Independent and Interconnected Interpenetrating Polymer Network Formation In Dual-Cure Thermoset Resins
Open Access
- Author:
- Kopatz, Jessica
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- December 07, 2020
- Committee Members:
- Michael T Lanagan, Dissertation Advisor/Co-Advisor
Michael T Lanagan, Committee Chair/Co-Chair
Michael Anthony Hickner, Committee Member
Robert John Hickey, III, Committee Member
Ramakrishnan Rajagopalan, Outside Member
Leah N Appelhans, Special Member
John C Mauro, Program Head/Chair - Keywords:
- Dual-Cure
Thermosets
Interpenetrating Polymer Networks
Hybrid Monomers
Direct Ink Write - Abstract:
- Thermoset polymers are of interest for many structural applications due to their mechanical, thermal, and chemical resiliency in comparison to thermoplastic polymers. However, utilizing thermosets in the realm of additive manufacturing is still a developing field, especially when using viscous thermosets where long cure times can limit the shape retention of 3D printed parts. There has been increased interest in the development of direct-ink-write (DIW) methods for thermoset resins in the last several years. The approaches generally fall into two categories; dual-cure systems or filled resins. Dual-cure resins are comprised of multiple thermoset polymers that are cured using different stimuli or variations of the same stimuli (i.e. temperatures, wavelengths, etc.). Common dual-cure thermoset resins consist of a photo-curable acrylate and a thermally-curable epoxy. For sequential dual-cure systems, the photo-cure mechanism can be rapidly completed immediately after extrusion from the print tip in order to lock-in the desired structure, while the sequential thermal cure forms the epoxy network and enhances the composite properties. Sequential curing of the resin, via UV exposure followed by thermal exposure, creates interpenetrating polymer networks (IPNs), which can enhance the overall thermomechanical properties compared to either parent material. IPNs consist of two or more networks that are interwoven on a molecular scale. Though the networks are not covalently bound to each other, they cannot be separated unless chemical bonds are broken. Independent IPNs exist where there are two separate but interwoven networks, while interconnected IPNs form by the addition of a heterobifunctional monomer that links the two networks together. Interconnected IPNs are expected to result in the best properties due to the tethering of the two otherwise separate networks. The filled resins approach to the DIW of thermosets generally require the material to possess high zero-shear viscosity, shear thinning behavior, and controlled curing. High zero-shear viscosity permits shape retention post-extrusion, while shear thinning enables the fluid to flow easily through the print head. To enable shear thinning, fillers such as clay or silica are added, where the filler is usually modified to enable better dispersion within the polymer matrix. Not only does the addition of inorganic fillers alter the rheology and printability of the resin, the fillers can also impact the thermomechanical properties of the composite. This thesis describes the influence of resin composition on the printability and ‘green strength’ of printed parts. After the sequential thermal cure in the dual-cure system, the impact of acrylate network formation on the evolution and formation of independent IPNs was investigated. Moving to a more simplified dual-cure acrylate-epoxy resin system, the impact of acrylate-functionalized and epoxy-functionalized filler particles as network crosslinkers was explored. Finally, the influence of heterobifunctional acrylate-epoxy hybrid monomers with differing backbones on the formation of interconnected IPNs was investigated.