Publications

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: We have developed an original experimental setup, coupling tribology, and velocimetry experiments together with a direct visualization of the contact. The significant interest of the setup is to measure simultaneously the apparent friction coefficient and the velocity of confined layers down to molecular scale. The major challenge of this experimental coupling is to catch information on a nanometer-thick sheared zone confined between a rigid spherical indenter of millimetric radius sliding on a flat surface at constant speed. In order to demonstrate the accuracy of this setup to investigate nanometer-scale sliding layers, we studied a model lipid monolayer deposited on glass slides. It shows that our experimental setup will, therefore, help to highlight the hydrodynamic of such sheared confined layers in lubrication, biolubrication, or friction on solid polymer. (C) 2016 AIP Publishing LLC.

Abstract: New-generation implants focus on robust, durable, and rapid tissue regeneration to shorten recovery times and decrease risks of postoperative complications for patients. Herein, we describe a new-generation thick nanofibrous implant functionalized with active containers of growth factors and stem cells for regenerative nanomedicine. A thick electrospun poly(epsilon-caprolactone) nanofibrous implant (from 700 mu m to 1 cm thick) was functionalized with chitosan and bone morphogenetic protein BMP-7 as growth factor using layer-by-layer technology, producing fish scale-like chitosan/BMP-7 nanoreservoirs. This extracellular matrix-mimicking scaffold enabled in vitro colonization and bone regeneration by human primary osteoblasts, as shown by expression of osteocalcin, osteopontin, and bone sialoprotein (BSPII), 21 days after seeding. In vivo implantation in mouse calvaria defects showed significantly more newly mineralized extracellular matrix in the functionalized implant compared to a bare scaffold after 30 days' implantation, as shown by histological scanning electron microscopy/energy dispersive X-ray microscopy study and calcein injection. We have as well bifunctionalized our BMP-7 therapeutic implant by adding human mesenchymal stem cells (hMSCs). The activity of this BMP-7-functionalized implant was again further enhanced by the addition of hMSCs to the implant (living materials), in vivo, as demonstrated by the analysis of new bone formation and calcification after 30 days' implantation in mice with calvaria defects. Therefore, implants functionalized with BMP-7 nanocontainers associated with hMSCs can act as an accelerator of in vivo bone mineralization and regeneration.

Abstract: The aim of this study is to understand time-dependant behaviour of polymeric surfaces and to characterize corresponding rheological mechanisms. One way to study this behaviour is by means of creep (including followed recovery) experiments. The first key point is the experimental characterization of the recovery phase. One of the specificity of the group is to develop original devices which allow us to record some mechanical and physical information on transparent polymers without being model dependant. The current step is to be able to study non transparent polymers with a new set-up where the normal load is applied by an UMT apparatus (Bruker) and the imprint recovery is measured by a deflectometry technique instrument developed by HOLO3. This new set-up allow to study voscoelastic and viscoplastic recovery kinetics.

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)

Abstract: Creep during loading and recovery phases after load removal are studied using a homemade experimental device that allows us to record in situ the evolution of the true contact area and of the residual imprint versus the time. Indentation tests are performed using a spherical indenter with a tip radius R 5 400 mu m onto amorphous polymeric surface poly(methylmethacrylate) (PMMA) at different contact durations (10-10(5) s) and controlled temperatures varying between -20 and 100 degrees C. Original experimental results are presented about the true evolution of the contact area during creep and recovery phases. An interesting experimental parameter, defined by the ratio a(t)/a(0), (with a(t), evolution of the contact radius with creep or relaxation time, and a(0), the initial value of the contact radius at the end of the loading phase or at the end of the creep phase) has been introduced to describe the evolution of imposed strain during indentation. As a function of the temperature and of the initial average strain imposed at the end of the loading phase, some nonlinear phenomena can be observed. Using two-dimensional axisymmetric finite element modeling, assuming only viscoelastic behavior, creep and recovering phases during indentation have been reproduced. The simulation results indicate that (i) the test is mainly controlled by the imposed strain and not by the contact pressure, and (ii) some plasticity could appear in the contact zone and as a function of the location and the size of the volume where the strain is maximal, the recovery is more or less limited.

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: Hybrid sol-gel films were prepared via a simultaneous organic-inorganic UV-curing process using a diaryliodonium salt as a superacid photogenerator. In this single-step procedure, an epoxy functionalized reactive resin mixed with a variable amount of either of two epoxy trialkoxysilane precursors was UV-irradiated, causing both the initiation of epoxy ring-opening copolymerization and the catalysis of trialkoxysilyl sol-gel reactions. The concomitant photo-induced sol-gel process was found to have a significant effect on the two related propagation mechanisms in competition for the oxirane ring-opening – the active chain-end and the activated monomer mechanisms – as proved by a systematic examination of the hybrid material microstructure through Si-29 and C-13 solid-state NMR spectroscopy. The effect of the oxo-silica network generation on the epoxy reaction kinetics was also evaluated using real-time Fourier transform infrared spectroscopy upon varying the epoxysilane structure and its concentration. Thermal and dynamic mechanical analyses were systematically performed on these hybrids, by studying thoroughly their structure-property interdependence. Other mechanical characterizations through tribological and scratch tests suggested that the present photopolymer-silica hybrid material provides a powerful tool to tailor mechanical property profiles. (C) 2010 Society of Chemical Industry