Enhancing the Dielectric Response of the ‘Hidden’ Secondary β-Relaxation in Polystyrene

The relaxation dynamics of polymers generally involve a series of molecular relaxations (α, β, γ…), and understanding these relaxations is important both for the fundamental understanding of polymer glass formation and the design of polymer glasses for specific applications. In particular, direct links have been identified between the secondary β-relaxation and the structural α-relaxation, where the latter defines the glass transition, suggesting that the β-relaxation is a precursor to structural dynamics in polymers. It is thus important to determine how the β-relaxation in polymers varies with parameters such as chain-length or `local’ chain characteristics. To achieve this, broadband dielectric spectroscopy (BDS) is a widely used technique that probes the reorientation of permanent dipoles over a wide frequency range. We here focus on the behaviour of one of the most investigated polymers - polystyrene (PS). PS is characterized by a relatively low segmental dipole moment, which makes the study of secondary relaxations using BDS difficult; BDS data for PS typically demonstrate a so-called excess wing on the high-frequency side of the dielectric α-relaxation response [1], and its interpretation is unclear. To address this, we add polar probe molecules to PS [2], and investigate the resulting effects on the BDS response. We find that addition of suitable polar probes leads to the observation of a molecular relaxation (here termed β*-relaxation). Comparisons of the properties of this β*-relaxation with results from small amplitude oscillatory shear rheology, dynamic mechanical analysis, as well as literature data, provide strong evidence that the dielectric β*-relaxation is directly related to a secondary β-relaxation that is present, but not fully observable, in BDS experiments on pure PS. Using the technique of probe addition, we determine the β-relaxation of PS for varying molecular weights, and we investigate the effects of probe characteristics on the observations.

References

[1] P. Lunkenheimer, U. Schneider, R. Brand, A. Loidl, Contemporary Physics 41, 15 (2000).

[2] J. Hintermeyer, A. Herrmann, R. Kahlau, C. Goiceanu, E.A. Rössler, Macromolecules 41, 9335 (2008).

[3] O. van den Berg, M. Wübbenhorst, S. J. Picken, W. F. Jager, Journal of Non-Crystalline Solids, 351, 2694 (2005).