Dynamics of polybutadienes with different microstructures .2. Dielectric response and comparisons with rheological behavior

A series of 1,2-1,4-polybutadienes with varying 1,2 (vinyl) content was investigated using time-domain dielectric spectroscopy, The time range was 10(-5)-300 s, which can be converted by Laplace transform to a frequency range of 10(-3)-6000 Hz. The samples were the same as those used in a previous rheological study from these laboratories. Therefore, a direct comparison of dielectric and mechanical responses was possible. Within experimental uncertainty, the ct relaxation observed by both methods shows the same temperature dependence but there is an offset between the characteristic times of both methods, which increases with increasing vinyl content. This result can be qualitatively understood from the difference of the size of the dipolar groups, viz, cis and vinyl monomeric units, in the context of the DiMarzio-Bishop model. In addition, the question of time-temperature superposition was studied using the dielectric data. In the cases of a vinyl content equal to or greater than 0.53 no deviations from time-temperature superposition-were detected. Only for the sample with the lowest vinyl content (0.07) does the attempt to construct a master curve from the dielectric loss data fail. In this case a fit with a combination of a Havriliak-Negami and a Cole-Cole functions suggests that this deviation from time-temperature superposition is an intrinsic feature of the ct alpha relaxation rather than an effect of its merging with the beta relaxation, The absence of indications for such, a deviation in the rheological study can be explained by the smaller frequency range of the latter. This stresses the necessity of a large dynamic range in experiments aimed at the examination of the time-temperature superposition principle. (C) 1997 American Institute of Physics.