My mission is to create innovative hybrid thin films for the design of sustainable materials through the surface engineering of polymer and glass substrates. Atmospheric plasma deposition (APD) is a versatile and straightforward technology, which enables to deposit – at room temperature and atmospheric pressure – various type of materials in a one-step process without the use of solvent or expensive vacuum systems.
In my research, I strive to develop innovative APD processes to synthesize multifunctional hybrid and nanocomposite thin films with optical and mechanical tunable properties to deliver advanced solutions for high-performance materials.
One of the main thrust in my current research is to provide multifunctional protective coatings on plastic to create lightweight polymer materials that will be stronger, lighter and tougher along with a better durability in order to replace conventional glass windows and metal frames for automotive or aeronautic industries. This will enable to save energy by reducing vehicle weight, improving aerodynamic design, as well as reducing greenhouse gas emissions or protect plastic that are exposed for very long period of time to solar radiation.
Previously, my work endeavored to promote thermal protective coatings to polycarbonate (PC) and polyamide-6 (PA-6) substrates for flame retardancy purposes. A first approach (Figure 1) consisted to elaborate siloxane-based coatings with embedded dopant-rich polyphosphates by APD. Deposition conditions were tailored to elaborate various thin films that can match the fire performance requirements. This approach enabled to deposit flame retardant coatings onto different polymer substrates, providing a versatile fireproofing solution for different nature of materials. The presence of an expanded charred layer at the surface (Figure 1b) acted as a protective barrier limiting heat and mass transfer. This latter retained and consumed a part of the PC or PA-6 degradation by-products and then minimized the released flammable gases. It also insulated the substrate from the flame and limited mass transfers of remaining volatile gases. Moreover, reactions in the condensed phase were also highlighted despite the relatively thin thickness of the deposited layers. As a result of these phenomena, excellent performances were obtained (Figure 1c), illustrated by a decrease of the peak of heat release rate (pHRR) and an increase of the time to ignition (TTI).
Figure 1. (a) Sketch of the atmospheric plasma deposition system. (b) Drawing representing a coated sample exposed to a radiant heater (i.e. cone calorimeter) before (top) and after (bottom) the film reacts with the substrate to form a carbonaceous layer. (c) Heat release rate (HRR, associated to the flammability of a material) of different coated polycarbonate and polyamide-6 in function of exposure time to an external heat flux of 50 and 35 kW·m2, respectively. Bare substrates are represented by the black curves, siloxane-coated substrate by the red curve (bottom) and the codeposits-coated (i.e. siloxane- and phosphate-containing films) substrates by the red and blue curves (top) and the blue curve (bottom).
A second approach involved the use of a nanopulsed plasma discharge to initiate free-radical polymerization at atmospheric pressure for the deposition of polymeric layers on cellulosic textile. Unipolar square pulses were employed to polymerize diethylallylphosphate (DEAP) via free-radical polymerization through a novel method: Atmospheric Pressure Plasma-initiated Chemical Vapor Deposition (AP-PiCVD, Figure 2). Whilst ultrashort high voltage pulses created large quantities of energetic electrons, ions, radicals and metastables – which can initiate the free-radical polymerization of the monomer reactive functional group, the long time-off periods enabled the propagation of the monomer free radicals to form a regular polymer (poly(diethylallylphosphate), PDEAP). This method, highly suitable for the treatment of natural biopolymer substrates was carried out on cotton textile to perform the deposition of an efficient and conformal protective coating. The surface of the treated cellulosic fabric was smooth, uniform and did not exhibit any stacking of polymer in between and above the fibers (Figure 3g), allowing the breathability of the fabric. The elaboration of a catalytic coating, which promoted the development of a char was achieved (Figure 3f). After burning, the structure of the fabric was perfectly retained (Figure 3h) even if the fibers were totally hollow (Figure 3i), confirming the presence of a homogeneous thin protective film on the surface of the fibers that, by reaction with the cellulose during pyrolysis, promoted the formation of a char.
Figure 2. Trace of the voltage pulse of an ultrashort square pulsed dielectric barrier discharge and its corresponding steps for the free-radical polymerization of the diethylallylphosphate (DEAP) monomer. Initiation occurs during the nanosecond pulse, whilst the propagation step appears during the time the plasma is not ignited (i.e. time-off period)
Figure 3. Photographs of (a) the pure cotton before the flammability testing, (b) after 5s of burning, (c) after 60s of burning, (d) after 120s of burning, (e) the pure cotton after the flammability testing and (f) the PDEAP-treated cotton after the flammability testing and its char residue. SEM pictures of PDEAP-treated cotton (g) before and (h) after the flammability testing. Picture (i) is a magnification of (h).
I also conduct research in metal oxide thin film processing, engineering and analysis for the development of transparent semiconductors and electro-active materials. My work focuses on the development of innovative synthesis methods based on atmospheric plasma deposition to achieve advanced optoelectronic functions which can ultimately be used in photonic systems.
My past achievements in that field include the synthesis of new colorimetric gas sensing surfaces containing metalloporphyrins. The research led to the design, synthesis and employment of novel dyes for incorporation into an atmospheric plasma-deposited layer suitable for food packaging. These intelligent foils detect spoiled food via colorimetric signal of the embedded pigment upon its interaction with volatile amines formed by bacterial decomposition of amino acids. In this work, different metalloporphyrins were successfully embedded in an organosilicon matrix by atmospheric plasma deposition. The integrity of the metalloporphyrins, followed by UV-visible spectroscopy, was successfully preserved and their aggregation prevented. The dyes were immobilized in a polymer matrix thanks to their simultaneous injection in the plasma discharge with the matrix precursor in a suitable solvent. Scanning electron microscopy (SEM) pictures (Figure 4a and b) did reveal the formation of mesoporous structures, which is of particular interest for gas sensing as accessible pore sizes and high surface areas are desired. Exposure to triethylamine, which reached the metalloprohpyrins through the pores of the organosilicate membrane, led to a shift in the absorption spectrum and confirms the gas sensing potential of such coatings (Figure 4d). The plasma deposition method was an effective route towards the immobilization of non-aggregated metalloporphyrins in a mesoporous structure and a useful alternative to classical wet methods employed to prepare such coatings. This process led to the formation of colorimetric gas surfaces. A good permeation kinetic of the analyte through the bulk of the film and the complexation of the analyte to the central metallic ion of the porphyrin was confirmed by gas detection experiments.
Figure 4. Scanning electron micrographs of the plasma polymerized CrIIICl(TPP)(H2O)-based coating at (a) intermediate magnification (× 100,000) and (b) high magnification (× 350,000). (c) Molecular structure of the CrIIICl(TPP)(H2O) metalloporhpyrin.(d) UV-vis absorption spectra of the as-deposited plasma polymerized CrIIICl(TPP)(H2O)-based coating (450 nm, black curve) and plasma polymerized CrIIICl(TPP)(H2O)-based coating exposed to a saturated triethylamine atmosphere (445 nm, blue curve).