Water absorption rate (%) = (Wwet -Wdry)/Wdry × 100 (1) Zeta Potential. The zeta potential of the PP NWFs was measured using an electrokinetic analyzer SurPASS (Anton Paar, Austria) equipped with an adjustable gap cell. The surface zeta potential was determined as a function of pH in a 1 mM KCl electrolyte solution by varying the solution pH by adding 0.1 M HCl or NaOH via the automatic titration unit of the instrument. Four measurements were taken for each pH value.