Molecular Dynamics Simulations of Cross‐Linked Phenolic Resins Using a United‐Atom Model

Abstract

because of their excellent physical properties such as mechanical strength, electrical insulation, heat resistance, and solvent resistance. [1] These properties result from their highly cross-linked network structures, which comprise phenolic and methy lene units. In these structures, three methylenes can connect to a phenolic ring such that one can be at the para (p) and two can be at the ortho (o) positions that are adjacent to the hydroxyl group of the phenolic ring, as shown in Figure 1. To further improve these properties, characterization of the relation between the network structure and the physical properties is one of the most important issues. However, this relation has not been completely understood experimentally because a fully cured resin is insoluble and infusible, which makes structural analysis difficult.In such cases, for studying cross-linked phenolic and other thermosetting resins that hold experimental constraints for structural analysis, a theoretical approach based on molecular dynamics (MD) simulations is expected to be effective. [2] Fueled by the evolution of computational sciences in polymer physics and the computational performance of desktop computers over the past two decades, many studies of epoxy resins using atomistic and coarse-grained MD simulations have been reported and the modeling methodology of cross-linked epoxy resins has been refined through these studies, revealing numerous molecularlevel findings that cannot be experimentally obtained. [2][3][4][5][6][7][8][9][10][11][12] Progress in the computational studies of epoxy resins over the last two decades has mainly been the result of rapid industrial growth of carbon-fiber reinforced plastics that use epoxy resins as matrix resins. However, detailed reports on atomistic simulations of cross-linked phenolic resins have been limited compared with the studies of epoxy resins. [13][14][15][16][17] We have investigated the application of atomistic MD simulation methods of epoxy resins to phenolic resins and were the first to propose a simple modeling method to generate highly cross-linked structures from a relaxed structure of phenolic oligomers. In the proposed method, a pair of nearest reactive carbons on the phenolic rings was connected via a newly inserted methylene unit, and this cross-linking reaction was repeated until a target degree of cross-linking was reached. [13] The structure modeling was successful, and a highly cross-linked structure (cross-linking degree of 0.92) was obtained. However, Molecular Dynamics SimulationsA new molecular modeling algorithm for conducting large-scale molecular dynamics simulation studies of cross-linked phenolic resins is developed using a united-atom model. A phenol-formaldehyde polycondensation system is simulated by a pseudoreaction algorithm taking into consideration (i) the difference in the experimental reaction rate constants at ortho and para positions of phenolic units and (ii) the geometry of the reactants. To avoid formation of locally strained cross-linked structures that c...

Dynamic Networks that Drive the Process of Irreversible Step-Growth Polymerization

“…4. a, Comparison of an 13 C NMR spectrum (HPE in DMF [45]) and the predicted degree distribution for the system with A 2 :B 3 = 1:1 at p A = 0.93. b, Comparison of the phenolic unit degree distribution (only out-edges) in a phenol-methylene system predicted by MD (dots) simulation [30] to the theory (solid lines). The gel point is predicted at p A = 0.58 by the MD simulation, the theory predicts it at p A = 0.5.…”

“…[45], and also, for a different polymerisation system, against Molecular Dynamics (MD) simulation data from Ref. [30].…”

“…Yet another approach that has been applied to polymer networks only recently, the molecular dynamics (MD) simulation, is especially attractive as it produces very detailed information on the structure of polymer networks [29,30]. Molecular dynamics simulations are notorious for being computationally expensive, and therefore, limited to small samples and short time scales.…”

A Novel Approach to Atomistic Molecular Dynamics Simulation of Phenolic Resins Using Symthons

“…Following bond formation, as proposed by Izumi et al, bond angles are checked to determine whether the angle is realistic: bonds with unrealistic angles (< 90 • ) are broken [29]. Note that the bond angle used is the methylene bridge angle, rather than using the angle defined across the phenol rings before bonding.…”

“…A key focus across all simulations of thermosets is enabling a high degree of crosslinking within short runtimes. Izumi et al have demonstrated extensively the potential for MD within phenolic resin research [27][28][29]. They have shown that large (>200,000 atom, or 10,000 phenol molecules) MD simulations are suitable for modelling phenolic resins for studying inhomogeneity.…”