Recent advances in vibrational spectroscopy and their applications to polymer science

Traditionally, vibrational spectroscopy, with its different ramifications (mid-IR, NIR, Raman) has represented a major tool for the molecular characterization of polymer systems. In recent years significant advancements have occurred in both experimental approaches and data-analysis methods, which have strongly impacted the amount and quality of information made available by these techniques. In the present lecture two distinct subjects will be discussed, which can be considered as exemplary cases of contemporary advanced applications. The first part will deal with the use of in-situ, time resolved FTIR experiments for investigating sorption processes in polymers. The rationale of the spectroscopic approach for diffusion studies lies in the wealth of available information at molecular level that is unavailable when relying on purely gravimetric data. Recent experimental setups for transmission measurements will be described, together with the advanced data processing tools that have been proposed for enhancing the resolution and the quantitative analysis. In particular, difference spectroscopy (DS), least-squares curve fitting (LSCF), and 2-D correlation spectroscopy (2DCOS), have allowed the isolation of the penetrant spectrum and have provided information on the nature, number, and dynamic behavior of the different molecular species present in the systems under investigation. Methods of general applicability for quantifying the population of the above species will be also outlined. First-principle simulation of the vibrational spectra (DFT-based normal coordinate analysis) as a method for deepening the interpretation of the observed spectral features will be also touched upon. The second part of the lecture will deal with Raman imaging techniques and, in particular, with Tip-Enhanced Raman Spectroscopy (TERS). This recently developed technique combines confocal Raman spectroscopy with Atomic Force Microscopy (AFM) to achieve Raman imaging at the nanoscale, far beyond the diffraction limit of the probing light. It utilizes the localized surface plasmons of a sharp metallic nanotip to selectively enhance the laser field in the vicinity of the tip apex, thus confining Raman excitation to a nanoscopic volume of the sample placed under the tip. By scanning the tip over the sample surface, one can obtain a Raman image of the sample with a nanometric spatial resolution (10 nm, in optimum situations). Several applications to polymer systems will be illustrated, namely studies of catalytically-active surfaces, polymer blends with nanoscale heterogeneity and photovoltaic solar cells.