POLYMER-SOLID INTERFACES - INFLUENCE OF STICKER GROUPS ON STRUCTURE AND STRENGTH

A model polymer-solid interface, aluminum-(Al-) carboxylated polybutad iene (cPBD), was designed to investigate the influence of the sticker group (-COOH) on the fracture energy (G(IC)). A model polymer, cPBD, w as synthesized through high-pressure carboxylation of polybutadiene (P BD) and contained -COOH randomly distributed along the length of the p olymer chains. T-peel tests were used to evaluate the interfacial frac ture energy. The effect of the concentration of the sticker groups (ph i) on the fracture energy was examined, and a critical concentration ( phi(c)), around 3 mol %, was found to give a maximum bonding strength, which was an order of magnitude stronger than the same interface with out sticker groups. The fracture energy of Al-cPBD-Al interfaces incre ased over a range of 10-1000 min annealing time, t, which is much long er than the characteristic relaxation time of PBD at room temperature. The fastest adhesion occurred for sticker group concentrations at phi (c), whereas chains with sticker groups at phi(c) +/- 1% required much longer surface rearrangement times. The dynamics of adhesion was foun d to be comparable to time-dependent surfaces restructuring, using dyn amic contact angle studies. Many of these results could be understood from a self-consistent lattice model developed by Theodorou, which we used to investigate how the sticker groups affect the structure of the interfacial chains. Sticker groups were found to have a strong tenden cy to segregate to the solid surface, resulting in a large concentrati on gradient near the solid surface. This phenomenon, together with the extremely slow surface restructuring process of cPBD chains, which re lax like tethered chains, partially accounts for the long time depende nce of the fracture energy of Al-cPBD-Al interfaces. Modeling also sho wed that the chain shape and the chain connectivity close to the solid surface was modified. With increasing concentration of sticker groups , the flatness of the chains near the solid substrate decreased at fir st and then increased, indicating an optimum concentration for efficie nt chain connectivity within the interfaces. These modeling results pr edicted a critical concentration of sticker groups for optimum bonding in the sense of cohesive strength, agreeing well with experimental re sults.