Alain Domard

Formation and characterisation of a physical chitin gel

Abstract The formation of N -acetyl chitosan gels in an acetic acid-water-propanediol solution was studied. Side reactions arising from the esterification of 1,2-propanediol by acetic anhydride, and hydrolysis of acetic anhydride were studied, as well as their possible role on the gel formation. The gels were studied by FTIR and 1 H NMR spectroscopy. Independently of the initial acetylation degree of chitosan, a minimal acetylation degree is required for gelation (80%). The final acetylation degree of the gels increases with the molar ratio acetic anhydride/amine, but does not reach 100%. An increase in temperature favours the molecular mobility, inter- and intra-molecular hydrogen bondings and hydrophobic interactions, responsible for gelation which appears to depend on two parameters: a critical acetylation degree and a critical molecular weight allowing the aggregation of the polymer chains. The nature of the solvent used to avoid chitosan side reactions during gelation (ethanol or propanediol) does not influence the chemical structure of the gel but only the kinetics of gelation. The formation of N -acetyl chitosan gels in acetic acid-water-propanediol solution is reported. The influence of parameters such as temperature, initial acetylation degree, molecular weight of chitosan and the nature of the alcohol, on the acetylation reaction and gelation is studied.

Chitosan carboxylic acid salts in solution and in the solid state

When chitosan is associated to simple carboxylic acids, such as formic, acetic, etc., electrostatic interactions occur, resulting in a salt formation, to a greater or lesser degree. Fourier transform infra-red (FTir) spectroscopy and potentiometric techniques were used to characterize the nature of such interactions, in aqueous solutions and in films. The evolution of solid state samples during storage and upon dehydration was studied, by FTir spectroscopy and transmission X-ray diffraction. Results showed that the carboxylic acid content of the films progressively decreased in relation with the physicochemical parameters of the acids (pKa, solubility, etc.). These results were compared with those obtained with a strong acid such as HCl or a complex carboxylic acid (lactic). In X-ray diffraction experiments, three crystalline structures of chitosan were found: a hydrated form, independent of the chemical nature of the salt, and two dehydrated forms depending on the chemical structure of chitosan.

Sorption and desorption studies on chitin gels.

The aim of this work was to study various transport phenomena in chitin gels obtained by N-acetylation of chitosan in a water-alcohol mixture. Three kinds of transport were investigated: the sorption of solutes interacting with chitin, the desorption of solutes without significant interaction with the polymer, and osmosis phenomena. In the case of interactive sorption, dyes having different chemical structures such as C.I. Acid Blue 74, C.I. Reactive Violet 5 or C.I. Direct Red 28 were tested. Sorptions of C.I. Acid Blue 74 and C.I. Reactive Violet 5 depend on the charge density of the polymer network and, as a consequence, on DA, pH and the dielectric constant of the media. This result reveals the importance of electrostatic interactions. On the other hand, the sorption of C.I. Direct Red 28 is mainly due to hydrophobic interactions and H-bonding, it is limited to the extreme surface of the gel. Concerning the non-interactive desorption, solutes of different steric hindrance such as PP vitamin, B1 vitamin and caffeine exhibit similar diffusion coefficients located within 3.7-5.6x10(-6) cm(2) s(-1). Finally, the osmotic behaviour of the gel immersed in a concentrated solution of gelatin allows us to multiply by 25 the concentration of chitin in the gel without any penetration of gelatin.

Chitosan behaviours in a dispersion of undecylenic acid

Abstract Chitosan is able to flocculate or stabilize aqueous dispersed media containing undecylenic acid. These flocculation/dispersion phenomena were studied by potentiometric, UV spectrometric and quasi-elastic light scattering techniques. Parameters such as pH, ionic strength of the media and total lipid concentration have a great influence on the interactions occurring between chitosan and the lipid particles. Adequate pHs, ionic strengths and a lipid concentration above a certain limit are required to obtain flocculated or dispersed systems. High ionic strengths, as well as dilution, tend to increase the particle sizes and destabilize the dispersions. On the other hand, high lipid concentrations yield smaller chitosan coated particles. A mechanism of adsorption of chitosan chains onto lipid particles is proposed, which explains the influence of the various parameters.

Glucosamine oligomers: 4. solid state-crystallization and sustained dissolution

When glucosamine oligomers are stored in the solid state they undergo a process of crystallization. The extent to which this occurs depends on whether the samples are isolated in the -NH3+ or -NH2 form, on the storage time, and on the degree of polymerization of the isolated oligomer. The allomorph obtained by this process seems to correspond to the so-called tendon-chitosan. Dissolution of such aged oligomer samples gives rise to a process of dissociation of the associated chains in the crystal, leading to the establishment of a pseudo-equilibrium between single and associated oligomer chains and hence the simultaneous presence of the monomeric, dimeric, trimeric, etc., forms of the oligomer. The phenomenon cannot be attributed to a process of aggregation in solution. The effects of various parameters on this behaviour have been investigated.

Physicochemical characterization of polypropylene films grafted by poly(acrylic acid). 1

Abstract Various analyses were used to characterize polypropylene films grafted with poly(acrylic acid): infra-red spectroscopy (by means of transmission, total attenuated reflection and microscopic techniques), permeation and optical microscopy on coloured samples. We have compared the results obtained when the film to be modified was initially treated to eliminate incorporated additives with those obtained when the additives were not removed. In addition to the progressive propagation of the grafting previously evidenced, we have shown that the solubility parameter plays the major part in the transport of the monomer molecules from the surface to the bulk.

Glucosamine oligomers: 2. N.m.r. studies on a DP3.

This paper concerns the characterization of the chemical structure of a DP3 glucosamine oligomer. The assignments of nearly all protons are reported. Variations of 1H chemical shifts with pD and temperature are correlated to the pKa of amino groups, and not with substantial conformational changes.

Chitosan behaviours in a dispersion of undecylenic acid. morphological aspects

Electron microscopy is used to complement our study of the interactions occurring between chitosan and undecylenic acid dispersions. Our previous papers reported investigations regarding the influence of physico-chemical and structural parameters, and a mechanism of interaction was proposed. We now confirm these studies by analysing the microscopic structure of chitosan/undecylenate flocs, using various TEM and SEM techniques. Catenary, fractal structures are observed, in agreement with a theoretical model established for the flocculation of spherical particles by polymer chains. Finally asymmetrical membranes were prepared and characterized. They result from the association of a chitosan undecylenate salt layer and chitosan/lipid aggregates in the redispersed state, according to a 3-step mechanism which is proposed.

Fine microstructure of processed chitosan nanofibril networks preserving directional packing and high molecular weight.

Crystalline chitosan nanofibril networks were prepared, preserving the native structural packing and the polymer high molecular weight. The fine microstructure of the nanomaterial, obtained by mild hydrolysis of chitosan (CHI), was characterized by using synchrotron small- and wide-angle X-ray scattering (SAXS and WAXS), transmission electron microscopy (TEM) and electron diffraction. Hydrolysis of chitosan yielded a network of crystalline nanofibrils, containing both allomorphs of chitosan: hydrated and anhydrous. The comparison of WAXS data in transmission and reflection mode revealed the preferential orientation of the CHI crystals when subjected to mechanical compression constrains. The results are in agreement with the existence of a network nanostructure containing fiber-like crystals with the principal axis parallel to the polymer chain axis. The evolution of the CHI allomorphic composition with temperature was studied to further elucidate the mechanism of structural transitions occurring during CHI nanofibril network processing.