Noncrystallizable elastomer

The strength and extensibility of a noncrystallizable elastomer depend on its viscoelastic properties (28,29), even when the stress remains in equilibrium with the strain until macroscopic fracture occurs. In theory, such elastomers have a time- or rate-independent strength and ultimate elongation, but such threshold quantities apparently have not been measured, though rough estimates have been made (28,30). 

Studies of uniaxial extension on noncrystallizable elastomer, poly(phenyl methyl siloxane) showed results which are consistent and comparable with those obtained for PDMS, suggesting that the crystallization is not important for this type of reinforcement [20]. Other examples for reinforcement effects achieved with the addition of silica fillers include polyisobutylene [24], poly(ethyl acrylate) [3], poly (tetra methylene oxide) [29,30], and some high-temperature polymers such as aromatic polyamides [14,33,34], polyi-mides [15,38,39], polybenzoxazoles [16,17], and polyben-zobisthiazoles [16,17]. Results indicated that the modulus increases with increase in silica content while the tensile... 

Schematic stress-strain isotherms in elongation for a unimodal elastomer in the Mooney-Rivlin representation of modulus against reciprocal elongation. The isotherms are represented as the dependence of the reduced stress ([f ] = f /(a - on reciprocal elongation. (f = f/A, f = elastic force, A = undeformed area, a = elongation). The top three are for a crystallizable network curve A for a relatively low temperature, B for an increased temperature, and C for the introduction of a swelling diluent. Isotherm D is for an unswollen unimodal network that is inherently noncrystallizable.

The two-state propagation mechanism cannot be tested by physical and spectroscopic properties of the ani-PP or kinetic results. We have reasoned that if the metallocene complex have dissimilar n -systems, and that their stereochemical controls of the states associated with them are different, then very different kind of PP may be produced. In particular, if one state is Isospeclfic and the other state is nonspecific, and if a chain propagates alternately via these two states, then the macromolecule would contain alternate crystallizable and noncrystallizable segments, such material would be a thermoplastic elastomer of homopolypropylene. 

The most active polymerization initiators for the polymerization of substituted oxiranes, particularly propylene oxide, produce a significant amount of product that is stereospecific and crystallizable. Most uses for polymers of propylene oxide, however, are in elastomeric systems (see chapter 2, table 4). For elastomers, the amorphous, noncrystallizable polymers are pre-... 

In the case of elastomers capable of undergoing strain-induced crystallization, such as CIS-1,4-polybutadiene networks (113), the higher the temperature, the lower the extent of crystallization and, correspondingly, the lower the ultimate properties. The effects of the increase in swelling parallel those for the increase in temperature, since diluent also suppresses network crystallization. For noncrystallizable networks, however, neither change is very important (114). 

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