The dielectric relaxation properties of poly(methylmethacrylate) (PMMA), polystyrene-block-poly(methylmethacrylate) (PS-b-PMMA) diblock copolymers and PS/PMMA blends were investigated by broadband dielectric spectroscopy in the frequency range from 0.01 Hz to 1 MHz and temperature range from - 40 to 150°C. By parametric fitting techniques, an α relaxation process associated with the glass transition, the secondary β relaxation process due to localized motions of side groups, and the conductivity at low frequencies were identified and modeled for all samples. Based on the modeling results, the characteristic relaxation time, fragility index, activation energies, and relaxation strength of PMMA and PS-b-PMMA diblock copolymers were compared. It was found that the structural α relaxation of the copolymers was suppressed compared with that of PMMA. The covalent chemical bonds that link PS and PMMA blocks in the diblock copolymers are therefore assumed to dominate over the PMMA-PMMA, PMMA-PS and PS-PS bonds that exist in the PMMA and PS/PMMA samples, accounting for the suppression of the cooperative structural α relaxation and for larger activation energies of the localized β relaxation of the diblock copolymers. Further, the activation energy barriers of localized β relaxations in the copolymers are observed to increase, above the glass transition temperature.
Bibliographical noteFunding Information:
We gratefully acknowledge funding support from the Air Force Office of Scientific Research ( FA9550-09-1-0388 ).
- Dielectric spectroscopy
- Havriliak-Negami model
- Molecular dynamics