Nanostiffening in Polymeric Nanocomposites

Selected elastic moduli of nanocomposites are higher than the elastic moduli of microcomposites. Molecular immobilization and crystallization at the interfaces had been proposed as potential causes, but studies suggested that these effects are minor and cannot be used to explain the magnitude observed in nanocomposites with >3nm particles. Alternately, molecular simulation of polymer deformation showed that rotation gradients can lead to additional molecular rotations and stiffen the matrix. The stiffening is characterized by the nanostiffening material parameter, l2. In this investigation, an analytical expression for nanostiffening in nanocomposites was developed using finite element analysis. The nanostiffening in nanocomposites was determined by the ratio of l2to the particle size r, and the expression was shown to be in good agreement with experimental data from the literature. The dependence on the ratio suggests that nanostiffening is significant only for nanocomposites with large l2/r but is negligible when l2/r is small.     

Effective strain rate sensitivity of two phase materials

Analytical expressions are derived for the effective strain rate sensitivity exponent of a two phase material when the behavior of both phases and of the composite itself can be described by power law relations between the stress and strain rates. The material is assumed to be plastically isotropic and obey to von-Mises type creep behavior. Two types of boundary conditions are considered: strain or stress-controlled. The obtained formulas are applied to a geological composite material (mixture of camphor and octachloropropane) with the help of different simple models of two phase composites.     

Polymeric photovoltaic materials

Recent developments in conjugated polymer-based photovoltaic elements have been reviewed. The photophysics of such photoactive devices is based on the photoinduced electron transfer from donor-type semiconducting conjugated polymers onto acceptor-type conjugated polymers or acceptor molecules such as Buckminsterfullerene, C₆₀. Photoinduced electron transfer in solid composite films of fullerenes embedded into conjugated polymers is reversible, ultrafast (within 300 fs) with a quantum efficiency approaching unity, and metastable. Similar to the first step in natural photosynthesis, this photoinduced electron transfer leads to a number of potentially interesting applications, which include sensitization of the photoconductivity and photovoltaic phenomena. Furthermore, using the conjugated polymer donors in polymer blends with another conjugated polymer acceptor, similar photovoltaic elements have been realized. Examples of photovoltaic architectures are discussed with their potential in terrestrial solar energy conversion.     

高分子纳滤膜材料研究进展

纳滤膜是介于反渗透膜和超滤膜之间的一种新型分离膜,其截留分子量范围相对较窄（200-1000）且孔径较小,膜材质的选择日益成为制备出高性能纳滤膜的关键。本文综述了国内外高分子纳滤膜材料发展、应用和改良的最新研究进展,并在此基础上展望了纳滤膜材料未来发展趋势。     

PTCR Characteristics in YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">x</Emphasis></Subscript>—Linear Low Density Polyethylene (LLDPE) Composite Materials

A series of superconductor/polymer composites is made using high T _c YBa₂Cu₃O_7−x (Y-123) phase and Linear Low Density Polyethylene (LLDPE). The resistivity of the composites is measured as a function of temperature between 293 K and 400 K. A large positive temperature coefficient (PTC) of resistance is observed near 395 K. An attempt has been made to relate the percolation behavior of the YBCO–LLDPE composites and the thermal expansion of the LLDPE matrix. The intensity of the PTC effect for these composites was found to be as large as eight orders of magnitude.     

Micromechanical modeling of damage and fracture of unidirectional fiber reinforced composites: A review

An overview of methods of the mathematical modeling of deformation, damage and fracture in fiber reinforced composites is presented. The models are classified into five main groups: shear lag-based, analytical models, fiber bundle model and its generalizations, fracture mechanics based and continuum damage mechanics based models and numerical continuum mechanical models. Advantages, limitations and perspectives of different approaches to the simulation of deformation, damage and fracture of fiber reinforced composites are analyzed.     

Synthesis,Characterization and A.C. Susceptibility of YBa<Subscript>2</Subscript>Cu<Subscript>3</Subscript>O<Subscript>7−<Emphasis Type="Italic">x</Emphasis></Subscript> / Polyethylene Composites

A series of flexible composites are fabricated by mixing a high-T _c YBa₂Cu₃O_7−x superconductor with Low Density Polyethylene (LDPE). The synthesized polymer composites are characterized by X-ray diffraction technique. Superconducting nature of the composites is confirmed by the Meissner effect. The surface morphology of the composites is studied by scanning electron microscopy. The crystallization temperature of the polymer and its composites is determined by using differential scanning calorimetry (DSC). The composites showed a large diamagnetic susceptibility that increases with increasing volume fraction of superconductor filler. Susceptibility measurements showed that the intrinsic diamagnetic properties of the superconducting materials are preserved in the composites and there is no change in the transition temperature of the superconductor. Potential applications in the areas of shielding and levitation are discussed.     

Influence of Scale Specific Features on the Progressive Damage of Woven Ceramic Matrix Composites (CMCs)

It is well known that failure of a material is a locally driven event. In the case of ceramic matrix composites (CMCs), significant variations in the microstructure of the composite exist and their significance on both deformation and life response need to be assessed. Examples of these variations include changes in the fiber tow shape, tow shifting/nesting and voids within and between tows. In the present work, the influence of many of these scale specific architectural features of woven ceramic composite are examined stochastically at both the macroscale (woven repeating unit cell (RUC)) and structural scale (idealized using multiple RUCs). The recently developed MultiScale Generalized Method of Cells (MSGMC) methodology is used to determine the overall deformation response, proportional elastic limit (first matrix cracking), and failure under tensile loading conditions and associated probability distribution functions. Prior results showed that the most critical architectural parameter to account for is weave void shape and content with other parameters being less in severity. Current results show that statistically only the post-elastic limit region (secondary hardening modulus and ultimate tensile strength) is impacted by local uncertainties both at the macro and structural level.     

Arg-Gly-Asp (RGD) Modified Biomimetic Polymeric Materials

The new generation of biomaterials focuses on the design of biomimetic polymeric materials that are capable of eliciting specific cellular responses and directing new tissue formation. Since Arg-Gly-Asp (RGD) sequences have been found to promote cell adhesion in 1984, numerous polymers have been functionalized with RGD peptides for tissue engineering applications. This review gave the advance in RGD modified biomimetic polymeric materials, focusing on the mechanism of RGD, the surface and bulk modification of polymer with RGD peptides and the evaluation in vitro and in vivo of the modified biomimetic materials.     

生物医用高分子材料及其应用

对目前广为应用的生物医用高分子材料的类型及其应用进行了综述 ,并对今后可能的发展趋势进行了展望     

Cement concrete and concrete–polymer composites: Two merging worlds: A report from 11th ICPIC Congress in Berlin, 2004

The search for durable and sustainable construction materials inspires the developments in the world of cement concrete, as well as in the world of concrete–polymer composites. Both worlds recognize, strive for and accept each other’s contribution to the synergetic effects that are realized by the combination of classical building materials and polymers. A better knowledge of materials behaviour, especially in the field of admixtures, and a better understanding of curing processes allowed the development of highly performing mineral or modified mineral concretes, mortars and grouts. CPC-science becomes an invaluable element in the development of sustainable construction materials. ICPIC brings together practitioners and scientists, dealing with concrete–polymer composites in all industrial fields, but with emphasis on construction industry. The 11th International ICPIC Congress took place in Berlin, 2–4th June 2004. New trends and evolutions have been presented and discussed. The highlights of the Congress, and the synergies for the construction world that emerge from this congress on polymers in concrete in combination with cement concrete, are presented.     

Creep behaviour of FRP-reinforced polymer concrete

Polymer concrete is a kind of concrete where natural aggregates such as silica sand or gravel are binded together with a thermoset resin, such as epoxy. Although polymer concretes are stronger in compression than cementitious concrete, its tension behaviour is still weak. The reinforcement of polymer concrete beams in the tension zone with pultruded profiles made of epoxy resin and glass fibers are a good compromise between stiffness and strength. In this paper it is reported an investigation of the creep behaviour of polymer concrete beams reinforced with fiber-reinforced plastics (pultruded) rebars. Four-point bending creep test were performed. An analytical model was applied to verify the experimental results.     

电学双稳态聚合物基非挥发存储器及其导电机理研究进展

本文较系统地介绍了电学双稳态聚合物基存储器的基本特性和结构,并对聚合物存储器中高低阻态之间相互转换的物理和化学机制进行了比较全面的综述,从而为进一步研究电学双稳态聚合物基非挥发存储器提出了有益的指导.     

Aerospace materials research opportunities

As we approach the second century of aerospace, increased emphasis is being placed on endurance, reliability, ease of manufacture, and lower cost, in addition to weight saving. This change in emphasis will have a major effect on not only the selection of materials, but on how materials are integrated into the total system design process. Subsonic aircraft will continue to play a major role in our future with emphasis on increased durability and lower cost. Non-metals that do not corrode are attractive; however, the issues of reliable fracture resistance to ensure safety and durability as well as ease of manufacture and inspection will be key. Higher performance engines and hypersonic aircraft will require higher temperature materials (including a substantial amount of non-metals) along with reliable toughness and ease of manufacture. In space, weight will continue to be a major driving force along with the need for long term vacuum and radiation stability. Ease of assembly and multifunctional use (e.g. electrically or thermally conducting structure) will be additional needs for spacecraft materials. We have reached a point in the evolution of structural materials where we are moving away from processing naturally occurring materials toward synthesizing designed microstructures to perform specific functions. The mathematical modeling of microstructure–property relationships and new chemical and biotechnical synthesis techniques appear to be critical technologies for the future. In addition, the future materials developer will need a broader understanding of the total structural life cycle so that the impact of utilization, maintenance, and training requirements in the design of new materials can be considered.     

Complex metallic alloys as new materials for additive manufacturing

Additive manufacturing processes allow freeform fabrication of the physical representation of a three-dimensional computer-aided design (CAD) data model. This area has been expanding rapidly over the last 20 years. It includes several techniques such as selective laser sintering and stereolithography. The range of materials used today is quite restricted while there is a real demand for manufacturing lighter functional parts or parts with improved functional properties. In this article, we summarize recent work performed in this field, introducing new composite materials containing complex metallic alloys. These are mainly Al-based quasicrystalline alloys whose properties differ from those of conventional alloys. The use of these materials allows us to produce light-weight parts consisting of either metal–matrix composites or of polymer–matrix composites with improved properties. Functional parts using these alloys are now commercialized.