Longitudinal acoustic mode studies of poly(ethylene oxide).

Normal mode calculations have been done for the exact structure of oligo-oxyethylenes to investigate the Raman active longitudinal acoustic mode (LAM) of helical polymers. Lateral interchain interactions were explicitly included through an atom-atom potential, and hydrogen bonding at the chain ends was included through perturbing forces. We find that it is impossible to reproduce observed LAM frequencies without incorporating interchain interactions. The results on model oligomers of poly(ethylene oxide) (PEO) show that the LAM of helical chains is much more sensitive to intermolecular interactions than that of planar zigzag chains. The importance of interchain interactions in the analysis of the LAM in helical chains provides a possible basis for explaining part of the difference between elastic moduli determined from Raman and x-ray experiments. The observed LAM bands of PEO were interpreted using the results of normal mode calculations on various chain models. The presence of a fold at the chain ends results in a significant decrease in the LAM intensity, but only a small decrease in the LAM frequency. SAXS and DSC studies were employed to help in interpreting the lamellar structure. In order to explain the observed LAM of PEO, we propose new models of folded-chain structure and lamellar structure.Ph.D.Polymer chemistryPure SciencesUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/128286/2/8907148.pd

Raman longitudinal acoustic mode (LAM) studies of folded-chain morphology in poly(ethylene oxide) (PEO). II. Normal mode analysis of folded-chain structures of PEO

Normal mode calculations were carried out on model folded-chain molecules to determine the effect of a fold on the LAM of PEO. The presence of a chain fold results in a decrease in the LAM frequency and intensity, the change in frequency being very much smaller than the change in intensity. End interactions do not seem to have much effect on the LAM when a fold is present at the chain ends. The effective chain length for determining the LAM frequency was found to be that part of the chain experiencing interchain interactions.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/38860/1/090280104_ftp.pd

Raman longitudinal acoustic mode (LAM) studies of folded-chain morphology in poly(ethylene oxide) (PEO). I. Normal mode analysis of LAM of a helical-chain oligomer of PEO

Normal mode analyses were done for the exact structures of model oligomers of PEO to investigate the longitudinal acoustic mode (LAM) in this helical polymer. It quickly became apparent that it is impossible to reproduce observed LAM frequencies without incorporating explicit interchain interactions. We have developed a force field that includes interchain interactions through an atom-atom potential and have shown that this force field reproduces very well the observed LAM frequencies of helical-chain oligomers of PEO. This result indicates that the LAM of helical chains is much more sensitive to intermolecular interactions than that of planar zigzag chains.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/38859/1/090280103_ftp.pd

Elastic modulus of poly(ethylene oxide) determined from the Raman longitudinal acoustic mode

The chain modulus of poly(ethylene oxide) is determined by the Raman longitudinal acoustic mode method with the aid of a normal coordinate analysis and found to be 1.3 × 10 11 dyn/cm 2 . Our normal mode calculations provide a clear basis for explaining the discrepancy between chain moduli determined from Raman and x-ray experiments. They show that the uncorrected Raman modulus of helical polymers reflects the presence of interchain interactions and therefore should not be taken as the correct elastic modulus of a single chain.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/38861/1/090280105_ftp.pd