Chain reorientation

Because the adsorbed HM-HEC molecules exhibit such slow rates of chain reorientation, the effects of molecular weight, amount of hydrophobic substitution and chain lengths of the hydrophobes on the interfacial properties of HM-HEC monolayers can be investigated by two kinds of dynamic experiments hysteresis and stress-jump, using a Langmuir trough film balance. 

Batchelder, L. S., Sullivan, C. E., Jelinski, L. W., and Torchia, D. A. (1982). Characterization of leucine side-chain reorientation in collagen fibrils by solid state 2H NMR. Proc. Natl. Acad. Sci. 79, 386—389. 

In order to get a more precise description of the molecular motions occurring in BPA-PC, it is interesting to check whether, in addition to the ring motions about the 1,4 axis already described, main-chain reorientation takes place. 

Methyl dipolar coupling has been applied [39] at room temperature to investigate the occurrence of main-chain reorientation in BPA-PC. Due to the... 

The carbonyl motions of the carbonate group can be accounted for by considering jumps between two sites, corresponding to a rotation around an axis perpendicular to the C = O bond in the plane formed by the three oxygen atoms, with an angle of 40° between the two sites, over which is superimposed a rotation of 15° about the C = O bond. This latter motion requires a main-chain reorientation motion. 

At higher temperatures, around - 100 °C at 1 Hz, the ft transition involves motions of both carbonates and phenyl rings, accompanied by main-chain reorientation, which can be described in more details as follows. 

Modified HRP with maleic and citraconic anhydrides An enhanced thermostability of modified HRP with respect to native HRP was observed, and the changes were attributed to side chain reorientations of the aromatic residues of the enzyme [66]... 

HRP. The existence of conformational changes was confirmed by circular dichro-ism studies, and the enhanced thermostability of modified HRP was attributed to side chain reorientations of aromatic residues [66]. 

Figure 20 The last frames (representative) of two 600 ps simulations of a self-assembled monolayer (SAM) on Au(l 11). When a defect was introduced by shifting roughly half of the chains to the next interstitial site on the Au(l 11) surface (this resulted in the loss of four chains in the system below), the chains reoriented to a helical conformation to fill the voids caused by the defect. A 1.5 nm layer of water was placed on top of the SAM, and the computations were carried out in the presence of 2-D periodic boundaries to mimic an infinite SAM. Only the non-water molecules and atoms of the primary system are shown for clarity...

In previous examples of substituted polysiloxanes, the relaxation of the side chains with the rigid backbone was assumed to describe the polymer chain in the amorphous glassy state. This assumption, relaxation of side chains only, can be used to study the crystalline states of PDES. Certainly, this simplification is extreme, but it can be usefiil to understand the available orientation of the pendant groups when the polymer chains undergo transition from one crystalline form to another. The present approach does not address the chain reorientation or the interchain interactions in the crystalline state. 

A simple model of an elastomer network is depicted in Fig. 7.1.8. The segmental motion of inter-cross-link chains is fast but anisotropic at temperatures of 100-150 K above the glass transition temperature The end-to-end vector R of such a chain reorients on a much slower timescale because it appears fixed between seemingly static cross-link points. As a result of the fast but anisotropic motion, the dipolar interaction between spins along the cross-link chains is not averaged to zero, and a residual dipolar coupling remains [Cohl, Gotl, Litl]. 

Further reaction of polar or other active species (e.g. transient radicals) producing linkages that hinder chain reorientation (specifically for silicones, surface silanol condensation is such a possibility). ... 

Thus, chain twists and distortions from the trans planar conformations are to be included in the model for understanding the spectral output of the hexagonal phase. Chain reorientation can also be present in phases with a degree of order higher than that of the hexagonal phase. For instance, a mechanism of rapid 180° chain flips about the chain axis has been described by NMR and makes compatible an orthorhombic phase with the presence of chain motion [15]. 

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