Abstract
The synthesis and characterization of new polymeric materials containing crown ethers in their structure is very interesting due to their possibilities of use in many technological applications: metal ion catalysts, ion-exchange membranes, surfactants, etc. The dielectric and mechanical relaxation behavior of two membranes of poly(methyl methacrylate-co-(1,4,7,10-tetraoxacyclododecan-2-yl) methyl methacrylate) (10/90 and 25/75) has been studied.
The samples were characterized by using Differential scanning calorimetry (DSC), Broadband Dielectric Spectroscopy (BDS) and Dynamic Mechanical Analysis (DMA). DSC measurements were performed between 203K to 418K, BDS studies were performed in a frequency range of 10-2 Hz to 106 Hz at temperatures ranging from 133 to 443K and DMA measurements were carried at five frequencies (0.3, 1, 3, 10 and 30 Hz) and temperature window 143 to 443K.
Both membranes have a similar mechanical spectrum. At 1 Hz and around 413K a Ñ-relaxation process associated with the glass transition temperature can be observed. In decreasing order of temperatures a Ò-relaxation process, probably related with the Ò-relaxation process of the MMA can be observed. This relaxation is intimately overlapped with other non-well defined ×-secondary process. At lower temperatures another Ô-secondary relaxation can be observed. In addition, dielectric spectrum is in good agreement with mechanical spectrum.
The Ñ-relaxation process presumably arises from cooperative motions of molecules, whereas internal motions of the backbone in a variety of environments produce the secondary process. However, at low frequencies and high temperatures there is a high contribution of the conductive processes to the dielectric loss. Therefore, in order to establish comparison between mechanical and dielectric spectra were necessary to remove ohmic conduction using Kramers- Kronig equations. All relaxation processes were characterized by using the empirical Havriliak-Negami model. From the Arrhenius plot the activation energies associated with the secondary relaxations have been evaluated. On the other hand, the relaxation associated with the glass transition followed a behavior defined by the Vogel-Fulcher-Tammann-Hesse equation. The barrier associated with the conductivity process was evaluated from the d.c. conductivity evaluated by dielectric measurements
