Stiffening effect

The rather knobbly side groups have a stiffening effect on the chain and result in high values for T (245°C) and Tg(50-60°C). Copolymerisation with hex-l-ene, oct-l-ene, dec-l-ene and octadec-1-ene which may be practised to reduce voidage causes some reduction in melting point and crystallinity as indicated in Table 11.9. 

Because of the chain-stiffening effect of the benzene ring the TgS of commercial materials are in the range 90-100°C and isotactic polymers have similar values (approx. 100°C). A consequence of this Tg value plus the amorphous nature of the polymer is that we have a material that is hard and transparent at room temperature. Isotactic polystyrenes have been known since 1955 but have not been of commercial importance. Syndiotactic polystyrene using metallocene catalysis has recently become of commercial interest. Both stereoregular polymers are crystalline with values of 230°C and 270°C for the isotactic and syndiotactic materials respectively. They are also somewhat brittle (see Section 16.3). 

Generally there is a stiffening effect in compression compared to tension. As a first approximation one could assume that tension and compression behaviour are the same. Thomas has shown that typically for PVC, the compression modulus is about 10% greater than the tensile modulus. However, one needs to be careful when comparing the experimental data because normally no account is taken of the changes in cross-sectional area during testing. In tension, the area will decrease so that the true stress will increase whereas in compression the opposite effect will occur. 

The contribution to the stress from electromechanical coupling is readily estimated from the constitutive relation [Eq. (4.2)]. Under conditions of uniaxial strain and field, and for an open circuit, we find that the elastic stiffness is increased by the multiplying factor (1 -i- K ) where the square of the electromechanical coupling factor for uniaxial strain, is a measure of the stiffening effect of the electric field. Values of for various materials are for x-cut quartz, 0.0008, for z-cut lithium niobate, 0.055 for y-cut lithium niobate, 0.074 for barium titanate ceramic, 0.5 and for PZT-5H ceramic, 0.75. These examples show that electromechanical coupling effects can be expected to vary from barely detectable to quite substantial. 

The first use of PAs was in fiber applications, in which fibers were produced by melt spinning. These materials have a high strength and good wear resistance and can be easily dyed. The tension stiffening effect of the PA melt made the production of fibers witii homogeneous thickness possible. 

The property possessed by certain colloids of forming a gel-like structure on standing but which revert to a liquid form when subjected to agitation. The term has also been applied to the stiffening effect shown by uncured rubber compounds containing fillers, particularly carbon black. 

Stress-strain characteristics. Linear chain polymers are quite flexible and subject to creep or stretch. Branching or rings in the backbone have a stiffening effect. For example ... 

Table 7.1 Chain-Stiffening Effects of Groups Present in Polymer Backbones...

Because of the stiffening effects of the phenyl and carbonyl groups, PEEK has a high heat deflection temperature (300 C). This high-performance polymer has high strength and excellent resistance to solvents. 

Thermal black Soft carbon black formed by the decomposition of natural gas (e.g., MT, medium thermal black). It has little stiffening effect, but imparts toughness, resilience, good resistance to tearing, and fair abrasion resistance. 

The unperturbed dimensions and characteristic ratios of PTMPS chains are calculated using the RIS approximation. The low experimental value of the characteristic ratio reported for this polymer is successfully Interpreted by these computations. The stiffening effect expected from introduction of thep-phenylene group into the chain backbone is apparently offset by its reducing the repulsive Interactions that would otherwise occur among the atoms or groups before and after it eiong the chain. 

These are ingredients which when added in very small proportions to the compound stock produce a slight stiffening effect or decrease in plasticity. Benzidine and para amino phenol are examples of... 

Reinforcement - The stiffening effect of solids, such as carbon black, on an unvulcanized elastomer mixture and the enhancement of the physical properties of the vulcanized compound, such as tensile, elongation, modulus, abrasion resistance, tear, etc. 

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