Determination by contact-angle measurements

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]. 

The polarity and adsorption data discussed above reveal some interesting aspects of the surface chemistry of vinyl acrylic latex surfaces. It is quite likely that the polarity of the latex films, expecially of the two co-polymers, determined by contact angle measurements may not correspond exactly with their respective latex surfaces in the dispersed state due to reorientation of polymer chains during film formation. But the surfactant adsorption data shows clearly that the three latex surfaces in their dispersed state do exhibit varying polarity paralleling the trend found from contact angle measurements. The result also shows that the surface of the co-polymer latex surface is a mixture of vinyl acetate and acrylate units. This result is somewhat unexpected in a vinyl acrylic latex, prepared by a batch... 

Wetting can be determined by contact angle measurements. It is governed by the Young equation, which relates the equilibrium contact angle 9 made by the wetting component on the substrate to the appropriate interfacial tensions ... 

TABLE 9 Surface Tension Components (in 10 N/m) of Various Solid Surfaces at 20C Determined by Contact Angle Measurements... 

Figure 9. Plot of the % HMS at the surface determined by XPS vs. the % HMS determined by contact-angle measurements...

All films prepared for contact angle studies were cast from an excess of dry polymer swollen and fluidized by a small quantity of added liquid (usually triple-distilled water, the last two distillations being in an allquartz apparatus). The pH of these weak gels at the initiation of film formation was always 5. All our surface films were gel-dried rather than sol-dried. Although swelling may not have reached equilibrium prior to film drying, the films were only about 1 mm thick and their surface properties, as determined by contact angle measurements, were always reproducible within 5°. 

The surface tension of sputter deposited cBN films was determined by contact angle measurements with different well known liquids [88]. The value of 40mNm is nearly the same as that of DLC (diamond like carbon) [89]. 

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. 

The component of the surfaces under investigation can be determined by contact angle measurements, using a nonpolar liquid as the interaction of a nonpolar liquid is restricted to dispersive forces [83], The nonpolar liquid should have a high surface tension (such as DIM) to avoid the spreading of the liquid. In the case of negligible acid-base interactions (i.e., for nonpolar solids), the total work of adhesion between a solid and a liquid... 

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 ... 

PLS has been widely used in blends with other polymers, and a lot of patents have been published on this topic. This stratified pseudo-multilayer structure gives the blend rather good water resistance, in comparison with PLS itself, due to the polyester surface protection. Water sensitivity, determined by contact angle measurements with water drops, decreases drastically for polyester contents lower than 10% in the blend. The biodegradabihties of such blends are modified, with degradation occurring from the starchy core toward the skin. 

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