Chain folded structure

The actual experimental moduli of the polymer materials are usually about only % of their theoretical values [1], while the calculated theoretical moduli of many polymer materials are comparable to that of metal or fiber reinforced composites, for instance, the crystalline polyethylene (PE) and polyvinyl alcohol have their calculated Young s moduli in the range of 200-300 GPa, surpassing the normal steel modulus of 200 GPa. This has been attributed to the limitations of the folded-chain structures, the disordered alignment of molecular chains, and other defects existing in crystalline polymers under normal processing conditions. 

The experiments of Kovacs et al. also demonstrate beautifully that although the extended chain crystal is the thermodynamically most stable form, when the undercooling is sufficient, kinetics favors folded chain lamellae. As we have seen, long chain polymers only crystallize at finite rates at high undercoolings, so only form folded chain structures. 

With regard to the present advances in the chemistry of cellulose an excellent review has been given by Shafizadeh (3). He also refers to the folded chain structure of cellulose (4, 5) and of the structure of elementary fibrils (6, ... 

The effect of crystallinity, on the other hand, strongly differentiates glasses and polymers. While crystallinity lowers the yield of trapped electrons in low molecular weight compounds, the opposite is observed with polymers. It is difficult to imagine that the larger number of cavities is associated with the crystalline region of the polymer. Tentative assignments of the voids in polyethylene to double bonds in the crystalline phase and physical cavities associated with the folded-chain structure have been discounted by Williams and co-workers [152]. 

Fig. 13.—Folded-chain Structure of Protofibrils (a) According to Manley and (b) According to Marx-Figini and Schulz. ...

The above results have obvious implications for the biosynthesis of cellulose mlcrofIbrlls. The parallel chain structure of cellulose I rules out any kind of regularly folded chain structure, and reveals the mlcrofibrils to be extended chain polymer single crystals, which leads to optimum tensile properties. Work by Brown and co-workers (22) on the mechanism of biosynthesis points to synthesis of arrays of cellulose chains from banks of enzyme complexes on the cell wall. These complexes produce a bundle of chains with the same sense, which crystallize almost immediately afterwards to form cellulose I mlcroflbrlls there is no opportunity to rearrange to form a more stable anti-parallel cellulose II structure. Electron microscopy by Hleta et al. (23) confirms the parallel sense of cellulose chains within the individual mlcroflbrlls stains reactive at the reducing end of the cellulose molecule stain only one end of the mlcroflbrll. 

Figure 10.83). Some attention has also been given to lamellar [267] and folded-chain structures [268]. Examination of rayon by the electron microscope has provided ample evidence for fibrillar structure in rayon. Although the fringed fibril structure shown in Figure 10.83 appears to fit best with the tendency of some rayons to fibrillate in the wet state under certain conditions the fringed micelle structure can also account for observed properties. 

Fractionation of the cellulose from the alga Valonia afforded a component of weight-average DP 4.4 x 10, which is thought to determine the size of the template required for the simthesis of the polysaccharide. The synthesis of cellulose molecules takes place on a flat template and leads directly to a folded-chain structure. Cellulose I crystallizes after it becomes detached from the site of synthesis. The crystal structure of native Ramie cellulose has been shown to be similar to that recently reported for Valonia cellulose. The R values, 0.158, 0.185, and 0.175, were obtained for anti-parallel, parallel-up, and parallel-down alignments respectively. Anti-parallel chain packing must still continue to receive consideration for cotton and Ramie cellulose despite contrary reports. 

Design of Folded-Chain Lamellae, Folded-chain lamellar crystals are a dominant morphological feature of natural and synthetic polymers of repetitive chemical structure. In synthetic polymers, the folded-chain structure is dictated by the kinetics, rather than the thermodynamics, of the crystallization process, and is metastable. In our early experiments... 

Experimental evidence has shown that the levels of crystallinity, based on the crystallizable blocks, are usually well below unity and are comparable to the values found with the corresponding homopolymers. The chain units of the crystallizable and noncrystallizable blocks mix in the noncrystalline phase. For sufficiently long chain lengths the observed melting temperatures of the crystalline components in block copolymers are comparable to and approach those of the corresponding homopolymers. These widespread observations are not consistent with nor can they be explained by regularly folded chain structures.(252)... 

Stereo-regular polymers usually for crystals of folded-chain structure and they can be extended to give highly oriented fibers. In this case, the period of the folded-chain crystal seems to be close to the diameter of the B-chain coil or an order of rig ll (i.e., 16nm). This fact suggests that crystals are formed by disentanglement of the gel with pseudo-cross-links of the size of 16 or B [27d, p. 186]. 

A schematic visualization of a spherulite is given in Fig, 4,36. Here the spherical nature is apparent and it is to be noted that the individual fi-brils/lamellae grow radially. The individual fibrils have a folded chain structure and the chain traverses both crystalline regions and amorphous regions as illustrated in Fig, 4,31 of the folded chain model. 

The crystalline structure of PVA has been discussed in detail by Bunn [14]. On a molecular level, the crystallites of PVA can be described as a layered structure [15, 16]. A double layer of molecules is held together by hydroxyl bonds while weaker van der Waals forces operate between the double layers. A folded chain structure of PVA chains leads to small ordered regions (crystallites) scattered in an unordered, amorphous polymer matrix. Values representative of the crystallinity and thermal properties of PVA have been reported [4]. The crystalline melting range of PVA is between 220 and 240 °C. The glass transition temperature of dry PVA films has been reported at 85 °C. In the presence of water (and other solvents), the glass transition temperature decreases significantly [2]. 

The effect of the end group on the chain folding of low molecular weight PEO (n approximately 45) has also been determined. Low molecular weight PEO crystallized in the extended chain form at r.t. end groups of seven or more atoms were required to stabilize folded chain lamellae. Also, it was found that the enthalpy of fusion can be used to differentiate between extended and folded chain structures, since it is essentially constant for a given structure and does not vary with the nature of the end groups. 

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