Water-Induced Conformational Transitions and Complex Formation in Concentrated Polyacrylonitrile–Dimethyl Sulfoxide Solutions

Understanding the molecular interactions between solvent components and polymer chains is crucial to advancing high-performance fiber technologies. Here, we combine vibrational spectroscopy, quantum-chemical modeling, rheological analysis, and controlled stretching experiments to unravel the fundamental role of dimethyl sulfoxide-water interactions in polyacrylonitrile (PAN) solutions. Infrared spectroscopy reveals distinct signatures of solvent-solvent and solvent-polymer hydrogen bonding, which are rationalized by quantum-chemical calculations and correlated to macroscopic rheological behavior. The molecular picture obtained explains the non-trivial dependence of viscoelasticity and fiber drawability on the water content. Moreover, stretching experiments under different humidity conditions directly demonstrate how nanoscale solvent structuring translates into the tunable mechanical performance of PAN fibers. This integrated approach establishes a clear link among molecular solvation, processability, and final fiber properties, opening new opportunities for the design of advanced precursors for carbon fibers.