Cross-linking crystallites

The presence of spherulites or smaller crystallites is comparable to cross-linking and affects not only the moduli and compliances, but also the ultimate properties such as yield strength and ultimate elongation. 

The mode of action of plasticizers can be explained using the Gel theory [35 ]. According to this theory, the deformation resistance of amorphous polymers can be ascribed to the cross-links between active centres which are continuously formed and destroyed. The cross-links are constituted by micro-aggregates or crystallites of small size. When a plasticizer is added, its molecules also participate in the breaking down and re-forming of these cross-links. As a consequence, a proportion of the active centres of the polymer are solvated and do not become available for polymer-to-polymer links, the polymer structure being correspondingly loosened. 

Elastomers are amorphous polymers that have the ability to stretch out and spring back to their original shapes. These polymers must have low Tg values and a small amount of cross-linking to prevent the chains from slipping over one another. In addition, the chains must have an irregular shape to prevent crystallite... 

Natural rubber (Chapter 7 Focus On) is the most common example of an elastomer. Rubber has the long chains and occasional cross-links needed for elasticity, but its irregular geometry prevents close packing of the chains into crystallites. Gutta-percha, by contrast, is highly crystalline and is not an elastomer (Figure 31.5). 

Figure 17 Schematic representation of heterogeneous portions of curdlan hydrogel (left) (A) liquid-like portion, (B) portion of intermediate mobility, and (C) triple-helical cross-links in the solid-like portion and crystallites as additional cross-links, and branched glucans (triple helical chains) (right). From Ref. 117 with permission.

The temperature dependence of the compliance and the stress relaxation modulus of crystalline polymers well above Tf is greater than that of cross-linked polymers, but in the glass-to-rubber transition region the temperature dependence is less than for an amorphous polymer. A factor in this large temperature dependence at T >> TK is the decrease in the degree of Crystallinity with temperature. Other factors arc the reciystallization of strained crystallites ipto unstrained ones and the rotation of crystallites to relieve the applied stress (38). All of these effects occur more rapidly as the temperature is raised. 

There is little crystal growth during carbonization, which always occurs in the solid phase. The solid cross-linking that occurs at this time does not lend itself to crystal growth. The glassy carbons are composed of random crystallites of the order of 5.0 nm across and are not significantly altered by ordinary graphitization heat treatment to 2800°C. 

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