Elsevier

Polymer

Volume 116, 5 May 2017, Pages 506-514
Polymer

Structure-mechanical property relationships in crosslinked phenolic resin investigated by molecular dynamics simulation

https://doi.org/10.1016/j.polymer.2017.02.037Get rights and content

Highlights

  • MD simulation was performed on mechanical property of crosslinked phenolic resins.

  • The tensile stress and moduli were analyzed according to the molecular interactions.

  • Tensile modulus of cured structure showed a good agreement with experimental result.

  • Bond potential and molecular orientation dominantly affect tensile modulus.

  • Structural inhomogeneity and hydrogen bonds are irrelevant to the linear elasticity.

Abstract

An atomistic molecular dynamics simulation was performed for crosslinked phenolic resins which were constructed from phenols and crosslinkers using a pseudo-reaction algorithm, in order to understand the structure-mechanical property relationships therein from an atomistic perspective. The tensile modulus was characterized from the linear elastic region of the stress-strain curves under uniaxial tensile deformation. Analysis of the relationships between the moduli and interatomic interactions indicated that bond interaction, especially bond orientation for the elongation axis, dominantly affects the tensile modulus in a range of strain 0 ≤ ε ≤ 0.05, while stress-concentration in specific domains and long-range interactions including hydrogen bonding do not.

Introduction

Phenolic resins are the typical thermosetting resins that form a three-dimensional network structure through covalent bonds, and they are widely used for many applications in the automotive and aerospace industries and in semiconductor devices owing to their excellent thermal and mechanical properties [1], [2], [3]. However, extension of their applications requires improvement of their mechanical properties, such as tensile modulus, strength, and toughness. To achieve these improvements, it is necessary to understand the structure-mechanical property relationships of these materials from an atomistic viewpoint and to control their crosslinked structure.

Clarifying the structure-mechanical property relationships in highly crosslinked thermosetting polymers is an attractive subject. For phenolic resins, de Boer and Houwink (1936) performed a theoretical model calculation regarding their tensile modulus and strength, which is analogous to that of crystalline materials. They reported significant discrepancies between the theoretical predictions and experimental results [4], [5], implying that the elasticity of crosslinked phenolic polymers is essentially different from crystal elasticity. Following their works, very few further theoretical or numerical studies on these structure-mechanical property relationships have been undertaken because of the difficulties in experimental structure analysis of cured resins owing to their insolubility and infusibility. Fractographic studies using electron microscope observation have indicated the existence of crosslink inhomogeneity and stress concentration in low-crosslink-density domains when external stress is induced [6], [7], [8], [9], [10]. However, there is no strong evidence to demonstrate these mechanisms. Therefore, the structure-property relationships in crosslinked thermosetting resins have not been well elucidated.

Molecular dynamics (MD) simulation is a useful tool for characterizing material properties and to understand the structure-material property relationships of thermosetting polymers [11]. Atomistic MD simulation enables us to obtain information about fundamental chemical structure, dynamics, and their external responses under equilibrium and non-equilibrium processes. In the 2000s, there have been many investigations into crosslinked thermosetting polymer networks concerning model construction and prediction of their thermal and mechanical properties. For phenolic resins, we previously investigated model construction and a methodology to characterize the mechanical properties of the crosslinked structure from monodispersed novolac resins, i.e., phenol-formaldehyde resins prepared in acidic conditions [12]. Following that study, we made further refinement of the methodology of model construction using pseudo-reaction, and verified the results in order to improve the precision of the MD simulations [13]. Compared to that for the structure of monodisperse novolacs [12], [14], [15], the constructed model structure of liquid phenols in the study exhibited better agreement with chemical structures obtained from 13C NMR spectroscopy and kinetic models. In addition, the calculated structure factor indicating atomic configuration in the MD simulations agreed well with the experimental results from small-angle X-ray scattering (SAXS) measurements in the range of q < 1 nm−1 on the condition that the number of atoms was ca. 232,000. This constructed model also suggested the existence and the development of crosslink density fluctuations after the gel point similar, as indicated by up-turn behavior in SAXS profiles, which indicate an increase in heterogeneity [16], [17], [18], [19]. However, the material properties related to this structure had not then been characterized, especially the mechanical properties.

In this work, we used MD simulation to investigate the mechanical properties of crosslinked phenolic resins as functions of strain in order to understand the structure-property relationships from an atomistic perspective. A full atomic MD simulation was performed with crosslinked structures containing 220,000–232,000 atoms including 16,000 phenols with different conversions and hydrogen bonds that were constructed in our previous study [13]. We characterized the structural changes in the network and tensile stress and modulus under tensile deformation, and analyzed the relationships between the moduli and the interactions. We believe that this investigation may play an important role in understanding the relationships between crosslinked structures and mechanical properties in thermosetting resins.

Section snippets

General

Atomistic MD simulations were performed for model construction and characterization of material properties in this work. The model structures of crosslinked phenolic resins were the same as those constructed in our previous study [13]. J-OCTA 1.8 (JSOL Corp, Japan) was used for model construction. Partial atomic charges were estimated using Gaussian 09 D (Gaussian, Inc., USA) and a restrained electrostatic potential (RESP) fitting program [20]. LAMMPS (Sandia National Laboratory, USA) [21] was

Stress-strain relationships

The calculated stress-strain curves for RN under uniaxial elongation with three different conversions are shown in Fig. 3. These results clearly show that the total stress increases with strain and conversion. The stress shows a linear change in a strain range of 0 ≤ ε ≤ 0.02 for all simulation results. Consequently, tensile modulus was calculated as the gradient of the fitting line in this region according to the definition in eq. (4). Poisson's ratio ν was also evaluated in this range.

Fig. 4

Conclusions

Structure-mechanical property relationships in highly crosslinked phenolic resins were investigated by atomistic MD simulation using uniaxial tensile deformation for the structures with different conversions and numbers of hydrogen bonds. The tensile stress and moduli were analyzed according to the molecular interactions in this model. The structural changes under the deformation process were investigated such as bond length, bond angle, hydrogen bond, and bond orientation. Spatial distribution

Acknowledgement

This research used computational resources of the K computer provided by the RIKEN Advanced Institute for Computational Science through the HPCI System Research project (Project ID: hp140097 and hp150096). The authors acknowledge to JHPCN research project for useful information and Prof. Hiroshi Takano for useful advices on simulations and analyses. The authors would also like to thank Dr. Satoshi Maji of S.B. Research Co., Ltd., Yuji Suzuki and Yusuke Watanabe of Sumitomo Bakelite Co., Ltd.

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