Study of dielectric relaxation and thermodynamic behaviour in poly(propylene glycol) using Time Domain Reflectometry

Temperature dependent dielectric relaxation and thermodynamic energy parameters of polypropylene glycols HO[CH(CH3)CH2O)(n)H with number average molecular weight 425(n = 7), 1000 (n = 17) and 2000 (n=34) g mol(-1) have been studied using Time Domain Reflectometry (TDR). The frequency dependence of the complex dielectric permittivity is analyzed by the Havriliak-Negami expression. The static dielectric constant epsilon(0), high frequency limiting dielectric constant epsilon(infinity) relaxation time tau(0) and thermodynamic energy parameters such as free energy, enthalpy of activation and entropy of activation have been determined. The average free energy of activation Delta F-tau for all the three PPG molecules was found to be in the range of 4-5 kcal mol 26(-1). The values of entropy Delta S-T for PPG-2000 molecules were found to be negative while entropy Delta S-tau for PPG-425 and PPG-1000 molecules found positive, which confirms that the configuration of PPG-2000 involved in the dipolar orientation has an activated state, which is more ordered than in the normal state compared to PPG-425 and PPG-1000 molecules. (C) 2011 Elsevier B.V. All rights reserved.