Mixed Crystals as Host-Guest Systems for Probing Molecular Interactions

A method is described for probing interactions between molecules at the surfaces of growing crystals. For this purpose, mixed crystals have been treated as unique host-guest systems where the guest molecules are occluded into the host crystal through stereospecific interactions occurring at the different growing surfaces of the host. Owing to the process of molecular recognition at such surfaces, the adsorption and occlusion of the guest molecules takes place at specific sites, resulting in a reduction in symmetry of the mixed crystal compared to the pure host. The concept of reduction in symmetry is general, and has been instrumental for probing subtle molecular interactions at the surfaces of growing crystals, for the transformation of centrosymmetric single crystals into crystals with polar arrangement, and for “absolute” asymmetric synthesis inside centrosymmetric crystals.     

Application of small system thermodynamics to polymer molecules. III. Molecular fractionation and tertiary nucleation

The statistical mechanical methods developed in Parts I and II in this series permit the postulation of a thermodynamic criterion for the molecular fractionation which occurs during crystallization. Using this criterion we define a “local equilibrium” melting temperature as that temperature at which a polymer molecule (considered as a small thermodynamic system) has the same free energy when crystallized into the lowest possible free energy conformation on a given crystal surface (or surfaces) as it does in a completely molten state but still in contact with the same surfaces. This temperature will be a function not only of molecular length but also of the nature of the crystal surfaces to which it is exposed. Lowest “local equilibrium” melting temperatures occur on large flat crystal surfaces (secondary nucleation sites), higher temperatures result from the intersection of two crystal surfaces (tertiary nucleation sites). A number of such potential tertiary nucleation sites have been investigated and the resulting temperatures satisfactorily cover the range over which molecular fractionation has been observed.     

XPS Studies of Polymer Surface Modifications and Adhesion Mechanisms

XPS has been used to elucidate the mechanisms of surface modification of low density polyethylene by electrical (“corona”) discharge treatment and by chromic acid treatment. The use of derivatisation techniques for improving the precision of functional group analysis is described. These techniques also allow the role of specific interactions in adhesion to discharge treated surfaces to be investigated. The role of residual Cr on the adhesion of deposited metal to polymer surfaces is discussed.     

Nonwetting,nonrolling, stain resistant polyhedral oligomeric silsesquioxane coated textiles

Cotton/polyester fabric surfaces were modified using nanostructured organic‐inorganic polyhedral oligomeric silsesquioxane (POSS) molecules via solution dip coating. Surface wetting characteristics of coatings prepared from two chemically and structurally different POSS molecules, a closed cage fluorinated dodecatrifluoropropyl POSS (FL‐POSS) and an open cage nonfluorinated trisilanolphenyl POSS (Tsp‐POSS), were evaluated with time and compared with Teflon. Surface analysis, including Atomic Force Microscopy, SEM/EDAX, and NMR revealed the presence of POSS aggregates on the fabric surface leading to a spiky topography, high roughness, and hysteresis. POSS coated fabrics showed complete reversal of surface wetting characteristics with contact angles higher than the benchmark Teflon surface. Water contact angle measured as a function of time showed equivalent or better performance for POSS‐coated surfaces in comparison to Teflon. Furthermore, FL‐POSS coated fabric exhibited exceptional stain and acid resistance along with a 38% reduction in relative surface friction. Additionally, “nonsliding” and high surface adhesion behavior of water droplets on the FL‐POSS coated fabric are reported. © 2009 Wiley Periodicals, Inc. J Appl Polym Sci, 2010     

Molecular dynamic modeling of interfacial energy

Molecular dynamic modeling is used to calculate the changes in the energy between two surfaces when the surfaces are first allowed to approach one another and, subsequently, separated. The equations of motion of the atoms, which were assumed to interact via a Lennard-Jones potential, were integrated using Verlet's algorithm. They were implemented in an environment of periodic boundary conditions, feedback loop temperature and pressure controllers, with direct computation of the stress and strain tensors. This approach allows one to calculate the temperature dependence of the 'leap-to-contact' phenomenon, the thermodynamic work of adhesion, the work needed to separate the surfaces, and the forces of attraction and separation. Effects that occur during approach and separation, such as surface roughening and vacancy formation, were included in the energetics calculations. Sound waves and the resulting thermal transients were also modeled. The adhesion hysteresis and irreversible behaviors during approach and separation that arise from these calculations are discussed in detail.     

Limit Cycles in Dynamic Adhesion and Friction Processes: A Discussion

Simple equations exist relating adhesion and friction forces. These apply to simple attachment-detachment processes and steady-state (smooth) sliding conditions. However, in the case of more complex, such as polymer, surfaces both the adhesion and friction can be very complex, irreversible, and nonlinear, exhibiting stringing and tack in the former and stick-slip sliding in the latter. We explore possible relationships between such nonlinear adhesion and friction processes. Based on recent experiments we find that certain types of “limit cycles,” relating the (normal) adhesion and (lateral) friction forces, F _⊥ and F _∥, to the relative velocities, V _⊥ and V _∥, of the surfaces during an attachment-detachment process or stick-slip sliding, bear a very similar resemblance to each other. We briefly discuss the theoretical and practical implications of describing such dynamic processes in terms of limit cycles.     

Further applications of carrier-mediated transport theory—A survey

Recent studies of “facilitated diffusion” or “carrier-mediated transport” in membranes and liquid films, mainly as models of biological mass transfer, have led to several advances in the theoretical understanding and analysis of such phenomena. It will be shown here that many of the concepts and methods are applicable to a variety of processes involving diffusion and convection accompanied by homogeneous chemical reaction such as blood oxygenation, metallurgical oxidation or sulphation of alloys, and heat transfer in reacting gases.Thus, the notions of carrier-mediation, global reactivity versus non-reactivity, reaction invariance, and equilibrium or “frozen” transport, according to the appropriate Damkohler criteria, find widespread applicability in characterizing various molecular transport systems. The main objective here is to discuss the applications to the regime of rapid, near-equilibrium reaction.It is concluded that the boundary-layer methods developed recently for facilitated diffusion in membranes may be highly useful in the treatment of convection as well. There are, however, several important exceptions, which generally involve singular reaction equilibria and detached reaction zones, typical of rapid irreversible reaction exhibiting the classical “thin-flame” structure.     

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, the Hamaker coefficient A, and Derjaguin's disjoining pressure . We have found that the STC theory is applicable to the systems involving either physisorption or chemisorption, whlie the EQS applies to those involving ony physisorption.     

INTERFACE/INTERPHASE ENGINEERING OF POLYMERS FOR ADHESION ENHANCEMENT: PART I. REVIEW OF MICROMECHANICAL ASPECTS OF POLYMER INTERFACE REINFORCEMENT THROUGH SURFACE GRAFTED MOLECULAR BRUSHES

This article reviews the theoretical principles of macromolecular design of interfaces between glassy polymers as well as those between rigid substrates and elastomers for maximizing adhesion and fracture performance of bonded assemblies. According to contemporary theories, macromolecular "connector molecules" grafted onto solid polymer surfaces effectively improve adhesion and fracture performance of interfaces between polymers by improving the interactions with adjacent materials through one or both of the following mechanisms: (1) interpenetration into adjacent polymeric phase, and (2) chemical reaction/crosslinking with the adjacent material.It is shown that the effectiveness of the interface reinforcement by surface-grafted connector molecules depends on the following factors: surface density of grafted molecules, length of individual chains of grafted molecules, and optimum surface density in relation to the length of connector molecules. The influence of the above-mentioned physico-chemical parameters of molecular brushes on the interphase-interface reinforcement is discussed and quantified by contemporary theories. Also, the optimum conditions for maximum adhesion enhancement are specified and verified by a range of experimental examples.Part II of this article demonstrates a novel and relatively simple, industry-feasible technology for surface grafting connector molecules and engineering of interface/interphase systems, which is discussed in detail and supported by a range of experimental examples. It is shown, in agreement with contemporary theories, that the use of chemically attached graft chemicals of controlled spatial geometry and chemical functionality enables a significant increase in the strength and fracture energy of the interphase, to the point of cohesive fracture of the substrate, or that of an adjacent medium such as adhesive, elastomer, or other material. This occurs even after prolonged exposure of investigated systems to adverse environments such as hot water.     

Interface/Interphase engineering of polymers for adhesion enhancement: Part I. Review of micromechanical aspects of polymer interface reinforcement through surface grafted molecular brushes

This article reviews the theoretical principles of macromolecular design of interfaces between glassy polymers as well as those between rigid substrates and elastomers for maximizing adhesion and fracture performance of bonded assemblies. According to contemporary theories, macromolecular "connector molecules" grafted onto solid polymer surfaces effectively improve adhesion and fracture performance of interfaces between polymers by improving the interactions with adjacent materials through one or both of the following mechanisms: (1) interpenetration into adjacent polymeric phase, and (2) chemical reaction/crosslinking with the adjacent material.It is shown that the effectiveness of the interface reinforcement by surface-grafted connector molecules depends on the following factors: surface density of grafted molecules, length of individual chains of grafted molecules, and optimum surface density in relation to the length of connector molecules. The influence of the above-mentioned physico-chemical parameters of molecular brushes on the interphase-interface reinforcement is discussed and quantified by contemporary theories. Also, the optimum conditions for maximum adhesion enhancement are specified and verified by a range of experimental examples.Part II of this article demonstrates a novel and relatively simple, industry-feasible technology for surface grafting connector molecules and engineering of interface/interphase systems, which is discussed in detail and supported by a range of experimental examples. It is shown, in agreement with contemporary theories, that the use of chemically attached graft chemicals of controlled spatial geometry and chemical functionality enables a significant increase in the strength and fracture energy of the interphase, to the point of cohesive fracture of the substrate, or that of an adjacent medium such as adhesive, elastomer, or other material. This occurs even after prolonged exposure of investigated systems to adverse environments such as hot water.     

Understanding surfaces and buried interfaces of polymer materials at the molecular level using sum frequency generation vibrational spectroscopy

This paper reviews recent progress in the studies on polymer surfaces/interfaces using sum frequency generation (SFG) vibrational spectroscopy. SFG theory, technique, and some experimental details have been presented. The review is focused on the SFG studies on buried interfaces involving polymer materials, such as polymer–water interfaces and polymer–polymer interfaces. Molecular interactions between polymer surfaces and adhesion promoters as well as biological molecules such as proteins and peptides have also been elucidated using SFG. This review demonstrates that SFG is a powerful technique to characterize molecular level structural information of complicated polymer surfaces and interfaces in situ. Copyright © 2006 Society of Chemical Industry     

Competitive pathways via nonadiabatic transitions in photodissociation

Photodissociation processes of molecules and radicals involving multiple pathways and nonadiabatic crossings are studied using the photofragment imaging technique and the core-sampling version of time-of-flight spectroscopy. Capabilities and challenges are illustrated by two systems. The isocyanic acid system demonstrates how interactions among potential energy surfaces can change during dissociation. The hydroxymethyl photodecomposition system highlights Rydberg-valence interactions common in free radicals. The cross-fertilization between theory and experiment is emphasized.     

The Role of the Entropy Factor in Chemical Fibre Technology

Examples of manufacturing processes involving use of the entropy factor are examined. A thermodynamic concept of entropy as the energy or orbital motion of molecules is formulated.     

Marine mussel adhesion: biochemistry,mechanisms, and biomimetics

Common blue mussel (Mytilus edulis) is a sessile organism that has unique ability to attach to a wide array of organic and inorganic marine surfaces using its holdfast structures. Strong adhesion to surfaces is essential for mussel survival, movement, and self-defense. Mussel proteins from byssal thread are structural components connecting soft mussel tissues to marine surfaces via an adhesive plaque in the distal end, while adhesive proteins from byssal plaque are responsible for mussel adhesion. Adhesive proteins are small molecules containing a high proportion of post-translationally modified amino acids such as 3,4-dihydroxyphenylalanine (DOPA). High DOPA content, small molecular size, protein flexibility, the presence of metal ions, and a high oxidation state enable strong mussel adhesion to surfaces. Mussel adhesion mechanisms depend on the composition and interactions of mussel proteins, as well as their interactions with the environment. Difficulties in the extraction of mussel adhesion proteins hamper mechanism studies and their practical applications. Development of recombinant mussel proteins and biomimetics will advance our understanding of adhesion mechanisms. In this paper, recent advances in the characterization of mussel adhesive proteins (MAPs), mussel adhesion mechanisms, application of MAPs, and the development of biomimetic biopolymers are reviewed.     

The domain model for heterogeneous catalysis

A domain theory inspired by non-uniformity of catalytic surfaces is introduced and a rigorous mathematical formulation is presented. This new analysis allows the classification of hysteresis systems into two distinct classes with qualitatively different behaviors for the first time. Heretofore unexplainable behaviors such as multistage extinction process, “memory” effects during induction period and discontinuous hysteresis can now be accounted for by the theory. Predicted hysteresis behaviors in an oscillating scan experiment, which has never been carried out, are also delineated by the theory. The classifications, verifications and predictions facilitated by the present formulation enhance the understanding of underlying system nonlinearities which are the origins of hysteresis as well as oscillatory and aperiodic dynamic behaviors.     

Surface reactivity of polyimide and its effect on adhesion to epoxy

Polyimides are commonly used as organic passivation layers for microelectronic devices due to their unique combination of properties such as low dielectric constant, high thermal stability, excellent mechanical properties and superior solvent resistance. Unfortunately, polyimides are well known to be difficult to bond to other materials, especially to epoxy resins. Many surface treatments have been developed to increase epoxy–polyimide adhesion. These treatments include exposure to ion beams, plasmas and chemical solutions. The goal of our research was to relate surface reactivity of epoxy and polyimide resins to the strength of epoxy–polyimide interfaces. The surface reactivity of four polyimides was studied and quantified using contact angle measurements, flow microcalorimetry (FMC), Fourier transform infrared (FT-IR) spectroscopy (using an attenuated total reflection (ATR) accessory) and X-ray photoelectron spectroscopy (XPS). Several ways of analyzing contact angles were tried and only a weak correlation between the polar component or the acid–base components of the surface free energy with the critical interfacial strain energy release rate (i.e., the interfacial fracture strength) was observed. FMC results suggest that the strength of epoxy–polyimide interfaces is related to the molecular interactions between the curing agent and polyimide. The molecular interactions between the curing agent and polyimide surfaces were found to be either greater than epoxy and polyimide interactions or more irreversible. Therefore, the curing agent (2,4-EMI) is thought to play a critical role in controlling adhesion strength.     

Luminescence studies of polymer behaviour

Luminescence techniques can be used to study polymer systems both as an analytical tool and as a means of studying molecular behaviour. In the latter context, some very interesting information may be obtained on a variety of molecular and energy transport processes. Measurements of luminescence intensity and quenching can be used to observe electronic energy transfer processes (both along polymer chains and from polymer chains to additive molecules). These have a technical importance in that they are the photophysical processes which precede the chemical phenomena important in photodegradation or photostabilisation of polymers. The basic criteria for energy transfer are introduced and examples are given of “down-chain” energy migration coefficients for a number of polymers. Studies of the depolarisation of luminescence can be used as a measurement of rotation in the electronic excited state of the photoactive group. Fluorescence depolarisation is introduced and examples are given of processes with nanosecond relaxation times. The concept of phosphorescence depolarisation is also mentioned, and the first use of this to study processes with millisecond relaxation times is illustrated.     

Interfacial agents for multiphase polymer systems: Recent advances

The rapid growth in the use of multiphase polymer systems (blends and composites) is undoubtedly related to the availability of methods of controlling the physical and chemical interactions at the interface. Compounds acting as interfacial agents are commonly known as “compatibilizers” in blends, or “coupling agents” in composites; their function is to promote adhesion and enhance overall properties. This paper is a review of recent advances in the use of these compounds in immiscible polymer blends and thermoplastic composites. Polymeric compatibilizers are classified according to their method of addition (in situformation vs. separate addition) and reactivity. Reactive low molecular weight compounds are also listed and their various coupling mechanisms are discussed. It is demonstrated that common routes to enhanced adhesion exist for some types of blends and composites. For example, reactive graft copolymers and certain crosslinklng agents are equally effective as adhesion promoters in blends and composites containing a polyolefin phase.     

Exploring the complexity of supramolecular interactions for patterning at the liquid-solid interface

The use of self-assembly to fabricate surface-confined adsorbed layers (adlayers) from molecular components provides a simple means of producing complex functional surfaces. The molecular self-assembly process relies on supramolecular interactions sustained by noncovalent forces such as van der Waals, electrostatic, dipole-dipole, and hydrogen bonding interactions. Researchers have exploited these noncovalent bonding motifs to construct well-defined two-dimensional (2D) architectures at the liquid-solid interface. Despite myriad examples of 2D molecular assembly, most of these early findings were serendipitous because the intermolecular interactions involved in the process are often numerous, subtle, cooperative, and multifaceted. As a consequence, the ability to tailor supramolecular patterns has evolved slowly. Insight gained from various studies over the years has contributed significantly to the knowledge of supramolecular interactions, and the stage is now set to systematically engineer the 2D supramolecular networks in a "preprogrammed" fashion. The control over 2D self-assembly of molecules has many important implications. Through appropriate manipulation of supramolecular interactions, one can "encode" the information at the molecular level via structural features such as functional groups, substitution patterns, and chiral centers which could then be retrieved, transferred, or amplified at the supramolecular level through well-defined molecular recognition processes. This ability allows for precise control over the nanoscale structure and function of patterned surfaces. A clearer understanding and effective use of these interactions could lead to the development of functional surfaces with potential applications in molecular electronics, chiral separations, sensors based on host-guest systems, and thin film materials for lubrication. In this Account, we portray our various attempts to achieve rational design of self-assembled adlayers by exploiting the aforementioned complex interactions at the liquid-solid interface. The liquid-solid interface presents a unique medium to construct flawless networks of surface confined molecules. The presence of substrate and solvent provides an additional handle for steering the self-assembly of molecules. Scanning tunneling microscopy (STM) was used for probing these molecular layers, a technique that serves not only as a visualization tool but could also be employed for active manipulation of molecules. The supramolecular systems described here are only weakly adsorbed on a substrate, which is typically highly oriented pyrolytic graphite (HOPG). Starting with fundamental studies of substrate and solvent influence on molecular self-assembly, this Account describes progressively complex aspects such as multicomponent self-assembly via 2D crystal engineering, emergence, and induction of chirality and stimulus responsive supramolecular systems.