Cross-linked polymers crystallite networks

With regard to the mechanical reactirai of a polymer network to a stress applied, it is important that loose ends of macromolecules in a network structure are as shmrt as possible and/or their concentration is low. As these ends mostly extend out of the lamellas of crystallites then, while crossUnking is taking place in an amorphous phase and with the simultaneous presence of crystallites, a network with small loose ends should be formed. The crosslink junctions stabilize the natural molecular network (entanglements and crystallites), and every chain in the system is potentially elastically operative and can contribute to the stress in a tensile experiment [33]. The stabilization effect of chemical crosslinks on entanglements and crystallites may be the direct cause of observed differences in the determination of the amount of chemical crosslinks from mechanical property measurements and sol-gel analysis of the cross-linked polymer. 

The chemical nature of the cross-link points is quite unimportant to typical cross-linked network properties such as elasticity and swelling in solvents. Most chemical cross-linking occurs via covalent bonds, but cross-linking can also be achieved with coordinate or electron-deficient bonds. Cross-link-like effects can also be caused by purely physical phenomena, for example, by crystallite regions in partially crystalline polymers, amorphous domains in block polymers, or molecular entanglements in amorphous polymers and polymer melts. 

The greater stability of the crystalline state of networks formed from unoriented but crystalline chains compared with networks formed from amorphous polymers, can be explained in the same way as for networks formed from axially oriented natural rubber. Although prior to network formation the crystallites are randomly arranged relative to one another, portions of chains are still constrained to lie in parallel array. The cross-linking of the predominantly crystalline polymer cannot, therefore, involve the random selection of pairs of units. The units that can be paired are limited by the local chain orientation imposed by the crystalline structure. An increase in the isotropic melting temperature of such networks would therefore be expected. It can be concluded that orientation on a macroscopic scale is not required for partial order in the liquid state to develop. Concomitantly a decrease in the entropy of fusion will result, which reflects the increase in molecular order in the melt. This is an important concept that must be kept in mind when studying the properties of networks formed in this manner. This conclusion has important implications in studying the properties of networks formed from unoriented crystalline polymers. 

The mechanical properties of crystalline materials can be viewed from two extreme positions. Materials of low crystallinity may be pictured as essentially amorphous polymers with the crystallites acting as massive cross-links, about 5-50 nm in diameter. The cross-links restrain the movement of the amorphous network just as covalent cross-links would. However, unlike the covalent bonds, the crystal crosslinks can be melted or mechanically stressed beyond a rather low yield point. At the opposite end of the crystallinity spectrum, one can regard a highly crystalline material as a pure crystal that contains numerous defects such as chain ends, branches, folds, and foreign impurities. Mechanical failure of highly crystalline nylon, for example, bears a great resemblance to that of some metals, with deformation bands rather than the ragged failure typical of amorphous polymers. 

The first three materials listed in Table 11.6 are amorphous thermoplastics. CR-39 is a cross-linked, amorphous network (see Section 17.2). Most highly crystalline polymers are hazy because the crystals and the amorphous phases do not have the same index of refraction and light is scattered at the interfaces. Poly(4-methyl-l-pentene) is unusual in that the two phases have nearly the same index of refraction. The haze in crystalline polymers can be reduced if the crystallite size is very small. A sorbitol-based clarifier for polypropylene is bis(3,4-dimethyldibenzylidene) [25]. It acts as a nucleating agent and makes it possible to produce a water bottle with PET-like clarity. 

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