Molecular threads

Rotaxanes-Threading Molecular Rings Rotaxanes are obtained by threading linear polymers through molecular rings such as cyclodextrins, crown ethers and cyclophanes. Molecular shuttles based on the rotaxane structure have been proposed. 

Having synthesized the simple rotaxanes using cucurbituril as a molecular bead we turned our attention to polyrotaxanes. Polyrotaxanes have been synthesized by threading molecular beads , particularly cyclodextrins, with organic polymers [19-21, 35], Typical examples are the ones prepared by Harada et al. [19] and Wenz et al. 

A very simple way of stoppering a bisammonium thread by using NaBPhi was proposed by Lawrence and Rao. [8a] They managed to obtain a threaded molecular loop - the per-2,6-dimethyl-j5-CD [2]rotaxane 7 - in an excellent 71 % yield by doing the self-assembly in aqueous solution (Fig. 2). Furthermore, the same methodology has been employed successfully [8b] for the construction of the [3]rotaxane 8 incorporating a tetraarylporphyrin unit. 

The linearized R-MPY version of the thread molecular closure condition of Eq. (6.7) can be shown to result in a nonlinear, self-consistent integral equation for the effective chi parameter [or equivalently the concentration fluctuation part of the collective structure factor 5(A )]... 

Figure 2 Reproduction from ref. 3 of the first threaded molecular complex a-cyclodextrin is threaded onto a benzene nucleus bearing long chain dilhiols.

Many simulations attempt to determine what motion of the polymer is possible. This can be done by modeling displacements of sections of the chain, Monte Carlo simulations, or reptation (a snakelike motion of the polymer chain as it threads past other chains). These motion studies ultimately attempt to determine a correlation between the molecular motion possible and the macroscopic flexibility, hardness, and so on. 

The theory of molecular symmetry provides a satisfying and unifying thread which extends throughout spectroscopy and valence theory. Although it is possible to understand atoms and diatomic molecules without this theory, when it comes to understanding, say, spectroscopic selection rules in polyatomic molecules, molecular symmetry presents a small barrier which must be surmounted. However, for those not needing to progress so far this chapter may be bypassed without too much hindrance. 

Traditional appHcations for latices are adhesives, binders for fibers and particulate matter, protective and decorative coatings (qv), dipped goods, foam, paper coatings, backings for carpet and upholstery, modifiers for bitumens and concrete, and thread and textile modifiers. More recent appHcations include biomedical appHcations as protein immobilizers, visual detectors in immunoassays (qv), as release agents, in electronic appHcations as photoresists for circuit boards, in batteries (qv), conductive paint, copy machines, and as key components in molecular electronic devices. 

Prior to deposition on a moving belt or screen, the molten polymer threads from a spinnerette must be attenuated to orient the molecular chains of the fibers in order to increase fiber strength and decrease extendibiUty. This is accompHshed by hauling the plastic fibers off immediately after they have exited the spinnerette. In practice this is done by accelerating the fibers either mechanically (18) or pneumatically (17,19,20). In most processes, the fibers are pneumatically accelerated in multiple filament bundles however, other arrangements have been described wherein a linearly aligned row(s) of individual filaments is pneumatically accelerated (21,22). 

Silk (qv) suture is made from the threads spun by the silkworm Bombjx mori. The fiber is composed principally of the protein fibroin and has a natural coating composed of sericin gum. The gum is usually removed before braiding the silk yams to make sutures in a range of sizes. Fine silk sutures may be made by simply twisting the gum-coated silk yams to produce the desired diameter. White silk is undyed. Silk is either dyed black with logwood extract or blue with D C Blue No. 9. The suture may be uncoated or coated either with high molecular weight polydimethylsiloxane or with wax. 

Considering a fiber or thread of nylon-66, which is an unoriented glassy polymer, its modulus of elasticity is about 2,000 MPa (300,000 psi). Above the Tg its elastic modulus drops even lower, because small stresses will readily straighten the kinked molecular chains. However, once it is extended and has its molecules oriented in the direction of the stress, larger stresses are required to produce added strain. The elastic modulus increases. 

Consider a threaded rod, representing a molecular enantiomer, that lies away from an observer. If the observer reaches out and spins a nut on the rod clockwise with his right hand, the nut will travel forward, away from the observer, and will shortly fly off the rod. Here, the angular momentum imparted to the nut (electron) by the observer s hand (photon) causes it to be ejected in a specific direction from the rod (molecular enantiomer) in the observer s reference frame. This is mediated by the interaction between the chiral thread of the rod and nut (the chiral molecular potential). If the rod is turned through 180° and the action repeated, the nut (electron) still departs in the same direction, away from the observer. Hence, the orientation of the rod (molecule) in the observer s frame does not alter the direction in which the nut (electron) is ejected. 

First approaches at modeling the viscoelasticity of polymer solutions on the basis of a molecular theory can be traced back to Rouse [33], who derived the so-called bead-spring model for flexible coiled polymers. It is assumed that the macromolecules can be treated as threads consisting of N beads freely jointed by (N-l) springs. Furthermore, it is considered that the solution is ideally dilute, so that intermolecular interactions can be neglected. 

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