Site-Dependent Segmental Dynamics Revealed Using Broadband Dielectric Spectroscopy on Well-Defined Functionalized Polystyrenes

Abstract

The site-dependent segmental dynamics of polystyrene has been investigated by using a combination of broadband dielectric spectroscopy (BDS) and well-defined, functionalized polystyrenes (Mn ≈ 2200 and 4200 g/mol) with strongly polar cyano (CN) groups precisely located either at the end of the chain (chain-end functionalization) or in the middle of the chain (in-chain functionalization). Since broadband dielectric spectroscopy (BDS) is sensitive to the local dipole moment fluctuations, it is possible to selectively probe the polymer motions responsible for the dipole moment fluctuations using these cyano-functionalized polymers. By means of rheology and differential scanning calorimetry (DSC), it was confirmed that neither chain-end nor in-chain CN-functionalization affected the overall polymer matrix properties as compared with an equivalent H-functionalization analogue. This was confirmed for important properties such as the glass transition temperature (Tg) and viscosity (η). Results obtained for the two different molecular weights show that the dielectric signal associated with the chain-end fluctuations exhibits an accelerated and more heterogeneous dynamics than the average segmental α-relaxation mainly at high temperature. Closer to the Tg, the time scale of the chain-end fluctuations and the overall dynamics approach each other, and the apparent decoupling disappears. In contrast, for the segments located in the middle of the chain, the dynamics display rather similar features as the mean response although it is distinctly slower approaching Tg. This has been interpreted as combination of the intrinsic dynamic segmental heterogeneity of the polymer chain combined with the increasing cooperativity length close to Tg. The functionalization with the CN groups also allows the observation of a rather weak low-temperature relaxation process that is interpreted as a specific, more localized movement of the functional groups. The activation energies obtained are typical for motions of single molecules or segments. Interestingly, the time scale associated with this relaxation seems to decrease with molecular weight, suggesting poorer local packing for larger chains. Furthermore, the time scale depends also on the functionalization site, indicating the relevance of intermolecular interactions.