Fabrication and physicomechanical performance of casein-hydroxypropyl methylcellulose nanofibers

Nanofibers were electrospun (20 kV, 6 mL/h, 10 cm, 8 h) from a phase-separated mixture of hydroxypropyl methylcellulose (HPMC) and molecularly dispersed casein. Associative phase separation resulted in a dope comprising a gel-like coacervate phase dispersed in a casein solution with a third phase comprising casein aggregates. Beadless fibers of 535 nm average diameter, a maximum specific surface area of 3.3 m²/g, and maxima in Young's modulus and tensile strength were spun from a dope containing 1.5 % w/v HPMC and 18.5 % w/v acid casein in 50 % v/v aqueous ethanol at pH 10 demonstrating a minimum in surface tension. Classic spindle-shaped beads resulting from Rayleigh instability were observed at lower HPMC concentrations as were thickened, irregular fibers likely resulting from the unique phase behavior at higher HPMC levels. At 100 % relative humidity, the fiber mats readily adsorbed moisture, causing their transformation into clear films. Reinforcement with HPMC produced casein-rich nanofibers with improved mechanical strength and potential utility in food, biomedical, or cosmetic applications.