There have been a number of review papers on layered silicate and carbon nanotube reinforced polymer nanocomposites, in which the fillers have high aspect ratios. Particulate–polymer nanocomposites containing fillers with small aspect ratios are also an important class of polymer composites. However, they have been apparently overlooked. Thus, in this paper, detailed discussions on the effects of particle size, particle/matrix interface adhesion and particle loading on the stiffness, strength and toughness of such particulate–polymer composites are reviewed. To develop high performance particulate composites, it is necessary to have some basic understanding of the stiffening, strengthening and toughening mechanisms of these composites. A critical evaluation of published experimental results in comparison with theoretical models is given.

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Effects of particle size, particle/matrix interface adhesion and particle loading on mechanical properties of particulate–polymer composites

In this study, the mechanical properties of epoxy nanocomposites with graphene platelets, single-walled carbon nanotubes, and multi-walled carbon nanotube additives were compared at a nanofiller weight fraction of 0.1 ± 0.002%. The mechanical properties measured were the Young’s modulus, ultimate tensile strength, fracture toughness, fracture energy, and the material’s resistance to fatigue crack propagation. The results indicate that graphene platelets significantly out-perform carbon nanotube additives. The Young’s modulus of the graphene nanocomposite was ∼31% greater than the pristine epoxy as compared to ∼3% increase for single-walled carbon nanotubes. The tensile strength of the baseline epoxy was enhanced by ∼40% with graphene platelets compared to ∼14% improvement for multi-walled carbon nanotubes. The mode I fracture toughness of the nanocomposite with graphene platelets showed ∼53% increase over the epoxy compared to ∼20% improvement for multi-walled carbon nanotubes. The fatigue resistance resu...

Enhanced mechanical properties of nanocomposites at low graphene content.

Engineering applications of synthetic polymers are widespread due to their availability, processability, low density, and diversity of mechanical properties (Figure 1a). Despite their ubiquitous nature, modern polymers are evolving into multifunctional systems with highly sophisticated behavior. These emergent functions are commonly described as “smart” characteristics whereby “intelligence” is rooted in a specific response elicited from a particular stimulus. Materials that exhibit stimuli-responsive functions thus achieve a desired output (O, the response) upon being subjected to a specific input (I, the stimulus). Given that mechanical loading is inevitable, coupled with the wide range of mechanical properties for synthetic polymers, it is not surprising that mechanoresponsive polymers are an especially attractive class of smart materials. To design materials with stimuli-responsive functions, it is helpful to consider the I-O relationship as an energy transduction process. Achieving the desired I-O linkage thus becomes a problem in finding how to transform energy from the stimulus into energy that executes the desired response. The underlying mechanism that forms this I-O coupling need not be a direct, one-step transduction event; rather, the overall process may proceed through a sequence of energy transduction steps. In this regard, the network of energy transduction pathways is a useful roadmap for designing stimuli-responsive materials (Figure 1b). It is the purpose of this review to broadly survey the mechanical to chemical * To whom correspondence should be addressed. Phone: 217-244-4024. Fax: 217-244-8024. E-mail: jsmoore@illinois.edu. † Department of Chemistry and Beckman Institute. ‡ Department of Materials Science and Engineering and Beckman Institute. § Department of Aerospace Engineering and Beckman Institute. Chem. Rev. XXXX, xxx, 000–000 A

Mechanically-induced chemical changes in polymeric materials

An overview of the achievements of the German priority program “Functionally Graded Materials (FGM)” in the field of processing techniques is given. Established powder processes and techniques involving metal melts are described, and recent developments in the field of graded polymer processing are considered. The importance of modeling of gradient formation, sintering and drying for the production of defect-free parts with predictable gradients in microstructure is discussed, and examples of a successful application of numerical simulations to the processing of functionally graded materials are given.

Processing techniques for functionally graded materials

Graphene, a single-atom-thick sheet of sp-bonded carbon atoms, has generatedmuch interest due to its high specific area and novel mechanical, electrical, and thermal properties. Recent advances in the production of bulk quantities of exfoliated graphene sheets from graphite have enabled the fabrication of graphene–polymer composites. Such composites show tremendous potential for mechanical-property enhancement due to their combination of high specific surface area, strong nanofiller–matrix adhesion and the outstanding mechanical properties of the sp carbon bonding network in graphene. Graphene fillers have been successfully dispersed in poly(styrene), poly(acrylonitrile) and poly(methyl methacrylate) matrices and the responses of their Young’s modulus, ultimate tensile strength, andglass-transition temperaturehave been characterized. However, to the best of our knowledge there is no report on the fracture toughness and fatigue properties of graphene–polymer composites. Fracture toughness describes the ability of a material containing a crack to resist fracture and it is a critically important material property for design applications. Fatigue involves dynamic propagation of cracks under cyclic loading and it is one of the primary causes of catastrophic failure in structural materials. Consequently, the material’s resistance to fracture and fatigue crack propagation are of paramount importance to prevent failure. Herein we report the fracture toughness, fracture energy, and fatigue properties of an epoxy polymer reinforced with various weight fractions of functionalized graphene sheets. Remarkably, only 0.125% weight of functionalized graphene sheets was observed to increase the fracture toughness of the pristine (unfilled) epoxy by 65% and the fracture energy by 115%.Toachievecomparableenhancement,carbonnanotube (CNT) and nanoparticle epoxy composites require one to two orders of magnitude larger weight fraction of nanofillers. Under fatigue conditions, incorporation of 0.125% weight of functionalized graphene sheets drastically reduced the rate of crack propagation in the epoxy 25-fold. Fractography analysis

Fracture and fatigue in graphene nanocomposites

Epoxy nanocomposites ¿ fracture and toughening mechanisms

Epoxy nanocomposites with high mechanical and tribological performance

It is well known that inorganic filler particles enhance the mechanical and tribological properties of polymers. The stiffness, toughness, and wear performance of the composites are extensively determined by the size, shape, volume content, and especially the dispersion homogeneity of the particles. In the present study, various amounts of micro- and nano-scale particles (titanium dioxide TiO 2 , 200-400 nm, calcium silicate CaSiO 3 , 4-15 μm) were introduced into an epoxy polymer matrix for its reinforcement. The influence of these particles on the impact strength, dynamic mechanical thermal properties, and block-on-ring wear behavior was investigated. Using only the nano-particles, the results demonstrate the best improvement in stiffness, impact strength, and wear resistance of the epoxy at a nano-particle content of 4 vol% TiO 2 . Therefore, this nanocomposite was used to act as a matrix for the CaSiO 3 micro-particles, in the hope of finding synergistic effects between the micro- and the nano-particles. Results show, in fact, a further improvement of wear resistance and stiffness, whereas the impact strength suffers. Geometrical properties of the particles, the homogeneous dispersion state, energy dissipating fracture mechanisms, and a transition of wear mechanisms mostly contribute to the increase in performance.

Impact and wear resistance of polymer nanocomposites at low filler content

Enhancements of the wear resistance of epoxy using various fillers, e.g. short carbon fibre (CF), graphite, polytetrafluoroethylene (PTFE) and nano-TiO 2 , have been systematically investigated in the present study. Wear properties were carried out on a block-on-ring apparatus. The best wear resistant composition was achieved by a combination of nano-TiO 2 with conventional fillers; as an example, epoxy+15 vol% graphite+5 vol% nano-TiO 2 +15 vol% short-CF exhibits a specific wear rate of 3.2×10 −7  mm 3 /Nm, which is about 100 times lower when compared to the neat epoxy. Worn surfaces were investigated using a scanning electron microscope and an atomic force microscope, from which it is assumed that a mechanism of nanoscale rolling governs this positive effect of the nanoparticles. The main concept of this paper is to strength the importance of integrating various functional fillers in the design of wear resistant polymer composites.

Frank Haupert

Papers

Epoxy nanocomposites ¿ fracture and toughening mechanisms

Epoxy nanocomposites with high mechanical and tribological performance

Impact and wear resistance of polymer nanocomposites at low filler content

Enhancement of the wear resistance of epoxy: short carbon fibre, graphite, PTFE and nano-TiO2

Ultrasonic dispersion of inorganic nanoparticles in epoxy resin.