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Persistent URL http://purl.org/net/epubs/work/32842
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Record Id 32842
Title Dynamics of propylene glycol and its oligomers confined to a single molecular layer
Abstract The dynamics of propylene glycol (PG) and its oligomers 7-PG and poly-propylene glycol (PPG), with Mw=4000 (?70 monomers), confined in a Na–vermiculite clay have been investigated by quasielastic neutron scattering. The liquids are confined to single molecular layers between clay platelets, giving a true two-dimensional liquid. Data from three different spectrometers of different resolutions were Fourier transformed to S(Q,t) and combined to give an extended dynamical time range of 0.3–2000 ps. An attempt was made to distinguish the diffusive motion from the methyl group rotation and a fast local motion of hydrogen in the polymer backbone. The results show that the average relaxation time ??d? of this diffusive process is, as expected, larger than the relaxation time ??? averaged over all dynamical processes observed in the experimental time window. More interesting, it is evident that the severe confinement has a relatively small effect on ??d? at T=300K, this holds particularly for the longest oligomer, PPG. The most significant difference is that the chain-length dependence of ??d? is weaker for the confined liquids, although the slowing down in bulk PG due to the formation of a three-dimensional network of OH-bonded end groups reduces this difference. The estimated average relaxation time ??? at Q=0.92Å?1 for all the observed processes is in excellent agreement with the previously reported dielectric ? relaxation time in the studied temperature range of 260–380 K. The average relaxation time ??? (as well as the dielectric ? relaxation time) is also almost unaffected by the confinement to a single molecular layer, suggesting that the interaction with the clay surfaces is weak and that the reduced dimensionality has only a weak influence on the time scale of all the dynamical processes observed in this study.
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Journal Article J Phys Chem 122 (2005): 244702. doi:10.1063/1.1943408 ms-clay-ppg-revised.pdf 2005