Tensions of Polymers

Before leaving the subject of interfacial behavior in polymers, it is instructive to consider the interfacial tension, and resulting interfacial density profiles. Making effective use of the Flory interaction parameter x, Helfand and Tagami (1972), Gaines (1972), Wu (1974), and others estimated the interfacial surface tension between incompatible polymer pairs (see Table 13.1). Also shown in Table 13.1 are theoretically estimated values of x-(See Section 4.7 and especially Sections 4.7.3 and 9.6 for related discussion.) Helfand and Tagami found that the characteristic thickness of the interface is proportional to x — y for small /. For a polystyrene/poly(methyl methacrylate) system, the value of / leads to an estimated interfacial thickness of 50 A. This value is much less than that estimated by Voyutskii and Vakula 

Tabulation of Typical Values of Interfacial Tensions between Polymers  

Poly(vinyl acetate) vs. others PVAc/PnBMA PVAc/PDMS PVAc/PTMO 

Poly(methyl methacrylate) vs. others PMMA/PnBMA PMMA/PtBMA 

Polydimethylsiloxane vs. others PDMS/PnBMA PDMS/PtBMA PDMS/PEO PDMS/PTMO 

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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. 

In the same manner, with decreasing of diffusion coefficient and interaction parameter, the spinodal is reached during the evolution of the system in the pregel stage. The very low values of interfacial tension in rubber modified epoxies (interfacial tension of polymer-polymer-solvent system were reported in range of 10-4-10-1 mN/m) therefore lead to an NG mechanism for phase separation. 

The properties of immiscible polymers blends are strongly dependent on the morphology of the blend, with optimal mechanical properties only being obtained at a critical particle size for the dispersed phase. As the size of the dispersed phase is directly proportional to the interfacial tension between the components of the blend, there is much interest in interfacial tension modification. Copolymers, either preformed or formed in situ, can localize at the interface and effectively modify the interfacial tension of polymer blends. The incorporation of PDMS phases is desirable as a method to improve properties such as impact resistance, toughness, tensile strength, elongation at break, thermal stability and lubrication. 

Table 3.4 Critical surface tension of polymer solids [10] ...

Table 8.2 compares the experimental values of the surface tension of polymers (obtained by different methods) and the calculated values, the latter being obtained by means of the parachor. The discrepancies between the different experimental values are reasonably small. The calculated values are, with a few exceptions, in reasonable agreement with the experimental values. 

TABLE 8.2 Experimental0 and calculated values of surface tension of polymers expressed in mj/m2 = mN/m... 

Wu SJ, "Surface and Interfacial Tensions of Polymers, Oligomers, Plasticizers and Organic Pigments" In Brandrup J, Immergut EH and Grulke EA (Eds) "Polymer Handbook", Part VI, p 521, Wiley, New York, 4th Ed, 1999. Zisman WA, In "Contact Angle, Wettability and Adhesion", Adv Chem Ser 43, Am Chem Soc 1964, pp 1-51 in Weiss P (Ed) "Adhesion and Cohesion", Elsevier, Amsterdam, 1962, pp 176-208. 

The analysis of variance and the method of regular regression are used in evaluating the results. Equation improvements are discussed. The difficulties in predicting surface tensions of polymers on the basis of bulk properties are emphasized. 

Tn recent years, developing interests in surface energetics and adhesion of liquid-like polymers, or polymer liquids, have prompted both theoretical and experimental work on surface tension. Unlike low molecular weight liquids, polymer liquids have not been extensively studied. Bondi and Simkin (1) mentioned surface tension in their study on high molecular weight liquids. Roe (28) applied both the cell theory of polymer liquids and the hole theory of surface tension of simple liquids to develop an approximate theory of surface tensions of polymer liquids. His approach has met some degree of success. Notably, both Bondi s and Roes work are somewhat related to the cell theory introduced by Prigogine and... 

The direct measurement of surface tensions of polymer melts (10, 27, 29, 30, 37) generally yields values somewhat greater than those obtained indirectly, depending on the method and the instrument. Unfortunately, structure correlation studies cannot be undertaken until more direct surface tension data have been obtained. 

Figure 2. Comparison of the observed and the calculated critical surface tensions of polymers (on the basis of solubility parameter)...

This method has been applied successfully in the determination of the surface tensions of polymer particles (140,141,1591, coal particles (160,161). and biological cells [162-168], Contact angles predicted from these surface tensions involving solids agree well with experimental contact angles f 159]. 

The introduced hydrophilic moieties tend to migrate into the deeper section of the surface state while the surface is kept dry. Such a migration occurs in the direction to minimize the interfacial tension of polymer/air interface. This process has been recognized as the decay of the surface modification effect. 

Surface dynamics is concerned with the kinetic aspect of this specific question of whether or not the surface tension of polymer changes by contacting with liquid water. If ys changes by the interaction of water with polymer molecules, Ysl also changes accordingly. 

The interfacial tension may be dealt by the differential surface tension, A(ys), (the difference between surface tension of polymer and that of contacting phase) ... 

This ratio is transformed to Fowkes equation when the polar term is negligibly small. Using Eq. (4) and experimental data on interfadal and surface tension of polymers, Wu calculated the polarity of some polymers. He estimates the contribution of the polarity to the surface tension of polymers to be considerable. Thus, for polylvinyl acetate) it takes up 33%, for poly (methyl methacrylate) 28%, and for polychloroprene 11%. 

The lattice fluid equation-of-state theory for polymers, polymer solutions, and polymer mixtures is a useful tool which can provide information on equa-tion-of-state properties, and also allows prediction of surface tension of polymers, phase stability of polymer blends, etc. [17-20]. The theory uses empty lattice sites to account for free volume, and therefore one may treat volume changes upon mixing, which are not possible in the Flory-Huggins theory. As a result, lower critical solution temperature (LCST) behaviors can, in principle, be described in polymer systems which interact chiefly through dispersion forces [17]. The equation-of-state theory involves characteristic parameters, p, v, and T, which have to be determined from experimental data. The least-squares fitting of density data as a function of temperature and pressure yields a set of parameters which best represent the data over the temperature and pressure ranges considered [21]. The method,however,requires tedious experiments to deter-... 

Extrapolation of the surface tension data for polymer melts to room temperature. This is often considered to be the most reliable method for estimating the y of a solid polymer. See Table 7.1 for surface tensions of polymers [2] measured by using this method. 

Table 7.2. Combined polar and hydrogen bonding components (yx) of the surface tensions of polymers, in dyn/cm [1,3,10]. Note that the differences are usually large when more than one yx value has been reported for a given polymer, underscoring the difficulty of making accurate and reproducible determinations of this quantity. 

Van Krevelen [3] and Carre and Vial [5] examined the methods used to estimate the interfacial tension in practical calculations. Wolf [15] also discussed some methods to estimate the interfacial tensions of polymer solutions and polymer blends, and commented on the influence of additives on the interfacial tension. 

The key contributions of our work to the calculation of the surface tensions and interfacial tensions of polymers involve simple empirical correlations of much broader applicability than those based on group contributions for predicting the surface tension and its components ... 

Erbil, H.Y. (1997). Surface Tension of Polymers. In Birdi, K.S. (ed.). Handbook of Surface and Colloid Chemistry. CRC Press, Boca Raton. 

Measurements of interfacial tensions of polymer melts were reviewed by Wu (55), Koberstein (65), and Demarquette (66). The measurements usually need long equilibrium time because of the high viscosities of polymer melts. The measurements can be divided into two groups static methods in which interfacial tension is calculated based on the equilibrium profile of the drops and dynamic methods that study the evolution of fiber or drop profiles with time. Static methods include pendant drop method, sessile drop method, and rotating drop method. Dynamic methods include breaking thread method, imbedded fiber method, and deformed drop retraction method. 

S. H. Wu, Surface and interfacial tensions of polymers, oligomers, plasticizers, and organic pigments, in Polymer Handbook, J. Brandmp, E. H. Immergut, and E. A. Grulke (eds.), Wiley, New York, 1999. 

There are two other important process parameters which can influence the interfadal tensions of polymer phases agauist the aqueous phase the initiator type and the type of surfactant added to the aqueous phase. Other authors have also tried to manipulate the composite particle morphology by altering the surface polarity of the particles when using ionic or nonionic initiator in the preparation of either the s or the shell polymers [56,57]. 

PVT data for polymers are important both from the academic and practical points of view. On the scientific side, PVT data are frequently needed for model considerations on polymer solutions and melts. On the industrial side, these data are needed for process design. An equally important thermodynamic quantity is the surface tension of polymer melts, due to their relevance in wetting, adsorption, and adhesion. It may strongly govern such surface processes as film formation or coating. Here we report on PVT data and surface tensions of different random copolymers. We also relate thermodynamic quantities describing bulk properties to surface tension of polymer melts. 

Kwok, D. Y., Cheung, L. K., Park, C. B., and Neumann, A. W., Study on the surface tensions of polymer melts using axisymmetric drop shape analysis, Polym. Eng. Sci., 38, 757-764 (1998). 

Wu, S., Surface and interfacial tensions of polymer melts I. Polyethylene, polyisobutylene, and polyvinyl acetate, J. Colloid Interface ScL, 31, 153 (1969). 

The slope constant Ke is influenced by the chemical nature of the end groups, as shown in Figure 12-2. 7 is independent of the molar mass and the nature of the end groups. Typical surface tensions of polymer liquids of finite molar mass are given in Table 12-1. The surface tensions do notvary very much with temperature. 

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