The kinetics of fiber growth from flowing solutions

Data are presented for the longitudinal and radial growth rates of polyethylene fibers grown from high molecular weight solutions using a seeding method in Poiseuille tube flow. Results indicate growth is a two-step process with the first step the formation of very thin untapered filaments ～25 μm in diameter which grow very rapidly the full length of the capillary tube. The second step involves radial thickening of the filament into a tapered shape, the kinetics of which have been measured. The discussion includes an analysis of the rapid longitudinal growth and a critical comparison to earlier experiments in both Poiseuille and Couette flow. A possible growth mechanism is suggested to explain the high growth rates found in these experiments as well as the unusual growth observed earlier for Couette rotor growth. A discussion of some kinetic data for the radial growth process is also given along with preliminary results which indicate thickening may only occur in flow of extensional kinematics.     

Apparent elongational viscosity of dilute polymer solutions

Apparent elongational viscosity studies were made on dilute solutions of high molecular weight polymers using a fiber spinning apparatus designed for low shear viscosity liquids with substantial elongational effects. The experimental method involved the flow of solutions of polyacrylamide and poly(ethylene oxide) from a tube into an evacuated vessel. Experimental results showed that the apparent elongational viscosity obtained from the jet shape increased linearly with the stretch rate.     

Rheological behavior and fiber orientation in slit flow of fiber reinforced thermoplastics

The flow behavior and fiber orientation in slit flow of a short fiber reinforced thermoplastic composite melt are investigated. A slit die with adjustable gap and interchangeable entrance geometries was designed and built. The slit die is fed by a single screw extruder. The bulk viscosity is calculated from the axial pressure profiles measured using three flush mounted pressure transducers. The effect of entrance geometry and gap dimensions on the fiber orientation and bulk flow behavior is specifically considered. A skin-core composite fiber orientation is observed in the thickness direction. Fibers are oriented in the flow direction and parallel to the walls in the skin region irrespective of the entrance geometry. Different fiber orientation distributions in the core region can be realized by using different entrance geometries. However, the changes in the core fiber orientation are not fully reflected by the measured viscosities, due to highly oriented skin layer. Exit pressures obtained by extrapolation of linear pressure profiles are found to be all positive, but dependent on the die geometry and entrance conditions, even for the unfilled melts.     

The role of elasticity in the formation of electrospun fibers

The role of fluid elasticity in the formation of fibers from polymer solution by electrospinning is investigated. Model solutions with different degrees of elasticity were prepared by blending small amounts of high molecular weight polyethylene oxide (PEO) with concentrated aqueous solutions of low molecular weight polyethylene glycol (PEG). The elastic properties of these solutions, such as extensional viscosity and the longest relaxation time, were measured using the capillary breakup extensional rheometer (CaBER). The formation of beads-on-string and uniform fiber morphologies during electrospinning was observed for a series of solutions having the same polymer concentration, surface tension, zero shear viscosity, and conductivity but different degrees of elasticity. A high degree of elasticity is observed to arrest the breakup of the jet into droplets by the Rayleigh instability and in some cases to suppress the instability altogether. We examine the susceptibility of the jet to the Rayleigh instability in two ways. First, a Deborah number, defined as the ratio of the fluid relaxation time to the instability growth time, is shown to correlate with the arrest of droplet breakup, giving rise to electrospinning rather than electrospraying. Second, a critical value of elastic stress in the jet, expressed as a function of jet radius and capillary number, is shown to indicate complete suppression of the Rayleigh instability and the transition from ‘beads-on-string’ to uniform fiber morphology.     

Bead‐to‐fiber transition in electrospun polystyrene

The morphological transition, namely bead‐to‐fiber transition, of electrospun polymer was examined for polystyrene, with its molecular weight ranging from 19,300 to 1,877,000 g/mol. Tetrahydrofuran and N,N‐dimethylformamide were used as solvents to examine the effects of solvent properties on the morphological variations. Polymer molecular weight and solvent properties had a significant effect on the morphology of beads as well as fibers. Observation of fiber diameter and its distribution suggested that the effect of molecular weight and solvent may be independent. The critical concentrations at which incipient and complete fibers were observed were found to decrease significantly with molecular weight, as can be expected. The effect of solvents on these critical concentrations was minimal for moderate to high‐molecular‐weight (>100,000 g/mol) solutions. For low‐molecular‐weight solutions, the transition occurred at concentrations much lower than those predicted by a model, based exclusively on chain entanglements. Rapid solidification of jet which is expected to occur with concentrated solutions may play a vital role in establishing stable fibers during electrospinning. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

超高分子量聚乙烯气辅挤出影响因素的数值模拟及分析

以超高分子量聚乙烯的圆形轴对称气辅口模挤出为研究对象,在采用Polyflow软件对气辅口模挤出时的等温流动进行数值模拟之后,就入口流率、松弛时间以及零剪切黏度等物性和工艺参数对挤出胀大、速度分布、口模压降和熔体外表面上剪切速率的影响进行了数值模拟和分析。分析表明:气辅挤出是克服超高分子量聚乙烯传统挤出时面临一系列困难的有效加工方式。     

RHEOLOGICAL AND MIST IGNITION PROPERTIES OF DILUTE POLYMER SOLUTIONS

A spinning disc atomizer has been used to characterize the mist flammability of Jet A and diesel fuels that contain high molecular weight polymers. The critical disc velocity required to produce significant flame propagation was shown to depend on polymer concentration, molecular weight, solvent viscosity, and polymer degradation.

The viscoelastic properties of these same polymer solutions have been characterized by a maximum Darcy viscosity measured from flow in packed tubes. For the polymers discussed in this paper, the maximum Darcy viscosity was independent of the bead size or tube length; however, it was strongly affected by the same variables that affected mist flammability; i.e., polymer concentration, molecular weight, solvent viscosity, and polymer degradation.

The critical ignition velocity of dilute polymer solutions is shown to depend on the Darcy viscosity in a similar manner as observed for viscous oils. At low viscosities, the ignition velocity is only slightly affected, but the dependence grows stronger as the viscosity (both shear and Darcy) increases. A close correspondence was also shown to exist between the ignition velocity of a polymer solution with a high Darcy viscosity and the ignition velocity of a Newtonian oil with approximately the same high shear viscosity.

Numerous similarities are described between flow-induced birefringence of dilute polymer solutions with opposed capillary jets and viscoelastic resistance of dilute polymer solutions in packed tubes. These similarities suggest that the maximum Darcy viscosity is associated with a condition of almost complete extension and alignment of the polymer molecules.     

热致性液晶聚芳酯纤维的后固相聚合宏观动力学

以热致性液晶聚芳酯(TLCP)初生纤维为原料,通过初生纤维后续的热处理(即后固相聚合)提高纤维力学性能,对纤维后固相反应的宏观动力学行为进行了研究。结果表明:在热处理初期,纤维质量损失率增加迅速,说明聚芳酯相对分子质量增长速率较快;当达到一定程度后,相对分子质量增长速率变慢;热处理温度对其影响显著,热处理温度越高,相对分子质量升高越快;热处理前后期的表观活化能分别为94.4 kJ/mol和38.4 kJ/mol。     

Elastic fracture of polystyrene solutions

Based on the assumption of a constant critical shear strain, an expression for the critical stress at the onset of entrance fracture as a function of polymer concentration has been developed. Experimental results with 10-25 percent narrow distribution, high molecular weight polystyrene-benzene solutions show the critical stress to be much lower than that for the polymer melt and in agreement with predicted values. This result is all the more impressive when the contrast in flow behavior at the capillary entrance for melts and solutions is observed. Instead of the rotating toroidal vortices surrounding a 90-deg material entrance cone observed with polystyrene melt, cine movies of the solution flow birefringence patterns in the capillary entrance region reveal only a stagnant zone surrounding a narrow cone less than 20 deg. At fracture, the cone axis moves in a rotary path circulating about the capillary axis without undergoing the flow discontinuities typical of melt behavior.     

Comments on the accuracy of zero shear intrinsic viscosity of high molecular weight polyacrylamide

The intrinsic viscosity of a polymer is traditionally measured with a capillary tube viscometer where the shear rate range is moderately high. Such method is valid when the polymers are non-ionic and have low to moderate molecular weight. The viscosity-shear rate curves obtained for dilute aqueous solutions of two high molecular weight polyacrylamides using two rotational viscometers indicate a strong shear-dependent viscosity in the medium to high shear rate regions. The zero shear intrinsic viscosity of the polymers determined by extrapolation from the high shear rate region to the zero shear condition may result in large errors. Its implication in predicting the molecular weight of polymers using the Mark-Houwink-Sakurada equation is discussed. A rheological equation for intrinsic viscosity as a function of shear rate is proposed.     

High Stiffness Cellulose Fibers from Low Molecular Weight Microcrystalline Cellulose Solutions Using DMSO as Co‐Solvent with Ionic Liquid

There is a need to develop high‐performance cellulose fibers as sustainable replacements for glass fibers, and as alternative precursors for carbon filaments. Traditional fiber spinning uses toxic solvents, but in this study, by using dimethyl sulfoxide (DMSO) as a co‐solvent with an ionic liquid, a novel high‐performance fiber with exceptional mechanical properties is produced. This involves a one‐step dissolution, and cost‐effective route to convert high concentrations of low molecular weight microcrystalline cellulose into high stiffness cellulose fibers. As the cellulose concentration increases from 20.8 to 23.6 wt%, strong optically anisotropic patterns appear for cellulose solutions, and the clearing temperature (T _c) increases from ≈100 °C to above 105 °C. Highly aligned, stiff cellulose fibers are dry‐jet wet spun from 20.8 and 23.6 wt% cellulose/1‐ethyl‐3‐methylimidazolium diethyl phosphate/DMSO solutions, with a Young's modulus of up to ≈41 GPa. The significant alignment of cellulose chains along the fiber axis is confirmed by scanning electron microscopy, wide‐angle X‐ray diffraction, and powder X‐ray diffraction. This process presents a new route to convert high concentrations of low molecular weight cellulose into high stiffness fibers, while significantly reducing the processing time and cost.     

Molecular entanglement for rigid rod molecular composites: Poly(p-phenylene-2,6-benzbisthiazole)/poly(hexamethylene adipamide)

The entanglement molecular weight necessary for integrity of processed fiber and film for rigid rod molecular composites is discussed. The case presented is one where the matrix flexible coil polymer has a marginal molecular weight for entanglement. The molecular weight of the rigid rod reinforcement was increased to offset this deficiency. No significant phase separation was observed for solutions undergoing high shear. Steady state shear measurements on rigid rod molecule solutions show that the zero shear viscosity results agree very well with recent theoretical rigid rod molecule behavior predictions.     

Solutions of drag-reducing polymers—diameter effect and rheological properties

Solutions (0.5% by weight) of two high molecular weight drag-reducing polymers, a polyethylene oxide and an acrylamide copolymer having anionic functionality, were prepared under low and high shear conditions. Rheological measurements demonstrated that high shear preparation causes a decrease in the size of the polymer units, the effect being markedly greater for the polyethylene oxide. These solutions were diluted to 25 ppm and drag reduction measured in 0.047-in. and 0.93-in. diameter test sections. The method of solution preparation showed no significant effect in the small tube measurements. In the large tube, however, low shear solution preparation led to drag reductions comparable to those found in the small tube, while high shear preparation gave markedly lower drag reductions. These results are discussed in terms of polymer supermolecular structure.     

Solid-state polymerization vis-à-vis fiber formation of step-growth polymers. I. Results from a study of nylon 66

This study is directed toward synergistic integration of processing of nylon 66 fibers with solid-state polymerization. The following two goals are sought through the incorporation of solid-state polymerization after the initial shaping operation: eliminating some of the processing problems in the production of high molecular weight, high performance industrial fibers and attainment of improved mechanical properties via high molecular weight. Successful solid-state polymerization has been achieved with as-spun fibers of nylon 66 and molecular weights up to 280,000 g/mol have been obtained from a starting molecular weight of 40,000 g/mol. It is shown that much of the ductility of the starting fiber can be retained, or even enhanced, with solid-state polymerization. Simulated drawing experiments using a thermal deformation analysis technique show an increase in the drawing potential of the solid-state polymerized fiber as compared to the starting material. This has important implications regarding the ultimate properties that can be achieved in fibers of condensation polymers. © 1994 John Wiley & Sons, Inc.     

Stirring-induced solution crystallization of ultra high molecular weight polyamide 6

Unstable solutions of ultrahigh molecular weight polyamide 6 have been prepared by adding a nonsolvent to the polymer solution. Crystallization of the polyamide from such a solution proceeds very slowly. It has been found, however, that vigorous stirring of the unstable solutions induces rapid fibrous crystallization of the polymer. The fiber mat consists of irregularly shaped fibers. A low temperature and a high stirring rate are among the conditions necessary to obtain a high yield of fibrous material. The fibers formed upon stirring have a higher molecular weight than the polyamide 6 molecules which remain in the solution. The melting point of the fibers depends on the speed of the paddle stirrer. The differential scanning calorimetry (DSC) thermogram reveals higher melting temperatures of the fibrous material if higher stirring rates have been applied.     

Enhanced Diffusional Separation in Liquids by Sinusoidal Oscillations

Abstract

A general analysis of the problem of enhanced diffusional separation of dilute liquid solutions contained within open ended capillary tubes and subjected to axial oscillations is presented. Results show that the mass diffusion flux of the various components is equal to the product of the molecular diffusion coefficient of the species in question, the magnitude of the species axial concentration gradient, the square of the ratio of the tidal displacement to the capillary radius, and a function of the Womersley number. The earlier results of Dryer are shown to be correct for small oscillation frequencies and tube diameters, but predict effective diffusion coefficients which are too low at higher Womersley numbers. Differential diffusion separation fluxes some six orders of magnitude larger than possible with the same geometry in the absence of axial oscillations appear to be achievable for typical aqueous solutions.     

PRELIMINARY FINDINGS ON THE EFFECT OF POLYACRYLAMIDE ON PARTICLE-LIQUID SEPARATION IN HYDROCYCLONES

Addition of polyacrylamide (264 ppm) to a 2 wt% water-clay suspension results in significant changes in the centrifugal separation efficiency of a 10 mm hydrocyclone operating at entrance Reynolds numbers larger than 10⁴. Because conical hydrocyclones have several flow structures which can be significantly influenced by small amounts of a high molecular weight polymer, the observed physical phenomena may be related to the underlying viscoelastic behavior of dilute polymer solutions.     

超高相对分子质量PPTA树脂及其高模量芳纶的研究

对高相对分子质量聚对苯二甲酰对苯二胺(PPTA)树脂进行了表征,开展了添加超高相对分子质量PPTA树脂与普通相对分子质量PPTA树脂共混进行液晶纺丝得到高强度和高模量芳纶的结构表征与性能试验,同时对芳纶的力学性能与其PPTA树脂相对分子质量的关系进行了研究。结果表明,芳纶的力学性能与其PPTA聚合体的相对分子质量紧密相关,如果PPTA树脂的相对分子质量不够高,加上液晶纺丝和高模量热处理过程分子链的进一步降解,高模量芳纶的制备就无法实现。在系统研究PPTA聚合反应规律,特别是聚合诱导相互转变规律及其影响因素研究基础上,通过调控连续聚合的反应条件,在1 000 t/a连续聚合生产线上制备出比浓对数粘度高达9.2 dl/g的超高相对分子质量PPTA树脂;用超高相对分子质量PPTA树脂与通用级PPTA树脂(比浓对数粘度6.8 dl/g)混合进行纺丝,制备出高强度的芳纶,并进一步热处理得到高强度和高模量的芳纶。     

Effects of non-gray thermal radiation on the heating of a methane laminar flow at high temperature

The effects of the non-gray thermal radiation on the heating of a methane/argon laminar flow at high temperature are investigated. The preheating zone of a tubular reactor is studied, where the thermal decomposition of methane does not occur. The laminar flow is simulated with the commercial Fluent code in an axisymmetric geometry. The discrete ordinates method is applied to the numerical simulation of radiative heat transfer. The non-gray gas radiative model used is the absorption distribution function (ADF) using high temperature methane radiative properties which were recently published. Several thermal entrance regions of tubular reactors are compared and the influence of methane participation in radiative heat transfer is studied. The results show that the temperature field is significantly influenced by radiation due to methane absorption. Furthermore, the average flow temperature increases when the wall temperature, the tube diameter or the methane mole fraction increases. Due to intense absorption bands of methane, it is shown that the influence of methane non-gray thermal radiation should be included in simulations of such reactors.     

EVALUATION OF POLYMER ADSORPTION-GEL FORMATION AND SLIP IN POLYMER SOLUTION FLOWS

A superposition model for evaluation of the effects of polymer adsorption-gel formation and slip of polymer solutions exhibiting both phenomena has been applied to the capillary flow of aqueous solutions of two molecular weight grades of hydroxyethyl cellulose (Natrosol 250, types G and HR, supplied by Hercules Powder Company). The flow behaviour of the solutions investigated was non-Newtonian. Evaluations are presented of the effective thicknesses of polymer adsorption-gel formation and pure solvent layers, as a function of the wall shear stress, tube radius and polymer concentration, corresponding to the determinations of the effective velocity at the wall.

The results of the analysis indicate the surface characteristics undergo a dramatic change from polymer adsorption-gel formation at the tube surface to the phenomenon characterized by slip in a narrow tube radius interval which has important implications in enhanced oil recovery by polymer solution floods. It also provides an explanation for the contrasting behaviours observed in the flow of aqueous Natrosol solutions through packed beds (Sadowski, 1963) and filter cakes (Kozicki et al., 1972).