Disodium 4,5,6-trihy­droxy­benzene-1,3-disulfonate dihydrate

In the title compound, 2Na+·C6H4O9S2 2−·2H2O, the benzene rings of the 4,5,6-trihy­droxy­benzene-1,3-disulfonate ions, which are stacked parallel to each other forming rods parallel to the a axis, are slightly deformed (planarity, symmetry) mainly because of the high degree of substitution. The two sodium ions, located within pockets of the anion rods, are coordinated by six and seven O atoms, resulting in octa­hedral and penta­gonal-bipyramidal coordinations, respectively. In addition to these coordinative bonds towards sodium, an extended network of intra- and inter­molecular hydrogen bonds occurs

Nanoscopic poly(ethylene oxide) strands embedded in semi-interpenetrating methacrylate networks. Preparation method and quantitative characterization by field-gradient NMR diffusometry

Networks of nanoscopic strands of linear, monodisperse poly(ethylene oxide) embedded in cross-linked methacrylate matrices were prepared. Depending on the choice of matrix constituents, the diameters of these strands can be varied considerably. The samples were characterized by DSC, TEM, SEM, and fringe field gradient NMR diffusometry with respect to the strand diameter. A formalism evaluating diffusive spin-echo attenuation curves based on the tube/reptation model is presented permitting the determination of the tube diameter. Values in the range from 8 to 58 nm were found in accordance with TEM micrographs of shadow-cast freeze-fractured surfaces of the samples

Reptation in artificial tubes and the corset effect of confined polymer dynamics

A spinodal demixing technique was employed for the preparation of linear poly(ethylene oxide) (PEO) confined to nanoscopic strands which in turn are embedded in a quasi-solid and impenetrable methacrylate matrix. Both the molecular weight of the PEO and the mean diameter of the strands are variable to a certain degree. Chain dynamics of the PEO in the molten state was examined with the aid of field-gradient NMR diffusometry (time scale: 10-2 s... 100 s) and field-cycling NMR relaxometry (time scale: 10 -9 s... 10-4 s). The dominating mechanism for translational displacements probed in the nanoscopic strands by either technique is shown to be reptation. A corresponding evaluation formalism for NMR difrusometry is presented. It permits the estimation of the mean PEO strand diameter. Depending on the chemical composition of the matrix, the diameters range from 9 to 58 nm. The strands were visualized by electron microscopy. On the time scale of spin-lattice relaxation time measurements, the frequency dependence signature of reptation, that is T1 ∝ νV 3/4, showed up in all samples. 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. This "corset effect" is traced back to the lack of the local fluctuation capacity of the free volume in nanoscopic confinements. The confinement dimension is estimated at which the cross-over from "confined" to "bulk" chain dynamics is expected

Reptation in artificial tubes and the corset effect of confined polymer dynamics

A spinodal demixing technique was employed for the preparation of linear poly(ethylene oxide) (PEO) confined to nanoscopic strands which in turn are embedded in a quasi-solid and impenetrable methacrylate matrix. Both the molecular weight of the PEO and the mean diameter of the strands are variable to a certain degree. Chain dynamics of the PEO in the molten state was examined with the aid of field-gradient NMR diffusometry (time scale: 10-2 s... 100 s) and field-cycling NMR relaxometry (time scale: 10 -9 s... 10-4 s). The dominating mechanism for translational displacements probed in the nanoscopic strands by either technique is shown to be reptation. A corresponding evaluation formalism for NMR difrusometry is presented. It permits the estimation of the mean PEO strand diameter. Depending on the chemical composition of the matrix, the diameters range from 9 to 58 nm. The strands were visualized by electron microscopy. On the time scale of spin-lattice relaxation time measurements, the frequency dependence signature of reptation, that is T1 ∝ νV 3/4, showed up in all samples. 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. This "corset effect" is traced back to the lack of the local fluctuation capacity of the free volume in nanoscopic confinements. The confinement dimension is estimated at which the cross-over from "confined" to "bulk" chain dynamics is expected

Field-cycling NMR relaxometry of polymers confined to artificial tubes: Verification of the exponent 3/4 in the spin-lattice relaxation dispersion predicted by the reptation model

2H field-cycling NMR relaxometry was applied to deuterated linear polyethyleneoxide (PEO) of different chain lengths confined in a porous matrix of cross-linked polyhydroxyethylmethacrylate (PHEMA). The PHEMA pore diameter was of the order of 10 nm, i.e. smaller than or similar to the dimension which the PEO coils would have in the unconfined melt. The frequency and molecular weight dependences of the spin-lattice relaxation time predicted by de Gennes as T1∝M0ω3/4 on a timescale short compared with the Rouse relaxation time are well reproduced experimentally

Polymer dynamics under nanoscopic constraints: The "corset effect" as revealed by NMR relaxometry and diffusometry

A spinodal demixing technique was employed for the preparation of linear poly(ethylene oxide) (PEO) confined in nanoscopic strands, which in turn are embedded in a quasi-solid methacrylate matrix impenetrable to PEO. Both the molecular weight of the PEO and the mean diameter of the strands are variable to a certain degree. Chain dynamics of the PEO in the molten state were examined with the aid of field-gradient NMR diffusometry and field-cycling NMR relaxometry. The dominating mechanism for translational displacements in the nanoscopic strands is shown to be reptation. A formalism for the evaluation of NMR diffusometry is presented, which permits the estimation of the mean PEO strand diameter. Samples of different composition revealed diameters in the range 9-58 nm, in reasonable agreement with electron micrographs. The time scale of the diffusion measurements was 10-300 ms. On the much shorter time scale of field-cycling NMR relaxometry, 10-9-10-4 s, a frequency dispersion of the spin-lattice relaxation time characteristic for reptation clearly showed up in all samples. An effective tube diameter of only 0.6 nm was found even when the strand diameter was larger than the radius of gyration of the PEO chain random coils. The finding that the tube diameter effective on the short time scale of field-cycling NMR relaxometry is much smaller than the diameter of the confining structure is termed the "corset effect", and is traced back to the lack of local free-volume fluctuation capacity under nanoscale confinements. The order of magnitude of the "pore" diameter, at which the cross-over from confined to bulk chain dynamics is expected, is estimated

The confined-to-bulk dynamics transition of polymer melts in nanoscopic pores of solid matrices with varying pore diameter

The confinement of polymer melts in nanoscopic pores leads to chain dynamics significantly different from bulk behaviour. This so-called 'corset effect' occurs both above and below the critical molecular mass and induces dynamic features as predicted for reptation. The confined-to-bulk dynamics crossover is treated analytically on the basis of general thermodynamic relations connected to the fluctuation of the number of particles (Kuhn segments) in a given volume. Bulk behaviour is shown to occur only if the pore diameter complies with the limit dpore ≫ (b3/k BTκT )1/3RF ≈ 10R F , where b is the Kuhn segment length, κT the isothermal compressibility, T the temperature, kB the Boltzmann constant and RF the Flory radius. For smaller pores, the confined polymer chains reptate along their own contours in tubes with an effective diameter d ≈ √b2ρskBTκ T ≈ 0.5 nm, where ρs is the number density of Kuhn segments. From the theoretical point of view, the crucial factors on which the corset effect is based are (i) impenetrable pore walls, (ii) low compressibility and (iii) the uncrossability of polymer chains

Intra- and inter-chain fluctuations in entangled polymer melts in bulk and confined to pore channels

It is known that topological restraints by "chain entanglements" severely affect chain dynamics in polymer melts. In this field-cycling NMR relaxometry and fringe-field NMR diffusometry study, melts of linear polymers in bulk and confined to pores in a solid matrix are compared. The diameter of the pore channels was 10 nm. It is shown that the dynamics of chains in bulk dramatically deviate from those observed under pore constraints. In the latter case, one of the most indicative signatures of the reptation model is verified 28 years after its prediction by de Gennes: The frequency and molecular mass dependencies of the spin-lattice relaxation time obey the power law T! ∝ M0v3/4 on a time scale shorter than the longest Rouse relaxation time τR. The mean squared segment displacement in the pores was also found to be compatible to the reptation law ∝ M-1/2t1/2 predicted for τR <t<τd, where τd is the so-called disengagement time. Contrary to these findings, bulk melts of entangled polymers show frequency and molecular mass dependencies significantly different from what one expects on the basis of the reptation model. The data can however be described with the aid of the renormalized Rouse theory

The "corset effect" of spin-lattice relaxation in polymer melts confined in nanoporous media

Linear polyethylene oxides with molecular weights Mw of 1665 and 10170 confined in pores with variable diameters in a solid methacrylate matrix were studied by proton field-cycling nuclear magnetic resonance relaxometry. The pore diameter was varied in the range of 9-57 nm. In all cases, the spin-lattice relaxation time shows a frequency dependence close to T1 ∝ ν3/4 in the range of ν = 3·10 -1-2·101 MHz as predicted by the tube-reptation model. This proton T1 dispersion essentially reproduces that found in a previous deuteron study (R. Kimmich, R.-O. Seitter, U. Beginn, M. Möller, N. Fatkullin: Chem. Phys. Lett. 307, 147, 1999). As a feature particularly characteristic for reptation, this finding suggests that reptation is the dominating chain dynamics mechanism under pore confinement in the corresponding time range. The absolute values of the spin-lattice relaxation times indicate that the diameter of the effective tubes in which reptation occurs is much smaller than the pore diameters on the time scale of spin-lattice relaxation experiments. An estimation leads to a value d* ∼ 0.5 nm. The impenetrability of the solid pore walls, the uncrossability of polymer chains (·excluded volume·) and the low value of the compressibility in polymer melts create the ·corset effect· which reduces the lateral motions of polymer chains to a microscopic scale of only a few tenths of a nanometer