Atomistic Modeling of Cross-linked Epoxy Polymer

Abstract

Molecular Dynamics simulations are used to study cross-linking of an epoxy polymer. OPLS force field parameters are used for modeling a 2:1 stoichiometric mixture of epoxy resin and the cross-linking agent. The model has 17,928 united atoms and a static cross-linking method is used along with molecular minimization and molecular dynamics techniques to achieve two different cross-link densities. The crosslinked models can be used for understanding various phenomenon occurring in cross-linked epoxy resins at the atomic scale. Glass-transition temperature ranges of two differently cross-linked samples have been predicted using the models. These models will be used for studying aging behavior at the atomic level in epoxy materials and understanding the influence of aging on mechanical properties. I. Introductionpoxy Resins are prime constituents in adhesives, sealants, and aircraft composite structural components. A wide range of studies have focused on epoxy-based materials to establish physical and mechanical properties. 1-3 The excellent specific-stiffness and specific-strength properties of epoxy-based composite materials are due to the complex microstructure of their constituent materials. There is significant interest in understanding the aging response of these material systems due to their wide-spread use in commercial aircraft. A. Computational Studies on Epoxy PolymersEpoxy resins are formed when epoxy monomers react with compounds known as cross-linking or curing agents with active hydrogens such as amines and anhydrides. 4 A trial-and-error approach to experimentally optimize the processing conditions of epoxy materials can become time-consuming and expensive. With the advancement of computational technology, computational modeling has provided an efficient route to study these polymer resins. [5][6][7][8][9][10][11][12][13][14]4,15 Molecular dynamics (MD) simulations based on the bead-spring model 10,11 and Monte-Carlo simulations based on the bond-fluctuation model 16,8,9 have been used in the last two decades for studying epoxy materials. The beadspring models did not take into account the details of the molecular structures and thus cannot predict the influence of specific groups of atoms on the physical properties. In the last few years, MD at the atomic scale has been quite successful in exploring different phenomena occurring at pico-to nano-second time scales in epoxy resins. 14 Many researchers have studied the formation of cross-linked epoxy resins using different approaches of simulated cross-linking. Doherty et al. 5 modeled PMA networks using lattice-based simulations in a polymerization MD scheme. Yarovsky and Evans 15 discussed a cross-linking technique which they used to crosslink low molecularweight, water-soluble, phosphate-modified epoxy resins (CYMEL 1158). The cross-linking reactions were carried out simultaneously (static cross-linking process). Dynamic cross-linking of epoxy resins was performed by Xu et al. 4