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Title:Sacrificial polymers for rapid manufacturing of bioinspired vascular materials
Author(s):Garg, Mayank
Director of Research:Sottos, Nancy R
Doctoral Committee Chair(s):Sottos, Nancy R
Doctoral Committee Member(s):Moore, Jeffrey S; Evans, Christopher M; Chen , Qian
Department / Program:Materials Science & Engineerng
Discipline:Materials Science & Engr
Degree Granting Institution:University of Illinois at Urbana-Champaign
frontal polymerization
Abstract:Synthetic vascular systems, inspired by hierarchical vascular networks for mediating heat and mass transport in biological structures, have gained tremendous interest in the fields of microfluidics, soft robotics, microelectronics, battery packaging, and civil infrastructure. Strategies for fabrication of vascular networks include pullout, dissolution, melting, or evaporation of sacrificial templates (metal wires, fugitive inks, photoresists) embedded in a host matrix to leave hollow conduits ranging from microns to meters in length. Most sacrificial materials cannot be easily templated into complex interconnected architectures or are unable to withstand thermomechanical stresses during integration into structural thermosets and fiber-reinforced polymer composites (FRPCs). Catalyst-assisted thermal depolymerization of poly(lactic acid) (PLA) templates containing an organometallic compound enables manufacturing of complex vasculature in thermosets and FRPCs for self-healing, thermal regulation, and electromagnetic reconfigurability applications. However, this vascularization method is a two-step energy-intensive process that requires sustained external heating to first polymerize the thermoset matrix, and then to depolymerize the embedded sacrificial PLA templates into volatile products (ca. 200 °C for 12 hours). Moreover, vaporization of sacrificial components (VaSC) with PLA is only suitable for host matrices that withstand these extreme conditions. Residual organometallic catalyst also remains on the channel walls, which may hamper subsequent functionalization of the microchannels. In this dissertation, we explore new sacrificial polymers that depolymerize at lower temperatures than PLA to reduce the energy consumption for making vascular structures and expand the scope of VaSC to a broader range of host matrices. We next investigate a more energy-efficient strategy for rapid manufacturing of vascular materials in a single-step by harnessing the energy from exothermic curing of the host matrix. New organometallic catalysts are pursued for lowering the VaSC temperature and time for PLA in epoxy-based matrices with a glass transition temperature (Tg) ca. 155 °C. Depolymerization of PLA films blended with various surface areas and concentrations of tin (II) oxalate and tin (II) acetate catalysts are compared via mass loss in a thermogravimetric analyzer (TGA). A 10x improvement in mass loss kinetics of PLA is observed with tin acetate. PLA films blended with tin acetate are then embedded in a host matrix and successfully evacuated in a heated oven at temperatures as low as 170 °C after 12 hours to create microchannels with less catalytic residue on the walls. A lower temperature limit for VaSC within a practical time (12 h) is reported near the melting temperature of PLA ca. 160 °C. Vascularization at lower temperatures with a five-fold reduction in thermal energy consumption than PLA is demonstrated with a metastable polymer known as cyclic poly(phthalaldehyde) (cPPA). cPPA is synthesized and solvent-processed into laser-cut films, fibers, and printed templates. Mass loss before (TGA) and after embedding into a host matrix (VaSC) is compared to determine successful vascularization protocols. A mass loss kinetics model derived from the TGA experiments reveals a ca. 32 kJ/mol reduction in activation energy for depolymerization of cPPA compared to PLA, allowing vascularization within 1 hour at 110 °C. Residue-free channel walls are obtained in epoxy-based matrices with Tg between 42-65 °C. A further 50 °C reduction in VaSC temperature is achieved with acid-catalyzed depolymerization of poly(propylene carbonate) (PPC) templates. PPC containing a latent photoacid generator (PAG) is melt-processed into fibers and filaments near 150 °C, which is easily depolymerized below the extrusion temperature after activating the PAG. This orthogonal trigger approach allows melt-extrusion into mechanically robust templates that sustain bending loads without brittle fracture and are readily incorporated in host matrices with low Tg after photoactivation. Residue-free vasculature is achieved after exposure to temperatures between 50-100 °C after 1-12 hours. A 100x increase in the pre-exponential factor for the Arrhenius-based mass loss kinetics of PPC compared to cPPA facilitates the faster depolymerization of PPC at such low temperatures. In the final part of the thesis, we investigate concurrent depolymerization of embedded sacrificial templates during frontal polymerization (FP) of a dicyclopentadiene (DCPD) matrix to create vascular thermosets and FRPCs within minutes under ambient conditions. The heat generated during FP of DCPD is sufficient for complete depolymerization of cPPA and PPC fibers and printed templates without an additional external heat source. A thermochemical computational model based on experimental parameters provides insights into the processing window for coupled polymerization and depolymerization reactions. Bioinspired thermosets with redundant vascular networks are fabricated with this single-step approach. In addition, vascular FRPCs are manufactured within seconds at room temperature, reducing fabrication time by three orders of magnitude and processing energy by four orders of magnitude compared to the current PLA VaSC method. This one-step, energy-efficient processing route can propel industrial-scale manufacturing of bioinspired vascular structures for biomedical, transportation, and energy applications.
Issue Date:2020-09-04
Rights Information:Copyright 2020 Mayank Garg
Date Available in IDEALS:2021-03-05
Date Deposited:2020-12

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