Most small-molecule organic liquids are mutually miscible and their mixtures do not form stable interfaces. Polymers are, however, usually immiscible and their mixtures form multiphase structures with stable interfaces. The dispersion, morphology, and adhesion of the component phases are affected by the interfacial energies, which thereby play an important role in determining the mechanical properties of a multiphase polymer blend. This chapter discusses interfacial energy, structure, and adhesion between polymers as related to the properties of polymer blends. It provides an overview of surface tension and interfacial tension, and discusses various theories of interfacial tension such as Antonoff's rule, theory of Good and Girifalco, theory of fractional polarity, and lattice theories. The interfacial tension varies more slowly with temperature than the surface tension and arises mainly from the disparity between the polarities of two phases. The extent of non-polar (dispersion) interaction between the two phases does not vary greatly from system to system but that of polar interactions can vary greatly. The polar interactions largely determine the magnitude of the interfacial tension. The polarity of a polymer can be calculated from the interfacial tension between the polymer and a non-polar polymer such as polyethylene, which can be regarded to be completely nonpolar. Thus, if the interfacial tension of a polar polymer against polyethylene is known, the polarity of the polar polymer can be calculated. Adhesion is the bonding or joining of dissimilar bodies, while autohesion or cohesion refers to joining of identical bodies. Several adhesion theories have been proposed. They have been a subject of much controversy. Most theories deal with the formation of the adhesive bond. The chapter reviews various factors affecting the formation and the fracture of adhesive bonds.

In this work the influence of the atmospheric pressure plasma treatment on the surface properties of polypropylene (PP) films was investigated. The film samples were modified by atmospheric pressure plasma treatment by diffuse coplanar surface barrier discharge (DCSBD) using ambient air as working gas. The contact angle measurement, the test pen method, atomic force microscopy (AFM) and attenuated total reflection technique Fourier transformed infrared spectroscopy (ATR-FTIR) were applied to analyze the changes of the surface of the polymer film. In all experiments, the contact angle of the treated polypropylene samples decreased and the surface energy of the samples increased in comparison with the plasma untreated samples. The proper surface energy for printing using solvent-based inks was detected by all the samples. There were not observed any significant changes in mechanical properties of the films after plasma treatment by measuring their tear parameters.

Low-rate dynamic contact angles of water, glycerol and methylene iodide on polystyrene and poly(4-(X = CH₃, (CH₃)₃, Cl, OH)-styrene) films were measured by axisymmetric drop shape analysis-profile (ADSA-P) for sessile drops. It was found that glycerol in the case of poly(4-chlorostyrene) and methylene iodide on all investigated surfaces did not yield constant contact angles: dissolving of the polymer during the contact with the liquid, penetration effects and stick/slip behavior were found. Water and glycerol yielded meaningful constant contact angles (except for glycerol on poly(4-chlorostyrene), for which we used formamide additionally). From these meaningful contact angles the solid surface tensions of the modified polymers were calculated using the equation of state approach [6]. The following values of γ_sv were determined: polystyrene γ_sv = 28.3 mJ/m², poly(4-methylstyrene) γ_sv = 25.8 mJ/m², poly(4-tert-butylstyrene) γ_sv = 22.0 mJ/m², poly(4-hydroxystyrene) γ_sv = 44.1 mJ/m², poly(4-chlorostyrene) γ_sv = 27.2 mJ/m².

The role of roughening and functionalization processes involved in modifying the wettability of poly(ε-caprolactone) (PCL) after treatment by an atmospheric pressure glow discharge plasma is discussed. The change in the ratio of CO/C–O bonds is a significant factor influencing the wettability of PCL. As the contact angle decreases, the level of CO bonds tends to rise. Surface roughness alterations are the driving force for lasting increases in wettability, while the surface functional species are shorter lived. We can approximate from ageing that the increase in wettability for PCL after plasma treatment is 55–60% due to roughening and 40–45% due to surface functionalization for the plasma device investigated.

Adhesion between polymeric phases like adhesives and plastic surfaces is critical in many technological and industrial applications. In the last decades much progress has been made to understand the impact of the surface properties of both phases on the adhesive strength between them. These surface properties influence processes like wetting, molecular adsorption and inter-diffusion which determine how an interface develops into an interphase after the two materials have been brought into contact. Ultimately, the properties of this interphase determine the overall adhesion strength of an assembly. In this paper important parameters in the adhesion process will be reviewed, including methods to engineer these parameters in order to attain adhesion strengths ranging from complete release to irreversible bonding.

The capability of the PDMS based antiadhesion surface treatment strategy for high resolution unconventional lithography using hard or soft molds as representatives of imprint lithography or soft lithography was investigated. A thin film of PDMs was used as an antiadhesion release layer as PDMS has a fairly low surface energy and allows for the easy release of the mold from the patterned polymer on the substrates. The surface of the Si wafer was coated with a thin film of PDMS and using this PDMS-coated Si wafer as a hard mold line/space patterns were printed on the SU-8-coated PET substrates. Using this photoresist replica mold as a template for a soft mold the same PDMS-based coating strategy was applied. The imprinting of nanostructure-patterned mold onto a polymer composed of the same chemical as the mold led to pattern collapse during the release of the assembly because of the extremely strong adhesion between the mold and the polymer.

An interpretation of solid surfaces is generated based on physical considerations and the laws of thermodynamics. Like the widely used Owens–Wendt (OW) method, the proposed method uses liquids for characterization. Each liquid provides an absolute lower bound on the surface energy with some uncertainty from measurement variations. If multiple liquids are employed, the largest lower bound is taken as the most accurate, with uncertainty due to measurement errors. The more liquids used, the more accurate is the greatest lower bound. This method links generalizations of the Good–Girifalco equation with a general thermodynamic inequality relating the three-interfacial tensions in a three-phase equilibrium system. The method always satisfies this inequality with better than a 65% certainty. However, the OW seldom, if ever, conforms to this inequality and even then, the degree of satisfaction is insignificant. A reconciliation of the two methods is proposed based on rescaling the OW surface energies to conform to the inequality. This enables interpretations of dispersion and polar components of the surface energy, which are thermodynamically self-consistent. The proposed method is also capable of dealing with material exchange between liquid and solid phases, when the surface tension and contact angle of the saturated liquids can be measured.

The curent state of problems connected with the definition and experimental determination of surface free energy and surface tension of polymers is discussed. An analysis of the application of some equations based on classical and modern thermodynamics of polymer solutions shows that present theories need an essential improvement to fit experimental data. The Zisman concept of critical surface tension and Fowkers' hypothesis of additivity in the contribution of polar and dispersion forces to surface tension are criticized and a new approach to the problem is proposed.

Methods for the surface free energy determination of solids based on wetting by liquids are reviewed. Some critical remarks and new ideas are included.

The effects of a thin liquid film on contact angles are studied using a simplified thermodynamic model. (in this model, the small transition zone between the liquid-vapour interface and the fiat thin liquid film is neglected). A set of mechanical equilibrium conditions have been derived for contact angle systems with a flat thin liquid film. The equilibrium condition at the three-phase intersection explicitly predicts the effects of the film tension, the disjoining pressure and the film thickness, on contact angles.

The number of degrees of freedom for a two-component solid-liquid-vapour surface system with a flat thin liquid film is shown to be three, implying the existence of an equation-of-state-type relationship among the solid-liquid interfacial tension, γ_sl, liquid surface tension, γ_lv, the disjoining pressure, Π, and the film tension, γ_f. An approximate, explicit form of such an equation of state has been derived. The combination of this equation of state with the equilibrium condition of the the three-phase intersection can be used to estimate the film tension, γ_f, and the solid-liquid interfacial tension, γ_sl, from the measured data for the vapour pressure, P_v, the film thickness, h, the curvature of the liquid-vapour meniscus, J, the liquid surface tension, γ_lv, and the contact angle, θ.

The effect of the thin film on the drop-size dependence of contact angles is also investigated and found to be negligible.

Recent progress in the correlation of contact angles with solid surface tensions are summarized. The measurements of meaningful contact angles in terms of surface energetics are also discussed. It is shown that the controversy with respect to measurement and interpretation of contact angles are due to the fact that some (or all) of the assumptions made in all energetic approaches are violated when contact angles are measured and processed. For a large number of polar and non-polar liquids on different solid surfaces, the liquid–vapor surface tension times cosine of the contact angle, γ_lvcosθ, is shown to depend only on the liquid–vapor surface tension γ_lv, and the solid–vapor surface tension γ_sv when the appropriate experimental techniques and procedures are used. Equations which follow these experimental patterns and which allow the determination of solid surface tensions from contact angles are discussed. Universality of these experimental contact angle patterns is illustrated; other reasons which may cause data to deviate from the patterns slightly are discussed. It is found that surface tension component approaches do not reflect physical reality. Assuming the fact that solid surface tension is constant for one and the same solid surface, experimental contact angle patterns are employed to deduce a functional relationship to be used in conjunction with Young's equation for determining solid surface tensions. The explicit form of such a relation is obtained by modifying Berthelot's rule together with experimental data; essentially constant solid surface tension values are obtained, independent of liquid surface tension and molecular structure. A new combining rule is also derived based on an expression similar to one used in molecular theory; such a combining rule should allow a better understanding of the molecular interactions between unlike solid–liquid pairs from like pairs. Existing static contact angles for 34 different types of solid surfaces from Zisman et al. are evaluated in terms of their solid surface tensions using experimental contact angle patterns. A FORTRAN computer program has been implemented to automate these procedures. It is found that literature contact angles do not have to be discarded completely; they can be used to determine solid surface tensions, with caution. The surface tensions for the 34 solid surfaces from Zisman et al. are also reported.