Dynamics of Rouse chains undergoing head-to-head association and dissociation: Difference between dielectric and viscoelastic relaxation

Abstract

Polymer chains having type-A dipoles parallel along the chain backbone exhibit slow dielectric relaxation reflecting their global motion. For monofunctionally head-associative Rouse chains having type-A dipoles, the Rouse equation of motion was combined with the association/dissociation kinetics to calculate the dielectric relaxation function, Φj(t) with j = 1 and 2 for unimer and dimer. Φ1(t) reflects the orientation of the end-to-end vector of the unimer, whereas Φ2(t) detects the orientation of two end-to-center vectors of the dimer (having symmetrical dipole inversion), both being in the direction of the applied electric field. The calculation was made by mapping the conformation of dissociating dimer onto the created unimer and vice versa with the aid of the Rouse eigenmode expansion of the bond vector of segments (Gaussian subchains). It turned out that Φj(t) is not influenced by the association/dissociation reaction. This result makes a striking contrast to the behavior of the viscoelastic relaxation function gj(t): gj(t) is strongly affected by the motional coupling between the unimer and dimer due to the reaction [H. Watanabe et al., Macromolecules 48, 3014–3030 (2015)]. This difference emerged because the dielectric Φj(t) corresponds to the vectorial first-moment average of the segmental bond vector at time t, u(n,t) with n being the segment index, whereas the viscoelastic gj(t) corresponds to the tensorial second-moment average. Because of this difference in the averaging moment, Φj(t) is subjected to cancelation in the conformational mapping but gj(t) is not, so that the reaction effect emerges only for gj(t). The experimental data of head-carboxylated high-cis polyisoprene chains (having the type-A dipoles), confirming this difference, are also presented.