Comparison of Surface and Bulk Properties of Pendant and Hybrid Fluorosilicones

Abstract

The most common fluorosilicone polymer commercialized to date is polymethyltrifluoropropylsiloxane. However, the low content of is the perfluorinated groups in the polymer 36.5 wt% does not fulfill the requirements of some high tech applications, particularly when swelling properties or degradation at high temperatures are concerned. A number of strategies have been employed to increase the fluorine content of fluorosilicone polymers. One elegant way is to introduce into the silicone chain, either as a pendant group or inside the backbone, perfluorinated groups of increasing size (typically C6 or higher). We refer to silicones with perfluorinated chains introduced as side groups as “pendant silicones” whereas those carrying fluorine atoms in the main backbone are called “hybrid silicones”. The most popular synthesis techniques of such polymers are briefly discussed here. A full fuller comparison is given of the two classes of polymers in terms of surface, mechanical, swelling and thermal properties.

An erratum to this chapter can be found at http://dx.doi.org/10.1007/978-94-007-3876-8_14

Appendices

Appendix A: Definition and Measurements of Surface Tension for Soft Polymers

5.1.1 A.1 Definition of Surface Tension

Surface tension is the force per unit length necessary to minimize the surface area between two immiscible media. This contraction force results from the propensity of bulk liquid molecules to attract those at the interface to ensure cohesion between them. For a liquid, the measurement of this parameter is easy and reliable, since an equilibrium state between the liquid and the surrounding gas or liquid can always be reached. For solid surfaces, the elastic force between that surface and a drop of liquid may not be at equilibrium, and the surface energy of the solid acts as an attractive force opposing contraction of the liquid. Consequently, liquid and solid surface tensions cannot be compared.

5.1.2 A.2 Measurement of Liquid Surface Tensions

Two techniques are used to measure the surface tension of liquid polymers; the pendant drop technique and the Wilhelmy plate technique (equations (5.3) and (5.4) and Fig. 5.27). The former is not frequently used in the papers reviewed in this chapter, since it requires an apparatus calibration component and the shape of the drop may not be perfectly round for viscous polymers. The Wilhelmy plate technique consists of measuring the pulling force on the plate introduced into the liquid. The simplest case is when a meniscus forms between the plate and the liquid where the contact angle is θ=0, and the surface tension is calculated knowing the perimeter of the plate, i.e. horizontal length and thickness (5.4). Note that a correction for the liquid buoyancy is avoided by performing the force measurement when the edge of the plate is at the same level with the liquid surface.

(5.3)

(5.4)

In these equations γ is surface tension of the liquid; ρ is density; g is specific gravity; H is a coefficient calibrated on the apparatus; f _w is the pulling force; and p is the plate perimeter.

Fig. 5.27

Sessile drop (left) and Wilhelmy plate (right) techniques to measure liquid surface tension

5.1.3 A.3 Measurement of Solid Surface Tensions

Solid surface tensions are almost exclusively determined using a contact angle technique as illustrated in Fig. 5.28. When a drop of a liquid is deposited on the polymer substrate the contact angle between the liquid, air and solid is given by Young’s equation:

$$ \gamma_{\mathrm{L}}\cos\theta =\gamma_\mathrm{S}-\gamma_{\mathrm{SL}} $$

(5.5)

where γ _L,γ _S and γ _SL are the liquid/air, solid/air and solid/liquid surface tensions, respectively. Since in this equation only γ _L and θ are known, this requires one to use semi-empirical equations to deduce the surface tension of the solid γ _S.

Fig. 5.28

Contact angle of a standard liquid drop on a flat (polymer) surface. By definition, if θ is less than 90°, the liquid wets the solid surface

Surface tensions can be divided into two components, a dispersive one ($\gamma_{\mathrm{S}}^{d}$) and a polar one ($\gamma_{\mathrm{S}}^{p}$), according to (5.6):

$$ \gamma_{\mathrm{S}}=\gamma_{\mathrm{S}}^d+\gamma_{\mathrm{S}}^p $$

(5.6)

Based on this, Owens and Wendt [157] derived equation (5.7) using a geometric approximation:

$$ \gamma_{\mathrm{L}}(1+\cos \theta)=2(\gamma_{\mathrm{S}}^d\gamma_{\mathrm{L}}^d)^{1/2}+2(\gamma_{\mathrm{S}}^p\gamma_{\mathrm{L}}^p)^{1/2} $$

(5.7)

Generally, two model liquids are necessary to determine both components. To determine the dispersive component, test liquids with no polar surface tension component, such as hexadecane, are chosen, while the polar component can be obtained from a polar liquid, typically water.

Another method of solid surface tension determination is given by the Girifalco–Good–Fowkes–Young equation [158, 159]:

$$ \cos\theta=2(\gamma_{\mathrm{S}}^d)^{1/2}(\gamma_\mathrm{L})^{-1/2}-1 $$

(5.8)

The so-called Zisman technique consists of determination of the surface tension by plotting cosθ versus γ _L for a series of liquid alkanes and extrapolating to cos θ=1; where $\gamma_{\mathrm{S}}^{d}=\gamma_{\mathrm{L}}$. This surface tension is referred to in this chapter as the dispersive critical surface tension, γ _c. The technique is believed to give solid surface tensions which depend little on the test liquids, although it was recently observed that short alkanes may partly swell the fluorosilicones.

One can also perform dynamic measurement of the contact angle: in this case the advancing angle (θ _A) is close but not similar to the one obtained by static measurements, i.e. “at equilibrium”, whereas the receding angle (θ _R) is measured after dewetting the surface (experimentally performed by sucking back a part of the liquid to decrease the droplet volume). The contact angle hysteresis (ω), i.e. the difference between the advancing and the receding contact angles, gives an indication either of the chemical rearrangement of the surface upon contact with the liquid, or of the surface roughness.

Appendix B: Swelling Measurements, Solubility Parameters and PDMS Case

A solvent is generally a liquid that dissolves another liquid, solid or gaseous solute, resulting in a uniform mixture called solution. Two substances are miscible if they show the same cohesion energy, c (cal cm⁻³). Since true solution requires complete separation of individual molecules, a cross-linked polymer can never dissolve but an appropriate solvent is likely to be absorbed by the network to give a swollen gel similar to a very viscous solution. The amount of swelling of the polymer depends on the competition between: (i) the free energy of the mixture on insertion of solvent molecules to solvate polymer segments; and (ii) the elastic retraction force acting opposite to the distortion, caused by the chain elongation in the swollen cross-linked network. Equilibrium of these two forces leads to an optimal volume swell.

Hildebrand solubility parameters δ (cal^1/2 cm^−3/2) describe interactions between different solvents and solutes. Swelling will be at maximum when the solubility parameter of the solvent δ _s and the polymer δ _p are numerically similar: δ _s≅δ _p. Many theoretical models of the solvent-polymer pairing have been proposed to explain their intrinsic interactions. However, the theories are limited because of the large variety of solvents available and their different chemical properties. In some studies, the solubility parameters are often divided into three components, describing hydrogen bonding, polarity and dispersive behavior of solutes; these theories are not considered here.

By definition, the solubility parameters do not include any hypothesis with regard to the association, the polarity, the solvation and the hydrogen bonding between solvent and polymer. However, Yerrick and Beck [126] classified solvents according to their electrostatic interactions with solutes into three categories: those inducing weak interactions (aliphatic, aromatic, fluorocarbon, chlorinated solvents); moderate (dipole–dipole) interactions (esters, ketones, ethers, nitriles); and strong (hydrogen bonding) interactions (aliphatic alcohols). For instance, the swelling of PDMS networks in different solvents is shown in Fig. 5.29 [126]. It can be seen from this figure that while hydrocarbon and chlorinated solvents have the same solubility parameter, chlorinated solvents tend to increase swelling while the hydrocarbons do not. Ethers, esters and ketones show reduced swelling abilities. In these solvents, interactions are mainly due to permanent dipole moments that tend to increase the efficient molecular volume and consequently to decrease their swelling ability. Plotting the volume swell as a function of the Hildebrand parameter yields the solubility parameter of PDMS (δ _PDMS=7.5 cal^1/2 cm^−3/2), as the maximum in the resulting curve. This value is in the range of earlier data in the literature (7.3 to 7.7 cal^1/2 cm^−3/2) [160–162] and is consistent with weak interaction forces characteristic for this polymer.

Fig. 5.29

Volume swells of PDMS elastomer as a function of solvent solubility parameters, for solvents with weak (▲), strong (○), and moderate (★) electrostatic interactions