Line shape analysis, polymer dynamics

In the solid, dynamics occurring within the kHz frequency scale can be examined by line-shape analysis of 2H or 13C (or 15N) NMR spectra by respective quadrupolar and CSA interactions, isotropic peaks16,59-62 or dipolar couplings based on dipolar chemical shift correlation experiments.63-65 In the former, tyrosine or phenylalanine dynamics of Leu-enkephalin are examined at frequencies of 103-104 Hz by 2H NMR of deuterated samples and at 1.3 x 102 Hz by 13C CPMAS, respectively.60-62 In the latter, dipolar interactions between the 1H-1H and 1H-13C (or 3H-15N) pairs are determined by a 2D-MAS SLF technique such as wide-line separation (WISE)63 and dipolar chemical shift separation (DIP-SHIFT)64,65 or Lee-Goldburg CP (LGCP) NMR,66 respectively. In the WISE experiment, the XH wide-line spectrum of the blend polymers consists of a rather featureless superposition of components with different dipolar widths which can be separated in the second frequency dimension and related to structural units according to their 13C chemical shifts.63... 

Variable temperature study with a subsequent line shape analysis enables a clear insight of the temperature-dependent dynamics of the system, where the glass transition temperature (Tg) of the amorphous phase, melting point or a crystallization onset of a present crystalline phase, or any other changes caused by altered dynamics of the involved species can be assessed. This was recognized by early investigations of polymer electrolytes and will be discussed in greater detail later. 

This chapter is organized as follows the fundamentals of ESR spectroscopy are described in Sections 2.08.2.1-2.08.2.3, with emphasis on the experimentally available parameters relevant for duddation of the polymer stmcture, dynamics, transport, and stability. line-shape analysis of nitroxide spin probes is given in detail in Section 2.08.2.4. Advanced ESR methods, induding HF-ESR and time-domain ESR as wdl as double resonance methods are outlined in Sections 2.08.2.5-2.08.2.7. The spin trapping method is described in Section 2.08.2.8. The transition from ESR spectroscopy to ESRI is presented in Section 2.08.3. Spedfic and important, applications of ESR spectroscopy and ESRI to polymeric systems are described in Sections 2.08.4.1-2.08.4.4. We will condude with an evaluation of the strengths of ESR methods, and our view on further applications of this approach and on areas that require further devdopment. 

Pulsed deuteron NMR is described, which has recently been developed to become a powerftd tool for studying molectdar order and dynamics in solid polymers. In drawn fibres the complete orientational distribution function for the polymer chains can be determined from the analysis of deuteron NMR line shapes. By analyzing the line shapes of 2H absorption spectra and spectra obtained via solid echo and spin alignment, respectively, both type and timescale of rotational motions can be determined over an extraordinary wide range of characteristic frequencies, approximately 10 MHz to 1 Hz. In addition, motional heterogeneities can be detected and the resulting distribution of correlation times can directly be determined. 

More recently, Noda has proposed the use of infrared two-dimensional correlation spectroscopy (2D-IR) to increase the information that can be extracted from a spectrum. This approach, essentially different from 2D-NMR spectroscopy, uses correlation analysis of the dynamic fluctuations caused by an external perturbation to enhance spectral resolution without assuming any line shape model for the bands. The technique was intended for the study of polymers and liquid crystals, and it has recently been applied to proteins. In the latter case, the perturbation can be achieved through changes in temperature, pH, ligand concentration and lipid-to-protein ratio. 

The ESR studies of local segmental dynamics of polymers benefited from the progress achieved in theoretical description of the effects of dynamics on line shape of nitroxide ESR spectra and in resulting software. The Schneider-Freed set of programs made possible the analysis of slow-motional spectra of nitroxides subjected to anisotropic rotational diffusion with rotation symmetry axis oriented quite arbitrarily with respect to the nitroxide axis system. The simulation of one such ESR spectrum with computers available in the 1980s required 20 min of computer time and the best fits to experimental spectra were found by visual comparison simulated spectra with experimental ones. 

We have performed an analysis of the entire MoBbauer line shape taking into aecount a density fluctuation correlation function of the Kohlrausch type 0(f) = aexp( r /r) with P = 1/2 and the relaxation time t. We could thus also follow the onset of the structural relaxation a process related to the dynamic glass transition. The relaxation times decrease from about 10 s at to 5 x 10 s at 300 K. Details of these studies especially concerning the behaviour of swollen polymers will be given elsewhere. 

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