Polymer -based engineered materials have become ubiquitous in major application fields not only because of the endless tailoring and modification possibilities offered by polymer themselves but also because synergistic combinations of properties can often be achieved through clever structuring, e.g. nano composites or hierarchical structures. Recent research at IMCN in these directions includes the development of mesoscopic models to determine the flow properties of complex macromolecules, the new routes to improve the toughness-stiffness balance of structural composite materials or coatings and the nano composites with hierarchical organization to control the absorption of electromagnetic radiation in the microwave range.
Nanocomposites and hybrid structures
Carbon nanofillers in polymer nanocomposites
Group of Christian Bailly, Arnaud Delcorte, Sophie Hermans, Isabelle Huynen (ICTeam)
Carbon-based nanofillers are very attractive for controlling functional properties of polymers, in particular electrical and thermal conductivity. Our research focuses on manipulating the dispersion and orientation of multiwall carbon nanotubes (MWNT), graphene and graphite nano-platelets in thermoplastic matrices.
In particular have shown how to trap unfunctionalized MWNT at the interfaces of phase-separated polymer mixtures. Stable interfacial confinement can be understood, based on interfacial thermodynamics and irreversible adsorption on the nanotubes. Interfacial confinement can be manipulated to yield a so-called triple percolation to maximize MWNT effectiveness for electrical conductivity. The confined MWNT can further be used to prevent coalescence of phase-separated blends by the “Pickering emulsion” mechanism. We have shown that selective adsorption of a polymer on MWNT can modify their crystallization nucleation properties, which in turn has a strong influence on the resulting conductivity of the nanocomposite. Recently, we have studied polymer films containing parallel graphite nanoplatelets, produced by squeezing the corresponding nanocomposites in the melt. Orientation of the conductive fillers is observed in the plane of the film. The samples show a tunable resonant absorption at a given frequency in the microwave region.
Left: TEM micrograph of a Polyamide 6 - Ethylene Acrylate copolymer blend showing interfacial confinement of multiwall carbon nanotubes.
Right SEM micrograph showing orientation by confinement of graphite nanoplatelets in polycarbonate; this structure shows resonant aborption at microwave frequencies
Hybrid superhydrophobic layers by LbL assembly
Group of Karine Glinel, Alain Jonas, Bernard Nysten
We are developing superhydrophobic, self-cleaning hybrid layers of improved transparency for application in glass technology (solar panels, mirrors, etc.). Our methodology is based on spray-assisted layer-by-layer assembly, using specific polymers and silica colloids, resulting in the fabrication of sturdy composite layers which can be deposited on virtually any surface. The process allows for the rational construction of composite layers with a vertically-tunable composition.
When a droplet of water impinges on such properly-designed surfaces, it rebounds as shown below, or rolls off, taking away dirt accumulated on the surface. We are investigating these fascinating properties by contact angle goniometry, rebound experiments performed with an ultrafast camera, and measurements of roll-off angles.
The project is led in collaboration with partners active in the field of polymer synthesis (C. Detrembleur, ULiège), optical properties of surfaces (O. Deparis, UNamur) and spraying (B. Kartheuser, CerTech, Seneffe), and also involves selected industrial partners.
Polymers for high-performance lithium batteries
Group of Jean-François Gohy
High specific energy, high power density, long cycle life, low cost and safer systems are required for next generation of Li-ion batteries with numerous targeted applications such as powering of electric vehicles and nomad devices. Current Li-ion batteries have high energy density but they suffer from low power density and relatively slow charge/discharge rates. Another concern is the requirement for flexible and/or miniaturized Li-ion batteries that could be easily handled and fabricated directly on various substrates. In this framework, we are developing new materials for the three basic compartments of a Li-ion battery: the anode, the cathode and the electrolyte separator.
As anode architecture, we develop different types of flexible materials based on various nanowire architectures.
Efforts are also devoted to the development of novel cathode materials based on organic stable radical polymers. We develop block copolymers containing a poly (2,2,6,6- tetramethylpiperidinyloxy-4-yl-methacrylate) (PTMA) block. PTMA displays rapid electron transfer kinetics resulting in the high power capability of these materials. By blending PTMA to LiFePO, we have constructed Li-ion batteries that can be fully recharged in a timeframe of a few minutes.
Novel types of electrolytes based on block copolymers are developed. The nanostructuration process of those systems leads to materials with good mechanical properties and ionic conductivities.
Finally, we develop original formulations to obtain paintable batteries suitable for a variety of supports.
Toughening of high performance epoxy composites by thermoplastic films and fibres
Group of Christian Bailly, Thomas Pardoen (IMMC)
Composite materials based on thermoset matrices and carbon fibers (CFRC) are by now widely used in aerospace applications because of their high specific stiffness and strength properties. However, highly cross-linked networks have poor fracture toughness and impact resistance in comparison to metal structures. These deficiencies can in principle be corrected by combining thermoplastics and/or nanofillers with the resin. However, direct mixing of the thermoset precursors with such additives makes processing of the CFRC by modern “out of autoclave” methods, including Resin Transfer Molding (RTM), very problematic.
Our group focuses on original strategies for combining epoxy precursors with thermoplastics and nanofillers, relying on the interdiffusion between the uncured resin and thermoplastic (nanocomposite) films positioned at strategic locations between the carbon fabrics.
In particular we have compared the interdiffusion of thermoplastics with contrasting Tg and compatibility, i.e. poly(ether sulfone) (PES) and phenoxy, in epoxy resin precursors processed by RTM, and analyzed the resulting morphologies after curing and the delamination toughness of the CFRC. The dramatic improvement found for the phenoxy-based system as compared to the pure thermoset reference can be ascribed to the broad morphology gradient only observed for phenoxy. We have also demonstrated that thermoplastic nanocomposite films can be used as carriers of nanofillers in RTM-processed CFRC. The proof of concept is extended to CFRC panels where nanocomposite phenoxy films are prepositioned between every odd carbon layer of the preform. Significant improvements of fracture toughness are found for the pure phenoxy as well as the nanofilled cases, especially with nanoclay.
Images from hot stage microscopy of a PES filament (initial diameter of 100μm) in RTM6 after isothermal treatment at (a) 100°C, (b) 120°C, (c) 140°C until gel time
Thermoplastic-thermoset interfaces : application to welding and composites surface modification
Group of Christian Bailly, Thomas Pardoen (IMMC)
The progressive replacement of metallic parts by composites in aerospace applications is generating new technological and related scientific challenges. Among them, increasing and understanding the reliability of thermoset composite joints is essential. Indeed, the methods currently used to join thermoset composite parts are directly derived from those dedicated to metals and hence not optimal for composites. A promising alternative to mechanical fastening methods, called Thermoset Composite Welding (see Figure), is receiving growing attention.
This method consists in thermally welding two thermoset composite parts together by partial fusion (step 2) of two thermoplastic layers previously placed at their surface during a preliminary co-curing step (step 1). Two key aspects are studied in our group. One is understanding the morphology developed at the thermoplastic/thermoset interface after reaction induced phase separation. This is being studied experimentally as well as by phase field simulations. Another key aspect is the damage resistance of the interfaces. We are building an in-depth understanding of the relation between the complex microstructure gradient in the interdiffusion zone and the corresponding damage mechanisms. A comparison between poly ethersulfone (PES)-epoxy and poly etherimide (PEI)-epoxy interfaces at given curing cycle and film thickness shows that the initiation toughness for PEI-epoxy interfaces is approximately 2 times higher than the value for PES-epoxy interfaces and 5 times higher than for pure epoxy. This observation can be directly linked to the longer interdiffusion lengths in the PEI case.
Concept of thermoset composite welding under study FIGURE 0 COMPETER
Development of models for relating Rheology and composition of polymeric systems
Modelling the dynamics of polymeric systems
From the comparison between theoretical predictions and experimental results on well-defined macromolecular systems, we are developing a general coarse-grained model, based on the tube concept, for describing the linear rheology of prototype macromolecular (linear and branched) architectures with increased complexity. The final aim is to predict the rheology of randomly branched polydisperse polymers.
In this picture, an entangled chain is confined in the melt by a topological constraining field, called the tube, which captures the effect of the molecular environment on its motions, and which becomes oriented when a deformation is applied. The tube diameter, which can increase upon relaxation, reflects the strength of the topological constraints on a chain (entanglements). In order to relax, the chains must free themselves from their initial tube, with the help of interrelated relaxation mechanisms called reptation, contour length fluctuations and constraint release. The elegance and efficiency of tube models arise from the universal mesoscale description of the chain, which avoids chemical details but is sensitive to macromolecular architecture, while using only a few material parameters.
We are now extending our tube model to nonlinear rheology, with the objective to further understand how stretch, strain hardening and spinnability of a polymer are related.
Development of statistical approaches for relating synthesis recipe and chains architecture
Group of Evelyne van Ruymbeke
For modeling the dynamics of polydisperse branched macromolecules, a detailed understanding of their architecture is essential. In this direction, we are developing statistical approaches, based on Monte Carlo assumptions, in order to get a detailed description of the molecules architecture, based on “average” information such as the branching density, average molecular weight or the synthesis chemistry.
This approach is particularly suitable for describing the statistical composition of polycondensate polymers. As an example, we apply it to predict the structure and rheology of randomly branched polyamide melts, on the basis of their initial reaction recipe.
We are also developing Monte Carlo algorithms in order to understand the statistical composition of any complex branched polymers obtained by the coupling of well-defined linear parent polymers. These algorithms allow us to determine the molar mass distribution of any possible architecture and in which proportion it is present in the final sample.
Combining rheology, statistical tool and tube-based model as a new characterization technique
Group of Evelyne van Ruymbeke
The combination of size exclusion chromatography, models based on the tube theory and rheological data represents a very powerful tool to point out the presence of low proportion of long chain branching in a polymer melt. We are therefore using these techniques in order to further characterize polymeric samples susceptible of containing few branches, which cannot be detected by more classical methods.
We also use tube models for well-defined branched polymeric systems, in order to detect the probable presence of unavoidable side-products (different architectures), which cannot be seen by standard methods, e.g., SEC analysis. To do so, we combine several levels of characterization: statistical tools are used in order to relate the information available from the synthesis protocols of a sample and its experimental molar mass distribution (obtained by temperature gradient interaction chromatography), while tube models are used to relate the statistical composition to the rheological behavior of the sample. By confronting the different results, we can therefore propose a detailed analysis of the sample composition.
In specific cases such as for linear polymers, we also use tube models in the inverse way, i.e. starting from the rheological data and predicting their molar mass distribution.