Investigating ionomer morphologies with STEM and SAXS: Toward rigorous processing -structure -property relationships

Due to their extraordinary chemical and physical properties, ionomers have found wide-ranging applications including chemically resistant thermoplastics, robust coatings, and selectively permeable ion-transport membranes. The unique properties of ionomers result directly from the self-assembly/organization of ionic functional groups and counterions into nanoscale aggregates which act as transient physical crosslinks. For more than a half century, significant effort has been devoted toward understanding these structurally complex multi-component polymers, however, a complete description of their processing-structure-property relationships remains elusive. Quantifying these relationships will provide an important step toward the rational design, synthesis, and preparation of superior ionomeric materials. In order to rigorously advance the study of ionomer morphology, we combine traditional small angle X-ray scattering (SAXS) approaches with cutting-edge real space imaging via scanning transmission electron microscopy (STEM). This technique has provides high resolution imaging capability in which the image contrast is generated by differences in local average atomic number. Our work has shown that these characterization methods can be used to obtain complementary morphological information regarding the size, shape, and spatial distribution of the nanoscale ionic aggregates that control the physical properties of ionomers. With this information, we evaluate the validity of prevalent structural/morphological models and systematically explore how the nanoscale morphology is affected by changes in polymer backbone structure, materials chemistry, and processing