Polymer, amorphous isomerizations

The separation of the crystalline and amorphous isomeric phases into their respective spectra has been carried out for a number of polymers including poly(ethylene terephthalate) [82], polystyrene [83], poly(vinyl chloride) [84], polyethylene [85,86], polypropylene [87], and poly(vinylidene fluoride) (PVDF) [88],... 

A suitable approach to the equilibration of an amorphous polymer system at bulk density becomes much more likely when the fully atomistic model in continuous space is replaced by an equivalent coarse-grained model on a lattice with sufficient conformational flexibility. Different strategies, which seek results at different levels of detail, can be employed to create an appropriate coarse-grained model. Section 4 (Doruker, Mattice) describes an approach which attempts to retain a connection with the covalent bonds in the polymer. The rotational isomeric state (RIS) [35,36] model for the chain is mapped into... 

Dendritic polymers are most often reported to be amorphous, which can be anticipated from their highly branched architecture. However, some exceptions are presented in the literature. Percec et al. [34,35] reported on liquid crystalline (LC) hyperbranched polymers where the LC-phase was achieved by conformational isomerism. Various repeat units of A2B type have been used where a flex-... 

Polyethylene has been studied spectroscopically in greater detail than any other polymer. This is primarily a result of its (supposedly) simple structure and the hope that its simple spectrum could be understood in detail. Yet as simple as this structure and spectrum are, a satisfactory analysis had not been made until relatively recently, and even then significant problems of interpretation still remained. The main reason for this is that this polymer in fact generally contains structures other than the simple planar zig-zag implied by (CH2CH2) there are not only impurities of various kinds that differ chemically from the above, but the polymer always contains some amorphous material. In the latter portion of the material the chain no longer assumes an extended planar zig-zag conformation, and as we have noted earlier, such ro-tationally isomeric forms of a molecule usually have different spectra. Furthermore, the molecule has a center of symmetry, which as we have seen implies that some modes will be infrared inactive but Raman active, so that until Raman spectra became available recently it was difficult to be certain of the interpretation of some aspects of the spectrum. As a result of this work, and of detailed studies on the spectra of n-paraffins, it now seems possible to present a quite detailed assignment of bands in the vibrational spectrum of polyethylene. 

It should be added that alternating ethylene/2-butene copolymers can exhibit stereoregularity namely the ethylene/cA-2-butene copolymer, which possesses an erythro-diisotactic structure and is a crystalline polymer. It may be interesting to note that from the formal point of view the alternating eryt/zro-diisotactic ethylene/cA-2-butene copolymer, i.e. erythro-diisotactic poly[ethylene- //-(c/.v-2-butene)], can be treated as isotactic head-to-head and tail-to-tail polypropylene. Isomeric trans-2-bu. ene gives atactic amorphous copolymers with ethylene [2,82]. 

Figure 10 presents the kinetic trans-cis photoisomerization process, under UV irradiation in the solid state, hi this case, significant differences appear between samples behaviour, as a function of the nucleobase chemical structures. It is interesting to note that, in the case of azo-polysiloxane substituted with adeiune (sample 2 -Table 1), the behaviours in the solid state and in solution are similar. That means that the polysiloxane chain flexibility, combined with the amorphous polymer ordering assure enough free volume for the trans-cis isomerization process. 

Numerous examples of constitution isomerisms that can be solved with the aid of the valence band spectra were given. Also, for specially synthesized and characterized compounds, it was possible to show a potentiality of the technique to evidence head-to-head linkages, stereoisomers ( ), tacticities and (alter-nant/block) structure of copolymers. If the influence of conformation in the valence band could also be evidenced, no success was obtained for differentiating crystalline and amorphous polymers. 

Since the polymer is semicrystalline, trans-cis azo isomerization must be restricted to the amorphous regions. Measurements at constant stress carried out at 200 °C indicate a deformation of about 0.6%. On the other hand, in experiments at constant length the... 

The chemical structure of a polymer determines whether it will be crystalline or amorphous in the solid state. Both tacticity (i.e., syndio-tactic or isotactic) and geometric isomerism (i.e., trans configuration) favor crystallinity. In general, tactic polymers with their more stereoregular chain structure are more likely to be crystalline than their atactic counterparts. For example, isotactic polypropylene is crystalline, whereas commercial-grade atactic polypropylene is amorphous. Also, cis-pol3nsoprene is amorphous, whereas the more easily packed rans-poly-isoprene is crystalline. In addition to symmetrical chain structures that allow close packing of polymer molecules into crystalline lamellae, specific interactions between chains that favor molecular orientation, favor crystallinity. For example, crystallinity in nylon is enhanced because of... 

Presented polymer mixtures are composed of amorphous macromolecules with different molecular architecture homopolymers and random copolymers, with different segments distributed statistically along the chain, form partly miscible isotopic and isomeric model binary blends. The mixing of incompatible polymers is enforced by two different polymers covalently bonded forming diblock copolymers. Here only homopolymers admixed by copolymers are considered. The diblock copolymer melts have been described recently in a separate review by Krausch [17]. 

As outlined before, the question of interest now is first, how the photochrome would react when incorporated in the backbone of amorphous and semicrystalline polymers such as segmented polyurethanes and if the isomerization behaviour of the photochrome would allow conclusions on the structure, morphology, and segmental mobility of this matrix. 

The rate constants k obtained for thermally differently treated polyurethanes are represented in the Arrhenius plot. Fig. lO. The apparent energy of activation E for the thermal cis-trans isomerization, as determined from the slopes in Fig. lO, is about 80 kj/mol (k ) in polymers with completely amorphous soft phase, a similar figure as found in solution in samples with partially crystallized soft segments, values of about 60 kJ/mol (k ) and 19 kj/mol (k.) are found for the two simultaneous first order processes (line 2 and 3, Fig. lO). The occuring of these two apparent energies of activation, which are similar to the energies required for rotational (crankshaft (15) ) and translational (3)... 

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