High-resolution solid-state carbon-13 NMR study of isotactic polypropylenes isothermally crystallized from the melt

Abstract

The phase structure of isotactic polypropylenes isothermally crystallized from the melt has been examined at different temperatures by solid-state high-resolution 13C NMR spectroscopy using VT/MAS (variable temperature/magic angle spinning). On the basis of 13C spin-lattice relaxation and computer line-shape analyses, it has been found that polypropylene samples are composed of not only crystalline and amorphous components but also another noncrystalline component with 31-helical conformation. Similar NMR analyses at different temperatures have revealed that a reversible structural change between the 31-helical and random conformations occurs in the noncrystalline region at 60-110 "C.

High-resolution solid-state NMR study of isotactic polypropylenes

“…Figure 1 shows that besides the lines observed at room temperature, the new resonance lines of CH 2 and CH carbons appear in the measured spectra with the rise of temperature. Similar effect was in the past observed in the spectra obtained using MAS DD 13 C NMR technique, i. e., without CP, on the PP crystallizing in $-form and these lines were assigned to the carbons within the chains of amorphous regions [7,11]. Therefore, additional lines detected for m-iPP at temperatures above 51°C can be associated with the CH 2 and CH carbons in amorphous regions of m-iPP.…”

“…CP MAS 13 C NMR spectra of m-iPP measured at temperatures as indicated, except the spectrum measured at 160°C for the molten sample using single pulse MAS 13 C NMR technique. CP MAS 13 C NMR spectra were normalized shown for m-iPP confirming that both iPP samples are in the hexagonal #-form [4,6,7]. However, small differences between the spectra measured at the same temperature can be found.…”

“…On the other hand, the sample crystallizing in monoclinic structure ($-form) prepared by an annealing procedure gives the spectra with the CH 2 and CH 3 lines split into the doublets. This splitting which appeared in NMR experiments carried out with CP as well as without the application of the CP technique, was shown to be associated with two different packing sites in crystals [4,7,11]. With increasing temperature the lines of the CH 2 and CH 3 doublets with lower chemical shifts were shown to be gradually shifted to the positions with higher chemical shifts and merged into singlet lines.…”

“…It was shown that the increase of the temperature gives rise to new resonance lines of the CH 2 and CH groups with chemical shifts between the lines observed at room temperature and those observed in the iPP solution. These lines detected as separated lines in the MAS DD spectra and as shoulders of the original lines in the CP MAS DD experiment, were assigned to the carbons in amorphous regions of the partially crystalline iPP [7]. To reveal if there are some differences in the structure and molecular motion between m-iPP and ZNiPP we report the high-resolution 13 C NMR study of the m-iPP and ZN-iPP crystallizing predominantly in the hexagonal #-form, based on CP MAS DD and single pulse MAS DD techniques at temperatures ranging from room temperature up to the melting point of the polymer.…”

“…NMR investigation is considered to be one of the basic relevant methods. The high-resolution solid-state 13 C NMR studies of iPP reported NMR spectra recorded with combination of magic angle spinning (MAS), cross polarisation (CP) and dipolar decoupling (DD) techniques [3][4][5][6][7][8][9][10][11][12][13][14]. Since iPP can crystallize in several crystalline forms, depending on the crystallization temperature and on the presence of specific nucleating agents, the shape of the spectra was found to depend strongly on the physical treatment of the sample and the temperature at which the NMR experiment was carried out.…”