Thermochemical and rheological characterization of highly reactive thermoset resins for liquid moulding.

Highly reactive thermosets are currently expanding the processability of high-performance structures for transportation industry. The short polymerization time makes it a suitable process to replace metallic structures with polymer matrix-based composite materials. The resin characterization is a fundamental step to obtain the properties and the associated constitutive models, which are required to design and optimize the manufacturing process parameters of composite materials. However, the short time on polymerization requires to use the characterization equipment at their performance capability limits. This work presents a comprehensive methodology to characterize the thermo-chemical properties of highly reactive resin systems, which are relevant for resin impregnation into the preform for liquid injection processes. Four different commercial resin systems are analyzed in this study. Experimental methodologies are analyzed and adapted for best data acquisition at high temperature isothermals. Based on the experimental data, Cure kinetics and viscosity equation-based models are used to describe the behaviour of these complex resin systems. Processing maps are developed based on the cure kinetics and viscosity models to predict the processability time for specific process conditions than can be used on liquid injection moulding processes.

Dry Synthesis of Binder-Free Ruthenium Nitride-Coated Carbon Nanotubes as a Flexible Supercapacitor Electrode.

Ruthenium nitride was successfully deposited on a multiwalled carbon nanotube (MWCNT) forest grown on a stainless-steel mesh substrate by radiofrequency plasma-assisted pulsed laser deposition. This novel dry fabrication method for flexible supercapacitor electrodes eliminates toxic byproducts and the need for any binder component. Experimental results show a successful thin film coating of the individual MWCNTs with RuNx under various synthesis conditions. The electrochemical characterization demonstrates a significant improvement in capacitance of the synthesized RuNx-MWCNT electrode compared to the bare MWCNT forest, with a large potential window of 1.2 V. Capacitance values as high as 818.2 F g-1 (37.9 mF cm-2) have been achieved.

ß-Myrcene/isobornyl methacrylate SG1 nitroxide-mediated controlled radical polymerization: synthesis and characterization of gradient, diblock and triblock copolymers.

ß-Myrcene (My), a natural 1,3-diene, and isobornyl methacrylate (IBOMA), from partially bio-based raw materials sources, were copolymerized by nitroxide-mediated polymerization (NMP) in bulk using the SG1-based BlocBuilder™ alkoxyamine functionalized with an N-succinimidyl ester group, NHS-BlocBuilder, at T = 100 °C with initial IBOMA molar feed compositions f IBOMA,0 = 0.10-0.90. Copolymer reactivity ratios were r My = 1.90-2.16 and r IBOMA = 0.02-0.07 using Fineman-Ross, Kelen-Tudos and non-linear least-squares fitting to the Mayo-Lewis terminal model and indicated the possibility of gradient My/IBOMA copolymers. A linear increase in molecular weight versus conversion and a low dispersity (D ≤ 1.41) were exhibited by My/IBOMA copolymerization with f IBOMA,0 ≤ 0.80. My-rich and IBOMA-rich copolymers were shown to have a high degree of chain-end fidelity by performing subsequent chain-extensions with IBOMA and/or My, and by 31P NMR analysis. The preparation by NMP of My/IBOMA thermoplastic elastomers (TPEs), mostly bio-sourced, was then attempted. IBOMA-My-IBOMA triblock copolymers containing a minor fraction of My or styrene (S) units in the outer hard segments (M n = 51-95 kg mol-1, D = 1.91-2.23 and F IBOMA = 0.28-0.36) were synthesized using SG1-terminated poly(ethylene-stat-butylene) dialkoxyamine. The micro-phase separation was suggested by the detection of two distinct T gs at about -60 °C and +180 °C and confirmed by atomic force microscopy (AFM). A plastic stress-strain behavior (stress at break σ B = 3.90 ± 0.22 MPa, elongation at break ε B = 490 ± 31%) associated to an upper service temperature of about 140 °C were also highlighted for these triblock polymers.

Piezoresistance in polymer nanocomposites with high aspect ratio particles.

In this paper, we address the problem of positive piezoresistance in high aspect ratio particle based polymer nanocomposites, a hybrid system at the center of research on flexible piezoresistive materials. We introduce a percolation theory based model relating the variation in electrical resistance to compressive strain and show that it gives accurate theoretical fits to experimental data presented in this paper, as well as to much of the available data in the literature. In contrast to existing theories, the model captures the characteristics of the particle network through experimentally definable parameters and does not rely on assumptions regarding the nature of the particles and/or the configuration of the network. It is further demonstrated that the presented theoretical framework is not limited to polymer nanocomposites with high aspect ratio particle but that it can explain piezoresistance in bulk electroconductive polymer nanocomposites in general. We find that the piezoresistive effect in such materials is rooted in a mechanical deformation induced change in the distribution of local conductances within the particle network, and we show that this change in the distribution of local conductances is well described by a strain dependent conductivity exponent, which scales with the magnitude of mechanical deformation. Besides, we demonstrate that these findings can be applied to the experimentally observed concentration dependence of the piezoresistance in polymer nanocomposites and, thus, to predicting the electric response to mechanical deformation at any particle concentration, which is expected to be highly instrumental in applied materials selection and performance evaluation.

Influence of the reaction stoichiometry on the mechanical and thermal properties of SWCNT-modified epoxy composites.

Previous studies suggest that carbon nanotubes (CNTs) have a considerable influence on the curing behavior and crosslink density of epoxy resins. This invariably has an important effect on different thermal and mechanical properties of the epoxy network. This work focuses on the important role of the epoxy/hardener mixing ratio on the mechanical and thermal properties of a high temperature aerospace-grade epoxy (MY0510 Araldite as an epoxy and 4,4'-diaminodiphenylsulfone as an aromatic hardener) modified with single-walled carbon nanotubes (SWCNTs). The effects of three different stoichiometries (stoichiometric and off-stoichiometric) on various mechanical and thermal properties (fracture toughness, tensile properties, glass transition temperature) of the epoxy resin and its SWCNT-modified composites were obtained. The results were also supported by Raman spectroscopy and scanning electron microscopy (SEM). For the neat resin, it was found that an epoxy/hardener molar ratio of 1:0.8 provides the best overall properties. In contrast, the pattern in property changes with the reaction stoichiometry was considerably different for composites reinforced with unfunctionalized SWCNTs and reduced SWCNTs. A comparison among composites suggests that a 1:1 molar ratio considerably outperforms the other two ratios examined in this work (1:0.8 and 1:1.1). This composition at 0.2 wt% SWCNT loading provides the highest overall mechanical properties by improving fracture toughness, ultimate tensile strength and ultimate tensile strain of the epoxy resin by 40%, 34%, 54%, respectively.

Quantitative dispersion analysis of inclusions in polymer composites.

The state of dispersion plays an important role on the performance of polymer nanocomposites. Dispersion is usually assessed based on the qualitative evaluation of microscopy micrographs. In this paper, a quantitative algorithm is introduced for analyzing the dispersion of inclusions in polymer composites using image analysis. In a binary image, on-pixels are considered as particle elements while off-pixels stand for matrix elements. To quantify the dispersion, the mean distance value between any matrix elements to their corresponding nearest neighboring particle element is measured. A dispersion index, DI, is then defined by comparing the image of interest with the associated uniformly dispersed case. Synthetic models were utilized to examine the sensitivity of the algorithm to various dispersity scenarios such as the effect of particle size, clustering, and cluster distribution. Optical micrographs of carbon nanotube modified epoxy with different states of dispersion were also employed to assess the applicability and functionality of the algorithm to real micrographs.

Dispersion stability in carbon nanotube modified polymers and its effect on the fracture toughness.

In this paper, the dispersion stability of multiwall carbon nanotubes (MWNTs) mixed with an epoxy resin is studied. An instrumented optical microscope with a hot stage was used to study the evolution of the carbon nanotubes (CNTs) dispersion during the cure of the resin. A new image processing approach is then introduced to quantify dispersion and identify the source of dispersion degradation during the cure. The results showed that the reduction of the resin viscosity at temperatures greater than 100 °C caused an irreversible re-agglomeration of the CNTs in the matrix. It was shown that the fine-tuning of the ratio and type of curing agent as well as the curing temperature directly affect the dispersion stability of MWNTs in the epoxy polymer. The dispersion quality was then directly correlated to the fracture toughness of the modified resin and a maximum of 20% improvement was achieved.

An interaction stress analysis of nanoscale elastic asperity contacts.

A new contact mechanics model is presented and experimentally examined at the nanoscale. The current work addresses the well-established field of contact mechanics, but at the nanoscale where interaction stresses seem to be effective. The new model combines the classic Hertz theory with the new interaction stress concept to provide the stress field in contact bodies with adhesion. Hence, it benefits from the simplicity of non-adhesive models, while offering the same applicability as more complicated models. In order to examine the model, a set of atomic force microscopy experiments were performed on substrates made from single-walled carbon nanotube buckypaper. The stress field in the substrate was obtained by superposition of the Hertzian stress field and the interaction stress field, and then compared to other contact models. Finally, the effect of indentation depth on the stress field was studied for the interaction model as well as for the Hertz, Derjaguin-Muller-Toporov, and Johnson-Kendall-Roberts models. Thus, the amount of error introduced by using the Hertz theory to model contacts with adhesion was found for different indentation depths. It was observed that in the absence of interaction stress data, the Hertz theory predictions led to smaller errors compared to other contact-with-adhesion models.

Interaction stresses in carbon nanotube-polymer nanocomposites.

A new technique of atomic force microscopy interaction measurement is used to obtain the three-dimensional stress field in nanocomposites made of single-walled carbon nanotubes (SWNT) and poly(methyl methacrylate) (PMMA) matrix. This original approach expands the current capability of AFM from imaging and force mapping to three-dimensional stress field measurements. Latest developments in the field have been limited to three-dimensional imaging at the surface only, and one value (adhesion) force mapping. The current work provides the interaction stress results for a PMMA-SWNT nanocomposite and shows a maximum estimated load transfer of less than 7 MPa (the maximum attraction stress estimated). This value is obtained for an unfunctionalized nanocomposite and hence the interaction stress is mainly based on van der Waals interactions. This means that for this system, carbon nanotubes behave similar to an elastic-fully plastic material with a yield stress of less than 7 MPa. This phenomenon illustrates why carbon nanotubes may not show their strong mechanical properties (yield strength of above 10 GPa) in polymeric nanocomposites.

Toughening of epoxy matrices with reduced single-walled carbon nanotubes.

Reduced single-walled carbon nanotubes (r-SWCNT) are shown to react readily at room temperature under inert atmosphere conditions with epoxide moieties, such as those in triglycidyl p-amino phenol (TGAP), to produce a soft covalently bonded interface around the SWCNT. The soft interface is compatible with the SWCNT-free cross-linked cured matrix and acts as a toughener for the composite. Incorporation of 0.2 wt % r-SWCNT enhances the ultimate tensile strength, toughness and fracture toughness by 32, 118, and 40%, respectively, without change in modulus. A toughening rate (dK(IC)/dwt(f)) of 200 MPa m(0.5) is obtained. The toughening mechanism is elucidated through dynamic mechanical analyses, Raman spectroscopy and imaging, and stress-strain curve analyses. The method is scalable and applicable to epoxy resins and systems used commercially.