In semicrystalline (SC) polymers, the mechanical and dielectric loss spectra generally show (at least) one absorption peak between the melting temperature Tm, of the crystalline phase [1–14] and the glass transition temperature Tg, of the amorphous phase. The first peak just below Tm, is called αc its amplitude increases with crystallinity and therefore is attributed to reorientation of the chains in the crystalline phase. This reorientation is commonly identified with the flipflop mechanism and/or the screw motion of methylene group in the crystal lattice. According to the soliton like model of Manfield and Boyd [12, 13] collective distortion of the C-C skeleton can propagate through the crystal lattice and in doing so causes trans- port of matter and hence diffusion. Infrared and Raman spectroscopy studies support this model [14]. This transition involving cooperative movements (of the whole crystalline stem) is quite different from the various phase transitions (first order) observed in paraffin crystals (for example, the rotator phases). It is well known that well below the melting temperature annealing effects occurs (change in density, crystallite thickness, long period). Several authors have described these effects as a gradual and re- versible passage toward melting and premelting [15–22]. In the Tαc region the mobility of the crystalline chain has been observed by NMR [18–22], and only a few authors [5, 6, 18] realized that the annealing effect so widely studied appears at that temperature. Obviously, the αc transition is unique to SC polymer; its characteristics will explain the unusual properties of these materials.