Very Low Frequency Measurements of Linear Alkanes

The study of vibrational spectra of linear alkanes has been used in the past to model the structure of polyethylenes. In particular, from these studies it has become clear that, in polyethylene, the longitudinal acoustic mode (LAM) frequency is inversely related to the length of polymer units between folds (lamellae), which is determined by sample history. This relationship had been developed by studying a series of linear alkanes. Spectra of samples of (C44H90), (C20D42), (C24D50), and (C36D74) are reported here. Although full spectra were recorded, we were more interested in the behavior in the low frequency region. Predictions for the LAM modes were in the range between 50 and 120 cm(-1), but we recorded spectra down to 4 cm(-1). In addition to the LAM modes, we observed sharp bands, some quite strong. An extensive literature search enabled assignment of these bands to librational modes (hindered rotations in the lattice). The ability to make this type of very low frequency measurement reliably can be used to predict physical properties of polymers and the crystalline polymorph of active pharmaceutical ingredients (API's), which has implications for bioavailability, shelf-life, stability, and protection of intellectual property (IP) for pharmaceutical manufacturers.