Polymer-polymer adhesion promotion by block-copolymer addition

Materials for new applications are often created by combining existing polymers into blends or composites. However most polymer pairs tend to be immiscible leading to weak adhesion and poor mechanical properties, which can outweigh any advantages gained by blending. Adhesion and mechanical properties can be improved in such systems by the addition of small amounts of suitable block copolymer to the interface. Adhesion is promoted, firstly by a reduction in the interfacial tension, leading directly to an increase in the thermodynamic work of adhesion, and secondly by increasing the interfacial width, leading to more entanglements between polymer chains and hence greater mechanical reinforcement. In some cases, adhesion is further enhanced by covalent bonds between the block copolymer and the homopolymers. As progress towards nanoscale technology continues in many fields, including biomaterials and electronics, a detailed knowledge of the fundamental origins of adhesion promotion is becoming ever more vital in the design of new materials. The purpose of the research presented in this thesis was to examine several important molecular aspects of polymer-polymer adhesion promotion by block copolymers involving rubber-rubber and glass-rubber interfaces. The origins of self-adhesion hysteresis in polydimethylsiloxane elastomers were investigated using the JKR contact mechanics technique. Hysteresis was only observed in extracted elastomers and in self-adhesion experiments, suggesting its origins arose from surface interactions and not bulk viscoelasticity. The hysteresis increased with decreasing cross-linking density of the elastomers. When the platinum catalyst used to synthesize the PDMS elastomers was poisoned the hysteresis disappeared implying that a hydrosilylation reaction across the interface was responsible. The JKR method was also used to investigate the contributions to adhesion promotion by chain pull-out and covalent bonding using polydimethylsiloxane and polybutadiene elastomers in contact with silane terminated poly(styrene-b-dimethylsiloxane), poly(styrene-b-butadiene) and various functional surfaces. No evidence of adhesion enhancement caused by chain interpenetration was observed over a wide range of experimental and molecular parameters. Adhesion was only enhanced when reactions across the interface were possible. Further experiments involving end-tethered PDMS brushes showed that significant adhesion could be obtained but that this could be explained by the occurrence of various chemical reactions.