Synthesis of new high molecular weight phosphorylated chitosans for improving corrosion protection

Abstract : Two grades of chitosan [chitosan 30000 g mol−1 (N-chitosan 30) and 250000 g mol−1 (N-chitosan 250)] were functionalized by the Kabachnik–Fields reaction. To obtain the highest phosphonic ester grafting rate (55% and 40% for the N-chitosan 30 and N-chitosan 250, respectively), the pH must be kept constant during the reaction (pH=5). Then, a partial hydrolysis of the ester functions was carried out in HCl medium to generate phosphonic acid functions up to 25% and 20% for the N-chitosan 30 and N-chitosan 250, respectively. It was shown that the grafting of phosphonic acids on chitosan significantly reduced the dynamic viscosity. Afterwards, electrochemical impedance measurements were performed in an aqueous solution (pH=5) in the presence of either N-chitosans or P-chitosans (3 wt.%). The two native N-chitosans were little adsorbed onto the carbon steel surface and the corrosion protection was low. In contrast, the impedance results in the presence of the 30000 g mol−1 phosphorylated chitosan (P-chitosan 30) evidenced the beneficial effect of grafted phosphonic acid on its adsorption on the steel surface. The lower efficiency of the 250000 g mol−1 (P-chitosan 250) was attributed to its high molecular weight which made difficult the interactions between the phosphonic groups and the metallic surface.
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Clément Coquery, Claire Negrell, Nicolas Caussé, Nadine Pébère, Ghislain David. Synthesis of new high molecular weight phosphorylated chitosans for improving corrosion protection. Pure and Applied Chemistry, De Gruyter, 2019, 17th International Conference on Polymers and Organic Chemistry (POC-17), 91 (3), pp.509-521. ⟨10.1515/pac-2018-0509⟩. ⟨hal-02067816⟩

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