Primary and secondary relaxations in bis-5-hydroxypentylphthalate

Broadband dielectric spectroscopy was used to study the relaxation dynamics in bis-5-hydroxypentylphthalate (BHPP) under both isobaric and isothermal conditions. The relaxation dynamics exhibit complex behavior, arising from hydrogen bonding in the BHPP. At ambient pressure above the glass transition temperature T(g), the dielectric spectrum shows a broad structural relaxation peak with a prominent excess wing toward higher frequencies. As temperature is decreased below T(g), the excess wing transforms into two distinct peaks, both having Arrhenius behavior with activation energies equal to 58.8 and 32.6 kJmol for slower (beta) and faster (gamma) processes, respectively. Furthermore, the relaxation times for the beta process increase with increasing pressure, whereas the faster gamma relaxation is practically insensitive to pressure changes. Analysis of the properties of these secondary relaxations suggests that the beta peak can be identified as an intermolecular Johari-Goldstein (JG) process. However, its separation in frequency from the alpha relaxation, and both its activation energy and activation volume, differ substantially from values calculated from the breadth of the structural relaxation peak. Thus, the dynamics of BHPP appear to be an exception to the usual correlation between the respective properties of the structural and the JG secondary relaxations.