Amorphous elastomers, tensile

The new polymers are intermediate in composition and crystallinity between the essentially amorphous EPR and the semicrystalhne iPP. The presence of the complementary blocks of elastomers for both ethylene and propylene crystallinity should not indicate a similarity, beyond the levels of the crystallinity in the properties of the E-plastomers and the P-plastomers. The E-plastomers and the P-plastomers differ in their stmctural, rheological, as well as their thermal, mechanical, and elastic properties. In a comparison of the tensile strength and tensile recovery (tension set) from a 100% elongation for a range of P-plastomers and E-plastomers, the former have lower tension set than EPR and iPP. However, for comparative E-plastomers and P-plastomers at equivalent tensile strength, the latter have significantly better tension set. In summary, P-plastomers are tough polyolefins which are uniquely soft and elastic. 

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. 

Cured polymers of butadiene with low cross-link density do not tend to cold flow and are useful elastomers. These vulcanized elastomers crystallize when stretched, but when the stress is removed, the restoring force is largely entropy and most of the crystals melt and the chains return to the random conformation.The tensile strength is increased dramatically when large amounts of carbon black or amorphous silica are added. 

In addition to the polyolefin blends designed for thermoplastic elastomer applications, a great deal of interest also has centered on other kinds of blends of polyolefins as has been reviewed recently (see chapter 21 of Ref. 10 by Plochocki). In a recent paper (84), we showed that blends involving polypropylene-high density polyethylene-low density polyethylene in various proportions and combinations exhibit additivity of tensile strength however, there are serious losses in ductility in some cases such that the blends are less ductile than either pure component. It is interesting to note, however, that these losses in ductility can largely be restored by addition of rather small amounts of an amorphous ethylene-propylene rubber (84). 

The answers to these questions can be gleaned from Table 13-2, which compares approximate values of the tensile modulus for various polymers. Rubbers or elastomer are also amorphous, of course, but they respond to a stress in an entirely different manner to all other types of materials. Because they have low Ts, at ordinary temperatures, they respond to a load by changing their distribution of chain conformations, the chains becoming more extended as the material is stretched. A rubber has to be extended many limes its original dimensions before the covalent bonds take the load. We will consider rubber elasticity as a separate topic later. 

The ionomer which was isolated from the neutralization of sample SBD-2 was a brown-colored elastic network of moderate strength. Ionomer samples SBD-1 and SBD-2, neutralized to the stoichiometric end point using KOH, were compression molded at 140°C and examined for tensile properties. The results, as shown in Figure 16, illustrate the profound influence of crystallinity on the elastomeric inner block. The semi-crystalline material (SBD-1) behaves much like a rigid plastic, while the amorphous sample (SBD-2) is an elastomer of moderate strength. 

On the other hand, a gradual increase in tensile strength and compression set values is observed for cast elastomers containing more amorphous hard segment contents. 

Amorphous thermoplasts find application below their glass transition temperature and crystalline thermoplasts are used below their crystallization temperature. So, in contrast to elastomers, the chain segments of thermoplasts at application temperatures exist in the frozen-in state. In this state, there are many physical bonds between segments, and so the tensile strength of thermoplasts is higher than for elastomers. The physical bonds, however, also reduce segmental mobility, and so, the extension at break of thermoplasts is less than for elastomers. 

Common examples of miscible blends are ethylene-propylene copolymers of different composition that result in an elastomer comprising a semicrystalline, higher ethylene content and an amorphous, lower ethylene content components. These blends combine the higher tensile strength of the semicrystaUine polymers and the favorable low temperature properties of amorphous polymers. Chemical differences in miscible blends of ethylene-propylene and styrene-butadiene copolymers can also arise from differences in the distribution and the type of vulcanization site on the elastomer. The uneven distribution of diene, which is the site for vulcanization in blends of ethylene-propylene-diene elastomers, can lead to the formation of two distinct, intermingled vulcanization networks. 

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