚酰胺6(LDPE/PA6)共混纤维中发现了分散相数目和尺寸呈现径向梯度分布,并提出了解释这一现象的机理假说。本文在此基础上,以PP/PS共混纤维为研究对象,进行共混纤维成形过程中两相形貌形成和演变机理研究,提出描述这些机理的数学模型体系,并进行数值求解。本文在以下几个方面展开工作并形成结论：(1)对原材料聚合物进行了细致的表征,包括分子量和分子量分布、基本热性能、剪切流变性能和拉伸流变性能,确定了流变经验方程中的常数,以及这些常数与流场强度之间的关系。研究表明,随着PS分子量的降低,共混体系粘度比下降。PP和PS均出现表观粘度随拉伸速率增大而降低的趋势,表现为“拉伸变稀”行为。PP和PS的拉伸粘流活化能和指前因子与拉伸速率的双对数值之间呈现良好的线性关系。在相同的拉伸速率下,温度越高,表观拉伸粘度比越高。提高拉伸速率,表观拉伸粘度比对温度的敏感性增强。这些结论将指导共混纤维纺丝工艺条件设计,以及后续的纺丝动力学研究和纤维形貌模拟。(2)通过共混熔融纺丝,制备了具有梯度形貌的基质-微纤型共混纤维。通过改变原材料聚合物的粘度比、纺丝速度,研究了加工条件对分散相形貌的影响。采用纺程取样的方法,获得了不同纺丝速度下,不同纺程位置的共混纤维,通过扫描电子显微镜对共混纤维形貌进行了表征,系统研究了基质-微纤形貌的形成和在纺程上的发展演变,讨论了加工条件对梯度化程度的影响,提出了形貌形成和演变的机理。研究表明,纤维的梯度相形貌强烈地依赖于体系的粘度比和纺丝速度。低纺丝速度下,纺程上只发生分散相液滴的形变,没有聚并。当纺丝速度超过一定的临界值时,液滴开始聚并。通过纺程取样分析发现,挤出丝中已经有一定程度的梯度形貌(液滴数量径向梯度αc=一1.04×10-3m-1、液滴直径径向梯度αd=2.72×10-3),随着纺丝过程进行,这种梯度形貌得到保持并发展。提高纺丝速度,基体纤维直径细化加剧,能够加剧梯度化,但是过高的纺丝速度(如1000m/min)会带来严重的聚并,弱化由于基体纤维直径细化对梯度化的贡献。挤出丝中出现梯度形貌是喷丝孔中非均匀剪切作用的结果,而纺程上梯度化程度增加则是分散相液滴非均匀形变、聚并和迁移的结果。(3)基于熔融纺丝动力学理论研究,推导了共混熔融纺丝过程中速度、速度梯度、纺程张力、结晶度和取向度沿纺程的轴向分布,以及温度、拉伸粘度、拉伸应力沿纺程的轴向和径向二维分布,建立了共混熔纺的动力学理论模型。(4)通过共混熔融纺丝过程中的微流变分析,建立了适合共混熔融纺丝过程中分散相液滴形变、破裂和聚并的数学模型。通过建立关联元胞方法,对液滴的初始状态,以及在纺程上的形变、破裂和聚并行为进行了数值模拟,并与实验结果进行了对比。研究表明,在纺程拉伸作用下,纺程上分散相液滴发生了仿射形变(约化毛细管数Ca*>4),由球形液滴形变成椭球形,最后形成微纤形貌。纺程上液滴的聚并是由液滴间基体相薄层的粘性破坏所控制的过程。理论模拟的基质-微纤形貌基本上与实验观测的结果一致。在研究的纺丝速度下,分散相液滴在纺程上不会发生破裂(Ca*>4)。本文通过实验与理论模拟结合的方法,对不相容聚合物共混物在非等温熔融纺丝过程中的基质-微纤形貌形成和发展进行了理论和实验的深入探讨,提出了描述形貌形成和发展行为的理论模型,尤其是分散相液滴仿射形变和基于基体相薄层粘性破坏控制的聚并理论,充实和拓展了聚合物共混和熔融纺丝理论。更多还原

【Abstract】 In the process of millions of years of evolution, organisms have developed numerous useful biological tissue structures in order to survive in the ruthless natural selection with limited resources, which is beyond human imagination and has aroused quite a few scientists’ interest. More than three thousand years ago, when graded steel was produced, the concept of gradient structure has been given in many organisms by nature force. Most natural composite materials formed by natural life body have structural hierarchy. Moreover, the dispersed phases or reinforcing phases can be found in non-uniform organization. These morphologies ensure that the natural composite materials have excellent mechanical properties to achieve complex physiological functions. Inspired by the special structure of organisms, material scientists have developed many functional materials, of which functionally graded material (FGM) is the most representative one.Polymer blending has been identified as one of the most versatile and economical method to produce new multiphase polymeric materials to meet the demands for complex performance. Development of the multiphase polymer materials by blending is strongly dependent on the control of morphology. Multiphase polymer material with matrix-fibril morphologies (MFMs) becomes one of the research hotspots in recent years. By controlling the processing flow field and the interfacial interaction between the polymer components, polymer blends with matrix-fibril morphologies can be prepared. Blend fibers with matrix-fibril morphologies can be utilized to improve the performance of conventional synthetic fibers, such as mechanical properties and dyeability. Moreover, super fine fibers and other fibers with special properties can be produced from matrix-fibril morphology. These enable a broad application of theoretical studies in this field.The published works on matrix-fibril type blend fibers focused on the effects of processing conditions on the morphology, while paid fewer attentions to the mechanism and modeling on the formations and evolutions of the morphology. A theoretical study on this theme could not only provide theoretical guides on the produce of matrix-fibril type blend fibers, but also enrich and expand the theory system of polymer blending and dynamics of melt spinning.Our research group has found that the dispersed phases in polypropylene/polystyrene (PP/PS) and low density polyethylene/polyamide6(LDPE/PA6) blend fibers show radial gradients on count and diameter, and proposed hypotheses of mechanism to explain this phenomenon in previous works. Based on these, this work focuses on the mechanism of two-phase morphology formation and evolution during melt spinning of PP/PS blend fibers, and proposes a system of mathematical models and solutions to the models.The main research contents and conclusions are summarized as below:Firstly, the polymers as raw materials are characterized; including molecular weight and its distribution, basic thermal properties, shearing and elongational rheological properties, meanwhile the constants for empirical equations on polymer rheology are determined, and the relationship between the rheological constants and flow field strength are also identified. The results show that the viscosity ratio of PP/PS blends decreases with the decrease of the molecular weight of PS. The apparent elongational viscosities of both PP and PS exhibit a decrease as the elongation rate increases, which is classified as so-called "elongation thinning" behavior. The elongation viscous flow activation energy and the pre-exponential factor for Arrhenius equation of both PP and PS show a good log-linear relationship with the applied elongation rate. As the elongation rate increases, the dependence of ratio of elongational viscosity on temperature becomes significant. These conclusions will guide the design of the processing conditions of melt spinning of blend fibers, and provides material parameters for the subsequent dynamic simulations and fiber morphology simulations of melt spinning of blend fibers.Secondly, blend fibers with gradient matrix-fibril morphology are prepared by melt blending and melt spinning. The effects of processing conditions on the morphology are studied by changing the viscosity ratio of polystyrene to polypropylene and the take-up velocity. Blend fibers in different position of the spinning line at various take-up velocities are captured, and are observed by scanning electron microscopy to characterize the fiber morphologies. The formation and evolution of the fiber morphologies along the spinning lines are studied systematically: the effects of processing conditions on the gradient are discussed in details, and the mechanism of the formation and evolution of the fiber morphologies is proposed. The results show that the morphologies of droplets dispersed in matrix-fibril type blend fibers are strongly controlled by the rheological properties (viscosity ratio) of raw material and the spinning conditions (take-up velocity). At low take-up velocities, droplets deformation occurs in the spinning line without coalescence. Coalescence of droplets occurs and fibril coarsens when the take-up velocity exceeds a critical value. A radial gradient of droplet morphology is found in extrudate fibers (with the radial gradient on the count-dispersion of droplets ac=-1.04×10-3m-1, and the radial gradient on the number-averaged diameter of droplets ad=2.72×10-3). And the gradient morphologies are maintained and developed along the spinning line. As the take-up velocities increase, the gradient morphologies are firmed due to the shrinkage of matrix fibers in diameter. But high take-up velocities (such as1000m/min) cause a serious coalescence, which weakens the effects of the shrinkage of matrix fibers in diameter on the gradient morphology. The gradient in extrudate fibers is formed from the non-uniform shear flow in the spinnerets, while the progress of the gradient in the spinning line is contributed by the non-uniform deformation and coalescence of droplets.Thirdly, based on the dynamics of melt spinning, this work calculates the axial distributions of velocity, gradient of velocity, spinning tension, crystallinity and orientation along the spinning lines, as well as the axial and radial distributions of temperature, elongational viscosity and elongational stress. A suitable system of mathematic models is established for the dynamics of melt spinning.Finally, based on micro-rheology, a suitable system of mathematical models is established to describe the deformation, break-up and coalescence of dispersed droplets in melt spinning of blend fibers. A linked cell method is developed to solve the models. The simulation results, including initial morphology of droplets, and the resulted morphologies of deformation, break-up and coalescence along the spinning lines, are compared with those of experimental results. The results show that the affine deformation of droplets occurs under the spinning tension in the spinning line (with the reduced capillary number Ca*>4), which prompts the spherical droplets to change their shapes to ellipsoidal, and finally to fibrils. The coalescence of droplets in the spinning line is decided by the cohesive break of the matrix film between two coalescing droplets. The results from theoretical simulations agree with the observed results by experiments quiet well. At the discussed take-up velocities, the break-up of droplets does not occur (with Ca*>4).In this work, a detailed research on the formation and evolution of matrix-fibril morphology in non-isothermal melt spinning of immiscible polymer blends is carried out by a combined method of experimental and numerical simulation. A system of mathematic models is proposed to describe the behavior of formation and evolution of morphologies, especially the models for the affine deformation of droplets and the coalescence controlled by the cohesive break of matrix film, which will enrich and expand the theory systems of polymer blending and melt spinning.更多还原