Small‐angle X‐ray scattering investigation of carbon nanotube‐reinforced polyacrylonitrile fibers during deformation

Structural changes during deformation in solution‐ and gel‐spun polyacrylonitrile (PAN) fibers with multi‐ and single‐wall carbon nanotubes (CNTs), and vapor‐grown carbon nanofibers were investigated using synchrotron X‐ray scattering. Previously published wide‐angle X‐ray scattering (WAXS) results showed that CNTs deform under load, alter the response of the PAN matrix to stress, and thus enhance the performance of the composite. In this article, we find that the elongated scattering entities that give rise to the small‐angle X‐ray scattering (SAXS) in solution‐spun fibers are the diffuse matrix‐void interfaces that follow the Porod's law, and in gel‐spun fibers these are similar to fractals. The observed smaller fraction of voids in the gel‐spun fibers accounts for the significant increase in the strength of this fiber. The degree of orientation of the surfaces of the voids is in complete agreement with those of the crystalline domains observed in WAXS, and increases reversibly upon stretching in the same way as those of the crystalline domains indicating that the voids are integral parts of the polymer matrix and are surrounded by the crystalline domains in the fibrils. The solution‐spun composite fibers have a larger fraction of the smaller (<10 nm) voids than the corresponding control PAN fibers. Furthermore, the size distribution of the voids during elongation changes greatly in the solution spun PAN fiber, but not so in its composites. The scattered intensity, and therefore the volume fraction of the voids, decreases considerably above the glass transition temperature (T_g) of polymer. Implications of these observations on the interactions between the nanotubes and the polymer are discussed. © 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2394–2409, 2009     

A novel method for making ultra-fine metal fibers and particles

A novel method based on plastic processing and equipment for preparing ultra-fine metal fibers and particles is reported. With this new method, metal fibers and particles can both be produced on the same equipment and the surfaces of the fibers and particles can be protected from oxidation by the polymers or solvents during the preparation process. Metal-alloy powders with lower melt point were filled into polymer by an extruder, followed by a die-drawing process at a temperature lower than the melt temperature of the metal alloy. Metal fibers or particles were obtained after the polymer matrix was washed away. Metal alloy fibers can be obtained when a polymer that strongly interacts with metal alloy, such as a special polyvinyl alcohol with a low alcoholysis degree, is used as the polymer matrix. Metal-alloy particles can be obtained when a polymer with weak interaction with metal alloy, such as polyethylene (PE), is used as the polymer matrix. Based on the principle of this new method, it is possible to produce finer or even nano-sized metal fibers and particles with higher melting points.     

Structure and characteristics of chitosan-based fibers containing chrysotile and halloysite

With the use of the methods of X-ray diffraction and electron microscopy, chitosan fibers prepared by coagulation into an alcohol-alkali mixture are shown to possess a two-phase structure containing C- and O-type crystallites. These fibers and composite fibers containing halloysite and Mg chrysotile nanotubes are characterized by anisotropic structure, i.e., by the orientation of both chitosan crystallites and Mg chrysotile particles along the fiber axis. A comparison of the rates of shear induced by passing of a polymer solution through a die and the data of rheological studies allows the conclusion that the structuring of chitosan solution under the applied field of shear stresses and the orientation of polymer macromolecules and filler nanotubes occur. An increase in the draw ratio during fiber spinning does not assist orientation of polymer crystallites but, in contrast, increases surface defectiveness and leads to the nucleation of longitudinal cracks; as a result, the strength of fibers decreases. The introduction of 5 wt % Mg chrysotile into the chitosan matrix markedly increases the mechanical characteristics of the composite fibers owing to the reinforcing action of oriented filler nanotubes.     

The role of the orientation tensor in the rheology of flexible polymers

The stress-strain relations for transversely isotropic deformations of linear and branched polymer melts as well as of (crosslinked) rubbers are discussed in terms of the orientation tensor. It is shown that orientation and network strand extension are decoupled, and that the relative tube diameter and its inverse, the molecular stress function f, can be extracted directly from experimental data, if the effect of network orientation is accounted for by the order parameter. The tension of the average network strand increases with increasing deformation. This is caused by an increasing restriction of lateral movement of polymer chains due to deformation. At small strains, f² is found to be linear in the average stretch for melts as well as for rubbers, which corresponds to a constant volume of the tube. At large strains, melts show maximum molecular tension, depending on the degree of longchain branching, while rubbers show maximum extensibility.     

面向共享的高分子材料网络数据库的设计和实现

以国家材料科学数据共享网为依托,我们建立了一个大型的高分子材料数据库并已向公众开放访问。数据库主要面向科研工作人员和工业企业等,提供基于高分子材料工业产品信息的数据共享服务。该数据库涵盖塑料、橡胶、纤维、涂料、胶粘剂、加工助剂等高分子领域主要的材料类型,目前已纳入7 000余个牌号的50 000多条数据。针对高分子材料类型的复杂性,该数据库采取了网状带冗余的分类方式以使一些组成复杂的材料可通过多种分类路径查询到。数据入库前,通过在数据生产、搜集、整合等多个过程中的评估以确保数据质量和可靠性。入库的数据也从数据来源、评估结果、修改记录等多个方面进行标记,以便后期进一步进行核对与评估。     

Effect of the Nanoparticle Functionalization on the Cavitation and Crazing Process in the Polymer Nanocomposites

The nanoparticle(NP) functionalization is an effective method for enhancing their compatibility with polymer which can influence the fracture property of the polymer nanocomposites(PNCs). This work aims to further understand the cavitation and crazing process, hoping to uncover the fracture mechanism on the molecular level. By adopting a coarse-grained molecular dynamics simulation, the fracture energy of PNCs first increases and then decreases with increasing the NP functionalization degree α while it shows a continuous increase with increasing the interaction ε_(pA) between polymer and modified beads. The bond orientation degree is first characterized which is referred to as the elongation. Meanwhile, the stress by polymer chains is gradually reduced with increasing the α or the ε_(pA) while that by NPs is enhanced.Furthermore, the percentage of stress by polymer chains first increases and then decreases with increasing the strain while that by NPs shows a contrast trend. Moreover, the number of voids is quantified which first increases and then decreases with increasing the strain which reflects their nucleation and coalescence process. The voids prefer to generate from the polymer-NP interface to the polymer matrix with increasing α o r ε_(pA).As a result, the number of voids first increases and then decreases with increasing α while it continuously declines with the ε_(pA). In summary, our work provides a clear understanding on how the NP functionalization influences the cavitation and crazing process during the fracture process.     

Properties of ultrahighly filled composites based on polymers and ground rubber

The stress-strain and strength properties of ultrahighly filled composites based on thermoplastic polymers and ground rubber wastes are studied. The content of the elastic filler is higher than 70 wt%. As is shown, introduction of minor amounts of the plastic polymer, which serves as the binder for the filler particles, makes it possible to improve the strength properties of ultrahighly filled composites and to prepare materials of a desired thickness. A correlation between the stress-strain properties of the plastic polymer-rubber systems and the effective viscosity of the matrix polymer is established. When a polymer with homogeneous deformation and good adhesion to the elastic filler is used as the matrix, the resultant composites are characterized by properties close to those of vulcanized rubbers. A new method is proposed for processing of ground rubber wastes and preparation of materials that are similar to hard rubbers.     

Relative Rates of Branching in Emulsion and Miniemulsion Polymerization

This communication describes the potential advantages of using a miniemulsion rather than an emulsion process for the polymerization of synthetic rubbers in which the polymerization is ended (short‐stopped) considerably before full conversion in order to limit excessive branching brought on by a high polymer‐to‐monomer ratio in the polymer particles. Because the polymer‐to‐monomer ratio in the particle at low monomer conversion is much lower in a miniemulsion, a miniemulsion can be polymerized to a significantly higher conversion than a conventional emulsion while maintaining an equivalent degree of branching. Short‐stopping at a higher monomer conversion will result in substantially reduced processing costs associated with recovery and recycle of unpolymerized monomer.     

Study of the anomalous particles formed during the surfactant-free emulsion polymerization of styrene

The presence of anomalous regions within polystyrene latex particles prepared in the absence of added emulsifier has been investigated. It appears that they arise through loss of monomer from particles consisting of a discrete monomer-rich region surrounded by a polymer shell. It is likely that in most cases, monomer is lost from the region on evacuation prior to electron microscope examination, although there is some evidence from γ-irradiated samples (prior to electron microscopy) that loss of monomer can also occur during storage and/or dilution of the sample. Scanning electron microscopy and carbon replication techniques have been used to determine the shapes of the voids. Gas adsorption studies and carbon replication have also served to illustrate that the presence of the regions was not due to electron-beam damage. The presence of extrusions on some samples has been attributed to incomplete loss of monomer on evacuation due to the thickness of the surrounding polymer shell.     

Thermodynamic state parameters of a system as a prognostic factor of strength properties of composites with different natures of a surface

It is shown that, upon the modification of filler particles by a polymer film of the same nature as the polymer matrix, intermolecular bonds become stronger in a boundary layer between a polymer and a filler and the level of mechanical characteristics of polymer composite increases correspondingly. In order to ensure the working capacity of the metal-epoxy primer-polyethylene system, along with cohesive and adhesive properties of epoxide polymers, it is also necessary to take into account acid-base properties of solid metal and polymer surfaces.     

Polymer–TiO2 composite nanorattles via RAFT‐mediated emulsion polymerization

In this work, successful synthesis of polymer nanorattles containing titanium dioxide pigment particles in the centers of air voids is reported. The method used amphiphilic macro‐RAFT copolymers as stabilizers for pigment dispersion and the subsequent encapsulation of the pigment with polymer. The particles were first encapsulated by a water swellable hydrophilic layer, followed by a hard hydrophobic layer. Nanorattles were formed by swelling of hydrophilic polymer layers on the surface of the encapsulated pigment particles in a basic solution at elevated temperature. After swelling, the outer hard polymer shell was crosslinked to improve its strength. Air void sizes of the nanorattles were found to be controlled by swelling time, temperature, and the hydrophilic polymer layer thickness. © 2011 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2012     

Swelling and Collapse of Swiss‐Cheese Polyelectrolyte Gels in Salt Solutions

A simple theory of swelling of microporous Swiss‐cheese polyelectrolyte gels (i. e., polyelectrolyte gels containing water voids) in the solution of 1–1 low‐molecular‐weight salt is developed. Due to the Donnan effect the overall concentration of salt within porous Swiss‐cheese gel can be significantly higher than that within the corresponding homogeneous gel due to the effective absorption of salt by the embedded voids. The degree of this absorption increases with the increase of average size of the voids and of their concentration. In the case of relatively large water voids with radii of about few μm the salt concentration within the water voids is approximately equal to that in the external solution, while the salt concentration in the polymer matrix can be much lower. It is thought that polyelectrolyte Swiss‐cheese gels are promising for use as suitable media for microreactors for nanotechnology.     

Effects of the surface pressure on the formation of Langmuir-Blodgett monolayer of nanoparticles

The effects of the surface pressure on the particle arrangement of Langmuir-Blodgett (LB) monolayers of alkanethiol-capped gold nanoparticles were studied. The LB monolayers were prepared from a highly concentrated particle solution, which increases film fabrication efficiency but readily causes small particle voids in the particle array. Overcompressing the LB monolayer to a high surface pressure restructured the particles and eliminated the voids. When the gold particles capped by dodecanethiol were 8.5 nm in diameter, the particle arrangement was vastly improved and a wafer-scale LB monolayer was transferred onto a substrate at the surface pressure of 20 mN/m.     

X-ray microtomography studies of nascent polyolefin particles polymerized over magnesium chloride-supported catalysts

Drastic changes occur during the initial stages of the α-olefin polymerization over heterogeneous catalysts. Fragmentation of the support takes place as polymer is formed at the active sites within the voids of the support/catalyst. Magnesium chloride-supported titanium catalyst/polymer particles have been analyzed employing high-resolution computed microtomography (CMT) using synchrotron radiation at Brookhaven National Laboratory. The changes in morphology, the spatial distribution of the support/catalyst fragments, porosity, and polymer distribution in single growing polypropylene and polyethylene particles have been studied. These studies documented considerable macroporosity ( > 2 μm in size) within the growing catalyst/support/polymer particles. The largest pores may be due to agglomeration of smaller subparticles. Our results confirm that the initial fragmentation of the support proceeds readily and uniformly to yield a multi-grain growth of subparticle agglomerates. The support/catalyst fragments appear to be distributed relatively uniformly within the growing polymer particle. The surface of the subparticle agglomerates is accessible through the void-space between growing catalyst/particle grains. This may facilitate monomer transport to the activate sites through the polymer/catalyst particles. © 1993 John Wiley & Sons, Inc.     

New technology of production of optically transparent polymer compositions containing capsulated nanosized aggregates of photoactive compounds

The article summarizes the results of the studies performed at the Institute of Chemical Physics, Russian Academy of Sciences, devoted to development of scientific foundations of adding nanosized particles of organic or inorganic photoactive compounds into polymers for production of new polymer nanocomposites of optical application. It is shown that luminescent europium complexes and photochromic indoline spirooxazines can be added to polymer matrices in the solubilized state at a threefold concentration compared to the process without Pluronics. The effect of an increase in the functional activity of solubilized luminescent europium complexes added to polymer compositions of various structures is discovered, which is related with a decrease in the size of aggregates of europium complexes at their solubilization by Pluronics. The solubilization by Pluronics exerts a sharp influence on the maximal content of indoline photochromes in the polymer matrix; the content of the colored form of photochrome in the 3D-crosslinked acryl matrix increases fourfold. This effect is related to both the decrease in the sizes of photochrome aggregates during solubilization and the plasticization of the polymer matrix by Pluronics.     

Synthesis of Polymer Nanoparticles via Vapor Phase Deposition onto Liquid Substrates

In this article, the growth of polymer nanoparticles formed at the liquid–vapor interface via vapor phase polymerization is studied. The particles grow by polymer aggregation, which is driven by the surface tension interaction between the liquid and polymer. It is demonstrated that the mechanism of particle growth is determined by whether polymer particles remain at the liquid–vapor interface or submerge into the liquid. The position of the particles depends on the interaction between the polymer and the liquid. For example, the deposition of poly(n‐butyl acrylate) onto poly(dimethyl siloxane) and Krytox liquids leads to the formation of nanoparticles that remain at the liquid–vapor interface. The size of these particles increases as a function of deposition time. The deposition of poly(4‐vinylpyridine) onto poly(dimethyl siloxane) and Krytox leads to the formation of nanoparticles that submerge into the liquid. The size of these particles does not significantly change with deposition time. Our study offers a new rapid, one‐step synthetic approach for fabricating functional polymer nanoparticles for applications in catalysis, photonics, and drug delivery.

     

Development of electrospun PVdF–PAN membrane-based polymer electrolytes for lithium batteries

Electrospun fibrous membranes of composites of polyvinylidene fluoride and polyacrylonitrile (PVdF–PAN–ESFMs) are prepared with different proportions of PAN (25, 50 and 75%, w/w). The morphology of the ESFMs is examined by field emission scanning electron microscopy (FESEM). FESEM image of PVdF–ESFM reveals that the fibers have uniform diameters and smooth surfaces. However, the fibers of PVdF–PAN–ESFMs are interconnected with large number of voids and cavities of different sizes. These voids are effectively utilized for the preparation of polymer electrolytes by loading lithium perchlorate dissolved in propylene carbonate. PVdF–PAN–ESFM with 25% PAN (designated as PVdF–PAN(25)–ESFM) could load a high amount of lithium salt with electrolyte uptake of more than 300%. PVdF–PAN(25)–ESFM electrolyte exhibits a high conductivity of 7.8 mS cm⁻¹ at 25 °C and electrochemically stable up to 5.1 V. Also, the addition of PAN into PVdF decreases the interfacial resistance with lithium electrode. PVdF–PAN–ESFM electrolytes have complementary advantageous characteristics of PVdF and PAN. The promising results reported here clearly indicate that polymer electrolytes based on PVdF–PAN–ESFMs are most suited for lithium batteries.     

Imaging c-PAM-induced flocculation of paper fibers

The flocculation of paper fibers by cationic polyacrylamides (c-PAM) was studied by imaging the fibers that remain free during flocculation. Studies with fibers of different lengths showed that the degree of flocculation increases with fiber length, with the best flocs being formed with mixtures of short and long fibers. Short fibers did not flocculate by themselves but were captured by flocs formed with longer fibers. The short fibers strengthen the floc and give it shear resistance. Shear had the expected effect of promoting flocculation at low Reynolds number but disrupting it at higher values. For a given polymer the maximum floc size for a mixture of fibers is dictated by the length distribution of the fibers. The polymer dose governs the rate of flocculation. The technique is especially useful in following the tail end of the flocculation process. At this stage a floc is almost fully grown and a small increase in its size would be very difficult to measure by conventional techniques. In contrast, the number of free fibers measured by single fiber imaging decreases rapidly at this point.     

Mechanism of TiO2 pigment retention on pulp fiber with polyelectrolytes

Pulp fibers and pigment dispersed in water are both negatively charged and, therefore, when a paper sheet is formed on a screen the pigment particles, which are of colloidal dimension, are not retained. Cationic polyelectrolytes are employed to promote pigment retention but the mechanism by which they operate is not well understood.Two cationic polyelectrolytes, based on polyethylenimine and polyacrylamide and used commercially as retention aids were examined to clarify their role in TiO₂ retention. From the comparison of polymer adsorption on both fiber and pigment, their electrophoretic mobility, and the extent of pigment retention it was concluded that the main mechanism is mutual attraction between oppositely charged fiber and pigment. In general, the fibers became positive at a polymer concentration where the pigment was still negative and maximum retention was realized when these negative pigment particles deposited on the already positive fibers. Homoflocculation of the pigment (and mechanical entrapment of pigment flocs in the forming sheet) also contributed to the overall retention and was more pronounced with the polyacrylamide polymer.     

The role of chain transfer agents in the emulsion polymerization of styrene

The effect of chain transfer agents on the nucleation and growth of polymer particles in the emulsion polymerization of styrene were examined extensively. The chain transfer agents used are carbon tetrachloride, carbon tetrabromide, and four primary mercaptans (C₂, n-C₄, n-C₇, and n-C₁₂). It is shown that with an increase in the amount of chain transfer agents charged the rate of polymerization per particle decreases progressively. The number of polymer particles formed, on the other hand, increases initially then decreases. These effects can be enhanced by using a chain transfer agent with higher values of chain transfer constant and solubility in water. It is also demonstrated that with increasing radical desorption from the particles, aided by chain transfer agents, the emulsifier dependence exponent for the number of polymer particles formed increases from 0.6 to 1.0 and the initiator dependence exponent decreases from 0.4 to 0. The effect of chain transfer agents on the nucleation and growth of polymer particles in the emulsion polymerization of styrene can be explained in terms of desorption of chain-transfered radicals from the polymer particles.