The BSMA division is active in the fields of stimuli-responsive macro- and supramolecular systems. The expertise covers many aspects such as synthesis, characterization, self-assembly in solutions and in solid state, and modelling.
Mechanically linked polymers
Group of Charles-André Fustin
The mechanical bond presents in catenanes and rotaxanes allows for controlled motions of large amplitude, and for the positioning of one of the interlocked component with respect to the other. These exceptional properties have made of catenanes and rotaxanes ideal candidates for devising prototypical molecular machines capable of performing well-defined tasks in response to various stimuli, or mechanically-linked polymers with peculiar (mechanical) properties.
In this context we are developing synthetic strategies toward new polymeric materials that contain mechanical links as junction between polymer chains. Examples of materials include “articulated” block copolymers, i.e. containg a single mechanical link at the junction of the two blocks, and slide-ring gels. They are studied under different aspects (self-assembly, mechanical and rheological properties,...) and at different scales, from the single molecule level to collective properties in solution and in bulk.
Stimuli responsive and self-healing supramolecular materials
Dynamics of supramolecular polymeric assemblies
Group of Charles-André Fustin, Evelyne van Ruymbeke, Jean-François Gohy
Supramolecular polymeric assemblies represent a very promising class of soft matter. Created from the association of macromolecules via non-permanent bonding, they exhibit reversible transitions between rubber-like and liquid-like properties. Their uniqueness derives from the fact that their characteristic relaxation times depend on many tunable parameters (such as the strength of the transient bonds, the temperature, or the building blocks architecture), which can change their dynamics by several orders of magnitude.
Our objective is to understand and control the structure and dynamics of supramolecular polymeric assemblies based on well-defined macromolecular building blocks, mainly linear and star-like polymers of various lengths and their blends, by combining experimental and modelling approaches.
In particular, we aim at understanding the combined effect on the dynamics of the associating groups (which depends on the strength, position, and nature of the bonds) and the internal dynamics of the building blocks (which depend on their capability to entangle and on their architecture), with a special focus on the rheology of supramolecular polymers based on metal-ligand interaction.
Storage and Loss moduli of supramolecular star PEO polymers based on metal-ligand interactions (Mbranch=10kg/mol.). a) Formation of a transient network; b) Influence of the strength of the transient bonds, which depends on the metal ions, c) Influence of the polymer concentration, using ethylene glycol.
Synthesis and characterization of supramolecular materials
Group of Charles-André Fustin, Jean-François Gohy, Evelyne Van Ruymbeke
New materials that have the ability to reversibly adapt to changes in their environment and possess properties such as self-healing, thixotropy,… are highly sought after. A promising way to design such materials is to exploit supramolecular interactions which are dynamic and reversible by nature.
This research topic deals with the development of highly tunable materials by combining polymers with supramolecular interactions, and in particular coordinative metal-ligand bonds.
The characteristics of such systems are then studied to be able to further design smart materials with desired (rheological) properties. The team has expertise in the synthesis and rheology of functionalized (co)polymers bearing ligands at selected and well-defined locations. The synthesis can be performed by direct polymerization of functional monomers by controlled radical polymerization methods (RAFT, ATRP, NMP) or by post-functionalization of the polymer. Addition of metal ions to these polymers reversibly links the polymer chains in different ways depending on the polymer architecture and location of the ligands, leading to networks, chain extension,… In depth rheological characterizations of the obtained materials are performed, in shear or extensional flow. Specific properties such as aging, thixotropy, or shear thickening are investigated. The response of these materials to various stimuli (temperature, pH, mechanical stress, competitive binders,…) is also studied.
As an example of realization, stimuli responsive gels have been obtained by linking together block copolymer micelles, decorated with terpyridine ligands, by metal-ligand bonds. These gels exhibit a rapid self-healing ability, recovering their initial rheological properties in a few seconds only.
Synthesis of redox copolymers for organic radical batteries
Group of Jean-François Gohy
Organic radical batteries (ORBs) have attracted a great interest over the past decade. Indeed, ORBs, due to the absence of environmentally unfriendly metals, are a cheap metal-free energy storage alternative to Li-ion batteries. Moreover ORBs offer several advantages over inorganic materials such as light weight, flexibility and unlimited organic resources. Due to the reversible and fast oxydo/reduction of stable organic radicals, ORBs allow a fast charge/discharge process, which might be a prerequisite for some applications.
In this context, we focus on poly(2,2,6,6- tetramethylpiperidinyloxy-4-yl-methacrylate) (PTMA). We have synthesized well-defined PTMA containing block copolymers. Such block copolymers allow to overcome the problem of the dissolution of PTMA in the electrolyte used in Li-ion batteries. Moreover the self-assembling properties of those block copolymers allow the development of new nanostructured cathodic materials for ORBs. We have also demonstrated the possibility to generate electrochemically active self-assembled micellar structures from PTMA containing copolymers dissolved in a typical battery electrolyte.
By combining lithium iron phosphate (LiFePO4), a high-energy density electrode, with PTMA, a high-power density redox capacitor, we construct a high performance hybridized Li-ion battery electrode. During charging, there is generation and co-existence of higher redox potential species (PTMA) in the oxidized form with lower redox potential species (LiFePO4) in the reduced form. When relaxed, an internal charge transfer process equilibrates the redox state of the hybridized species leading primarily to charging of LiFePO4. This new hybridization scheme allows for 90% state-of-charge in the hybrid battery within a five minutes time window of current pulse and relaxation sequences.
Synthesis and self-assembly of stimuli responsive block copolymers
Group of Charles-André Fustin, Jean-François Gohy
Responsive polymers attract an ever-increasing interest due to their ability to change their properties in response to a small change of their environment. Polymers may be devised to respond to many different stimuli such as pH, temperature, light, redox, ionic strength,… In addition, block copolymers can self-assemble in solution or in the bulk to form well-defined nanostructures.
Different aspects are covered by the research, from the synthesis to the self-assembly and characterization. The team has expertise in the synthesis of block copolymers by controlled radical polymerization methods (RAFT, ATRP, NMP) as well as in post-functionalization of polymers. The self-assembly of these block polymers is studied both in solution (formation of micelles) and in thin films.
As examples of realisations, block copolymers bearing a cleavable junction between the two blocks have been synthesized and used as precursors for the preparation of functionalized nanoporous thin films. A photocleavable moiety, a metal-ligand complex, and an ionic bond have been used as junctions. In other examples, light has been used to induce the micellization of block copolymers or the disruption of preformed micelles, accompanied by the encapsulation or release of a cargo, respectively.
Modelling the dynamics of complex macromolecules
Modelling the dynamics of supramolecular systems
Group of Charles-André Fustin, Evelyne van Ruymbeke, Jean-François Gohy
Our objective is to study and model the dynamics of transient polymeric networks in which both disentanglement process association/disassociation dynamics of the supramolecular junctions play a role. In particular, we would like to investigate and model the influence of the building block architecture.
In this direction, we developed a general algorithm for predicting the linear rheology of telechelic star and linear entangled polymer melts. We are now studying the tunability of other (semi-) telechelic linear and star systems such as functionalized PEO chains able to create metal-ligand associations.
a) example of the telechelic star and linear polymers with zwitterions at the branches extremities. b) Storage and loss moduli for a star Polyisoprene melt composed of three branches of Mw=20kg/mol: experimental data with (o) or without (square) telechelic bonds, theoretical predictions (--) obtained by using the model I have developed.
Our second objective is to include in our tube model the influence of sticky groups along the chain backbone, which delay the relaxation of the chains.
Tube-based models to predict the rheology of complex macromolecules
Group of Evelyne van Ruymbeke
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.