The Different Interaction Mechanisms of Carbon Dioxide in Macromolecular Environments: A 2DCOS Study

Carbon dioxide is a molecule of fundamental interest: besides the environmental issues related to its accumulation in the atmosphere, which demand reliable storage-release devices, it is attracting much interest as green-chemistry solvent. In particular, it has been demonstrated that supercritical CO2 has considerable potential in the field of polymer processing, as well as for separations, extraction and fractionation. Recently it has also been proposed as a medium for polymer synthesis [1]. However, despite the numerous studies on the subject, the molecular interactions formed by CO2 with the substrates in which it is absorbed, remain an open issue. Several mechanisms, either concurrent or mutually exclusive, have been proposed, ranging from purely physical phenomena (Henry's type solubility) to electron-donor/acceptor interactions, to weak Hydrogen bonding. The current perception is that the molecular interactions occurring in a specific system depend on the nature of the polymer host and of its interaction sites. Infrared spectroscopy represents a powerful tool for molecular characterization; in particular, time-resolved diffusion measurements allow us to detect the spectrum of the probe molecule, whose vibrational response is directly related to its geometric/electronic state, thus providing relevant information on the interactional behavior. At the same time, the perturbation of the IR spectrum of the host is useful to identify the active sites present in the polymer structure and the interaction mechanism. These two pieces of information, taken together, form a comprehensive molecular picture, which in some cases may be completed by a population analysis, i.e. by evaluating the concentration of the different molecular species identified spectroscopically [2]. However, the analysis of FTIR spectra poses nontrivial problems, mainly related to a considerable band broadening, which limits and often prevents the resolution of the individual components. In this respect, the application of 2D-FTIR correlation spectroscopy (2D-COS) to diffusion studies has been demonstrated to be very powerful. In fact, it brings about a significant enhancement of resolution, it helps to identify the individual molecular species and the signals they produce and may provide details on the dynamic behaviour of the system, making use of the asynchronous spectrum. In the present contribution the FTIR spectra collected in situ during sorption-desorption cycles of CO2 in two different macromolecular systems are examined by means 2D-COS.