USING SELF-ASSEMBLED MONOLAYERS TO EXPLORE THE RELATIONSHIPS BETWEEN INTERFACIAL INTERACTIONS AND FRACTURE IN STRUCTURAL ADHESIVE JOINTS

摘要

A problem of longstanding interest in adhesion science is to understand the relationship between the nature and spatial distribution of fundamental interfacial interactions and engineering fracture quantities such as fracture stress or fracture toughness .These relationships depend strongly on the mechanical properties of the adhesive, and in this work we focus on highly crosslinked, glassy structural adhesives. These relationships are clearly important from the standpoint of designing interfacial chemistry sufficient to provide the level of mechanical strength required for a particular application. However, as has been recognized for some time, a relatively low level of interface strength is sufficient to produce strong joints, and in general joint strength is not limited by interfacial chemistry. On the other hand, it is very important to be able to identify the relatively rare cases when interfacial interactions may be a factor. Interfacial interactions can be important even for strong joints, as loading conditions may force cracks to initiate or propagate along a bimaterial interface. Traction separation laws are being investigated as a method for describing interfacial separation in such cases. The predictions resulting form this approach depend strongly upon the local stress that can be supported by the interfacial interactions, along with other factors. Finally, the in-plane distribution of interfacial interactions is important from the standpoint of stress concentration. Regions of an interface lacking strong specific interactions will not carry load, resulting in a concentration of load over the remaining area. Important questions remain to be answered regarding the conditions for flaw sensitive fracture behavior, such as how the magnitude of stress concentration and crack initiation are related to the in-plane distribution of interactions, the mechanical properties of the adhesive, and the mode of loading. To address these questions, we have been employing a model experimental system involving an epoxy adhesive on smooth substrates coated with self-assembling monolayers. Several methods are used to vary the nature and in-plane distribution of interfacial interactions. Lithography is used to deposit self-assembling monolayers (SAMs) of octadecyltrichlorosilane (ODTS) into domains of various sizes. Mixed monolayers of methyl terminated molecules and molecules terminated with various functional groups are used to vary the number density of strongly interacting sites as well as the nature of the strong interaction (covalent, ionic, hydrogen bonding, and polar) with an epoxy. In this way a range of interface strength is achieved that covers most, if not all, engineering systems and specific details can be examined quantitatively. A wide range of fracture tests are employed to study both fracture stress and fracture toughness. Below we report results from one of these studies. We deposited mixed monolayers of dodecytrichlorosilane (DDTS) and 1-bromo-11-undecyltrichlorosilane (BrUTS) at variable composition onto silicon wafers and quartz rods. The epoxy interacts with the bromine on BrUTS through specific ionic interactions, but interacts through only weak van der Waals (VDW) interactions with the methylated tails of the DDTS molecules. Fracture studies are reported below using the napkin-ring (NR, pure shear) and cruciform (CR, predominantly tensile) geometries. We then use the temperature dependence of the fracture stress to map the interface strength of several engineering systems onto the model system.