Farid Khelifa

Free-Radical-Induced Grafting from Plasma Polymer Surfaces

With the advances in science and engineering in the second part of the 20th century, emerging plasma-based technologies continuously find increasing applications in the domain of polymer chemistry, among others. Plasma technologies are predominantly used in two different ways: for the treatment of polymer substrates by a reactive or inert gas aiming at a specific surface functionalization or for the synthesis of a plasma polymer with a unique set of properties from an organic or mixed organic-inorganic precursor. Plasma polymer films (PPFs), often deposited by plasma-enhanced chemical vapor deposition (PECVD), currently attract a great deal of attention. Such films are widely used in various fields for the coating of solid substrates, including membranes, semiconductors, metals, textiles, and polymers, because of a combination of interesting properties such as excellent adhesion, highly cross-linked structures, and the possibility of tuning properties by simply varying the precursor and/or the synthesis parameters. Among the many appealing features of plasma-synthesized and -treated polymers, a highly reactive surface, rich in free radicals arising from deposition/treatment specifics, offers a particular advantage. When handled carefully, these reactive free radicals open doors to the controllable surface functionalization of materials without affecting their bulk properties. The goal of this review is to illustrate the increasing application of plasma-based technologies for tuning the surface properties of polymers, principally through free-radical chemistry.

Free radical generation and concentration in a plasma polymer: the effect of aromaticity.

Plasma polymer films (PPF) have increasing applications in many fields due to the unique combination of properties of this class of materials. Among notable features arising from the specifics of plasma polymerization synthesis, a high surface reactivity can be advantageously used when exploited carefully. It is related to the presence of free radicals generated during the deposition process through manifold molecular bond scissions in the energetic plasma environment. In ambient atmosphere, these radicals undergo autoxidation reactions resulting in undesired polymer aging. However, when the reactivity of surface radicals is preserved and they are put in direct contact with a chemical group of interest, a specific surface functionalization or grafting of polymeric chains can be achieved. Therefore, the control of the surface free radical density of a plasma polymer is crucially important for a successful grafting. The present investigation focuses on the influence of the hydrocarbon precursor type, aromatic vs aliphatic, on the generation and concentration of free radicals on the surface of the PPF. Benzene and cyclohexane were chosen as model precursors. First, in situ FTIR analysis of the plasma phase supplemented by density functional theory calculations allowed the main fragmentation routes of precursor molecules in the discharge to be identified as a function of energy input. Using nitric oxide (NO) chemical labeling in combination with X-ray photoelectron spectroscopy analysis, a quantitative evaluation of concentration of surface free radicals as a function of input power has been assessed for both precursors. Different evolutions of the surface free radical density for the benzene- and cyclohexane-based PPF, namely, a continuous increase versus stabilization to a plateau, are attributed to different plasma polymerization mechanisms and resulting structures as illustrated by PPF characterization findings. The control of surface free radical density can be achieved through the stabilization of radicals due to the proximity of incorporated aromatic rings. Aging tests highlighted the inevitable random oxidation of plasma polymers upon exposure to air and the necessity of free radical preservation for a controlled surface functionalization.

Healing by the Joule effect of electrically conductive poly(ester-urethane)/carbon nanotube nanocomposites

Recent demands for polymers with autonomous self-healing properties are being constantly raised due to the need for high-performance and reliable materials. So far, the advances in this field are limited to the production of self-healing materials requiring a high energy input. Therefore there is an urgent need to develop self-healing polymer systems, in which healing can be easily and specifically induced by external stimuli for economical and viable applications. In the current work we demonstrate, for the first time to our knowledge, the possibility to heal local macroscopic damage by a confined temperature increase arising from the Joule effect. The damage healing is promoted by the resistance to an electrical current at the crack tip. This new concept is studied on thermo-reversible and electrically conductive poly(ester-urethane)/carbon nanotube nanocomposites derived from thermo-reversible Diels–Alder reactions between furfuryl- and maleimide-functionalized poly(e-caprolactone) (PCL)-based precursors. Electrically conductive materials are then obtained after incorporating multi-walled carbon nanotubes into the thermo-reversible networks using reactive extrusion. Under mild electrical conditions, temperature in the range of the retro-Diels–Alder reaction can be obtained near the damaged site. The obtained results reveal the potential of this new approach for healing materials locally while maintaining the overall material properties.

Derivatization of free radicals in an isopropanol plasma polymer film: the first step toward polymer grafting.

Plasma-polymerized films (PPF) synthesized by plasma-enhanced chemical vapor deposition (PECVD) find increasing applications in biomedicine and differ in many ways from conventional polymers. One of the most specific properties of the PPF is the high reactivity of its free-radical-rich surface, arising from the deposition mechanism. Although generally considered as a disadvantage leading to the aging of the PPF, reactivity of the plasma-treated polymers and PPF surfaces can be beneficially employed, for example, for grafting of a specific chemical functionality or short polymer chains. The quantitative evaluation of the surface radical density of the PPF is thus considered as the necessary preparatory step toward any subsequent grafting reaction. In the present study, the surface radical density of an isopropanol-based PPF was quantitatively determined by a combination of NO chemical derivatization and X-ray photoelectron spectroscopy (XPS). Once the derivatization conditions were optimized, the radical density, derived from at % N determined by XPS, was evaluated as a function of the deposition power. It was found out that the surface density of free radicals presents a maximum for the deposition power of 200 W (~2.3 × 10(14) spin/cm(2)) and it stabilizes (~2.1 × 10(14) spin/cm(2)) with further power increase. XPS findings were supported by in situ FTIR measurements that provided additional information about the degree of plasma fragmentation denoting fragmentation saturation for a deposition power of 200 W. By fitting the N1s peak it was possible to identify primary, secondary and ternary radicals and to study their respective evolutions with different deposition conditions. Angle-resolved XPS analysis allowed the in-depth distribution of radicals to be addressed, revealing that on the top surface, primary, and secondary radicals are dominating, whereas more tertiary radicals are present in the subsurface region. Finally, some preliminary chemical grafting experiments have allowed the relevance of derivatization results to be cross-checked.

Free radical-induced grafting from plasma polymers for the synthesis of thin barrier coatings

Plasma polymer films (PPF) are attracting a great deal of attention for application in various fields due to several remarkable properties, such as good adhesion to different substrates, improved mechanical/chemical stability and a high surface reactivity. This reactivity, associated with the presence of free radicals and originating from the PPF growth mechanism based on many fragmentation and recombination reactions, is often, however, a potential source of trouble. Oxidation of the PPF promptly begins in aerobic conditions via reactions of surface free radicals with oxygen molecules and causes a deterioration of its intrinsic properties in the surface region leaving a nonspecifically functionalized surface in the long-term. Recently a novel approach to functionalize plasma polymer films through the grafting reaction initiated from free radicals trapped on the PPF surface was developed. The present work investigates the potential to employ such an approach in a corrosion protection context. Characterization methods, including Electrochemical Impedance Spectroscopy (EIS) tests, demonstrate that the controlled consumption of surface free radicals via polymer grafting, instead of oxidation, has a beneficial effect on the corrosion protection behavior of the PPF layer deposited on clad 2024 aluminum alloy.

Use of free radicals on the surface of plasma polymer for the initiation of a polymerization reaction.

A novel approach to functionalize plasma polymer films (PPFs) through the grafting polymerization initiated from free radicals trapped in the film was developed in this work. 2-Ethylhexyl acrylate (EHA) was chosen as radically polymerizable monomer given the wide use of its corresponding polymer in coating and adhesive applications. The occurrence of the grafting was first confirmed by time-of-flight secondary ion mass spectrometry (ToF-SIMS) and X-ray photoelectron spectroscopy (XPS). Then grafted chains were studied in more detail. The thickness of grafted chains was quantitatively estimated by angle-resolved XPS (ARXPS), while their morphology and interfacial behavior were qualitatively investigated by atomic force microscopy (AFM), contact angle measurements, and quartz crystal microbalance (QCM). The latter technique provided additional insights regarding the swelling behavior of the grafted layer and its stability upon exposure to challenging environments. Reported scientific findings suggest to use this approach for the covalent binding of a very thin layer on the top surface of a PPF without affecting its bulk properties.

Effect of cellulosic nanowhiskers on the performances of epoxidized acrylic copolymers

The aim of the present work is to synthesize new nanocomposites based on an acrylic polymeric matrix loaded with cellulose nanowhiskers (CNWs). The acrylic copolymer was prepared by free-radical copolymerization of two monomers widely used for coating applications, namely 2-ethylhexylacrylate (EHA) and glycidyl methacrylate (GMA) bearing epoxy moieties, while CNWs were extracted from ramie fibers by the means of acid hydrolysis. A series of nanocomposites with various CNW loadings were obtained by solvent casting and were further crosslinked by an UV-induced curing. The extent of the crosslinking was substantiated by Fourier transform infrared spectroscopy (FTIR) whereas morphological, thermo-mechanical as well as optical properties were evaluated before and after the crosslinking by Scanning Electronic Microscopy (SEM), Differential Scanning Calorimetry (DSC), Thermal Gravimetric Analysis (TGA), Dynamical Mechanical Analysis (DMA) and Polarized Optical Microscopy (POM). Thus it was demonstrated that strong interactions occur between the polymeric matrix and CNW, contributing to a significant improvement of the thermal stability and mechanical properties of the nanocomposites. More importantly, nanocomposites exhibit at certain CNW loading interesting optical properties.

A multilayer coating with optimized properties for corrosion protection of Al

In the context of a universal search for alternatives to chromate-based coatings, which are toxic for both human beings and the environment, a multilayer coating for Al protection purposes is developed and investigated in the current work. The first layer is selected to be a hexamethyldisiloxane-based plasma polymer film (PPF), deposited by Plasma Enhanced Chemical Vapor Deposition (PECVD), due to a number of interesting features that are characteristic of plasma polymers. The second layer is synthesized via the polymerization of the 2-ethylhexyl acrylate monomer initiated from the free radicals trapped on the surface of the PPF during its growth. A subsequent layer of a copolymer of 2-ethylhexyl acrylate and glycidyl methacrylate, poly(EHA-co-GMA), is deposited by spin coating to increase the corrosion resistance of the coating. An improvement in the anti-corrosion properties of the multilayer coating by approximately three orders of magnitude as compared to the uncoated Al substrate is observed. Further enhancement of the adhesion and scratch resistance properties is addressed via UV-crosslinking and the incorporation of in situ generated silica nanoparticles into the final layer.

Effect of photo-crosslinking on the performance of silica nanoparticle-filled epoxidized acrylic copolymer coatings

The effect of UV-crosslinking on the morphological, mechanical and barrier properties of a hybrid coating based on an epoxidized acrylic polymer filled with silica nanoparticles, incorporated in situ through the sol–gel method, was studied. A systematic comparison of coatings loaded with various amounts of silica nanoparticles subjected or not to UV-curing was reported. The morphological and mechanical properties were investigated by atomic force microscopy (AFM) and tribology tests, while the corrosion resistance of thin films applied on aluminum alloys was measured by electrochemical impedance spectroscopy (EIS). Our findings indicate that a synergic effect is brought about by the combination of silica nanoparticles and UV-curing, significantly enhancing the mechanical properties of the coating, although the corrosion protection was compromised. The resulting coating may be used in applications requiring high mechanical performance.

Epoxy Monomers Cured by High Cellulosic Nanocrystal Loading

The present study focuses on the use of cellulose nanocrystals (CNC) as the main constituent of a nanocomposite material and takes advantage of hydroxyl groups, characteristic of the CNC chemical structure, to thermally cross-link an epoxy resin. An original and simple approach is proposed, based on the collective sticking of CNC building blocks with the help of a DGEBA/TGPAP-based epoxy resin. Scientific findings suggest that hydroxyl groups act as a toxic-free cross-linking agent of the resin. The enhanced protection against water degradation as compared to neat CNC film and the improvement of mechanical properties of the synthesized films are attributed to a good compatibility between the CNC and the resin. Moreover, the preservation of CNC optical properties at high concentrations opens the way to applying these materials in photonic devices.