Infrared and Raman spectroscopy of polypropylene

Vibrational spectroscopy is one of the most versatile methods of polymer characterization [1]. Other methods may be better in a certain area, but the advantage of vibrational spectroscopy is that it supplies several types of information, while being a rapid and inexpensive method. Infrared (IR) and Raman spectroscopy probe the vibrational spectrum by absorption and inelastic scattering, respectively. In many cases these two techniques are complementary. For a noncentrosymmetric molecule, such as isotactic PP (iPP) in a 31 helix, bands which are strong with one technique are often weak with the other. From a practical point of view, the requirements for sample preparation are different. IR spectroscopy, being an absorption method, is mostly performed on thin films in transmission mode. However, reflection methods or photoacoustic detection can be utilized to analyze thick or opaque samples, with a range of penetration depths. In principle, no sample preparation is required for Raman spectroscopy. Fluorescence has been a problem when analyzing polymers with this technique. However, this problem is almost eliminated with Fourier transform (FT) Raman instruments, which excite in the near infrared (NIR) range (∼4000-11000 cm-1) and analyze the scattered light in a modified FT-IR instrument.