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

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.