Amorphous elastomers

The properties of elastomeric materials are also greatly iafluenced by the presence of strong interchain, ie, iatermolecular, forces which can result ia the formation of crystalline domains. Thus the elastomeric properties are those of an amorphous material having weak interchain iateractions and hence no crystallisation. At the other extreme of polymer properties are fiber-forming polymers, such as nylon, which when properly oriented lead to the formation of permanent, crystalline fibers. In between these two extremes is a whole range of polymers, from purely amorphous elastomers to partially crystalline plastics, such as polyethylene, polypropylene, polycarbonates, etc. 

Two different substituents at the P of different length Amorphous elastomers... 

The true value of the chloropolymer (I) lies in its use as an intermediate for the synthesis of a wide variety of polytorgano-phosphazenes) as shown in Figure 1. The nature and size of the substituent attached to the phosphorus plays a dominant roll in determining the properties of the polyphosphazene. Homopolymers prepared from I, in which the R groups are the same or, if different, similar in molecular size, tend to be semi-crystalline thermoplastics. If two or more different substituents are introduced, the resulting polymers are generally amorphous elastomers. (See Figure 1.)... 

Amorphous elastomers are obtained when phosphazene is refluxed with nucleophiles, such as sodium trifluoroethoxide or sodium cresylate, and secondary amines. Difunctional reactants such as dihydroxybenzenes (hydroquinone) produce cross-linked phosphazenes. 

Although hdpe and it-PP are crystalline, the commercial random copolymer of ethylene and propylene (EP) is an amorphous elastomer. The most widely used EP copolymer (EPDM) is produced by the copolymerization of ethylene and propylene with a small amount of an alkyldiene this permits cross-linking or vulcanization. 

These highly amorphous elastomers have relatively low Tt values (—73 C) and tend to crystallize when stretched. The cold flow of these thermoplastic polymers is reduced when they are crosslinked (vulcanized) with a small amount (2%) of sulfur. Since these polymers of isoprene have a solubility parameter of 8.0 H, they are resistant to polar solvents but are soluble in many aliphatic and aromatic hydrocarbon solvents. The cross-linked derivatives swell but do not dissolve in these solvents. 

This copolymer is an amorphous elastomer. Because of its outstanding resistance to heat, solvents, and corrosives, and in spite of its brittleness at low temperatures, this copolymer is used for gaskets and (brings. 

Polymers of epichlorohydrin and copolymers of epichlorohydrin with ethylene oxide am atactic, flexible, amorphous elastomers with the following repeating unit ... 

Ultimate stress-strain properties of amorphous elastomers... 

The nature of the polymer slightly affects the solubility and is probably related to the solubility parameter of the polymer. For amorphous elastomers without strong polar groups (and even for amorphous polymers in general ) Eq. (18.12c) may be used as a first approximation (with an accuracy of 0.25). 

Crystallinity can be reduced by disruption of the order in the chain by copolymerization.14 For example, both polyethylene and polypropylene are crystalline plastics, whereas ethylene-propylene rubber produced at about a 50 50 ratio is an amorphous elastomer. Compositional excursions much outside this range lead to crystalline materials.15 For some materials, such as natural rubber, that are close to crystallizing, stretching the chains can align them sufficiently for crystallization to occur. Such polymers can exhibit excellent gum properties and improved strength in the uncured state that greatly facilitate processing. 

SBR is an amorphous elastomer, with irregular chains. It does not exhibit crystallisation either on stretching or cooling and therefore exhibits negligible gum strength, unless it is reinforced with a fine particle size carbon black. It is compounded much like natural rubber. 

The phenomenology of failure of amorphous elastomers (crosslinked amorphous polymers at T>Tg) differs from the scheme shown in Figure 11.9. It will be discussed later. 

Furthermore, monomers from which crystalline homopolymer can be produced, such as high-density polyethylene and polypropylene, can be copolymerized to produce resins with controllably reduced crystallinity and thus greater transparency. The ethylene/propylene copolymers may range from partially crystalline plastics to amorphous elastomers. 

New Reactive Polyolefins Containingp-Methylstyrene Ranging from Semicrystalline Thermoplastics to Amorphous Elastomers... 

PIB has a high bulk density for an amorphous elastomer, which leads to low gas permeability and high hysteresis at a given temperature. 

FIGURE 10.13 Fracture energy, G, versus rate of tearing, R, reduced to Tg for six unfilled amorphous elastomers. (From Mullins (1959).)... 

FIGURE 10.16 Fracture energy, G, for an amorphous elastomer (SBR) reinforced with 30% by weight FT carbon black. (From Greensmith (1956).)... 

Although amorphous elastomers are found to tear steadily, at rates controlled by the available energy for fracture, G (as shown in Figures 10.13 and 10.14), strain-crystallizing elastomers do not tear continuously under small values of G, of less than about 10" J/m for natural rubber, for example (see Figure 10.15). Nevertheless, when small stresses are applied repeatedly, a crack will grow in... 

Amorphous elastomers show more crack growth under intermittent stressing than under a steady stress, and the additional growth step per stress cycle is found to depend on the available energy for fracture, G, in substantially the same way as for natural rubber. The principal difference is that over region 3, the exponent, a, in Eq. (10.21) is about 4 for SBR in place of 2 for NR (Lake and Lindley, 1966). 

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