Introduction.One of the most prominent features of polycarbonate (PC) is its ductility, leading to a yield point at approximately 60 MPa stress accompanied by a strain of 7 % .l Dynamic mechanical measurements were performed to obtain knowledge about the molecular motions involved. From these studies it was already concluded that the low-temperature y relaxation has to be connected with the phenylene motions, since the substitution of the ring protons changes the temperature of this maximum significantly while variations of the other groups cause only slight shifts, if at alL2 Only a few alterations of the isopropylidene group exhibit larger effects, which are supposed to be generated by steric hindrance or reduced cooperativity of the phenylene ring motions. Loss modulus spectra of strained PC exhibit intensification and broadening of the y peakwith increasing strain, but no significant temperature shift is ob~erved.~ Thus, the relaxation strength is enhanced, which means that the phenylene motions should be enlarged without alteration of the correlation times. Deuteron as well as 13C NMR experiments demonstrated that rapid 180' flips of the phenylene rings around their l,4-axis, augmented by small-angle fluctuations, occur in the glassy state on the time scale of microseconds toThe mixing of PC with additives like p-dichlorobenzene or polychlorinated biphenyls causes the material to become brittle and slows down the ring motion significantly by broadening the distribution of correlation times and shifting its mean value to lower freq~encies.~ Simultaneously, the y relaxation peak is suppressed: indicating that these properties are correlated.Further, infrared dichroism measurements of PC at small strains (up to 2 5% ) suggest that the transition from linear (below 1 % strain) to nonlinear behavior is accompanied by the onset of rotations around backbone bonds.1° All these methods lack direct information about the geometry of the involved motions. Therefore, in this investigation the deuteron two-dimensional (2D) exchange NMR experiment was applied to study ultraslow reorientations of the phenylene groups in PC under tensile stress up to 50 MPa (3% strain). By this means, the geometrical information can be obtained directly and model-free by inspecting the off-diagonal intensities in the 2D plane," a method that has been successfully applied already to study chain motions in crystalline polymers12 as well as amorphous polymers13 at their glass transition.In this paper only the main results of this study are presented, namely, those concerning the 2D 2H NMR experiment and the spin alignment echo14 measurements for the determination of slow motions on the time scale of milliseconds to hundreds of milliseconds and their dependence on applied tensile stress. Further details as well as solid-echo spectra and the results of TI relaxation measurements under hydrostatic pressure (NMR techniques sensitive t o faster motions) are published elsewhere.15J6 Sample Preparation and Instrumentation. Phenylene-deuterated, amorph...

The fouling behavior of microstructured hollow fibers was investigated in constant flux filtrations of colloidal silica and sodium alginate. It was observed that the fouling resistance increases faster with structured fibers than with round fibers. Reversibility of structured fibers' fouling was similar during silica filtrations and better in sodium alginate filtrations when compared with round fibers. The deposition of two different silica sols on the membranes was observed by NMR imaging. The sols had different particle size and solution ionic strength and showed different deposition behaviors. For the smaller particle-sized sol in deionized solution (Ludox-TMA), there was more deposition within the grooves of the structured fibers and much less on the fins. For the alkali-stabilized sol Bindzil 9950, which had larger particles, the deposition was homogeneous across the surface of the structured fiber, and the thickness of the deposit was similar to that on the round fiber. This difference between the deposition behavior of the two sols is explained by differences in the back diffusion, which creates concentration polarization layers with different resistances. The Ludox sol formed a thick polarization layer with very low resistance. The Bindzil sol formed a slightly thinner polarization layer; however, its resistance was much higher, of similar magnitude as the intrinsic membrane resistance. This high resistance of the polarization layer during the Bindzil sol filtration is considered to lead to quick flow regulation toward equalizing the resistance along the fiber surface. The Ludox particles were trapped at the bottom of the grooves as a result of reduced back diffusion. The fouling behavior in sodium alginate filtrations was explained by considering the size-dependent deposition within the broad alginate size distribution. The better reversibility of fouling in the structured fibers is thought to be the result of a looser deposit within the grooves, which is more easily removed than a compressed deposit on the round fibers.

Axial dispersion and wall effects in narrow fixed beds with aspect ratios < 10 were investigated, both by classical methods and by NMR imaging. The residence time distribution (RTD) in the center and at the wall was measured, system water/ NaCl-solution as tracer, and subsequently compared with radial velocity profiles based on NMR imaging. The influence of the aspect ratio and Re p on dispersion and on the degree of non-uniformity of the velocity profile was studied. The NMR results are consistent with the RTD and also with literature data of numerical simulations. For low aspect ratios, dispersion/wall effects have a strong influence on the reactor behavior, above all, in cases where a low effluent concentration is essential, as proven by breakthrough experiments with the reaction of H 2 S with ZnO.

Introduction. Molecular order and the ratio between crystalline and amorphous portions are important factors determining the macroscopic properties of polymeric materials.1 Spatial variations of the molecular parameters, depending on the processing conditions, lead to nonuniform and spatial-dependent macroscopic behavior. For a systematic improvement of the manufacturing process, methods which record the spatial dependence of order and crystallinity are in need. On a phantom of deuterated polyethylene we have recently shown that solid-state 2H NMR spectroscopy, in combination with magneticfield gradients for spatial encoding, can indeed provide the desired information for bulk samples.2 It is the purpose of this paper to demonstrate that spectroscopic images with 13C in natural abundance can be recorded for a "real" polymer material, where spatial heterogeneities are known to exist.

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