Engineering Chemistry of Electrospun Nanofibers and Interfaces in Nanocomposites for Superior Mechanical Properties

The novelty of this work is based on designing the chemistry of the electrospun nanofibers, so that the resultant composites substantially benefit from cross-linking between the nanofibers and the polymer matrix. Specifically, the solution of in-house synthesized copolymers polystyrene-co-glycidyl methacrylate P(St-co-GMA) is electrospun to produce mats of surface reactive nano-to-submicron scale fibers that are accompanied later by spraying over the ethylenediamine (EDA) as a supplementary cross-linking agent for epoxy. The P(St-co-GMA)/EDA fiber mats are then embedded into an epoxy resin. Analysis of the three-point-bending mode of the composites reveals that the storage modulus of P(St-co-GMA)/EDA nanofiber-reinforced epoxy are about 10 and 2.5 times higher than that of neat and P(St-co-GMA) nanofiber-reinforced epoxy, respectively, even though the weight fraction of the nanofibers was as low as 2 wt %. The significant increase in the mechanical response is attributed to the inherently cross-linked fiber structure and the surface modification/chemistry of the electrospun fibers, that results in cross-linked polymer matrix−nanofiber interfacial bonding

Pointwise Bias Error Bounds for Response Surface Approximations and Min-Max Bias Design

Two approaches addressing response surface approximation errors due to model inadequacy (bias error) are presented, and a design of experiments minimizing the maximal bias error is proposed. Both approaches assume that the functional form of the true model is known and seek, at each point in design space, worst case bounds on the absolute error. The first approach is implemented prior to data generation. This data independent error bound can identify locations in the design space where the accuracy of the approximation fitted on a given design of experiments may be poor. The data independent error bound can easily be implemented in a search for a design of experiments that minimize the bias error bound as it requires very little computation. The second approach is to be used posterior to the data generation and provides tightened error bound consistent with the data. This data dependent error bound requires the solution of two linear programming problems at each point. The paper demonstrates the data independent error bound for design of experiments of two-variable examples. Randomly generated polynomials in two variables are then used to validate the data dependent bias-error bound distribution

Effects of electrospinning parameters on polyacrylonitrile nanofiber diameter: an investigation by response surface methodology

Effects of material and process parameters on the diameter of electrospun polyacrylonitrile fibers were experimentally investigated. Response surface methodology (RSM) was utilized to design the experiments at the settings of solution concentration, voltage and the collector distance. It also imparted the evaluation of the significance of each parameter on the resultant fiber diameter. The investigations were carried out in the two-variable process domains of several collector distances as applied voltage and the solution concentration were varied at a fixed polymer molecular weight. The mean diameter and coefficient of variation were modeled by polynomial response surfaces as functions of solution concentration and voltage at each collector distance. Effect of applied voltage in micron-scale fiber diameter was observed to be almost negligible when solution concentration and collector distance were high. However, all three factors were found statistically significant in the production of nano-scale fibers. The response surface predictions revealed the parameter interactions for the resultant fiber diameter, and showed that there is a negative correlation between the mean diameter and coefficient of variation for the fiber diameter. A sub-domain of the parameter space consisting of the solution concentration, applied voltage and collector distance, was suggested for the potential nano-scale fiber production

Poly(vinylidene fluoride)/zinc oxide smart composite material

This work aimed at fabrication and electromechanical characterization of a smart material system composed of electroactive polymer and ceramic materials. The idea of composite material system is on account of complementary characteristics of the polymer and ceramic for flexibility and piezoelectric activity. Our preliminary work included Polyvinylidene Fluoride (PVDF) as the flexible piezoelectric polymer, and Zinc Oxide (ZnO) as the piezoelectric ceramic brittle, but capable to respond strains without poling. Two alternative processes were investigated. The first process makes use of ZnO fibrous formation achieved by sintering PVA/zinc acetate precursor fibers via electrospinning. Highly brittle fibrous ZnO mat was dipped into a PVDF polymer solution and then pressed to form pellets. The second process employed commercial ZnO nanopowder material. The powder was mixed into a PVDF/acetone polymer solution, and the resultant paste was pressed to form pellets. The free standing composite pellets with electrodes on the top and bottom surfaces were then subjected to sinusoidal electric excitation and response was recorded using a fotonic sensor. An earlier work on electrospun PVDF fiber mats was also summarized here and the electromechanical characterization is reported

Multiscale reinforcing interlayers of self-same P(St-co-GMA) nanofibers loaded with MCF for polymer composites and nanocomposites

Electrospinning has become a proven technique to introduce polymeric sub-phases into composites. The sub-phases such as nanofibers can also be used as a carrier platform for reinforcing particles at different scales, enabling a multiscale reinforcement approach. However, the polymeric nanofibers may lose their intended fibrous morphology during the composite curing at elevated temperature. As such, polymeric sub-phase can not contribute effectively as fibers to the mechanical properties of the composite. This paper exemplifies introduction of milled carbon fibers (MCF) carried by electrospun polymeric nanofibers and the use of the resultant multi-scale reinforcement as interlayer within conventional structural composites. The issue of polymeric nanofibers exposed to elevated temperature curing is circumvented by implementing a novel self-same nanofibrous strategy. While a base polymer for the nanofibers is chosen as epoxy compatible P(St-co-GMA), its derivative by a cross-linker Phthalic Anhydrate, P(St-co-GMA)/PA is also incorporated by dual-electrospining, i.e. simultaneous electrospinning of the two polymers. It was shown that the nanofibers of the base polymer melt and fuse over the cross-linkable nanofibers forming the self-same nanofibrous morphology during the heat treatment in accordance with the cure cycle of the epoxy resin in this study. MCFs were mixed into the cross-linkable polymer solution and electrospun with the P(St-co-GMA)/PA nanofibers. The dual polymer and MCF loaded nanofibrous structures were analyzed morphologically before and after heat treatment. Homogenous distribution of particles in the fibrous structures, melting of the neat copolymer, crosslinking of the polymer mix, and selfsame fibrous structure were characterized. The nanofiber mats were used as the reinforcement to epoxy resin films and as interlayers for carbon fiber-reinforced composites. In the case of nanocomposites, MCF enhanced the elastic modulus by about 9%. In the use of multiscale nanofibrous mats as interlayers of continuous carbon fiber composites, they improved the ultimate tensile strength of a cross-ply laminate by 9%

High strain rate response of nanofiber interlayered structural composites

Nanofibrous interlayer toughening strategy for laminated composite materials typically demonstrated at quasi-static loading is here evaluated under high strain rate deformation. Carbon fiber reinforced composite laminates of (0/90)25s stacking sequence are interlayered by polystyrene-co-glycidyl methacrylate (P(St-co-GMA)) nanofibers which are chemically tuned for interfacial compatibility when embedded in epoxy matrix. The cubical composite specimens are cut and subjected to high strain-rate deformation via Split Hopkinson pressure bar testing. Specimens are hit at their through-the-thickness (stacking) and side-to-side (in-plane) directions. The change in the dissipation of energy due to altered interlaminar microstructure is monitored and reported. Enhancement in the capacity of the energy dissipation due to the nanofibrous interlayers is as high as 80% in-plane and 40% through thickness directions, depending on the strain rate. The results overall suggest that interlayer toughening strategy used in this work prevents the formation of critical matrix cracks that can cause the formation of instantaneous mode II delamination. Incorporation of the nanofibers without causing notable weight penalty effectively toug

Designed-in molecular interactions lead to superior thermo-mechanical properties in nanocomposites

The effect of the nanofiller chemistry on the mechanical behaviour of thermoset polymer matrix nanocomposites is investigated. The interaction between a crosslinked polymer resin and the reinforcing nanofibers driven by their chemistry is revealed by molecular dynamics simulations. Specifically, crosslinked network systems of neat epoxy and epoxy-P(St-co-GMA) are modeled to discuss the effect of various molecular interactions as a function of temperature on a molecular basis. At 433 K, incorporation of single molecule of bonded P(St-co-GMA) and nonbonded P(St-co-GMA) lead to increase in Young’s modulus by 10% and 6%, respectively, compared to neat epoxy system

PVA/PANI/rGO ternary electrospun mats as metal-free anti-bacterial substrates

Successful performance of biocompatible hybrid systems in various biomedical applications such as wound healing patches, and scaffolds for stem cell preparation have been reported. However, relatively poor structural properties and further bacterial infection have been the major drawbacks for their commercialization. In order to improve the antimicrobial property of such structures, transition metals have been previously added to the media. However, the potential risk of metal pollution as well as hardship of processing has put this approach into obsolescence. Herein the ternary polyvinyl alcohol/reduced graphene oxide/polyaniline fibrous nanocomposites, as substitute for transition metal-containing nanocomposites, were prepared via electrospinning. The mats' structural properties (e.g. rheological, morphological, electrical and mechanical properties) and their antibacterial properties against E. coli bacteria cultures after two different treatments (including thermal and acid doping approaches) were systematically investigated. It was shown that in addition to significant structural improvement, an over 80% antibacterial property enhancement in treated mats in comparison to pristine PVA fibers were achieved. Finally the interaction and main effect analyses were used for suggesting the optimum antibacterial specimen conditions

Design of and with thin-ply non-crimp fabric (NCF) as building blocks for composites

New generation non-crimp fabric (NCF) offers an attractive thin and lightweight building block alternative in the design of composite materials and structures. Pre-assembly of multiple plies of parallel fibers, each laying in a different orientation would not require crimping of the fibers and would enable one-axis lay-up that can substantially reduce the labor, scrap, and manufacturing costs. A state-of-the-art tow-spreading technique enables ply thickness to be reduced to as low as one-third of the typical commercial high quality pre-preg ply thickness. The thin-ply NCF stacks result in well-dispersed plies of different fiber orientations and creates the so-called homogenized laminates without ply clustering. As an option, bi-angle thin-ply NCF offers two different fiber orientations with one being off-axis, e.g. at φ°, along with an on-axis 0° forming (0/φ) assembly. This allows to design in anisotropic properties within the NCF building block. An overview of several aspects of the thin-ply bi-angle NCF composites is provided to address associated benefits and opportunities in the lightweight structural composites design process