Nanocomposite fibres: a strategy for stronger materials?

The scientific literature is full of claims for the exceptional potentialities of nanoscale reinforcing materials, such as nanoparticles, nanofibres, nanotubes and nanoplatelets, to improve the mechanical properties of polymer matrices. However, most of the reliable data on the effective properties of polymer nanocomposites are somewhat disappointing, particularly when compared to structural composites reinforced with high-performance continuous fibers. The main reasons advocated to explain the discrepancies with theoretical predictions include poor dispersion, inadequate alignment of nanofillers, and bad stress-transfer ability at the interface (i.e. poor adhesion with polymer matrix). A possible route for introducing nanofillers into polymer matrices, and retaining a certain control on orientation and improving the dispersion level, can be offered by polymer fibres technology. In fact, the elevated draw ratios and elongational flow involved in typical fibre production processes can concurrently promote a disruption of the aggregates and a strong orientation of the nanofillers along the fibre axis. A variety of processing techniques can be adopted for the production of nanocomposite fibres, including spinning from polymer melts or solutions, gel-spinning, melt-blowing and electrospinning. This approach has been proven to be extremely efficient when nanofillers with an elevated aspect ratio, such as nanofibres or nanotubes, are concerned. In fact, in the last decade several papers have been published documenting the extraordinary reinforcing efficiency of single- or multiwalled carbon nanotubes in highly oriented polymer fibres or tapes. More recently, some successful attempts have been made to extend this approach to nanoplatelets, in particular layered silicates and graphene. Among the drawback to overcome, one can list the marked viscosity increments generally induced by nanofillers in polymer matrices, and the consequent necessity to identify proper processing conditions for the fibre production. Moreover, the decrease of elongation at break (which is also reflected in the molten state) typically induced by some nanofillers may also limit fibre preparation possibilities. The outstanding properties of nanocomposite fibres could improve traditional textile products, or be exploited in some advanced processing technologies, such as single- or all-polymer composites and commingled yarns composites

The way to autonomic self-healing polymers and composites

composites may cause irreversible damages such as surface scratches, micro cracks or small internal flaws that can seriously impair the load carrying ability of structural components. In fact, these cracks can grow under subcritical loading conditions (such as fatigue or creep) and ultimately lead to failure. Repairing these small damages is often a difficult task since it requires non-destructive inspections an intervention under maintenance procedures. A big challenge for polymer scientists and engineers is to develop new polymeric matrices having the built-in ability to partially repair damages occurring during its service life time. An ideal self-healing polymer is able, after having been damaged, to autonomously restore its initial mechanical performances without the need of external stimuli like temperature, radiation, pressure, etc. This process is quite common in nature, since damage to a living tissue generally elicits a healing response. Self-healing applied to thermosetting matrices and composites is an emerging area of research, with the first significant papers being published in early 2000's and with the 1st International Conference on Self-Healing Materials organized by the Delft University of Technology (NL) in 2007. The first report of a man-made self healing polymer was by the group of prof Scott White of the University of Illinois at Urbana-Champaign. They developed an epoxy system containing microcapsules filled with a healing agent (liquid monomer). When a crack propagates, the microcapsule will rupture, the monomer will fill the crack and eventually undergo a polymerisation process, initiated by (Grubbs') catalyst particles dispersed in the system. This model system proved to work quite well, as the service life time of such material under fatigue conditions is significantly improved. On the other hand, it also presents a number of drawbacks related to the Grubbs' catalyst (high cost, partial deactivation by the amine curing agent, decomposition above 120°C) and to the stability of the microcapsules (diffusion and leakage of the healing agent, decomposition of the capsules above 170°C). A number of research groups world wide is trying to improve the microencapsulation-based approach or to develop new approaches to self-healing. A possible alternative strategy is based on the development of reversible crosslinked polymers through selfassembly chemistry in which the material is repaired by the re-association of the self-assemblable bonds broken by the crack. At a present, there are no commercially available products having a self-healing ability and the challenge for materials scientists and engineers is fascinating

Improving fibre/matrix interface through nanoparticles

strength controls several mechanical properties of composite materials, in particular the matrix-dominated ultimate parameters. In the last thirty years an impressive number of experimental and modelling efforts have been focused on the understanding of fibre/matrix interfacial bond with the aim to improve it. Over the years, two main strategies emerged for polymer composites: i) the development of specific fibre sizings/coatings/treatments and/or ii) the addition of coupling agents to the matrix resin. In the recent years new strategies came to light: in fact, the availability of various types of nanoparticles offered the possibility to tailor the fibre/matrix interactions at a nanoscale level. In particular, some recent investigations proved that nanoparticles homogeneously dispersed in a polymer matrix can play a beneficial role on the fibre/matrix interfacial adhesion in several types of structural composites. For example, the introduction of organo-modified clays in an epoxy matrix led to the formation of a stronger interface with E-glass fibres, with an increase of the interfacial shear strength of about 30% for a filler content of 5 wt% (DOI: 10.1177/ 0021998311420311). Concurrently, the evaluation of the fibre/matrix contact angle revealed an improved wettability when organo-modified clays were added, and a simultaneous enhancement of the fracture toughness of the resin matrix. This approach could be also adopted to add specific functional properties to composite materials, such as damage controlling capabilities (DOI: 10.1016/j.compositesa.2012.03.019). Another approach relies on the possibility to confine nanoparticle in the interfacial region, with the advantage of localizing their presence in the area where stress transfer takes place, thus reducing the overall quantity of nanoparticle required. As an example, a sizing containing single or multi-walled carbon nanotubes has been used for coating glass fibres (DOI: 10.1016/j.compscitech.2007.10.009). Two simultaneous results have been reached, to 'heal' surface flaws and to enhance interfacial adhesion in a polypropylene matrix, indicating nanotube related interfacial toughening mechanisms. An amazing amount of research has gone, and still goes, into the understanding of the properties of nanoscale particles and their usage to improve engineering materials. Development of polymer composites can surely benefit from this research, including the 'old issue' of fibre/matrix interface

Proposal of the Boltzmann-like superposition principle for nonlinear tensile creep of thermoplastics

Abstract The Boltzmann superposition principle (BSP) valid for "standard linear solids" is presented in all textbooks on viscoelasticity. In practice, the BSP is not applicable to viscoelastic polymers because (i) the apparent limit (if any) of the stress–strain linearity is very low, (ii) real deformations (stresses) are not infinitesimal, and (iii) tensile deformations give rise to additional free volume, which affects all currently running deformation processes. Consistent application of the free volume approach, including the strain-induced free volume, allowed us to derive and verify a new type of the internal time–tensile strain superposition for a series of single-step nonlinear creeps [J. Kolařik, A. Pegoretti, Nonlinear tensile creep of polypropylene: time–strain superposition and creep prediction, Polymer 47 (1) (2006) 346]. The Boltzmann-like superposition principle for multistep nonlinear tensile creep, proposed in this paper, consists of (i) the separation of individual creeps, (ii) their reconstruction for the initial free volume by introducing a specific internal time, and (iii) the superposition of the reconstructed creeps. The procedure is demonstrated using creep data for three types of commercial polypropylene

Nanofiller Aggregation as Reinforcing Mechanism in Nanocomposites

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Phase structure and tensile creep of recycled poly(ethylene terephthalate)/short glass fibers/impact modifier ternary composites

Binary and ternary composites of recycled poly(ethylene terephtalate) (rPET), short glass fibres (SGF) and/or impact modifier (IM) were prepared by melt compounding and injection moulding. SEM images of rPET/IM fracture sur- faces indicated that IM particles of about 1-2 μm in diameter were uniformly distributed in the rPET matrix, but with a poor adhesion level. Microphotographs of PET/SGF composites evidenced brittle fracture proceeding through the matrix and strong adhesion between components. In ternary composites SGF were evenly distributed, while IM particles were no longer detectable. Tensile creep of rPET and prepared composites was investigated under short and long term testing conditions at various stress levels. Main part of the tensile creep corresponded to the elastic time-independent component, while the time- dependent component was rather limited even at relatively high stresses. While SGF accounted for a significant decrease in the overall creep compliance, the incorporation of IM induced a small decrease in the compliance and a non-linear vis- coelastic behavior. In ternary composites, the reinforcing effects of SGF was dominating. Under a constant stress, the log- arithm of compliance grew linearly with the logarithm of time. The creep rate, which resulted to be generally very small for all tested materials, was slightly reduced by SGF and increased by IM

Polydopamine-Coated Paraffin Microcapsules as a Multifunctional Filler Enhancing Thermal and Mechanical Performance of a Flexible Epoxy Resin

This work focuses on flexible epoxy (EP) composites containing various amounts of neat and polydopamine (PDA)-coated paraffin microcapsules as a phase change material (PCM), which have potential applications as adhesives or flexible interfaces with thermal management capability for electronics or other high-value-added fields. After PDA modification, the surface of PDA-coated capsules (MC-PDA) becomes rough with a globular appearance, and the PDA layer enhances the adhesion with the surrounding epoxy matrix, as shown by scanning electron microscopy. PDA deposition parameters have been successfully tuned to obtain a PDA layer with a thickness of 53 ± 8 nm, and the total PDA mass in MC-PDA is only 2.2 wt %, considerably lower than previous results. This accounts for the fact that the phase change enthalpy of MC-PDA is only marginally lower than that of neat microcapsules (MC), being 221.1 J/g and 227.7 J/g, respectively. Differential scanning calorimetry shows that the phase change enthalpy of the prepared composites increases with the capsule content (up to 87.8 J/g) and that the enthalpy of the composites containing MC-PDA is comparable to that of the composites with MC. Dynamic mechanical analysis evidences a decreasing step in the storage modulus of all composites at the glass transition of the EP phase, but no additional signals are detected at the PCM melting. PCM addition positively contributes to the storage modulus both at room temperature and above Tg of the EP phase, and this effect is more evident for composites containing MC-PDA. As the capsule content increases, the mechanical properties of the host EP matrix also increase in terms of elastic modulus (up to +195%), tensile strength (up to +42%), Shore D hardness (up to +36%), and creep compliance (down to −54% at 60 min). These effects are more evident for composites containing MC-PDA due to the enhanced interfacial adhesion

Mechanical properties and strain monitoring of glass-epoxy composites with graphene-coated fibers

An engineered interphase can improve the mechanical properties of epoxy/glass composites simultaneously inducing a piezoresistive response. To prove this concept, E-glass fibers were coated with graphene oxide (GO) by electrophoretic deposition, while reduced graphene oxide (rGO) coated fibers were obtained by subsequent chemical reduction. The fiber-matrix interfacial shear strength (measured by the single-fiber fragmentation test) increased for both GO and rGO coated fibers. Unidirectional composites with a high content of both uncoated and coated fibers were produced and mechanically tested under various configurations (three-point bending, short beam shear and mode-I fracture toughness, creep). Composites with coated fibers performed similarly or better than composites prepared with uncoated fibers. Finally, composites with rGO coated fibers were tested for their piezoresistive response under both static and dynamic conditions. The electrical resistance changed proportionally to applied strain thus confirming the possibility of using composites with rGO coated fibers as strain sensors in load-bearing components