Motional environments, segmented

The spectra shown in Figure 5 clearly indicate that there are two distinct motional environments in the segmented copolymers. The broad component of the spectrum arises from the majority of the hard segments which show motional characteristics similar to that of poly (butylene terephthalate). The sharp component is attributed to hard segments which reside in the soft segment matrix, either by virtue of being very short blocks, or because they form loops on the surfaces of the hard segment lamellae. 

The results presented here show that there are two distinct motional environments for the "central" deuterons of poly (butylene terephthalate) in the segmented copolyesters containing poly (butylene terephthalate) as the hard segment. One of the motional environments is identical to that observed in the poly (butylene terephthalate) homopolymer, in which Helfand-type motions (6) about three bonds occur with a correlation time of 7 x 10-6 s at 20 °C. The Tj for these deuterons is ca. 60 ms at 20 °C. Approximately 90% of the hard segments reside in these organized lamellar environments in the segmented copolymer with 0.87 mole fraction hard segments. 

The broad, featureless nature of the Si NMR signal is likely due to one of three phenomena that are well-known to cause such effects in NMR spectroscopy a multiplicity of chemical and physical environments about the silicon atoms restricted segmental motion in polycyclic or cage-like molecules or ne broadening due to ySi magnetic interactions with the numerous N quadrupoles. 

The motion of R1 in helices has been studied in detail, and this is the case relevant for analysis of rhodopsin. At helix surface sites where R1 has no interactions with other side chains or main-chain atoms, the motion is anisotropic and can be accurately modeled by a single order parameter (.S ) and effective correlation time (tc) (Columbus etal., 2001). This simple anisotropic motion is expected to be the same at all helical surface sites unless modulated by direct interactions of R1 with other groups in the protein and/or by local backbone fluctuations. Interactions of R1 with the environment and local backbone fluctuations are qualitatively distinguishable by their opposite effects on motion the former reduces and the latter increases the mobility relative to a noninteracting reference on the surface of a rigid helical segment. 

In a real situation, the motion of the segments of a chain relative to the molecules of the solvent environment will exert a force in the liquid, and as a consequence the velocity distribution of the liquid medium in the vicinity of the moving segments will be altered. This effect, in turn, will affect the motion of the segments of the chain. To simplify the problem, the so-called free-draining approximation is often used. This approximation assumes that hydrodynamic interactions are negligible so that the velocity of the liquid medium is unaffected by the moving polymer molecules. This assumption was used in the model developed by Rouse (5) to describe the dynamics of polymers in dilute solutions. 

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. 

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