Crystalline polymers direction

Structurally the difference between PEN and PET is in the double (naphthenic) ring of the former compared to the single (benzene) ring of the latter. This leads to a stiffer chain so that both and are higher for PEN than for PET (Tg is 124°C for PEN, 75°C for PET is 270-273°C for PEN and 256-265°C for PET). Although PEN crystallises at a slower rate than PET, crystallization is (as with PET) enhanced by biaxial orientation and the barrier properties are much superior to PET with up to a fivefold enhancement in some cases. (As with many crystalline polymers the maximum rate of crystallisation occurs at temperatures about midway between Tg and in the case of both PEN and PET). At the present time PEN is significantly more expensive than PET partly due to the economies of scale and partly due to the fact that the transesterification route used with PEN is inherently more expensive than the direct acid routes now used with PET. This has led to the availability of copolymers and of blends which have intermediate properties. 

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. 

Plastic molecules that can be packed closer together can more easily form crystalline structures in which the molecules align themselves in some orderly pattern. During processing they tend to develop higher strength in the direction of the molecules. Since commercially perfect crystalline polymers are not produced, they are identified technically as semicrystalline TPs (normally up to 85% crystalline and the rest amorphous). In this book and as usually identified by the plastic industry, they are called crystalline. 

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. 

In extraction from a polymer/additive solid matrix the rate-determining step in the extraction process is governed by the interaction of the solvent of sufficient dissolution power with the matrix and the removal of the analyte (cf. Section 3.4.1.3). There appears to exist a direct relationship between degree of swelling and efficiency of extraction. The amount of C02 absorbed depends on temperature, pressure and the polymer concerned. Crystalline polymers are-not surprisingly-plasticised less... 

Vc crystalline Va, amorphous). The densities of the pure crystalline (pc) and pure amorphous (pa) polymer must be known at the temperature and pressure used to measure p. The value of pc can be obtained from the unit cell dimensions when the crystal structure is known. The value of pa can be obtained directly for polymers that can be quenched without crystallization, polyfethylene terephtha-late) is one example. However, for most semi-crystalline polymers the value of pa is extrapolated from the variation of the specific volume of the melt with temperature [16,63]. 

Here m is the mode order (m — 1,3,5. .., usually 1 for polyethylenes), c the velocity of light, p the density of the vibrating sequence (density of pure crystal) and E the Young s modulus in the chain direction. The LAM band has been observed in many polymers and has been widely used in structural studies of polyethylenes [94—99,266], as well as other semi-crystalline polymers, such as poly (ethylene oxide) [267], poly(methylene oxide) [268,269] and isotactic poly(propylene) [270,271], The distribution of crystalline thickness can be obtained from the width of the LAM mode, corrected by temperature and frequency factors [272,273] as ... 

Many industrial semi-crystalline polymer materials like polypropylene, polyamides, or polyesters contain nucleating agents or clarifiers which form needle-shaped aggregates already in the polymer melt. "For this purpose the pattern is desmeared using the measured primary beam. For a less involved treatment it may be sufficient to know the integral width of the primary beam profile in fiber direction. 

Anisotropic materials have different properties in different directions (1-7). 1-Aamples include fibers, wood, oriented amorphous polymers, injection-molded specimens, fiber-filled composites, single crystals, and crystalline polymers in which the crystalline phase is not randomly oriented. Thus anisotropic materials are really much more common than isotropic ones. But if the anisotropy is small, it is often neglected with possible serious consequences. Anisoiropic materials have far more than two independent clastic moduli— generally, a minimum of five or six. The exact number of independent moduli depends on the symmetry in the system (1-7). Anisotropic materials will also have different contractions in different directions and hence a set of Poisson s ratios rather than one. 

Electron Microscopy can be used for resolution of smaller objects the practical limit of resolution being a few angstrom units. Electron Microscopy has been used in the study of the morphology of crystalline polymers. The usual techniques of replication, heavy-metal shadowing, and solvent etching are widely used. The direct observation of thin specimens, like polymer single crystals, is also possible and permits the observation of the electron-diffraction pattern of some specimen area, which is invaluable for... 

While no direct evidence of liquid crystallinity in PET-BB copolymers has been reported, the high-BB-content copolymers have been shown to possess morphologies similar to those of liquid crystalline polyesters [40], and show major changes in both melt relaxation times and fiber tensile moduli, suggestive of structural organization in a frustrated liquid crystalline polymer (LCP) (Table 6.3 and Figure 6.4) [41, 42],... 

Fouquey, C. Lehn, J.-M. Levelut, A.-M. Molecular recognition directed self-assembly of supramolecular liquid crystalline polymers from complementary chiral components. Adv. Mater. 1990, 2, 254-257. 

The chain stiffness inflnences the height of the glass-rnbber transition temperatnre (and of the melting point), bnt not the stiffness of the polymer below Tg (in the glassy state). Extremely stiff chains show the effect of the formation of LCP s (liqnid-crystalline polymers), by which very high stiffness is reached, bnt only in the direction of the orientation. 

Gulikkrzywicki T, Fouquey C, Lehn JM. Electron-microscopic study of supramolecular liquid-crystalline polymers formed by molecular-recognition-directed self-assembly from complementary chiral components. Proc Natl Acad Sci USA 1993 90 163-167. 

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