Local segmental dynamics

Our experimental measurements of the orientation autocorrelation function on sub-nanosecond time scales are consistent with the theoretical models for backbone motions proposed by Hall and Helfand(ll) and by Bendler and Yaris(12). The correlation functions observed in three different solvents at various temperatures have the same shape within experimental error. This implies that the fundamental character of the local segmental dynamics is the same in the different environments investigated. Analysis of the temperature dependence of the correlation function yields an activation energy of 7 kJ/mole for local segmental motions. 

We can analyze the temperature dependence of the local segmental dynamics in terms of the Kramers picture of diffusion over a barrierdS, 19). Presumably, more than one fundamental process contributes to the local motion observed in these experiments. In this case, we can write an expression for the rate wj of the ith process which contributes to the reorientation of the chromophore s transition dipole, in terms of the viscosity n and the activation energy E-j ... 

The above treatment implicitly assumes that the local segmental dynamics are not affected by changes in global chain... 

Plazek, 1995) become the local segmental dynamics at the highest frequencies prior to the onset of glassy behavior. 

Local Segmental Dynamics of Polyacrylates in Concentrated Chloroform Solutions... 

This article describes stochastic computer simulations of the local segmental dynamics of synthetic polymers. Particular attention is given to local dynamics in solution. Related work involving experimental methods, analytical theory, and molecular dynamics simulations will also be discussed. An introduction to the concepts involved in stochastic simulations will be presented. Methods of characterizing local segmental dynamics will also be described. The main portion of the article describes various features of the simulated dynamics. The approximations inherent in stochastic approaches and their influence on the observed dynamics will be discussed. Issues of cooperativity in conformational transitions will be highlighted. 

We close this section with a few comments about the organization of this review. Section 2 describes the BD technique and its relatimiship to other computer simulation methods. Section 3 introduces methods of characterizing local segmental dynamics. Most of the results on local dynamics obtained from BD simulations are discussed in Sect. 4. Section S highlights issues c cooperativ-ity in lcx al pofymer dynamics. The questicms posed in this introduc on will be addressed there. Although the material in Sect. S logically could have been included in Sect. 4, it has been treated separately because of the central importance ctf cooperativity in lcx al dynamics. 

Many different methods have been used to characterize the local segmental dynamics of polymer chains. In this section we review some of these methods. The material discussed in this section will be used in Sects. 4 and S. 

Helfand has characterized local segmental dynamics in polyethylene using orientation autocorrelation functions and a set of molecule fixed vectors [28]. 

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. 

When comparing various mentioned techniques used for characterization of local segmental dynamics in polymers, only NMR techniques do not require the presence of any label that can affect the chain dynamics on the other hand, its applicability requires appearance of suitable lines in the NMR spectrum of the polymer enrichment is frequently needed). The presence of a bulky chromophore label bonded in the main chain, which was required when applying the fluorescence technique, affects both main-chain conformation and dynamics in the vicinity of the signal group significantly. The ESR technique requires the presence of a spin label. 

Waldow DA, Ediger MD, Yamaguchi Y, Matsushita Y, Noda I (1991) Viscosity dependence of the local segmental dynamics of anthracene-labeled polystyrtme in dilute-solution. Macromolecules 24(11) 3147-3153. doi 10.1021/ma00011a018... 

Glowinkowski S, Gisser DJ, Ediger MD (1990) C-13 nuclear-magnetic-resonance measurements of local segmental dynamics of polyisoprene in dilute-solution— nonlinear viscosity dependence. Macromolecules 23(14) 3520-3530. doi 10.1021/ma00216a021... 

Hyde PD, Ediger MD, Kitano T, Ito K (1989) Local segmental dynamics of polyisoprene in concentrated-solutions and in the bulk. Macromolecules 22(5) 2253-2259. doi 10.1021/ ma00195a044... 

Naturally, a theory of polymer-blend dynamics is less desirable if it is constructed just for explaining the local segmental dynamics of polymer blends, but has no utility for the consideration of problems related to local segmental dynamics in homopolymers. Some examples of challenging problems regarding homopolymers will be discussed in Sections 2.5 and 2.6. 

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