Recent developments in mid-infrared (MIR) spectroscopic ellipsometry enabled by quantum cascade lasers (QCLs) have resulted in a drastic improvement in signal-to-noise ratio compared to conventional thermal emitter based instrumentation. Thus, it was possible to reduce the acquisition time for high-resolution broadband ellipsometric spectra from multiple hours to less than 1 s. This opens up new possibilities for real-time in-situ ellipsometry in polymer processing. To highlight these evolving capabilities, we demonstrate the benefits of a QCL based MIR ellipsometer by investigating single and multilayered polymer films. The molecular structure and reorientation of a 2.5 µm thin biaxially oriented polyethylene terephthalate film is monitored during a stretching process lasting 24.5 s to illustrate the perspective of ellipsometric measurements in dynamic processes. In addition, a polyethylene/ethylene vinyl alcohol/polyethylene multilayer film is investigated at a continuously varying angle of incidence (0∘- 50∘) in 17.2 s, highlighting an unprecedented sample throughput for the technique of varying angle spectroscopic ellipsometry in the MIR spectral range. The obtained results underline the superior spectral and temporal resolution of QCL ellipsometry and qualify this technique as a suitable method for advanced in-situ monitoring in polymer processing.
Keywords: ellipsometry; in-line monitoring; mid-infrared spectroscopy; polymer films; polymer processing; quantum cascade laser; real-time.