Supramolecular ionics: electric charge partition within polymers and other non-conducting solids

Fernando Galembeck,Carlos A R Costa,André Galembeck,Maria Do Carmo V M Silva

doi:10.1590/s0001-37652001000400003

Abstract

Electrostatic phenomena in insulators have been known for the past four centuries, but many related questions are still unanswered, for instance: which are the charge-bearing species in an electrified organic polymer, how are the charges spatially distributed and which is the contribution of the electrically charged domains to the overall polymer properties? New scanning probe microscopies were recently introduced, and these are suitable for the mapping of electric potentials across a solid sample thus providing some answers for the previous questions. In this work, we report results obtained with two of these techniques: scanning electric potential (SEPM) and electric force microscopy (EFM). These results were associated to images acquired by using analytical electron microscopy (energy-loss spectroscopy imaging in the transmission electron microscope, ESI-TEM) for colloid polymer samples. Together, they show domains with excess electric charges (and potentials) extending up to hundreds of nanometers and formed by large clusters of cations or anions, reaching supramolecular dimensions. Domains with excess electric charge were also observed in thermoplastics as well as in silica, polyphosphate and titanium oxide particles. In the case of thermoplastics, the origin of the charges is tentatively assigned to their tribochemistry, oxidation followed by segregation or the Mawell-Wagner-Sillars and Costa Ribeiro effects.

Highlights

Electric charges are transferred, accumulated and dissipated in contacting dielectrics, and many related observations are very well known to the layperson
The analytical solution of the Poisson-Boltzmann equation is achieved only in specific cases, often under rather drastic simplifications (Hunter 1991). Another more specific open question is the following: which is the distribution of electric potentials across the bulk or surface of any given object made out of an organic polymer or ionic non-conducting material, and how does it contribute to the mechanical, optical, adhesion and electrically insulating properties of the solid? This question is usually neglected, and almost all the literature on plastics, rubbers and glasses normally assumes that dielectrics are electrically neutral, in the macroscopic scale and at the micro- and nanometric scales
It is not surprising that these polymers contain ionic charges, but current thinking on all polymers and most other solids is based on the idea of prevalence of electroneutrality (Wu et al 1999), following which charges should be present as ion pairs or small ion clusters, and larger domains with opposite charges could be found but only as electrical double layers. This is not confirmed by microscopy results; Figure 4 shows AFM, scanning electric potential microscopy (SEPM) and elemental maps of the poly latex

Summary

Introduction

Electric charges are transferred, accumulated and dissipated in contacting dielectrics, and many related observations are very well known to the layperson. Another more specific open question is the following: which is the distribution of electric potentials across the bulk or surface of any given object made out of an organic polymer or ionic non-conducting material, and how does it contribute to the mechanical, optical, adhesion and electrically insulating properties of the solid?

Results

Conclusion