Extended chain crystallinity

Similady, hquid-crystal polymers exhibit considerable order in the hquid state, either in solution (lyotropic) or melt (thermotropic). When crystallized from solution or melt, they have a high degree of extended-chain crystallinity, and thus have superior mechanical properties. Kevlar (Du Pont) is an aromatic polyamide (atamid) with the repeating unit designated as (2). It is spun into... 

A) is much smaller than the lamellar length of ordinary PE crystals ( 100A), thus the PE chains are prevented from folding. In this manner, extended-chain crystalline nanofibers of linear polyethylene with an ultrahigh molecular weight (6,200,000) and a diameter of 30 to 50 nm are formed. 

Extended-chain crystallinity, 20 398-399 Extended-DLVO theory, 21 668 Extended failures, 26 981 Extended HMO theory (EHT), 16 736 Extended patent families, 18 207 Extended shelf life (ESL) packaged products, 13 32... 

Figure 1.60 Schematic illustration of extended chain crystallinity in polymers (a) polyethylene and (b) polypropylene.

These materials tend to be very brittle because, although they are highly crystalline, there are few inter-crystalline linking molecules. Figure 5.9 shows an electron micrograph of a replica of a fracture surface of extended-chain crystalline material of polychlorotrifluoroethylene. The lamellae are about 1-2 pm thick and there appears to be no limit on the lateral dimensions of the crystallites except space and availability of material. 

For extended-chain crystallization to occur, the polymer molecule must either be inherently stiff and extended in solution, or be extended by an elongational flow field immediately prior to crystallization, usually at low supercooling. Pennings and coworkers have studied flow-induced crystallization. The extended chain crystals could form a fibrous network in which the junctions are simply common extended-chain crystalline regions (Fig. 12a), or regions of common chainfolded crystal overgrowth (Fig. 12b). Hoffman has developed a... 

Both Kevlar (Section 4.7) and extended-chain hnear polyethylene (Section 4.2) fibers contain a high degree of extended-chain crystallinity, yet the former is somewhat stronger than the latter. Why ... 

The formation of supramolecular liquid crystalline order was recently reported in suspensions of (extended-chain) crystalline whiskers of the flexible macromolecule poly(tetrafluoroethylene) (PTFE) (22). This finding will be briefly reviewed, because it once more illustrates that spontaneous ordering is not the exclusive property of solutions or melts of (semi) rigid molecules. Flexible macromolecules, when assembled into asymmetric objects, such as extended-chain whiskers, also may exhibit the fascinating and useful selfalignment phenomenon that leads to liquid crystalline order. It is believed that this class of soft condensed matter will provide new avenues for processing (flexible) macromolecules into highly ordered materials with anticipated superior performance. 

As noted above, if polymer chains can be aligned in the melt prior to crystallization, high degrees of extended-chain crystallinity may be formed, imparting remarkable mechanical properties, at least in the chain direction. Another way of doing it is by starting with molecules that show a degree of order in the liquid phase. Such liquid-crystalline materials have been known for years, but until fairly recently were limited to relatively small molecules. Now, however, several types of polymers are known which exhibit liquid-crystalline order, either lyotropic (order in solution) or thermotropic (order in the melt). ... 

Examples of polymers which form anisotropic polymer melts iaclude petroleum pitches, polyesters, polyethers, polyphosphaziaes, a-poly- -xyljlene, and polysdoxanes. Synthesis goals iaclude the iacorporation of a Hquid crystal-like entity iato the maia chaia of the polymer to iacrease the strength and thermal stabiHty of the materials that are formed from the Hquid crystal precursor, the locking ia of Hquid crystalline properties of the fluid iato the soHd phase, and the production of extended chain polymers that are soluble ia organic solvents rather than sulfuric acid. 

Miscibility or compatibility provided by the compatibilizer or TLCP itself can affect the dimensional stability of in situ composites. The feature of ultra-high modulus and low viscosity melt of a nematic liquid crystalline polymer is suitable to induce greater dimensional stability in the composites. For drawn amorphous polymers, if the formed articles are exposed to sufficiently high temperatures, the extended chains are retracted by the entropic driving force of the stretched backbone, similar to the contraction of the stretched rubber network [61,62]. The presence of filler in the extruded articles significantly reduces the total extent of recoil. This can be attributed to the orientation of the fibers in the direction of drawing, which may act as a constraint for a certain amount of polymeric material surrounding them. 

Hence, the extension of an isotropic unoriented partially crystalline polymer leads to the formation of a highly organized material with a characteristic fibrillar structure. The anisotropy of the sample as a whole is expressed by a higher modulus, tenacity and optical anisotropy. It would seem that the increase in strength in the drawing direction suggests that the oriented samples consist of completely extended chains. However, while the strength of such perfect structure for polyethylene has been evaluated as 13000 MPas), the observed values for an oriented sample are 50 to 30 MPa. 

As compared to ECC produced under equilibrium conditions, ECC formed af a considerable supercooling are at thermodynamic equilibrium only from the standpoint of thermokinetics60). Indeed, under chosen conditions (fi and crystallization temperatures), these crystals exhibit some equilibrium degree of crystallinity at which a minimum free energy of the system is attained compared to all other possible states. In this sense, the system is in a state of thermodynamic equilibrium and is stable, i.e. it will maintain this state for any period of time after the field is removed. However, with respect to crystals with completely extended chains obtained under equilibrium conditions, this system corresponds only to a relative minimum of free energy, i.e. its state is metastable from the standpoint of equilibrium thermodynamics60,61). 

Unlike the crystalline cycloamylose complexes, combining ratios of host to guest in solution are usually 1 1. A notable exception is the interaction of the cycloamyloses with long chain aliphatic carboxylic acids. Solubility plots suggest that as many as four cycloheptaamylose molecules may interact with a single molecule of dodecanoic acid (Schlenk and Sand, 1961). In analogy to the crystalline state, cycloamyloses may form channels in solution in order to accomodate extended chains. 

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