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...

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SYNOPSISTwo series of segmented polyurethane elastomers based on 4,4'-diisocyanato dicyclohexylmethane were investigated with respect to their thermal properties and deformation behavior. We used a crystallizable soft segment, 1,4-poly( tetramethylene glycol), for one series and a noncrystallizable soft segment, 1,2-poly(propylene glycol), for the other. Both systems exhibited mechanical self-reinforcement that depended strongly on the deformation rate. We propose a mechanism for the observed stress/strain behavior in terms of two competing processes: ( 1 ) the buildup of orientation caused by deformation and ( 2 ) the loss of orientation caused by plastic slippage and segmental relaxation during deformation. The kinetics of these processes depend strongly on the deformation rate and overall molecular weight.

Pulse propagation in SF 6 has been investigated using 200-nsec pulses of a C0 2 laser. Operating at A. = 10.53 jxni (PI4 transition) we have observed a breakup of the pulse envelope, an oscillatory energy transmission, and an energy-dependent pulse delay. We interpret our data in terms of the existing theory of self-induced transparency; the corresponding Bloch-type equations were derived assuming thermal equilibrium between rotational M sublevels.