Lik-ho Tam

A molecular dynamics investigation on the cross-linking and physical properties of epoxy-based materials

Epoxy-based materials are extensively used in industry due to their excellent mechanical and thermal stability. As the epoxy products are getting smaller and smaller nowadays, there are difficulties arising from the material processing and measurement, which need to be carefully considered in the design stage. SU-8 photoresist, which is commonly used in micro-electro-mechanical systems (MEMS), is chosen as a representative of epoxy-based materials in this study. Here, we propose an effective dynamic cross-linking algorithm, which can be used to construct the SU-8 epoxy network with cross-linking degree higher than 80%. Using an equilibration process incorporating successive pressure controls, the density of the cross-linked structure can be accurately obtained. By performing the dynamic deformations in the molecular dynamics simulations, elastic properties of the equilibrated SU-8 photoresist can be determined, which are in good agreement with the experimental measurements. The good predictions of the physical properties demonstrate a strong mechanical stability of the SU-8 structure at the nano-scale. The dynamic cross-linking algorithm described in the present study can be applied in other polymeric material investigations involving the cross-linked network model through homopolymerization.

Moisture effect on interfacial integrity of epoxy-bonded system: A hierarchical approach

The epoxy-bonded system has been widely used in various applications across different scale lengths. Prior investigations have indicated that the moisture-affected interfacial debonding is the major failure mode of such a system, but the fundamental mechanism remains unknown, such as the basis for the invasion of water molecules in the cross-linked epoxy and the epoxy-bonded interface. This prevents us from predicting the long-term performance of the epoxy-related applications under the effect of the moisture. Here, we use full atomistic models to investigate the response of the epoxy-bonded system towards the adhesion test, and provide a detailed analysis of the interfacial integrity under the moisture effect and the associated debonding mechanism. Molecular dynamics simulations show that water molecules affect the hierarchical structure of the epoxy-bonded system at the nanoscale by disrupting the film-substrate interaction and the molecular interaction within the epoxy, which leads to the detachment of the epoxy thin film, and the final interfacial debonding. The simulation results show good agreement with the experimental results of the epoxy-bonded system. Through identifying the relationship between the epoxy structure and the debonding mechanism at multiple scales, it is shown that the hierarchical structure of the epoxy-bonded system is crucial for the interfacial integrity. In particular, the available space of the epoxy-bonded system, which consists of various sizes ranging from the atomistic scale to the macroscale and is close to the interface facilitates the moisture accumulation, leading to a distinct interfacial debonding when compared to the dry scenario.

Molecular Mechanics of the Moisture Effect on Epoxy/Carbon Nanotube Nanocomposites

The strong structural integrity of polymer nanocomposite is influenced in the moist environment; but the fundamental mechanism is unclear, including the basis for the interactions between the absorbed water molecules and the structure, which prevents us from predicting the durability of its applications across multiple scales. In this research, a molecular dynamics model of the epoxy/single-walled carbon nanotube (SWCNT) nanocomposite is constructed to explore the mechanism of the moisture effect, and an analysis of the molecular interactions is provided by focusing on the hydrogen bond (H-bond) network inside the nanocomposite structure. The simulations show that at low moisture concentration, the water molecules affect the molecular interactions by favorably forming the water-nanocomposite H-bonds and the small cluster, while at high concentration the water molecules predominantly form the water-water H-bonds and the large cluster. The water molecules in the epoxy matrix and the epoxy-SWCNT interface disrupt the molecular interactions and deteriorate the mechanical properties. Through identifying the link between the water molecules and the nanocomposite structure and properties, it is shown that the free volume in the nanocomposite is crucial for its structural integrity, which facilitates the moisture accumulation and the distinct material deteriorations. This study provides insights into the moisture-affected structure and properties of the nanocomposite from the nanoscale perspective, which contributes to the understanding of the nanocomposite long-term performance under the moisture effect.

Characterizing Mechanical Properties of Polymeric Material: A Bottom-Up Approach

Polymeric materials have received tremendous attention in both industrial and scientific communities, and can be readily found in applications across a large range of length scales, ranging from the nanoscale structures, such as the photoresist lithography in the micro-electro-mechanical systems, to the macroscale components, such as the adhesive bonding in the aerospace industry and civil infrastructures. The durability of these applications is mainly determined by the mechanical L.-h. Tam Department of Architecture and Civil Engineering, City University of Hong Kong, Hong Kong, China D. Lau (*) Department of Architecture and Civil Engineering, City University of Hong Kong, Hong Kong, China Department of Civil and Environmental Engineering, Massachusetts Institute of Technology, Cambridge, MA, USA e-mail: denvid@mit.edu # Springer Nature Singapore Pte Ltd. 2018 C.-H. Hsueh et al. (eds.), Handbook of Mechanics of Materials, https://doi.org/10.1007/978-981-10-6855-3_5-1 1 reliability of the constituent polymeric materials. In this chapter, a review of the bottom-up approach to investigate the mechanical properties of the polymeric materials is provided. A dynamic algorithm is developed to achieve the crosslinking process of the atomistic network,which possesses themechanical properties in a good accordance with the experimental measurements. Meanwhile, the moisture effect on themechanical properties is studied based on the atomisticmodel, and it is found that themechanical properties of the solvatedmodels show no significant deterioration. Furthermore, the predicted mechanical properties at the atomistic level are used to develop the cross-linked network at the mesoscale, which enables the investigation of the effect of the structural voids on the polymeric materials. The simulation results demonstrate the strong mechanical reliability of the synthetic polymeric materials during the long-term service life. The multiscale method summarized in this chapter provides a versatile tool to link the nano-level mechanical properties of the polymeric materials to the macro-level material behaviors.

Molecular dynamics study on the effect of salt environment on interfacial structure, stress, and adhesion of carbon fiber/epoxy interface

ABSTRACT Epoxy resins are widely used as matrices for bonding carbon fiber tightly together in fabricating carbon fiber reinforced polymer (CFRP), which has been increasingly used in marine and offshore applications. To analyze CFRP performance in salt environment, it requires a fundamental understanding of the behavior of carbon fiber/epoxy interface under saltwater exposure. Here the molecular interface model of carbon fiber/epoxy bonded system is constructed to analyze the interfacial integrity in salt environment. The simulation results show that salt solution leads to largest loss of interfacial adhesion, which correlates with structural and mechanical degradation of bonded interface, as indicated by the decreased epoxy density close to interface and reduced interfacial stress as compared to dry and wet case. Through examining interfacial structure and stress during pulling process, it is observed that epoxy detaches in a sequential manner and the final debonding occurs more easily under saltwater ingress. By simultaneously capturing mechanical degradation and interfacial deterioration, it is clear that strong mechanical properties of carbon fiber/epoxy bonded interface are critical for long-term interfacial integrity and performance of CFRP in salt environment. This study provides microscopic information of interfacial deterioration in CFRP composite, which forms basis for predicting performance degradation of macroscopic CFRP in consideration of environmental exposure. Graphical Abstract