Pre-Existing Order

The structure of amorphous polymers has often been compared with that of a mass of cooked spaghetti i.e. without correlation between the end-to-end vectors of segment sequences. A number of experimental data, however, point to the existence of more ordering, than implied by this picture, perhaps even in the equilibrium state. 

Because the handling of the samples in the electron microscope introduces some uncertainties, supplementary evidence from other methods of investigation is urgently needed. In the case of optically clear polymers 

It must be concluded that chain ordering may be a reality at least in a number of networks and should be taken into account as a possible source of deviations from Gaussian network behaviour (see (IV-3)). 

Clustering (association or aggregation) as a result of dissimilar parts within one chain molecule is well-known in the field of biopolymers a recent example in the field of synthetic polymers is provided by poly (p-nitrophenylmethacrylate) which in dimethylformamide (105) forms threefold clusters, presumably because of the difference in polarity of backbone and ester groups. Liquori (118) has presented proof for a double-stranded helix of isotactic and syndiotactic polymethylmethacrylate. 

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If we exclude the case of pre-existing order, we have so far considered a network as a random, but completely homogeneous structure. It should now be mentioned that the crosslinking process itself may give rise to "aggregation of network elements and therefore, in the swollen state, to significant fluctuations in segment density. 

It is the opinion of the present authors that frequently too much emphasis is placed in network studies on the influence of network defects (Chapter II, Section 2), while in reality pre-existing order, inhomogeneous crosslinking, composite network formation, or microsyneresis may play an important role in the mechanical behaviour, as well as in most other network properties. Examples of this will be given in Chapter IV. 

It thus seems fair to conclude that explanations for C2 have to be sought in the equilibrium rather than the time dependent behaviour, although the actual magnitude of the C2 contribution may often be determined by the perhaps time dependent pre-existing order in the polymer, which is trapped by the crosslinking process. 

The ruthenium carbene catalysts 1 developed by Grubbs are distinguished by an exceptional tolerance towards polar functional groups [3]. Although generalizations are difficult and further experimental data are necessary in order to obtain a fully comprehensive picture, some trends may be deduced from the literature reports. Thus, many examples indicate that ethers, silyl ethers, acetals, esters, amides, carbamates, sulfonamides, silanes and various heterocyclic entities do not disturb. Moreover, ketones and even aldehyde functions are compatible, in contrast to reactions catalyzed by the molybdenum alkylidene complex 24 which is known to react with these groups under certain conditions [26]. Even unprotected alcohols and free carboxylic acids seem to be tolerated by 1. It should also be emphasized that the sensitivity of 1 toward the substitution pattern of alkenes outlined above usually leaves pre-existing di-, tri- and tetrasubstituted double bonds in the substrates unaffected. A nice example that illustrates many of these features is the clean dimerization of FK-506 45 to compound 46 reported by Schreiber et al. (Scheme 12) [27]. 

Dislocations as introduced in Section 3.2 have been postulated in order to explain the low yield strength of a crystal (if we compare it to the theoretical shear strength). Likewise, cracks are postulated in order to explain fracturing of crystals well below the theoretical tensile strength of the atomic bonds. Pre-existing cracks can easily magnify low applied stresses at their tips to the maximal atomic bond strength. 

Experimental evidence was reported for the existence of various additional phases a pre-cholesteric order in the form of a network of double-twisted cylinders, analogous to the thermotropic blue phases [27], a hexatic phase that replaces the hexagonal columnar in very long DNA fragments [31], and a structure with orthorhombic symmetry appearing in the transition to crystalline order [27]. 

This article will outline the experimental techniques we have used in the laser spectroscopy of these atoms and briefly indicate current plans for the refinement of these measurements. As Fig. 1 shows, the laser spectroscopy of positronium and muonium is not competitive with comparable measurements in hydrogen, largely due to the low density sources of these atoms. In the case of positronium, the first measurements were done at peak densities of a few atoms/cm3 during a laser pulse. The muonium work was limited by atom densities 10 2 atom/cm3 per laser pulse. As feeble as these sources might seem to spectroscopisis of less exotic atoms, one must remember that these instantaneous densities represent many orders of magnitude improvement of above pre-existing sources of thermal positronium and muonium. Clearly, improved sources will lead to more precise measurements. 

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