Studies on the internal medium-range ordering and high pressure dynamics in modified ibuprofens

Broadband dielectric spectroscopy (BDS), combined with X-ray diffraction (XRD) and Fourier transform infrared (FTIR) techniques were used to study the dynamics of primary (α) relaxation process and slow mode (SM), as well as structural properties and intermolecular interactions in the methyl-, isopropyl-, hexyl-, and benzyl derivative of a well-known pharmaceutical, ibuprofen (IBU). Unexpectedly, the XRD and FTIR methods revealed the formation of a medium-range ordering together with some molecular organization, which probably leads to the creation of small aggregates at the scale of several microns at lower temperatures. Unexpectedly, high pressure dielectric experiments revealed that the SM (observed at ambient pressure data) is not detected in the loss spectra of compressed IBU esters, which is consistent with the results reported previously for propylene carbonate and dioxolane derivatives. This finding can be interpreted as connected to either comparable time scale of the structural and slow mode or suppression of the motions responsible for the latter process at elevated pressure. Additionally, it was found that the pressure coefficient of the glass transition temperature (dT_g/dp) and activation volume (ΔV) change with molecular weight (Mw) in a non-monotonic way. It might be related to various chemical structures, conformations, intermolecular interactions, and possibly different architecture of supramolecular aggregates in the investigated compounds.