Vibrational spectroscopic study of structural changes in isotactic polypropylene below the melting point

In situ FTIR spectroscopic study of the regularity bands and partial-order melts of isotactic poly(propylene)

“…Based on the changes of relative intensity, the various regularity bands can be rearranged according to the degree of order: 940, 1 220, 1167, 1 303, 1 330, 841, 998, 900, 808, 1100 and 973 cm -1 . The sequence of 1 220, 841, 998 and 973 cm -1 is in agreement with those reported in literature [5][6][7][8][9][10][11][12] whereas the sequence of 1167, 900 and 808 cm -1 is different from the conclusions of Zichy 18) and Hendra 19) . Some regularity bands still remain, when the temperature is much higher than the melting temperature (165.3 8C), which accords with those reported in the literature 4,19) .…”

“…She obtained that the minimum sequence length necessary for the appearance of a particular regularity band increases in the order: 1 256, 900, 998, 841, 809 and 1 220 cm -1 . Subsequently, Hendra et al 19) found a transition temperature that was 30 8C smaller than the melting temperature of iPP and corresponds to the beginning of the molecular disordering process. Having considered the different temperature coefficients above and below the transition temperature, they suggested that the order of regularity bands increases in the order: 998, 1168, 900, 1 256, 809, 841 and 1 220 cm -1 .…”

Variations of regular conformation structures in melt of syndiotactic polypropylene

“…This process lasts till 166 C. It can be deduced that 868 cm À1 band corresponds to long ordered structures of helical conformation in sPP melt, which exist above T m but attenuate quickly with increasing temperature until complete disappearance at 166 C. This process is in good consistence with DSC trace (see Fig. 1), i.e., both FTIR and DSC results indicate that long ordered structures of helical conformation in sPP melt decrease almost linearly with T f before 166 C. Similar phenomenon has also been observed in iPP melt, in which the ordered structures at this stage were considered as a partially ordered melt, with the degree of ordering much lower than its crystalline state, but higher than the unperturbed melts [20,29,30]. Opposite to 868 cm À1 band, the intensity ratio of 963 cm À1 band increases slowly after sPP crystals are melted at about 128 C, implying that more planarezigzag structures are formed.…”

Fourier Transform Infrared Spectroscopy study of polymorphism in propylene-co-1-pentene copolymers: Trigonal form identification

“…These latest vibrations involve, then, conformational arrangements and are commonly designated as regularity bands or helix bands. The majority of the absorption bands that appear at frequencies below 1400 cm À1 are regularity bands [32] in the case of iPP, these bands being very important because the frequency at which vibrational coupling is observed depends on the helical sequence length [32][33][34][35][36][37][38] (see Table 1). Its significance is also related to the absence of true crystalline absorptions (resulting from intermolecular vibrational coupling of atoms within a crystal lattice) in the iPP infrared spectrum since the TGTGTG (3 1 ) helical conformation of its macromolecular chains is identical in the long-established polymorphs (including the mesomorphic modification [39]).…”