Thermo-responsive metallo-supramolecular gels based on terpyridine end-functionalized amphiphilic diblock copolymers
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Thermo-responsive metallo-supramolecular gels based on terpyridine end-functionalized amphiphilic diblock copolymers
Jérémy Brassinne, Charles-André Fustin* and Jean-François Gohy* Institute of Condensed Matter and Nanosciences (IMCN), Bio and Soft Matter (BSMA), Université catholique de Louvain (UCL), Place L. Pasteur 1, 1348 Louvain-la-Neuve, Belgium. E-mail: [email protected], [email protected] Tel./Fax: +32-10-479269.
ABSTRACT A thermo-responsive hydrogel was prepared on the basis of terpyridine endfunctionalized polystyrene-block-poly(N-isopropylacrylamide) diblock copolymer. As a first level of assembly, the copolymer was dissolved in a selective solvent to yield micelles bearing terpyridine ligands at the extremity of the coronal chains. The second level of self-assembly was triggered upon addition of metal ions to the micellar solution. Mechanical properties of the accordingly obtained micellar gel were finally characterized by rotational rheometry, below and above the lower critical solution temperature. INTRODUCTION Today, smart polymer gels are one of the most studied classes of polymeric materials [1, 2]. Indeed, they undergo physico-chemical changes in response to variations in environmental conditions, which in turn affects their mechanical properties. Stimuli can be of chemical or physical nature. Some complex polymeric architecture can even be characterized by two or more responsive properties, while different signals can be applied in order to induce responses in the so-called multi-responsive systems [3, 4]. In practice, multi-responsive hydrogels can find applications in tissue engineering [5], delivery of therapeutics [6], sensors [7] or actuators [8]. A classical way to impart adaptive properties to polymeric architectures consists in introducing stimuli-sensitive polymer sequences. In this respect, temperature-responsive poly(Nisopropylacrylamide) (PNIPAAm) constitutes a widely used polymer that exhibits a lower critical solution temperature (LCST) at about 32°C [9]. As a consequence, hydrogels incorporating PNIPAAm sequences in their architecture undergo a shrinking-type volume phase transition in response to heat [10]. In addition, those hydrogels are also known to be sensitive to the presence of salts [11] or surfactant [12]. Another conventional strategy takes advantage of secondary intermolecular interactions to impart responsive properties into macromolecular architectures. Indeed, those interactions are generally weaker than covalent bonds but kinetically more labile and dynamically more flexible [13], which bestows adaptable and self-restructuring properties to non-covalent assemblies [14]. In this context, supramolecular chemistry [15] offers a straightforward approach for the construction of highly complex and dynamic structures by assembling small building blocks, over one or more levels of assembly [16, 17]. Among them, polymer gels, where cross-links are
provided by hydrogen bonding [18], hydrophobic effect [19], π-π stacking [20], metal–ligand coo
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