Trail of pore shape and temperature-sensitivity of poly(N-isopropylacrylamide) hydrogels before and after removing Brij-58 template and pore formation mechanism

Abstract

To investigate the change upon structure and temperature-sensitivity of porous hydrogels before and after removing Brij-58 and confirm the pore formation mechanism, a series of hydrogels (abbreviated as CLH) based on N-isopropylacrylamide (NIPAM) serving as functional monomer and polyoxyethylene-20-cetyl-ether (Brij-58) acting as template were obtained. The chemical composition, crystal structure and temperature-sensitivity of PNIPAM hydrogels before and after removing Brij-58 were compared and confirmed by FTIR, XRD and DSC. The optical performance, pore shape and (de)swelling behavior of prepared CLH hydrogels were investigated by UV/vis, SEM and gravimetric method. The disappearance of the characteristic absorption peak at 1062–1146 cm⁻¹ and coincidental absorption peak at 195 nm, and recovery of diffraction peaks at 2θ = 7.8° and 2θ = 20.75° all confirmed that Brij-58 plays a template role and could be removed after 48 h. With increasing mass ratios of Brij-58/NIPAM from 0 to 2, the critical time of optical transmittance of CLH hydrogels dropping to 0 from 100 decreases to 7.5 min from 22 min, while maximum equilibrium swelling ratios increase from 6.3 to 10.31 g g⁻¹. SEM images show that, with increasing mass ratios of Brij-58/NIPAM from 0 to 2, CLH hydrogels first form homogeneous and smaller pores, and then pore sizes increase from 4–10 μm to 50–100 μm together with interconnected pore structure. With increasing mass ratios of Brij-58/NIPAM, thermo-responsibility of CLH hydrogels containing Brij-58 template disappeared gradually and then recovered after removing template. The deswelling rates of CLH hydrogels depend on the mass ratios of Brij-58/NIPAM as r = Brij-58/NIPAM ≤ 1/10, the hydrogels lose over 70% water within 60 min, while the hydrogel loses over 70% water within 10 min as r = Brij-58/NIPAM = 2. This kind of templated hydrogel with adjustable porous structure has much potential in the fields of drug delivery, tissue engineering and chemical separation.