Chain conformational dynamics

While thin polymer films may be very smooth and homogeneous, the chain conformation may be largely distorted due to the influence of the interfaces. Since the size of the polymer molecules is comparable to the film thickness those effects may play a significant role with ultra-thin polymer films. Several recent theoretical treatments are available [136-144,127,128] based on Monte Carlo [137-141,127, 128], molecular dynamics [142], variable density [143], cooperative motion [144], and bond fluctuation [136] model calculations. The distortion of the chain conformation near the interface, the segment orientation distribution, end distribution etc. are calculated as a function of film thickness and distance from the surface. In the limit of two-dimensional systems chains segregate and specific power laws are predicted [136, 137]. In 2D-blends of polymers a particular microdomain morphology may be expected [139]. Experiments on polymers in this area are presently, however, not available on a molecular level. Indications of order on an... 

The conformational dynamics of chain segments near the head groups is more restricted than that of those far from the micellar core [8]. Moreover, to avoid the presence of energetically unfavorable void space in the micellar aggregate and as a consequence of the intermolecular interactions, surfactant molecules tend to assume some preferential conformations and a staggered position with respect to the micellar core [9]. A schematic representation of a reversed micelle is shown in Figure 1. 

Finally, there is a large body of experimental and theoretical contributions from investigators who are mainly interested in the dynamic and conformational properties of chain molecules. The basic idea is that the cyclisation probability of a chain is related to the mean separation of the chain ends (Morawetz, 1975). Up to date comprehensive review articles are available on the subject (Semiyen, 1976 Winnik, 1977, 1981a Imanishi, 1979). Rates and equilibria of the chemical reactions occurring between functional groups attached to the ends or to the interior of a flexible chain molecule are believed to provide a convenient testing ground for theories of chain conformations and chain dynamics in solution. 

This author is perfectly aware that he could add very little to the work done by these workers if an attempt was made to focus on intramolecular catalysis phenomena or on the relevance to cyclisation of available models of chain conformation and chain dynamics instead, the aim will be the presentation of a general treatment of the subject, namely, one that includes the cyclisation of very short chains as well as that of very long chains of, say, 100 atoms or more. With a subject as vast as this, an encyclopaedic review would be a hopeless task. Therefore, the subject will be treated in a systematic and critical way, with more concentration on reaction series with regular and wide variations in structure, rather than on scattered examples. The aim will be to show that the field of intramolecular reactions is a mature area in which the merging of concepts from both physical organic chemistry and polymer chemistry leads to a unified treatment of cyclisation rates and equilibria in terms of a few simple generalisations and theories. 

Robinson, A. J., Richards, W. G., Thomas, P. J. and Hann, M. M. (1995). Behavior of cholesterol and its effect on head group and chain conformations in lipid bilayers a molecular dynamics study, Biophys. J., 68, 164-170. 

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... 

There has been extensive effort in recent years to use coordinated experimental and simulation studies of polymer melts to better understand the connection between polymer motion and conformational dynamics. Although no experimental method directly measures conformational dynamics, several experimental probes of molecular motion are spatially local or are sensitive to local motions in polymers. Coordinated simulation and experimental studies of local motion in polymers have been conducted for dielectric relaxation,152-158 dynamic neutron scattering,157,159-164 and NMR spin-lattice relaxation.17,152,165-168 A particularly important outcome of these studies is the improved understanding of the relationship between the probed motions of the polymer chains and the underlying conformational dynamics that leads to observed motions. In the following discussion, we will focus on the... 

Chain conformation (gauche vs. Irons) interchain coupling Surface topography Chain dynamics... 

The dynamics of a generic linear, ideal Gaussian chain - as described in the Rouse model [38] - is the starting point and standard description for the Brownian dynamics in polymer melts. In this model the conformational entropy of a chain acts as a resource for restoring forces for chain conformations deviating from thermal equilibrium. First, we attempt to exemphfy the mathematical treatment of chain dynamics problems. Therefore, we have detailed the description such that it may be followed in all steps. In the discussion of further models we have given references to the relevant literature. 

By their nature, and in contrast with microscopic and scattering techniques that are used to elucidate long-ranged structure, spectroscopic methods interrogate short-range structure such as interactions between fixed ions in side chains and counterions, main chain conformations and conformational dynamics, and the fundamental hopping events of water molecules. The most common methods involve infrared (mid-IR and to a much lesser extent near- and far-IR) and solid-state NMR spectroscopies, although other approaches, such as molecular probes, have been utilized. 

Morphology based on chemical environment can be probed using F NMR spectroscopy because the chemical shifts of F atoms in the side chains are considerably separated from those in the backbone. Conformational dynamics as affected by domain-selective solvent incorporation are reflected in the widths of static F peaks. These conformational motions, in turn, can influence the migration of solvent penetrants. 

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