Synthesis and characterization of poly(N-isopropylacrylamide) films by photopolymerization

Abstract

A novel method used for the preparation of poly(N-isopropylacrylamide) (PNIPAAm) films of varying crosslink density under homogeneous/heterogeneous conditions is described in this paper. Photopolymerization of the N-isopropylacrylamide (NIPAAm) monomer in water (homogeneous at ∼7°C and heterogeneous at ∼40°C) or a mixture of water/ethanol (50:50, heterogeneous at ∼7°C) was carried out using 1-[4-(2-hydroxyethoxy)-phenyl]-2-hydroxy-2-methyl-1-propane-1-one (hydrophilic) or 2-hydroxy-2-methyl propiophenone (hydrophobic) photo-initiator. In order to investigate the effect of temperature and crosslink density, polymerization was carried out at ∼7°C [below lower critical soluble temperature (LCST)] and ∼40°C (above LCST) using varying amounts of N,N′-methylene bisacrylamide (BIS) ranging from 1–4 wt%. Degree of swelling (determined by optical microscopy), phase transition temperature [determined by differential scanning calorimetry (DSC)] as well as morphology (scanning electron microscopy) were found to be dependent on solvent system (homogeneous/heterogeneous), temperature of polymerization and crosslink density. Hydrogels prepared at ∼7°C using hydrophobic photo-initiator and water/ethanol (50:50) as solvent, showed much higher degree of swelling at all levels of crosslink density as compared to hydrogel prepared at ∼7°C using hydrophilic photo-initiator and water as solvent. Hydrogels were used for patterning which may find applications in microfluidic devices. Copyright © 2006 John Wiley & Sons, Ltd.