Abstract

This work is focused on understanding how the properties of chitosan-g-poly(N,N-diethylacrylamide-co-N,N-dimethylacrylamide) and chitosan-g-poly(N,N-diethylacrylamide-co-N-ethylacrylamide) copolymers are affected by varying their compositions. Poly(N,N-diethylacrylamide), PDEAm, is a thermosensitive polymer with a reversible coil-to-globule transition in aqueous solution near the human body temperature. The transition temperature can be tuned by varying the amount of two more hydrophilic units: N-ethylacrylamide (NEAm) or N,N-dimethylacrylamide (DMAm). While the former can participate in hydrogen bonding as a proton donor or acceptor, the latter is only an acceptor and a very-well-known hydrophilic unit with no phase transition. The gelation process was followed by rheological measurements, showing the general features of the chemical gelation and the formation of a strong gel. A reaction autoacceleration was observed, which is interpreted by the high viscosity of the reaction mixture due to the presence of chitosan, although a greater effect was found in NEAm copolymers due to the higher hydrogen-bonding capacity of NEAm. A relationship was found between the compositions of both copolymers, the strength of the hydrogels, and the calculated pore size. The pore sizes range from 32 to 18 nm. Thermogravimetric analysis of the hydrogels showed the typical decomposition stages of polyacrylamides. Neither the temperature of the maximum decomposition rate nor the weight loss associated displayed a clear trend with the copolymer composition. There was an increment in the equilibrium swelling values with the content of NEAm units (from 16 to almost 28 g water/g polymer), but the copolymers with DMAm remained unaffected (swelling values around 23 g water/g polymer). The hydrogen-bonding interaction affected the gelation process and swelling behavior of the hydrogels. This can be explained by the greater ability of NEAm to form hydrogen bonds compared to DMAm.

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