Dielectric alpha-relaxaton and ionic conductivity in propylene glycol and its oligomers measured at elevated pressure

摘要

Structura dynamics and volume were measured as a function of both temperature and pressure for a propylene glycol and its oligomers (PPG),and the results compared withprevious data on higher molecular weight polypropylene glycols.PPG is of special interest because the terminal groups form hydrogen bonds;thus,by studying different molecular weights,the manner in wich hydrogne bonding influences the dynamics in the supercooled regime can be systematically investigated.The fragility (T_g-normalized temperature dependence) of the dimer and trimer of PPGincreases with pressure,similar to results for other H-bonded liquids,but different from van der Waals glass formers.This behavioris believed to be due to the effect of pressure in decreasing the extent of hydrogen bonding.From the combined temperature and volume dependences of the relaxation times,the relative degree to which thermal energy and volume dependences of the relaxation times,the relative degree to which thermal energy and volume govern the dynamics which thermal energy and volume govern the dynamics was quantfied.With decreasing molecular weight,the relative contribution of thermal energy to the dynamics was foudn to strongly increase,reflecting therole of hydrogen bonding.By comparing the ionic conductivity and the dielectric relaxation times,a decoupling between rotational and translational motions was observed.Interestingly,this decoupling was independent of both pressure and molecular weight,indicating that hydrogen bonds have a negligible effect on the phenomenon.