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Dissertation
Highly Ductile Epoxy Systems Obtained by Network Topology Modification and Its Influence on Secondary Phase Tougheners
Doctor of Philosophy (Ph.D.), Drexel University
Sep 2021
DOI:
https://doi.org/10.17918/00000509
Abstract
Epoxy toughening has been investigated for many decades. The addition of a second dispersed particulate phase (soft and rigid particles) is widely used to improve the toughness of epoxy thermosets. But, the resulting increased viscosity and the large particle size limit their use in many applications. Moreover, the toughening performance of the secondary phases depends highly on the epoxy matrix ductility, and the improvement of fracture toughness is minimized for the high Tg (high crosslink density) thermosetting systems. Initial investigations by our group showed that adding partially reacted substructures (mPRS) to tailor network topology resulted in high Tg systems (> 100°C) capable of exceptional strain at failure, though this modification also resulted in decreased yield strength and Tg. Herein it is shown that the enhanced ductility is a result of altered network topology on not simply Tg reduction, and that the topologically enhanced ductility persists at strain rates as high as 6000/s. A synergistic effect on fracture toughness improvement was observed when epoxy systems modified with mPRS were used in conjunction with traditional second phase tougheners including rigid 20 nm nano silica particles as well as compliant 200 nm rubber particles. The highly ductile mPRS epoxy matrix improves nano silica particle debonding and subsequent void growth increasing fracture toughness greatly. The high matrix ductility of the mPRS matrix also promotes rubber particle cavitation and plastic shear deformation thus improving fracture toughness. In both cases materials with Tg greater than 120 °C and KIC greater than 1.2 MPa*m1/2 were obtained. In order to improve the ductility of the high Tg epoxy-amine systems without sacrificing Young's modulus and yield strength, another PRS topology-based toughening approach was developed using thermally reversible Diels-Alder (DA) chemistries, whereby DA linkages are present in the network at room temperature imparting high Young's modulus and yield strength but were assumed to cleave upon heating induced by plastic deformation to produce a more ductile network. Experiments conducted over a range of temperatures confirmed this behavior. Moreover, it was found that the spatial distribution of DA linkages and therefore the network topology influences stimulus-responsive ductility. Room temperature compression experiments conducted over a range of strain rates up to 5000/s and all showed enhanced ductility without affecting room temperature modulus. Analyses of strain rate, thermal diffusion, and reaction timescales indicate that DA bond cleavage is mechanically activated.
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Details
- Title
- Highly Ductile Epoxy Systems Obtained by Network Topology Modification and Its Influence on Secondary Phase Tougheners
- Creators
- Jian Gao
- Contributors
- Giuseppe R. Palmese (Advisor)Cameron F. Abrams (Advisor)
- Awarding Institution
- Drexel University
- Degree Awarded
- Doctor of Philosophy (Ph.D.)
- Publisher
- Drexel University; Philadelphia, Pennsylvania
- Number of pages
- xxiii, 274 pages
- Resource Type
- Dissertation
- Language
- English
- Academic Unit
- Chemical and Biological Engineering; College of Engineering; Drexel University
- Other Identifier
- 991015469406804721