【作者】 张磊；

【导师】 赵国群；

【作者基本信息】 山东大学 ， 材料加工工程， 2019， 博士

【摘要】 资源的加速消耗和环境的严重破坏已成让整个社会开始前所未有的关注节能减排问题。对于以消耗石油和白色污染为代价的聚合物材加工行业,实现节能减排已经迫在眉睫。然而,由于长链分子的高构象熵、强的分子间相互作用效应和分子间的拓扑缠结,聚合物材料的成形成性过程存在工艺窗口窄、流动行为复杂和产品缺陷多等问题。上述问题的解决可使聚合物材料的加工技术进一步降能降耗、节省原料和提高产品质量。二氧化碳是一种无毒、廉价、生物兼容和可调性强的流体,将二氧化碳引入到聚合物加工工艺中,可以实现对聚合物微观凝聚态演变的干预和介观结构形成的调控,从而提升聚合物产品力学性能和服役性能。这种优势引起传统聚合物加工行业的高度重视,开发了包括聚合物发泡注塑工艺在内的多种二氧化碳辅助的聚合物成形工艺。此外,这种优势还吸引了众多科研工作者开展聚合物/二氧化碳体系的相关研究,探讨其在组织工程、药物输运、电磁屏蔽、超级隔热、吸声、吸油等领域中的应用潜力。目前,人们围绕聚合物/二氧化碳的二元体系,在聚合物流场中气体相的形态演变和聚合物在高压二氧化碳环境中的凝聚态演变等方面开展了许多研究工作。然而,仍存在诸多关键问题亟待研究和解决。气泡在聚合物注塑流场中形态演变的全过程尚未探明,聚合物发泡件表面缺陷的形成机理及其消除方法尚不明确,工艺参数对聚合物/二氧化碳体系的影响规律缺乏理论解释;加压二氧化碳对聚合物结晶的影响尚未探明,如何通过改变二氧化碳压力实现对聚合物晶体形貌的控制缺乏理论指导。围绕上述问题,本文开展了关于聚合物/二氧化碳体系的动态相演变与结晶行为的研究,其主要研究工作和取得的研究成果如下:(1)建立了一种不可压缩、非等温、非稳态三维多相流数学模型,提出了一种模具型腔排气边界条件设置方法,将聚合物熔体的人为损耗降低到1 ‰以内。采用能量方程/PIMPLE耦合算法,解决了大黏度比两相界面温度求解发散问题;采用基于场量的自适应网格划分技术,提高了宏观尺度流场中微小气泡界面追踪的精度。基于该模型,本文研究了在注塑流场厚度截面上温度场和速度场对气泡形态演变过程的影响规律,预测了在剪切和泉涌流场中不同初始大小和位置的球形气泡的变形、破裂和溃灭过程;结合聚合物发泡注塑(Polymer Foaming Injection Molding,PFIM)短射实验,揭示了发泡注塑制件表面泡坑、银纹、塌陷形貌的形成机理。(2)建立了一种基于有限体积法的非等温非稳态的多相-VOF模型,提出一种采用隐式区域耦合算法同步求解模具和型腔区域温度场的方法,并进行快速热循环(Rapid Heat Cycle Molding,RHCM)辅助的PFIM工艺实验研究。基于模拟和实验结果,本文分析了超临界二氧化碳、聚合物熔体、空气在模具型腔内的瞬态流动行为,研究了模具温度对气泡在泉涌流场中的变形、破裂和塌陷的影响,揭示了 RHCM/PFIM工艺所成形的塑件表面缺陷的形成机理。(3)基于两相流模型开发了一种非等温流固耦合模型。该模型采用隐式耦合传热算法考虑注塑模具与聚合物熔体之间的耦合传热。模型精确预测了RHCM/PFIM工艺过程中的温度场,分析了 RHCM/PFIM工艺过程中热响应特征,结合数值模拟结果和实验获得的泡孔结构,揭示了 RHCM/PFIM工艺过程中的塑件内部多孔结构的形成机理。(4)开发了一种原位高压显微系统,该系统包含一个温度和压力可以闭环控制的样品池。在不同样品厚度、分子量、温度、二氧化碳压力条件下,本文采用该系统研究了 PLLA样品在加压二氧化碳中的晶体生长过程。通过合理选择实验参数,本文研究了雪花晶体形成过程的初始阶段。结合原子力显微镜,阐明了雪花状PLLA晶体的生长模式。(5)建立了一种原位高压多光学观测系统,该系统由光学、偏振光学和小焦激光散射三部分组成,可以研究0.1 μm-1 cm尺度范围内的聚合物凝聚态演变过程。本研究利用该系统获得了左旋聚乳酸(Poly(L-lactic acid),PLLA)在二氧化碳中的晶体尺寸和晶体数量密度的统计数据。结合原子力显微镜,分析了树枝状晶体在高压二氧化碳中的生长行为,发现了一种通过节奏式生长形成的竹节状树枝晶。(6)将超临界二氧化碳引入到PLLA样品的熔融等温结晶中,制备了一种可以排除链分子和晶间缠结的螺旋梯田状晶体。根据原子力显微镜、透射电子显微镜、核磁共振、X射线衍射的表征结果,确定了晶间相分子的链构象,研究了无定形链的构象状态对多层片晶形貌的影响,最终给出了关于晶片外翻起源的理论解释。(7)利用自建的原位高压显微系统,实现了一种可以在线升压的结晶实验。通过这种单变量的实验方法研究了二氧化碳压力对PLLA晶体长大的影响规律,基于两步形核模式提出一种用于计算在二氧化碳中聚合物晶体二次形核速率的新模型。结合实验结果,通过对聚合物/二氧化碳体系中结晶自由能、扩散活化能、混合能、吸附能、平动能的定量计算,本文给出了高压二氧化碳对聚合物晶体二次形核的影响机理。更多还原

【Abstract】 The accelerated consumption of resources and the severe environmental damage have let the whole society begin to pay unprecedented attention to the issues of energy conservation and emission reduction.For the polymer materials processing industry at a cost of oil consumption and white pollution,it is urgent to realize energy conservation and emission reduction.Because the high conformational entropy,strong molecular interactions effect and intermolecular topological entanglement of long-chain molecule,there are many questions such as narrow process window,complex flow behavior and product defects in the forming and functionalization process of polymer materials.Solution of the above problems can make the processing of polymer materials further reduce energy consumption,save raw materials and improve the service performance of products.CO2 is a non-toxic,cheap,bio-compatible and adjustable fluid.The incorporation of CO2 into the forming process of polymer materials can intervene the evolution of micro condensate state and regulate the formation of mesoscopic structure of polymer.As a result,the mechanical and service performances of polymer product can be enhanced.Such advantages cause the attention of the traditional polymer processing industry,and a variety of CO2-assisted polymer forming processes,including the polymer foaming injection molding process,have been developed.Furthermore,these advantages also attract many researchers to research the polymer/CO2 system,and investigate its application potentiality in the field of tissue engineering,drug transport,electromagnetic shielding,super heat insulation,sound absorption,oil absorption and so on.At present,in terms of the polymer/CO2 binary system,a lot of researches have been carried out in the aspects of morphology evolution of gas phase in polymer flow field and condensed state evolution of polymer in high-pressure CO2 environment.However,there are still several key issues need to be studied and solved.The whole morphological evolution process of bubbles in the polymer injection flow field has not been explored,the formation mechanism and elimination method of surface defects of polymer foam parts are not clear,and the influence rule of process parameters on polymer/CO2 system lacks theoretical explanation.The influence of pressurized CO2 on polymer crystallization has not been explored,and there is no theoretical guidance on how to control the morphology of polymer crystals by changing the CO2 pressure.Focus on the above issues,this paper investigates the dynamic phase evolution and crystallization behavior of the polymer/CO2 system.The main research work and achievements are as follows(1)An incompressible,non-isothermal,and unsteady three-dimensional mathematical model of multiphase flow was established.A setting method of boundary conditions with the exhaust function on the mold cavity walls was proposed,and the artificial loss of polymer melt was reduced to less than 1‰.The problem of temperature solution divergence on the interface between two phases with high viscosity ratio was solved through the coupling algorithm of energy equation and PIMPLE loop.The tracking accuracy of micron grade bubbles interface in macroscopic scale flow field was improved though adaptive meshing refining technique.Based on this model,the influence law of temperature field and velocity field on the bubble morphological evolution in the thickness cross-section of the injection flow field was obtained.The deformation,burst and vanishing process of bubbles with different initial sizes and locations in the shear and fountain flow field was predicted.Combined with the short shot experiment of polymer foaming injection molding(PFIM),the formation mechanism of pits,silver marks and collapses on the product surface manufactured by PFIM was revealed(2)A non-isothermal transient multiphase-VOF model based on finite volume method was established,and the implicit domain coupling algorithm was proposed to synchronously calculate the temperature field of mold and cavity regions.A in the Rapid Heat Cycle Molding(RHCM)assisted PFIM process(RHCM/PFIM)was also realized.Based on the simulation and experiment results,the transient flow behavior of supercritical CO2,polymer melt and air in mold cavity was analyzed.The effect of mold temperature on the deformation,bursting and collapsing process of bubbles in fountain flow field was revealed.The formation mechanism of the surface defects of RHCM/PFIM plastic part was also revealed(3)A non-isothermal fluid-solid coupling model based on the two-phase model was developed.The coupling heat transfer between the injection mold and the polymer melt is considered by using the implicit domain coupled algorithm.This model can predict the temperature field very accurately.The thermal response characteristics in RHCM/PFIM process were further analyzed.By combining the simulation results with the cellular morphology obtained by experiments,the formation mechanisms of the cellular morphology in RHCM/PFIM process was revealled.(4)An in-situ high-pressure microscopic system was developed with a sample cell whose temperature and pressure can be close-loop controlled.The crystal growth process of Poly(L-Lactic Acid)(PLLA)film was recorded by this system in pressurized CO2 environment with different film thicknesses,different molecular weights,different temperatures and different CO2 pressures.With a reasonable selection of experimental parameters,the preliminary stage of formation process of snowflake-shaped crystal was researched by using the self-established system.With the help of atomic force microscope,this research illuminated the growth mode of the snowflake-shaped PLLA crystal.(5)An in-situ high-pressure multiple-optical observing system composed of ordinary optics,polarization optics and small angle laser scattering was established to record the evolution process of polymer condensate state in pressurized CO2 environment with a scale range from 0.1 μm to 1 cm.The statistical results of Poly(L-lactic acid)(PLLA)crystal size and crystal number density obtained in CO2 were obtained by using the this system.Combining with atomic force microscopy,the growth behavior of PLLA dendritic crystal in high-prssure CO2 was recorded and analyzed.And a bamboo-shaped dendrite formed by rhythmic growth has been discovered.(6)The supercritical CO2 was introduced to intervene in the melting isothermal crystallization of PLLA film.A screw terrace crystal excluding the tie molecules and intercrystalline entanglements was prepared.Based on the results of Atomic Force Microscopy,Transmission Electron Microscopy,Nuclear Magnetic Resonance and X-ray Diffraction,intercrystalline phases on the lamella surface were determined,and the influence of amorphous chain state on the morphology of multilayer lamellae was researched.Finally,a theoretical explanation about the origination of the lamellar splaying was concluded.(7)By using an the self-established in-situ high-pressure microscopic system,a kind of crystallization experiment with online rising pressure was realized to research the polymer crystallization in CO2.Through this univariate experimental method,the effect of CO2 pressure on the PLLA crystal growth was investigated experimentally.A new model evaluating the polymer crystal nucleation rate was proposed based on the two-step mode.Combining with the experimental results,this research concluded the influencing mechanism of pressurized CO2 on the polymer crystal secondary nucleation through quantitatively calculating crystallization free energy,diffusion activation energy,mixing energy,adsorption energy and translational energy in polymer/CO2 system更多还原