To determine if full-strain measurements are sufficient in measuring interfacial phenomena in additively manufactured thermosets, we devised a testing method to examine the effect of interface history (i.e., time between interface formation and original material deposition) on localized mechanical properties. Interfaces were formed in diglycidyl ether of bisphenol A (DGEBA)/diethylenetriamine (DETA) epoxy resins with various DETA concentrations. Times to the gel point and full cure were assessed by rheology and Fourier transform infrared spectroscopy (FTIR) for each composition. From here, dogbone samples were fabricated with an interface formed at either the gel point or at full cure for each composition. Key findings show that tensile strength and Young’s modulus deteriorate globally with the presence of an interface regardless of the history or initiator (DETA) concentration. Samples with an interface demonstrate high strain regions near the interface prior to fracture. Micro X-ray computed tomography revealed high density regions at the interface that increased in number with both cure times and initiator concentrations. FTIR revealed that the interface demonstrated a higher cure completion than the sample interior, resulting in a stiffer epoxy localized at the interface versus the bulk. These findings were confirmed by atomic force microscopy modulus mapping at the interface. Finally, computational modeling of epoxy in uniaxial tension with an increasing number of stiff inclusions demonstrated that inclusion contents correlated with increased, localized stress concentrations. These findings will aid in the understanding of fracture phenomenon in additively manufactured thermosets and point to digital image correlation as a useful tool in epoxy interface detection.

Filaments composed of aluminium powder and poly(vinylidene fluoride) (PVDF) were produced by melt-processing to investigate the effect of particle size and loading on decomposition behavior and burn performance. Thermal analysis revealed that nanoscale Al samples decompose PVDF in one step through interactions with the Al particle surface. Microscale samples presented with two distinct decomposition steps: (1) accelerated decomposition through interactions with the Al particle surface and (2) pyrolysis. This behavior occurs due to the drastic change in Al specific surface area. The burn test revealed that the filaments experience a maximum flame speed near the stoichiometric concentration for each fuel size. Although there are variations in decomposition and burn behavior between particle sizes, burn product analysis shows that all melt-processed filaments result exclusively in AlF 3 formation in open-air burns. This behavior is unique to melt-processed energetic composites and may provide more insight to binder-particle interactions and the effect on burn properties in energetic composites.

The cover image is based on the Research Article Manipulating polymer decomposition to alter burn performance in aluminium/poly(vinylidene fluoride) filaments by Jared W. Strutton et al., https://doi.org/10.1002/pi.6129. image

This work studies the effect of interlayer adhesion on mechanical performance of fluorinated thermoplastics produced by fused deposition modeling (FDM). Here, we study the anisotropic mechanical response of 3D-printed binary blends of poly (vinylidene fluoride) (PVDF) and poly (methyl methacrylate) (PMMA) with the isotropic mechanical response of these blends fabricated via injection molding. Various PVDF/PMMA filament compositions were produced by twin-screw extrusion and, subsequently, injection-molded or 3D printed into dog-bone shapes. Specimen mechanical and thermal properties were evaluated by mode I tensile testing and differential scanning calorimetry, respectively. Results show that higher PMMA concentration not only improved the tensile strength and decreased ductility but reduced PVDF crystallization. As expected, injection-molded samples revealed better mechanical properties compared to 3D printed specimens. Interestingly, 3D printed blends with lower PMMA content demonstrated better diffusion (adhesion) across interfaces than those with a higher amount of PMMA. The present study provides new findings that may be used to tune mechanical response in 3D printed fluorinated thermoplastics, particularly for energy applications.