Poly geometric factor

Geometric factors, such as the symmetry of the backbone and the presence of double bonds on the main chain, affect Tg. Polymers that have symmetrical structure have lower Tg than those with asymmetric structures. This is illustrated by two pairs of polymers polypropylene vs. polyisobutylene and poly(vinyl chloride) vs. poly(vinylidene chloride) in Table 4.4. Given our discussion above on chain stiffness, one would have expected that additional groups near the backbone for the symmetrical polymer would enhance steric hindrance and consequently raise Tg. This, however, is not the case. This discrepancy is due to conformational requirements. The additional groups can only be accommodated in a conformation with a loose structure. The increased free volume results in a lower Tg. 

Poly(ester-urethane) and poly(L-lactide) mixtures were purposed as biodegradable vascular prostheses and biodegradable nerve guides. The two polymers are well known to degrade differently, and mechanical properties and degradation rate can be optimized as function of composition and geometric factors. 

The apparent surfaces of poly crystals, measured geometrically, are often 2—3 times smaller than the real area because the latter is relatively rough—it has hills and valleys that are invisible to unaided sight (see Fig. 7.32). Because various metals—and different samples of the same metal—may have different roughness factors ... 

We shall examine the range of stability of the ordered structures of copolymers containing an amorphous polystyrene, polybutadiene or poly(ethyl methacrylate) block and acrystallizable polyethylene oxide) (PEO) or poly(e-caprolactone) (PCL) crystallizable block and the factors that determine the existence and the geometrical parameters of such periodic structures. 

Inclusion of transition metals cations into cavity of macrocycle poly-efiier is proved by now by various physical-chemical methods. At that the concrete structure of complex is determined not only by geometric accordance of metal ion and crown-ether cavity but by the whole totality of electron and spatial factors created by metal, polyether and other ligand atoms and also by solvent [87]. 

The column packed with Sil-ODA shows very unique separation in RP-HPLC. Especially the uniqueness is emphasized when the solutes are PAHs. An extremely high separation factor, as compared with conventional ODS columns, is observed at temperatures below Tci e.g., the separation factor pentacene/chiysene IS 17.6 and 1.6 for Sil-ODA and conventional ODS columns respectively. To explain this unusual selectivity, we have proposed the multiple tt-tt interaction mechanism between PAHs and carbonyl groups of acrylate moieties in the ordered state.This interaction is quite possible according to our previous calculations and experiments 1) Fig. 3a shows the temperature dependencies of the separation factor a for geometrical isomers of stilbene. The poly(methyl acrylate) phase is less hydrophobic than Sil-ODA and ODS, as well as in a disordered state because of the absence of any long-chain alkyl groups, but the selectivity is distinctly higher than that in ODS. 2) A carbonyl-7T-benzene-TT interaction was simulated by the ab initio study. 

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