Direct-writing of fiber networks for 3D printing soft composites

Abstract

Fiber-reinforced soft composites (FrSCs) are an emerging class of composites that are made up of polymeric fibers with specific material properties and hierarchical length-scales, embedded within another soft-polymer matrix. The research presented in this thesis studies the fundamental manufacturing issues associated with direct-writing of fibrous networks for 3D printing soft composites. First, a novel 3D printing platform is presented for fabricating FrSC structures with characteristic length-scales in the tens of millimeters range. This platform combines the conventional inkjet-based printing of ultraviolet (UV) curable polymers with the deposition of either aligned or random fiber mats, in between each printed layer. The fiber mats used for 3D printing are generated using two complementary techniques, viz., (i) a far-field electrospinning process that produces rolls of fiber mats en mass, and (ii) a near-field electrospinning-based “direct-write” system that produces both highly aligned as well as random nylon fiber mat coupons. During the 3D printing process, a stamping operation is used to transfer the electrospun fibers onto the polymer layer being printed. The printing process has been proven to manufacture multi-material FrSCs having different 3D geometries. The dimen-sional accuracy of the parts is seen to be a function of the interaction between the different UV-curable polymer inks.

The process study efforts presented in this thesis are aimed at systematically studying the effect of key processing parameters viz., fiber mat alignment, area coverage, and surface energy of the fiber carrier substrate, on the tensile properties and failure mechanisms seen in 3D printed FrSCs. The fiber mats are characterized for their diameter distributions, effective area coverage, number density, and tensile properties. The surface energy of the fiber carrier substrate is found to be critical to the fiber transfer efficiency of the stamping operation used in the 3D printing process, with polytetrafluoroethylene (PTFE)-coated aluminum films being more effective due to their low surface energy. Tensile testing results show that the addition of the fibers improves the mechanical properties of the FrSCs, with the Young’s modulus and the ultimate breaking stress showing the most improvement. Depending on the extent of alignment and the fiber content present in the 3D printed composite, the elastic modulus values for the FrSCs show a 40%-260% improvement over that of the base UV-curable polymer. The compo-sites also show evidence of characteristic failure mechanisms seen in the domain of nanocomposite materials, viz., fiber-induced local plastic deformation (crazing), crack arrest and deflection, fiber strengthening, and fiber pull-out. The evidence of fiber pull-out also points to the formation of an interfacial polymer sheath around the fibers. The elastic modulus of this sheath is estimated to be an order of magnitude higher than the base polymer.

The current design of the fiber stamping stage of the FrSC 3D printer has a short-coming, in that, it yields low fiber transfer efficiency values (< 50%) for mats with effective area coverage values (defined as the percentage area of the aluminum carrier substrate that is covered by the fibers in a mat) less than 55 %. In order to increase the fiber transfer efficiencies encountered during the 3D printing of FrSCs, it is critical to first understand the mechanics of the fiber transfer process using a suitable stamping process model. To this end, the objective of the computational thrust presented in this thesis is to develop a cohesive zone-based finite element model that captures the compe-tition between the ‘fiber-aluminum carrier substrate’ adhesion and the ‘fiber-polymer matrix’ adhesion encountered during the stamping process. The cohesive zone model parameters are first calibrated using fiber peeling experiments involving the aluminum substrate and the polymer matrix. The predictions of the calibrated model are then validated using fiber transfer experiments. The model predictions are seen to be accurate only at the higher end of the experimentally measured values indicating that the model predictions are conservative. The validated model is used to perform parametric studies that provide design insights into improving the fiber transfer efficiency for a given fiber/substrate combination.