Improved Adhesion of Elastomeric Silicone Adhesives through Surface-Grafted Connector Molecules

Surface-Grafted Connector Molecules Interpenetrating into the Adjacent Adhesive

The theoretical principles of adhesion enhancement through surface-grafted connector molecules, interpenetrating the cross-linked network of an adjacent

0

CH3 / СНз СНз ОССНз о

I I I I II

HO—Si —o —Si—Ol-Si – OH+ ZCH3— Si — О— ССНз I I I I

CH3 СНз CH3 ОССНз

FIG. 2—First-stage reaction of hydroxy-terminated PDMS with the acetoxy cross-linker [1].

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TABLE 2—Typical functional groups of cross-linkers used in silicone adhesives [3].

Acetate4

Amines4

Benzamide4

Order of commercial usage Substituent Formula with functional group

Oximes4

Alkoxides4

Octoate4

NH-

Amine =Si—OH + (CH3)2N—Si=——————————– ► = Si—О—Si = + (CbbfeNH

FIG. 3—Schematic representation of the reaction of typical cross-linkers in one – component moisture curing silicone adhesives [1].

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adhesive, were developed by de Gennes [5-7] and his co-workers [8-12] (see Fig. 4 for schematic illustration of the mechanism of interactions between surface-grafted macromolecular chains and an elastomeric silicone adhesive).

According to de Gennes’ theory, in the simplest case of interface reinforce­ment involving chain pull-out in the presence of van der Waals interactions in the macromolecular chain/elastomer system (where connector chains and adhe­sive polymer are identical), the fracture energy associated with deformation and extraction of the connector chains is

G = 2y(1 + aN) (1)

where c is the surface energy of the polymer and that of connector chains and a is the surface density of connector chains.

It has been subsequently demonstrated that the normalized increase in the fracture energy of the interface between a solid substrate “tethered" with surface-grafted connector molecules and the PDMS elastomeric adhesive is as follows [9]:

G – W ffi cNr(1 – a2/3N!=3) (2)

where G is the fracture energy of the interface reinforced with surface-grafted connector molecules, W is the energy of adhesion (W = 2y) between the chemi­cally identical PDMS adhesive and PDMS connector molecules, N is the degree

FIG. 4—Schematics of the mechanism of interactions between the “tethered substrate surface” (a substrate with surface-grafted macromolecular chains) and an elastomeric silicone adhesive (adopted with changes from [7]).

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of polymerization (N = MW/m) of connector chain molecules, Nc is the degree of polymerization of cross-links of the PDMS adhesive.

The following can be seen from Eq 2:

• For r>N—1/2, the connector molecules completely separate from the PDMS network and G reduces to W.

• The optimum adhesion occurs for:

rOPT ffi 0.465 N—1/2 (3)

• The corresponding value of the optimum fracture energy, GOPT, is:

N

Gopt — W = 0.186c —2 (4)

N1/2

Figure 5(a) illustrates the results for Eq 2 solved numerically for N = 2300 and Nc = 230, and c = 21.6 mJ/m2 at 25°C [11]. These results indicate that there is a distinct optimum (rOPT) in the surface density (r) of the connector molecules which has to be achieved to maximize the adhesion between a surface-modified substrate and the adhering polymeric material, i. e., adhesive.

The theoretical model described by Eq 2 has been verified experimentally.

An example of this is illustrated in Fig. 5(b) [10]. It shows the normalized fracture energy (G – W)/W) for the interphase comprising molecular brushes interacting with the elastomeric silicone adhesive through van der Waals forces as a function of surface density of molecular chains, r, for a cross-linked PDMS elastomer (Nc = 230) in contact with a silicon wafer grafted with PDMS connec­tor chains exhibiting N = 2300. W is the thermodynamic work of adhesion and W = 2c, where c is the surface energy of PDMS (c = 21.6 mJ/m2 at 25°C).