Chain ends/ loops

In 1944, Flory (3) noted that the moduli of cross-linked butyl rubbers generally differ somewhat from values calculated from the crosslink density according to the kinetic theory of rubber elasticity. In many cases, the modulus also depends on the primary (uncross-linked) molecular weight distribution of the polymer. He attributed both observations to three kinds of network defects chain ends, loops, and chain entanglements. The latter are latent in the system prior to cross-linking and become permanent features of the network when cross-links are added. 

Molecular motions (polymer chain ends, loops and their flexibility)... 

The interdiffusion of polymer chains occurs by two basic processes. When the joint is first made chain loops between entanglements cross the interface but this motion is restricted by the entanglements and independent of molecular weight. Whole chains also start to cross the interface by reptation, but this is a rather slower process and requires that the diffusion of the chain across the interface is led by a chain end. The initial rate of this process is thus strongly influenced by the distribution of the chain ends close to the interface. Although these diffusion processes are fairly well understood, it is clear from the discussion above on immiscible polymers that the relationships between the failure stress of the interface and the interface structure are less understood. The most common assumptions used have been that the interface can bear a stress that is either proportional to the length of chain that has reptated across the interface or proportional to some measure of the density of cross interface entanglements or loops. Each of these criteria can be used with the micro-mechanical models but it is unclear which, if either, assumption is correct. 

All-or-none transitions occur if the chain length is relatively short (n < 15 tripeptide units) and if the cooperativity is high (a < 1) since in this special case, the concentration of intermediates is negligibly low. Besides, in the case of short chains we may conclude that back folding and oligomerization are negligibly small because of the shortness of the chain ends beyond the helical part. A further simplification is the assumption that only one helical sequence exists, which excludes the formation of loops within a helical part, because of reasons of stability. Under these circumstances, only two different products exist in a measurable concentration at equilibrium. 

Free chain ends (unreacted functionalities) reduce the number of active network chains in a network compared with the same network without free ends. Disregarding possible presence of loops and entanglements, C — 1 C crosslinks are necessary according to Flory (55) to connect C chains into one giant macromolecule. Additional crosslinks will be elastically effective. Their number is given by... 

The chain and most of the associated side chains are not well defined in the following regions residues 2, 65-72, and 119-123. The chain is very poorly defined or not visible at all in regions residues 1, 18-20, 21-23, and 124. Except for the small peptide loop, these regions are all associated with the chain ends. Cursory inspection of the sums of the electron densities in the backbone and /3-carbon positions shown residues 3 through 9 to be systematically low, and preliminary calculations show that a simple shift of these residues by 0.6 A in the crystallographic z direction would bring these sums into line with those for the remainder of the structure. 

Fig. 1. Various conformation models for macromolecules adsorbed on an interface, a) chain lying totally on the interface b) loop-train conformation c) loop-train-tail conformation d) adsorbed at one chain end e) random coil adsorbed at a single point f) rod-like macromolecules adsorbed at one end g) rod-like macromolecules adsorbed located totally on an interface...

How does the rigid body movement of the switch-2 cluster comply with its interactions with the rest of the motor domain Regarding the main chain connectivity, such a movement requires flexible adaptors at both ends of the cluster. At the N-terminus, loop LI 1 obviously fulfills this function. At the other end, loop L13 (between a5 and strand jS8 of the central /i-sheet) may provide sufficient flexibility. Loop L13 contains two... 

In sec. 5.2 we emphasised the fact that polymers have a large number of interned degrees of freedom. Adsorption must imply changes in this number. The usual description of conformations at an adsorbing Interface, first proposed by Jenkel and Rumbach and depicted schematically in fig. 5.1, is in terms of three types of subchains trains, which have all their segments In contact with the substrate, loops, which have no contacts with the surface and connect two trains, tind tails which are non-adsorbed chain ends. 

Schnell at al. argue that the low degree of interpenetration necessary to activate the plastic deformation mechanisms could be indicative of the formation of a significant number of loops at the interface rather than chain ends, as shown schematically in Fig. 42. This argument would apply even more strongly for the results on PS-r-PVP/PS interfaces recently reported by Benkoski et al. [70] but would not apply to PMMA interfaces where the transition from regimes I and II occurs for much wider interfaces [69]. 

Creep. One of the most remarkable aspects of the deformation of polydiacetylenes is that it is not possible to measure any time-dependent deformation or creep when crystals are deformed in tension parallel to the chain direction (14,24). This behviour is demonstrated in Figure 3 for a polyDCHD crystal held at constant stress at room temperature and the indications are that creep does not take place at temperatures of up to at least 100 C (24). Creep and time-dependent deformation are normally a serious draw-back in the use of conventional high-modulus polymer fibres such as polyethylenes (28). Defects such as loops and chain-ends allow the translation of molecules parallel to the chain direction in polyethylene fibres. In contrast since polydiacetylene single crystal fibres contain perfectly-aligned long polymer molecules (cf Figure lb) there is no mechanism whereby creep can take place even at high temperatures. 

The occurrence of disorder-to-order transitions involving segments of a protein, the remainder of which has a well-defined conformation, has been documented in crystal structures of a variety of systems. The segments involved may be at the N terminal or the C terminal end (disordered arm ) or in the middle of the polypeptide chain (disordered loop ). The length of the disordered segment varies from five or six amino acids to a sizable portion of the... 

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