The alternate temperature-change cleaning behaviors of PNIPAAm grafted porous polyethylene membrane fouled by proteins

Based on the reversible volume phase transition of Poly(N-isopropylacrylamide) (PNIPAAm) around LCST, a simple temperature-change cleaning method for PNIPAAm grafted porous polyethylene membrane fouled by bovine serum albumin (BSA) was proposed. It was found that after the temperature-change cleaning program between 25 and 35 C, the fouling BSA could be eluted out easily by water from the graft membrane surface, and a flux recovery of 97.6% relative to that of the graft membrane before fouling was reached. The hydrophilicity and the rapidly stretching of PNIPAAm chains are considered responsible for loosing and shaking off the adsorbed BSA on graft membrane surface. However, the streaming potential measurements indicated a secondary BSA fouling occurring on the hydrophobic substrate membrane surface that was freshly exposed by the stretching PNIPAAm chains. The secondary BSA fouling was cleaned effectively with a streaming potential recovery of 99.1% by an alkali solution of 0.1 wt.% KOH also under the assistance of temperature-change cleaning method, and it is assumed that a higher enough graft density of PNIPAAm on substrate membrane surface was necessary to prevent the secondary BSA fouling. In the end, the above fouling condition and cleaning effects on graft membrane surface were further verified by ATR FT-IR analyses.

3.3. FT-IR analyses The BSA fouling and cleaning processes were simulated according to the cleaning protocol shown in Fig. 1 and analyzed by FT-IR. The ATR FT-IR spectra for the cleaning effects are shown in Fig. 6. Both BSA and PNIPAAm graft chains have the same amide structure exhibiting the overlapped amide characteristic bands at 1650 cmÀ1 (amide I) and 1548 cmÀ1 (amide II). After BSA fouling, the two bands are found slightly increasing, but the band at 1014 cmÀ1 enhances more obviously (Fig. 6, step A), which is considered the responsible structure for BSA binding to the surface of graft membrane [24,25]. Under the temperature-change cleaning steps from B to D, all the three bands are found the gradually decreasing tendency, however, the band 1014 cmÀ1 still exists indicating the residual BSA adsorbed on graft membrane surface. The band 1014 cmÀ1 becomes finally invisible only under the alkali solution elution along with the assistance of the temperature-change cleaning Substrate membrane Step F Reflectance (%) Step D Step C Step B Step A By grafting PNIPAAm on the surface of porous polyethylene membrane, a temperaturechange cleaning method for BSA fouled membrane surface was proposed based on the reversible volume phase transition of PNIPAAm. The cleaning method is simple just by the temperature-change water elution around LCST of PNIPAAm graft chains. After the alternate temperature-change cleaning program between 25 and 35 C, the water flux recoveries of 97.6% was reached relative to that of the graft membrane before fouling. The flux measurement results indicate that most of the fouling BSA has been eluted out from the graft membrane surface. The hydrophilicity and the rapidly stretching of PNIPAAm chains are considered responsible for loosing and shaking off the adsorbed BSA on graft membrane surface. However, a secondary BSA fouling adsorbed on the freshly naked substrate membrane surface exposed by the stretching PNIPAAm chains was detected by the susceptible streaming potential measurement. These residual BSA were eventually cleaned out by the alkali solution elution also with the assistance of the temperature-change cleaning program, after which a streaming potential recovery of 99.1% was reached. It is assumed that the secondary BSA fouling could be prevented or relieved by a higher enough graft density of PNIPAAm. In the end, the above fouling conditions and cleaning effects were further verified by FT-IR analyses.