Semicrystalline polymers secondary transitions

Higher values can be reached for semi-crystalline polymers below Tg the crystalline phase is stiffer than the glassy amorphous phase (e.g. PEEK, E 4 GPa). Semicrystalline polymers above Tg have, however, a much lower E-value, such as PE (0.15 to 1.4 GPa) and PP 1.3 GPa) E is, in these cases, strongly dependent on the degree of crystallinity and on the distance to Tg. Sometimes a low modulus is also found for semi-crystalline polymers below Tg, due to the effect of one or more secondary transitions a strong example is PTFE (E = 0.6 GPa ). 

Thus plots of log aT versus l/T for a secondary relaxation will yield straight lines, not curves as in the WLF case. This fact has been used to distinguish the main glass transition from other relaxations occurring in semicrystalline polymers. ... 

The higher crystalline, cold compressed sample shows a so-called crystalline phase (a) transition at about 130°C, a (weak) glass-rubber (S) transition at about 50°C and a secondary, amorphous phase (y) transition at -75°C. This weak glass-rubber transition effect is typical for a semicrystalline polymer. It indicated already that it would be difficult to detect this effect by DSC. 

Secondary Transitions. Both amorphous and semicrystalline polymers display multiple relaxation transitions. It is customary to label relaxation transition in polymers as a, y, S, and so on, in alphabetical order with decreasing temperature, a-relaxation being the glass transition. One should, however, note that there is a lot of confusion in the classification of relaxation transitions and their interpretation. This situation is described in detail elsewhere (16,141,142). 

In this case, an apparent activation energy is determined, and it has higher values than secondary relaxations 100-300 kJ/mol for urethane-soybean oil networks (Cristea et al. 2013), 200-300 kJ/mol for polyurethane-epoxy interpenetrating polymer networks (Cristea et al. 2009), more than 400 kJ/mol for semicrystalline poly(ethylene terephtalate) (Cristea et al. 2010), and more than 600 kJ/mol for polyimides (Cristea et al. 2008, 2011). The glass transition temperature is the most appropriate reference temperature when applying the time-temperature correspondence in a multifrequency experiment. The procedure allows estimation of the viscoelastic behavior of a polymer in time, in certain conditions, and is based on the fact that the viscoelastic properties at a certain tanperature can be shifted along the frequency scale to obtain the variation on an extended time scale (Brostow 2007 Williams et al. 1955). The shift factor is described by the Williams-Landell-Ferry (WLF) equation ... 

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