Influence of the slow wave on the relation between the angular resonances and the leaky guided modes properties for a poroelastic plate embedded in water

A numerical study of the guided modes in a water-saturated poroelastic plate that obeys the Biot theory is presented. In the first part, we study the leaky guided modes and the angular resonances when the slow wave does not propagate. Two types of guided modes exist. The first ones occur from coupling of the fast longitudinal wave with the shear wave; most of them propagate whatever the frequency is, provided that it is not close to their cut-off frequencies. The leaky guided modes of the second type occur from coupling of the two longitudinal waves and the shear wave. These modes do not propagate (they are highly damped) as long as the slow wave remains diffusive. We show that the characteristics of the angular resonances can be linked to the leaky guided waves of the first type in the same way as for an elastic plate. The guided modes of the second type may not be associated to angular resonances. In the second part, we consider a thinner plate in a higher frequency range so that the slow wave can propagate. Once again its influence is studied both on the leaky guided modes and on the angular resonances.     

UV light impact on ellagitannins and wood surface colour of European oak (Quercus petraea and Quercus robur)

Two European oak species (Q. petraea and Q. robur) have a high content of phenols which may participate in the alteration of colour upon UV irradiation. To study the photodegradation process of oak surfaces, the two oak species extractives, vescalagin, castalagin, ellagic acid and gallic acid were analysed quantitatively by HPLC before and after UV irradiation. Irradiation time was altered between 3, 24, 72, 96, 120, 144, 192 and 216 h. In parallel, any colour changes of Oak wood surface was followed after 120 h of UV-irradiation by measuring CIELAB parameters (DL*, Da*, Db* and DE*). We observed that 60% of total phenol content of extractives decreased after the maximal exposure time. Our findings also showed that castalagin and gallic acid were destroyed after 216 h and vescalagin and ellagic acid after 72 h. This study proves the photosenibility of oakwood extractives which, supplementary to lignin degradation, would strongly result in the discolouration of oak heartwood.     

Effect of the surface free energy on the behaviour of surface and guided waves

The aim of this paper is to evaluate the influence of the surface free energy upon the propagation of the eigenmodes of structures, by studying successively (a) the Rayleigh wave for an elastic half-space, (b) the Lamb waves for an elastic layer, and (c) the guided modes for a tri-layer structure (e.g., metal/adhesive/metal). The surface free energy is a parameter which appears in the jump conditions of stresses and displacements at each interface, and which consequently modifies the eigenmodes, solutions of the boundary conditions system. As expected, the Rayleigh wave is dispersive and its velocity increases when the surface free energy increases. In the same way, the velocity of Lamb waves also increases except at normal angle of propagation where the surface free energy does not arise. Moreover, near the Rayleigh angle, the behaviour of the A₀ and S₀ Lamb modes varies strongly according to the surface free energy. Similar results are observed for the tri-layer structure.     

Study of the in vitro degradation of poly(ethyl glyoxylate)

In vitro degradation of poly(ethyl glyoxylate) (PEtG), a functionalised polyacetal, was investigated. First, the thermodynamic polymerization parameters and the ceiling temperature (T_c) were determined (ΔH_p = 28 ± 3 kJ mol⁻¹, ΔS_p = 98 ± 7 J mol⁻¹ K⁻¹, T_c = 310 ± 4 K). Secondly, PEtG hydrolysis was investigated using potentiometry, weight loss measurements, SEC and ¹H NMR. The results show that PEtG is stable for at least 7 days in aqueous media. Then degradation occurs and releases ethanol and glyoxylic acid hydrate as final products. A scheme for the degradation mechanism involving chain scission and ester hydrolysis is proposed.     

Empirical rheological model for rough or grooved bonded interfaces

In the industrial sector, it is common to use metal/adhesive/metal structural bonds. The cohesion of such structures can be improved by preliminary chemical treatments (degreasing with solvents, alkaline, or acid pickling), electrochemical treatments (anodising), or mechanical treatments (abrasion, sandblasting, grooving) of the metallic plates. All these pretreatments create some asperities, ranging from roughnesses to grooves. On the other hand, in damage solid mechanics and in non-destructive testing, rheological models are used to measure the strength of bonded interfaces. However, these models do not take into account the interlocking of the adhesive in the porosities. Here, an empirical rheological model taking into account the interlocking effects is developed. This model depends on a characteristic parameter representing the average porosity along the interface, which considerably simplifies the corresponding stress and displacement jump conditions. The paper deals with the influence of this interface model on the ultrasonic guided modes of the structure.     

Normal modes of a poroelastic plate and their relation to the reflection and transmission coefficients

A poroelastic plate that obeys the Biot theory is considered. Compact new forms of its reflection and transmission coefficients, similar to those of the resonance scattering theory for an elastic plate, are derived. A numerical comparison of the reflection coefficient modulus with the plate normal modes, at low frequency, shows that a study of the reflection or transmission coefficient does not provide the same kind of information on the poroelastic plate than an investigation of guided leaky waves propagation.     

Extraction and characterization of cellulose nanocrystals from post-consumer wood fiberboard waste

This study investigates the potential of wood wastes, specifically post-consumer fiberboards, as a new source for cellulose nanocrystals (CNC). This underused resource has currently no commercially viable way to recycle it and so the volumes of fiberboard waste are growing rapidly. A sequential chemical fractionation was used to separate the three main constituents of wood, namely cellulose, hemicelluloses and lignin, and the non-wood components present in fiberboards, such as resins and finishes (e.g. varnishes, paints, plastics, laminates, etc.). Most of the non-cellulosic components and non-wood elements were removed by an alkali treatment followed by bleaching, resulting in a cellulosic fraction which is suitable for the further isolation of CNC by an acid hydrolysis step. The intermediate and final products were characterized by chemical composition, microscopic, spectroscopic and X-ray diffraction methods. The CNC obtained from wood waste are totally devoid of traces of contaminants and possess comparable characteristics and quality to those extracted from virgin wood fibers. The results indicate that fiberboard wastes can be used as promising alternative source for nanocelluloses production.     

Unprecedented polymerization of trimethylene carbonate initiated by a samarium borohydride complex: mechanistic insights and copolymerization with epsilon-caprolactone

Poly(trimethylene carbonate) (PTMC) was synthesized through ring-opening polymerization by using a rare-earth borohydride initiator, [Sm(BH(4))(3)(thf)(3)]. This initiator shows a high activity to give high-molar-mass PTMCs with molar-mass distributions ranging from 1.2 to 1.4, and with a regular structure void of ether linkages. The polymers were characterized by (1)H and (13)C NMR spectroscopy, (1)H-(1)H COSY, (1)H-(13)C HMQC NMR spectroscopy, size-exclusion chromatography (SEC), viscosimetry, and MALDI-TOF MS analyses. A coordination-insertion mechanism was established based on detailed NMR characterizations, especially of the polymer chain end-functions. The monomer initially coordinates the samarium to give [Sm(BH(4))(3)(tmc)(3)], 1. The monomer then opens up through cleavage of the cyclic ester oxygen--acyl bond and inserts into the Sm--HBH(3) bond resulting in an alkoxide complex, [Sm{O(CH(2))(3)OC(O)HBH(3)}(3)], 2, or [Sm{O(CH(2))(3)OC(O)H}(3)], 2', which then propagates the polymerization of TMC to give the active polymer [Sm({O(CH(2))(3)OC(O)}(n)O(CH(2))(3)OC(O)HBH(3))(3)], 3 or [Sm(O(CH(2))(3)OC(O){O(CH(2))(3)OC(O)}(n)O(CH(2))(3)OC(O)H)(3)], 3'. Finally, acidic hydrolysis of 3 or 3' gives HO(CH(2))(3)OC(O)[O(CH(2))(3)OC(O)](n)O(CH(2))(3)OC(O)H, 4. This novel alpha-hydroxy,omega-formatetelechelic PTMC represents the first example of a formate-terminated polycarbonate. TMC and epsilon-caprolactone (CL) were copolymerized to afford both random PTMC-co-PCL and block PTMC-b-PCL copolymers that were characterized by (1)H NMR spectroscopy, SEC, and differential scanning calorimetry (DSC). The structure of the block copolymers depends on the order of addition of monomers: if CL is introduced first, dihydroxytelechelic HO-PTMC-b-PCL-OH polymers are formed, whereas introduction of TMC first or simultaneous addition of comonomers leads to hydroxyformatetelechelic HC(O)O-PTMC-b-PCL-OH analogues.