Modulus aging effect

The three polymers studied display remarkable physical aging effects [24], with a strong increase of modulus and changes in the location and intensity of the glass transition. This phenomenon is exemplified in Fig. 17 where the changes of dynamic moduli in a PDEB sample aged for 14 months are very apparent. 

The ultimate tensile strength and the modulus at various strains are always quoted in technical literature. These are very good quality control values and give an indication to the type (ether, ester, MDI, IDI) and nature of the prepolymer system used. These tests are also valuable in evaluating the chemical and aging effects on polyurethanes. Where polyurethanes are used in tension, the amount they are stretched is normally no more than 30%. The highest extension is on the order of 100%. 

A modulus value increase upon storage under ambient conditions is also reported for other semi-crystalline polymers like, for instance, polypropylene. Struik [11] measured for PP a continuously increasing dynamic stiffness at 20°C in combination with a decrease of the intensity of the glass-rubber (S) transition of PP (the temperature location of the S-transition did not change). Struik called this phenomenon an amorphous phase ageing effect a densification process of the amorphous PP phase due to a free volume relaxation effect. 

In secondary drawing operations, the aging properties of the spun yam must be considered. Because polypropylene fibers have a low Tg, the spun yam is re-stmctured between spinning and drawing this is more important as the smectic content is increased (43). The aging process depends on whether the yam is stored on bobbins under tension or coiled in cans with no tension on the fiber. The aging of quick-quenched (smectic) polypropylene films has been studied (43). Stored at room temperature, the increase in yield stress is 5% in 24 hours. Similar data on polypropylene spun fibers have not been published, but aging effects are similar. Drawn fiber properties, such as density, stress relaxation modulus, and heat of fusion, age because of collapse of excess free volume in the noncrystalline fraction (44). 

Because of the simplicity of the first test method, most of the comparisons are made usiag this technique. The effects of the aging process are usually measured on tensile properties such as tensile strength, elongation, and stress (modulus) at 300% elongation (42). 

For partially crystalline ionomers, such as those based on copolymers of ethylene and methacrylic acid, even time or aging at room temperature can have an effect on mechanical properties. For example, upon aging at 23°C, the modulus of the acid form of the copolymer increased 28%, while in the ionomer form, the increase ranged up to 130%, with the specific gain in modulus depending on the degree of conversion and on the counterion that was present [17]. 

In conclusion, the different thermal histories imposed to PTEB have a minor effect on the /3 and y relaxations, while the a. transition is greatly dependent on the annealing of the samples, being considerably more intense and narrower for the specimen freshly quenched from the melt, which exhibits only a liquid crystalline order. The increase of the storage modulus produced by the aging process confirms the dynamic mechanical results obtained for PDEB [24], a polyester of the same series, as well as the micro-hardness increase [22] (a direct consequence of the modulus rise) with the aging time. 

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