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Author ORCID Identifier
Campus-Only Access for One (1) Year
Doctor of Philosophy (PhD)
Polymer Science and Engineering
Year Degree Awarded
Month Degree Awarded
Polymer and Organic Materials | Polymer Chemistry
The importance of synthetic polymers in everyday life continues to grow, owing to their societal importance for improving our standard-of-living and enabling advances spanning medicine, electronics, construction materials, transportation. While niche applications occupy a small fraction of the overall volume of polymers produced, large scale applications tend to employ lower cost materials, such as polyethylene, polypropylene, and polystyrene. In addition to environmental considerations connected to these polymerized hydrocarbons, produced in excess of 380 million tons per year worldwide, their inherent flammability creates additional requirements associated with their manufacturing and use. Societal benefits of such polymers are extensive, and thus, there is a pressing need to develop non-toxic and safe polymers and additives.
Chapter 1 overviews the fundamental aspects of flame retardants and state of the art flammability characterization techniques. In addition, we will highlight recent academic and commercial efforts to develop non-halogenated polymers and additives. Specifically, bishydroxydeoxybenzoin (BHDB), a fully hydrocarbon bis-phenol monomer, has emerged as an excellent candidate to achieve nonflammable polymers by its facile integration into a variety of polymers, such as polyesters, polyurethanes, and epoxies. Developing functional deoxybenzoin small molecules, monomers, and polymers allows access to new opportunities to achieve non-halogenated, low flammability polymers and networks that are not accessible with conventional deoxybenzoin structures.
In Chapter 2, we discuss the synthesis of a novel deoxybenzoin-based AB2 monomer. AB2 monomers present opportunities to conduct one-pot syntheses of highly branched or “hyperbranched” polymers, which are known for their distinct physical and chemical properties relative to linear polymers. We describe the synthesis of a deoxybenzoin-containing AB2 monomer and its use in step-growth polymerization to prepare branched aromatic polyesters. Highly soluble deoxybenzoin polymers were obtained with degrees of branching reaching 0.36 and estimated molecular weights approaching 20 kDa. The phenolic chain-ends of the polymer allowed for post-polymerization modification by silylation and esterification chemistry. Thermogravimetric analysis (TGA) and microscale combustion calorimetry (MCC) revealed these novel aromatic polyesters to possess the critically important characteristics of flame-retardant polymers, such as high char yield and low heat release. Hyperbranched-containing blends were prepared and characterized by rheological methods to reveal surface migration behavior of the low viscosity, hyperbranched additives due to energetic driving forces that lead to energy dissipation.
In Chapter 3, we discuss the design and synthesis of a multifunctional epoxide monomer. Multifunctional monomers present opportunities to design cross-linked networks with tunable thermal and mechanical properties. We describe the synthesis of a novel, multifunctional deoxybenzoin-based epoxide molecule containing pendant allyl groups positioned on the aromatic rings, and its use in the preparation of epoxide networks. We demonstrate the use of these functional groups in orthogonal thiol-ene and epoxide-amine reactions to facilitate double network formation. The resultant networks exhibited tunable Tg values, moduli, and crosslink densities. TGA and microscale combustion calorimetry revealed these double networks to have impressively low heat release properties, ~50% lower than traditional bisphenol A-based networks, and very high char residues that reached ~60%.
In Chapter 4, we present an alternative synthetic route to deoxybenzoin-like polymers. We show the polymerization of commercial and novel AB monomers and their one-step polymerization to achieve fully hydrocarbon, poly(aryl ketone)s. These polymers exhibit high char residues (~60%) and extremely low heat release properties (FGC ~ 55 J/g-K). The resultant polymers were blended with commercial PET and showed a 40% reduction in heat release properties with just 10 wt% poly(aryl ketone) incorporated. Our results show that low concentrations of deoxybenzoin-like polymers employed as additives significantly reduce the heat release properties.
Finally, in Chapter 5, we utilize multifunctional organophosphorus additives to engineer epoxy networks with both enhanced mechanical properties and ultra-low flammability. We describe the use of dimethyl methylphosphonate (DMMP) as an additive in conventional and inherently low flammability epoxy resins. TGA of DMMP-containing networks showed that DMMP promoted considerable char formation, with char residues reaching high levels, up to 55%, for DMMP-containing deoxybenzoin networks. Microscale combustion calorimetry of the DMMP-containing networks exhibited 50% lower heat release capacity and total heat release rate values relative to formulations without DMMP corresponding well with vertical burn tests that demonstrated self-extinguishing characteristics of DMMP-containing formulations. Mechanical characterization revealed 50% higher elastic modulus and comparable yield stress for networks containing DMMP relative to those without DMMP. This organophosphorus additive represents an opportunity to combine materials chemistry with mechanical enhancement mechanisms to achieve low heat release properties without the need for halogenated flame-retardant additives. In addition, phosphorus-containing deoxybenzoin monomers and polymers were synthesized through the addition of phosphorus across C=C bonds by UV irradiation. Phosphorus-containing deoxybenzoin polymers showed little, if any, decrease in heat release properties, in fact, pendant unsaturated alkene groups appear to promote char and decrease heat release, thereby bypassing the need for halogen or phosphorus additives, and paving the path for continued study and use of fully hydrocarbon flame retardants. While synthetic analogs are synthetically challenging, organophosphorus additives are effective in reducing flammability and improving mechanical properties.
Stubbs, Elizabeth, "Designing Nonflammable Polymers and Blends Containing Deoxybenzoin Derivatives" (2022). Doctoral Dissertations. 2478.
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