PAA

Linear relationships have been established on one hand between the Rf and pAa values of these azaaromatic bases (in the absence of steric hindrance of the ring nitrogen) and on the other hand, between the... 

The terms Po, Pa, Pt, Pat, Paa, and Pt,t, are adjustable parameters whose values are determined by using linear regression to fit the data to the equation. Such equations are empirical models of the response surface because they have no basis in a theoretical understanding of the relationship between the response and its factors. An empirical model may provide an excellent description of the response surface over a wide range of factor levels. It is more common, however, to find that an empirical model only applies to the range of factor levels for which data have been collected. 

Poly(acrylic acid) and Poly(methacrylic acid). Poly(acryHc acid) (8) (PAA) may be prepared by polymerization of the monomer with conventional free-radical initiators using the monomer either undiluted (36) (with cross-linker for superadsorber appHcations) or in aqueous solution. Photochemical polymerization (sensitized by benzoin) of methyl acrylate in ethanol solution at —78° C provides a syndiotactic form (37) that can be hydrolyzed to syndiotactic PAA. From academic studies, alkaline hydrolysis of the methyl ester requires a lower time than acid hydrolysis of the polymeric ester, and can lead to oxidative degradation of the polymer (38). Po1y(meth acrylic acid) (PMAA) (9) is prepared only by the direct polymerization of the acid monomer it is not readily obtained by the hydrolysis of methyl methacrylate. 

Aqueous Acrylamide, Forms 260-951-88, Analytical Method PAA 44, Chemical and Metals Department, The Dow Chemical Company, Midland, Mich., 1976. 

Polymer/Polymer Complexes. PVP complexes with other polymers capable of interacting by hydrogen-bonding, ion-dipole, or dispersion forces. For example mixing of PVP with poly(acryHc acid) (PAA) in aqueous solution results in immediate precipitation of an insoluble complex (113). Addition of base results in dismption of hydrogen bonding and dissolution (114—116). Complexes with a variety of poly-acids (117) and polyphenols (118) have been reported. The interest in compatibiHty on a molecular level, an interesting phenomenon rarely found to exist between dissimilar polymers, is favored by the abiHty of PVP to form polymer/polymer complexes. 

One of the simplest molecules found to inhibit the repHcation of DNA vimses in animals is phosphonoformic acid [4428-95-9] (PEA, 1) CH O P. Both PEA (as the trisodium salt CNa O P, foscamet [63585-09-1] audits homologue phosphono acetic acid [4408-78-0] (PAA, 2) C2H O P, were developed by Astra Pharmaceuticals (6) and show selective inhibition of DNA polymerase in various herpes vimses. 

Some P-adrenoceptor blockers have intrinsic sympathomimetic activity (ISA) or partial agonist activity (PAA). They activate P-adrenoceptors before blocking them. Theoretically, patients taking P-adrenoceptor blockers with ISA should not have cold extremities because the dmg produces minimal decreases in peripheral blood flow (smaller increases in resistance). In addition, these agents should produce minimal depression of heart rate and cardiac output, either at rest or during exercise (36). 

Leadley and Watts also investigated the interaction of polyacrylic acid (PAA) with oxidized metal substrates [26]. Through careful curve fitting of the C(ls) spectra, three specific types of interaction between PAA and the oxidized metal... 

Fig. 31. XPS spectrum of a freshly prepared PAA/Al oxide surface. Reproduced by permission of Chapman and Hall Ltd. from Ref. [38].

Fig. 4, Cross-sectional electron micrographs of (a) primed PAA interphase and (b) unprimed PA oxide. showing complete penetration of primer into the oxide pores [9].

Fig. 8. Surface behavior diagram showing hydration of the PAA surface. Hydration occurs in three stages 1, reversible adsorption of moisture II, hydration of the AHOr to AlOOH and 111, further hydration to Al(OH)s. The numbers represent hours of exposure to high humidity. Adapted from Refs. 138,40].

Fig. 9. Wedge test results of aluminum adherends with the following surface preparations FPL, PAA, and FPL followed by an NTMP treatment. Adapted from Ref. [42].

Another means of providing a hydration-resistant surfaee is its treatment with a hydration inhibitor [41]. Fig. 9 shows wedge tests results for a Forest Produet Laboratory (FPL) bond [43], an FPL bond pretreated with nitrilotrismethylenephos-phonie (NTMP) aeid [42,44,45], and a PAA bond. The monolayer eoverage of NTMP stabilizes the FPL surfaee against hydration and provides wedge test bond performanee similar to that of PAA-treated adherends. 

A more recent process, the P2 etch [60], which uses ferric sulfate as an oxidizer in place of sodium dichromate avoids the use of toxic chromates, but still provides a similar oxide surface morphology (Fig. 15) allowing a mechanically interlocked interface and strong bonding [9]. The P2 treatment has wide process parameter windows over a broad range of time-temperature-solution concentration conditions and mechanical testing confirms that P2-prepared surfaces are, at a minimum, equivalent to FPL-prepared specimens and only slightly inferior to PAA-prepared surfaces [61]. 

PAA. Phosphoric acid anodization (PAA) was developed by the Boeing Company in the late 1960s and early 1970s to improve the performance of bonded... 

Fig. 16. Top High-rcsolution stereo SEM micrograph of the PAA oxide on 2024 aluminum. Bottom Schematic drawing of the oxide structure. Diagram from Refs. [9,59].

The chemistry of the CAA oxide also differs from that of PAA. Although the upper portion is amorphous AI2O3, like that of PAA, indications are that the lower part is crystalline [68-70,73,80,81]. Also, in contrast to the phosphate incorporated in the PAA oxide, little chromate is incorporated in or on the CAA oxide [68,69,82]. In spite of this, hydration of the CAA oxide generally occurs at a slower rate than the PAA which may be due to the crystallinity or the greater thickness [83] but the evidence is not clear on this point. 

As a consequence of these factors, it is important to realize that success in developing good bonds to a CAA surface depends critically on the type of surface preparation used immediately prior to anodization. When this is taken into account and a FPL or PAA treatment precedes anodization, then the CAA process can yield excellent results. Moreover it has also been shown that the CAA oxide, overall, is less friable, i.e., less susceptible to damage, than PAA, or other thinner oxides [29,84]. 

Although the most popular of the processes described above (PAA and CAA) perform very well, there is a continuing need to develop processes that are less hazardous (no chromates, no strong acids or bases) or that are suitable for treatment of repair areas in the field. Two of these treatments are grit blasting coupled with silane coupling agents and sol-gel. 

Fig. 17. High-resolulion stereo SEM micrograph of CAA oxide (a) with FPL prelrealmenl (b) with PAA prelrealmenl (c) production CAA surface, (a) and (b) are from Ref. [9].

---