Lieng-Huang Lee

Dual mechanism for metal-polymer contact electrification

Abstract Contact charging and triboelectrification are considered a non-equilibrium two-step process consisting of instantaneous bond-forming and bond-breaking. Our proposed concept of a dual mechanism for metal-polymer contact electrification is based on a coexistence model with electrons (or holes) as charge carriers in the formation of a donor-acceptor complex which involves two terms: Coulombic or electrostatic and frontier orbital or charge transfer. The former is long-range and ionic, while the latter is short range ( ⩽ 4 A ) and electronic. Therefore, for the bond-forming step, both ionic and electronic transfer mechanisms are involved as in a continuum. For the bond-breaking step, air breakdown and bond fracture have been found by Dickinson to be accompanied by the electron emission (EE) and positive ion emission (PIE). Recently, a similar dual mechanism of bond-breaking for frictional electrification has been shown by Sakaguchi and Kashiwabara to involve both mechanoradicals and mechanoions. Thus, our concept of a dual mechanism for contact electrification between metal and polymer is consistent with the two-step non-equilibrium process, and this concept may be applied to help understand some unanswered questions about contact charging and charge accumulation.

Roles of molecular interactions in adhesion, adsorption, contact angle and wettability

This study is aimed at understanding the controversy between the surface tension component (STC) theory and the equation of state (EQS) approach for interfacial tensions. We attempt to relate molecular interactions to various components of surface tension. Molecular interactions consist of electrostatic (ES), charge transfer (CT), polarization (PL), exchange-repulsion (EX), dispersion (DIS), and coupling (MIX) components. These interactions can be the basis for the STC theory involving Lifshitz-van der Waals (LW) and the short range acid-base (AB) or donor-acceptor interaction. Each of these components is shown to contain two major parameters. New equations for the interaction energy and surface tension for polar molecules are proposed to include the ES and EX parameters, which happen in some cases to balance each other or nearly cancel out without being detected. The roles of molecular interactions on adhesion, adsorption, contact angle, and wettability are illustrated through the spreading coefficient S...

Adhesives, Sealants, and Coatings for Space and Harsh Environments

This review discusses new developments in adhesives, sealants, and coatings for space and harsh environments. Several papers presented to the Symposium on the subject matter held in Denver, Colorado in April, 1987 will be included in this review. Other developments reported through 1987 will be briefly mentioned.

Relevance of the density-functional theory to acid-base interactions and adhesion in solids

In this paper, the acid-base interaction in terms of the molecular interaction is discussed from a broader background. Thus, the overall nature of the acid-base interaction consists of electrostatic, charge (or electron) transfer, exchange, polarization, and dispersion components. Among them, the electrostatic (or ionic) and charge transfer (or covalent) are the two major components. It is pointed out that one of the important criteria determining whether there is any molecular interaction is the interatomic distance that affects each of these components in different ways. The optimum distance appears to be around 2 A. To complement existing acid-base theories, we further demonstrate the relevance of the density-functional theory to surface interactions and solid adhesion. On the basis of the density-functional theory, two chemical parameters, i.e. chemical potential μ and absolute hardness η, will be shown to govern an acid-base interaction. From these, the number of transferred electrons ΔN can be estim...

Molecular Bonding and Adhesion at Polymer-Metal Interphases

Abstract The purpose of this review is to demonstrate that there are well established molecular bonding and strong interactions between monomers or polymers and metals. We discuss both theoretical and experimental work related to adsorption and adhesion at polymer-metal interphases. Firstly, we briefly describe the fractal nature of polymer-metal interphases, and the effect of chemisorption on fractal dimension. Secondly, we mention several theoretical studies related to the models and the conformation of polymer segments to metal surfaces. Recent theoretical work by others with molecular modeling has provided some insight about the interfaces; however, this type of work is still at an early stage. Thirdly, we cite the experimental work by others with XPS, SERS (surface-enhanced Raman scattering spectroscopy), Mossbauer emission spectroscopy, etc., on chemisorption, molecular bonding, redox interaction, restructuring of polar groups, and contact oxidation of polymers on metal surfaces. Among them, SERS an...

The Chemistry and Physics of Solid Adhesion

This review about bond formation resembles a jigsaw puzzle because we cannot see the overall picture until we put most of the pieces together. There have been many questions about solid adhesion. However, it is surprising to learn how little we do know about this subject matter.

Adhesion and cohesion mechanisms of lunar dust on the moon's surface

This review is based on many publications from the US and some from the former Soviet Union. It briefly describes the sources of lunar dust. The problems posed to the past Apollo missions are summarized. The physical properties of lunar soils, such as cohesion, angle of repose, angle of friction, and bearing capacity, as reported by scientists from these two nations, are compared. The composition of lunar soils is described in terms of acidic and basic components. Since lunar soils are mainly silicates, we are the first to point out the importance of the Lewis acid-base interaction to the cohesion and adhesion of lunar dust. The adhesion of lunar dust is similar to that of a xerographic developer. For the adhesion between lunar dust and dissimilar materials, both van der Waals and electrostatic interactions can be the driving force, depending on the size of the dust particles. For particles smaller than 50 μm, van der Waals interaction predominates, while for those larger than 50 μm, electrostatic interac...

Adhesion and Surface-Hydrogen-Bond Components for Polymers and Biomaterials

Abstract In this paper, we briefly discuss several ways to determine the work of adhesion and the requirements for achieving maximum adhesion and spontaneous spreading. Specifically, we will concentrate on the methodology developed by van Oss. Chaudhury and Good [5–7] for the determination of the work of adhesion and interfacial tension. Recently, Good [4] has redefined the surface interaction components γ+ and γ− as hydrogen bond acidic and basic parameters. We have related the surface−hydrogen−bond components γ+ and γ− to the Taft and Kamlets [28, 29] linear solvation energy relationship (LSER) solvatochromic α and β parameters. We [8, 9] have found that, for water at ambient temperature, α [hydrogen-bond-donating (HBD) ability] and β [hydrogen-bond-accepting (HBA) ability] are not equal, and the ratio for the normalized α and β is 1.8. This new ratio is assumed to be equal to that of γ+ & γ− for water at 20°C. On the basis of the new ratio, we will present our recalculated surface-hydrogen-bond compon...

Molecular Bonding Mechanism for Solid Adhesion

Abstract The purpose of this study is to understand why and how solids can be bonded together with and without an adhesive. Beside van der Waals interactions and chemical bonding, there are some intermediate interactions, such as the Coulombic and charge-transfer interactions. These interactions are also called molecular interactions. Thus, molecular bonding mechanism for solids deals mainly with the formation of an adhesive bond through molecular interactions. The driving forces for molecular interactions are discussed in terms of adhesive energy and separation distance. The functions of electrons are illustrated with molecular orbitals. Moreover, some unique interactions between a molecule and the surface of a solid are demonstrated with the results found by Hoffmann.

Surface Hydrogen-Bond Components and Linear Solvation Energy Relationship Parameters

Abstract In 1987, van Oss, Chaudhury and Good introduced the Lewis acid (or hydrogen-bond acidic) component, γ+, and Lewis base (or hydrogen-bond basic) component, γ−, and assumed the ratio of γ+ and γ− for water at 20°C to be 1.0. With that ratio, the base components, γ−, for other liquids and polymers appeared to be overestimated. Recently, we unexpectedly found a correlation between γ+ and γ− and the linear solvation energy relationship (LSER) parameters α (hydrogen-bond-donating ability, HBD) and β (hydrogen-bond-accepting ability, HBA), introduced by Taft and Kamlet in 1976. Interestingly, we found the ratio for the normalized α and β for water at ambient temperature to be 1.8 instead of 1.0. Based on this new ratio for the corresponding γ+ and γ−, the calculated total surface tensions for other liquids and polymers at 20°C are generally unchanged, as expected, despite the favorable changes in the γ+ and γ− ratio to make them less basic. In addition, the implications of other LSER parameters, e.g. ∏∗...