Interfaces and Dynamics in Polymeric 3D Printing and Crystalline Polymer Blends

This dissertation presents experimental work that provide a foundation to rationally improve fused filament fabrication (FFF) and immiscible blend compatibilization. Objects generated from additive manufacturing processes, such as FFF, have intrinsic structural weaknesses which include two project specific examples: structural anisotropy and irreversible thermal strain. Due to low adhesion between individual print layers that results in macroscopic defects, the mechanical strength of printed objects when force is applied perpendicular to the build orientation is drastically reduced. In the first dissertation chapter, we present a protocol to produce interlayer covalent bonds by depositing multi-amine additives between individual layers of a print to strengthen interlayer adhesion during the FFF printing process. Upon deposition, the amines then react with the deposited filament via functional groups produced by oxygenation to form the covalent crosslinks. Results demonstrate that the majority of amine crosslink reactions occur shortly after filament deposition from the residual thermal energy of extrusion, and that the amine reactivity is not the prevailing factor in determining the direct changes to print strength and interlayer fracture energy. The focus of our next project is to evaluate the effects of nanoscale additives on irreversible thermal strain (ε) build-up within 3D printed poly (lactic acid) (PLA) monoliths by measuring the change in this irreversible thermal strain with annealing. Once the contributions of internal void spaces were removed, the results of the modified strain value, ε_z*, indicates that incorporation of low molecular weight 3-arm star PLA chains into the print filament increases print ε while pure PLA or graphene modified filaments have similar ε. By correlating the strain measurements with results of a filament calorimetry experiment, it was shown that print irreversible thermal strain is influenced directly by how an additive changes the viscosity and molecular dynamics of the deposited polymer chains. In the third project, we examine the use of telechelic oligomers to optimize the physical properties of crystalline polymer blends via reactive compatibilization. The results demonstrate that the blend properties are influenced by three molecular level processes that are governed by the loading of oligomers used for compatibilization. Ultimately, the detrimental effects of plasticization from excess oligomer in the blend can be avoided and optimized alongside the synergistic relationship of blend crystallinity and reactive compatibilization by using a targeted concentration of telechelic additive.