Rheology of polyacrylonitrile‐based precursor polymers produced from controlled (RAFT) and conventional polymerization: Its role in solution spinning

Polymer solutions in dimethyl sulfoxide (DMSO) as a solvent, made from reversible addition fragmentation chain transfer (RAFT)‐mediated polyacrylonitrile (RAFT^¥ PAN) terpolymer with molecular weight (MW) of 260,000 g/mol and dispersity (Ð) of 1.29, behave differently under applied shear stress than polymer solutions made from conventional PAN (Control PAN) with similar MW (258,000 g/mol) but Ð of 2.05 in the same solvent. The unique rheology of RAFT PAN is because of the reduced amount of high MW polymer fractions. Specifically, a 25% (w/v) polymer solution of RAFT PAN had a viscosity of 198 Pas while the equivalent control PAN polymer solution had a viscosity of 968 Pas at a shear rate of 1 s^?1. Also, RAFT PAN polymer solutions had a longer Newtonian plateau than control PAN polymer solutions. This exhibits more liquid character in RAFT PAN polymer solutions than control PAN polymer solutions at same temperature and concentration. In dynamic tests, RAFT PAN polymer solutions gelled slower than their equivalent control PAN polymer solutions because of their longer polymer chain relaxation times. Slow gelling and higher liquid character in RAFT PAN polymer solutions can result in obtaining stronger and finer precursor fibers during wet spinning. Since RAFT PAN polymer solutions exhibit low viscosity and higher liquid character when compared to its equivalent control PAN at same concentration and temperature, these can allow a wider working window for wet spinning and can also allow higher solid content in the polymer solutions that remain easy to wet spin. This is expected to lead to compact and finer fibers with less voids and higher strength. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44273.     

In situ study on kinetic behavior during asymmetric membrane formation via phase inversion process using Raman spectroscopy

Asymmetric membrane formation has been studied by using an in situ analysis developed with a Micro Raman spectroscopy, which emphasizes kinetic aspects of the phase inversion process. Changes in composition with time was successfully measured on the gelation bath‐side as well as inside the precipitated phase for the polymer/solvent/nonsolvent system of polysulfone/1‐methyl‐2‐pyrrolidinone/ethanol. The results shows that resulting relative mass transfer rates of solvent and nonsolvent during the phase inversion process strongly influence the final membrane morphologies. In addition, ternary compositions at which phase separation initiates were explored by analyzing the coagulation front. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 75: 135–141, 2000     

Diffusion dynamics of ionic liquids during the coagulation of solution spinning for acrylic fibers

The coagulation dynamics of acrylic polymer (PAN) with 1‐butyl‐3‐methylimidazolium chloride [BMIM]Cl as solvent for PAN and H₂O as nonsolvent was investigated in detail. On the basis of Fick's second law of diffusion, a mass‐transfer model of [BMIM]Cl from concentrated PAN/[BMIM]Cl solution was established as verified with the experimental data. The established model has a good fit with the experimental data and the diffusion coefficient D of [BMIM]Cl was calculated according to the model. The diffusion coefficient D decreased a little when the concentration of solution increased. As increasing the coagulation bath concentration, the diffusion coefficient D initially increased and then decreased, reaching a maximum of 5 wt% in the coagulation bath. The diffusion coefficient D decreased with the coagulation bath temperature. From the diffusion coefficient and the structure of the coagulated filament, it can be concluded that the diffusion rate of [BMIM]Cl from PAN concentrate solutions is relatively slow, which is prospective to prepare uniform‐structure fibers. POLYM. ENG. SCI., 48:184–190, 2008. © 2007 Society of Plastics Engineers     

Stimuli‐responsive molecularly imprinted hybrid polymer gel as a potential system for controlled release

The purpose of this study is to develop a stimuli‐responsive hybrid polymer gel system with an improved mechanical stability as a controlled drug delivery carrier that can undergo phase transition by the stimulation of ethanol–water mixture. For this aim, trimethoxysilane terminated poly(propylene glycol) by coupling of 3‐isocyanatopropyl‐triethoxysilane with the hydroxyl end groups of poly(propylene glycol) through urethane bonds was synthesized. Hybrid polymer gels prepared in the presence of tryptophan (Trp), as a model of drug, were characterized and gelation time of polymer network was obtained by monitoring the fluorescence emission of Trp in pre‐gel solution. Swelling, solvent uptake and release kinetic of polymer gels were evaluated depending on time. The diffusional exponents (n) and diffusion constants (k) of each gel were calculated by using the swelling kinetic data. The effect of precursors as a monomer on Trp release profile was analyzed. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 42913.     

Study on the Coagulation Mechanism of Wet-Spinning PAN Fibers

The mechanism of coagulation process of polyarylonitrile (PAN) wet-spinning was studied based on the numerical analysis of the coagulation of viscous polymer solution by diffusional interchange with a bath. Experiments were performed in order to determine the diffusion coefficient of solvent and non-solvent. The numerical solutions of Fick’s second law equations for diffusion were obtained by using the MOL method. The microstructure of nascent fibers is investigated by Transmission Electron Microscope (TEM) and Electron Probe Microanalyser (EPMA). It is found that the speed of outflow solvent is more rapid than that of influent non-solvent. The speed of concentration change of solvent and non-solvent is rapid in outside layers of filament (0.2R) and slow in inner layers. The solvent concentration is keeping a high value at the center of filament. TEM and EPMA show that the microstructure is a typical layered structure. The thickness of dense skin is about 0.2R. The core region of fiber is very loose.     

Numerical simulation of the self‐assembly of a polymer–polymer–solvent ternary system on a heterogeneously functionalized substrate

The spinodal decomposition of a polymer–polymer–solvent ternary blend spin coated on heterogeneously functionalized substrate is studied in a three‐dimensional numerical model. The Cahn‐Hilliard equation was used to describe the free energy profile of the domain. The mechanism of the morphology evolution was studied quantitatively. The well‐established linear relationship of the characteristic length, R(t), with t^1/3 can be observed in the simulation results. The functionalized substrate greatly affected the morphology evolution of blends with different solvent concentrations. The results indicated that a critical time can be observed, at which the evolution rate changes abruptly, also after which the compatibility of the surface morphology to the functionalized substrate pattern increases at a much lower pace in the diluted solution. In the condensed solution, the compatibility actually decreases beyond the critical time. The influence of solvent evaporation is investigated and a sharper interface was observed in the case with solvent evaporation and film thickness reduction. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers     

Modelling study of emulsion latex coagulation processes in coagulators

This paper presents a numerical study of emulsion latex coagulation processes in continuous coagulators based on the full computational fluid dynamics approach. The RANS approach together with the k‐ε turbulence model was used to describe the detailed flow field in the coagulators. The coagulant mixing process was modelled by the convection‐diffusion equation and the emulsion latex coagulation process was formulated by the population balance equation of the particle size with a coagulation kernel including a perikinetic and orthokinetic combined mechanism. The flow and coagulation models were independently validated by means of comparing simulated results to the relevant experimental data from the literature. A series of simulations were carried out to study the effects of coagulator bottom shape, salt solution feeding location, residence time and agitation speed, as well as the influence of four typical scale‐up criteria on the latex particle coagulation process. The presented results would be helpful for the relevant process design, development, and scale‐up of continuous latex coagulators.     

The formation of the S‐shaped edge‐on lamellae on the thin porous polylactic acid membrane via phase separation induced by water microdroplets

Biodegradable polymers have received increased attention due to their potential application in the medicine and food industries; in particular, poly(lactic acid) (PLA) is a primary biopolymer because of its excellent biocompatibility and absorbability features. In this study, a porous PLA membrane was fabricated by phase separation using water microdroplets produced by an ultrasonic atomizer as a coagulation bath. The formation of S‐type clusters was attributed to the slow exchange rate between solvent and coagulant, which provided time for the movement of polymer molecules. The effect of preparation conditions on the structure of S‐type clusters, including polymer concentration in solution and ambient temperature was investigated. The PLA porous membrane prepared from water micro‐droplets with different morphology compared with the sample fabricated from distilled water has potential application in fields of tissue engineering and artificial organ generation. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43355.     

Curing kinetics of a “green” thiol‐containing resin: Oligo(ethylene‐2‐mercaptosuccinate)

This work focuses on examining the curing process of neat oligo(ethylene‐2‐mercaptosuccinate) using differential scanning calorimetry (DSC), rheology, and Fourier transform infrared (FTIR) spectroscopy. The thiol‐containing resin offers much promise as a bioabsorbable polymer in medical field and as a reusable thermoset in sustainable applications. Although curing between thiol groups has been investigated in solutions, studies of neat materials without solvent are rare. Here, the evolution of glass transition temperature (T_g), complex shear modulus (G*), gelation, and chemical structure are monitored as a function of isothermal curing time and temperature. Both T_g and G* increase with curing, indicating the formation of polymer networks. The conversion of the cure is determined from the DiBenedetto equation and is found to follow a second‐order plus second‐order autocatalytic reaction model. Importantly, the intensity of the S–H bond absorption decreases with the extent of curing, which confirms the curing mechanism, i.e., disulfide formation between the thiol groups. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43205.     

Modeling and simulation of the phase‐inversion process during membrane preparation

The objective of this work is to present a simple mathematical model to describe the main features of phase inversion by immersion precipitation. The model was developed for planar geometry and is used to simulate membrane synthesis. Two systems commonly used for membrane preparation were used for simulations: cellulose acetate/acetone/water and polyetherimide/N‐methylpyrrolidone/water. The influence of nonsolvent addition to initial polymer casting solutions, solvent addition to the coagulation bath, and geometric variables, such as polymer film thickness, on the final precipitation conditions were studied through simulation and compared to available experimental data. The results are in good agreement with published results. 1 - 3 It is shown that polymer film composition profiles at the moment of precipitation may give important information about the structure and substructure of formed membrane. It is also shown, for both polymeric systems investigated in this work, that the dynamics of the mass transfer process seems to be much more important than the influence of the concentration on the diffusion and thermodynamic partition coefficients, as fair agreement with available experimental data was obtained even when these coefficients were assumed to be constant. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 3036–3051, 2001     

Experimental study and numerical simulation the air jet flow field of a dual slot sharp blunt die in the melt blowing nonwoven process

In this work, the physical model of a polymer in a melt blowing process is established and solved by introducing the numerical computation results of the air jet flow field of the dual slot sharp inset die. The influence of the melt blowing processing parameters and the die design parameters on the fiber diameter is also studied. A lower polymer throughput rate, higher polymer melt initial temperature, higher air initial temperature, higher air initial velocity, smaller angle between slot and axis of the spinneret, smaller width of the die head, and larger width of the slot can all produce finer fibers. At the same time, the air jet flow field model of the dual slot sharp inset die of polypropylene polymer nonwovens fabrics in melt blowing process was also established. The air jet flow field model was solved by using the finite difference method. The computational simulation results of the distributions of the z‐components of air temperature and air velocity along the spinline during melt blowing process are in accordance with the experimental data. The air drawing model of melt blowing process was simulated by means of the numerical simulation results of the air jet flow field. The predicted fiber diameter agree with the experimental data. The effects of the air initial velocity and air initial temperature on the fiber diameter were studied and discussed. The results demonstrate that a higher air initial velocity and a higher air initial temperature are beneficial to the air drawing of the polymer melt and thus to reduced fiber diameter. The results show the great potential of this research for computer assisted design in melt blowing nonwoven process and technology. POLYM. ENG. SCI., 57:417–423, 2017. © 2016 Society of Plastics Engineers     

Guidelines for dryer design based on results from non‐Fickian model

In polymer solution coatings below the glass transition temperature of the pure polymer, the coating can go undergo a glass transition and develop stresses during drying. When stresses develop, a non‐Fickian model accurately describes solvent mass transport in drying polymer coatings. The non‐Fickian model includes the solvent transport due to both stress and concentration gradients. This article presents a non‐Fickian model, which predicts a lower residual solvent than does the corresponding Fickian model. We showed in an earlier article that the non‐Fickian model predicts trapping skinning (higher residual solvent under more intense operating conditions) at higher drying gas‐flow rates. In this article, the non‐Fickian model was used to investigate how the gas‐flow rate, dry film thickness, and substrate thickness affect the residual solvent for a single‐zone dryer. This work recommends guidelines for choosing gas‐flow rates, gas temperatures, and substrate thickness to minimize the residual solvent. The model predictions show that, at any gas temperature, the residual solvent is minimum at an intermediate gas‐flow rate. The trapping skinning effect is less evident in thicker coatings and substrates. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 87: 477–486, 2003     

Microporous anisotropic phase inversion membranes from bisphenol A polycarbonate: Effect of additives to the polymer solution

Phase inversion is a very flexible technique to obtain membranes with a large sort of morphologies. Membrane properties can vary greatly depending on the kind of polymer system used. Bisphenol A polycarbonate (PC) could be used as a phase inversion membrane base polymer, and presents very good properties. Nevertheless, very little information on membrane preparation using PC and the phase inversion process can be found in the literature. In this work flat‐sheet microporous membranes were obtained by the phase inversion process using the immersion precipitation technique. A new polymer system was studied, consisting of polycarbonate, N‐methyl‐2‐pyrrolidone as solvent, water as the nonsolvent, and an additive. The influence of some parameters on membrane morphology, such as polymer solution composition, exposition time before immersion into the precipitation bath, and the kind of additive was investigated. Precipitation was followed using light transmission experiments and membrane morphology was observed through Scanning Electron Microscopy (SEM). The viscosity and cloud points of all polymer solutions were also determined. The results were related to the studied synthesis parameters, using the basic principles of membrane formation by the phase inversion technique, looking forward to establishing criteria to control the morphology of flat‐sheet membranes using polycarbonate as the base polymer. The results showed that both additives were able to increase pore interconnectivity and even suppress macrovoid formation. The decrease in the miscibility region of the polymer system and increase in mass transfer resistance are found to be the determining factors during polymer solution precipitation. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 86: 3085–3096, 2002     

Wetting kinetics of polymer solutions and force‐based contact angles

The effect of the shear thinning behavior and elasticity of polymer solutions on the dynamic contact angles are investigated. Under dynamic conditions, the contact angle of a liquid on a solid surface changes significantly with the substrate velocity from its equilibrium value. The dynamic contact angles for polyethylene oxide (PEO) solutions of two molecular weights 3 × 10⁵ and 4 × 10⁶ have been measured using a polyethylene terephthalate (PET) plate. The three‐parameter Ellis model to fit the rheological data to obtain shear thinning power n, characteristic shear stress, and the zero‐shear viscosity is used. The theory indicates that dynamic contact angles follow power law in this instance instead of showing Newtonian behavior with zero‐shear viscosity when the shear thinning effects are considered. The elastic effect becomes important at larger polymer concentrations that reduces the dependence on capillary number, that is, reduces n keeping with the experiments. © 2016 American Institute of Chemical Engineers AIChE J, 62: 2533–2541, 2016     

Polyamide/polyacrylonitrile thin film composites as forward osmosis membranes

Thin film composites (TFCs) as forward osmosis (FO) membranes for seawater desalination application were prepared. For this purpose, polyacrylonitrile (PAN) as a moderately hydrophilic polymer was used to fabricate support membranes via nonsolvent‐induced phase inversion. A selective thin polyamide (PA) film was then formed on the top of PAN membranes via interfacial polymerization reaction of m‐phenylenediamine and trimesoyl chloride (TMC). The effects of PAN solution concentration, solvent mixture, and coagulation bath temperature on the morphology, water permeability, and FO performance of the membranes and composites were studied. Support membranes based on low PAN concentrations (7 wt %), NMP as solvent and low coagulation bath temperature (0 °C) demonstrated lower thickness, thinner skin layer, more porosity, and higher water permeability. Meanwhile, decreasing the PAN solution concentration lead to higher water permeance and flux and lower reverse salt flux, structural parameter, and tortuosity for the final TFCs. Composites made in N,N‐dimethylformamide presented lower permeance and flux for water and salt and higher salt rejection, structural parameter, and tortuosity. FO assay of the composites showed lower water permeance values in saline medium comparing to pure water. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44130.     

Model Optimization for the Dynamic Simulation of Reactive Absorption Processes

The optimal design of reactive separations is impossible without reliable process models. Especially for the dynamic simulation and the model‐based control of complex reactive absorption processes the model development leads to a contradiction between the required model accuracy to reflect the process complexity and the feasibility of process simulations regarding the computation time. In this respect, we have developed a new rigorous dynamic two‐phase model based on the two‐film theory as a first step, which takes into account the influence of chemical reactions and additional driving forces in electrolyte systems on mass transfer considering thermodynamic nonidealities as well as the impact of column internals on the process hydrodynamics. For a model optimization, we have performed an analysis of different model approaches for complicated industrial absorption processes and determined an appropriate model complexity. Based on results of sensitivity studies, we have accomplished different model modifications leading to a stabilization of the numerical solution without affecting the good agreement between simulation results and the experimental data. This time‐optimized model can be considered superior as compared to previous approaches and facilitates for the first time a rigorous dynamic simulation of entire reactive absorption columns and the application within an on‐line process control system.     

Mechanism of bubble formation in the drying of polymer films

A common problem in making thin polymer films by solution processing is the undesirable formation of bubbles during the drying process. These bubbles appear well below the boiling point of the solvent. Experience indicates, however, that the degassing of the polymer solutions reduces bubble formation, indicating a relationship with the presence of air. This work is based on a hypothesis that if the solubility of air in the polymer solution increases with solvent concentration, then the solution can become supersaturated with air as the concentration of the solvent is reduced during the drying process. To test this hypothesis the system poly(vinyl acetate)‐toluene‐nitrogen was chosen. Previously published data on the solubility and diffusion of nitrogen in the polymer‐solvent system were used. Different diffusion models based on the friction coefficients and free‐volume model were then used to correlate the diffusivity data so that the diffusion behavior of the ternary system can be predicted over a broad range of conditions. Finally, the thermodynamic and diffusivity correlations were incorporated into a multicomponent drying model which included main and cross‐diffusion terms to predict saturation behavior in the polymer solution during the drying process. The model without the cross‐diffusion terms represents the ideal system in which the diffusion of one component does not affect the diffusion of others. The drying model did not predict supersaturation of nitrogen when cross‐diffusion terms were neglected. Supersaturation of nitrogen was predicted, however, when the cross‐diffusion terms are included. Therefore, the cross‐diffusion terms in the mass transfer model are essential for the development of nitrogen supersaturation. Also different diffusion models based on the friction coefficients led to qualitatively similar predictions for the supersaturation of nitrogen. The simulation's results supported our experimental observations regarding bubble formation. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2009     

Mathematical modeling of a moving bed reactor for post‐combustion CO2 capture

A mathematical model for a moving bed reactor with embedded heat exchanger has been developed for application to solid sorbent‐based capture of carbon dioxide from flue gas emitted by coal‐fired power plants. The reactor model is one‐dimensional, non‐isothermal, and pressure‐driven. The two‐phase (gas and solids) model includes rigorous kinetics and heat and mass transfer between the two phases. Flow characteristics of the gas and solids in the moving bed are obtained by analogy with correlations for fixed and fluidized bed systems. From the steady‐state perspective, this work presents the impact of key design variables that can be used for optimization. From the dynamic perspective, the article shows transient profiles of key outputs that should be taken into account while designing an effective control system. In addition, the article also presents performance of a model predictive controller for the moving bed regenerator under process constraints. © 2016 American Institute of Chemical Engineers AIChE J, 62: 3899–3914, 2016     

Droplet formation at megahertz frequency

Droplet formation has been a fascinating subject to scientists for centuries due to its natural beauty and importance to both scientific and industrial applications, such as inkjet printing, reagent deposition, and spray cooling. However, the droplet generation frequency of common drop‐on‐demand (DOD) jetting techniques is mostly limited to ～10 kHz. This article presents an investigation of the possibility of jetting at megahertz frequencies to boost the productivity of DOD material deposition by ～100 times. The focus of this article is to understand the limitations of generating droplets at a megahertz frequency and to explore possible solutions for overcoming these limitations. A numerical model is first developed for the simulation of droplet formation dynamics. The numerical model is validated against available experimental data from the literature. Aided by insights gained from scaling analysis, the validated model is then used to study the effects of different parameters on high frequency jetting. The study finds energy density input to the nozzle is the key to achieve megahertz frequency droplet breakup. © 2016 American Institute of Chemical Engineers AIChE J, 63: 2367–2377, 2017     

乳酸提纯新工艺动态建模仿真和控制系统设计

基于非平衡级和拟均相假设,建立了乳酸提纯反应精馏新工艺实验装置的动态机理模型.通过改进的数值计算方法提高了模型的求解效率,设计并实现了包含物性估算系统的模型仿真平台,以促进新工艺的工业化应用.利用仿真平台对新工艺装置进行了动态特性分析,在此基础上设计了两种单端质量控制方案:直接物料平衡和间接物料平衡方案.在不同类型和幅度的过程扰动下,分析比较了两种控制方案的调节性能.结果表明直接物料平衡方案控制品质优于间接物料平衡方案,可在不同扰动情况下满足过程的产品质量和转化率的联合控制要求.非平衡级动态机理模型能够反映反应精馏过程的动态特性,分析发现反应精馏过程有着独特的过程特性,基于机理模型的仿真平台是分析反应精馏特性的有效工具.