Surface Tension of Liquid Polymers

Liquids have a surface tension with respect to the gaseous phase and an interfacial tension with respect to other liquids. Such interfacial and surface tensions may be measured by a whole series of different methods in the case of low-molar-mass liquids. But polymer liquids have very high viscosities, and so, only a few methods of measurement are suitable. The capillary method and the wire ring method are not suitable since the measured surface tension depends on the speed at which the test is carried out. All of what are known as static methods are suitable, i.e., the suspended drop method and the Wilhelmy plate method. 

The Wilhelmy-plate method consists in partially immersing a plate in a wetting liquid. The surface tension of the liquid 7/v acts downward on the plate. When the plate is wetted and its lower edge just resides on the liquid surface, the force acting on the plate is 7/v /per, where /per is the perimeter of the plate. By measuring the restraining force on the plate in air and in contact with 

The shape of a suspended drop depends on the surface tension as well as on the gravitational force. The drop is photographed and the diameter at various positions is measured. A consistent shape factor can be evaluated when hydrodynamic equilibrium is reached. 

The surface tension of a liquid polymer depends on its end groups, its molar mass, and on the temperature. A theoretical derivation of the molar mass dependence based on free volume theory leads to 

The interfacial tensions between two liquid polymers are generally small. They increase in size with stronger polarity differences between the two liquids (Table 12-1). 

---

Slow, K. S., and Patterson, D., Prediction of surface tensions of liquid polymers. Macromolecules, 4, 26 (1971). 

For Yiv > YPv> where y v and Ypv are the surface tensions of liquid and protein, respectively, AFads increases with increasing ysv, predicting decreasing polymer adsorption. An example of this is phosphate buffer saline where y]v = 72.9 mJ/m2 and Ypv is usually between 65 and 70mJ/m2 for most proteins [5]. Therefore, supports for gel-permeation and affinity chromatography should be as hydrophilic as possible in order to minimize undesirable adsorption effects. 

In order to calculate polymer/filler interaction, or more exactly the reversible work of adhesion characterizing it, the surface tension of the polymer must also be known. This quantity is usually determined by contact angle measurements or occasionally the pendant drop method is used. The former method is based on the Young, Dupre and Eowkes equations (Eqs. 21,8, and 10), but the result is influenced by the surface quality of the substrate. Moreover, the surface (structure, orientation, density) of polymers usually differs from the bulk, which might bias the results. Accuracy of the technique maybe increased by using two or more liquids for the measurements. The use of the pendant drop method is limited due to technical problems (long time to reach equilibrium, stability of the polymer, evaluation problems etc.). Occasionally IGC is also used for the characterization of polymers [30]. 

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 ... 

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. 

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... 

Owens and Wendt applied only two liquids to form drops in their experimental surface tension determinations. They used fw = 21.8 and y(v =51.0 for water, and y= 49.5 and y[v = 1.3 mj m 2 for methylene iodide, in their calculations. After measuring the contact angles of these liquid drops on polymers, they solved Equation (693) simultaneously for two unknowns of yfv and y( v, so that it would then be easy to calculate the total surface tension of the polymer from the (ysv = yfv + 7sv) equation. Later, Kaelble extended this approach and applied determinant calculations to determine ysv and y(v- When the amount of contact angle data exceeded the number of equations, a non-linear programming method was introduced by Erbil and Meric in 1988. 

CALCULATION OF CRITICAL SURFACE TENSIONS OF POLYMERS AND SURFACE TENSIONS OF LIQUIDS FROM CHEMICAL STRUCTURE ONLY. 

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. 

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. 

Fox and Zisman [62,63] characterised some polymer surfaces by measuring the contact angles for a series of liquids, and plotting the data in the form of cos 0 against the surface tension of the liquids. When 9 = 0 (cos9= 1), the liquid spreads on the surface and the surface tension of the liquid is then equal to the critical surface tension of the polymer. A plot is shown in Fig. 15 for liquids on some fluorinated copolymers the values of 7. are obtained from the intercept with the upper abscissa. The values of critical surface tension for some polymers are given in Table 10. 

Table 14.4. Liquid surface tension, /iv, at which maximum (Max) or minimum (Min) in sedimentation volume occurs, for several polymers PTFE, polytetrafluoroethylene PVDF, poly(vinylidene fluoride) PVF, poly(vinyl fluoride) HOPE high-density polyethylene PA 66, poly(hexamethylene adipamide) (nylon 6, 6) PSF, polysulfonate. Experiments were performed at room temperature, and the extrema are to be understood as being the surface tensions of the polymers (from refs (65, 66)) ... 

Schonhom H, Ryan FW, Sharpe LH (1966) Surface tension of a molten polychlorotrifluor-oethylene. J Polym Sci A-2 4 538-542 133. Edwards H (1968) Surface tensions of liquid polyisobutylenes. J Appl Polym Sci 12 2213 Wilhelmy L Ueber die Abhangigkeit der Capillaritats-Constanten des Alkohols von Sub-stanz und Gestalt des benetzten festen Korpers. Ann Phys 195 177-217 Dettre RH, Johnson RE Jr (1966) Surface properties of polymers I. The surface tensions of some molten polyethylenes. J Colloid Interface Sci 21 367-377... 

To apply Eq 2.52, one needs to know the surface tension of adhesive. There are methods which allow one to estimate this value. The methods are based on the measurement of contact angles. To find the surface tension of a polymer Yp, using various liquids with surface tension, Yl, the following equation is used ... 

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 ... 

Edwards H (1968) Surface tensions of liquid polyisobutylenes. J Appl Polym Sci 12 2213... 

---