Structural and Thermal Analysis of Copper-Doped Poly(N-isopropylacrylamide) Films

Abstract

In the present study, the chemical structure of thermoresponsive copper-doped poly(N-isopropylacrylamide) thin films was investigated. The polymer thin films were deposited by spin-coating from a solution containing the polymer on silicon windows. Spin-coating was carried out at certain conditions yielding films of nanometric-scale thickness (170–250 mn). Thermal transitions such as low critical solution temperature (LCST) and glass transition temperature (Tg) of polymer samples with respect to copper concentrations were studied by the differential scanning calorimetry (DSC) technique. Thermograms show that thermal transitions shift to higher temperatures after doping polymer with Cu+2. Heat capacity (Cp, J/g.°C) also increases as the concentration of Cu+2 increases. Fourier transform-infrared (FT-IR) spectrum of pure poly(N-isopropylacrylamide) film exhibits several characteristic stretching bands attributed to Vas (NH), amide I (C=O), and amide II (C-N) respectively. The infrared spectrum of the corresponding Cu+2-doped polymer thin films showed a significant shift in the characteristic bands compared to that of pure polymer, indicating a strong interaction between Cu+2 and poly(N-isopropylacrylamide). The UV-visible spectrum of Cu+2-doped poly(N-isopropylacrylamide) shows the creation of a new band positions at 276 nm and 278 nm. These bands are absent in the pure polymer spectrum, indicating a complex formation between Cu+2 and poly(N-isopropylacrylamide). Coil to swollen aggregate formation was investigated by using the tapping mode atomic force microscopy (AFM) technique. Addition of copper ions to the polymer shows a clear change in the morphology of the polymer thin films compared to the morphology of pure poly(N-isopropylacrylamide) prepared with water as solvent, resulting in clusters that approach single nanoparticle behavior.