Temperature dependence of aggregation behavior and dye adsorption of poly(N-isopropylacrylamide) hydrogel/mesoporous silica composites

Abstract

This study aimed to synthesize mesoporous silica (MS) with various amounts of poly(N-isopropylacrylamide) (PNIPAM) hydrogel using two procedures and investigate the influence of the PNIPAM hydrogel on the aggregation performance of the synthesized PNIPAM hydrogel/MS composite at temperatures above the lower critical solution temperature (LCST) of PNIPAM (i.e., 32 °C) and adsorption behavior of the composite below the LCST. To rapidly recover an MS adsorbent from water, we synthesized the MS coated with hydrogel composed of PNIPAM as a temperature-responsive polymer and N, N′-methylenebisacrylamide (BIS) as a cross-linker. We characterized our samples using ultraviolet-visible spectroscopy, X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), nitrogen adsorption and desorption experiment, and thermogravimetric analysis (TG). Using TG, we found that the amount of immobilized PNIPAM hydrogel could vary from 6.6 to 57.5 wt% by changing the amount of BIS and the immobilization procedure. The XRD patterns revealed that the PNIPAM hydrogel coating had slight influence on the MS structure. The higher the amount of PNIPAM hydrogel in the composite, the faster the turbidity rate decreased based on the turbidity measurement of the supernatant solution during the PNIPAM hydrogel/MS composite sedimentation test in water at 50 °C. The coating by PNIPAM hydrogel of the MS surface only slightly affected the amount of methyl orange adsorbed at 30 °C.