Poly stretch

The analogous coupling between the antisyimnetric stretch and bend is forbidden in the H2O Hamiltonian because of syimnetry.) The 2 1 resonance is known as a Femii resonance after its introduction [ ] in molecular spectroscopy. The 2 1 resonance is often very prominent in spectra, especially between stretch and bend modes, which often have approximate 2 1 frequency ratios. The 2 1 couplmg leaves unchanged as a poly ad number the sum ... 

In polymers made of dis-symmetric monomers, such as, for example, poly(propylene), the stmcture may be irregular and constitutional isomerism can occur as shown in figure C2.1.1(a ). The succession of the relative configurations of the asymmetric centres can also vary between stretches of the chain. Configuration isomerism is characterized by the succession of dyads which are named either meso, if the two asymmetric centres have the same relative configurations, or racemo if the configurations differ (figure C2.1.1(b )). A polymer is called isotactic if it contains only one type of dyad and syndiotactic if the dyad sequence strictly alternates between the meso and racemo fonns. 

The next significant strength improvement followed the 1950 Du Pont (19) discovery of monoamine and quaternary ammonium modifiers, which, when added to the viscose, prolonged the life of the ziac cellulose xanthate gel, and enabled even higher stretch levels to be used. Modifiers have proliferated siace they were first patented and the Hst now iacludes many poly(alkylene oxide) derivatives (20), polyhydroxypolyamines (21—23), and dithiocarbamates (24). 

In conventional tenter orientation, the sequence of steps is as described above (MD—TD). In some cases it is advantageous to reverse the draw order (TD—MD) or to use multiple draw steps, eg, MD—TD—MD. These other techniques are used to produce "tensilized" films, where the MD tensile properties are enhanced by further stretching. The films are generally unbalanced in properties and in extreme cases may be fibrillated to give fiber-like elements for special textile appHcations. Tensilized poly(ethylene terephthalate) is a common substrate for audio and video magnetic tape and thermal transfer tape. 

HoUow-fiber fabrication methods can be divided into two classes (61). The most common is solution spinning, in which a 20—30% polymer solution is extmded and precipitated into a bath of a nonsolvent, generally water. Solution spinning allows fibers with the asymmetric Loeb-Soufirajan stmcture to be made. An alternative technique is melt spinning, in which a hot polymer melt is extmded from an appropriate die and is then cooled and sohdified in air or a quench tank. Melt-spun fibers are usually relatively dense and have lower fluxes than solution-spun fibers, but because the fiber can be stretched after it leaves the die, very fine fibers can be made. Melt spinning can also be used with polymers such as poly(trimethylpentene), which are not soluble in convenient solvents and are difficult to form by wet spinning. 

Soft-drink bottles made from poly(ethylene terephthalate) (PET) are usuady made by stretch-blow mol ding in a two-step process. Eirst, a test-tube-shaped preform is molded, which is then reheated to just above its glass-transition temperature, stretched, and blown. Stretching the PET produces biaxial orientation, which improves transparency, strength, and toughness of the botde (54,56). A one-step process is used for many custom containers that are injection stretch-blow molded. 

Fibers. Poly(vinyl alcohol) fibers possess excellent strength characteristics and provide a pleasant feel in fabrics. The fiber is usually spun by a wet process employing a concentrated aqueous solution of sodium sulfate as the coagulating bath. Water insolubiUty, even in boiling water, can be obtained by combining stretching, heat treatment, and acetalization with formaldehyde. Super hydrolyzed PVA is the preferred material for fiber production. 

Benzonatate [104-31 ] (46) is a unique compound which appears to have both central and peripheral antitussive effects. Stmcturally it is a derivative of ji)-aminoben2oic acid and contains a long poly(ethylene glycol) side chain. The peripheral effects ate the result of local anesthetic action on the pulmonary stretch receptors. Clinical activity was first reported in 1955 (65). 

Biaxial orientation effects are of importance in the manufacture of films and sheet. Biaxially stretched poly(ethylene terephthalate) (e.g. Melinex),... 

Poly(ethylene terephthalate) film is produced by quenching extruded film to the amorphous state and then reheating and stretching the sheet approximately three-fold in each direction at 80-100°C. In a two-stage process machine direction stretching induces 10-14% crystallinity and this is raised to 20-25% by... 

This polymer has a slightly stiffer chain and hence slightly higher melting point and heat distortion temperatures than poly(ethylene terephthalate). Films are available (Kodel-Kodak) which have been biaxially stretched about 200% from polymer with molecular weights of about 25 000. They are similar electrically to poly(ethylene terephthalate), are weaker mechanically but have superior resistance to water and in weathering stability. Some properties are given in Table 25.6. 

There has been considerable interest in the structure and properties of poly(tetra-methylene terephthalate) 4GT, stemming from the observation that when the oriented polymer is stretched, the crystalline regions are transformed from one structure to a new one30,3l). It appears from the X-ray diffraction data that in the structure... 

The elasticity of a polymer is its ability to return to its original shape after being stretched. Natural rubber has low elasticity and is easily softened by hearing. Flowever, the vulcanization of rubber increases its elasticity. In vulcanization, rubber is heated with sulfur. The sulfur atoms form cross-links between the poly-isoprene chains and produce a three-dimensional network of atoms (Fig. 19.17). Because the chains are covalently linked together, vulcanized rubber does not soften as much as natural rubber when the temperature is raised. Vulcanized rubber is also much more resistant to deformation when stretched, because the cross-... 

Similarly, oriented crystallisation can be induced by stretching sheets or films of polymers in two directions simultaneously. The resulting materials have biaxially oriented polymer crystals. Typical examples of such materials are biaxially stretched poly(ethylene terephthalate), poly(vinylidene chloride), and poly (propylene). Since the oriented crystals do not interfere with light waves, such films combine good strength with high clarity, which makes them attractive in a number of applications. 

Subsequent work by Johansson and Lofroth [183] compared this result with those obtained from Brownian dynamics simulation of hard-sphere diffusion in polymer networks of wormlike chains. They concluded that their theory gave excellent agreement for small particles. For larger particles, the theory predicted a faster diffusion than was observed. They have also compared the diffusion coefficients from Eq. (73) to the experimental values [182] for diffusion of poly(ethylene glycol) in k-carrageenan gels and solutions. It was found that their theory can successfully predict the diffusion of solutes in both flexible and stiff polymer systems. Equation (73) is an example of the so-called stretched exponential function discussed further later. 

Recent interest has focused on acidic phosphoproteins, such as bone sialoprotein, acting as sites of nucleation. These proteins contain motifs (eg, poly-Asp and poly-Glu stretches) that bind calcium and may provide an initial scaffold for mineralization. Some macromolecules, such as certain proteoglycans and glycoproteins, can also act as inhibitors of nucleation. 

When X = Y, as in polyethylene, poly-(tetrafluoroethylene), polyisobutylene, and poly -(vinylidene chloride), the polymers are highly crystalline products with sharply definable melting points (except for polyisobutylene, which crystallizes readily on stretching but with difficulty on cooling). Oriented specimens of high strength may be obtained, exactly as in the crystalline condensation polymers. 

The head-to-tail arrangement in poly-(vinyl alcohol) is further confirmed by its X-ray diffraction pattern in the crystalline state. Likewise, analysis of the X-ray diffraction of crystalline poly-(vinylidene chloride), (—CH2—CCI2—)x, and of crystalline (stretched) polyisobutylene, [—CH2—C(CH3)2—]x, shows the units to be arranged in these cases also in the expected head-to-tail forms. 

Unlike polybutadiene, polyisoprene prepared at low temperatures shows little or no inclination to crystallize either on stretching or cooling. This may seem surprising in view of the even greater preponderance of trans-1 4 units in polyisoprene than in poly butadiene. The explanation for the contrasting behavior in this respect between low temperature synthetic polyisoprene, on the one hand, and guttapercha and low temperature polybutadiene, on the other, probably is to be found in the appreciable occurrence of head-to-head and tail-to-tail sequences of 1,4 units of the former. 

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