Threading efficiency

In addition to the types of structure and chemical compositions, the properties of a polyrotaxane are determined by the amount of cyclic incorporated. To define such quantities, the min value was introduced [7, 12], Min, file threading efficiency, was defined for systems of Types 4-6, 9, and 10 as the average number of cyclic molecules per repeat unit [7, 12]. However, this definition seems a little awkward for polyrotaxanes of Types 7, 8, 11, and 12, because in these polyrotaxanes the linear component penetrates through the polymeric cyclic instead of the cyclic being threaded on to the linear species. To fit all the types in Table 1, we redefine min as the proportion of rotaxane repeat units in the polymer. 

To study the effect of the BG on threading process, triphenylmethyl-based azo compounds were designed as BG initiators for the preparation of polystyrene rotaxane 32 [67-69]. The BG fragments were shown to end up at both chain ends of the polystyrene since termination occurs nearly exclusively by combination. However, the threading efficiencies did not increase with 29 as cyclic and increased very little in other instances relative to those without BG. 

In 1979, Maciejewski et al. also explored Method 3 for the preparation of main-chain poly(vinylidene chloride-/ -CD rotaxane) 35 [74, 75]. Radiation polymerization and AIBN-initiated solution polymerization of the complex of vinyli-dene chloride with 21 gave products with min = 0.34 and 0.49, respectively. However, the polyrotaxane via Method 1 had a much lower min (0.087) with much lower CD/monomer feed ratio than for those via the polymerization of the complex (1 1 ratio). Therefore, the reported min values are not comparable, so the difference between the two methods in terms of threading efficiencies cannot be distinguished. Although the authors did not see any threading via Method 2 for the same polyrotaxanes, Ogino and coworkers prepared a true CD-based polyrotaxane of Type 5 using metal complexes as stoppers [76]. It was also found that... 

Because of their novel topologies, polyrotaxanes have properties different from those of conventional polymers. Solubility, intrinsic viscosity, melt viscosity, glass transition, melting temperature and phase behavior can be altered by the formation of polyrotaxanes. The detailed changes are related both to the properties of the threaded cyclics and to the backbone and the threading efficiency. 

Poly(urethane-rofaxa-bis(p-phenylene)- 34-crown-10 Segmented polyurethane host-guest complexation approach 80% threading efficiency. [240]... 

Poly(urethane)-rofaxa-(60-crown-20) Poly(urethane)-rofaxa-(36-crown-12) Statisticai threading approach threading efficiency 57% with 60Cr20 and 16% with 36Cr12. Tg of rotaxanes ( 28°C for 60Cr20 and 19°C for 36Cr12) iower than the parent poiymer (51 °C). [241]... 

Polymeric rotaxanes, consisting of crown ethers threaded onto a polyurethane backbone, were synthesized by allowing the crown ether to equilibrate with tetrakis(ethylene glycol) before its reaction to bis(p-isocyanato-phenyl)methane. The threading efficiency, as measured by xln, the mole ratio of the cyclics per repeat unit, increased linearly with ring size from 36- to 60-membered crown ethers. Up to 63% by mass of the macrocycle was incorporated. The physical properties of the polyrotaxanes differ from those... 

Prices of spandex fibers are highly dependent on thread size selling price generally increases as fiber tex decreases. Factors that contribute to the relatively high cost of spandex fibers include (/) the relatively high cost of raw materials, (2) the small size of the spandex market compared to that of hard fibers which limits scale and thus efficiency of production units, and (J) the technical problems associated with stretch fibers that limit productivity rates and conversion efficiencies. 

I. The Hershberg stirrer, shown in part in Fig. 2, provides very efficient agitation of this or other pasty mixture. Two glass rings are sealed to the end of a stirrer shaft at right angles to one another, and each is threaded with B. and S. No. 18... 

The axon emerges from a neuron as a slender thread and frequently does not branch until it nears its target. In contrast to the dendrite and the soma, the axon is myelinated frequently, thus increasing its efficiency as a conducting unit. Myelin, a spirally wrapped membrane (see Ch. 4), is laid down in segments, or internodes, by oligodendrocytes in the CNS and by Schwann cells in the PNS. The naked regions of axon between adjacent myelin internodes are known as nodes of Ranvier (see below). 

The nebulizer capillary position may be adjustable on a screw thread to permit optimization of sample uptake and drop size. Alternatively or additionally, an impact bead may be placed in the path of the initial aerosol to provide a secondary fragmentation and so improve the efficiency of nebulization. Such a device is illustrated in Fig. 2.9. 

The choice of clamping system is very much dependent on the rate of production. Conventional threaded bolts are the simplest choice, but they may also be the most cost efficient for low volume production with cycle times in the range of hours. On the other hand, for high-volume production mold closing as well as mold opening must be achieved much quicker, and the choice may be a hydraulically driven mold carrier. 

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