Publications

Abstract: During the formation of open-cell foams the relevant driving force besides gas pressure comes from surface tension which controls area minimization of the topology. Accordingly, the Surface Evolver software expressly developed to shape liquid surface by minimizing their energy given various constraints such as bubble volumes allows computing very realistic models of equilibrium foam microstructures. However, when considering non equilibrium foam (dense foam) or the formation of closed-cell foams where films between adjacent cells are thick, a more complete model would have to consider the expansion process as a whole and include the viscous flow of the suspending fluid, which eventually solidifies to form the solid phase of the foam. This situation has not been investigated computationally because the problem size is too large and despite several attempts there remain significant challenges to integrate the foaming process knowledge into mathematical formulas. In a continuing effort to establish structure-properties relationships, an ad-hoc model is detailed to generate numerical closed-cell foam microstructures that resemble the real ones for a medium to a high porosity range. Inspired from a physical description of the manufacturing process making use of chemical blowing agent, a dynamic approach taking into account the influence of the temperature on the physical material characteristics is proposed. It allows the nucleation of cells, their growth through gas expansion, and the interactions between growing cells to be taken into account simultaneously. The large variety of generated microstructures with porosities spanning the few to 95 percents range is compared to real polymeric closed-cell foams whose microstructures were determined by X-ray microtomography at high spatial resolution. These structures are then used as templates to generate realistic finite element models and study the mechanical properties.

Abstract: Glass formation and glassy behavior remain as the important areas of investigation in soft matter physics with many aspects which are still not completely understood, especially at the nanometer size-scale. In the present work, we show an extension of the �nanobubble inflation� method developed by O�Connell and McKenna [Rev. Sci. Instrum. 78, 013901 (2007)] which uses an interferometric method to measure the topography of a large array of 5 mu m sized nanometer thick films subjected to constant inflation pressures during which the bubbles grow or creep with time. The interferometric method offers the possibility of making measurements on multiple bubbles at once as well as having the advantage over the AFM methods of O�Connell and McKenna of being a true non-contact method. Here we demonstrate the method using ultra-thin films of both poly(vinyl acetate) (PVAc) and polystyrene (PS) and discuss the capabilities of the method relative to the AFM method, its advantages and disadvantages. Furthermore we show that the results from experiments on PVAc are consistent with the prior work on PVAc, while high stress results with PS show signs of a new nonlinear response regime that may be related to the plasticity of the ultra-thin film. Published by AIP Publishing.

Abstract: The formation of polydopamine composite membranes at the water/air interface using different chemical strategies is reported. The use of either small molecules (urea, pyrocatechol) or polymers paves the way to understand which kind of compounds can be used for the formation of PDA-composite free-standing membranes produced at the water/air interface. On the basis of these screening results, we have found that alginate grafted with catechol groups allows the formation of robust free-standing films with asymmetric composition, stimuli-responsiveness, and self healing properties. The stickiness of these membranes depends on the relative humidity, and its adhesion behavior on PDMS was characterized using the JKR method. Thus, alginate-catechol polydopamine films appear as a new class of PDA composites, mechanically robust through covalent cross-linking and based on fully biocompatible constituting partners. These results open the door to potential applications in the biomedical field.

Abstract: We describe a novel combination of orthogonal reactions based on UV-driven thiol-ene and alkoxysilyl condensation reactions to form a single-step route toward thioether-bridged silsesquioxane films. Our chemical strategy consists of using two bifunctional (meth) acrylate (E) and propanethiol (T) trimethoxysilyl precursors containing two complementary functional moieties for thiol-ene coupling and sol-gel process. The reaction kinetics revealed that c.a. 85% of thiol and ene conversions were consumed concomitantly. Meanwhile, a complete hydrolysis was accomplished, affording ultimately a high degree of condensation (81%). Emphasis was placed on differences of mechanical properties between sol-gel hybrids resulting from thiol-ene reaction (E-T mixture) and ene homopolymerization (E only) using scratch test measurements. For the methacrylate system, the formation of thioether linkages within a vitreous silica network emerged as a useful strategy for the formation of a uniform, low-stress and flexible crosslinked hybrid structure. Enhanced mechanical properties were manifested by an expanded elastic domain, and better resistance to cracking. Moreover, there are clear indications that mechanical properties can be easily tuned upon varying the ratio of the two hybrid precursors.

Abstract: This work presents a new instrumentation dedicated to the study of contact creep and recovery of non transparent polymeric samples. The aim is to be able to analyze viscoelastic properties of polymeric surfaces recording the apparent contact area for the contact creep phase and the residual imprint area for the recovery phase. This gives valuable information not to be model dependant in the analysis compared to classical techniques. Indeed, for the creep phase many methods have been proposed to estimate the contact area, but rarely perform direct vizualisation as we propose. For the recovery phase two main challenges arise: avoid contact probe not to influence on recovery kinetics; get recording at early times to have information before recovery is quite finished. This study focus on indentation problems to prove the concept of viscoelastic analysis directly from experimental data without modeling. But the problematic is more larger than that and widen to more complicated rheological surface studies including indentation and scratching. (C) 2016 Elsevier B.V. All rights reserved.

Abstract: Nanocomposite films possessing multiple interesting properties (mechanical strength, optical transparency, self-healing, and partial biodegradability) are discussed. We used Layer-by-Layer assembly to prepare micron thick wood-inspired films from anionic nanofibrillated cellulose and cationic poly(vinyl amine). The film growth was carried out at different pH values to obtain films of different chemical composition, whereby, and as expected, higher pH values led to a higher polycation content and also to 6 times higher film growth increments (from 9 to 55 nm per layer pair). In the pH range from 8 to 11, micron thick and optically transparent LbL films are obtained by automated dipping when dried regularly in a stream of air. Films with a size of 10 cm(2) or more can be peeled from flat surfaces; they show tensile strengths up to about 250 MPa and Youngs moduli up to about 18 GPa as controlled by the polycation/polyanion ratio of the film. Experiments at different humidities revealed the plasticizing effect of water in the films and allowed reversible switching of their mechanical properties. Whereas dry films are strong and brittle (Youngs modulus: 16 GPa, strain at break: 1.7%), wet films are soft and ductile (Youngs modulus: 0.1 GPa, strain at break: 49%). Wet film surfaces even amalgamate upon contact to yield mechanically stable junctions. We attribute the switchability of the mechanical properties and the propensity for self-repair to changes in the polycation mobility that are brought about by the plastifying effect of water.

Abstract: The objective of this work is to investigate a novel top-down synthesis route toward the elaboration of nanopatterned polymer surfaces by combining thin polymer structuring (polymer-demixing) and plasma etching techniques. Thanks to this original approach, the adjustment of the parameters during the wet chemical and plasma steps allows independent tuning of the lateral dimension of the polymer structures (diameter) and the height of the pillars. The nanopatterning description of two different polymers, namely polyethylene naphthalate and polyimide, is reported. Johnson-Kendall-Roberts (JKR) adhesion tests are carried out to compare the adhesive property of the patterned and non-patterned polymer surfaces. These measurements allow highlighting the importance of the polymer viscoelasticity for future development of bioinspired polymer-based dry adhesives.

Abstract: Oligomers containing the peptide sequence cysteine-any-cysteine (CXC) were attached, at specific locations, to a linear chain of polystyrene. The polymer-bound peptide motifs were then oxidized under dilute conditions to afford a complex bio-hybrid bi-cyclic topology via intramolecular twin disulfide bridge formation.

Abstract: This paper describes the conditions for contact formation between soft elastic hemispheres and soft elastic substrates micropatterned with pillars. These microstructured substrates were often proposed in the two past decades for the control of adhesion. In this study the hexagonal micropillars are arranged in hexagonal arrays and the width L of the pillars as well as the distance D between the pillars (with L=D for a sample) are both varied from one sample to another in order to keep the overall planar surface fraction of the pillars constant. Additionally, two pillars of heights h=4 mu m and h=16 mu m are considered here. As expected from previous studies different contact morphologies are found as a function of the aspect ratio and the contact pressure. The contact may be formed (i) only at the top of the pillar, (ii) both at the top and between the pillars, or (iii) simultaneously at the top of the pillars and in between the pillars at the centre of the contact for which the pressure is the highest. Unexpectedly it is shown in this last case that the same contact morphology is obtained with the same pressure if the surface pattern aspect ratio L/h is varied homothetically, suggesting a scale invariant behaviour of the contact formation between the hemisphere and the soft elastic micropatterned substrate. (C) 2013 Elsevier Ltd. All rights reserved.

Abstract: The theological behavior at the interface between a polymer surface and a rigid indenter during dry friction is linked to the local pressure and the roughness of the surface of the indenter. This study analyzed experimental results for the friction coefficient mu versus the mean contact pressure p(mean) obtained at ambient temperature on polycarbonate. The rigid indenters consisted of spherical glass tips of several radii having surfaces of two types: polished to tight tolerance (optical lenses) or nano-roughened (chemical etching). Below the yield stress (p(mean) < 100 MPa), the intrinsic friction coefficient followed a master curve for smooth indenters whereas nano-roughnesses reduced the friction (from 5.5 to 140 nm Rrms). To explain these experimental results an original model based on the classical laws of contact mechanics was developed and provides a basis to study the rheology of confined polymer layers. (c) 2013 Elsevier B.V. All rights reserved.

Abstract: The aim of this work was to investigate how specificities of thin films prepared from aqueous polymer colloids (latexes) influence their friction properties, tested with a sliding stainless steel spherical tip. Two acrylic latexes containing either 1 wt% or 4 wt% of acrylic acid (AA) were used at pH 2 or 10. Bulk mechanical properties were also studied in order to improve the interpretation of friction results. Increasing AA concentration and pH increases the overall film rigidity, pH being more effective than AA concentration. Contact pressures are directly correlated to bulk mechanical properties whereas surface shear stresses are also strongly influenced by molecular interactions with the sliding tip. Friction coefficients are rather high, peaking at 5, because of important viscoelastic dissipation in the films as well as strong polar interactions introduced by AA, especially at high pH. The paper also addresses the question of the relevant characteristic length in this kind of friction study. (C) 2012 Elsevier Ltd. All rights reserved.

Abstract: This paper studies the rheology of weakly entangled polymer melts and films in the glassy domain and near the rubbery domain using two different methods: molecular dynamics (MD) and finite element (FE) simulations. In a first step, the uniaxial mechanical behavior of a bulk polymer sample is studied by means of particle-based MD simulations. The results are in good agreement with experimental data, and mechanical properties may be computed from the simulations. This uniaxial mechanical behavior is then implemented in FE simulations using an elasto-viscoelasto-viscoplastic constitutive law in a continuum mechanics (CM) approach. In a second step, the mechanical response of a polymer film during an indentation test is modeled with the MD method and with the FE simulations using the same constitutive law. Good agreement is found between the MD and CM results. This work provides evidence in favor of using MD simulations to investigate the local physics of contact mechanics, since the volume elements studied are representative and thus contain enough information about the microstructure of the polymer model, while surface phenomena (adhesion and surface tension) are naturally included in the MD approach.

Abstract: A difunctional methacrylate oligomer was mixed with a variable amount of a MAPTMS precursor in the presence of both a radical and a cationic photoinitiator. The simultaneous photolysis of both photosensitive molecules upon UV irradiation allowed the single- step generation of a type-II polymethacrylate/ polysiloxane nanocomposite film. Methacrylate and methoxysilyl conversions during irradiation were efficiently monitored by FTIR spectroscopy. The inorganic structure of the resulting silica-based hybrid films was characterized using 29 Si solid-state NMR. Finally, the reinforcement ability of the resulting hybrid films was also assessed by using a unique range of characterization techniques: DMA, scratch test, and nanoindentation.

Abstract: Scratch durability of polymer surfaces is becoming crucial for the growing use of these materials in innovative applications, replacing former materials with more resistant surfaces. A variety of physicochemical processes such as annealing and a range of ion plantation techniques, generally applied to improve surface properties, cannot be applied to transparent polymers like PMMA due to their darkening outcome. Enhancement of the scratch resistance must be investigated for the most part as a consequence due to decreasing the friction coefficient. The recovery of the groove left on the surface increases if the tip is smooth or if the local friction coefficient is low. The effect of three fatty acid amides namely Erucamide, Behenamide and Stearamide on both surface and bulk mechanical properties of PMMA was studied at a wide range of temperature. Experimental results show that a decrease in friction coefficient is possible by the introduction of appropriate plasticizer without having a significant effect on its bulk behaviour and this decrease in friction depends upon the nature and the content of plasticizer. Moreover, Erucamide has been proved more effective in decreasing friction of PMMA than Behenamide or Stearamide. The values of the rear contact angle and in situ photographs during scratching confirm that the decrease in friction is associated with the decrease of yielding of the contact between the tip and the polymer surface. The surface energy analysis shows that the decrease in friction coefficient is without any significant change in surface energy of PMMA. (C) 2011 Elsevier B.V. All rights reserved.

Abstract: Nanocomposite materials prepared from radically photocurable hybrid sol-gel precursors have been widely developed within the last decade, especially to devise novel optical devices and coatings. For their synthesis, a preferential route has involved in the successive sol-gel process of acrylate trialkoxysilane precursors followed by radical photopolymerization. In contrast, this work presents an original one-step synthesis based on the association of two different photoinitiators (PIs) in the same formulation: the photolysis of a hydroxyphenylketone (radical PI) affords polyacrylate chains while that of a diaryl iodonium salt (cationic PI) generates powerful superacids catalyzing the sol-gel reactions of the alkoxy functions. The behavior of methacrylate and acrylate trimethoxysilane precursors was compared to highlight the effect of the organic moiety functionality on the reaction kinetics (Fourier transform infrared spectroscopy) and the film microstructure (C-13 and Si-29 solid-state nuclear magnetic resonance). Interestingly, evidence of local organization in these hybrid films was also given by X-ray analysis. In a last part, their thermomechanical properties were discussed thoroughly using a range of techniques: DSC, scratch-resistance test, nanoindentation. (c) 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 48: 4150-4158, 2010