Sub-TG relaxations in LiClO4-poly (propylene glycol)-2000 solutions

The dielectric permittivity and loss of LiClO₄ solutions in poly (propylene glycol (PPG)), molecular weight 2000, have been measured over a concentration range up to a ratio of Li⁺ to oxygen atoms in PPG of 33.3:100, between 77 and 350 K. The data have been analyzed in both the permittivity and electrical modulus formalisms. Addition of LiClO₄ to poly (propylene glycol) first increases the height of the β-relaxation peak, and ultimately a second sub-T_g relaxation peak at a higher temperature emerges. This is in addition to the β-relaxation peak due to the reorientation of PPG dipoles, whose strength decreases from that in pure PPG-2000. For a fixed temperature, the dc conductivity initially decreases with increasing Li⁺ concentration up to 20 Li⁺ per 100 O atoms and thereafter increases. This concentration corresponds to that at which the T_g of the solution reaches its limiting value of ca. 310 K. It is concluded that the formation of ion pairs causes a second and slower sub-T_g relaxation process and that the increase in the efficiency of chain packing reduces the strength of the β-relaxation of the polymer.