The Role of Diffusion Phenomena in Polymer-to-Polymer Adhesion
S. S. Voyutskii and V. L. Vakula
Lomonosov's Institute of Fine Chemical Technology, Moscow, U.S.S.R.
"Only high molecular (weight) substances with their long-chain molecules
have pronounced adhesive properties."
"Earlier authors attempted to relate adhesion to wetting and incorrectly
approached the bonding as a thermodynamically reversible phenonmenon."
"According to McClaren, the formation of adhesive bond can be divided
into two stages.
- The first stage consists of the migration of large polymer molecules
from the solution to the surface of the adherend as a result of
micro-Brownian motion. As a consequence polar groups of macromolecules
of the adhesive approach the polar groups of the adherend. Through
pressure and lowering of viscosity during heating, the chain segments
can approach the surface very closely even in the absence of solvent.
- The second stage of adhesion consists ofthe sorption process. When
distance between molecules of adhesive and adherend becomes less than 5
angstroms, intermolecular forces come into play. This includes the
whole "force spectrum" starting with dispersion forces (about 10^2
cal/mole) up to hydrogen bonding with energyies in the range of 10^4
cal/mole."
Deryagin and Krotova showed that "the peeling work of the adhesive film
can reach values as high as 10^4 to 10^6 ergs/cm, but the work required
to overcome molecular forces does not exceed 10^2 to 10^3 ergs/cm. In
other words, the real work of adhesion is sveral magnitudes higher than
that expected from molecular forces. The attempts at explaining this
by attributing it to the expenditure of work of film formation which
peeling failed.
It was established by Deryagin and Krotova that "adhesion depends upon
the peeling rate of the adhesive film while the work required to
overcome the molecular forces must not depend on the separation rate
of the molecules."
"It is known that the more similar the nature of high polymers, the
higher their adhesion."
"If the adhesive is applied in solution, and the adherend is capable of
swelling or dissolving in this solution, a remarkable diffusion of the
molecules of adherend into the adhesive may occur. Both processes to the
disappearance of the boundary between the phases and to the formation
of a bond presenting a gradual transition from one polymer to the
other. Hence, adhesion, according to the diffusion theory, essentially
presents a volume phenomenon."
"The diffusion of molecules of one polymer into another is nothing but
solubility.
."
"The penetration of the molecules of adhesive into the adherend to a negligible
depth (on the order of 10 angstroms) is sufficient to increase manifold
the adhesive strength. This is supported by Patrikeev's elementary
calculations."
"Neither adsorption nor electrical theory can provide the satisfactory
interpretation of the increasing adhesion strength with the time
represented in Figure 1, and only the diffusion theory easily explains
such a dependence by the slow penetration of the macromolecules of adhesive
into the adherend."
"Increasing the adhesion on heating of the bonded samples cannot be attributed
to an increase in true contact area or to the evaporation of remaining
solvent since the increase in adhesion is observed for the bonded samples
prepared from the copolymer solution (for which the area of real contact should
be maximum) as well as for those prepared by bringing into contact the
adherend and the film of adhesive without the solvent."
"The diffusion theory easily explains the reason for increased adhesion through
the increased thermal motion of macromolecules or their segments and through
the increase of their diffusive penetration into the adherend with the rise
in temperature."
"Apparently when the molecular weight is low and the size of the macromolecules
is commensurable with the segment length, the number of free ends (i.e.,
end segments) is relatively great, and consequently the probability of their
diffusion into an adherend is also great. In keeping with purely steric
reasons, the probability of the diffusion of end segments of macromolecules
into a bulk polymer is more than that of the middle segments. Therefore
under these conditions the adhesion will be determined almostly completely
by the diffusion of free ends of the macromolecules. As the molecular
weight increases, the relative proportion of end and middle segments changes
abbruptly in favor of the latter, thus decreasing adhesion. Finally, at
some definite value of molecular weight, the number of ends of
macromolecules becomes so small in comparison with the total number of segments
in the macromolecule that the participation of the former in diffusion
becomes imperceptible. Hence, at high molecular weights adhesion will
be determined mainly by the diffusion of middle segments, and insofar as
the size of the segment does not depend on the length of the
macromolecule, adhesion ceases to be dependent on the
molecular weight of the polymer."
"Since with decreasing molecular weight of polymeric adhesives their cohesive
strength fails, polymers of medium molecular weight must obviously have
optimum bonding ability; they are able to provide good adhesion and at the
same time a sufficiently high cohesive strength of adhesive."
"Polydispersity of the commercial polymers also explains why the adhesion
of such polymers is always higher than that of high molecular fractions
separated from them."
Last Update- September 21, 1995- wld