ABSTRACT
In their early findings of the role of acid-base interactions in adhesion science, Fowkes et a!. (6) demonstrated that the amount of PMMA (electron donor or Lewis base) adsorbed on silica (electron acceptor or Lewis acid) is much greater than that of adsorbed PVC (Lewis acid). When the substrate is CaC03 (Lewis base), PVC adsorbs with a greater amount than PMMA. Acid-base interactions are of great benefit for adsorption and mixing of polymers for they are exothermic and may overcome the positive or the negligibly small negative entropic term -T~S, respectively. On the contrary acid-acid and base-base interactions do not improve adhesion for they are of the van der Waals type only (7-9). This has quantitatively been proved with simple molecular (10) or complex systems (8,9) and the old general wisdom stating that polar materials yield a non-zero value of the polar component of the reversible thermodynamic work of adhesion is ruled out (see section III.A.l.). Despite the advances achieved in adhesion science many scientists still consider acid-base interactions in terms of dipolar interactions. It has actually been demonstrated for organic compounds in 1960 by Pimentel and McClellan in their book "The Hydrogen Bond", that the heat of H bonding adduct formation between two distinct molecules is related to the acid
strength of the proton donor (or electron acceptor) and to the base strength of the proton acceptor (or electron donor) and completely unrelated to their dipolar moments (II). Fowkes' work is actually an extension to the findings of Drago (12), Pearson (13) and Bolger (14) but with application to complex formulated materials such as polymers and inorganic solid surfaces. If the dispersive properties of polymers are well established and tabulated, unfortunately this is not the case for acid-base properties and it is the concern of our polymer and materials science community to determine acid-base parameters for polymers. These may be Drago's E and C constants, Gutmann's donor and acceptor numbers (DN and AN), "(andy, acid and base contributions to the surface energy y, pKa etc. Although they are seldom encountered in the literature such parameters may however be determined since several techniques have been tested for this purpose and methods are now established by several groups. The arsenal of techniques available to the polymer scientist and engineer to quantitatively assess acid-base properties of polymers (fillers and inorganic substrates) includes microcalorimetry (15-17), microgravimetry (I 0), inverse gas chromatography (IGC) (18), contact angle measurements (5,8, 19,20), infrared spectroscopy (10, 21), nuclear magnetic resonance (NMR) (22-25), and X-ray photoelectron spectroscopy (XPS) (26-36).
