Solid-State 13C-NMR Investigation of the Disorder in Crystalline Syndiotactic Polypropylene

Abstract

A quench-precipitated sample of highly syndiotactic polypropylene (s-PP) (with 86% fully syndiotactic pentads), which shows an X-ray spectrum near that calculated for an "ideal" orthorhombic C-centered structural model of packing as proposed by Corradini et al. (with chains in a helical (TTGG), conformation), has been analyzed by high-resolution solid-state 13C-NMR spectroscopy. The 13C-NMR CP MAS spectra of this sample, in addition to the resonances already reported in the literature for s-PP (in the B-centered structure), exhibit additional peaks in the regions of the methyl (at 18.9 and 22.4 ppm) and methylene (at 44.9 ppm) groups. It is established that these additional resonances belong indeed to nuclei in well-defined conformational environments fixed in rigid structures placed at the interface of the crystallites and within the strained or disordered area of the crystallites themselves. In particular, the peak at 44.9 ppm in assigned to CH2 carbon atoms in a TX.YG conformational environment, whereas the resonance at 18.9 ppm is assigned to CH3 in trans-planar portions of the chain (y-gauche effect). This indicates local disorder in the C-centered crystalline s-PP sample, aue to the presence of conformationally disordered chains. Models of s-PP chains comprising defective but energetically feasible portions of chains in a trans-planar conformation (disturbing the ('M'GG)n dominating conformation) are imagined as packed according to a C-pseudocentered structure. In order to keep the methyl groups in a crystalline register and the chains well interdigitated, the defects should be clustered on planes which cross 3D ordered portions in the crystals. The chemical shiRs of the carbon atoms in the regions comprising the defect for model chains have been calculated on an ab initio level through the IGLO method, and the results are compared with the experiments. The calculations provide a good explanation of the peculiarities of the experimental 13C-NMR spectra of the s-PP sample under study. Thus, detailed information on chain defects can be obtained by combining X-ray scattering, solid-state NMR, and computer simulations.

Variations of regular conformation structures in melt of syndiotactic polypropylene

“…Based on the X-ray diffraction results, some basic concepts have been constructed: there are at least three crystalline forms in sPP, with ggtt conformations in helical form I, all-trans in form II, and (g 2 t 2 g 2 t 6 ) n in form III [1e3]. The crystallization behavior of sPP, in fact, is more complicated with respect to the above mentioned three polymorphs, which have been intensively investigated by Lotz et al and De Rosa et al [2,3]. However, there is only little information regarding one of the crucial parts for sPP, namely, the conformation structures and its variation in the melt.…”

Dynamic FT-IR Analysis of the Crystallization to the Planar Zigzag Form for Syndiotactic Polypropylene

“…The former two modifications are known to take on the most stable conformation. [3][4][5][6][7][8][9][10][11][12][13][14] A conventional method for crystallization to form III with the planar zigzag form required the cold-drawing process. [15][16][17][18] However, we recently reported that this conformational form could be obtained by holding in ice-water for several hours after quenching into ice-water from the melt.…”

“…19 It is worth noting that this process needs no elongation process. Our previous research 19 was performed at room temperature by solid-state high-resolution 13 C NMR and wide-angle X-ray diffraction methods. The contribution due to form I with the t 2 g 2 conformation in addition to form III was also observed.…”

Local Packing Disorders in a Polymer Crystal by Two Dimensional Solid-State NMR

“…6,7,[11][12][13][14][15][16][17][18] Structural disorders such as conformations and regio-and stereodefects in the crystalline region have been quantatively characterized by high-resolution SS NMR. 6,7 The packing structures of the crystalline polymers were also investigated by highresolution SS NMR [12][13][14][15][16][17][18] However, the observed NMR line shapes were attributed simply to symmetry of space group in full limit order, were not related with the packing disorders due to up and down orientations. 18 This might be attributed to very minor structural change due to substitution of up-and downward chains and insufficient NMR spectral resolutions.…”