Interfacial Properties of Polymers

The interfacial tension between a polymer and a condensed phase, such as a solvent or another polymer, is of great importance to the production and application of many polymeric products such as elastomers, plastics, textiles, foams, polymer blends, coatings, adhesives, and sealants.

The interfacial energy is defined as the work required to increase the interface between two phases by one unit area. It is usually expressed in mJ/m2 or erg/cm2. The force pulling in all directions parallel to the interface is called interfacial tension and is usually epressed in mN/m or dyne/cm. The interfacial tension and interfacial free energy are numerically and dimensionally identical for liquids but not for solids because solid surfaces do not necessarily assume an equilibrium shape, thus, the interfacial energy between a polymer in its solid state and a liquid or another solid can be (much) larger than between two liquids.

Wetting, spreading and adhesion phenomena are often described in terms of contact angles of liquids on a given surface. The figure below shows a solid surface (S) in contact with a droplet of liquids (L); we can distinguish between three different cases; if the liquid L fully wets the solid it tends to spread right over the surface. Thus, the contact angle θ is zero. In the second case the liquid does not spread over the surface but does wet it. The contact angle is between 0 and π/2. The third case, describes a liquid that does not wet the surface, i.e it tends to shrink away from the solid. For this situation, the contact angle is greater than π/2.

Contact Angle of Liquids at Solid Interfaces

Equilibrium contact angles of liquids on solids are often described with the Young’s equation:

γsv = γsl + γlv cos θ

or

γlv cos θ = (γs - γsl) - (γs - γsv) =  (γs - γsl) - πeqγs - γsl

γlv cos θ is called the adhesion tension. Complete wetting occurs when cos θ = 1 or θ = 0. For this situation, the so-called spreading coefficient S, defined as

S = γsv - γlv - γsl,

is zero. Thus, for S < 0 the droplet will assume a finite contact angle and for S > 0 it will spread. Obviously, wetting is favored by a low interfacial free energy, a high solid surface energy and a low liquid surface free energy or surface tension. Unfortunately, only λLV and θ can be directly measured. However, in order to predict adhesion phenomena, we also have to know λs and λsl. Fox and Zisman (1952) found that for a homologous series of liquids on a given solid surface a plot of cos θ versus λlv yields generally a straight line. Zisman was the first to introduce the concept of critical surface tension of wetting λcr, which is defined as the value of λlv, at the intercept of the cos θ - λlv plot with the horizontal line cos θ = 1 (Zisman plot). A liquid having a surface tension, λlv, less than the critical tension, λcr, will spread on the surface. Usually, λcr is very close to λs.

Another important relation has  been suggested by Girifalco and Good (1957):

γsl =γsv + γlv - 2·Φ·(γsvγlv)1/2

where

Φ = 4(Vm,s·Vm,l)1/3 / (Vm,s1/3 + Vm,l1/3)2

where Vm,l and Vm,l are molar volumes of the liquid and the solid. In many cases, Φ is of the order of unity, which is the case for aliphatic compounds where only dispersion forces are present:

γslγsv + γlv - 2·(γsvγlv)1/2

Another equation was suggested by Owens and Wendt (1969):

γslγsv + γlv - 2·[(γsvdγlvd)1/2 + (γsvpγlvp)1/2

In this equation, γsvd is the dispersion force component and and γsvp the polar force component of the surface free energy, i.e. γsv = γsvd + γsvp.

References
1. H.W. Fox and W.A. Zisman, J. Colloid Sci. 5, 514-531 (1950); 7, 109-121 and 428-442 (1952)
2. L.A. Girifalco and R.J. Good et al., J. Phys. Chem. 61, 904 (1957)
3. D.K. Owens and R.C. Wendt, J. Appl. Polym. Sci. 13, 1741 (1969)
4. True and accurate surface tensions of polymers are excedingly difficult to obtain. A clean and smooth plastic surface is required which is difficult to maintain, because even a low energy surface will pick up contaminations from the environment during the manufacturing process and storage. If such contaminations are present, even only as a monolayer, it can act as a weak boundary layer and seriously affect the interfacial tension with other phases.

• Summary

Interfacial Energy and Tension

The interfacial energy is defined as the work required to increase the interface between two phases by one unit area. The interfacial tension is the line force acting normal to the interface.

• The interfacial tension and interfacial free energy are numerically identical for liquid-liquid interfaces but not for solid-solid interfaces.

• Wetting, spreading and adhesion phenomena are often described in terms of contact angles θ of liquids on a given polymer surface. Complete wetting occurs when cos θ = 1 (θ = 0).

• Wetting is favored by low interfacial free energy, high solid surface energy and low liquid surface free energy or surface tension.

• Crystalline and semi-crystalline polymers have a higher interfacial tension than amorphous polymers.

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