Molecular Modeling

About 10,000 years ago, humans began to domesticate plants and animals. Now, it is time for us to domesticate molecules. In today’s polymer materials science, many scientists have synthesized a large number of synthetic polymers, which have been widely used in people’s lives. In a sense, natural polymers, like natural rubber, cellulose, etc., are of less importance than ever before. It is well known that the properties of the materials are closely related to their composition and bulk state at the molecular level. However, regarding the experimental studies, most of the breakthroughs in polymer materials are still based on a try-error-try loop, and it indeed wastes the resources and energies to a great extent. To the best of our knowledge, molecular modeling may be the only method that can reveal the nature of materials directly at the molecular level. It could be much more effective if we combine the traditional experimental study with molecular modeling. For these reasons, molecular modeling has evolved from an academic curiosity to an essential predictive tool for materials design within the past 20 years (Figure 1) [1]. However, in this chapter, we confine our discussion on the basic concept of molecular modeling and its applications to polybenzoxazine. What is molecular modeling?Molecular modeling is the science of representing molecular structures numerically and simulating their behavior with the equations of quantum and classical physics [2]. Using computer programs, polymer scientists could directly generate and obtain molecular data including geometries (bond lengths, bond angles, torsion angles), energies (heat of formation, activation energy, etc.), electronic properties (charges, electron affinity), spectroscopic properties (vibrational modes and intensities), and bulk properties (volumes, diffusion, viscosity, modulus, etc.) [3]. Today’s mainstream simulation methods include the ab initio quantum chemistry method, molecular mechanics (MM), molecular dynamics (MD), and Monte Carlo (MC) methods. The ab initio method, which has the highest accuracy among the above methods, is based on the Schrödinger equation, and it was often applied to deal with the electronic structure of organic molecules and their conformation and spectra properties. However, in terms of the polymer, it cannot be simulated directly by the ab initio method because of its high molecular weight. By contrast, MM and MD simulation, which is based on classical mechanics, can solve this problem well. They have been widelyused to simulate the conformation of a single polymer chain and bulk properties of polymers. The MC method is a statistical method, which can be used to predict the preferred conformations of polymers. It can simulate many more atoms in polymer system, but its primary disadvantage is that dynamic information cannot be obtained by theMCmethod. In the following parts of chapter, we review some applications of molecular modeling in polybenzoxazine. Section 2 concerns the chemical reactions of benzoxazine. Section 3 focuses on the structure analysis of polybenzoxazine with different level structures. Section 4 addresses the prediction of properties of polybenzoxazine using MD modeling.