Molecular dynamic simulations of polyacrylonitrile in ethanol and water solvents

Abstract

For some decades Polyacrylonitrile (PAN) fibers have been calling the attention of many researchers due to a large application of this polymer in several areas of science and industry. However, its structural characterization in different solvents is unclear. Thus, we have used molecular dynamic simulations to study the PAN in ethanol and water solvents. Our dihedral analysis show that the trans/gauche ratio is ∼25% lower than in solid state experiments, which can be attributed to the solvent effects in the PAN structure. From the radius of gyration analysis, our results show the PAN adopts a folded and unfolded conformation in ethanol and water solvents, respectively. The folded form is stabilized by the nitrogen-nitrogen intramolecular interaction, with two maximum peaks in the radial distribution function (RDF): one in 0.35 nm and the second in 0.60 nm. In addition, the –CH₂ group from PAN is stabilized by the –CH₃ group from the ethanol molecules. The results presented here provide new insight into the PAN structure, and our results are in agreement with experimental data.

Introduction

Several works have already been done with the PAN polymer (C₃H₃N)_n in science and engineering and shown possibilities of great applications in several areas such as linear actuators and artificial muscles [1], having in mind the PAN’s great capacity of supporting big tensions and flexibility [2]. While the pure PAN presents the above properties, doped ones can present electrical properties very useful to the development of sensors, activators and accumulators [3], [4], [5]. In reference to its capacity to support big tensions and electrical properties, a mixture of carbon nanotubes is for sure promising [5], [6]. Although many works have been done to characterize the PAN (pure and doped) and its physical and chemical properties, just a few of them aim at understanding and foreseeing its behavior. As an example, an interesting experiment was carried out in our laboratory, recently [7]. This experiment shows that the PAN polymer can contract and expand its structure in water and ethanol solvent, respectively. Clearly, this behavior is related to the PAN structure: the process of folding and unfolding. As a result, the PAN conformation has been studied for a long time using experimental techniques [8], [9], [10], [11], [12], [13], [14], [15], [16] and theoretical methods [16], [17], [18], [19]. Analyzing all these papers, we do not find an explanation about the PAN conformation in different solvents like ethanol and water. For instance, most of the studies reported above considered the solid state. Therefore, due to the large application of the PAN polymer, in the present work we propose to study it in ethanol and water solvents via molecular dynamic simulations. In the last decade, molecular dynamic (MD) simulations have become a powerful tool in the investigation of molecular systems at the atomistic level. For instance, MD can provide insight into structural and dynamical features of assembled molecules as a time function.