| 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. |
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