Elucidating the Influence of Surface Treatments, Material Functionalization, and Additives on Adhesion at Buried Interfaces.

다국어 초록 (Multilingual Abstract)

Adhesion is important to a variety of applications and for a number of industries. Adhesion occurs at buried interfaces and is mediated by interfacial structure. It is difficult to probe buried interfacial structure in situ nondestructively. In this thesis, sum frequency generation (SFG) vibrational spectroscopy was applied to study molecular structures of buried interfaces to understand molecular mechanisms of polymer adhesion at buried interfaces in situ. In addition to the studies of enhancing adhesion of polymer adhesives to substrates, research on a reversible adhesive for polymer recycling and sustainability was also conducted. Soldering flux treatment on copper is an important procedure to ensure bonding of interconnects in an electronic device but can disrupt adhesion of packaging polymers. Chapter 2 reports SFG studies on the effect of model and commercial flux on polymer adhesion. It was found that with surface treatments of the flux treated copper leads to disordered polymer at the buried copper/polymer interfaces, increasing the adhesion. Chemical reactions occurring at the buried interface between two polymers in situ were studied in Chapters 3 and 4. First, the buried ethylene vinyl alcohol (EVOH)/maleic anhydride (MAH) grafted polyethylene (MAHgEO) interface was studied, showing that interfacial chemical reactions occurred, greatly enhancing the adhesion. The chemical reactions can greatly increase adhesion between nylon and MAHgEO was also observed, evidenced by the disappearance of the reactants and appearance of reaction products in the SFG spectra. SFG experiments were also performed to follow the reaction kinetics and measure the activation energy of the interfacial reaction. Chapter 5 studied the influence of an amino silane adhesion promoter on the interfacial structure and adhesion of butyl acrylate/methylmethacrylate copolymers (BAMMAs) incorporated with methyl acrylic acid (MAA) to silica. SFG results elucidated the mechanisms of adhesion enhancement by the adhesion promoter, showing: (1) ionic bonds formed between the tail amine group and acid functionality of BAMMA, (2) migration of more ester C=O groups to the interface with order, and (3) disordered or reduced levels of CH groups at the interface. A self-assembled monolayer (SAM) for debonding adhesives on demand was developed in Chapter 6. The SAM was applied to a silica surface and characterized with SFG and x-ray photoelectron spectroscopy (XPS). Once the SAM was fully prepared, the UV sensitivity was also tested with SFG. Before debonding the UV sensitive group on the SAM, adhesives were applied resulting in an adhesive interface that can be selectively broken. Adhesion tests were performed on silica/adhesive and silica/SAM/adhesive samples before and after UV irradiation exposure, which showed that the debonding could reduce the adhesion strength. SFG has been developed into a powerful tool to characterize interfaces but most of the SFG research has focused on simple model systems. This research demonstrated that SFG can be applied to study many complex polymer interfaces of commercial materials, adding an important technique to the toolbox used to study “real-life” interfaces important to industry. This research showed that for commercial flux, packaging polymers, and polymer resins that disordering, covalent bond formation, and ionic bond formation at buried interfaces could greatly enhance adhesion. This study also reported an easy way to create reversible adhesives, important for recycling. The methods developed in this thesis are generally applicable to study important interfaces of many polymer systems in the future.