Abstract

Dynamic covalent chemistry provides the functional basis for the efficient and reliable exchange of reversible binding groups which enable self-repair thus avoiding structural impairment or permanent damage of material properties. Covalent adaptable networks (CANs) have garnered attention for the sustainable development of polymers capable of mending damages thus facilitating eco-friendly recyclability. This work aims to address the role of reaction pathway and manufacturing process on the subsequent properties of CANs bearing hindered urea bonds (HUBs). HUBs have proven easy to incorporate into polymers and exhibit moderate temperature thermo-reversibility and creep resistance. In this investigation we study the effects of preparing CANs bearing HUBs using both free radical and thiol-ene chemistries. A novel HUB-based poly(urea urethane) prepolymer (PUUP) with diacrylate functionality enables self-healing under thermal stimulus. The PUUP was incorporated into networks with thiol building blocks at different stoichiometries; the diacrylate functionality enabled the formation of networks using either free radical photo-initiator or base catalyst. By preparing distinct networks with various molecular weight per elastically effective network chain (MC), this work addresses the interdependencies of network topology, mechanical properties, and self-healing resulting from these two synthetic strategies. Dynamic mechanical analysis (DMA) was utilized to measure the modulus of prepared networks, which enabled an assessment of crosslink density. Mechanical testing was then employed to investigate the bulk and self healing properties of these networks. A wide range of mechanical behavior was exhibited, depending on the preparation strategy: for photo-polymerized networks, elongation at break (emax) ranged from 88 to 152% and toughness values (UT) ranged from 0.45 to 0.62MJ/m3. On the other hand, base catalyzed networks exhibited elongation to break as high as 904%, and toughness as high as 2.75MJ/m3. A high healing efficiency of 100% recovery of toughness (hU) was exhibited by the base catalyzed sample with highest MC whereas photopolymerized networks exhibited significantly reduced performance (hU=29%). The reduced performance of photopolymerized networks is attributed to a competition between the desirable thiol-ene reaction and the competing free radical polymerization of acrylate groups. Additionally, intrinsically self-healing polymers are constructed via masked stereolithographic additive manufacturing (MLSA AM). The presence of strong hydrogen bonding between urea and urethane groups of the PUUP was reduced by incorporating reactive diluent tetrahydrofurfuryl acrylate (THFA), thus enabling the preparation of resins with sufficiently low viscosities (< 10Pa.s) for vat polymerization. Resin formulations relied on traditional free radical polymerization or thiol-ene chemistry, where the incorporation of thiol building blocks facilitated a dual cure approach to building networks. The developed CANs are designed to contain HUBs within the side-chain (ladder) or main-chain (thiol). The polymerization mechanics of thiol systems resulted in networks with enhanced self-healing performance (83% < he < 98%) in comparison to ladder networks (he < 21%). We further note that thiol networks exhibit high self-healing efficiencies over a wide range of HUB concentrations. These results suggest that thiol-ene chemistry can act as a powerful tool in generating AM network structures which enable sufficient topologic- and self-diffusion of HUBs to facilitate healing without a strong dependence on HUB concentration. Ultimately, this investigation has facilitated the development of advanced functional materials amendable to vat polymerization AM with utility in biomedical engineering, conductive devices/sensors, soft robotics, and for the extended shelf-life of energetics. This research provides the comprehensive characterization of the synthesis, scale-up, and utilization of AM to prepare CANs. By introducing a novel prepolymer capable of thermo-reversibility, the development of traditional cast-and-cure polymers are explored. These materials are further expanded towards use in the AM paradigm. An intrinsically healable HUB-based synthon was integrated into networks and scaled sufficiently to enable the 3DP workflow. This work highlights how this technology can be translated across multiple domains, acting to bridge the gap between small scale synthesis and AM of CANs.

Library of Congress Subject Headings

Polymeric composites; Chemical bonds; Urea; Self-healing materials

Publication Date

7-28-2025

Document Type

Dissertation

Student Type

Graduate

Degree Name

Biomedical and Chemical Engineering (Ph.D)

Department, Program, or Center

Biomedical Engineering

College

Kate Gleason College of Engineering

Advisor

Christopher L. Lewis

Campus

RIT – Main Campus

Plan Codes

BMECHE-PHD

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