聚苯乙烯/CO2发泡体系的气泡生长过程模拟

以聚苯乙烯/CO2发泡体系为研究对象,利用细胞模型推导出了基于牛顿流体本构方程、幂律流体本构方程、dewitt黏弹性本构方程3种本构方程的气泡生长数学模型,通过比较分析得出了最适合该发泡体系的气泡生长模型,并研究了熔体弹性和非牛顿性质等物性参数对气泡生长过程的影响。结果表明,dewitt模型能更加准确地反映实际的气泡生长的过程;聚合物熔体弹性在生长初期对气泡生长有明显的促进作用;聚合物熔体的非牛顿性质对于气泡生长全过程都有影响。     

振动力场作用下聚合物熔体本构关系的研究进展

从加工和理论两方面总结和评述了振动力场作用下聚合物熔体本构关系的研究成果,着重介绍了高分子链缠结动力学的结构网络本构理论。同时指出经典的连续介质本构方程不能精确描述振动力场作用下的聚合物熔体流变行为,而分子链缠结动力学瞬态网络理论可以精确描述大振幅振动剪切场下的聚合物流变行为。     

单螺杆挤出过程熔体输送段数值模拟方法

综述了应用于螺杆挤出成型过程熔体输送段数值模拟的各种计算方法，根据挤出过程聚合物流场控制方程，介绍了几种有代表性的聚合物本构关系，包括线性黏弹性和非线性黏弹性本构方程。概述了有限差分法、有限元法、有限体积法和边界元法等数值计算方法的特点和应用范围，并针对聚合物熔体流动模拟中的迭代收敛性和稳定性、耦合和解耦合、高Weissenberg数等问题进行了讨论，指出应该根据研究目的和实际情况选择适宜的本构方程和数值方法。     

高速微注射成型中熔体充填模式及裹气机理研究

塑料为了准确模拟聚合物熔体在型腔中的流动及前沿位置和形态,建立了熔体、气体两相流流动模型,构造了熔体流动的黏弹性本构关系,用无量纲方法建立了熔体流动前沿的气体、熔体流动的统一控制方程和本构方程,并采用水平集方法预测和跟踪熔体流动前沿,模拟了熔体在低速、中速、高速条件下的流动状态和充填模式,分析了高速微注射成型中气孔产生的原因和可能出现的位置,开展了实际产品的高速微注射成型实验,比较了模拟结果和实验结果。研究表明,熔体充填模式与注射速度、材料特性、型腔尺寸密切相关,在喷射充填模式下可能产生裹气。     

聚合物熔体脉振传递过程的协同学研究

利用协同学分析方法,研究了典型的平板振动剪切流熔体黏滞动力学特性(如垂直叠加的振动拖曳流),在脉振力场作用下聚合物熔体扰动本构方程的基础上,分别提出了在以上脉振动剪切流场中聚合物熔体的序参量控制方程,并建立了单螺杆脉振传递过程熔体动力学模型。     

振动剪切作用下聚合物熔体的非仿射网络结构模型 Ⅱ．非仿射网络结构模型

假设聚合物熔体的缠结网络形变是非仿射的，运用瞬态网络结构原理，采用在本课题第一部分^[1]中所建立的动态速率方程，并对上随体Maxwell本构方程加以修正来建立一个非仿射网络结构模型，利用这一模型来研究振动剪切作用下LDPE熔体的流变行为，研究表明，随着应变振幅和频率的增加，LDPE熔体的剪切应力也增加，同时指出了非仿射网络结构模型的精确变比仿射结构模型有较大提高，这表明在振动力场作用下，网络形变发生了非仿射形变，因此在建立振动力场作用下聚烯烃熔体本构方程时，不能假设其网络是仿射形变的。     

压出过程中混炼胶锥口型流动的研究

本文讨论了聚合物熔体在圆锥口型中的流动问题。应用改进的二阶流体的本构方程,建立起描述熔体入口流动中流速分布、剪切应力分布和法向应力差分布的数学模型,在此基础上,对熔体椎入口流动进行了数值模拟,并以混炼胶为试样,作了相应的实验研究。     

基于Giesekus黏弹性本构的注射成型数值算法研究

针对熔体的黏弹性特征,考虑熔体流动过程中的惯性作用,基于Giesekus模型建立了注射成型中熔体流动的黏弹性理论模型。将黏弹性本构方程整合为含瞬态项、对流项和源项的偏微分方程,基于有限体积法离散,提出了黏弹性本构方程的离散方法。借鉴有限元框架下的黏弹性离散分裂算法（DEVSS）,发展了以有限体积法为基础的黏弹性离散分裂算法（FVM-DEVSS）,开发了三维黏弹性流动的计算程序。用经典充分发展的4∶1收缩流验证方法的有效性,再结合充模流动实验,正确预测了熔体流动前沿、浇口压力及熔接线位置,通过光弹实验结果再次证明了本文方法的有效性。结果表明,本研究所发展的黏弹性算法可以有效表达熔体的黏性和弹性对流动压力的影响,而且能准确预测两股熔体相遇时熔接线的位置及形成过程。此外,本文黏弹性算法有效准确地计算了熔体充模过程中的流动诱导应力。     

聚合物熔体二维非等温脉动流场模型

基于螺杆轴向振动,建立单螺杆挤出机计量段熔体流动模型,推导聚合物熔体二维非等温幂率本构方程,计算螺杆径向熔体温度分布和沿螺槽方向熔体速度,结果表明:在螺杆计量段,熔体沿螺杆径向分布存在一定温度差且为非线性,熔体温差的存在会对熔体流场产生影响。     

塑料电磁动态逆化挤出机的挤出功率研究与模拟

本文采用自行修正的Tanner本构方程研究了塑料电磁动态塑化挤出机中熔体输送挤出功率对振动力场的响应。建立了振支力场作用下熔体输送挤出功率的理论模型,以此为基础模拟塑料电磁动态逆化挤出设备的设计和生产应用提供重要的理论指导依据。     

粘弹性流体非等温本构方程

本文将Giesekus提出的单模式平均构型分子模型推广到非等温情形,然后利用人们熟知的粘温关系式来计及温度和热历史对应力张量的影响,得到一个描述聚合物浓溶液和熔体非等温流变性质的本构方程,不必在分子模型中引入人为的假定来考虑非等温效应.对于非等温应力发展的计算表明,这一本构方程能避免以前研究者提出的本构方程的缺点,合理地描述聚台物流体的非等温流变性质.     

Rheological characterization of controlled-rheology polypropylenes using integral constitutive equations

Controlled-rheology polypropylene melts were prepared via molecular modification of a commercial polypropylene resin. A peroxide-initiated degradation was performed, resulting in materials with different molecular weight distributions. These resins were subjected to rheological characterization, and an integral constitutive equation of the K-BKZ type was used to study the effect of molecular weight characteristics on their rheological properties. Data for the linear viscoelastic spectrum and shear viscosities was used to obtain the model constants. The same constitutive equation has been used to predict the stress and Trouton ratios for simple shear and simple elongational flows, thus giving a quantitative assessment of the viscoelastic character of the melts. The results show the effect of the molecular modification on the rheological behavior of the melts. Polymers produced at higher peroxide concentrations exhibit reduced viscoelasticity manifested in less shear-and strain-thinning behavior. The present work clearly shows the potential of integral constitutive equations in fitting and interpreting experimental data and, thus, giving a much better understanding of the rheological behavior of commercial polymers. © 1996 John Wiley & Sons, Inc.     

The non-linear viscoelastic properties of molten polymers and concentrated solutions

For polymer melts and concentrated solutions highly nonlinear effects dominate all aspects of both the transient and steady state mechanical behavior. The first part of this paper will review the salient features of the experimental results and focus attention on those common features which should be understood in a coherent manner. The various constitutive equations, especially the integral representations that have been proposed to describe polymer melts will also be discussed. They fall roughly into three classes: those where the relaxation function depends on the deformation rate, deformation, or stress history. Evidence will be presented which indicates that the stress history rather than the deformation history may be the more relevant variable to account for the non-linearities. The last part of the paper will briefly review the theories that have been proposed to understand the mechanism for non-linear behavior at the microscopic level and the influence of molecular weight and concentration.     

Review of some useful rheological equations

A variety of constitutive equations have been proposed in recent years to describe the behaviour of polymer solutions and melts. Rather complete reviews of most of the differential and integral equations proposed prior to 1967 have been given by Spriggs, Huppler and Bird⁽¹⁾ and by Bogue and Doughty^(2,3). Since then, numerous efforts have been directed towards the development of equations capable of predicting adequately nonlinear and transient properties of polymer solutions and melts. Considerable information can be found in the recently published works by Bird et al.^(4,5). Some of the more successful models are presented here in an uniform notation. The models are evaluated in terms of their ability to simultaneously describe some or most of the rheological functions such as the non-Newtonian viscosity, the normal stress difference functions, the components of the complex viscosity, stress growth and stress relaxation.     

Systematic coarse graining flowing polymer melts: thermodynamically guided simulations and resulting constitutive model

Complex fluids, such as polymers, colloids, liquid-crystals etc., show intriguing viscoelastic properties, due to the complicated interplay between flow-induced structure formation and dynamical behavior. Starting from microscopic models of complex fluids, a systematic coarse-graining method is presented that allows us to derive closed-form and thermodynamically consistent constitutive equations for such fluids. Essential ingredients of the proposed approach are thermodynamically guided simulations within a consistent coarse-graining scheme. In addition to this new type of multiscale simulations, we reconstruct the building blocks that constitute the thermodynamically consistent coarse-grained model. We illustrate the method for low-molecular polymer melts, which are subject to different imposed flow fields like planar shear and different elongational flows. The constitutive equation for general flow conditions we obtain shows rheological behavior including shear thinning, normal stress differences, and elongational viscosities in good agreement with reference results.     

Shear rate and wall slip velocity functions of polyvinyl chloride melts based on slit die viscometry data

Shear stress versus shear rate and wall shear stress versus slip velocity relations of polymer melts are key material property functions needed in the design of polymer processing equipment. These properties are normally measured using capillary or slit viscometers. The presence of slip greatly complicates the processing of the resulting viscometry data. This paper describes the conversion of the viscometry data of a number of polyvinyl chloride melts through slit dies with different gaps into these two material property functions. The conversion procedure, based on Tikhonov regularization, has the advantage that it is independent of rheological constitutive equation and does not require extrapolation of the experimental data. It also has the ability to cope with the unavoidable noise in the experimental data. Consequently, the property functions thus obtained are likely to be closer to true material properties than those from some of the existing methods. For each of the polyvinyl chloride melts investigated, these properties' functions will be compared with published results and their differences discussed.     

Converging flow analysis,entrance pressure drops,and vortex sizes: Measurements and calculated values

The use of converging flows for the determination of the elongational behavior of polymer melts is interesting provided that the solving of the inverse problem is fast. For this purpose, this paper shows that the entrance pressure drops that occur during the axisymmetric flow of a polymer melt from the reservoir of a capillary rheometer into a cylindrical die with flat entry can be rapidly calcylated using a Wagner constitutive equation and Protean coordinates. This calculation technique performs particle tracking, which is absolutely necessary when using integral constitutive equations. In turn, the method also enables the determination of the particle path in the entrance zone and gives the boundary of the vertex. The appropriate solution is obtained through the minimization of the power consumption in the entrance zone. Calculation results are compared with experimental pressure drops and vortex pictures using two well‐characterized polyethylenes and show good agreement.     

聚合物口模挤出计算机模拟中的高We数问题研究

讨论了聚合物口模挤出计算机模拟中高Weissenberg(We)数问题 (HWNP)产生的可能的原因 ,指出本构方程本身固有的缺陷、流变方程的双曲线型、局部高应力梯度都有可能导致HWNP。从本构方程的选择、应力求解方法的改进和网格离散的精细化等方面详细分析了现有研究成果中提高临界We数所采取的措施。为今后解决聚合物口模挤出计算机模拟中的HWNP提供了借鉴。