Polypropylene Secondary relaxations

The effect of temperature on microhardness at low temperatures (for PE between —60 and 25 °C and for isotactic polypropylene (i-PP) between -20 and 25 °C) has also been studied (Perena et al, 1989 Martin et ah, 1986). While for i-PP it is possible to detect accurately the glass transition temperature, for PE it is concluded that the use of only H for recognizing the secondary relaxations in PE does not allow their precise temperature location. Here the joint use of dynamic mechanical techniques and microhardness is recommended (Perena etal, 1989). 

Usually the primary (P) glass-rubber relaxation cannot be resolved from the secondary relaxation at hypersonic frequencies. However, this is not always the case. The Brillouin frequencies Awd) and tan 8 for polypropylene glycol (PPG) (13) are plotted versus temperature in Figure 9. Two tempratures of maximum loss are observed. The higher temperature loss at 100 °G and a frequency of 4.40 GHz correlates very well with the primary glass-rubber relaxation line determined by dielectric relaxation at gigahertz frequencies (13), The lower temperature loss at 50°G and a frequency of 5.43 GHz correlates with an extension of the secondary transition line. The transition map is shown in Figure 10. 

Cold rolling led also to an increase in the creep strain and secondary creep strain-rate. The creep activation energy was found to increase with increasing rolling reduction. Within the secondary creep stage, the creep process in polypropylene is mainly due to the a-relaxation process and most of the creep strain was recoverable. 

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). 

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