Publications

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: In this work, we discuss quantitatively two basic relations describing the wetting behavior of microtopographically patterned substrates. Each of them contains scale invariant topographical parameters that can be easily expressed onto substrates decorated with specifically designed micropillars. The first relation discussed in this paper describes the contact angle hysteresis of water droplets in the Cassie?Baxter regime. It is shown that the energy at the origin of the hysteresis, that has to be overcome for moving the triple line, can be invariantly expressed for hexagonal pillars by varying the pillars width and interpillar distance. Identical contact angle hystereses are thus measured on substrates expressing this scale invariance for pillar widths and interpillar distances ranging from 4 to 128 ?m. The second relation we discuss concerns the faceting of droplets spreading on microtopographically patterned substrates. It is shown in this case that the condition for pinning of the triple line can be fulfilled by simultaneously varying the height of the pillars and the interpillar distance, leading to faceted droplets of similar morphologies. The invariance of these two wetting phenomena resulting from the simultaneous and homothetic variation of topographical parameters is demonstrated for a wide range of pattern dimensions. Our results show that either of those two wetting behaviors can be simply achieved by the proper choice of a dimensionless ratio of topographical length scales.

Abstract: The effect of a nanostructured architecture of a rubber-reinforced block copolymer PMMA on both volume and surface mechanical properties is studied. The rubber inclusions ordered at the nanoscale enhance the ductile behavior of the triblock copolymer. This acrylic glass can therefore be easily thermoformed even. Besides, polymeric surfaces are sensitive to scratches which have direct influence on optical transparency. Scratches, which left irreversible grooves on the surface, appear at higher strains with rubber inclusion ordered at the nanoscale compared to classical PMMA or high impact cast PMMA. Eventually, the studied rubber-reinforced block copolymer PMMA is a good candidate to acrylic glass dedicated to automotive glazing. (C) 2014 Elsevier B.V. All rights reserved.

Abstract: This article attempts to analyze the viscoelastic behavior of an amorphous polymer during a microindentation test. The viscoelastic behavior of an amorphous polymer (poly(methyl methacrylate), PMMA) is derived from different relaxation tests performed under different applied true strains. A generalized Maxwell model is then used to identify the mechanical parameters of the viscoelastic behavior. The numerical results display good correlation with experiments during the creep phase. The uniaxial relaxation test used to identify the viscoelastic behavior is chosen in relation to the experimental conditions of indentation. The results obtained for the recovery phase allow a first analysis of the strain and von Mises equivalent stress fields during indentation test. The recovery of the imprint left on the surface seems to depend on the location of the strain maxima. If a strain level of 10 % or more reaches the surface of the deformed volume, a permanent imprint is obtained. Otherwise the residual imprint may be considered to be completely healed even if the subsurface has partially yielded during the loading phase or creep time.

Abstract: The second part of this work aiming at investigating how specificities of thin films prepared from aqueous polymer colloids (latexes) influence their friction properties is devoted to the role of surfactants. Two acrylic latexes containing either 1 wt% or 4 wt% of acrylic acid were used, to which 0 to 9 wt% of sodium dodecyl sulfate (SDS) was post added before film formation by drying. Bulk mechanical properties were also studied in order to improve interpretation of friction results. Increasing the amount of SOS in the films has the effect of increasing Young's and shear storage moduli and decreasing tan delta. SOS, forming crystallized aggregates in the bulk of the films, behaves like a reinforcing filler, except at 9 wt% where the SDS phase percolates under stress and allows film deformation by shear bands at very low stress. Friction coefficients dramatically decrease with increasing SDS concentration, especially at high strain rate. Surface shear stress is strongly decreased due to lubrication by SDS having migrated to the film surface. In order to gain more insight in the lubrication mechanisms, SOS layers were deposited on glass in increasing amounts. This part of the study seemed to indicate that organization of the SOS surface layers has more impact than their amount. (C) 2012 Elsevier Ltd. All rights reserved.

Abstract: This work demonstrates a possible route to connect a particle (chain) based understanding with continuum mechanical questions about contact mechanics. The bond orientation, chain conformation and stress field of a polymer film were analyzed during scratch tests (tangential contact) using a molecular dynamics (MD) simulation approach. Scratch tests with a conical tip at constant scratching velocity were simulated on linear amorphous polymer surfaces at various temperatures and roughnesses of the tip and for various interactions between the tip and the particles of the polymer chains. The second Legendre polynomial (computed for small domains around the tip) gave the bond orientation inside the polymer film during sliding of the tip. The gyration tensor (layer-resolved in the direction of the polymer film thickness) provided information about the conformation of the polymer chains. These results allowed us to argue in favor of Briscoe's hypothesis (thin film sheared vs. “bulk” compressive behavior) concerning the friction properties of the polymer surfaces. Finally, the first stress measurements of the virial stress tensor (in sub-boxes placed in the MD cell) revealed a complex combination between compressive hydrostatic pressure and shear stress, which may be interpreted as a complex sheared domain at the interface.

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: The scratch behavior of a thermoset solid polymer exhibiting brittle behavior in tension is investigated. The surfaces are scratched and an imaging system is used to record real time photographs. The 3D crack pattern is analyzed using fluorescence confocal laser scanning microscopy. A finite element simulation is performed to determine the indenter/specimen contact conditions. They are used as input data for the 3D crack network analysis based on combined 3D localized multigrid and X-FEM/level set techniques. The computed stress distributions within the 3D cracked specimen are of great interest to understand the crack network formation observed during the experiments. (C) 2010 Elsevier Ltd. All rights reserved.

Abstract: This paper is devoted to an analysis of the creep and recovery occurring on the surface of an amorphous polymer during indentation, a characteristic of the self-healing performance of the material. Creep and recovery were studied using a home made experimental device which allows one to record in situ the evolution of the imprint created by a probe. The influence of the temperature, the initial imposed strain and the creep duration were analysed. The evolution of the true contact area with time was followed during micro-indentation. We demonstrate the non-linear behaviour of the amorphous polymer studied during the creep and recovery phases. The results obtained allow calculation of the activation energy and activation volume of the polymer. These results are also compared with the predictions of the viscoelastic model of Lee and Radok and viscoplasticity is demonstrated for a reasonable initial mean contact strain as defined by Tabor. Nevertheless, it is still difficult to quantify the recovery and to define the conditions for the appearance of plasticity.

Abstract: This work presents a mechanical analysis of tangential contact using molecular dynamics simulations. Scratch tests with a conical tip on amorphous polymer surfaces were simulated at various temperatures, scratching velocities, roughnesses of the tip and with various interactions between the tip and the monomers. The local friction coefficient for the different contact conditions (temperature, tip roughness and scratching velocity) was calculated by studying the apparent friction and the contact asymmetry. The results are in good agreement with experimental data from tests on classical polymer surfaces on a microscopic scale. (C) 2011 Elsevier B.V. All rights reserved.

Abstract: The synthesis and characterization of bridged polysilsesquioxane films was performed via a photoacid-catalyzed sol-gel method using a series of three precursors with different organic moiety structures.

Abstract: In most cases. scratching of the surface of a polymeric glass elicits brittle behavior. A common way to improve the scratch resistance of a sensitive surface is to coat it with a thin film. Further work is required to explain the improvement in scratch resistance due to coating technique and predict the cracking in anti-scratch coatings. Moreover, the substrate/thin film adhesion must be well controlled and measurable. The present study contributes to these aims. Using a single-asperity scratching device allowing in situ observation of the scratch, the fracturing of a thin nano-composite coating deposited on a polycarbonate substrate was investigated under different conditions of temperature and scratching speed. Four types of fracture mechanisms were observed, depending on these two variables. A global energy balance model of the blistering process which is obtained for some experimental conditions permits one to determine the adhesion of the system. The adhesion can be measured by following the delaminated area (quantified by image analysis) as a function of the scratching distance during blistering. The particular case of an experimental stable blistering process was studied and the corresponding substrate/thin film adhesion was derived using the global energy balance model. (C) 2009 Elsevier Ltd. All rights reserved.

Abstract: The scratch behavior of two amorphous polymers is investigated to understand how the materials characteristics affect the scratch resistance. A thermosetting resin and a thermoplastic polymer are studied using both an experimental set-up allowing in situ observations of the contact area during indentation and scratching with spherical tips, and a finite element modeling (FEM). The theological properties of polymer surfaces are modeled assuming a linear elastic behavior and a plastic law with high strain hardening, described by G'Sell-Jonas equation. The local friction coefficient mu(loc) at the interface between the indenter and the material was modeled with a Coulomb's friction coefficient, for each computed ratio a/R, where a being the contact radius, and R the tip radius. A description of the plastic strain field, beneath the indenter during scratch is proposed as a function of the ration all? set between 0.1 and 0.4 and the friction coefficient varying between 0 and 0.4. The results obtained by FEM are in good correlation with experimental observations and show that the plastic strain gradient beneath the indenter depends clearly on the ratio a/R, on the local friction coefficient and also the theological parameters of tested material. An equivalent average plastic strain is calculated with FEM over a representative volume deformed plastically. The average plastic strain increases with the ratio a/R, as predicted by the empiric Tabor's rule, but also with the local friction coefficient, for a given ratio a/R, while the strain hardening ability tends to decrease plastic strain imposed during the contact. Clear correlations are demonstrated between the average plastic strain and geometrical parameters, classically used to describe the geometry of the contact area. (C) 2009 Elsevier Ltd. All rights reserved.