Elizabeth F. McCord

Synthesis of a new fluoroalkylated diamine, 5-[1H,1 H-2-bis(trifluoromethyl)- heptafluoropentyl]-1,3-phenylenediamine, and polyimides prepared therefrom

Abstract The preparation of a new perfluoroalkylated diamine, 5-[1H,1H-2-bis(trifluoromethyl)heptafluoropentyl]-1,3-phenylenediamine, is described. Poly(amic acid)s and polyimides were prepared from this new diamine and polyimides were evaluated as films for electronics applications. The polyimides evaluated were found to yield low dielectric constant and low moisture absorption, but were found to have exceptionally high thermal expansion coefficient. The materials are envisioned to have applications in electronics, where a low and humidity-stable dielectric constant is required.

2D-NMR studies of a model for Krytox® fluoropolymers

Multiple two‐dimensional nuclear magnetic resonance (2D‐NMR) techniques have been used to study the structures of Krytox® perfluoro(polyalkyl ether) and its mechanism of polymerization. Model compound K4, containing four Krytox® fluoropolymer repeat units, was analyzed to interpret the multiplet patterns in the NMR spectra from the polymer model. 19F {13C}‐Heteronuclear single‐quantum correlation experiments, performed with delays optimized for 1JCF and 2JCF, provided spectra that permitted identification of resonances from individual monomer units. Selective, 19F‐19F COSY 2D‐NMR experiments were performed with different excitation regions; these experiments were combined with selective inversion pulses to remove 19F‐19F J couplings in the f1 dimension. The resulting COSY spectra were greatly simplified compared with standard 19F‐19F COSY spectra, which are too complicated to interpret. They give information regarding the attachments of monomer units and also provide insights into the nature of the stereoisomers that might be present in the polymer. Both infrared and NMR spectra show peaks identifying chain end structures. With the help of these studies, resonances can be assigned, and the average number of repeat units in the polymer chain can be calculated based on the assignments obtained. Copyright

2-D NMR Studies of Chain-End Structures in Poly(Hexafluoropropylene Oxide) Fluoropolymers

2-D NMR techniques were used to study the structures of poly (hexafluoropropylene oxide) (PHFPO) and its mechanism of polymerization. Model compounds K4, containing four hexafluorpropylene oxide repeat units, and E4 and E2, containing four and two ether oxygens respectively, were analyzed to interpret the resonance patterns in the NMR spectra from the polymers. Selective 19 F- 13 C gradient HSQC experiments provided information about the C-F attachments within monomer units of the backbone and chain ends. A series of 19 F- 19 F selective COSY 2-D NMR experiments were performed to determine the connections and arrangements of each hexafluoropropylene oxide unit. When selective experiments were combined with selective inversion pulses to remove most of the JFF couplings in the f1 dimension, greatly simplified 19 F- 19 F COSY 2-D NMR spectra were obtained. This made it possible to resolve signals from various stereo sequences and monomer inversions. Resonance chain-end structures detected in the polymer have been identified and attributed to the various mechanisms for initiation and termination of polymerization. Unambiguous evidence of inverse addition and isopropyl end-group structures was obtained.

Use of Paramagnetic Relaxation Agents in the Characterization of Acrylic Polymers: Application of Chromium (III) Acetylacetonate and Iron (III) Acetylacetonate

The quantitation of acrylate polymers by high-resolution 13C-NMR requires a knowledge of the spin-lattice relaxation times. For signals from non-protonated carbons, such as carbonyls, these relaxat...

Failure analysis methods applied to PV module reliability

In the testing of photovoltaic materials and modules, failure analysis provides insights into the specific mechanism of performance breakdown and offers opportunities to improve performance by materials or process modification. We review various analytical methods applied to photovoltaic modules and test structures to better understand the nature of failure, including several methods not previously discussed in failure analysis literature as applied to photovoltaic devices. Included in this discussion will be the use of environmental scanning electron microscopy (ESEM) and x-ray microtomography to investigate the failure mechanism in electrical impulse testing of a candidate PV module.

19F DOSY diffusion-NMR spectroscopy of fluoropolymers

A new pulse sequence for obtaining 19F detected DOSY (diffusion ordered spectroscopy) spectra of fluorinated molecules is presented and used to study fluoropolymers based on vinylidene fluoride and chlorotrifluoroethylene. The performance of 19F DOSY NMR experiments (and in general any type of NMR experiment) on fluoropolymers creates some unique complications that very often prevent detection of important signals. Factors that create these complications include: (1) the presence of many scalar couplings among 1H, 19F and 13C; (2) the large magnitudes of many 19F homonuclear couplings (especially 2JFF); (3) the large 19F chemical shift range; and (4) the low solubility of these materials (which requires that experiments be performed at high temperatures). A systematic study of the various methods for collecting DOSY NMR data, and the adaptation of these methods to obtain 19F detected DOSY data, has been performed using a mixture of low molecular weight, fluorinated model compounds. The best pulse sequences and optimal experimental conditions have been determined for obtaining 19F DOSY spectra. The optimum pulse sequences for acquiring 19F DOSY NMR data have been determined for various circumstances taking into account the spectral dispersion, number and magnitude of couplings present, and experimental temperature. Pulse sequences and experimental parameters for optimizing these experiments for the study of fluoropolymers have been studied. Copyright