Adhesive and rheological properties of lightly crosslinked model acrylic networks

The viscoelastic and adhesive properties of a series of model, lightly crosslinked acrylic polymer networks have been investigated. The model networks were statistical copolymers of 2-ethyl-hexyl acrylate and acrylic acid or terpolymers of 2-ethyl-hexyl acrylate, acrylic acid, and stearyl acrylate synthesized in solution. All were lightly crosslinked after the polymerization was completed to obtain typical properties of pressure-sensitive adhesives. The bulk rheological properties of the networks were characterized by dynamical mechanical spectroscopy and in uniaxial extension. Their adhesive properties were tested with an instrumented probe tester fitted with a cylindrical steel probe. The presence of acrylic acid in the copolymer caused an increase in both elastic modulus and resistance to interfacial crack propagation characterized by the critical energy-release rate G(c) and the incorporation of stearyl acrylate caused a decrease in both modulus and G(c). In both cases, however, the modification of G(c) controlled the overall behavior. The analysis of the nonlinear elastic properties of the adhesives with the Mooney - Rivlin model provided new insights on the role played by the ratio between entanglements and crosslink points in controlling the formation and extension of the bridging fibrils observed upon debonding.