Date of Award

Fall 12-2014

Degree Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Department

Polymers and High Performance Materials

Committee Chair

Dr. Jeffrey Wiggins

Committee Chair Department

Polymers and High Performance Materials

Committee Member 2

Dr. William Jarrett

Committee Member 2 Department

Polymers and High Performance Materials

Committee Member 3

Dr. Sergei Nazarenko

Committee Member 3 Department

Polymers and High Performance Materials

Committee Member 4

Dr. James Rawlins

Committee Member 4 Department

Polymers and High Performance Materials

Committee Member 5

Dr. Robson Storey

Committee Member 5 Department

Polymers and High Performance Materials

Abstract

This dissertation presents the investigation of the glassy state molecular motions in isomeric thermoset epoxies by means of solid-state deuterium (2H) NMR spectroscopy technique. The network structure of crosslinked epoxies was altered through monomer isomerism; specifically, diglycidyl ether of bisphenol A (DGEBA) was cured with isomeric amine curatives, i.e., the meta-substituted diaminodiphenylsulfone (33DDS) and para-substituted diaminodiphenylsulfone (44DDS). The use of structural isomerism provided a path way for altering macroscopic material properties while maintaining identical chemical composition within the crosslinked networks.

The effects of structural isomerism on the glassy state molecular motions were studied using solid-state 2H NMR spectroscopy, which offers unrivaled power to monitor site-specific molecular motions. Three distinctive molecular groups on each isomeric network, i.e., the phenylene rings in the bisphenol A structure (BPA), the phenylene rings in the diaminodiphenylsulfone structure (DDS), and the hydroxypropoyl ether group (HPE) have been selectively deuterated for a comprehensive study of the structure-dynamics-property relationships in thermoset epoxies.

Quadrupolar echo experiments and line shape simulations were employed as the main research approach to gain both qualitative and quantitative motional information of the epoxy networks in the glassy state. Quantitative information on the geometry and rate of the molecular motions allows the elucidation of the relationship between molecular motions and macro physical properties and the role of these motions in the mechanical relaxation. Specifically, it is revealed that both the BPA and HPE moieties in the isomeric networks have almost identical behaviors in the deep glassy state, which indicates that the molecular motions in the glassy state are localized, and the correlation length of the motions does not exceed the length of the DGEBA repeat unit. BPA ring motions contribute to the low temperature (around -80 to -50 °C) region, and HPE chain motions to the even lower temperature range (-110 °C) of the mechanical relaxation as detected by DMA. The differences in the physical properties of the isomeric epoxies are mostly attributed to the DDS moieties. The occurrence of 44DDS ring motions decreases the modulus and the swept-out space from its ring axis fluctuation explains the higher hole-size free volume of the para-substituted networks. 33DDS rings do not exhibit large amplitude motions but only undergo fast small-angle fluctuations, which results in a decrease in the magnitude of the high temperature part of the γ relaxation of 33A, a phenomenon often seen in the anti-plasticization process.

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