Effects on Crystallinity

ABS, polycarbonate and polysulphone) but large effects on crystalline polymers. It is particularly interesting, as well as being technically important, that for many crystalline polymers the unfilled polymer has a heat deflection temperature (at 1.82MPa stress) similar to the Tg, whereas the filled polymers have values close to the T (Table 9.2). 

The large effect on crystalline polymers is, on the one hand, trivial and self-evident on the other hand it appears that reinforcing fibres bring about such an increase in the stiffness at increasing temperatures that the temperature region of application is substantially widened. 

Symmetry is another factor to affect Tm. The salts with symmetric ions generally show higher Tm than those with asymmetric ones. For example, 1,3-dimethylimidazolium tetrafluoroborate showed higher Tm than 1-methylimi-dazolium or l-ethyl-3-methylimidazolium salts, as shown in Figure 3.1. In the case of tetraalkylammonium salts, their Tm also increased with increasing symmetry of the cation structure [18]. This tendency is understood to relate to the structural effect on crystallinity [19], i.e., highly symmetric ions are more efficiently packed into the crystalline structure than unsymmetric ones. Other kinds of chain structures such as polyether [20], perfluorocarbon [21], etc. [22] are obviously also effective in influencing thermal properties. 

Adsorption of transition metal species during processing, cellulose, 388 Advisory panels for standards for archival materials, potential benefits, 311 Age of cellulosic textiles determination from single-fiber creep measurements, 19-38 effect on crystallinity, 417 effect on rate of degradation, 416-17 Aged paper... 

Table 8 Effects on crystalline morphology of alternate heating in oxygen and purified hydrogen ... 

The COH ] must be maintained at a certain minimum value (pH 10) in order to synthesize crystalline pyrochlore from solution. This implies that a minimum solubility is required for crystallization and that this crystallization of the pyrochlore directly out of alkaline solution may involve a solution-reprecipitation mechanism. Once the restriction of minimum pH has been satisfied, COH ] does not seem to have a significant effect on crystallinity as long as oxidizing conditions are maintained. The solubility of lead rapidly increases with hydroxide concentration therefore, when all else is held constant, the lattice parameter of the product pyrochlore decreases as the pH of the synthesis medium increases. 

The densities of these polymers, and particularly those listed in Table 60, are not reliable indicators of branching because of the extreme changes in MW, which can also have an effect on crystallinity. The large amount of low-MW wax in some samples tends to increase density, although some branching is present. The wax chains tend to crystallize... 

DRX studies of SmP04 and SmP04, Cs2P04 calcinated at various temperatures (between 200 and 800°C) show that cesium has no visible effect on crystalline structure of products ... 

However, it is more important to ascertain that incorporation of other ingredients changes the crystallinity only by the thermodynamic interactions, while other effects on crystallinity are negligibly small. Blending affects crystalhnity in diverse ways. 

Blends of PEO/PMMA were oriented by solid-state coextrusion in a capillary rheometer [Kim and Porter, 1988]. Addition of PMMA to POM dramatically decreased its drawabUity from a draw ratio A, = 36 for pure PEO to A. =5 for a blend of PEO/PMMA = 40/60 wt%. Drawing has a strong effect on crystallinity, %, and melting point, T. In pure PEO, % increases from 83% at A, = 1 to X = 95% at = 36, while increased from 62.4 to 70°C, indicating an increase in crystal size and perfection. A different behavior was found in the blends. The crystallinity remained at = 70%, while T decreased not only with increasing PMMA content but also with X. This may be taken as an indication that in POM/ PMMA blends the latter polymer gets trapped between PEO lamellae and disrupts the crystalline morphology. 

On the other hand, controlled branching in crystalline, homochain polymers, such as polyethylene, has been used as a strategy to broaden the distribution in crystallite size, lower the overall degree in crystallinity, and increase compliance. A similar but more difficult to implement approach to achieving such an effect on crystallinity, as alluded to above, has been used specifically in the production of linear segmented and block heterochain copolymers such as ... 

Some polymer structures will now be considered for their effect on crystallinity and general properties. 

Isotacticity level has a stronger effect on crystallinity and related properties of polypropylene. PP grades with an isotactic index (II) of 0.96-0.98 are obtainable which can offer a 20% increase in flexural modulus in comparison to standard grades (II = 0.93-0.95). This enhanced stereoregularity has very interesting consequences on the properties of bio-rientated PP films (BOPP). A 25% increase in flexural stiffness and 25%... 

Zhang, L., Zhang, W., Zhang, Z., et al. (1992) Radiation effects on crystalline polymers - I. Gamma-radiation-induced crosshnking and structural characterization of polyethylene oxide. International Journal of Radiation Applications and Instrumentation. Part C. Radiation Physics and Chemistry, 40 (6), 501-505. 

Each zeolite type has a typical silica/alumina ratio related to the crystal stmcture. It is possible, to increase the silica/alumina ratio however, by removing aluminum atoms from the Y-zeolite framework, with no effect on crystallinity. This, of course, modifies catalyst performance by changing the nature and... 

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