A molecular dynamics study of water transport inside an epoxy polymer matrix Sudharsan Pandiyan a , Jakub Krajniak b , Giovanni Samaey b , Dirk Roose b , Erik Nies a,⇑ a Division of Polymer Chemistry and Materials, Department of Chemistry, Katholieke Universiteit Leuven, Celestijnenlaan 200F, B-3001 Heverlee, Belgium b Scientific Computing Research Group, Department of Computer Science, Katholieke Universiteit Leuven, Celestijnenlaan 200A, B-3001 Heverlee, Belgium article info Article history: Received 9 December 2014 Received in revised form 22 April 2015 Accepted 24 April 2015 Keywords: Molecular dynamics Epoxy network Water transport abstract Classical all-atom molecular dynamics simulations were used to build and study a polymer network model of EPON-828 as an epoxy and diethylenetriamine as a cross-linker. A cut-off based cross-linking algorithm was adopted to make the cross-linking bonds of the epoxy network. A step-wise cross-linking process was implemented to achieve an epoxy polymer network with a maximum conver- sion of 0.8. Based on the uniaxial stress–strain response the elastic, bulk and strain moduli were deter- mined and found to match with previous experimental and simulation studies. Water transport inside epoxy networks was analyzed by preparing the epoxy polymer network models containing different amounts of water. The water–epoxy hydrogen bonding interactions strongly influence the diffusion of water molecules in the polymer. Radial distribution functions, volume swelling analysis and mean squared displacements indicated two different types of water molecules in the polymer matrix. Water-matrix hydrogen bonding dominates during the initial sorption process and then the water–water interactions gradually increase their influence on the diffusion process by forming bigger water clusters. Ó 2015 Elsevier B.V. All rights reserved. 1. Introduction Epoxy polymers are the products of the reaction between an epoxy resin and a so-called curing agent such as a polyfunctional amine [1]. The properties of the resulting thermosetting polymers from the epoxy–curing agent reactions are strongly related to the functionality of the monomers and the molar-ratio of epoxy and curing agent involved in the reaction [2,3]. Hence, by altering either the molar-ratio of the monomers or the functionality of the monomers the properties of the epoxy polymer can be fine-tuned [4,5]. For example, poly-functional amines lead to highly cross-linked polymers and the di-functional amines are gen- erally used as chain extenders for producing linear polymers [6]. The cross-linked structure of the epoxy polymers leads to their higher mechanical strength and makes them important ingredient in various industrial sectors [7]. The global business of epoxy resin in year 2013 is US$ 18.6 Billion and expected to grow to US$ 25 Billion in next 5 years [8]. Epoxy polymers are mainly used as adhesives or as coating materials in different branches of industry ranging from bio-medical to aerospace [9,10]. In most of these applications, the epoxy polymers are used to protect the inner layers of the com- posite materials from corrosion [11,12]. Moisture adsorption inside these epoxy polymer matrices is one of the major issues which leads to a lot of potential problems such as interfacial delamination [13,14], swelling [15,16], plasticization [17] and induced corrosion [18,19]. In order to make better polymers with higher protection capacity it is important to understand the underlying relationship between the polymer structure and the diffusion of small mole- cules. A number of experimental and simulation studies have been devoted to understand the moisture transport inside epoxy poly- mers [20–36]. However the molecular level understanding of the moisture induced plasticization effects are still an area of interest to many industrial processes. In the present work, we analyzed the water diffusion inside a bulk epoxy polymer matrix using molecular dynamics simulations and our intention is to shed some light on the plasticization effect induced by the moisture sorption at the atomistic level. Previously, both experimental and simulation studies con- firmed the presence of strong hydrogen bond interactions between the epoxy matrix and the water molecules [22,37]. Various exper- imental studies reported at least two types of water molecules namely, bound and free water molecules to be present in the sys- tem. The bound molecules are the ones found to have one or more hydrogen bond interactions with the epoxy matrix, the free http://dx.doi.org/10.1016/j.commatsci.2015.04.032 0927-0256/Ó 2015 Elsevier B.V. All rights reserved. ⇑ Corresponding author. E-mail address: erik.nies@chem.kuleuven.be (E. Nies). Computational Materials Science 106 (2015) 29–37 Contents lists available at ScienceDirect Computational Materials Science journal homepage: www.elsevier.com/locate/commatsci