Date of Award

Spring 5-8-2015

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Polymers and High Performance Materials

Committee Chair

James Rawlins

Committee Chair Department

Polymers and High Performance Materials

Committee Member 2

Jeffrey Wiggins

Committee Member 2 Department

Polymers and High Performance Materials

Committee Member 3

Sergei Nazarenko

Committee Member 3 Department

Polymers and High Performance Materials

Committee Member 4

Derek Patton

Committee Member 4 Department

Polymers and High Performance Materials

Committee Member 5

William Jarrett

Committee Member 5 Department

Polymers and High Performance Materials

Abstract

Covalent bonds in organic molecules can be produced, altered, and broken through various sources of energy and processes. These include photochemical, thermochemical, chemical, and mechanochemical processes. Polymeric materials derive their physical properties from the time scale of motion, summation of intermolecular forces, and number of chain entanglements and crosslinks. Glassy thermoset polymers experience mechanical fatigue during dynamic stress loading and properties diminish with inevitable material failure at stress levels below the ultimate tensile strength (UTS). Damage modeling has been successful in predicting the number of cycles required to induce failure in a specimen due to stress. However, it does not directly provide an explanation of the origin of fatigue in polymers. It is hypothesized herein that mechanical failure at stress levels below the ultimate strength property is due to the accumulation of mechanically induced homolytic chain scission events throughout the glassy thermoset network. The goal of this research will be to quantify homolytic chain scission events with fatigue cycles with the ultimate goal of correlating mechanical property loss with degradation of covalent network structure.

To accomplish this goal, stable free nitroxyl radicals were incorporated into an epoxy-amine matrix to detect homolytic chain scission resulting from fatigue. Chapter II discusses a successful synthesis and characterization of the nitroxyl radical molecule, a product of 4-hydroxy-2,2,5,5-tetramethylpiperdin-1-yl-oxyl (TEMPO) and isophorone diisocyanate designated as BT-IPDI. In Chapter III, the epoxy-amine reaction was determined to be unaffected by incorporation of up to 5 wt% of BT-IPDI. Although 50% UTS fatigue studies produced property degradation and fatigue failure as shown in Chapter IV, analysis of BT-IPDI through EPR did not detect homolytic chain scission. Chapter V reveals that mechano-radicals were produced from cryo-grinding the glassy epoxy-amine thermoset, and although the mechano-radicals reacted through recombination at elevated temperatures, the reaction between mechano-radicals and the BT-IPDI was not detected to occur within the glassy state.

During mechanical testing, observations of unusual tensile yield behavior were coupled with production of atypical fracture surfaces. In Chapter VI, physical aging was used as an investigative tool to verify that viscous deformation (plastic flow) was required to produce the atypical fracture surfaces. Atomic force microscopy and scanning electron microscopy of the fracture surface both revealed a tendril nodule morphology. It is our hypothesis that this morphology produces the unusual mechanical behavior. In Chapter VII, NIR, AFM, and SEM were used to measure the conversion and morphology of the epoxy-amine thermoset correlated with mechanical properties. The thermal cure profile of the epoxy-amine thermoset affects the size and formation of the nodular nanostructure. Eliminating vitrification during thermoset polymerization forms a more continuous phase, reduction in size of the nodules, and eliminates the capacity of the material to yield in plastic flow. Specific findings of this research reveal that morphology control through thermal cure design may indicate a route in which thermoplastic type failure mechanisms can be incorporated into glassy epoxy thermosets.

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