Anne Bergeret

Micron-sized main-chain liquid crystalline elastomer actuators with ultralarge amplitude contractions.

Responsive surfaces composed of cylindrical or square micrometer-sized thermoresponsive pillars made of thiol-ene nematic main-chain liquid crystalline elastomers (LCEs) are produced by replica molding. The individual pillars behave as microactuators, showing ultralarge and reversible contractions of around 300-400% at the nematic to isotropic phase transition. The nematic main-chain LCE microactuators described here present contractions as large as the best macroscopic systems reported in the literature. Moreover, the contraction observed for this new system outperforms the best values already reported for other LCE microsystems.

The hygrothermal behaviour of glass-fibre-reinforced thermoplastic composites: a prediction of the composite lifetime

Abstract A study in an accelerated environment of the mechanical properties of glass-fibre-reinforced thermoplastic composites based on polyamide 66, poly(ethylene terephthalate) and poly(butylene terephthalate) is reported. Results showed a decrease of −90% to −50% in ultimate stress to failure and impact strength with ageing according to the nature of the matrix. Increasing the ageing temperature resulted in a faster degradation rate. Depending on the matrix nature, the mechanisms of ageing seemed to be different. Polyamide 66 and poly(butylene terephthalate) composites showed effects of both physical ageing through plasticisation and chemical degradation through chain scission. Poly(ethylene terephthalate) composites did not give evidence of any plasticisation. The extent of hydrolysis was quantified through end-group analysis and gel permeation chromatography measurements. Scanning electron microscopy observations showed that hygrothermal ageing reduced the effectiveness of the interfacial bonds.

Polystyrene-glass bead composites : Maxwell-Wagner-sillars relaxations and percolation

Dielectric spectrometry experiments are performed on a series of polystyrene-glass bead composites with volume filler content from 0 to 50% and with three particle diameters ( 5μm, 20 μm, and 90 μm) in order to study the Maxwell-Wagner-Sillars (MWS) relaxations and the percolation phenomena. In the high-temperature region (130 to 220°C ), the experimental data give evidence of MWS relaxations for all the composite systems, whatever the bead size and the filler content are. A good agreement is found between the experimental values of the maximum loss factor frequency and the theoretical ones drawn from the van Beek formula, especially for low contents. A percolation phenomenon is shown in the low-temperature region (40 to 120°C for high-content/low-size composites. The percolation threshold, determined by considering the critical interparticle distance, is below 15.0% for the 5 μm glass bead composites and above 47.3% for the 90 μm composites; it lies between 20.5 and 28.6% for the 20 μm composites. Two schematic models, based on a distribution of the sizes and on a random dispersion of the beads, are developed to show how MWS and percolation phenomena can both be observed for the high-content/low-size composites.

Stimuli-responsive topological change of microstructured surfaces and the resultant variations of wetting properties.

It is now well established that topological microstructures play a key role in the physical properties of surfaces. Stimulus-induced variations of topological microstructure should therefore lead to a change in the physical properties of microstructured responsive surfaces. In this paper, we demonstrate that roughness changes alter the wetting properties of responsive organic surfaces. Oriented nematic liquid crystalline elastomers (LCEs) are used to construct the microstructured surfaces via a replica molding technique. The topological microstructure of the surfaces covered with micropillars changes with temperature, due to the reversible contraction of the LCE pillars along the long axis at the nematic-to-isotropic phase transition. This is directly observed for the first time under environmental scanning electron microscopy (E-SEM). A high boiling point liquid, glycerol, is used to continuously monitor the contact angle change with temperature. The glycerol contact angle of the microstructured surfaces covered with small pillars decreases from 118° at room temperature to 80° at 140 °C, corresponding to a transition from Cassie state to Wenzel state.

Natural Fibre-Reinforced Biofoams

Starches and polylactic acids (PLAs) represent the main biobased and biodegradable polymers with potential industrial availability in the next decades for “bio” foams applications. This paper investigates the improvement of their morphology and properties through processing and materials parameters. Starch foams were obtained by melt extrusion in which water is used as blowing agent. The incorporation of natural fibres (hemp, cellulose, cotton linter, sugarcane, coconut) in the starch foam induced a density reduction up to 33%, a decrease in water absorption, and an increase in mechanical properties according to the fibre content and nature. PLA foams were obtained through single-screw extrusion using of a chemical blowing agent that decomposed at the PLA melting temperature. A void content of 48% for PLA and 25% for cellulose fibre-reinforced PLA foams and an improvement in mechanical properties were achieved. The influence of a fibre surface treatment was investigated for both foams.

Study of the degradation of fire-retarded PP/PE copolymers using DTA/TGA coupled with FTIR

Abstract A PP/PE copolymer was successively flame retarded using Mg(OH) 2 , then using brominated trimethylphenyl indane associated with Sb 2 O 3 (Br/Sb), and finally using blends of equal weights of this last combination with Mg(OH) 2 or talc-containing non-hydrated fillers. Decompositions of pure and additive-containing copolymer were studied by DTA/TGA coupled with FTIR. A good correlation exists between the maxima of Gram–Schmidt curves and the derivatives of TGA curves. The coupling of techniques shows that the incorporation of the Br/Sb flame retardant limits strong exothermic phenomena due to sample ignition. In the case of Mg(OH) 2 associated with Br/Sb, the decomposition of the hydrated mineral occurs at a lower temperature than the reaction between brominated trimethylphenyl indane and Sb 2 O 3 . This delays the action of Br/Sb flame retardant towards higher temperatures, improving the thermal stability of the polymer. A good agreement is also found between DTA/TGA-FTIR conclusions and fire resistance tests carried out on standardized samples. When magnesium hydroxide is replaced by the fillers, the interest in using a pure talc, which appeared in fire resistance tests, is not strongly confirmed by DTA/TGA-FTIR. This discrepancy may be ascribed to the reduced influence of mass diffusion phenomena due to the small weight of the sample used in thermal analysis experiments.

Incorporation of elastomer into poly(ether ether ketone) an attempt to improve the damping factor

The incorporation of an elastomer phase into a poly(ether ether ketone) (PEEK) matrix was carried out using two different processing methods (melt blending (MB) and dry blending) to improve the damping factor (tan δ) of the composite with a minimal change in the PEEK stiffness. A cross-linked fluoroelastomer (CFE) was carefully chosen according to its high glass transition temperature (T α), high thermal stability and high modulus. The blends were characterized by scanning electron microscopy (SEM), dynamic mechanical thermal analysis (DMTA), modified Oberst test, flexural test and pyrolysis combustion flow calorimeter. According to SEM micrographs, an original well-dispersed PEEK-elastomer composite was obtained. The tan δ of the materials was evaluated using DMTA and modified Oberst test. Both techniques indicate that the incorporation of 5–20 wt% of CFE fine powders only slightly increased the tan δ of the material. Moreover, a decrease in flexural modulus and thermal stability of the blends was detected when there was an increase in the CFE content. Even if the properties are not yet significantly improved, it was well ascribed that the MB method was suitable to mix elastomer particles within a PEEK matrix. Poor interfacial adhesion has been identified as the main key parameter, which should be improved in further work.

Coupled hydro-mechanical aging of short flax fiber reinforced composites

One of the challenges in the widespread use of biocomposites for engineering applications is the influence of environmental conditions on their mechanical properties, particularly for a combination of aging factors such as temperature, moisture, and mechanical stresses. Thus, the purpose of this paper is to study the influence of coupled aging factors by focusing on a 100% bio-based and biodegradable composites made of flax/poly(lactic acid) with several fiber contents. The development of a specific testing setup enabled continuous in-situ measurements and allowed comparing the effects of combined aging factors to those of uncombined aging factors. It was confirmed that the aging temperature in wet conditions led to a loss of elastic properties, especially for higher fiber fractions. While creep tests in dry conditions resulted in little decrease of elastic properties, it was observed that mechanical loading of the materials combined with water immersion resulted in a strong synergistic effect on the loss of stiffness. Finally, the presence of fibers reduced environmental stress cracking mechanisms and increased the time to failure.

Sodium alginate adhesives as binders in wood fibers/textile waste fibers biocomposites for building insulation

Alginate derived from seaweed is a natural polysaccharide able to form stable gel through carbohydrate functional groups largely used in the food and pharmaceutical industry. This article deals with the use of sodium alginate as an adhesive binder for wood fibres/textile waste fibres biocomposites. Several aldehyde-based crosslinking agents (glyoxal, glutaraldehyde) were compared for various wood/textile waste ratios (100/0, 50/50, 60/40, 70/30 and 0/100 in weight). The fully biomass derived composites whose properties are herewith described satisfy most of the appropriate requirements for building materials. They are insulating with a thermal conductivity in the range 0.078-0.089 W/m/K for an average density in the range 308-333 kg/m3 according to the biocomposite considered. They are semi-rigid with a maximal mechanical strength of 0.84 MPa under bending and 0.44 MPa under compression for 60/40 w/w wood/textile waste biocomposites with a glutaraldehyde crosslinking agent.

Modelling and Experimental Analysis of Multi-Coating Effect on Thermal Expansion and Thermal Stresses of Polymer Fiber-Reinforced Composites

Due to Thermal Expansion Coefficient (TEC) mismatch between the fiber and matrix, thermal stresses can appear during cooling from the processing temperature to room temperature. In this paper, we (1) analyze experimentally the effect of coating on the TEC of fiber composite and (2) present a thermal self-consistent model to predict thermal stresses and TEC in multi-coated fiber. Over a large range of temperature (40°C to 200°C), the experimental results show several interesting features that result from the presence of (Silane and Polyvinylacetate) coating. These results are then modeled theoretically using the proposed thermal self-consistent model. The results show that variations in thermal stress may initiate damage in the vicinity of the fiber or promote interface contact during curing of the composite.