Chain dynamics in mesoscopically confined polymer melts. A field-cycling NMR relaxometry study |
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Authors: | R Kausik C Mattea R Kimmich and N Fatkullin |
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Institution: | (1) Universit?t Ulm, Sektion Kernresonanzspektroskopie, 89069 Ulm, Germany;(2) Department of Molecular Physics, Kazan State University, Kazan, 420008 Tatarstan, Russia |
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Abstract: | Polymer chain dynamics were studied with the aid of
field-cycling NMR relaxometry (time scale: 10-9s...
10-4s) supplemented by field gradient NMR diffusometry
(time scale: 10-4s...100s). Three sorts of samples
of mesoscopically confined polymer melts were examined. In the first
sample series, linear poly(ethylene oxide) was incorporated in
strands embedded in a quasi-solid and impenetrable methacrylate
matrix. The strand diameters ranged from 10 to 60 nm. It was shown
that chain dynamics becomes dramatically different from bulk
behavior. This so-called “corset effect” occurs both above
and below the critical molecular mass and reveals dynamic features
predicted for reptation. On the time scale of spin-lattice
relaxation, the frequency and molecular weight, signature of
reptation, T1 ∼M0 ν3/4, that is limit II of
the Doi/Edwards formalism corresponding to the mean squared segment
displacement law 〈r2 〉∼M0 t1/4, showed
up. A “tube” diameter of only 0.6 nm was concluded to be
effective on this time scale even when the strand diameter was
larger than the radius of gyration of the PEO random coils.
The corset effect is traced back to the lack of the local
fluctuation capacity of the free volume under nanoscopic
confinements. The confinement dimension at which the cross-over from
confined to bulk chain dynamics is expected was estimated to be
micrometers. Using the so-called roll-coating technique, micrometer
thick polymer melt layers between Kapton foils were prepared.
Perceptible differences from the bulk materials were found. The
polymer species studied in this case was perfluoropolyether with
Flory radii in the order of 7 nm. Remarkably, the confinement
effect was shown to reach polymer-wall distances of the order 100
Flory radii. As a third confinement system, melts of
perfluoropolyether were filled into a porous silica glass (Vycor;
4 nm nominal pore size). In this case, a crossover from Rouse
dynamics in the bulk to reptation in the Doi/Edwards limit III
(T1∼M-1/2 ν1/2 corresponding to 〈r2
〉∼M-1/2 t1/2) was observed. |
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Keywords: | |
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