Enhanced scattering induced by electrostatic correlations in concentrated solutions of salt-free dipolar and ionic polymers

TL;DR

This paper develops a generalized theory explaining enhanced ultralow wavevector scattering in salt-free dipolar and ionic polymer solutions, attributing it to electrostatic fluctuations and polarization effects without requiring aggregation or phase separation.

Contribution

The study introduces a comprehensive theoretical framework accounting for electrostatic correlations and polarization effects in concentrated dipolar and ionic polymers, explaining experimental scattering phenomena.

Findings

Enhanced scattering at ultralow wavevectors explained by electrostatics.

Structure factor peaks at finite wavevector for dipolar polymers.

No aggregation or phase separation needed for observed scattering.

Abstract

We present a generalized theory for studying static monomer density-density correlation function (structure factor) in concentrated solutions and melts of dipolar as well as ionic polymers. The theory captures effects of electrostatic fluctuations on the structure factor and provides insights into the origin of experimentally observed enhanced scattering at ultralow wavevectors in salt-free ionic polymers. It is shown that the enhanced scattering can originate from a coupling between fluctuations of electric polarization and monomer density. Local and non-local effects of the polarization resulting from finite sized permanent dipoles and ion-pairs in dipolar and charge regulating ionic polymers, respectively, are considered. Theoretical calculations reveal that, similar to the salt-free ionic polymers, the structure factor for dipolar polymers can also exhibit a peak at a finite wavevector and enhanced scattering at ultralow wavevectors. Although consideration of dipolar interactions leads to attractive interactions between monomers, the enhanced scattering at ultralow wavevectors is predicted solely on the basis of the electrostatics of weakly inhomoge- neous dipolar and ionic polymers without considering the effects of any aggregates or phase separation. Thus, we conclude that neither aggregation nor phase separation is necessary for observing the enhanced scattering at ultralow wave-vectors in salt-free dipolar and ionic polymers. For charge regulating ionic polymers, it is shown that electrostatic interactions between charged monomers get screened with a screening length, which depends not only on the concentration of free counterions and coions but also on the concentration of adsorbed ions on the polymer chains. Qualitative comparisons with the experimental scattering curves for ionic and dipolar polymer melts are presented using the theory developed in this work.