Examining chemical structure at the interface between a polymer binder and a pharmaceutical crystal with neutron reflectometry

The mechanical properties of many composites are determined in part by the chemical structure and bonding at the interface between constituents in the microstructure. The study of these interfaces in molecular crystal - polymer composites is difficult using traditional techniques such as electron microscopy or X-ray scattering because of weak or detrimental interactions between the probe and materials. Here, the interface between acetaminophen and a poly(ester urethane) copolymer is analyzed using ellipsometry, infrared spectroscopy, and neutron reflectometry. These materials were chosen for their relevance to pharmaceutical tablets and plastic-bonded explosives. The acetaminophen was shown to dissolve into the polymer coating and creates an interphase region between the two materials; this mixing is almost certainly produced by typical formulation conditions, and likely affects mechanical response of the composite. Additionally, reflectometry shows that plasticizing the polymer alters this interphase region. These techniques can be applied to similar composites to reveal the relation between formulation conditions, constituent interface microstructure characteristics, and bulk mechanical response.