PROGRAM | Materials Science & Engineering

By: Derek Bischoff Chair: Michael Mackay

ABSTRACT

Fused filament fabrication (FFF) is a type of additive manufacturing used to create polymeric objects by adding material in a layer-by-layer manner. The fabricated objects often suffer from inferior mechanical properties due to inherent weaknesses encountered at the interface between layers from incomplete polymer chain diffusion/reptation. This leads to a high degree of mechanical anisotropy, upwards of 60% in typical thermoplastics like polylactic acid (PLA). This value is typically determined by fabricating tensile testing specimens with varying raster angles that orient the interfaces either parallel or perpendicular to the direction of stress. While several attempts have been made to improve the interlaminar strength, the most successful approaches have employed covalent adaptable networks (CANs). CANs are crosslinked polymer networks that contain reversible covalent bonds that exchange when exposed to a stimulus like heat. CANs form covalent bonds between layers to form strong interfaces relying on the diffusion of short dangling chain ends across the interface instead of full-length thermoplastic chains. In this dissertation, a blend comprised of a highly crosslinked Diels-Alder (DA) thermoreversible CAN and polycaprolactone (PCL) is shown to reduce the anisotropy in 3D printed objects without any specialized hardware or post-print processing.

First, the rheology of the constituent materials and blend are established proving that they are amenable to the 3D printing process and to establish the time-dependent structural development of the material. High temperatures cause the DA adducts to dissociate, enabling material flow and extrusion from the nozzle. 3D printing the filament results in objects with a ~10% degree of anisotropy, and a toughness of ~50 MJ/m³. The same DA CAN is used to fabricate glass microsphere composites using up to 60 wt% of filler. The use of a maleimide surface-functionalized microsphere filler is shown to improve the interfacial adhesion using electron microscopy and the viscosity is lower than those at the same wt% using the unmodified filler, aiding in the processability of the composites.

Next, inverse vulcanization is used to synthesize high sulfur content (≥50wt%) chalcogenide hybrid inorganic/organic polymers (CHIPs) where elemental sulfur is polymerized with organic unsaturated monomers. Upon exposure to heat, the S-S bonds can dissociate and exchange with other chains making linear CHIPs “living” polymers and crosslinked CHIPs CANs. These materials find use primarily in infrared transparent optics and in this work are shown to impart improved flame-retardant properties in polystyrene. The rheological properties of a series of CHIPs are established for the first time with the surprising result that certain compositions follow the simple Maxwell model of viscoelasticity indicating that their terminal stress relaxation can be described by a single, dominant relaxation time. This is true of both linear and crosslinked CHIP architectures, indicating that the S-S bond exchange common to both systems is the driving force. For Maxwellian stress relaxation to occur, the system must have a sufficient glass transition temperature as to activate the S-S bond exchange while not being overly crosslinked leading to too many topological constraints. The flow behavior of the CHIPs makes them amenable to melt processing techniques like compression molding which is used to fabricate optical windows of varying thicknesses to compare the mid-wave infrared transparency of the materials with typical acrylic. The CHIP material retains transparency in windows up to 5 mm thick whereas only 1 mm is achieved by acrylic. Finally, the CHIPs are 3D printed using FFF for the first time by blending them with thermoplastics typically used in FFF including PLA and polystyrene. The blends are subject to a UL 94 flame test, revealing that adding 10wt% of poly(S₅₀–r-DIB₅₀) into polystyrene results in a V2 rating, indicating that the flame self-extinguishes after 30 seconds whereas polystyrene never does.

Back >