Surface energy of solid polymers

Three ways are available for the estimation of ys, the surface tension of the solid. The first is the method measuring the contact angle between the solid and different liquids and applying Eq. (8.9). The second is the determination of ycr according to Zisman (1964), with the assumption that ys ycr. The third way is the extrapolation of surface tension data of polymer melts to room temperature (Roe, 1965 Wu, 1969-1971). 

Estimation of surface tensions of solid polymers from the parachor 

Due to the fact that the extrapolation of surface tensions of melts to room temperature leads to reliable values for the solid polymer, the surface tension of solid polymers may be calculated from the parachor per structural unit by applying Eq. (8.5). The molar volume of the amorphous state has to be used, since semi-crystalline polymers usually have amorphous surfaces when prepared by cooling from the melt. We have found that the original group contributions given by Sugden show the best correspondence with experimental values for polymers. 

If no experimental data are available, calculation by means of the group contributions to the parachor gives a reliable approximation. A still higher accuracy can be reached if the methods of standard properties or standard substances , discussed in Chap. 3, are applied. 

From what has been said about the surface tensions of liquids it may be expected that a relation also exists between the surface tension and the cohesive energy density of solid polymers. This proves to be so with y expressed in mj/m2 and ecoh in mj/m3, the following empirical expression may be used  

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Measurement of the contact angle at a solid-liquid interface is a widely used method for the determination of the surface energy of solid polymers. Fowkes [1] first proposed that the surface energy of a pure phase, y y could be represented by the sum of the contribution from different types of force components, especially the dispersion and the polar components, such that ... 

An Analytical Method for Determining the Surface Energy of Solid Polymers... 

R.N. Shimizu, N.R. Demaiquette, Evaluation of surface energy of solid polymers using different models. J. Appl. Polym. Sci. 76(12), 1831-1845 (2000)... 

The specific surface energy of a polymer can be estimated by means of an additive quantity, the Parachor. Alternatively, it maybe calculated from the molar cohesive density (which is also additive). Rules are given for the estimation of the interfacial tension and the contact angle of a liquid on a solid. 

This too predicts that yc will depend on the nature of the liquids used, as this will influence the value of fi. The critical surface tension will only be the same as the surface energy of the polymer if the interaction parameter is unity this can occur with a non-polar liquid on a non-polar solid. More usually, the value of is less than one. 

Once the dispersion component of surface energy of a liquid is known, the polar component can be obtained from the surface tension using Eqn. 10. The approach based on Eqns. 9 and 10 can then be used to estimate surface energies of solids, particularly polymers, very much in the same way as Ys values are obtained from Eqn. 8. Here, yi2 is eliminated between Eqn. 10 and the Young equation, giving... 

The degree of wetting is controlled by thermodynamic and kinetic factors. The driving force for wetting a substrate is dependent on the surface energies of the adhesive and the substrate. The Zisman concept of critical surface tension provides the basis for estimating the surface energy of solids [212]. Typical values of critical surface tension (in mN/m) for polymer substrates follow [216] ... 

An alternative method uses a concept called critical surface tension, proposed by Fox and Zissman to characterize the surface energy of solids. A plot cosine of the contact angle (cos 6), and liquid—vapor surface tension (yiv), yields a straight line for a homologous series of liquids (Fig. 3.5). Nonho-mologous liquids yield a curved line that may not be easily extrapolated. The intercept of the line at cos (0) equal to one is defined as the critical surface tension of the polymer (yc). Values of 18 dynes/cm for... 

The qualitative thermodynamic explanation of the shielding effect produced by the bound neutral water-soluble polymers was summarized by Andrade et al. [2] who studied the interaction of blood with polyethylene oxide (PEO) attached to the surfaces of solids. According to their concept, one possible component of the passivity may be the low interfacial free energy (ysl) of water-soluble polymers and their gels. As estimated by Matsunaga and Ikada [3], it is 3.7 and 3.1 mJ/m2 for cellulose and polyvinylalcohol whereas 52.6 and 41.9 mJ/m2 for polyethylene and Nylon 11, respectively. Ikada et al. [4] also found that adsorption of serum albumin increases dramatically with the increase of interfacial free energy of the polymer contacting the protein solution. 

The existence of the mesophase layer has been proved by infra-red spectroscopy, ESP, NMR, electron microscopy and other experimental methods. Moreover, it has been also proved that the thickness of this layer depends on the polymer cohesion energy, free surface energy of the solid, and on the flexibility of the polymer chains. 

Aid in the uniform dispersion of additives. Make powdered solids (e.g. particulate fillers with high energy and hydrophilic surface) more compatible with polymers by coating their surfaces with an adsorbed layer of surfactant in the form of a dispersant. Surface coating reduces the surface energy of fillers, reduces polymer/filler interaction and assists dispersion. Filler coatings increase compound cost. Fatty acids, metal soaps, waxes and fatty alcohols are used as dispersants commonly in concentrations from 2 to 5 wt %. 

In the third part of the chapter the solid state properties of our block copolymer are examined. The surface energies of these materials are characterized by contact angle measurements. The organization of the polymer chains in the solid state phase is investigated by small-angle X-ray scattering (SAXS) and the gas selectivity of porous membranes coated with these block copolymers is characterized by some preliminary permeation measurements. 

These data show the surface characteristics as related to ycr. In many cases, the surface of a solid may not behave as desired, and therefore it treated accordingly, which results in a change of the contact angle of fluids. For instance, the low surface energy of polymers (polyethylene [PE]) is found to change when treated with flame or corona (as shown in the following table). 

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