2.2. The preparation of PVDF–PVP membranes
The PVDF membranes were chemically modified by grafting
PVP via a three-step reaction described by Xu [17]. The modification
process is listed in Scheme 1 as follows. In the first step, pristine
PVDF membranes are defluorinated and double bond is
generated in alkaline and strong oxidizing condition, as indicated
in Scheme 1(a). Then polyalcohol is generated in acidic and nucleophilic
condition, as shown in Scheme 1(b). Finally PVP is introduced
onto polyalcohol membrane surface via hydrogen bond
interaction, as illustrated in Scheme 1(c).
Detailed modification process was carried out as follows. Firstly,
the PVDF70K membrane and PVDF100K membrane were fully immerged to KOH (0.5 mol/L) and KMnO4 (1.0 wt.%) aqueous solution
for 5 min at 55 C, and the membrane surface became brown.
Secondly, the membranes were rinsed with NaHSO3 (1.5 wt.%)
acidic aqueous solution. Thirdly, graft polymerization with PVP
was carried out by soaking the membranes in PVP (0.5 wt.%) aqueous
liquid for 5 min. Finally, the modified PVDF membranes,
denoted as PVDF70K–PVP and PVDF100K–PVP membrane, respectively,
were repeatedly washed with DI water to remove residual
PVP.