Quantitative relation between thickness and grafting density of temperature-responsive poly(N-isopropylacrylamide-co-acrylamide) thin film using synchrotron-source ATR-FTIR and spectroscopic ellipsometry

Poly(N-isopropylacrylamide-co-acrylamide) (PNIAM-co-AM) grafted onto tissue culture polystyrene surfaces (TCPSs) with different thicknesses and grafting densities were prepared by varying the spin-coating speed and UV irradiation time for use in cell sheet engineering. The thickness of the PNIAM grafted surface, less than 30 nm, was believed to influence protein adhesion and cell attachment. However, methods for determining the thicknesses of a thin surface (nanometer scale) are still a challenge. In this study, a model was developed to relate the thickness of the PNIAM-co-AM film to its grafting density. Spectroscopic ellipsometry (SE) and attenuated total reflection-Fourier transform infrared spectroscopy (ATR-FTIR) with synchrotron radiation were used to quantify the thickness and the grafting density, respectively. The amount of the grafted copolymer was determined from quantitative ATR-FTIR by relating the signals at wavenumber 1,654 to 1,600 cm(-1) (A(1,654)/A(1,600)) to the known grafting density. A combination of SE and ATR-FTIR was used to construct a linear model (h = 6.29 sigma + 2.24), to obtain the grafting density (sigma) from the film thickness (h) with R-2 = 0.93. To validate the model, random samples were prepared with different thicknesses and grafting densities. The linear model was used to predict the grafting density, which was nearly equal to the measured values.