Thickness measurement of nanoscale polymer layer on polymer substrates by attenuated total reflection infrared spectroscopy

For the first time, attenuated total reflection (ATR)-Fourier transform infrared (FT-IR) spectroscopy was utilized to measure the thickness (d0) of a nanoscale polymer layer on polymer substrate with significant credibility. First, a mathematical formula, A/A0 = 1 - 2d0/ d(p), was derived based on a self-defining subsection function (where d(p) was defined as depth of penetration of ATR and A and A0 were defined as the absorption band area of the characteristic functional group only contained in bulk substrate with a thin polymer layer attachment and the same group in blank substrate, respectively). On the mathematical model, through changing incidence angles, a series of values of A (A0) and corresponding d(p) were obtained, and when plotting A/A0 versus 2/d(p), d0 was obtained as the slope. With polystyrene (coating)/olypropylene (substrate) as a model system, we obtained the relevant values (d0). Comparing the results with the values of practical coating thickness (calculation and TEM observation), we found that this method was able to characterize well the thickness of a thin polymer layer on a polymer substrate in the range from 10 to 110 nm. Errors in the measurement were given and analyzed. Furthermore, this method was well applied in the thickness measurement of a polyacrylamide graft layer on a polypropylene film surface. The effect of pressure in the ATR technique on the coating thickness measurement was also discussed. In comparison with other methods such as XPS, SEM, TEM, and AFM, this approach based on a universal ATR technique was very convenient and fast. This method is expected to widen the application of the ATR-FT-IR technique and stimulate the further development of many fields such as surface self-assembly and surface functionlization.