Block Copolymers as Adhesives

Abstract

Block copolymers form a new class of molecular composite materials by the phase separation of incompatible hard and soft segments which form their macro-molecular structure. Thermoplastic elastomers where the soft segments form the continuous phase have been extensively investigated by means of an adsorption-interdiffusion (A-I) model for the interfacial phase which bonds the hard and soft phases. The molecular structure and rheological activity of the interfacial phase in thermoplastic elastomer block copolymers is shown to play a dominant role in nonlinear viscoelastic response, mechanical hysteresis and energy absorption. Creation of elastomeric microphases in epoxy structural adhesives has been recently identified with in situ block copolymerization between carboxy terminated nitrile (CTBN) rubber and the diepoxide. In this case, the soft phase is discontinuous and acts as a toughening agent for the hard epoxy continuum. The A-I model is shown applicable to describing the rheology and fracture properties of the CTBN modified epoxy adhesives. The specific advantages of developing block copolymers as elastomeric or structural adhesives is discussed in terms of molecular design factors such as composition, block structure and molecular weight. Adhesion properties unique to block copolymer adhesives are discussed and summarized.