Relaxation processes polypropylenes

Recent work has shown that the creep and recovery behaviour of ultra high modulus polypropylenes is very similar to that of LPE. Again the Sherby-Dorn plots form a good entry to the detailed examination of the creep response. Plateau creep behaviour similar to that of LPE has been observed, and the high stress process correlates well with the a-relaxation process in terms of its activation energy. 

Polymers with a flexible chain, such as polyethylene (PE), polypropylene, poly(tetrafluorethylene), or poly(methylene oxide), exhibit relaxation processes directly related to the presence of their crystalline Action, For PE, by far the most important system in this context, such processes may be dielectrically active, provided that the sample is "decorated" widi a few C-Cl or C=0 dipoles, by chlorination or oxidation [166]. 

Takayanagi s first comparison between the predictions of his model and the observed mechanical behaviour covered a wide range of crystalline polymers, including polyethylene, polyvinyl alcohol, polytetra-fluoroethylene, polyamide, polyethylene oxide, polyo. ymethylene and polypropylene. Attempts were made to define relaxation processes as associated with either the crystalline regions or the non-crystalline regions, and in the former case specific molecular mechanisms were proposed, e.cj. a local twisting mode of molecular chains around their axes and a translational mode of molecular chains along their axes. 

Men Y, Rieger J, Strobl G (2003b) Role of the entangled amorphous network in tensile deformation of semicrystalline polymers. Phys Rev Lett 91 955021-955024 Men Y, Strobl G (2002) Evidence for a mechanically active high temperature relaxation process in syndiotactic polypropylene. Polymer 43 2761-2768 Plazek DJ, Chay I, Ngai KL, Roland CM (1995) Visoelastic properties of polymers. 4. Thermo-rheological complexity of the softening dispersion in polyisobutylene. Macromolecules 28 6432-6436... 

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. 

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. 

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