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Author(s): V. Percec (corresponding author) [1]; M. Glodde [1]; T. K. Bera [1]; Y. Miura [1]; I. Shiyanovskaya [2]; K. D. Singer [2]; V. S. K. Balagurusamy [1, 3]; P. [...]

Orientation molecular dynamics were investigated in a series of “defect-free” oligofluorenes by depolarized dynamic light scattering and dynamic NMR spectroscopy. Typical liquid crystalline pretransitional dynamics were observed upon cooling the isotropic phase to the liquid crystalline phase with strong increase of the scattered intensity and slowing down of the characteristic time of the probed collective relaxation. This is well accounted for by the Landau-de Gennes theory, however, with a strong temperature dependence of the viscosity coefficient, reflecting the proximity of the glass transition. For the trimer the two transitions almost overlap and the molecular orientation coincide with the α-relaxation associated with the glass transition. The NMR measurements confirm that the time scale of the dynamics is completely governed by the glass process, yet the geometry of the motion is anisotropic, yielding order parameters ranging from 0.15 to 0.25 for the long axis in the liquid crystalline phase. The glass transition is therefore geometrically restricted with poorly ordered mesophase which is consistent with the weak transverse phonons in the light scattering experiment down to Tg+20 K. [ABSTRACT FROM AUTHOR]

The local, temperature dependent structure of poly-(ethylmethacrylate) was studied with wide-angle x-ray scattering. The results were set into context to recent wide-angle neutron scattering results as well as to the results from a multitude of studies of the dynamics of this polymer. The temperature dependence of the wide angle x-ray results point to the development of local order which is backed by the neutron scattering results and which is connected to characteristic temperatures of the relaxation dynamic Tg and Tc. The poly-(ethylmethacrylate) was studied in its predominantly syndiotactic as well as predominantly isotactic state displaying vastly different local structures as manifested in the x-ray results. © 2005 American Institute of Physics. [ABSTRACT FROM AUTHOR]

Analytical expressions for transverse electron spin relaxation induced by small angle motion were derived for the first time within an anisotropic model for rotational diffusion by using an approximation of the spin Hamiltonian and its variation during reorientation that is valid close to the canonical orientations. The dependence of the decay of the stimulated echo on such motion was studied by extensive Monte Carlo simulations and regimes were identified in which the time constant of this decay is related to parameters of the anisotropic diffusion model by simple equations. For testing these theoretical findings and obtaining insight into hierarchical cage dynamics in soft matter, high-field electron paramagnetic resonance (EPR) measurements were performed at a frequency of 94 GHz where the canonical orientations for nitroxide spin labels are well resolved. A combination of continuous wave EPR, saturation recovery measurements, and measurements of the decay of primary and stimulated electron spin echoes was employed to cover time scales from a few picoseconds up to several microseconds. Ionic spin probes attached by electrostatic interactions to the surface of ionic clusters in ionomers were used as a model system in which slow cage reorientation can be studied in the glass transition region of the polymer (0.64g<1.05). Three hierarchical reorientation processes of the spin probe were observed on different time scales. The spin probe undergoes fast intramolecular libration on the time scale of a few picoseconds, it experiences a local rearrangement of the cage on the time scale of hundreds of nanoseconds and it performs cooperative reorientation coupled to the structural relaxation of the glassy matrix over time scales comparable to or longer than several microseconds in the glass transition region. © 2003 American Institute of Physics. [ABSTRACT FROM AUTHOR]

The complex dynamics in supramolecular structures of poly(p-phenylenes) with short ethylene oxide (EO) side chains (known as "hairy rods") have been studied by combining temperature and pressure dependent dielectric spectroscopy (DS) with different nuclear magnetic resonance (NMR) techniques. The system forms a periodic lattice of layers of the main chains separated by layers of the amorphous EO chains. Since the dipole moment is located on the EO side chain, DS is probing mainly the side-chain motions and only indirectly the backbone dynamics. On the other hand, the different NMR techniques are also sensitive to the backbone mobility and provide the molecular origin of the dynamic processes. Two dielectrically active processes exist, a "fast" β (which differs from the usual Johari-Goldstein β process found in homopolymers and low molecular weight glass-forming liquids) and a "slow" α process with the latter carrying most of the intensity. Comparison of the NMR and dielectric results identifies the outer EO units as solely responsible for the dielectric β process whereas both the short EO segments and the unfreezing of the internal backbone dynamics including bending motions contribute to the α process. The unfreezing of the backbone dynamics does not lead to the dissolution of the structure that only happens at even higher temperatures. Hairy rod molecules thus can serve as model systems which provide the means of studying the effect of confinement on the dynamics. In this respect, the effect of confinement is mainly on the temperature dependence of the α-process relaxation times. [ABSTRACT FROM AUTHOR]

The two-dimensional (2D) exchange nuclear magnetic resonance (NMR) experiment is applied to study ultraslow as well as faster chain motions in amorphous polymers in the glass transition range. Acquisition of the time domain data with a four-pulse sequence leads to new characteristics in the corresponding 2H 2D line shapes if the correlation times of the motion are in the intermediate dynamic range. From the asymmetric 2D line shapes, the width of the correlation time distribution can be determined with higher accuracy than through conventional 1D NMR methods. Experimental data are presented on two amorphous polymers—atactic polypropylene and cis-1,4-polyisoprene—and are analyzed in terms of isotropic rotational diffusion. Deviations from this simple model due to the presence of conformational transitions within the polymer backbone are detected. Close to Tg, the mean correlation times extracted from 2D exchange NMR exhibit strongly nonArrhenius behavior usually described by the Williams–Landel–Ferry (WLF) equation. In addition, the width of the correlation time distribution is found to decrease with increasing temperature. [ABSTRACT FROM AUTHOR]