This thesis is broadly aimed at developing a better strategy to achieve enhanced interfacial properties in polymer–metal/metal oxide joints using computational techniques such as density functional theory (DFT), molecular dynamics (MD), and dissipative particle dynamics (DPD) methods. Our calculations provide support to a valuable design principle that increasing polymer stiffness can improve the strength of polymer–metal/metal oxide joints even when interfacial failures are observed. Additionally, we reveal that, as the polymer stiffness increases, the chemical functionality within polymers can work more significantly to improve the interfacial strength of polymer–metal/metal oxide joints.

Bonding technologies between polymers and metals/metal oxides are required in many industries such as the transportation sector, including automotive, aviation, and maritime industries, where multi-material architectures should be achieved with the aim of weight reductions to decrease global greenhouse gas emissions. However, there is not yet a full understanding of the contributions of influential parameters such as stiffness and chemical functionality of polymers to the interfacial properties, which makes designing joining processes difficult. This thesis focuses on polymer–metal joints consisting of isotactic polypropylene (iPP), iPP grafted with maleic anhydride (iPPgMA), or iPP grafted with amine groups (iPPgNH₂) and hydroxylated γ-Al₂O₃, which is a model for an oxidised aluminium surface, and investigates the contributions of the stiffnesses of iPP, iPPgMA, and iPPgNH₂, and chemical functionalities (MA and NH₂ groups) in iPPgMA and iPPgNH₂ to the interfacial failure behaviours.

In Part I containing between Chapters 2 and 4, using joints models between iPP, iPPgMA, or iPPgNH₂ and a flat surface of hydroxylated γ-Al₂O₃, we investigate the influences of the stiffness of a polymer component and the chemical functionalities within two grafted polymers, iPPgMA and iPPgNH₂, on the interfacial failure behaviours on a flat surface. Our calculations reveal that higher Young’s moduli of iPP, iPPgMA, and iPPgNH₂ lead to a higher tensile strength of the joint models even in interfacial failures. Moreover, as the Young’s moduli of iPPgMA and iPPgNH₂ increase, their functional groups of MA and NH₂ groups improve their interfacial strengths more significantly.

Additionally, based on these findings, we suggest a protocol that requires much reduced computational resources to compare different functional groups within polymers with regard to their bonding abilities to metal/metal oxide substrates. This proposed procedure successfully provides consistent results that iPPgNH₂ shows the highest interfacial strength with an alumina surface, followed by iPPgMA, and then non-grafted iPP, relative to experimental observations.

In Part II consisting of between Chapters 5 and 7, a coarse-grained joint model between iPP and an alumina surface is developed as a preparation for investigating the effect of the stiffness of iPP on the interfacial failure behaviours on a porous surface in the DPD method. DPD parameters between iPP beads and those between iPP and surface beads are derived by Bayesian optimisation and validated from the comparison between MD and DPD methods.

Finally, in Chapter 8 in Part III, using a coarse-grained joint model between iPP and a porous alumina surface, the effect of the stiffness of iPP on the interfacial failure behaviours on a porous surface is examined. The results from DPD simulations demonstrate that a higher Young’s modulus of iPP results in an increased interfacial strength on a porous surface even in interfacial failures.

Generally, when interfacial failures are observed in mechanical tests on polymer–metal/metal oxide joints, improving the interfacial interactions may seem to be the most effective way to enhance the strengths. Nevertheless, our findings offer another useful strategy of increasing the polymer stiffness.

Some chapters of this thesis are based on the following manuscripts published in journals.

1. Y. Suganuma and J. A. Elliott. Effect of Varying Stiffness and Functionalization on the Interfacial Failure Behavior of Isotactic Polypropylene on Hydroxylated γ-Al₂O₃ by MD Simulation. ACS Applied Materials and Interfaces, 15(4): 6133–6141, 2023. https://doi.org/10.1021/acsami.2c19593
2. Y.Suganuma and J.A.Elliott. Isolating the Effect of Crosslink Densities on Mechanical Properties of Isotactic Polypropylene Using Dissipative Particle Dynamics. Macro- molecular Theory and Simulations, 2300014, 2023. https://doi.org/10.1002/MATS.202300014