Stretching crystallization

Most elastomers are amorphous, but those with regular structures can crystallize when cooled to extremely low temperatures. Vulcanized soft rubber, which has a low cross-link density, when stretched crystallizes in a reversible process, and the oriented polymer has a high modulus (high stress for small strains, i.e., stiffness) and high tensile strength. 

E20.22(b) Tension reduces the disorder in the rubber chains hence, if the rubber is sufficiently stretched, crystallization may occur at temperatures above the norma crystallization temperature. In unstretched rubber the random thermal motion of the chain segments prevents crystallization. In stretched rubber these random thermal motions are drastically reduced. At higher temperatures the random motions may still have been sufficient to prevent crystallization even in the stretched rubber, but lowering the temperature to 0°C may have resulted in a transition to the crystalline form. Since it is random motion of the chains that resists the stretching force and allows the rubber to respond to forced dimensional changes, this ability ceases when the motion ceases. Hence, (he seals failed. 

Fig. 8.19 Elastomeric behavior of PET in compression at T = 353 K, 6 K above Tg. Unloading after true strains of 1.2, 1.5, and 1.7 shows effects of stretch crystallization (from Zaroulis and Boyce (1997) courtesy of Elsevier).

Examination of Fig. 8.19 more closely, however, shows other important effects. For unloading of the stretched PET from strain levels of 1.2, 1.5, and 1.7 there is strong hysteretic recovery behavior with a momentary permanence of the extension that is not completely relieved at zero stress. This is indicative of some semipermanent stretch crystallization and departure from ideal elastomeric behavior. 

It has already been stated (p, 38) that sulphur forms chain molecules on quick coolii after beiii kept at 250 C. If plastic sulphur, obtained on pouring the melt into cold water, is stretched, crystals are formed, built up from oriented polymers, composed of Sg-units Very pronounced, highly elastic properties and a considerable tensile strength can be observed at this stage, depending upon the temperature of preparation and upon the time elapsing after the cooling process. 

The strain or the stretching crystallization of rubbers is another important property to be considered in sealing applications. The stretching crystallization of various rubbers is given Table 5.4. 

Name Chemical name Vulcanizing a nt Stretching crystallization Gum tensile strength... 

As indicated earlier, the essential structural feature of a rubber vulcanizate is its flexible three-dimensional network. It is this arrangement which leads to the characteristic elastic behaviour. Comparative values for some properties of typical vulcanizates of common elastomers are given in Table 18.1. When natural rubber is stretched, crystallization of the highly regular chains occurs and the material shows a high tensile strength. The addition of fillers such as carbon black results in some increase in strength but the effect is not so marked as with elastomers for which stress-induced crystallization is not possible. 

Under ordinary conditions natural rubber is an amorphous material. When frozen or stretched, it crystallizes in the cis form. It is crystallization that gives rubber its self-reinforcing effect. As the rubber is stretched, crystallization increases, and this raises the ultimate tensile strength. Shipments of rubber when held for extended periods of time at cool temperatures — say 5 °C — can become frozen. In this state, rubber is rock hard and impossible to plasticize mechanically. Luckily, the crystallization process is completely reversible. At plants, the rubber can be stored in a hot room with temperatures over 45 °C and its original softness brought back. The time required for this is largely dependent upon how well the bales are separated, for natural rubber is a poor conductor of heat. 

In Chapter III, surface free energy and surface stress were treated as equivalent, and both were discussed in terms of the energy to form unit additional surface. It is now desirable to consider an independent, more mechanical definition of surface stress. If a surface is cut by a plane normal to it, then, in order that the atoms on either side of the cut remain in equilibrium, it will be necessary to apply some external force to them. The total such force per unit length is the surface stress, and half the sum of the two surface stresses along mutually perpendicular cuts is equal to the surface tension. (Similarly, one-third of the sum of the three principal stresses in the body of a liquid is equal to its hydrostatic pressure.) In the case of a liquid or isotropic solid the two surface stresses are equal, but for a nonisotropic solid or crystal, this will not be true. In such a case the partial surface stresses or stretching tensions may be denoted as Ti and T2-... 

Figure Bl.22.1. Reflection-absorption IR spectra (RAIRS) from palladium flat surfaces in the presence of a 1 X 10 Torr 1 1 NO CO mixture at 200 K. Data are shown here for tluee different surfaces, namely, for Pd (100) (bottom) and Pd(l 11) (middle) single crystals and for palladium particles (about 500 A m diameter) deposited on a 100 A diick Si02 film grown on top of a Mo(l 10) single crystal. These experiments illustrate how RAIRS titration experiments can be used for the identification of specific surface sites in supported catalysts. On Pd(lOO) CO and NO each adsorbs on twofold sites, as indicated by their stretching bands at about 1970 and 1670 cm, respectively. On Pd(l 11), on the other hand, the main IR peaks are seen around 1745 for NO (on-top adsorption) and about 1915 for CO (tlueefold coordination). Using those two spectra as references, the data from the supported Pd system can be analysed to obtain estimates of the relative fractions of (100) and (111) planes exposed in the metal particles [26].

Stretching a polymer sample tends to orient chain segments and thereby facilitate crystallization. The incorporation of different polymer chains into small patches of crystallinity is equivalent to additional crosslinking and changes the modulus accordingly. Likewise, the presence of finely subdivided solid particles, such as carbon black in rubber, reinforces the polymer in a way that imitates the effect of crystallites. Spontaneous crystal formation and reinforcement... 

For large deformations or for networks with strong interactions—say, hydrogen bonds instead of London forces—the condition for an ideal elastomer may not be satisfied. There is certainly a heat effect associated with crystallization, so (3H/9L) t. would not apply if stretching induced crystal formation. The compounds and conditions we described in the last section correspond to the kind of system for which ideality is a reasonable approximation. 

The disks are assumed to lie in the same plane. While this picture is implausible for bulk crystallization, it makes sense for crystals grown in ultrathin films, adjacent to surfaces, and in stretched samples. A similar mathematical formalism can be developed for spherical growth and the disk can be regarded as a cross section of this. 

At the molecular level, stretching extends polymer coils and facilitates crystallization. 

Orientation. Most articles made of HDPE, including film, fiber, pipes, and injection-molded articles, exhibit some degree of molecular and crystal orientation (21). In some cases, orientation develops spontaneously for example, during melt flow into a mold and its subsequent crystallisation. When blown HDPE film and fiber are manufactured, orientation can be introduced dehberately by stretching. 

Polymorphism. Many crystalline polyolefins, particularly polymers of a-olefins with linear alkyl groups, can exist in several polymorphic modifications. The type of polymorph depends on crystallisa tion conditions. Isotactic PB can exist in five crystal forms form I (twinned hexagonal), form II (tetragonal), form III (orthorhombic), form P (untwinned hexagonal), and form IP (37—39). The crystal stmctures and thermal parameters of the first three forms are given in Table 3. Form II is formed when a PB resin crystallises from the melt. Over time, it is spontaneously transformed into the thermodynamically stable form I at room temperature, the transition takes about one week to complete. Forms P, IP, and III of PB are rare they can be formed when the polymer crystallises from solution at low temperature or under pressure (38). Syndiotactic PB exists in two crystalline forms, I and II (35). Form I comes into shape during crystallisation from the melt (very slow process) and form II is produced by stretching form-1 crystalline specimens (35). 

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