Elastomers physical constants

Physical Constants of Poly(oxytetramethylene-oxyterephthaloyl) and Copolymers with Tetramethylene Oxide Thermoplastic Elastomers ... 

These copolymers were made by anionically polymerizing 1,3-butadiene with n-Buli followed by the addition of isoprene to the live cement. The molecular weight was varied in the 1,H poly(bd) block to produce the maximum physical properties. The content of the Bd/isoprene in the copolymer was varied 30/70. Similarly, (Table VI) the molecular weight of the diblock was kept constant at 60 AO Bd isoprene ratio, while the molecular weight of the individual block was varied. In Tables V and VI the physical properties of the di block of the conjugated diene rubber showed elastomeric properties typical of that of the uncrossed elastomer. 

In a stress-relaxation experiment, the sample under study is deformed by a rapidly applied stress. As the stress is normally observed to reach a maximum as soon as the material deforms and then decreases thereafter, it is necessary to alter this continually in order to maintain a constant deformation or measure the stress that would be required to accomplish this operation. The apparatus used varies in complexity with the physical nature of the sample, being simplest for an elastomer and becoming more sophisticated when the polymer is more rigid. 

In contrast to the extensive experimental investigations of nematic, cholesteric and chiral SmC phases, comparatively little work has been done on tbe characterization of the physical properties of SmA and SmC elastomers. The elongation, A, has been measured as a function of temperature for constant external load for a number of different loads in SmA sidechain elastomers [6]. It was found that A increases monotonically as a function of temperature for a material, which has a SmA-I transition. In addition it was shown that the elasticity modulus, E, decreases monotonically with temperature. X-ray investigations on SmA phases in side-chain LCE have been performed [70]. It was found that for the family of compounds studied, the orientation of the mesogenic groups was always perpendicular to the direction of stretching. 

Amorphous polymers when heated above Tg pass from the hard to the soft state. During this process, relaxation of any internal stress occurs. At the Tg many physical properties change abruptly, including Young s and shear moduli, specific heat, coefficient of expansion and dielectric constant. For hard polymeric materials this temperature corresponds to the highest working temperature, for elastomers, it represents the lowest working temperature. 

In many applications, AFLAS outperforms other elastomers because of the following characteristics (1) High temperature resistance (400 F long term 550-h°F shorter term) (2) Resistance to a wide range of chemicals (including acids, bases, steam, sour (H2S) oil and gas with amine corrosion inhibitors, oils and lubricants, hydraulic fluids of all types, brake fluids, bleaches, oxidizing agents, alcohol, etc.) (3) Durable physical properties (4) Excellent electrical resistance-Dielectric constant at 60 Hz of 2.5. 

The increasingly demanding requirements for applications under the hood extend beyond the physical service limits of conventional oll-resistant elastomers, such as nitrile rubber (NBR) and halogenated rubbers. Therefore, they are replaced by temperature resistant elastomers such as hydrated nitrile rubbers (HNBR), Figure 5.239. In addition to the required temperature resistance, FINBR exhibits the combination required for hose materials such as high strength and strain, constant modulus, dynamic resistance, abrasion resistance and chemical resistance, see Section 5.5 [796],... 

Rory (pages 5-9 of ref. 22) reported three types of experiments from which he deduced no evidence for structure (1) stress-temperature coefficients, (2) vapor pressure of a PIB-diluent system, and (3) ring-chain equilibrium constants between cyclic and linear siloxanes. In each case the systems were evaluated far above their respective T/ s. Such results are not pertinent to our present inquiry. We have searched sporadically but without success for physical measurements which span a temperature region across Tu in elastomers. Finally, we note that because elastomers tend to be flexible hydrocarbons, Tu should be weak and may not have a great influence on physical properties. The marked exception to this generalization is PIB with its stiff, stereoregular backbone. Tu in PIB has been discussed recently in great detail, with Tu 250 K, Tip 290 K. 

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