STUDY OF THE ADHESION OF CERTAIN PHOTOSENSITIVE POLYMERS TO METALS

V. D. Filimonov, E. E. Sirotkina

(Presented by the scientific-methodological seminar of the Chemical-Technological Faculty)

At present, polymers possessing semiconductor properties are attracting ever closer attention from scientists and engineers. Among the many possible applications of such polymers, their use in electrophotography as photosensitive coatings is important.

A necessary condition for obtaining highly sensitive photoelectric layers is their high adhesion to the metallic substrates onto which polymer films are applied. The present work was undertaken in order to study the adhesive capacity of certain polymers.

Since the polymers we selected gave a brittle, weak film, it was impossible to determine adhesion by peeling the polymer film from the metal, as is usually done. In this connection, we measured adhesion during peeling of a thin metallic foil from the polymer layer. The measurements were carried out using Deryagin’s angular adhesiometer [3]. The work of adhesion of the polymer to the metal was evaluated in erg/cm² by the formula

\[ A=\frac{P}{b}(1-\cos\alpha), \tag{1} \]

where

\(P\) — weight of the load peeling off the foil;
\(b\) — width of the foil strip;
\(\alpha\) — angle of deflection of the rotating plate of the adhesiometer.

Experimental Part

The adhesion to brass and aluminum of the following photosensitive polymers was determined:

1. Poly-9-vinylcarbazole (PVK), an oligomer obtained by the transvinylation reaction of vinyl acetate with carbazole [1], molecular weight 500–1000, softening temperature 110–120°C; the molecular weights of all the oligomers studied were determined by the cryoscopic method [12].

2. Poly-9-vinylcarbazole, obtained by suspension polymerization of the monomer in the presence of benzoyl peroxide, molecular weight = 20,000, softening temperature 250–260°C; the molecular weights of polymers obtained by polymerization of monomers were determined viscosimetrically in benzene solution [12].

Table 1

Adhesion to brass and aluminum of a series of polymers

1. Poly-9-vinylcarbazole, telomer, obtained by telomerization of 9-vinylcarbazole in carbon tetrachloride in the presence of benzoyl peroxide, molecular weight = 11,500, softening temperature 190–200°C.

2. Poly-9-vinyl-3-chlorocarbazole (3 Cl PVK), oligomer, obtained by the transvinylation reaction of vinyl acetate with 3-chlorocarbazole [1], molecular weight = 500–1000, softening temperature 120–130°C.

3. Poly-9-vinyl-3-chlorocarbazole, obtained by polymerization of the monomer in solution in the presence of benzoyl peroxide, molecular weight = 15,000–20,000, softening temperature 280°C.

4. Polyvinyldiphenylamine (PVDFА), obtained by transvinylation of vinyl acetate with diphenylamine [2], molecular weight = 1500–2000, softening temperature 80–90°C.

5. Polymers obtained as a result of the transvinylation reaction with vinyl acetate of mixtures of diphenylamine and carbazole in various ratios, molecular weight 500–1000, softening temperature 85–100°C.

The thickness of the brass foil was 0.04 mm, and that of the aluminum foil, 0.05 mm. The foil was obtained by etching thick brass foil 0.2 mm thick with dilute nitric acid, and thick aluminum foil with a concentrated solution of caustic potash to the required thickness. The width of the brass bonds was 4 cm, and that of the aluminum bonds, 3 cm.

The polymer was applied to the working sides of the foil strips by pouring from various solutions of specified concentration. Immediately before application of the polymer, the foil surface was degreased with acetone. After application of the solution, the bonds were dried to constant weight at temperatures on the order of the melting temperatures of the polymers. The bonds were then folded with the treated sides together and, under the pressure of a 500 g load, were subjected to heat treatment at a temperature somewhat above the melting temperatures of the corresponding polymers. After completion of the heat treatment, the bonds were cooled for several hours together with the thermostat.

The prepared bond was fastened with special clamps to the rotating plate of the adhesiometer; a load $P$ was suspended from the thin foil. The weight of the load in the various experiments, depending on the magnitude of the work of adhesion, ranged from 50 to 500 g. The adhesiometer plate was rotated at a rate of 1° per second until the foil began to delaminate; this value of the angle was then substituted into equation (1).

The values of the work of adhesion given in Table (1) are averages of 6–12 measurements of adhesion for identical bonds.

Because instruments permitting accurate measurement of the thickness of thin films were lacking, the latter was determined from the amount of polymer, in grams, deposited on 1 cm$^2$ of foil surface.

Discussion of Results

The dependence between the magnitude of adhesion of the polyvinylcarbazole oligomer to brass and aluminum and the temperature of heat treatment of the bonded joints was studied. It was shown that the maximum adhesion value is reached at a temperature of 120–140°C, and that with a further increase in temperature this value does not change. It was also found that the time required to establish maximum adhesion of the PVC oligomer to brass at a heat-treatment temperature of 140°C is 1.5 h. This time apparently corresponds to the disappearance of the interface between two layers of the polymer and also, probably, to the establishment of adsorption–desorption equilibrium of sections of the polymer macromolecules on the brass surface, i.e., to achievement of the maximum possible degree of adsorption. Figure 1 shows the dependence of the adhesion of the polyvinylcarbazole oligomer to brass on the film thickness.

Fig. 1. Dependence of adhesion of PVC oligomer to brass on the thickness of the polymer film.

It is seen that adhesion increases with decreasing film thickness up to a maximum value at a film thickness of 0.03 g/cm². Such a dependence of adhesion on film thickness is a widespread and theoretically substantiated phenomenon in adhesive-bonding technology [4, 5]. With a further decrease in film thickness, the adhesion forces begin to exceed the cohesive forces in the film, and separation then proceeds within the film itself, which breaks in the process, as a result of which nonreproducible results are obtained.

We found that the adhesion of films of the polymers studied is practically independent of the type of solvent from which these films were obtained. Benzene, dichloroethane, and acetone were used. This seems somewhat strange, since it has been shown [6] that adsorption of polymer macromolecules from solution strongly depends on the degree of structuring of the polymers in solution, i.e., the degree of ordering of macromolecules in the surface layer on the substrate surface and the density of their packing ultimately depend on the type of solvent, and the magnitude of polymer adhesion depends on these same factors. Our data probably indicate that the individual structure of the surface layers of the polymers studied, which depends on the type of solvent, disappears upon melting of the polymer during heat treatment and drying.

It is seen from Table 1 that the adhesion of PVDFА is almost 20 times greater than the adhesion of the other polymers. This can be explained by the much greater mobility of the segments of PVDFА macromolecules; since the degree of orientation of molecules in the boundary layer depends both on the adsorption field of the substrate and on the mobility of the macromolecule segments, the higher the degree of orientation, the greater the number of macromolecule–substrate contacts and, consequently, the stronger the adhesive bond between

polymer and the substrate [7]. Segments of polyvinylcarbazole macromolecules, however, are practically incapable of rotation because of conjugation through the carbazole nuclei; i.e., its macromolecule is extremely rigid and inelastic, which is confirmed by the values of the elastic moduli, comparatively high for thermoplastic polymers [8]. In addition, as Rutler [9] has shown, some polymers that contain nitrogen atoms in their macromolecules can, through their unshared electron pairs, form coordination bonds with the metal atoms of the substrate, which increases the overall value of adhesion. In PVPhA the formation of such bonds is possible, for example, with aluminum, which is precluded for PVK macromolecules because of steric hindrance.

$$ \begin{gathered} -\mathrm{Al}^{+}\;-\;\mathrm{Al}^{+}-\\ \ \ \ \backslash \mathrm{O}^{-}\qquad \mathrm{O}^{-}\\ \ \ \ \uparrow\\ \mathrm{H}_{5}\mathrm{C}_{6}-\mathrm{N}-\mathrm{C}_{6}\mathrm{H}_{5}\\ \ \ \ \mid\\ (-\mathrm{CH}-\mathrm{CH}_{2}-)_{n} \end{gathered} $$

Furthermore, as our data have shown [8], the values of internal stresses in PVK films are many times greater than the values of internal stresses in PVPhA films, and internal stresses in coatings greatly weaken the adhesion of the coating to the substrate.

Of interest is the sharp drop in adhesion in products obtained by the perevinylation of vinyl acetate with a mixture of carbazole and diphenylamine, whose adhesion approaches the values of the work of adhesion of pure PVK already at a carbazole content of 5.23% in the mixture. The corresponding values of the elastic moduli [8] of these products did not change substantially in comparison with the elastic moduli of pure PVPhA. The adhesion of artificial mixtures of PVPhA and PVK oligomers is close to the adhesion of the products of the joint perevinylation of carbazole and diphenylamine and is practically equal to the adhesion of pure PVK. On the basis of these data it may be assumed that, in the reaction of perevinylation of mixtures of diphenylamine and carbazole with vinyl acetate, predominantly not copolymers are formed, but a mixture of PVPhA and PVK. It may further be assumed that, from a solution of mixtures of these polymers, PVK macromolecules are adsorbed more rapidly on the metal surface, forming a surface layer that determines the adhesive bond of the polymer film as a whole with the metal.

It is interesting to compare data on the adhesion of identical polymers, but obtained by polymerization of the monomer and by the perevinylation reaction with vinyl acetate. It has been shown [10] that carbonyl groups are present in molecules of the PVK oligomer; consequently, they should be more polar than ordinary PVK polymers. From the literature data [4, 5] it is known that the adhesion of polymers, as a rule, is the higher the greater their polarity. However, the data of Table 1 show the opposite; moreover, it was found [8] that the internal stresses in polymer films are significantly higher than in oligomer films; hence, the ratio of the true intermolecular forces polymer—metal to the intermolecular forces oligomer—metal is even greater than is shown by the works of adhesion determined by the peel method. In our view, one of the reasons for such a paradoxical phenomenon, when the adhesion of a polar polymer to a metal is lower than the adhesion of a comparatively nonpolar one, is that oligomers obtained by the perevinylation method always contain acetic acid, which during drying and heat treatment reacts with the oxide film of the metal surface, dissolving it. As a result, a loose, weak layer with poor ad-

... adhesion to metals, the acetate layer, which peels off together with the polymer and which, in fact, determines the adhesion of the entire polymer coating.

It is evident from Table 1 that the adhesion of the overwhelming majority of the polymers studied is noticeably higher to brass than to aluminum. However, in both cases it remains low.

On the basis of current views on the nature of the adhesion of polymers to metals, it may be assumed that polymers possessing semiconductor properties probably cannot exhibit good adhesion (except in cases of chemical interaction). According to the adsorption theory [5], adhesion will be low because of the low flexibility of the macromolecules, which is apparently a characteristic feature of polymers possessing semiconductor properties, owing to the presence of conjugation along the macromolecular chain. The influence of the flexibility of polymer chains on adhesion was considered above.

According to the electrical theory [3], adhesion is determined by the magnitude of the potential of the electric double layer formed at the polymer–metal interface as a result of the transfer of electrons from the metal into the polymer. The higher the density of the “electron gas” in the polymer, the higher the energy barrier at the interface for electron transfer, the lower the potential of the electric double layer and, consequently, the weaker the adhesion of the polymer to the metal. In semiconducting polymers, the density of the “electron gas” is comparatively high [11]. Consequently, good adhesion to metals cannot be expected for such polymers either.

Conclusions

1. A method has been proposed that makes it possible to determine the work of adhesion of brittle polymer films to metals.

2. The work of adhesion of several polymer films to brass and aluminum has been measured.

3. The influence on the magnitude of adhesion of several factors has been shown: the thickness of the polymer film and the duration of heat treatment.

Literature

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3. B. V. Deryagin, N. A. Krotova. Adhesion. Publishing House of the USSR Academy of Sciences, 1949.

4. S. S. Voyutskii. Autohesion and Adhesion of High Polymers. Rostekhizdat, 1960.

5. Adhesion. Glues, Cements, Solders, edited by N. Debroin and R. Guvink. Inostrannaya Literatura Publishing House, 1954.

6. Yu. S. Lipatov. Adsorption of polymers from solutions and its relation to the adhesion of polymers to surfaces. In: Adhesion of Polymers. Publishing House of the USSR Academy of Sciences, 1963.

7. H. Alter, W. Soller. Ind. Eng. Chem., 50, 923, 1958.

8. V. D. Filimonov, E. E. Sirotkina. Investigation of thermal internal stresses in certain polymer coatings. (The present collection).

9. Rutzler, Adh. Age., 2, No. 6, 1959.

10. G. N. Ivanov. Dissertation, Tomsk, 1969.

11. Organic Semiconductors. Collection of articles edited by V. A. Kargin, Moscow, Nauka Publishing House, 1968.

12. K. Uberreiter, Z. Springer, phys. chem. Neue Folge, 36, 299–319 (1963).