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Confinement Effects on Chain Dynamics and Local Chain Order in Entangled Polymer Melts.

Ok S, Steinhart M, Serbescu A, Franz C, Vaca Chávez F, Saalwächter K - Macromolecules (2010)

View Article: PubMed Central - PubMed

Affiliation: Institut für Chemie, Universität Osnabrück, Barbarastr. 7, D-46069 Osnabrück, Germany.

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Consequently, a reduced interchain entanglement density has been discussed as the reason for enhanced flow of confined chains... In contrast, a “sticky” surface or local orientation effects and thus anisotropic dynamics of the segments close to an interface are thought to be the reason for enhanced elasticity or reduced diffusion coefficients... The only significant change is the overall upward shift of C(t/τe) by a factor of about 3, corresponding to a √3 ≈ 1.7-fold increase of the constraint-induced order... It may be expected that this increase is not homogeneous across the melt droplet, yet the featureless nDQ data in these samples did not allow for any further conclusions... This may be rationalized by a (rather long-ranged) geometrically wall-induced anisotropy acting on top of the tube constraint... For a rough estimate of the range of such an effect, we can assume that the center parts of the droplets are bulklike and that the confined parts do not show isotropic chain-end signals at all... At the lowest temperature of 263 K, which is about 90 K above Tg (just as for the lowest temperature of our PDMS samples), we observe only little confinement effects (the 20 nm sample exhibits a somewhat quicker initial rise), which we explain with the dominance of the much stronger entanglement-induced anisotropy to chain motion... In order to reconcile the different observations for PDMS and PB, we need to keep in mind that the estimates of the constrained fraction were based on different observations, respectively, i.e., missing isotropic chain-end parts (which were not significant for highly entangled PB) and the amount of polymer involved in significantly anisotropic segmental motion (which is too severely affected by terminal relaxation in lowly entangled PDMS)... More conclusive results will be drawn from our ongoing study of a larger molecular-weight range and from an in-depth analysis of the experimental data... For now, we hypothesize that the length scale of geometrically induced anisotropy of segmental motion might be related to the entanglement separation (Me) rather than the radius of gyration, explaining the rather large length scale of the effect observed for PDMS in the nanodroplets... Further, a recent inelastic neutron scattering study focusing on fast segmental motions of poly(ethylene oxide) in self-ordered AAO channels of 40 nm diameter concluded no significant confinement effects on translational diffusion (apart from the expected immobilized absorption layer), which, however, as pointed out in a recent rebuttal by Kimmich and Fatkullin, cannot be taken as evidence for the absence of a “corset effect” affecting rotational dynamics... Finally, the effect observed by us is significantly more pronounced than orientation effects described by Deloche and co-workers, who did not detect any changes in moderately entangled PDMS films above 25 nm thickness... However, their effect in films below 25 nm could be quantitatively attributed to immediately geometrically induced segmental anisotropy in combination with diffusive averaging, and our preliminary T2 relaxation experiments on macroscopically ordered membrane stacks confirmed that the observed anisotropy is orientation-dependent, suggesting a similar origin of the phenomena... In contrast, cylindrical confinement of highly entangled PB (M/Me ≈ 50) leads to an almost temperature-independent response indicating network-like segmental anisotropy in an estimated ∼3 nm surface layer.

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(a) Exponential signal tails related to isotropically mobile chain ends, as fitted to Iref − IDQ for different PDMS samples at 280 K. (b) Chain-end fractions as a function of temperature for all PDMS samples. The dashed lines are interpolations of the bulk melt data, scaled down by factors of 0.8, 0.45, and 0.3 to match the confined-sample data.
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fig2: (a) Exponential signal tails related to isotropically mobile chain ends, as fitted to Iref − IDQ for different PDMS samples at 280 K. (b) Chain-end fractions as a function of temperature for all PDMS samples. The dashed lines are interpolations of the bulk melt data, scaled down by factors of 0.8, 0.45, and 0.3 to match the confined-sample data.

Mentions: It may be expected that this increase is not homogeneous across the melt droplet, yet the featureless nDQ data in these samples did not allow for any further conclusions. However, another very relevant observation concerns the separable signal tail of Iref, which has been attributed to unentangled, isotropically moving chain ends. It presumably arises from contour-length fluctuation effects,(36) and in a bulk melt, it contributes to a decrease in elasticity by diluting the entangled fraction. Figure 2a shows sample fits of this tail fraction, and Figure 2b demonstrates its temperature dependence in all samples. In the bulk melt, it increases with temperature up to a range of 30%, which then roughly corresponds to the 2/7 fraction expected for the two outermost chain segments with lengths corresponding to Me of a 7-fold entangled polymer chain. At lower temperatures, the chain dynamics is not fast enough to render the full Me outer part isotropic on the NMR time scale.


Confinement Effects on Chain Dynamics and Local Chain Order in Entangled Polymer Melts.

Ok S, Steinhart M, Serbescu A, Franz C, Vaca Chávez F, Saalwächter K - Macromolecules (2010)

(a) Exponential signal tails related to isotropically mobile chain ends, as fitted to Iref − IDQ for different PDMS samples at 280 K. (b) Chain-end fractions as a function of temperature for all PDMS samples. The dashed lines are interpolations of the bulk melt data, scaled down by factors of 0.8, 0.45, and 0.3 to match the confined-sample data.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2875088&req=5

fig2: (a) Exponential signal tails related to isotropically mobile chain ends, as fitted to Iref − IDQ for different PDMS samples at 280 K. (b) Chain-end fractions as a function of temperature for all PDMS samples. The dashed lines are interpolations of the bulk melt data, scaled down by factors of 0.8, 0.45, and 0.3 to match the confined-sample data.
Mentions: It may be expected that this increase is not homogeneous across the melt droplet, yet the featureless nDQ data in these samples did not allow for any further conclusions. However, another very relevant observation concerns the separable signal tail of Iref, which has been attributed to unentangled, isotropically moving chain ends. It presumably arises from contour-length fluctuation effects,(36) and in a bulk melt, it contributes to a decrease in elasticity by diluting the entangled fraction. Figure 2a shows sample fits of this tail fraction, and Figure 2b demonstrates its temperature dependence in all samples. In the bulk melt, it increases with temperature up to a range of 30%, which then roughly corresponds to the 2/7 fraction expected for the two outermost chain segments with lengths corresponding to Me of a 7-fold entangled polymer chain. At lower temperatures, the chain dynamics is not fast enough to render the full Me outer part isotropic on the NMR time scale.

View Article: PubMed Central - PubMed

Affiliation: Institut für Chemie, Universität Osnabrück, Barbarastr. 7, D-46069 Osnabrück, Germany.

AUTOMATICALLY GENERATED EXCERPT
Please rate it.

Consequently, a reduced interchain entanglement density has been discussed as the reason for enhanced flow of confined chains... In contrast, a “sticky” surface or local orientation effects and thus anisotropic dynamics of the segments close to an interface are thought to be the reason for enhanced elasticity or reduced diffusion coefficients... The only significant change is the overall upward shift of C(t/τe) by a factor of about 3, corresponding to a √3 ≈ 1.7-fold increase of the constraint-induced order... It may be expected that this increase is not homogeneous across the melt droplet, yet the featureless nDQ data in these samples did not allow for any further conclusions... This may be rationalized by a (rather long-ranged) geometrically wall-induced anisotropy acting on top of the tube constraint... For a rough estimate of the range of such an effect, we can assume that the center parts of the droplets are bulklike and that the confined parts do not show isotropic chain-end signals at all... At the lowest temperature of 263 K, which is about 90 K above Tg (just as for the lowest temperature of our PDMS samples), we observe only little confinement effects (the 20 nm sample exhibits a somewhat quicker initial rise), which we explain with the dominance of the much stronger entanglement-induced anisotropy to chain motion... In order to reconcile the different observations for PDMS and PB, we need to keep in mind that the estimates of the constrained fraction were based on different observations, respectively, i.e., missing isotropic chain-end parts (which were not significant for highly entangled PB) and the amount of polymer involved in significantly anisotropic segmental motion (which is too severely affected by terminal relaxation in lowly entangled PDMS)... More conclusive results will be drawn from our ongoing study of a larger molecular-weight range and from an in-depth analysis of the experimental data... For now, we hypothesize that the length scale of geometrically induced anisotropy of segmental motion might be related to the entanglement separation (Me) rather than the radius of gyration, explaining the rather large length scale of the effect observed for PDMS in the nanodroplets... Further, a recent inelastic neutron scattering study focusing on fast segmental motions of poly(ethylene oxide) in self-ordered AAO channels of 40 nm diameter concluded no significant confinement effects on translational diffusion (apart from the expected immobilized absorption layer), which, however, as pointed out in a recent rebuttal by Kimmich and Fatkullin, cannot be taken as evidence for the absence of a “corset effect” affecting rotational dynamics... Finally, the effect observed by us is significantly more pronounced than orientation effects described by Deloche and co-workers, who did not detect any changes in moderately entangled PDMS films above 25 nm thickness... However, their effect in films below 25 nm could be quantitatively attributed to immediately geometrically induced segmental anisotropy in combination with diffusive averaging, and our preliminary T2 relaxation experiments on macroscopically ordered membrane stacks confirmed that the observed anisotropy is orientation-dependent, suggesting a similar origin of the phenomena... In contrast, cylindrical confinement of highly entangled PB (M/Me ≈ 50) leads to an almost temperature-independent response indicating network-like segmental anisotropy in an estimated ∼3 nm surface layer.

No MeSH data available.