Simultaneous effects of polymer concentration,jet-stretching,and hot-drawing on microstructural development of wet-spun poly(acrylonitrile) fibers

Investigation of structural development of acrylic fibers during the early stages of the wet-spinning process has great importance both in carbon fiber and textile industries. The simultaneous effects of increasing polymer concentration, jet-stretching and hot-drawing on porosity, morphology, and mechanical properties of wet-spun poly(acrylonitrile) fibers were studied. The detailed microstructure of the voids was characterized by electron microscopy, porosimetry, and thermoporometry. The effects of jet-stretching/hot-drawing on the overall porosity of the fibers were negligible below a threshold polymer concentration. Increasing polymer concentration from 10 to 20 vol.% reduced the total porosity. Hot-drawing was more effective in reducing the overall porosity of the fibers in comparison with jet-stretching. Stretching and drawing replaced the macrovoids by dense ligaments but did not change the volume fraction of nanovoids, however, shifted nanovoids size distribution toward smaller values. In general, Young’s modulus and elongation at break increased by decreasing overall porosity, however, they depended also on the distribution of voids size and chain orientation along the fiber axis. Strength–diameter correlation showed a good agreement with the Griffith’s theory.     

基于显微CT技术的碳纤维复合材料体孔隙率测量

提出了一种基于显微CT技术的碳纤维复合材料体孔隙率测量的新方法,分析了采用显微CT技术测量孔隙率的实验原理,对实验结果进行了图像处理,并统计体孔隙率。实验结果表明,显微CT技术是一种行之有效的碳纤维复合材料体孔隙率测量技术,通过图像灰度进行阈值分割可以清晰地分辨材料内部基体与孔隙,且测量过程中应选择足够大的试样体积,测量值才能真实反映材料内部的体孔隙率。     

Study of Porosity in Corroded Refractories

In this work, computer image analysis was used in order to characterize the complex porosity found in ceramic refractory materials. The porosities of SiC and Al₂O₃ refractory bricks before and after slagging tests have been examined by both mercury porosimetry and image analysis techniques; the results obtained by these two methods were compared and their differences discussed. Mercury porosimetry underestimated the volume fraction of pores larger than 100 μm, which contributed significantly to the total pore volume of analyzed materials. In contrast, image analysis did not reveal finer porosity, the contribution of which was, however, less important, Image analysis appeared to have the advantage of quantification of local heterogeneities of porosity, and the capabilities of this technique to quantify the variations in the volume of pores adjacent to reaction zones are documented and supported by microscopic observations.     

粒径可控的小晶粒Y型沸石的合成

采用乳液聚合法制备单分散的聚合物(聚苯乙烯)微球,将Y沸石晶种溶液填充到聚合物微球的间隙中,成功合成粒径和形貌可控的小晶粒Y沸石.采用DLS表征聚合物的粒径,XRD、N2吸附和SEM表征结果表明,合成的样品为纳米到亚微米级Y沸石.主要原因:(1)聚合物之间的间隙可限制晶体的生长;(2)Y沸石晶种溶液的引入可以缩短诱导期...     

Activity and long-term stability of PEDOT as Pt catalyst support for the DMFC anode

The feasibility of using poly(3,4-ethylenedioxythiophene) (PEDOT) as Pt catalyst support for direct methanol fuel cell (DMFC) anodes was investigated. Measurements with freshly prepared Pt-PEDOT/C electrodes showed poor activity for methanol oxidation in a half-cell and a DMFC. A substantial enhancement in that activity was evident after either electrochemical over-oxidation of PEDOT or long-time storage of the Pt-PEDOT/C gas diffusion electrode (GDE) in air. Both procedures led to a reorganization and increase in porosity of the reaction layer, which obviously contributed to better methanol accessibility to Pt catalyst active centres. The effects of electrochemical activation and long-time storage in air on the morphology and elementary composition of the Pt-PEDOT layer were investigated by means of Hg porosimetry and SEM/EDAX. It was found that the increase in porosity was due to degradation of PEDOT characterized by a significant depletion of sulphur and oxygen in the conducting polymer matrix.     

Use of mercury porosimetry for characterization of poly(vinyl chloride) suspension grades with regard to particle type,particle heterogeneity,and surface area

The correction for polymer and mercury compressibility is essential to the use of mercury porosimetry for the evaluation of the porosity of poly(vinyl chloride) (PVC) suspension granules. The polymer compressibility, however, varies with the morphology type of the granules. The technique outlined in this paper allows the appropriate correction to be made without the need for the prior determination of the polymer compressibility. Bulk-compressibility factors for the polymer samples are derived from the porosimetry data. Surface areas of PVC granules, as derived from porosimetry data using two fundamentally different techniques, are in error because of the presence of “ink-bottle” shaped pores.     

碳纤维复合材料孔隙率及其检测方法

本文分析了碳纤维复合材料孔隙形成的原因,指出孔隙的定量指标孔隙率及其两种定义方法即面积孔隙率和体积孔隙率,分别阐述了孔隙对碳纤维复合材料剪切性能和弯曲性能的影响程度,同时介绍了密度测量法、吸水测量法、显微照相法三种破坏性检测法和射线检测法、超声衰减法、导波法、超声声阻抗法、极角背反射法五种无损检测法,简述两类孔隙率的检测方法的基本原理,并对两类方法进行了比较分析.     

Mercury Porosimetry of Pharmaceutical Powders and Granules

Effect of scanning speed and moisture content of the sample on the result of mercury porosimetry analysis of mannitol and microcrystalline cellulose (Emcocel® 50M) powders and granules produced by wet granulation were studied. In high-pressure porosimetry analysis, the smallest pores (diameter <20 nm) of the powders and granules could not be detected accurately when high scanning speeds were used. If the total pore volume is the only parameter of interest, fast scanning speed can be used, because the scanning speed does not affect this value. In high-pressure porosimetry analysis, the volume of the smallest pores (diameter <40 nm) of mannitol and microcrystalline cellulose granules increased with increasing water content. For powders, no effect of moisture on the volume of the smallest pores was observed. Thus, the increase in the volume of the smallest pores of granules with increasing moisture is related to the structure of the granules. Measurement of the water content of the samples together with proper drying of the samples before the porosity measurement is extremely important.     

Porosity formation by biaxial stretching in polyolefin films filled with calcium carbonate particles

Porous polymer films were generated by biaxial stretching of polypropylene (PP) and high‐density polyethylene (HDPE) filled with various amounts of calcium carbonate particles. The porosity of the films was measured by mercury porosimetry, and the obtained results were related to the processing conditions and to the morphology development during the biaxial stretching. The results showed that increasing the calcium carbonate (CaCO₃) concentration and the draw ratio resulted in porosity increase for PP‐based composite films and in a decreased porosity in HDPE‐based composite films. Such peculiar behavior was connected to interfacial specific interactions between the matrix and the dispersed particles as well as to the crystallinity of the films. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Characterization of multi-scale porosity in cement paste by advanced ultrasonic techniques

The effectiveness of advanced ultrasonic techniques to quantitatively characterize the capillary porosity and entrained air content in hardened cement paste is examined. Direct measurements of ultrasonic attenuation are used to measure the volume fraction and average size of entrained air voids and to assess variations in intrinsic porosity - as influenced by water-to-cement ratio (w/c) - in hardened cement paste samples. For the air entrained specimens, an inversion procedure based on a theoretical attenuation model is used to predict the average size and volume fraction of entrained air voids in each specimen, producing results in very good agreement with results obtained by standard petrographic methods and by gravimetric analysis. In addition, ultrasonic attenuation measurements are related to w/c to quantify the relationship between increasing porosity (with increasing w/c) and ultrasonic wave characteristics.     

Internal structure of porous glass coated with a polymer layer of poly (2,3-epoxypropyl methacrylate)

A change in the internal structure of porous glass due to the formation of a polymer layer of poly(2,3-epoxypropyl methacrylate) by polymerization on its surface was investigated. By using the methods of dynamic desorption of nitrogen and mercury porosimetry, the character of the polymer layer and its effect on the final porosity for various types of the original material were determined.     

Optimizing the nanofibrous morphology of electrospun poly[(butylene succinate)‐ co‐(butylene adipate)]/clay nanocomposites and revealing the effect of the fibre nano‐dimension on the attained material properties

Electrospun mats of biodegradable polymer/clay nanocomposites were prepared in order to investigate the qualitative and quantitative correlations between electrospinning‐related parameters and the fibrous morphology of the mats. A full factorial design of experiments was used for that purpose and scanning electron microscopy was employed for dimensional characterization and determination of the chosen responses. Statistical analysis revealed that solution concentration and clay loading of nanocomposite were the most important parameters affecting the morphology of the fibrous webs. This, subsequently, allowed prediction of the domain of these two parameter settings where purely fibrous morphology can be achieved and further allowed optimization of the process in the framework of response surface methodology (RSM). The structure and thermal behaviour of the nanocomposites were also characterized before and after electrospinning using X‐ray diffraction, thermogravimetric analysis and differential scanning calorimetry. A multi‐analyte platform is provided by the combination of RSM results for improving the nanofibrous quality and post‐spinning characterization for predicting the overall performance of the electrospun web. Copyright © 2011 Society of Chemical Industry     

Preparation of interconnected poly(ε-caprolactone) porous scaffolds by a combination of polymer and salt particulate leaching

This paper examines a new technique for the preparation of porous scaffolds by combining selective polymer leaching in a co-continuous blend and salt particulate leaching. In the first step of this technique, a co-continuous blend of two biodegradable polymers, poly(ε-caprolactone) (PCL) and polyethylene oxide (PEO), and a certain amount of sodium chloride salt particles are melt blended using a twin screw extruder. Subsequently, extraction of the continuous PEO and mineral salts using water as a selective solvent yields a highly porous PCL scaffold with fully interconnected pores. Since, the salt particles and the co-continuous polymer blend morphology lead to very different pore sizes, a particular feature of this technique is the creation of a bimodal pore size distribution. Scanning electron microscopy, mercury intrusion porosimetry and laser diffraction particle size analysis were carried out to characterize the pore morphology. The prepared scaffolds have relatively homogeneous pore structure throughout the matrix and the porosity can be controlled between 75% and about 88% by altering the initial volume fraction of salt particles and to a lesser extent by changing the PCL/PEO composition ratio. Compared to the conventional salt leaching technique and to its different variants, the proposed process allows a better interconnection between the large pores left by the salt leaching and a fully interconnected porous structure resulting from the selective polymer leaching. The average compressive modulus of the different porous scaffolds was found to decrease from 5.2 MPa to about 1 MPa with increasing porosity, according to a power-law relationship. Since, the blending and molding of the scaffold (prior to leaching) can be made using conventional polymer processing equipment, this process seems very promising for a large scale production of porous scaffold of many sizes and in an economic way.     

Microstructural Characterization of Alumina and Silicon Carbide Slip-Cast Cakes

The effect of solids loading, particle-size distribution, and suspension viscosity on the resultant microstructure of slipcast monolithic ceramics prepared from aqueous suspensions of alumina and silicon carbide was studied. Unimodal alumina suspensions (average particle size = 0.6 μm) were prepared at 35, 37, and 42 vol%. Silicon carbide suspensions (average particle size = 0.7 μm) were produced with different quantities of dispersant at 37 vol%. Similarly, aqueous alumina suspensions of 42 and 50 vol% were produced with a bimodal particle-size distribution. The slip-cast microstructures were characterized by mercury porosimetry and small-angle neutron scattering, which provided pore size (distribution), pore fraction, and pore morphology. Essentially, the combination of these techniques deciphered packing differences obtained in the cake microstructures. For the alumina cakes produced from the 35,37, and 42 vol% suspensions, the individual characterization techniques, mercury intrusion, and the neutron scattering measurements showed that the cake microstructures were similar in pore size and quantity. However, comparison of the techniques and their assumptions showed differences in the pore shape. Mercury porosimetry and neutron scattering showed bimodal porosity for the cake produced from a mixture of 85% 6-μm particles and 15% 0.6-μm particles. Pore volume fraction and pore size increases were correlated with increased viscosity in the silicon carbide suspensions. In addition, the silicon carbide cake microstructures were measured, and homogeneity was evaluated as a function of position in the cast.     

Small-Angle Neutron Scattering from Porosity in Sintered Alumina

A new approach for studying porosity in ceramic or metallic materials has been developed using multiple neutron scattering in the small-angle regime. This technique enables the measurement of voids from 0.08 to 10 μm in size with 1% to 50% porosity. Whereas a fully dense alumina sample showed no evidence of multiple neutron-scattering effects, a more porous alumina exhibited striking neutron wavelength dependence. The voids in the latter material are 1.48 μm in mean diameter and 10.5% in volume fraction.     

The effect of porosity on elastic moduli of polymer foams

Constitutive equations are reported for the effect of porosity on the elastic moduli and longitudinal sound speed of polymer foams. These relations are grounded on the asymptotic homogenization method combined with the integration embedding scheme. Observations are analyzed on open-cell and closed-cell porous polymers manufactured by (1) foaming with chemical blowing agents, (2) foaming with inert gases, and (3) emulsion-templating foaming. Numerical analysis of experimental data shows that changes in the elastic moduli with porosity are correctly predicted by the governing relations for foams with spherical voids. Porous polydimethylsiloxane (PDMS) elastomers prepared by the emulsion-templating method provide an exception from this rule. The difference between these materials and the other cellular polymers is caused by production of hydrogen gas under curing, which induces severe deformation of pores. Simulation reveals that experimental data on cellular PDMS elastomers are described by the differential equations for foams with spheroidal voids whose aspect ratio depends on porosity. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48449.     

Microstructural comparison of porous oxide ceramics from the system Al2O3–ZrO2 prepared with starch as a pore-forming agent

In this paper we show examples of microstructures of porous oxide ceramics prepared by traditional slip casting (TSC) and starch consolidation casting (SCC) and present results obtained using different microstructural characterization techniques; Archimedes method (open and total porosity), shrinkage measurement, mercury intrusion porosimetry (pore size distribution) and microscopic methods – optical microscopy with microscopic image analysis (pore size distribution) and scanning electron microscopy (detailed investigation of the local microstructure). In particular, microstructures are compared for porous ceramics from the system Al₂O₃–ZrO₂ prepared with rice and corn starch. It is shown that maximum values of the total porosity of porous ceramics prepared with starch as a pore-forming agent were approx. 50%. A major finding by using SEM with respect to starch-produced porous ceramics is the existence of pore fillings in the form of small sintered ceramic shell inside the pores, as a result of starch granule shrinkage during the drying and burn-out steps.     

Microstructural aspects in a polymer-modified cement

Scanning electron microscopic observations of polymer-free and polymer-modified cements have shown that the polymer particles are partitioned between the inside of hydrates and the surface of anhydrous cement grains. Differential thermal analysis, thermogravimetric analysis, and porosimetry experiments show the retardant effect of the polymer and provide information on changes in porosity.     

A new approach for the characterization of the pore structure of dual porosity rocks

For the realistic representation of the pore space of dual porosity rocks, a new method of pore structure characterization is developed by combining experimental Hg intrusion/retraction curves with back-scattered scanning electron microscope (BSEM) images and inverse modeling algorithms. The pore space autocorrelation function measured by processing the digitized BSEM images is combined with the surface fractal dimension estimated from the high pressure Hg intrusion (MIP) data to derive a synthetic small-angle neutron scattering (SANS) intensity function, the inversion of which provides a volume-based pore body radius distribution (PBRD). The volume-based PBRD is fitted with a multimodal number-based PBRD consisting of two component distributions: one representing the macroporosity and another one representing the microporosity. Based on arguments of percolation theory, analytical mathematical models are developed to describe the Hg intrusion in and retraction from dual pore networks in terms of the complete PBRD, pore throat radius distribution (PTRD) of macroporosity, drainage accessibility functions (DAFs) of both porosities, and imbibition accessibility functions (IAFs) of both porosities. Inverse modeling of the Hg intrusion data set enables us to estimate the PTRD and DAFs. Inverse modeling of the Hg retraction datasets enables us to estimate a set of primary and secondary IAFs. The method is demonstrated by the pore structure characterization of four outcrop samples of carbonate and sandstone rocks. Analytic approximate equations developed from the critical path analysis (CPA) of percolation theory enable us to calculate explicitly the absolute permeability and the formation factor of the porous rocks using the estimated parameters (PBRD, PTRD, DAF) of the macroporosity. The measured permeability of cores is predicted satisfactorily and observed discrepancies may be attributed to large length-scale macro-heterogeneities which are not evident in BSEM images and Hg porosimetry data.     

Solubility parameter estimation and phase inversion modeling of bentonite-doped polymeric membrane systems

Effects of bentonite concentration on morphology and permeation characteristics of bentonite-doped polysulfone membranes were investigated. Solubility sphere for bentonite was constructed to estimate its solubility parameter. Thermodynamic modeling of phase inversion of this system was carried out using Flory–Huggins theory. The trade-off between thermodynamic and kinetic parameters was used to predict the membrane morphology for bentonite concentration varying from 0 to 5 wt %. The porosity of bentonite-doped membranes decreased up to 3 wt % that increased thereafter. Morphological analysis showed dense cross section with finger-like macrovoids at 3 wt % beyond which it changed to honeycomb structure with large circular voids. Permeability of 3 wt % membrane was the lowest (5.6 × 10⁻¹² m/Pa s) with 95% bovine serum albumin rejection. Contact angle of the membranes decreased from 83 to 66° with bentonite addition making the membrane more hydrophilic. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48450.