Kai Yu The George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332 Qian Shi State Key Laboratory for Strength and Vibration of Mechanical Structures, School of Aerospace Science, Xian Jiaotong University, Xian 710049, China Tiejun Wang State Key Laboratory for Strength and Vibration of Mechanical Structures, School of Aerospace Science, Xian Jiaotong University, Xian 710049, China Martin L. Dunn SUTD Digital Manufacturing and Design (DManD) Centre, Singapore University of Technology and Design, Singapore 138682, Singapore H. Jerry Qi 1 The George W. Woodruff School of Mechanical Engineering, Georgia Institute of Technology, Atlanta, GA 30332 e-mail: [email protected] A Computational Model for Surface Welding in Covalent Adaptable Networks Using Finite-Element Analysis Covalent adaptable network (CAN) polymers can rearrange their macromolecular net- work by bond exchange reactions (BERs), where an active unit attaches to and then replaces a unit in an existing bond and forms a new bond. When such macromolecular events occur on the interface, they can contribute to surface welding, self-healing, and recycling of thermosetting polymers. In this paper, we study the interfacial welding and failure of CANs involving both interfacial normal and shear stresses. To do this, we incorporate our recently developed multiscale model for surface welding of CANs with a cohesive zone modeling approach in finite-element method (FEM) simulation. The devel- oped FEM paradigm involves a multiscale model predicting the interfacial chain density and fracture energy, which are transferred to a cohesive zone model to establish the sur- face traction-separation law. The simulations show good agreement with experimental results on the modulus and strength of welded samples. They also provide understanding of the interactions between surface welding and material malleability in determining the final mechanical properties of polymer structures. The developed FEM model can be applied to study other complex welding problems, such as polymer reprocessing with nonregular particle size and shape. [DOI: 10.1115/1.4033682] Keywords: covalent adaptable network, vitrimers, cohesive zone model, finite-element method, recycling of thermosetting polymers 1 Introduction Thermosetting polymers are ideal candidates for various structural or composite applications [1] due to their stable thermo- mechanical properties at high temperature and their resistance to environmental attacks, such as heat, chemicals, UV irradiation, etc. [2]. However, their permanently cross-linked network makes reforming and recycling these materials inherently difficult. Disposal of such polymer waste typically requires landfills, high temperatures, or toxic chemicals, all of which lead to significant environmental concerns [3]. In view of these challenges, new reprocessing and recycling methods have been successively pro- posed. The recently developed CANs (or dynamic covalent net- works) represent a unique paradigm in cross-linking polymers, as these can rearrange the network when a stimulus is given while preserving the system’s integrity and cross-linking density. One typical mechanism in such dynamic networks involves the use of exchangeable bonds in the polymer chains in order to enable so-called bond exchange reactions (BERs). Figure 1(a) illustrates the process of a BER, where an active unit (left dot connected to a dangling chain dot) attaches to an existing bond (two connected dots), then kicks off one unit (top dot). CANs exhibit two funda- mental features not found in traditional thermosetting polymers: malleability resulting from significant stress relaxation [4] and the surface welding. Moreover, the latter enables the self-healing, reprocessing, and recycling of the thermosetting polymer [5–9]. Previously, the dynamics and malleability of CANs were stud- ied by using macromolecular theories such as polymer sticky reptation theory [10,11], particle patching [12], continuum mechanics models [13,14], and molecular dynamics (MDs) simu- lations [15]. However, our understanding of the surface welding effect is still limited. Stukalin et al. [16] studied the welding kinetics by using scaling theory. There, the diffusion of polymer chains across the interface is correlated with their pervaded vol- ume during the Rouse type motion, which is subsequently used to scale the interfacial chain density and network penetration. By using a full atomistic MD simulation, Yang et al. [17] investigated the BER-induced surface welding effect, as well as the influence of welding conditions and material properties, such as welding time, temperature, network cross-linking density, etc., on the performance of welded samples. In our recent work [18], a multiple length scale constitutive model of the surface welding in CANs was developed. As shown in Fig. 1, a simple model is first developed at the macro- molecular network scale, based on the kinetics of BERs (Fig. 1(a)); this is then used in a lattice model to capture the evolution of chain density across the interface and its connection with BERs in the bulk material (Fig. 1(b)). The chain density evolution is then transferred to a continuum level interfacial model that considers surface roughness and welding pressure in order to predict the mechanical properties of welded thermoset- ting polymers (Figs. 1(c) and 1(d)) as functions of temperature and pressure conditions. The model does this with only four parameters, three of which can be measured directly through bulk material testing. By using the developed multiscale model, we are able to predict the effective elastic modulus and interfacial fracture energy of 1 Corresponding author. Contributed by the Applied Mechanics Division of ASME for publication in the JOURNAL OF APPLIED MECHANICS. Manuscript received March 31, 2016; final manuscript received May 17, 2016; published online June 22, 2016. Editor: Yonggang Huang. Journal of Applied Mechanics SEPTEMBER 2016, Vol. 83 / 091002-1 Copyright V C 2016 by ASME Downloaded From: https://appliedmechanics.asmedigitalcollection.asme.org on 01/01/2019 Terms of Use: http://www.asme.org/about-asme/terms-of-use
A Computational Model for Surface Welding in Covalent Adaptable Networks Using Finite-Element Analysis
Jun 14, 2023
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