Nitrogen Corona Activation of Polyethylene

Experiment has shown that the nitrogen corona-induced autohesion of polyethylene and the nitrogen-corona induced sorption of iodine by polyethylene both follow similar mechanisms. The controlling factor is postulated to be the formation of short-lived electrets within the polymer surface.     

Polyparaxylylene electrets usable at high temperatures

Thin coatings of various polyparaxylylenes were transformed into electrets by corona charging them and then overcoating with more polyparaxylylene to protect the charge layer. For such electrets, the extrapolated ten-year use temperature depended on the thermo-oxidative stability of the polyparaxylylene layer and was greater than 100°C for the most stable electrets. The voltage levels and surface charge densities were always greater than or equal to those of charged Teflon electrets, similar to those used in commercial devices.     

Outstanding performance of parylene polymers as electrets for energy harvesting and high-temperature applications

Parylene C is used in many applications due to its high properties but it remains a material with moderate performance as long as it is intended for use as an electret. Hence, the generally accepted idea, rightly so, in the scientific and industrial community not to necessarily select parylene (i.e., parylene C) for applications where the endurance of the electret is a strong criterion. Our study provided a new perspective on the performance of parylenes as electret. In this case, we will talk about fluorinated Parylenes of the VT-4 type and especially AF-4 variant. Their thermal stability is outstanding and a charge stability is almost total up to 100 °C. A 50% reduction in the charge is recorded at a temperature as high as of 220 °C (9 μm thick Parylene AF-4), making it one of the most efficient polymer electrets to date. Negatively and positively charged Parylene AF-4 electrets presented similar performance over long durations, which is out of ordinary for the commonly employed polymeric electrets. Finally, these fluorinated polymers are therefore particularly promising new candidates for applications in electret-based converters for energy harvesting. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48790.     

Electret characteristics of melt-blown polylactic acid fabrics for air filtration application

Polylactic acid (PLA) is a biodegradable and biocompatible polymer. Melt-blown PLA electret fabrics have become attractive materials for air filter applications. Its filtration performance mainly depends on the electrostatic effect originating from electrets. How to improve their electret behavior and then to enhance their filtration efficiency has always been challenging. In this article, three melt-blown PLA fabrics with different morphological or crystal structure are made from different process conditions or ingredient. The electrets are formed by means of corona charging technology. Their crystalline and morphological structure was analyzed, and filtration performance was measured. Their charge-trapping feature was studied by thermal stimulating discharge technology. The results proved that the air filtration efficiency of three melt-blown samples improved a lot when electrets were formed. Different crystalline structures of PLA fabrics will result in differences in their electret charge-trapping feature. The PLA fabrics with semi-crystalline characteristics display a regular charge-trapped level distribution and amorphous structure hints the diversity of charge-trapped levels. The deference in filtration resistance is mainly ascribed to the different fiber diameters and their distributions. More narrow distribution of fiber diameter leads to more compact fabrics and to higher filtration resistance. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 48309.     

Desalination by electret technology

A purification process of making use of electret technology for efficient but cost effective deionization was discussed in this paper. Douglas MacGregor claimed that PTFE electret works as a means of ions separation for desalination due to the polarization of an electrolyte and ions migration under a permanent electrostatic field generated by a pair of electrets with opposite polarities. The electrostatic field attracts counter-ions to the charged solid surface, and thus generates a purified effluent in the mainstream. Theoretical and experimental analyses were made to investigate the practicality of desalination by electret technology. It is found that MacGregor's design is meaningful only for a dilute electrolyte or a system with very high surface charge density because the number of the counter-ions attracted to the surface is limited and cannot exceed that of the surface charges at the electret.     

Influence of TiO2 nanofiller geometry on the rheological and optical properties of poly(acrylic acid)-based nanocomposite hydrogels

The properties of macroscopic polymer nanocomposites are highly dependent on the nanoparticle–polymer interfacial region, which varies with the morphology of the nanoparticles. Herein, we used TiO₂ nanofillers with very low aspect ratio, the same chemical composition, and surface functions, but different geometries (nanospheres and nanoplates) dispersed in a poly(acrylic acid) matrix to analyze the influence of the nanoparticle geometry on the properties of nanocomposite hydrogels. The geometry was found to affect the swelling and rheological properties of the nanocomposite polymers. Particularly, the yield strain (from 25% to 130%), modulus (from 17,500 to 25,000 Pa), and brittleness of nanoplates based nanocomposite increased more significantly. Finite-difference time-domain simulations demonstrated that nanoplates increased the wavelength of the absorption maxima (224 nm for NS to 240 nm for NP) and charge distribution. The results obtained in this research indicate that the nanofiller shape markedly influences the rheological properties of the nanocomposite polymers, opening the door to further research focused on polymer–nanofiller interactions, and their effect on the macroscopic properties of the nanocomposites.     

Studies of polymer electrets. III. Charge decay behavior in polar polymer homoelectrets

Charge stabilities of various polar polymer homoelectrets were determined. Although these electerts are reasonably stable in dry environments, they rapidly discharge when exposed to humidity. The rate of charge decay was found to depend directly on the ability of these polymers to absorb water under equilibrium conditions. Protection from humidity is obtained if these polar polymers are coated on both sides with nonpolar polymers. If, however, two different polymer films are laminated, the electret behavior follows a pattern that can be explained on the basis of charging at the interface. Difference in stabilities of the electrets of polar and nonpolar polymers is attributed to the differing natures of the charge traps present in these two classes of polymers.     

Piezoelectric and Electrostatic Properties of Electrospun PVDF‐TrFE Nanofibers and their Role in Electromechanical Transduction in Nanogenerators and Strain Sensors

Piezo‐ and ferroelectric nanofibers of the polymer poly(vinylidenefluoride) (PVDF) have been widely employed in strain and pressure sensors as well as nanogenerators for energy harvesting. Despite this interest, the mechanism of electromechanical transduction is under debate and a deeper knowledge about relevant piezoelectric or electrostatic properties of nanofibers is crucial to optimize transduction efficiency. Here poly(vinylidenefluoride‐trifluoroethylene) nanofibers at different electrospinning conditions are prepared. Macroscopic electromechanical response of fiber mats with microscopic analysis of single nanofibers performed by piezoelectric and electrostatic force microscopy are compared. The results show that electrospinning favors the formation of the piezoelectric β‐phase in the polymer and leads directly to piezoelectric properties that are comparable to annealed thin films. However, during electrospinning the electric field is not strong enough to induce direct ferroelectric domain polarization. Instead accumulation of triboelectric surface charges and trapped space charge is observed in the polymer that explain the electret like macroscopic electromechanical response.     

Multiscale modeling for polymer systems of industrial interest

Atomistic-based simulations such as molecular mechanics (MM), molecular dynamics (MD), and Monte Carlo-based methods (MC) have come into wide use for materials design. Using these atomistic simulation tools, one can analyze molecular structure on the scale of 0.1–10 nm. Although molecular structures can be studied easily and extensively by these atom-based simulations, it is less realistic to predict structures defined on the scale of 100–1000 nm with these methods. For the morphology on these scales, mesoscopic modeling techniques such as the dynamic mean field density functional theory (Mesodyn) and dissipative particle dynamics (DPD) are now available as effective simulation tools. Furthermore, it is possible to transfer the simulated mesoscopic structure to finite element modeling tools (FEM) for calculating macroscopic properties for a given system of interest. In this paper, we present a hierarchical procedure for bridging the gap between atomistic and macroscopic modeling passing through mesoscopic simulations. In particular, we will discuss the concept of multiscale modeling, and present examples of applications of multiscale procedures to polymer–organoclay nanocomposites. Examples of application of multiscale modeling to immiscible polymer blends and polymer–carbon nanotubes systems will also be presented.     

基于微观孔隙结构的聚驱方案调整方法研究

以往国内外对于微观孔隙结构的研究,主要集中于不同储层微观孔隙结构的表征以及探讨微观孔隙结构与宏观性质的关系这两个方面。大庆油田开展聚驱以来,发现油层发育相似的井聚驱效果存在一定的差异。因此,应用微观孔隙特性,采用油田聚驱方案的调整方法,取得了良好的效果。     

Application of the Kelvin Probe method for screening the interfacial reactivity of conducting polymer based coatings for corrosion protection

In our recent studies we could show that intrinsically conducting polymers definitely possess promising potential for application in intelligent corrosion protection coatings. One prerequisite for this was shown to be that macroscopic networks of the conducting polymers have to be avoided in the coating in order to avoid predominant and disastrously fast cation incorporation during the corrosion induced reduction of the polymer. Only then anions serving as inhibitors and safely stored in the conducting polymer will be efficiently released during a corrosive attack. This mechanism is more or less independent of the metal that has to be protected, i.e. it is a property of the composite coating derived from dispersing micro-clusters of conducting polymer in a non-conducting matrix and unspecific for the metal onto which it is to be applied. In this paper we focus on specific electrochemical reactions at the interface between the conducting polymer and the metal that were found to define further criteria for successful application. The aim should be that the conducting polymer is in electronic contact with the passive metal surface. However, the formation of an insulating interface, i.e. loss of electronic contact and hence functional inactivity of the conducting polymer, and enhanced corrosion are also possible. We will show how by application of the Kelvin Probe method a fast and easy screening between these three cases can be achieved.     

Computer modeling of polymer crystallization - Toward computer-assisted materials' design

Crystalline polymers are very interesting and useful materials with great versatility through their potential morphology control. Recent surge in computer modeling studies has its origin both in increasing need for efficient methods of materials' design and in tremendous developments in computer power that is expected to meet the need. In this paper, we briefly survey the present state of computer modeling of polymer crystallization with the aim to foresee future developments. We first review simulations of crystallization in simple polymers under quiescent conditions where most of the efforts have hitherto been devoted. We also examine recent studies on crystallization under flow or large deformation. Then we present our ambitious plans to extend the simulation methods to polymers having complex chemical structures, though it is still an uncultivated field of research. We also refer to the new modeling strategies which integrate macroscopic and microscopic methods, and to the possibilities of molecular modeling in polymer nanotechnologies. Though our goal seems very far, there are obviously very fertile lands for the computer simulation studies.     

CHEMICALLY INDUCED MIGRATION OF PARTICLES ACROSS FLUID STREAMLINES

The velocity of a colloidal particle that moves because of a gradient of concentration of a molecular solute depends on the concentration field at the surface of the particle. Effects of macroscopic convection of the suspending fluid on two such transport phenomena, capillary-driven movement of fluid particles and diffusiophoresis of rigid particles, are considered here. In the case of fluid particles our results also apply to motion caused by a temperature gradient. If the particles are in a laminar flow with the solute gradient directed perpendicular to the direction of flow, as might arise in the boundary layer near a surface to which the particles are being deposited, the local solute concentration field around each particle is disturbed from that of pure diffusion of the solute. Using published results for these concentration disturbances in a simple-shear flow, we determine the effect of the imposed velocity gradient on the speed of the particles in the direction of the solute gradient. For both fluid and rigid particles, the correction due to macroscopic shear is 0(Pe^3/2:) where Pe is the Peclet number based on particle radius and fluid shear rate; this effect opposes the zero-shear particle velocity. A possible consequence of this result is that by increasing the shear rate in a laminar boundary layer in the hope of enhancing the rate of particle adsorption, one may actually be decreasing the rate.     

Metal/polymer interfaces with designed morphologies

The morphology of a metal/polymer interface is important for many properties, e.g. its adhesional strength. Starting from the basic processes occurring in the initial stages of metal/polymer interface formation, it is possible to obtain different morphologies by variation of the preparation conditions. In this report we present selected examples from our own work of how metal/polymer interfaces with different morphologies can be prepared by evaporating noble metals (Au, Ag, Cu) onto chemically different polymers, i.e. bisphenol-trimethyl cyclohexane polycarbonate (TMC-PC), pyromellitic dianhydride-oxydianiline (PMDA-ODA) polyimide (PI), and on Teflon AF 1601. The interfaces were characterized using transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and atomic force microscopy (AFM). The combination of these techniques allows one to determine morphological parameters such as the concentration and distribution of metal clusters at the surface and in the near-surface region. Using low deposition rates and elevated temperatures, spread-out metal/polymer interfaces can be formed, whereas the use of high deposition rates and moderate temperatures results in relatively sharp interfaces. Another approach to obtain a defined morphology is to form large metal clusters of 10-30 nm diameter on the polymer surface and embed them into the polymer in a controlled manner by a subsequent annealing process. First experiments on the macroscopic adhesion of Au and Cu on TMC-PC showed that the initially low peel strength could be increased substantially by subsequent annealing above the glass transition temperature.     

Heterogeneous photooxidation of solid polymers

Photooxidation of polymer can undergo heterogeneous effects, a complication which impacts attempts to extrapolate artificial accelerated tests to natural outdoor exposures. The main cause of heterogeneous degradation results from oxygen diffusion-limited effects. Several experimental techniques can be used to monitor macroscopic heterogeneous oxidation in polymers. These techniques are based on i.r. spectroscopy: micro FTi. r. spectroscopy permits to determine oxidation profiles in polymer materials till a resolution around 10 microns and photoacoustic (PAS) -FTi. r. spectroscopy is an interesting method for analyzing the oxidation of the surface layers.     

Beyond classical theory: Predicting the free energy barrier of bubble nucleation in polymer foaming

Based on the Gibbs‐Tolman‐Koenig formalism, we considered the Tolman correction to the free energy barrier of bubble nucleation in polymer‐gas binary mixtures. For this class of systems, the correction may be estimated with a reasonable accuracy using experimentally accessible macroscopic thermodynamic quantities only. Although the Tolman correction is applicable only in the low supersaturation regime, a simple ansatz regarding the supersaturation dependence of the Tolman length can be made to extend the usefulness of this approach and to yield the free energy barrier that vanishes at the mean‐field spinodal as demanded by thermodynamic considerations. © 2013 American Institute of Chemical Engineers AIChE J, 59: 3042–3053, 2013     

Effect of temperature gradient on polymer migration

Polymer migration in a flow field is a well-known phenomenon, and it has been verified that the sources of such phenomena are nonhomogeneity of the flow field, concentration effects, and hydrodynamic interactions between the polymer molecules and walls. In this study, we investigate the temperature gradient effect on polymer dynamics due to an imposed flow field. The diffusion equation and concentration profile of the polymer molecules induced by a temperature gradient using a two-bead dumbbell model are obtained from the Fokker-Planck equation. From the concentration equation constructed for a dilute solution in a general flow geometry, we find that there are significant effects on the polymer migration not only due to the nonhomogeneity of the flow field, but also due to temperature gradients.     

Thermomechanical analysis and modeling of the extrusion coating process

During the extrusion coating process, a polymer film is extruded through a flat die, stretched in air, and then coated on a substrate (steel sheet in our case) in a laminator consisting of a chill roll and a flexible pressure roll. The nip, i.e. the area formed by the contact between the pressure and the chill rolls, constitutes the heart of the extrusion coating process. Indeed, in this region, some of the most critical properties, such as adhesion, barrier properties, optical properties, are achieved or lost. In this article, we first present an experimental investigation of the coating step, which enables to characterize the leading thermomechanical phenomena. It is shown that there is no polymer macroscopic flow in the nip, but a local flow within the asperities of the steel substrate surface. This microscopic flow, at the interface between the film and the substrate, is slowed by strong cooling conditions in the nip. Several models are then proposed, giving access to the temperature profile through polymer thickness and substrate, the pressure distribution in the nip as well as the behavior of the polymer melt in the nip at the interface with the substrate. POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers     

Measurement of charge distribution in polymer electrets by a new pressure-pulse method

Summary The charge distribution in the thickness direction of 10 to 100 m thick polymer electrets can be determined with a new method utilizing a <1 ns laser pulse to launch a pressure pulse in the sample. Propagation of the pressure pulse through the film causes electrode currents which yield the charge distribution. The method has been applied to electron-beam charged PETP and FEP samples.     

自由基聚合反应器中多尺度流场的模拟进展

自由基聚合过程中,由于混合、传递及聚合反应的相互作用使得反应器内部存在复杂的多尺度流场,例如宏观尺度的速度、浓度、温度分布,介观尺度的液滴粒径分布,微观尺度的聚合反应速率、聚合物分子量和多分散性指数分布。这些复杂的多尺度流场分布使得聚合反应器的模型化研究成为难题。本文较为系统地介绍了自由基聚合反应器中存在的多尺度现象;简述了微观尺度聚合物性质流场分布的模型化与模拟研究方法;从悬浮聚合和乳液聚合两个方面介绍了介观尺度液滴粒径分布的模拟研究进展;从非理想混合的角度阐述了宏观尺度多相流流场分布的研究进展。最后,本文分析了多尺度模型的耦合求解方法。本综述也有本文作者对这个领域的初步观点,可为聚合反应器的设计、优化和放大提供参考。