Search

This study aims to understand the effects of toughness and hardness on two-body wear of nanostructured carbide-free bainitic steels. For this purpose, three different steel grades were austempered at 250 °C to obtain carbide-free bainitic microstructures with different mechanical properties. The mechanical properties were determined in terms of fracture toughness and hardness. The wear tests were carried-out at three different loads with two different types of abrasive papers. The results show that a carbide-free bainitic steel with an optimum combination of hardness and fracture toughness exhibits the highest wear resistance. It has been seen that hardness is not the only parameter in determining the abrasive wear resistance and retained austenite has a beneficial effect on two-body abrasive wear.

Chitosan-g-N-isopropylacrylamide (NIPAm) water-soluble copolymers were synthesized and characterized by FTIR and (1)H NMR spectroscopies combined with conductometric and potentiometric titrations. Their thermoresponsive, fully reversible, behavior in aqueous solutions was characterized by means of microcalorimetry and rheology. During heating of copolymer solutions there is a well-known endothermic effect, which coincides with a marked increase in G' and a moderate decrement in G'' due to the formation of a hydrophobic network at the expense of the net amount of sol fraction. It was also found that a straight dependence between the values of G' above the LCST and the enthalpies associated with the transition reflecting that the connectivity in the gel network is governed by the net number of formed enthalpic-hydrophobic driven-junctions. Both the LCST and the enthalpy change vary with the ionic strength of copolymer solutions, but no dependence was found with the neutralization of the polyelectrolyte chain.

Le comportement de solutions aqueuses de methylcellulose (MC) a ete etudie en fonction de la temperature. Les solutions semi-diluees de MC ont la particularite de former un gel lorsqu'elles sont chauffees. Le processus de gelification est reversible mais presente un effet cinetique et une hysterese importants. Differents cycles de temperature ont ete realises et ont demontre la formation progressive d'interactions au sein de l'echantillon. Des etudes en regime dilue ont permis de mettre en evidence l'apparition d'agregats dans les solutions pour des temperatures superieures a 35°C. Par ailleurs, le role de la structure chimique (repartition des substituants et valeur du degre de substitution) sur les proprietes rheologiques a ete etudie. La gelification est ainsi attribuee aux interactions hydrophobes.

The determination of the radius of gyration and intrinsic viscosity as function of molecular weight for xanthan polysaccharide allows for a better understanding of the conformation of this polymer in solution. The native ordered conformation of xanthan, which corresponds to that obtained during its biosynthesis appears to be a single chain characterized by a persistence length of approximately 400 A. In contrast, the renatured ordered conformation restored after the first denaturation of the native conformation exhibits behavior suggesting doublestranded conformation as described previously in the literature. In fact, this conformation seems to correspond to a folded xanthan chain.

cited By 44; International audience; The sorption of moisture by different cellulose derivatives was examined as a function of relative humidity, to evaluate the influence of counter-ions and different charged groups on the moisture adsorption mechanism. Studies were made on cellulosic materials containing sulfate (-O-SO3H) and carboxylic (-COOH) groups in different ionic forms (H+, Li+, Na+, K+ and Ca2+). Uncharged cellulose and amylose were also examined for comparison. It was shown that the choice of counter-ions had a large influence on the moisture uptake by the studied polymers. Moisture adsorption by cellulose containing sulfate acid groups was favoured by the counter-ion H+, rather than, in order of decreasing efficiency, by Li+, Na+, Ca2+ and K+. Two types of differently adsorbed water were found: non-freezing bound water and freezing bound water. Most of the differences in total moisture content were due to differences in the freezing water content. The counter-ion favouring water adsorption in the case of the cellulose containing carboxylic acid groups was Li+, followed, in decreasing order of efficiency, by Na+, K+, Ca2+ and H+. In this case, the differences in total water content were found in both the non-freezing and freezing water. It was furthermore shown that the presence of charged groups was necessary for the adsorption of freezing water when the relative humidity was less than 100%. The large influence of the counter-ions on water adsorption may be associated with the ionic radius (size of the counter-ion), which influences the degree of hydration. Differences in water adsorption for the protonated form may be explained by the observation that carboxylic acids are weak acids with limited dissociation, whereas the stronger sulfate acid groups facilitate a more complete dissociation. © 1994 Butterworth-Heinemann Ltd.

Publisher Summary This chapter reviews fundamental knowledge on gellan structure and properties. In addition, a few applications in food and non-food industries are discussed. Native gellan behaves as a loose gel in aqueous solutions for polymer concentration larger than 10g/L. At lower concentrations, it forms highly viscous solutions. On the loose gel state, a transition to sol behavior occurs around 60oC, but the moduli are controlled by the levels of salt. Native gellan adopts a double helix conformation characterized by a temperature of conformational change around 75oC; Tm is only slightly displaced when the ionic strength increases. The rheological behavior is largely imposed by polymer concentration and temperature but has no hysteresis over a temperature cycle. After deacylation, gellan forms a stronger gel with hysteresis over a temperature cycle. The conformation is also a double helix with a different conformation compared with the native gellan. The conformational change is located at a temperature Tm much lower than for the native gellan and is very sensitive to the salt excess. At low polymer and ion concentrations, deacylated gellan behaves as a dispersed polymer, but when the ionic strength increases, a coil-to-double helix transition occurs with no ionic selectivity between monovalent counterions. Then, double helices aggregate to form a stiff network. The degree of aggregation is determined by the thermodynamics (temperature, ionic concentration) but also exhibits an ionic selectivity: K+ counterions promote the strongest gelation among monovalent counterions; divalent cations are very effective to promote gelation. Throughout, kinetics of aggregation controls the gel properties.

This paper concerns the main properties of water soluble pectins in sol and gel states. First of all, the methods of purification and characteization are discussed. The method of steric exclusion chromatography equipped with different detectors is demonstrated as the most useful to determine the macromolecular characteristics of these polymers; the role of aggregation is pointed out. The worm like chain behaviour of the pectins is developped allowing to predict the dimensions of the chain as soon as the persistence length is known. The experimental data are compared with the theoretical prediction obtained from conformational analysis. The polyelectrolyte properties of pectins are breefly exposed; specially the role of the carboxylic groups distribution along the chain is demonstrated to control the electrostatic properties. The viscometric behaviour depends on the ionic concentration and on the nature of the counterions in relation with electrostatic repulsions. Then, the ionic selectivity is discussed and related to the mechanism of crosslinking with divalent counterions. The sol-gel transition is then examined for LM and HM pectins and the mechanisms described in these two cases. The physical properties of the gels are related to the microstructure of the polymers and few data are examined.

Many polysaccharides are recognized as valuable food additives due to their original physicochemical properties. The polymers used for food applications are mainly thickening or gelling polysaccharides. Some are also used to stabilize emulsion or to suspend solid particles1-5

An analysis of dynamic scattering behavior of branched polymers in the high-wave vector region is presented. In particular the first cumulant of the dynamic structure factor Ω(q) is examined. This quantity is calculated using the standard formula which defines Ω(q) within a proportionality constant as a ratio of the generalized mobility M(q) and the static structure factor S(q), where q is the amplitude of the scattering wave vector. The latter quantity has already been calculated by Benoit et al. who obtained a general formula for a branched polymer with an arbitrary architecture. Here, this calculation is extended to the mobility M(q) including hydrodynamic interaction with the angular pre-averaged Oseen tensor. This result may be useful in the cases where the measurement of the first cumulant by quasi-elastic scattering techniques can be made to complete the information obtained by the absolute intensity measurements. © 1994 John Wiley & Sons, Inc.

International audience

This paper concerns the behavior in dilute and demidilute solutions of deacetylated gellan. The conformational transition, controlled by temperature and ionic strength, is investigated. It corresponds to a double-helix single-chain transition. Large ionic selectivity is observed in the helical conformation th at controls the degree of aggregation upon gelation. Potentiometry and conductivity measurements are interpreted in terms of the Manning polyelectrolyte theory in the sol state.

International audience

This paper gives experimental evidence for two different ordered conformations in solution for the xanthan molecules. Their hydrodynamic characteristics are quite different and their existence is controlled by experimental conditions.

Dans ce travail, nous mettons en evidence la formation de paires d’ions entre l’ion potassium et le κ-carraghenane ; cette selectivite originale apparait deja lorsque le polymere presente une conformation desordonnee avec un parametre de charge faible (λ = 0,68). La deshydratation de la paire d’ions sulfate-potassium est mise en evidence par mesure de l’absorption ultrasonore en solution diluee.

The kinetics of γ-benzyl-L-glutamate NCA's polymerizations in dioxane as solvent are discussed. The partial orders respective to [A], the NCA concentration and [I0], the initial initiator concentration are given; the rate constants are deduced and a mechanism is proposed to justify a rate of polymerization Vp = k[A][I0]2. The dependence of the rate constants on the conformation of the growing chains is demonstrated; the acceleration is attributed to the ordered structures favorable to hydrogen bonding. The kinetics of aging have also been examined and discussed; it is shown that they cannot modify the kinetics of polymerization. The DPn were obtained on the same samples before and after debenzylation; it is proved that at any concentration, DPn ≃ [A0]/[I0]. The molecular-weight distributions were obtained by chromatography and a polydispersity lower than 1.3 was deduced for each sample. The trimodal distribution, appearing as soon as [A0]/[I0] is larger than 3, is attributed to the existence of the three structures σ, β, and α. The weight fraction of each of the structures was correlated to the kinetics of polymerization.

We intend to present first a set of experimental data concerning the equilibrium properties (pK, activity of counterions) of polyelectrolytes in salt free solutions; in a second part, these results are compared with theoretical values obtained when a rod-like model is adopted in the Lifson-Katchalsky, Manning and Oosawa treatments. The polyelectrolytes tested are respectively polysaccharides (alginic and pectic acids), carboxymethylated derivatives of cellulose (CMC), dextran (CMD), amylose (CMA), and sulphonic derivatives of polyvinyl alcohol. The essential difference between them is local rigidity. It is possible to vary by synthesis the degree of substitution and consequently the linear charge density; our data are given for dilute aqueous solutions (2 × 10-3 N−5 × 10-3 N) at 25°C.

Viscosity measurements have been done on concentrated aqueous solutions of the polyelectrolytic polysaccharide xanthan in the range where the solutions of xanthan exhibit a transition between isotropic and cholesteric phases. The critical volume fractions of the polymer corresponding of the both transitions (isotropic-biphasic ; biphasiccholesteric) are discussed in term of the Flory theory for rigid rod like molecules.

Eleven samples of carboxylic polysaccharides were studied. The activity coefficients γ have been measured for monovalent (Na+) and divalent (Ca2+) counterions. There is no specific influence of the structure of the chain on γ values. Agreement with theoretical values confirms the rigidity of the chain; for low charge density, the theoretical treatment seems to be incorrect. Selectivity is discussed in term of selectivity coefficient K and free energy of exchange ΔG; ΔG is linearly related to the charge density but K which characterizes the competition of the two counterions is sensitive to the nature of the chain. The carboxymethylamyloses present a larger selectivity whose origin is not discussed here. The last point treated is the intrinsic constant of dissociation of polyacids. The pK0 values are practically independent of the nature of the polyelectrolyte and of the charge density; the values are close to the pK0 of monomeric unit and are not affected by the position of COOH in the anhydroglucose ring.

The influence of ionic strength &) on the intrinsic viscosity of xanthan is established; extrapolation to infinite ionic strength leads to the relation [?I = 5.66 X 10"' M'. The dependence of [q] on p is related to the stiffness of the molecule following empirical treatment proposed by Smidsr~d.~J~ In this paper, the theory of Odijk12J3 is introduced to interpret the experimental data and allow for the determination of an intrinsic Dersistence length eaual to 310 A. The behavior of the xanthan presents many analogies with that of DNA. "1 Introduction For a long time now, there have been discussions in the literature concerning the behavior of xanthan in dilute solution. It was suggested to be a rigid rodlike polymer' and then more recently to be a wormlike chain charac- terized by a persistence length from 400 to 1200 A.213 In addition to proposing a theoretical model which permits us to understand the behavior of this linear polymer, there remains the problem of whether the con- formation is given by a single-helical structure or a dou- ble-stranded helix. We recently reconsidered these questions in our labo- ratory using electron microscopy4 and viscosity measure- ment~.~ The object of this paper is to study the effect of the addition of inorganic salt on the viscosity of dilute aqueous xanthan solutions; from these data it is possible to extrapolate to infinite ionic strength, in order to obtain information about the extension of the xanthan molecule in an effectively uncharged state, the degree of extension being mainly caused by the intrinsic stiffness of the polymer backbone and to a lesser extent by electrostatic expansion caused by the charged groups. Data will be discussed in terms of the wormlike chain model.

The experimental results given in this paper concern the preparation of spin labelled amylose and amylopectins. These polymers are then used to investigate the adsorption on a calcite dispersed in water. From analysis of the electron spin resonance spectrum, the fractions of monomers involved in trains (p) are deduced ; the dependence of p on the molecular weight of the amylopectins and the molecular structure of the polymers are discussed.

The activity coefficients of colons Cl − , of counterions Na + and Ca 2+ have been determined on Na-carboxymethylcellulose-NaCi and Ca-carboxymethylcellulose-CaCl 2 systems for two charge densities of the polyelectrolytes (λ stretched = 1.38 and 3.45); the mean activity coefficients of the external salt are then deduced. Their variations with the equivalent ratio X are investigated and the experimental values are compared to the theoretical ones given by the Manning treatment.

The polysaccharides form a very important class of biopolymers as much for their biological activities than the industrial applications.

L'etude porte sur les proprietes d'ultrafiltration d'une membrane homogene anionique ; la distribution des rayons de pores a ete determinee par thermoporometrie. Les taux de retention de solutes neutres et ioniques ont ete determines et compares a la fraction de pores accessibles y obtenue par thermoporometrie. En absence d'adsorption, on montre que le taux de retention d'un solute donne est egal a (1 — yR ) avec yR la fraction de pores de rayon superieur au rayon R de la molecule consideree ; un modele de tamis rend alors bien compte du comportement de la membrane. En presence d'adsorption meme faible (cas des dextranes dans l'eau), le taux de retention du solute determine a partir des flux de solvant et de solute est superieur a la valeur prevue a partir du volume hydrodynamique du solute.Avec des solutes ioniques, nous mettons de plus en evidence un effet d'exclusion electrostatique lorsque la force ionique de l'eluant est inferieure a 10— 2 N.

The chain association of pectic molecules with different levels and patterns of esterification during calcium-induced gelation was studied by methods such as light scattering, viscometry, and determination of calcium activity coefficient and of calcium transport parameter. A gel point can be determined, assuming a power law including a critical ratio (equivalent concentration of calcium ions/pectin carboxylic groups) and quasi-critical exponents varying in the range 0.50–1.26. The values of these critical parameters were discussed as a function of polymer concentration, of level and pattern of esterification, and of the nature of the divalent counterion. The lower these values are, the better, apparently, the gel-forming ability of the pectins is.