The detailed structure of defects in soft materials like polymers has received relatively little attention compared to the vast literature on defects in hard crystalline materials (metals, ceramics and semiconductors), where high-resolution electron microscopy and other techniques have quantitatively imaged the new arrangements of the atoms caused by the various kinds of defects, thus enabling a very detailed understanding of the key role of different kinds of defects on controlling important material properties such as mechanical strength and electrical conductivity. This research will explore the use of new, highly advanced electron microscopic and tomographic techniques in an attempt to provide valuable data for comparison to theoretical simulations of the morphology of microdomain interface morphology in important classes of polymeric materials called A-B diblock copolymers. For these materials the interplay of minimization of interfacial area between the A and B domains versus the stretching of the polymer molecules within the respective domains controls the shape of the interface. Additionally, the PI will extend and verify advanced microscopy techniques to explore the 3D nature of grain boundary and line defects that can frequently occur in technologically important diblock polymeric materials and can limit their performance. This research will also contribute to the training of postdoctoral scholars and graduate and undergraduate students in advanced electron microscopic and tomographic techniques. Results from this project will also be incorporated into relevant graduate-level courses.
This research seeks to image the 3D shape of the microdomain interface in network phases and around grain boundary and dislocation defects in block copolymers (BCPs). In noncrystalline BCPs, the order is not in the atoms but in the interfaces separating the component blocks (the so called Intermaterial Dividing Surface, IMDS). Previous limitations to 3D reconstructions will be overcome via incoherent imaging using high-angle annular dark-field scanning transmission electron microscopy (HAADF STEM). Computational work will define the limitations of the reconstruction techniques by simulations using ideal morphological models combined with the known technique limitations (missing data wedge, strong oscillations in the contrast transfer function for bright-field imaging, defocus across tilted specimens). 3D imaging of the morphology is crucial but very challenging -- the inherent lack of contrast and the susceptibility of organic materials to electron beam damage are formidable obstacles. For some morphologies, reliable reconstructions may not be realizable due to experimental limitations. The few literature studies featuring 3D reconstructions of BCPs display some seemingly unrealistic features of the microdomain interface. Should the approaches turn out to be unable to overcome such experimental limitations, the focus will shift to setting limits on "interpretable resolution" and cautioning that the reconstructions in the published literature to date may have been "overinterpreted". No matter what the outcome, these results will be important for the polymer materials community to view and to understand. For ordered materials, the presence of defects can contribute positively or negatively to material performance. With such fundamental understanding, it should be possible to eventually learn to manipulate the defect types and their numbers in BCP systems and accordingly influence transport properties applicable to fuel cells, battery membranes, etc., where the light weight, flexibility, and low cost of polymers makes them highly attractive.
