Elmar Fischer

Characterization of polymer networks using the dipolar correlation effect on the stimulated echo and field-cycling nuclear-magnetic resonance relaxometry

Chain dynamics in a series of styrene-butadiene rubbers (SBR) was studied with the aid of the dipolar correlation effect (DCE) and field-cycling NMR relaxometry (FCR). The typical time scales of the two techniques are t>10−4 s and t<10−3 s, respectively, and therefore complementary. The crosslink density of the polymer networks was varied in a wide range. In order to prevent sinusoidal undulations of the stimulated-echo attenuation curves due to spin exchange between groups with different chemical-shift offsets, the DCE of the samples was examined using a modified radio frequency pulse sequence with additional π pulses inserted in the free-evolution intervals. Residual dipolar couplings can thus be probed in samples where chemical-shift and dipolar interactions are of the same order. The dipolar correlations probed with the DCE in SBR networks turned out to exist on a time scale exceeding 300 ms. The short-time fluctuations (probed by FCR) and the long-time dynamics (probed by DCE) can be approached by po...

Spin diffusion in melts of entangled polymers

Based on theoretical considerations [N. F. Fatkullin, Sov. Phys. JETP 72, 563 (1991)], immaterial spin diffusion mediated by flip–flop transitions of dipolar coupled spins on different macromolecules was predicted to influence the diffusion coefficient measured in nuclear magnetic resonance field-gradient experiments. In order to test this hypothesis, we have carried out supercon fringe field proton magnetic resonance diffusometry experiments with polyethylene oxide melts (Mw=438 000) using field gradients of up to 60 T/m. The polymer chains were dispersed in a matrix of deuterated chains of an equivalent molecular mass. The time-dependent segment diffusion coefficients measured in the diluted and undiluted polymer coincided for diffusion times below about 200 ms. However, increasing the diffusion time up to 1 s leads to a reduction of the diffusion coefficient in the deuterated matrix by a factor of about 2 relative to the undeuterated system. The long-time diffusion coefficient measured with long polyme...

Shear flow affects secondary and tertiary structures in hyaluronan solution as shown by rheo-NMR

Abstract 13 C NMR spectra of hyaluronan (HA) in aqueous solution have been recorded under shear in a Couette viscometer. In shear-thinning conditions as determined in mechanical rheometry experiments the chemical shifts of glycosidic linkage carbons moved to higher frequency. This result is consistent with a more stretched conformation for the chain. Shear-dependant broadening and extension to downfield of the acetamido CO resonance are compatible with a partial switch from β-sheet (tertiary structure) NH H-bonded forms to non-H-bonded states, suggestive of some disruption of the β-sheet. By contrast, the chemical shifts of sugar ring carbons did not deviate from values found in the solution at rest.

NMR field gradient diffusometry of segment displacements in melts of entangled polymers

Segment diffusion in a polyethyleneoxide melt (Mw = 5 000 000) was studied with the aid of the supercon fringe field version of field‐gradient NMR diffusometry. The evaluation based on the second moment of the probability density function, i.e., the mean squared displacement, shows reasonable agreement with the predictions of the tube/reptation model. However, taking into account the whole probability density function, leads to substantial discrepancies.

Characterization of director fluctuations in liquid crystals using the dipolar correlation effect of the stimulated echo

Abstract The purpose of this work is to show that the stimulated-echo pulse sequence can be employed for the examination of slow collective motions in ordered systems. In nematics of low molecular weight (4′- n -pentyl-4-cyanobiphenyl) the attenuation of the ratio of the stimulated and primary echo amplitudes was measured as a function of the time delay between the first and the second 90° pulses. The attenuation of the echo quotient as well as the atypical (compared to the isotropic phase) oscillating behavior of the attenuation curves are described by means of stochastically time dependent interactions of dipolar coupled proton pairs. Slow director fluctuations are considered as a mechanism tending to average these interactions. Good agreement with the experimental data was obtained.

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 … 10 0 s) and fieldcycling 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 diffusometry 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 3/4 1 ν ∝ T , 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.