Determining the interphase thickness and properties in polymer matrix composites using phase imaging atomic force microscopy and nanoindentation

In polymer matrix composites, the interface between the reinforcing phase a nd the bulk phase is paramount to the overall performance of the composite as a structural material. This interface is now thought to be a distinct, t hree-dimensional phase surrounding the reinforcing phase called the interph ase. The developments of the atomic force microscope and nanoindentation de vices have facilitated the investigation of the interphase. Previously, for ce modulation atomic force microscopy (AFM) and nanoindentation were the pr imary methods used to determine the size of the interphase and its stiffnes s relative to the bulk phase. The present investigation utilized phase imag ing AFM and nanoindentation to examine the interphase in a glass fiber-rein forced epoxy matrix composite. Nanoindentation experiments indicated that t he relatively stiff fiber might have caused a gradient in the modulus acros s the interphase region. Specifically, the modulus next to the fiber approa ched that of the fiber and decreased to that of the bulk polymer as the dis tance away from the fiber increased. Once the fiber was removed by chemical etching, this gradient reversed itself; hence, nanoindentation, due to the fiber bias, was not found to be adequate for measuring actual interphase p roperties. It was found that phase imaging AFM was a highly useful tool for probing the interphase, because it involves much lower interaction forces between the probe and the sample than force modulation or nanoindentation. The interphase in the model composite investigated was found to be softer t han the bulk phase with a thickness of 2.4-2.9 mum, and was independent of fiber silane pretreatment, for silane pretreatments between 0.1% and 5.0% ( initial aqueous concentration).