Cooling Flexural modulus

Flexural modulus increases by a factor of five as crystallinity increases from 50 to 90% with a void content of 0.2% however, recovery decreases with increasing crystallinity. Therefore, the balance between stiffness and recovery depends on the appHcation requirements. Crystallinity is reduced by rapid cooling but increased by slow cooling. The stress—crack resistance of various PTFE insulations is correlated with the crystallinity and change in density due to thermal mechanical stress (118). 

In theory, the flexural modulus and the tensile modulus should be the same. In practice this is only approximately so, because plastics materials are seldom (if ever) isotropic throughout the thickness. DifTerenlial cooling rates, variations in extrusion or injection rates, changes in flow patterns, etc., all contribute to nonuniform properties throughout the thickness. When coupled with the nonuniform stresses already mentioned, it is hardly surprising that inconsistencies with the tensile test arise. 

Isotactic polybutene crystallizes into three different forms. When it cools from the melt, it initially crystallizes into a metastable crystalline one. After several days, however, it transforms into a different form. Noticeable changes in melting point, density, flexural modulus, yield, and hardness accompany this transformation. The third crystalline form is due to crystallization from solution. The polymer exhibits good impact and tear resistance. It is also resistant to environmental stresscracking. 

Toughness is the energy a film can absorb before rupturing, and it is measured by the area under the stress-strain curve. Brittleness is the lack of toughness. Amorphous and semicrystalline polymers become brittle when cooled below their glass transition temperature. Tests for tensile properties are described in the ASTM D 882, for flexural strength in ASTM D 790, and for flexural modulus in ASTM D 790M. 

Flexibility is an important end-use property for elastomers. The Tg, which is the point at which an elastomer (on cooling) goes from a flexible more rubber-like form to a more rigid inflexible form, is a critical parameter in determining the elastomer s suitability for specific applications. Plots of flexural storage modulus G (Pa) versus specimen temperature are very useful in evaluating the stiffness and flexibility of polymeric materials. 

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