Multiscale model development for composites such as plastic-bonded explosives (PBXs) is critical for assessing structural integrity and behavior and for developing new materials. For models to be successful they must reproduce bulk material responses while incorporating important microstructural features. These features arise at the mesoscale during processing, and while they have commensurate effects on composite material response they are not easily identified or characterized. Here, we study Composition A-3 as a representative PBX, chosen both for relevance to the field and as a tractable formulation to model. Composition A-3 is formulated by mixing micron-scale explosive crystals with an emulsified polyethylene solution, then breaking the emulsion to form small crystal/polyethylene agglomerates suitable for pressing and machining. Key aspects of this formulation process that may potentially affect the microstructure were identified. Specifically, the emulsification chemicals in a model system were found to partially dissolve or degrade the explosive and create a diffuse interface between the crystal and binder. The diffuse interfaces, along with some of the chemicals that remain in the composite after manufacture, create a heterogeneous multicomponent system that likely influences adhesion, void formation, and crack formation. The observed interfaces may be difficult to model. These results are compared with previous interfacial studies in other PBX materials, and the necessity of including such data in mesoscale models is discussed.