Polymer/carbon nanotube composites for liquid sensing: Selectivity against different solvents

An electrically conductive polymer composite (CPC) based on polycarbonate filled with 1.5 wt.% carbon nanotubes (CNTs) was investigated regarding its solvent selectivity when used as a sensor material for liquid detection. Based on the electrical response characteristics of the CPC when immersed in different solvents, “good” and “bad” solvents out of 59 test liquids were detected and the Hansen solubility parameters of the CPC were calculated using the “Hansen software” to be δ_D = 18.4 ± 0.2 MPa^0.5, δ_P = 10.9 ± 0.8 MPa^0.5, and δ_H = 4.1 ± 0.5 MPa^0.5.Based on the CPC's and solvents' Hansen solubility parameters the affinity between CPC and solvent represented by the distance in Hansen space was determined. As a second parameter the molar volume of the solvent molecules was used to describe the selectivity of the CPC by means of a quarter circle like area clearly separating “good” and “bad” solvents in a so-called solvent map.Whereas the Hansen solubility parameters are based on thermodynamics, the influence of the solvents' molar volume on the CPC's selectivity can be explained by diffusion processes. When increasing the molar volume of solvents having a similar chemical structure, a limiting value of the molar volume was found above which no solvent diffusion into the CPC was observed.Using this large number of solvents it could be clearly shown that the electrical response kinetics upon immersion into “good” solvents cannot be correlated with the Hansen solubility parameters of the CPC or their difference between CPC and solvent but is determined by the diffusion kinetics, which is governed by the solvents molecule size.     

A Unique Double Percolated Polymer Composite for Highly Efficient Electromagnetic Interference Shielding

This study has developed a carbon nanotube (CNT)/ethylene vinyl acetate (EVA)/ultrahigh molecular weight polyethylene (UHMWPE) composite with a unique double percolated conductive structure, in which only 20 wt% of CNT enriched EVA is needed to form a continuous conductive network. Compared with conventional double percolated conductive polymer composites (CPCs) which require filler‐enriched polymer content up to 50 wt%, the low CNT/EVA content gives rise to an unprecedentedly increased effective CNT concentration in the CNT/EVA/UHMWPE composite. The double percolated composite exhibits electrical conductivity comparable to that obtained in CNT‐loaded single EVA composite with five times of CNT content. Only 7.0 wt% CNT gives the composite an electromagnetic interference (EMI) shielding effectiveness of 57.4 dB, much higher than that of mostly reported CNT and graphene based CPCs. Absorption is demonstrated to be the primary shielding mechanism due to the numerous interfaces between UHMWPE domains and CNT/EVA layers facilitating multiple reflection, scattering, and absorption of the incident microwaves. The construction of unique double percolated structure in this work provides a promising strategy for developing cost‐effective and high‐performance CPCs for use as efficient EMI shielding materials.

     

A transport model for the absorption of mixed solvents in glassy polymer membrane

Transport model for mixed solvents in glassy polymer membrane is rare in literature. In our previous work, a new experimental method has been developed and absorption kinetic curves for two mixed solvent systems (ethanol/1,2-dichloroethane and ethanol/ethyl acetate) in polyurethane (PU) membrane at have been measured. In this work, based on Liu et al.'s transport model for single solvent/polymer membrane system, a transport model for the absorption of mixed solvents in glassy polymer membrane is established. Three model parameters in this model can be obtained by correlating the experimental data of the corresponding single solvent/polymer membrane systems; the other three should be determined by correlating the experimental data of mixed solvents/polymer membrane system. The effect of model parameters on diffusion is studied by numerical simulation. The correlated results agree well with the experimental absorption curves. The model has the ability to predict the transport phenomena of mixed solvents in polymer membrane.     

以聚乳酸为基体的导电高分子复合材料研究进展

概括了导电填料种类、结构及成型方法、工艺条件等因素对单相聚乳酸(PLA)基导电高分子复合材料(CPCs)电学性能的影响,介绍了PLA基多相CPCs中导电填料选择性分布机制和较低逾渗值的机理,总结了PLA基CPCs在有机溶剂中的响应行为,并对材料的应用前景进行了展望。     

Quantitating the Effect of Solvent Type on Neutral Macrocycle-Mediated Cation Transport in Liquid Membranes

The recently confirmed diffusion-limited transport model for neutral macrocyle-mediated cation transport in membranes has been used to quantify the membrane solvent effects on the fundamental parameters in the model and their relative importance. The fundamental parameters involved are the extraction equilibrium constant, macrocycle partition coefficient, and diffusion coefficient. Experimental measurements and predictive correlation calculations have been made to quantify these fundamental parameters. Membrane stability has also been quantified by looking at solvent solubilities and solvent compatibility with various membrane types. It was found that membrane stability vs. rapid transport is the major choice to be made in choosing a membrane solvent. Retention of the macrocycle in the membrane and diffusion coefficients are greatest with short carbon chain solvents containing few chlorine atoms. On the other hand, high boiling points and low water solubility are obtained with long carbon chain solvents containing increasing numbers of chlorine atoms. Extraction equilibrium constants also decrease with increasing carbon chain lengths and increasing chlorination, but the decrease is not uniform like the changes in the other parameters. Little effect of solvent type on membrane selectivity was observed. The transport model and quantification of these fundamental parameters should allow for accurate predictions in the choice of solvent in designing stable membranes with optimal transport properties.     

Effect of solvent exposure on the properties of hydroxy‐terminated polybutadiene‐based polyurethanes

Hydroxy‐terminated polybutadiene‐based prepolyurethanes and diamine chain extended polyurethane‐ureas were prepared and treated with various organic solvents in the moisture‐cured state in order to modify their ultimate strength. FTIR studies with solvent‐treated polyurethanes and polyurethane‐ureas confirmed that organic solvents penetrated inside the polyurethane hard segments and affected hydrogen bonding. The polar and non‐polar solvents showed different abilities to penetrate into polyurethane hard segments. Solvent treatment after moisture curing increased the tensile strength of these polyurethanes and polyurethane‐ureas with respect to control samples. The stress–strain behaviour of solvent‐treated polyurethane follows the constrained junction model. The change in hard segment crystallinity on solvent treatment has been explained by wide‐angle X‐ray diffraction study. The better orientation in polybutadiene soft segments evidenced from SEM (scanning electron microscopy) pictures is believed to be the main reason behind the improved tensile properties of solvent‐treated polyurethane samples. The effect of solvent treatment, as well as stretching, on the diffusion coefficient of hexane in polyurethanes was investigated. Copyright © 2003 Society of Chemical Industry     

Effects of thermodynamic models on the predictions of free volume diffusion theory for concentrated polymer solutions

Three thermodynamic models were used to demonstrate the effects of model choice on solvent–polymer binary diffusion coefficients predicted by free volume theory. Poly(vinyl acetate) and four solvents were used as typical solutions for these calculations. Thermodynamic models affect the predictions the most at high solvent weight fractions and for solutions which exhibit positive enthalpic interactions. For solutions dilute in solvent where Henry's law might describe phase equilibria, diffusion coefficients can be estimated without reference to thermodynamic data.     

Solute diffusion through swollen polymer membranes

Experiments were designed to study the role of the solvent in the transport of a solute through a solvent-swollen polymer membrane. A single solute (an organic dye), a single polymer (cross-linked natural rubber), and 24 different organic solvents were used for this purpose. The solute diffusion coefficient D was calculated from the measured permeability P and distribution coefficient K, and was compared to the diffusion coefficient of the solute in the pure solvent. The main parameters of the solvent were shown to be its viscosity and the degree it swells the polymer. At high swelling, the results are in agreement with a model that pictures the resistance to solute diffusion as hydrodynamic interaction with the solvent while the polymer acts as an obstruction that increases the tortuosity of the diffusion path. At very low swelling, the diffusion coefficient approaches an asymptotic limit which is independent of solvent viscosity. However, even with as low as 10% solvent, some effects of viscosity are still seen. These results are discussed in terms of a quantitative theory for the obstruction effect proposed by Meares and compared to other literature data.     

Multi-Zone Drying Schemes for Lowering the Residual Solvent Content during Multi-Component Drying of Semicrystalline Polymers

The development of a glassy skin in multicomponent semicrystalline polymer systems limits the diffusion of solvents out of the system and increases residual solvent levels. Based on the results of a mathematical model that we had previously developed, we have proposed a multi-zone drying scheme aimed at lowering the residual solvent levels by taking into account the effect of interactions between the various solvents as predicted by the model. This article focuses on the application of this model to develop optimal drying schemes and to verify the effectiveness of these predictions using experimental techniques. The mathematical model developed previously to study the diffusion of multiple solvents and changes in the crystallinity of semicrystalline polymer systems during drying incorporates many features including Vrentas-Duda diffusion theory, solvent-induced crystallization kinetics, as well as glass transition effects and skinning of the film. The multi-zone drying system was developed by varying the drying temperature in each zone as well as changing the partial pressure of individual solvents during the drying process. The effectiveness of the multi-zone drying schemes predicted by the model was validated experimentally using thermogravimetric methods. The polymer-solvent system chosen was a poly(vinyl alcohol)-water-methanol system. Our experimental data suggested that the multi-zone drying schemes were superior to a single-zone drying system through direct comparison. Further examination of the mathematical model yielded individual solvent profiles and these data reaffirmed our conclusions that a multi-zone drying scheme has the ability to reduce the effect of solvent trapping and thus lower the overall residual solvent content.     

Multi-Zone Drying Schemes for Lowering the Residual Solvent Content during Multi-Component Drying of Semicrystalline Polymers

The development of a glassy skin in multicomponent semicrystalline polymer systems limits the diffusion of solvents out of the system and increases residual solvent levels. Based on the results of a mathematical model that we had previously developed, we have proposed a multi-zone drying scheme aimed at lowering the residual solvent levels by taking into account the effect of interactions between the various solvents as predicted by the model. This article focuses on the application of this model to develop optimal drying schemes and to verify the effectiveness of these predictions using experimental techniques. The mathematical model developed previously to study the diffusion of multiple solvents and changes in the crystallinity of semicrystalline polymer systems during drying incorporates many features including Vrentas-Duda diffusion theory, solvent-induced crystallization kinetics, as well as glass transition effects and skinning of the film. The multi-zone drying system was developed by varying the drying temperature in each zone as well as changing the partial pressure of individual solvents during the drying process. The effectiveness of the multi-zone drying schemes predicted by the model was validated experimentally using thermogravimetric methods. The polymer-solvent system chosen was a poly(vinyl alcohol)-water-methanol system. Our experimental data suggested that the multi-zone drying schemes were superior to a single-zone drying system through direct comparison. Further examination of the mathematical model yielded individual solvent profiles and these data reaffirmed our conclusions that a multi-zone drying scheme has the ability to reduce the effect of solvent trapping and thus lower the overall residual solvent content.     

CPCs在应力场下的性能演变及机理研究进展

综述了导电高分子复合材料(CPCs)在压缩应力场、拉伸应力场及应力-应变循环力场中电阻率随应力、应变、循环次数及时间等的变化规律,分析了CPCs在上述应力场中电阻率变化与其微观结构演变之间的关系,结合导电粒子在高分子基体中的位置改变及导电粒子与高分子基体间相互作用,归纳了CPCs电性能在上述外力场中的演变机制。针对目前该复合材料在上述循环应力场中出现的磁滞效应和可重复性差等问题,归纳了相应的改善方法。简要概括了CPCs的工业应用。     

Electrospinning of polylactide and its composites with carbon nanotubes

Electrospinning of the biodegradable polylactide (PLA) and its composites containing carbon nanotubes (CNTs) was studied in terms of solution concentrations and solvents effects as well as CNT loadings. The results reveal that the PLA fibers obtained from the solutions using the mixed solvents of chloroform/assistant solvent (v/v 3/1) show better morphologies than those from the solutions using chloroform as the single solvent. This is due to the synergistic effect by the improved conductivity and altered viscosity with addition of assistant solvent. Moreover, the surface structure of fibers depends on the volatility of assistant solvents strongly. Using volatile acrylonitrile or acetone as the assistant solvents, the columned fibers with porous surface structure are obtained; while the flat fibers with fluted surface are formed using nonvolatile dimethyl sulfoxide as the assistant solvents. As for electrospinning of the PLA/CNT composites, the morphology of obtained fibers is closely related to the dispersion of CNTs in the fibers. At low loading levels, the CNTs can be well embedded in the PLA matrix and oriented along the fiber axis, forming nanowire structure. At high loading levels, the CNTs are mainly dispersed as entangled bundles along the fiber axis, and as a result, the obtained fibers show tortuous or misshaped morphologies. Compared with that of the neat PLA fibers, the overall morphologies of the composite fibers are more or less degraded because the presence of some small CNT aggregates in the solutions easily leads to the formation of beaded fiber structure during electrospinning. The conductivity of the obtained composite fiber mats was further studied in terms of CNT loadings. POLYM. COMPOS., © 2011 Society of Plastics Engineers.     

Prediction of diffusion in a ternary solvent–solvent–polymer blend by means of binary diffusion data: Comparison of experimental data and simulative results

Multicomponent diffusion of solvents in polymeric systems is not completely understood, despite many scientific contributions to the topic. Literature scarcely offers measurement data on diffusion for model validation in such systems. In this work, the ternary systems consisting of poly(vinyl acetate) and the solvents toluene and methanol was investigated experimentally and numerically. By means of inverse micro Raman spectroscopy (IMRS) concentration gradients in drying thin films have been measured. Initial composition of the samples has been varied systematically in order to detect mutual influence of the solvents' diffusive behavior. It was shown that the mobility of the different species is increased in the presence of other solvents as predicted by theory. This experimental data is provided for model validation. A new expression to calculate the diffusion coefficients in ternary mixtures is proposed which only requires binary data. This expression is tested by means of a model‐based simulation to predict the drying of ternary polymer solutions in terms of concentration profiles and residual solvent content. The results are in very good agreement with the experiments. Cross terms diffusion coefficients and thermodynamic factors were not found to be necessary for a satisfying prediction. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43899.     

反相气相色谱法测定小分子溶剂在聚乙烯粒子中的无限稀释扩散系数

基于经典线性非平衡色谱过程的矩分析理论，用反相气相色谱法测定了323.15～358.15 K温度范围内二氯甲烷、三氯甲烷、四氯甲烷3种不同相对分子量的同系物小分子及正己烷在聚乙烯粒子中的无限稀释扩散系数。采用聚合物粒子直接填充的色谱柱，考察了温度、同系物小分子分子量及聚乙烯结晶度对扩散系数的影响。实验结果表明，对同一种小分子溶剂/聚乙烯体系，扩散系数均随温度升高而增大。不同相对分子量的同系物小分子在同一种聚乙烯中扩散系数随分子量增加而减小，聚乙烯结晶度增加也会导致扩散系数减小。采用文献中所报道的Krevelen扩散系数预测模型的计算值与实验测量值较为吻合，表明本文所采用的以聚合物粒子直接填充色谱柱的反相气相色谱扩散分析具有一定的可靠性。     

Carbon nanotubes‐polymer nanocomposites for controlled heating materials

Nanocomposites of modified carbon nanotubes (CNT) and either polyurethane (PU), poly(vinyl acetate) (PVAc), or silicone materials were synthetized and characterized for thermal mat application. The obtained results revealed that the polymer used as a matrix had an impact on the electrical resistance of the mats. The lowest results of 32 Ω of resistance was registered with silicone‐based mats containing 5 wt % of CNT. For the same CNT content the mats based on PVAc and PU displayed values of 55 and 60 Ω, respectively. The low resistance properties of silicone‐based materials were due principally to the good compatibility of both polymer and functionalized CNT. Because of the low resistance values, this mat was subjected to thermovision analysis revealing that the samples reached temperature of about 60 °C in 9 min and 70 °C after 27 min of 27 V of applied potential. The results showed an almost uniform temperature distribution in the samples’ surface with some high and low temperature spots, which were attributed to nonuniform distribution of CNT in the polymer matrix. In summary, all the obtained results confirm that silicone‐CNT are very promising materials that can be used as low‐voltage heating mats. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 44194.     

A diffusion coefficient model for polymer devolatilization

Polymer devolatilizers are in widespread use in the polymer industry for removing solvents and monomers from polymer melts prior to product fabrication. Design equations for describing the solvent flux usually include both the diffusion coefficient of the solvent in the polymer melt and the equilibrium concentration of the solvent at the polymer-vapor interface. Several models make the as sumption that the solvent diffusivity is constant over the ranges of solvent concentrations and temperatures in the devolatilizer. This is a critical assumption that may be difficult to check without obtaining diffusivity data at the operating temperatures and concentrations of the process equipment. There are three models that can be used for diffusion coefficients in devolatilizer design: the free volume model developed by Duda, Vrentas, and coworkers; a new linear model proposed in this study; and a constant diffusivity model, The linear model is obtained by combining a new correlation for solvent activity coefficients in molten polymers with free volume theory and linearizing the resulting equation. The error between using the complete free volume theory and using the linear model, or alternatively, using a constant diffusion coefficient, is calculated for several solvent-polymer systems. The linear model is convenient to use for determining the effects of the solvent activity coefficient on the diffusion coefficient. A method is presented for determining whether the complete model, the linear model, or the constant diffusivity model is appropriate for a given devolatilizer design.     

Estimation of solvent diffusion coefficient in amorphous polymers using the Sanchez-Lacombe equation-of-state

A modified free-volume model was proposed to predict the solvent diffusion coefficient in rubbery polymers without knowledge of any diffusion data. With the introduction of the Sanchez-Lacombe (SL) equation-of-state (EOS) into the Vrentas-Duda model, this model is an attempt to bridge the gap between the thermodynamic and transport properties of polymer solutions. The free volume provided by polymers for solvent diffusion can be estimated solely using the parameters of the SL EOS characteristics and the polymer glass transition temperature; thus the proposed model avoids the need to use polymer viscoelastic data in determination of polymer free-volume parameters. The other parameters in the Vrentas-Duda model remain applicable. Calculated results of solvent self- and mutual-diffusion coefficients of four common solvents in two polymers indicated that the modified model can give reliable predictions. In addition, it can reflect the effect of pressure on solvent diffusivity for concentrated polymer solutions.     

Three-dimensional solubility parameters and chemical protective clothing permeation. II. Modeling diffusion coefficients,breakthrough times,and steady-state permeation rates of organic solvents in Viton® gloves

Three-dimensional (3-D) solubility parameters are used in separate models of the solubility, S, and diffusion coefficient, D, of organic solvents in polymers. Modeled values of these variables are then combined in Fickian diffusion equations to estimate solvent breakthrough times (BT) and steady-state permeation rates (SSPR). Published data on the permeation of 18 solvents through commercial Viton® glove samples are used to test the accuracy of the approach. Estimates of S are determined based on the model described in the preceding article. Of several empirical correlations investigated to model D, best results are achieved using the product of the solvent molar volume, V₁, and either the weighted solvent-Viton 3-D solubility parameter difference, A_w, or the Flory interaction parameter, X, also calculated from 3-D solubility parameters. To account for the change in the value of D over the course of the permeation test, D values are evaluated at breakthrough and steady state. Modeled BT values within a factor of three of experimental values (typically within a factor of two) are obtained for the 15 solvents for which analytical detection limits were reported. Modeled SSPR values within a factor of six of experimental values (typically within a factor of four) are obtained for the 15 solvents with valid SSPR measurements. © 1993 John Wiley & Sons, Inc.     

Growth model for plasma-CVD growth of carbon nano-tubes on Ni-sheets

The plasma enhanced chemical vapor deposition (PECVD) of carbon nano-tubes (CNT) on nickel sheets is considered as efficient production method of great technologically interest. Different morphologies of CNT on Ni-sheets can be achieved by a variation of the process parameters, like partial pressure of the acetylene and ammoniac inlet gas mixture, the total gas pressure, and temperature. The results are summarized in a detailed growth model based on thermodynamic considerations. The conclusion from this model is that the production of single-wall nano-tubes (SWCNT), which is more desirable than multi wall variants (MWCNT) or graphite nano-fibers (GNF), requires the fastest reaction velocity for CNT formation. The PECVD supports the CNT formation, but for enhancement of the reaction velocity additional inhibitors are required.