Surface Tension of Solid Polymers

The surface tension of polymers (synthetic polymers such as plastics, biopolymers such as proteins and gelatin) is indeed of much interest in many areas. In industry where plastics are used, the adhesion of these materials to other materials (such as steel, glass) is of much interest. The adhesion process is very complex since the demand on quality and control is very high. This is also because adhesion systems are part of many life-sustaining processes (such as implants, etc.). The forces involved in adhesion need to be examined, and we will consider some typical examples in the following text. 

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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. 

Several atomic and structural contribution tables, derived from the observed surface tensions of organic liquids, are available for the calculation of P 16-8], Van Krevelen [3,4] found that when these tables are used to estimate the surface tensions of solid polymers instead of liquids and melts, the original table of group contributions developed by Sugden [6] provides better agreement than the two revised and improved tables [7,8] with the "experimental" values of y extrapolated for solid polymers from various types of measurements. Ele also provided an... 

It is not permissible to adopt data acquired from changes of the liquid resin s surface tension for solid state polymer. Zisman [72] does it supposing that the reversible adhesion work of the solid polymer must be close to that estimated for the liquid state. The conclusion follows from the assumption that the forces that act on the phase separation boundary spread out to a depth that does not exceed the size of some molecules. As a result, the interaction on the bound y cannot depend on the change of state of the substance. One must accept this because the determined v dues of the surface tension of solid polymers significantly exceed those for the liquid oligomers. If we deal with undercured products the v dues usually exceed those for the polymer surface tension acquired by wetting agent critic d surface tension methods. 

As shown e u lier, the dependence of the surface tension of solid polymers on the surfactant concentration exhibits a minimum and maximum, and the increase in surface tension under the influence of surfactant is rather significant (Fig. 2.33). This is apparently due to the demonstrated structure-forming action of surfactant, which is possible only at stages of reaction when sufficiently flexible molecules... 

Water and methylene iodide are two liquids that are used for the determination of the components of surface tension of solid polymers by measuring contact angles. Table 2.7 gives the values of the components of their surface tension. 

The surface tension of solid polymers is most conveniently determined by contact angle measurements, which consist of measuring the contact angle [6) of a homologous series of liquids of known surface tension (yj on a plane polymer solid surface. The different values of the contact angle (0) (Fig. 10.104) lead to different effects ... 

The extensive use of the Young equation (Eq. X-18) reflects its general acceptance. Curiously, however, the equation has never been verified experimentally since surface tensions of solids are rather difficult to measure. While Fowkes and Sawyer [140] claimed verification for liquids on a fluorocarbon polymer, it is not clear that their assumptions are valid. Nucleation studies indicate that the interfacial tension between a solid and its liquid is appreciable (see Section K-3) and may not be ignored. Indirect experimental tests involve comparing the variation of the contact angle with solute concentration with separate adsorption studies [173]. 

In Table III, critical surface tensions of thirty-nine polymers are reported, calculated on the basis of Equation 5 by assuming < = 1. In other words, these calculated results are equivalent to the calculated solid surface tensions (y8). Solubility parameters of several polymers listed in the table were calculated on the basis of Small s constants (35), and all molar volumes were calculated on the basis of the molecular weight of the repeat unit and the density of the polymer. The results in Table III were used to prepare a graph (Figure 2) for the comparison between the calculated and the observed critical surface tensions of these polymers. The data are rather scattered, and the calculated values are generally lower than those observed directly. The following factors may be contributing to the deviations ... 

Our restriction to simple fluids was meant to emphasize general laws and phenomena. For this reason, we did not discuss theories of the surface tension of solids, for which a variety of models have been elaborated. One of the considerations for omitting these was that such tensions cannot be measured, so that a check of the quality is edso impossible. We also consciously excluded the surface tensions of liquid metals, liquid crystals, molten crystals and polymer melts. However, spread and adsorbed polymer layers will be considered in chapter 3 and 4, respectively. For similar reasons, and because most practical applications involve ambient temperatures, we did not extensively discuss critical phenomena, notwithstanding their Intrinsic Interest. Under critical conditions the surface energy - surface entropy balance differs considerably from that at lower temperatures, emphasized in this chapter. 

The present investigation describes the successful modification of the surface properties of polymeric solids by the adsorption of appropriate partially fluorinated compounds at polymer-air interfaces during the formation of the polymer surfaces. The extent of additive adsorption was foxmd to be dependent upon the molecular structure, fluorine content, and solubility of the additives in the solute—i.e., their organophilic-organophobic balance with respect to the solute. Certain effective additives were able to decrease the critical surface tension, of such polymers as poly(methyl methacrylate) and polyacrylamide to 20 and 11 dynes per cm., respectively. These low values correspond to surfaces containing closely packed CF2 and CF3 groups. 

The contact angles of water and suitable solvents at the solid/liquid/gas interface allow the determination of the surface tension of solids as well as the dispersive (y ) and polar (yP) components. A semiquantitative prediction of the hy-drophilicity and hydrophobicity of polymer surfaces has already been achieved by contact angle determination with water [74,75]. 

Ratner BD (1988) The surface characterization of biomedical materials. In Ratner BD (ed) Progress in biomaterials engineering, vol 6. Elsevier, Amsterdam, p 13 Vidrine DW (1982) Photoacoustic Fourier transform infrared spectroscopy of solids and liquids. In Fourier transform infrared spectroscopy Fries T (1994) Deutscher Verband fiir Materialprufung, p 127 Sacher E (1988) The determination of the surface tensions of solid films. In Ratner BD (ed) Progress in biomaterials engineering, vol 6 Surface characterization of biomaterials. Elsevier, Amsterdam, p 53 Owens DK, Wendt RC (1969) J Appl Polym Sci 13 1741... 

Thus by contact angle measurements using three different liquids (L), of which two must be polar, with known y Y and y values, the ys", Ys and ys of any solid (S) can, in principle, be determined. The value of yl must be known or determined independently [108]. The apolar component of the surface tension of solids (yj" ) can be determined by contact angle measurements using strictly apolar liquids for which yL = y These surface tension components can be related to experimentally determined pull-off forces between chemically modified AFM tips and an oxyfluorinated isotactic polypropylene surface in CFM approaches [110]. It was observed that the pull-off force measured with carboxylic acid tips in ethanol depended hnearly on the basic term of the surface tension (y,") on the modified polymer surface. 

FIGURE 2.19 Experimental [109] (symbols) and calculated (solid lines) surface tensions of pure polymers. 

From the analysis presented above, it becomes pertinent that when a polymer is in contact with a solid having higher surface tension, the increase in the surface tension of a polymer will be observed due to polarization. It is also evident that the value Wc should be considered as a cohesion energy of the interphase region but not of pol5mier in bulk. This value may or may not coincide with the cohesion energy of polymer far from the phase border. The same conclusion follows from the analysis of an interaction between sohd and hquid along... 

Explain briefly how the solid surface tension of a polymer as well as its van der Waals and Lewis acid and Lewis base components can be estimated using the van Oss et al. theory. How... 

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 behavior of polymer solutions, which exhibit phase coexistence between a polymer-rich liquid (L) and a polymer-poor vapor (V), is qualitatively similar. Only the separation of energy scales between the surface tensions of the polymer and vapor in contact with the solid and the Uquid-vapor interfacial tension is less pronounced because the Uquid-vapor interface also is narrow and the cohesive van der Waals interactions inside the Uquid are strong. 

These equations show that in order to obtain Yg it may be preferable to plot COS0 as a function of Ylv 2 (instead of Ylv explain why Yc may vary from one liquid series to another. The surface tension of a polymer can be identified with the critical surface2 tension if the solid-liquid system is regular (( >=1). This occurs when the interactions at the interface are exclusively of the London type (in addition to TTg 0). ... 

This closeness of 0 to zero explains the existence of a gas-oversaturated solution area in the polymer melt, when P < Pg, but the entire volume of gas remains in the solution. The degree of oversaturation, particularly upon free foaming (not in flow) can be 2- to 3-fold. In real polymer compositions, there are always solid admixtures, which have poor wetting areas. This reduces the degree of oversaturation at the interface melt-molding tool. Moreover, bubble nuclei can result from fragmentation of gas bubbles in the polymer [16]. Another factor that promotes the formation of bubble nuclei is the presence of localized hot points in the polymer melt they act as nuc-leation centres. Hot points appear either after a chemical reaction in the melt polymer [17], or in overheated areas on the surface of metal equipment [18]. Density of nucleation can be improved via introduction of various agents that reduce tension of the polymer [19]. 

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