Smart interfaces with thermo-reversible properties – INTHERMO

To design smart interfaces with thermoreversible properties, thermally-reversible Diels-Alder chemistry has been chosen. This chemistry is a good candidate to reach such properties since it exhibits dynamic covalent bonding under certain conditions and is also classified among click chemistry reactions, being efficient and selective. The fabrication of the smart surfaces was assisted by plasma polymerization which enables the synthesis of functional polymer films on a wide variety of substrates (various natures of substrates including thermosensitive materials), being thus considered as a universal surface modification process. To get the Diels-Alder reactive coating, it was planned to fabricate of a functional polymer thin film, followed by the subsequent grafting of Diels-Alder reactive groups on the plasma polymer. The appropriate choice or synthesis of original Diels-Alder reactive compounds was motivated by the two main objectives of this project, namely the understanding of the interfacial Diels-Alder reaction and also the fabrication of prototypes to illustrate the concept of thermoreversible strong adhesion.

A thermodynamics methodology has been developed to characterize thoroughly thermoreversible reactions on coatings fabricated via a green, universal process. The reversibility of this interfacial reaction under mild conditions has been successfully achieved, providing a proof of concept of robust, reversible, strong adhesion. It has also enabled to build an international cluster of researchers and students working on thermoreversible strong adhesion, who frequently discussed together, met for a workshop, published together and also submitted a proposal for future financial support on this topic.

In addition to the methodology developed to understand and control interfacial DA reaction, this project provided the proof of concept of a possible strong but reversible adhesion, in particular between two macroscopic materials, via the presence of nanometric thin films engineered at the interface. The nanometric dimension of these thin films and the universality of the deposition process make these systems original and really innovative (compared to the state-of -the-art and the existing commercial solutions).
The development of this concept at an industrial scale however needs additional improvements, including an increase in the adhesion force. This requires to advance the understanding on adhesion mechanisms within these systems at the nanometric scale and to optimize the chemical but also physical contributions of the thin films in the adhesion process.

This project led to 3 publications already published in international journals (Advanced Functional Materials, Langmuir, The Journal of Physical Chemistry, Part C) and 3 publications that will be submitted in the coming months, as well as to 15 communications (3 as posters), including 7 oral communications in international conferences, 1 being invited and 1 receiving the best communication price. Discussions with the SATT Connectus have been initiated for 2 patent applications that finally did not succeed.

Stimuli-responsive materials have emerged but their industrial applications remain limited. The whole composition of the system is usually specifically formulated to react to environmental conditions although many phenomena locally occur at the surface of the material. This strategy is thus economically non-viable because only few percents of the material volume are exploited for their smart properties. Consequently, industrial renewal can be stimulated by the fabrication of stimuli-responsive coatings that will cover the material, preserving the characteristics of the bulk material and limiting the cost of these additional smart properties while modifying its sensitivity to the surrounding environment. Two challenges appear: i) the development of efficient interfacial dynamic systems, controlled by an external stimulus, and ii) the incorporation of these systems in a ‘universal’ surface modification process.

In that context, the INTHERMO project aims at designing smart interfaces with switchable properties, in particular thermo-reversible properties, via a substrate-independent process. These surfaces will react through thermally-reversible Diels-Alder (DA) chemistry. This chemistry is a good candidate to reach such properties since it exhibits dynamic covalent bonding under certain conditions and has also been classified among click chemistry reactions, being efficient and selective. In addition, the elaboration of such surfaces will be assisted by plasma polymerization which enables the fabrication of functional polymer films on a wide variety of substrates (various natures of substrates, including thermo-sensitive materials, various shapes), thus using a universal surface modification process. Grafting of interfacial DA compounds on these reactive coatings will bring the required properties to the polymer film and enable the production of smart interfaces with thermo-reversible properties.

The INTHERMO project develops the concept of thermo-reversible covalent bonding on surfaces by i) thoroughly investigating interfacial DA chemistry to elucidate the dependence of the plasma polymer properties on the surface reactivity and ii) developing new DA couples for interfacial thermo-reversible chemistry, reacting under mild conditions (relatively low temperatures for direct and reverse DA reactions, reactions in water). The results of these works will lead to the fabrication of samples illustrating the concept of original interfacial DA chemistry for two precise industrial applications. First, this concept can be applied to reversible covalent adhesion of materials to consider easy replacement of damaged pieces (for instance in composite materials or microelectronics). Covalent assembly of materials at low temperature (via direct DA reaction) and disassembly of elements at a higher temperature, that doesn’t degrade many common materials (via reverse DA reaction), is a great challenge for recyclability of complex system assemblies. Secondly, thermo-reversible DA reaction confined in thin, functional polymer coatings can be used for thermally-controlled immobilization and release of (bio)molecules at/from a substrate. This strategy can find interest for the separation of (bio)molecules from a complex medium while addressing an efficient regeneration of the substrate after use.

This 36-month research project, coordinated by a Young Researcher working at the Institute of Materials Science of Mulhouse (IS2M), will target a unique combination of original interfacial DA chemistry, operating under mild conditions, with a substrate-independent process based on plasma polymerization, so that smart interfaces become a reality for specific industrial applications.