Peculiar relaxation dynamics of propylene carbonate derivatives

The aim of this work is to analyze in detail the effect of the alkyl chain length on the dynamics of glass-forming propylene carbonate (PC) derivatives. Examined samples are low-molecular weight derivatives of the PC structure, i.e., the 4-alkyl-1,3-dioxolan-2-one series, modified by changing the alkyl substituent from methyl to hexyl. The molecular dynamics (MD) has been analyzed based on experimental data collected from differential scanning calorimetry, broadband dielectric spectroscopy (BDS), X-ray diffraction (XRD), and nuclear magnetic resonance relaxometry measurements as well as MD simulations. The dielectric results show in samples with the propyl- or longer carbon chain the presence of slow Debye-like relaxation with features similar to those found in associative materials. Both XRD and MD reveal differences in the intermolecular structure between PC and 4-butyl-1,3-dioxolan-2-one liquids. Moreover, MD shows that the probability of finding one terminal carbon atom of the side chain of BPC in the vicinity of another carbon atom of the same type is much higher than in the case of PC. It suggests that there is a preference for longer hydrocarbon chains to set themselves close to each other. Consequently, the observed slow-mode peak may be caused by movement of aggregates maintained by van der Waals interactions. Reported herein, findings provide a new insight into the molecular origin of Debye-like relaxation.