(417b) Monitoring Nanoconfined Inorganic-Polyepoxy-Inorganic Adhesive Interfacial Changes and Molecular Forces during Curing at Various Environmental Conditions
Polyepoxides are increasingly being used to replace heavier materials in structural roles, used as bonding agent between dissimilar substrates, or matrices for high performance composite materials, relevant for military and commercial applications. Physical (e.g., roughening) and chemical (e.g., coupling agents) surface treatments dictate, to great extent, the durability and effectiveness of these interfaces, together with environmental curing conditions (e.g., relative humidity). Therefore, it is of crucial interest to identify, in real time, any changes at the polyepoxy-inorganic substrate interfaces that can become the source of failure, and correlate this with molecular forces. In this work, we used a stoichiometric mixture of bisphenol A diglycidyl ether (BADGE) and amine-terminated polyoxypropylene glycol (D-230) as a polyepoxy model confined between smooth alumina substrates as an inorganic substrate model, in combination with a Surface Forces Apparatus (SFA) under different relative humidity environments. The SFA and alumina fabricated surfaces allowed us to conduct real time surface imaging and refractive index changes of the confined polyepoxy using fringes of equal chromatic order (FECO). Simultaneously, we monitored curing strains and curing stresses from prior-to gel-point conversion to fully-cured over 48 hrs. Results support the following: 1) low imposed tensile forces favor full cure of the polyepoxy at various relative humidity environments, and molecular forces dominate the system during curing; 2) at high imposed tensile forces, regardless of relative humidity, no full cure was ever observed unless the load (force) experience by the polyepoxy consisted of a compressive load; 3) upon exposure of the polyepoxy to high relative humidity, water diffusion takes place within seconds, increasing the degree of plasticization, preventing cure. Finally, to provide a full and detailed quantitative picture of the adhesion and failure mechanisms of the polyepoxy-inorganic substrate system studied, we complemented with destructive nano-tensile tests and failure analysis imaging, revealing mainly cohesive (polymer-polymer) rather than adhesive (polymer-substrate) failure modes. Overall, we present a new interface quality measurement method to investigate polyepoxy-inorganic substrate systems expected to be of interest to a wide range of technological applications.