Nanofibrous membranes from aqueous electrospinning of carboxymethyl chitosan

Carboxymethyl chitosan (CMCS) with varying molecular weights (M_v = 40-405 kDa) and degrees of substitution (DS = 0.25-1.19) has been synthesized by alkalization of chitosan, followed by carboxymethylation with monochloroacetic acid. At DS up to 1.19, the locations where carboxymethylation took place were influenced by the alkalization temperature, i.e., both C2 and C6 substitution at ambient temperature (N,O-carboxymethylated) and mainly C6 substitution at the lower temperature, -15°C (O-carboxymethylated). Generally, carboxymethylation was more favorable at the primary C6 hydroxyl than the C2 position with increasing DS. CMCS synthesized from 405 kDa CS was soluble in water at DS≥0.73 whereas those from 40 and 89 kDa became water-soluble at lower DS of 0.25 and 0.36, respectively. Electrospinning of aqueous solutions of CMCS was facilitated with the addition of water-soluble polymers, including PEO, PAA, PAAm and PVA. The optimal fiber formation was observed at equal mass composition of O-CMCS (89 kDa at 0.36 DS) and PVA, producing nanofibers with an average diameter of 130 nm. Heat-induced esterification (at 140°C for 30 min) produced inter-molecular covalent cross-links within and among fibers, rendering the fibrous membrane water-insoluble. Membranes containing higher CMCS carboxyl to PVA hydroxyl ratio retained better fiber morphology upon extended water exposure, indicating more favorable inter-molecular cross-links. The fibrous membranes generated with less substituted CMCS were more hydrophilic and retained a greater extent of the desirable amine functionality.