G. Fleischer

Translational and rotational diffusion in supercooled orthoterphenyl close to the glass transition

Self diffusion coefficients in supercooled orthoterphenyl (OTP) have been determined down toDt=3·10−14 m2s−1 using a1H-NMR technique applying static field gradients up to 53T m−1 In a range of more than two decades theDt values agree with those of photochromic tracer molecules of the same size determined by forced Rayleigh scattering down to the glass transition temperatureTg. A change of mechanism is found for translational diffusion atTc≈1.2Tg whereDt is proportional to the inverse shear viscosityη−1 atT>Tc butDt ∼ηξ with ξ=0.75 atT<Tc. Rotational correlation times determined by2H-NMR stimulated echo techniques in deuterated OTP remain proportinal toη−1 down toTg. Our results are discussed in relation with mode coupling theory and with models of cooperative motion at the glass transition.

Self-diffusion of water in cartilage and cartilage components as studied by pulsed field gradient NMR.

Pulsed field gradient (PFG) nuclear magnetic resonance (NMR) was used to investigate the self‐diffusion behavior of water molecules in cartilage, polymeric cartilage components, and different model polymers. The short‐time self‐diffusion coefficients (diffusion time Δ ≈ 13 msec) are found to decrease steadily with decreasing water content. This holds equally well for cartilage and cartilage components. The short‐time diffusion coefficients are subjected to a rather nonspecific obstruction effect and mainly depend on the water content of the sample. The long‐time diffusion coefficients in cartilage (Δ ≈ 500 msec), however, reflect structural properties of this tissue. Measurements with varying observation times as well as experiments involving enzymatic treatment of articular cartilage suggest that the collagenous network in cartilage is likely to be responsible for the observed restricted diffusion. Magn Reson Med 41:285–292, 1999.

Temperature dependence of self diffusion of polystyrene and polyethylene in the melt. An interpretation in terms of the free volume theory

SummaryThe temperatur dependence of the self diffusion coefficients of polystyrene and polyethylene in the melt was measured with the pulsed field gradient nmr technique. The temperature and molar mass dependences of the self diffusion coefficients can be described by the free volume model. Taking into account the matrix effect we detected the beginning of the break of the reptation process for polystyrene at low molar masses. The activation energies of the self diffusion process are comparable with those observed for viscosity.

Self Diffusion in Melts of Polystyrene and Polyethylene Measured by Pulsed Field Gradient NMR

SummaryThe self diffusion coefficient of melts of four monodisperse molecular weight polystyrenes and nine fractions of linear and branched polyethylene was measured by pulsed field gradient nmr. The relation D∼N−2 holds in the range of N (number of monomeric units) from about 20 up to 1500, i.e. the diffusion goes via reptation. No break was observed at the critical molecular weight MC. Due to long chain branches in polyethylene the self diffusion coefficient is lowered considerably.

Ion transport in gel electrolytes

Abstract Gel electrolyte films were prepared by a redox initiated copolymerization of oligo(ethylene glycol) 23 dimethacrylate and acrylonitrile in the presence of oligo(ethylene glycol) 11 dimethyl ether as a plasticizer and LiCF 3 SO 3 . The influence of the polar comonomer acrylonitrile on the conductivity and the cationic transference number was investigated. The materials show conductivities between 10 −4 and 10 −5 S cm and transference numbers in the range of 0.5 at room temperature. Pulsed field gradient-NMR (PFG-NMR) measurements were carried out to determine the diffusion coefficients of the plasticizer by 1 H-NMR and of LiCF 3 SO 3 by 19 F-NMR.

A pulsed field gradient NMR study of diffusion in semicrystalline polymers: self-diffusion of alkanes in polyethylenes

By means of the pulsed field gradient NMR technique the self-diffusion of six alkanes (from n-butane to n-pentadecane) in three low density polyethylenes and one high density polyethylene differently thermally treated was examined. The concentration dependence could be described very satisfactorily with the free volume theory in the form of Fujita (Adv. Polymer Sci. 3(1961) 1). The parameter B of the diffusants and the fractional free volumef2 of the polyethylenes were determined from the experimental data. The fractional free volumesf2 show a strong dependence on the type of polyethylene, the main influence results from the different content of CH3 groups or short chain branches. The diffusion coefficient extrapolated to zero diffusant concentration is proportional to the eighth power of the amorphous content. This strong dependence shows that the free volumes of the amorphous parts of the polyethylenes are intimately connected with crystallinity, both determined by the different degrees of short chain branching. The pre-exponential factor in the free volume expression decreases with increasing amorphous content of the polyethylenes and increases with increasing length of the diffusants. It was found that the spherulite boundaries in the polyethylenes do not act as diffusion barriers.

The chain length dependence of self-diffusion in melts of polyethylene and polystyrene

The self-diffusion coefficients in melts of polyethylene fractions and polystyrene standards were measured by the NMR pulsed field gradient technique and compared with those measured by other techniques. The data agree very well if one takes into account the molar mass distribution of the samples and the free volume of the matrix. For molar masses much higher than the critical molar massMc, reptation is confirmed,D ∼M−2 holds. BelowMe=Mc/2 the self-diffusion coefficients corrected for constant free volume show approximately the dependenceD ∼M−1 confirming Rouse-like diffusion. This result was also obtained by investigating the self-diffusion of the molecules with different molar masses of a polyethylene fraction with a rather broad molar mass distribution aroundMe andMc, i. e. diffusion in a constant matrix. In the molar mass region betweenMc and about 3 ·Mc the observed molar mass dependence of self-diffusion can be explained by tube formation. The constraint release model of Graessley seems to slightly overestimate the self-diffusion coefficients.

Dynamics of solutions of triblock copolymers in a selective solvent: Effect of varying copolymer concentration

We have used static and dynamic light scattering and pulsed field gradient NMR to study the effect of varying concentration on the dynamics of the triblock copolymer, polystyrene block poly(ethylene, butylene) block polystyrene (PS-PEB-PS), dissolved in n-heptane, a selective solvent for the middle block. The correlation function for a dilute solution with c = 0.49 % (w/v) corresponds to the translational diffusion of micelles. At intermediate concentrations [1.1 ≤ c ≤ 2.6 % (w/v)], the correlation functions can be fitted to the sum of a single exponential and stretched exponential functions. The slower mode is due to the diffusion of polydisperse clusters formed by random association of triblock copolymer molecules and the faster one again represents micelles. A complex behavior is observed in the semidilute region [4.0 ≤ c ≤ 6.9 % (w/v)]. Three dynamic processes can be extracted from the correlation function: (i) The fast diffusive mode is the collective diffusion mode in the physical gel, (ii) the middle, relaxational mode, is probably due to the local movement of insoluble domains trapped in the network of the physical gel, and (iii) the slow diffusive mode implies the existence of large-scale inhomogeneities in the system.

NMR as a Generalized Incoherent Scattering Experiment

Two types of stimulated NMR echo experiments, 2H-spin alignment and field gradient NMR, are formulated in terms of a “generalized dynamic scattering function”. The analogies to incoherent quasielastic neutron scattering are discussed. The concept is illustrated by selected examples covering molecular reorientations and self diffusion in molecular crystals and supercooled liquids, anomalous diffusion in linear chain polymers and restricted diffusion of molecules in confined geometries.

NMR diffusion studies in heterogeneous systems

Abstract Pulsed field gradient NMR is a powerful tool for the investigation of molecular diffusion. Being able to trace molecular displacements in the micrometer range over observation times of typically milliseconds to seconds, this technique is particularly suitable for the study of molecular propagation in heterogeneous systems. After an introduction to the fundamentals and limitations of the technique, its versatility is illustrated by recent examples of application to the study of molecular transport in interface systems and polymers.