Local mobility

The investigation of structural dynamics of CP is particularly topical in connection with the establishment of correlation between local intramolecular mobility and the reactivity of chain fragments. It has been established that groups located in the most mobile parts of the polymer chain exhibit the greatest reactivity [48], The chemical heterogeneity in relationship to local mobility is particularly... 

Burns, A. R., Frankel, D. J. and Buranda, T. (2005) Local mobility in hpid domains of supported bilayers characterized by atomic force microscopy and fluorescence correlation spectroscopy. Biophys. J., 89, 1081-1093. 

The last feature about micellar dynamics is related to the local mobility of chain segments in the core or in the corona of the micelles. SAXS, SANS, and fluorescence techniques have proven to be effective for obtaining information about chain conformation and dynamics in the different micellar... 

Here we describe the model selection algorithm that is used to derive microdynamic (model-free) parameters for each NH group from 15N relaxation data. It is implemented in our program DYNAMICS [9]. Given the overall rotational diffusion tensor parameters (isotropic or anisotropic) derived as described above, this analysis is performed independently for each NH-group in order to characterize its local mobility. 

Which model provides the best representation for local mobility in a particular group remains unclear, as a detailed picture of protein dynamics is yet to be painted. This information is not directly available from NMR measurements that are necessarily limited by the number of experimentally available parameters. Additional knowledge is required in order to translate these experimental data into a reliable motional picture of a protein. At this stage, molecular dynamic simulations could prove extremely valuable, because they can provide complete characterization of atomic motions for all atoms in a molecule and at all instants of the simulated trajectory. This direction becomes particularly promising with the current progress in computational resources, when the length of a simulated trajectory approaches the NMR-relevant time scales [23, 63, 64]. 

The reduction of the long-range diffusivity, Di by a factor of four with respect to bulk water can be attributed to the random morphology of the nanoporous network (i.e., effects of connectivity and tortuosity of nanopores). For comparison, the water self-diffusion coefficient in Nafion measured by PFG-NMR is = 0.58 x 10 cm s at T = 15. Notice that PFG-NMR probes mobilities over length scales > 0.1 /rm. Comparison of QENS and PFG-NMR studies thus reveals that the local mobility of water in Nafion is almost bulk-like within the confined domains at the nanometer scale and that the effective water diffusivity decreases due to the channeling of water molecules through the network of randomly interconnected and tortuous water-filled domains. ... 

Polarized Luminescence. The polarized luminescence technique gives information about local mobility of polymer sequences, and so about microscopic structure of the system (18-21. ... 

The mere fact that the reaction proceeds in the glass-like state implies that for the chemical reaction to occur a small-scale local mobility (possibly P-relaxation) is sufficient, whereas the segmental mobility provides for a translational diffusion of the functional groups in the system3,70,71). 

Within the frame of a program on simulation of the Brownian motion of chain molecules, the conformational static and dynamic properties of a model of PE are studied. In the present paper the same properties are systematically derived by using the RIS theory. As expected, there Is good agreement for static properties such as conformational averages and chain dimensions, in addition the local mobility of the chain Is favorably compared by the aid of the two approaches. 

N 123 "The Long-Range Stiffness and Local Mobility of Double Stranded DNA ... 

Theoretical conformational analysis provides a basis for understanding the unique features of double-stranded DNA In terms of its chemical architecture. The well-known stiffness of the chain as a whole derives from the sequence of heterocyclic bases, while the local mobility of the constituent nucleotides reflects the structural complexity of the sugar-phosphate backbone. 

Polymer networks can be formed by chemical reactions between polymer chains (cross-linking) or by using trifunctional comonomers during the polymerisation. If such a network is dissolved in a second monomer and this second monomer is again polymerized into a second network, one obtains a structure in which both polymers are intertwined. These polymer chains only have very local mobility. In cases where both polymers are partially or completely immiscible the L1/L2 phase-separation is reduced to a very small scale. The properties of such an IPN are completely different from the uncross-linked polymer blend [15]. 

As pointed out by C. Moonen (National Institutes of Health), For discrimination of healthy and diseased tissues, adequate contrast is essential. Such contrast depends not only on differences in water concentration, but also on the NMR relaxations, which, in turn, are related to local mobilities and interactions. ... 

Microscopies offer a more integral response. Other techniques such as thermal and thermomechanical analysis, and methods sensitive to local mobility such as nuclear magnetic resonance (NMR), can also be used. 

Generally speaking, these properties are almost independent of the crosslink density in the same way as they are almost independent of the chain length in linear polymers. Thus, there are no fundamental differences between thermosets and amorphous thermoplastics in the glassy state, although certain second-order effects linked to crosslinking can be observed, sometimes, on packing density and local mobility. 

Parameters related to local mobility are more difficult to predict, although there have been significant advances in this field (Ngai, et al. 1986). A complete understanding of these properties, including phenomena such as antiplasticization, will probably require the knowledge of factors such as the spatial free volume fluctuations. 

As in linear polymers, the relative influence of the molecular structure (scale of nanometers and monomers), and the macromolecular structure (crosslink density), on network properties, depends on temperature, as shown in Fig. 10.9. In the glassy state, the physical behavior is essentially controlled by cohesion and local molecular mobility, both properties being mainly under the dependence of the molecular scale structure. As expected, there are only second-order differences between linear and network polymers. Here, most of the results of polymer physics, established on linear polymers, can be used to predict the properties of thermosets. Open questions in this domain concern the local mobility (location and amplitude of the (3 transition). 

Figure 11.5 Temperature variation of G for a network with low local mobility (UP) and a network with high local mobility (EPO) forthe latter, (AG/G) 1/2.

Polystyrene (PS) and bisphenol A polycarbonate (PC), are good examples of linear polymers with respectively low (PS) and high (PC) local mobility. Their characteristics are summarized in Table 11.1 (Porter, 1995). 

The role of local mobility on the temperature or frequency variations of v appears clearly when one compares amine-crosslinked epoxies to unsaturated polyesters, at ambient temperature under usual tensile testing conditions (s 10-4 s ) ... 

Yielding needs cooperative chain motions and local mobility. The motions involved in the /3 transition are often regarded as precursors of the yielding... 

---