Study of 3,3' vs. 4,4' DDS isomer curatives on physical properties and phenyl ring motions of DGEBA epoxy via molecular dynamics, deuterium NMR, and dielectric spectroscopy (2010)

The purpose of this research is to develop a multiscale understanding of crosslinked amorphous matrices, connecting molecular level events to macroscopic properties. To accomplish this goal, our methodology was to identify network architectures that influence molecular level energy dissipation through mechanisms such as bond rotations, torsions, and ring flips and then relate those molecular motions to macroscopic properties. Studies were accomplished on two aerospace grade matrices: the epoxy, diglycidyl ether of bisphenol A (DGEBA) cured with two amines, 3,3’- diaminodiphenyl sulfone (33DDS) and 4,4’-diaminodiphenyl sulfone (44DDS). The 33DDS/DGEBA and 44DDS/DGEBA served both to provide a baseline for experimental testing of aerospace matrices and to enable the comparison of a meta-substituted vs. parasubstituted diamine in chemically isotropic systems. The results presented in the first seven chapters of the dissertation focus on these two matrices. Molecular Dynamics (MD) simulations provide a tool to quickly create networks with alterations in network architectures, such as crosslink density, aromaticity, sulfone content, pendant bulky groups, etc. MD can then be used to predict thermomechanical properties of these matrices and determine the effect of network architecture on properties. In this work, molecular dynamics simulations were used to accurately predict thermomechanical properties of 33DDS/DGEBA and 44DDS/DGEBA. Additionally, modifications to these baseline matrices were made in order to study the effect of network architecture and chemical composition of matrices. To bridge the gap in understanding between network architecture and ultimate matrix performance, molecular motions resulting from the network architecture and responsible for ultimate properties, must be understood. To analyze these molecular motions, various solid state Nuclear Magnetic Resonance (NMR) spectroscopic techniques, Dielectric Spectroscopy (DES), and Dynamic Mechanical Analysis (DMA) are employed. Deuterium (2H) NMR spin-lattice relaxation studies and lineshape analyses are powerful tools in determining the motional behavior of targeted chemical moieties within the glassy state. Solid state deuterium NMR studies were used to selectively study the phenyl ring motions of 33DDS, 44DDS, and DGEBA in 33DDS/DGEBA and 44DDS/DGEBA networks. Phenyl ring deuterated diglycidyl ether of bisphenol A (d8- DGEBA), 3,3’-diaminodiphenyl sulfone (d8-33DDS), and 4,4’-diaminodiphenyl sulfone (d8-44DDS) were all synthesized. Deuterated DGEBA was reacted with non-deuterated 33DDS and 44DDS to study the effect of the amine on the motions of the epoxy rings. Deuterated 33DDS and 44DDS were reacted with non-deuterated DGEBA to study the phenyl ring motions of the amines. Carbon NMR spectroscopy was also used to provide additional information about phenyl ring motions of the epoxies. While solid state NMR techniques can be used to elucidate the motions of specific chemistries and architectures within the network, secondary, or sub-Tg, relaxations seen in dielectric spectroscopy and dynamic mechanical analysis are often associated with mechanical properties, including modulus, toughness, and strength. The association of the chemistries responsible for specific motions and the contributions of these motions to secondary relaxations help connect the molecular scale to the macroscale. In this work, phenyl ring motions have been correlated with sub-Tg transitions observed with DES and DMA, furthering understanding of the relationship between chemical composition and mechanical properties in polymers. DES and DMA have also shown distinct differences in 33DDS/DGEBA and 44DDS/DGEBA. Solid state NMR techniques were used to study the kinetics of reaction of 33DDS and 44DDS with DGEBA. The reaction of most primary amines before reaction of secondary amines was observed in 33DDS/DGEBA and 44DDS/DGEBA. This provides information on network architecture by showing that DDS/DGEBA systems form linear oligomers prior to crosslinking. The effect of modification with an octafunctional polyhedral oligomeric silsesquioxane (POSS) on network properties of on an ambient cure epoxy was studied. The results show that POSS modification provides substantial increases in mechanical properties at ambient conditions and elevated temperatures. Finally, the surface chemistry of industrially manufactured aerospace composites was analyzed to gain insight on preparing proper surfaces for bonding. Release agents used in the fabrication process were found in trace quantities on the surfaces. The presence of low surface energy release agents on surfaces is counterproductive to the bonding of those surfaces.