Model adhesive studies using block copolymers
The diversity of adhesive/adherend pairs has created innumerable unique combinations of thermodynamic, kinetic, chemical, physical and rheological contributions with which to explain adhesion processes. So complex are the interrelationships of these contributions to the overall composite system that little information can be extracted about the role of the adhesive properties themselves. This study attempts to simplify the number of variables influencing the strength of adhesive joints by employing a series of styrene-isoprene-styrene linear triblock copolymers as model hot-melt adhesives for titanium 6-4 alloy substrates. The block copolymer samples have narrow molecular weight distributions and styrene contents ranging from 20 to 60% by weight. For these systems a finite number of "dominant" variables are defined including 1) adherend wettabf lity, 2) mechanical properties, 3) rheology, 4) adhesive and adherend contamination, and 5) temperature-pressure cycles for joint formation. Characterization of the block copolymer series in each of these areas is presented. Copolymer morphology emerges as an important variable affecting the material and adhesive properties of these systems.
The adhesive joints prepared from these samples are simple lap shear specimens. Comparative joint strengths and joint fracture energies of the series of copolymer adhesives are rationalized in terms of styrene content, styrene domain connectivity and domain orientation. Scanning electron microscopy is employed to examine the surface characteristics of the fractured adhesives. Distinctive failure features are associated with the styrene content and dissipative capacities of the adhesives.