5782
Macromolecules 1993,26, 5782-5789
MAS NMR Characterization of Syndiotactic Polypropylene: Crystal Structure and Amorphous Phase Conformation Piero Sozzani* Facoltii di Ingegneria, Universitb di Brescia, via Branze 38,25133 Brescia, Italy
Roberto Simonutti Istituto Ronzoni, via G. Colombo 81,20133 Milano, Italy
Maurizio Galimberti Centro Ricerche di Ferrara, HIMONT Italia s.r.l., piaz. Donegani 12,44100 Ferrara, Italy Received April 7, 1993; Revised Manuscript Received June 18, 199P
ABSTRACT The high-resolution solid-state 13CNMR characterization of highly syndiotactic polypropylene (sPP) was performed. Each carbon atom of the repeating unit shows at different temperatures several resonances, which were fully assigned to conformational sequences in different phases or different states of the same phase. The helix chain conformation was confirmed to be the only one present in the crystal structure. Fine details about the crystalline packing were obtained from the methyl signals and explained by the introduction of statistical disorder in the distribution of the chirality of the chains within the lattice. The average chemical shift of the mobile amorphous phase was determined at high temperature, giving for the first time an accurate measurement of the conformer distribution and supporting a model described by a considerable trans content in the amorphous phase. On the other hand, the conformations in the amorphous phase below the glass transition (at 203 K) were singly identified. A further phase interpreted as a rigid amorphous phase, showingdifferent relaxation properties and temperature dependence, could be also detected. The as-polymerizedpolymer was shown to be rich in this phase, which lies probably at the interfaces, associated with a less regular crystallization. The characterization reported in the following will deal with three different specimens obtained by the same synthesis and different thermal treatments. Viscosity. The inherent viscosity of the samples, measured in tetralin solution at 408 K (0.25 wt %), was 1.20 dL/g. GPC. Gel permeation chromatography was performed on a Waters 150-C GPC instrument equipped with Progel TSK columns, working at 408 K with o-dichlorobenzene as a mobile phase. The polymer shows a weight-averagemololecular weight M, of 164 OOO, with a dispersion index MwJMn= 2.2. DSC. A Perkin-Elmer DSC-7 calibrated with indium and tin standards was used for the calorimetric analysis, in the range 253-453 K at a heating rate of 10 K/min. X-ray Analysis. X-ray diffraction spectra were recorded on a Philips automatic diffractometer, scanning the range of 28 from 5 O to 35O by steps of 0.05O 20. Ni-filtered Cu Ka radiation was used. N M R Solution State. Solutionstate 13CNMRspectra were obtained at room temperature in 1,2-dideuteriotetrachloroethane on a Bruker AM 300 instrument operating at 75.5 MHz. The following pentad distribution was obtained: rmmr = 1.3% ;mmrr =2.6%;rmrr=1.3%;rrrr=91.2%;rrrm= 3.3%;mrrm=0.3%; intensities of the other pentads are zero. Solid State. MAS 13CNMR analysis was performed using a Bruker CXP 300 spectrometer operating at 75.5 MHz. The samples (50-100 mg) were spun in a range between 203 and 393 K in zirconia rotors at a rate of 4500 Hz. For CPMAS spectra, the best contact time was found at ca. 2 ma; for MAS without cross-polarization, a 90° pulse for carbon of 3.8 ps was used. High-power decoupling (DD)of 15 G was applied, and delay between pulses was 5 s for CPMAS and 2 and 5s for MAS. For each spectrum 5Oo-lOoOtransients were collected. The resolution was checked on glycine (width at half-height = 26 Hz). Crystalline polyethylene (PE) was used as an external reference at 33.63 Experimental Section ppm from tetramethylsilane (TMS). Chemical shift correction with temperature was applied; PE spectra were run before and Synthesis. Syndiotactic polypropylene was obtained by Ewen’s catalyst system, Me~C(Cp)(9-Flu)ZrCl~[-Al(CH30-1,, after each experiment. accordingto the procedure already reported by one of the authom2
Introduction The presence of even minor microstructural defects is known to produce dramatic changes in the phases and properties of the materials. Syndiotactic polypropylene (sPP) of high structural purity is now available, being synthesized by a new family of Ziegler-Natta homogeneous catalysts, and is becoming more attractive for possible applications.lP2 However, as characterization methods continue to improve, they may reveal unexpected details about the phase structure of the materials. Therefore, we reexamined the solid-state NMR characterization of sPP, which constituted one of the early examples of the application of MAS NMR to po1yme1-s.~ In the case of sPP, the sensitivity of the methylene 13C chemical shift to the main-chain conformation is quite exceptional. Therefore, this polymer constitutes a good candidate for the following of crystal phase transitions and of the conformation and the mobility of the amorphous phase. Along this line, we recently presented a case in which the macroscopic stretching of the sPP specimen produces an “extended-chain” crystal and, as a consequence, the carbon signals due to the “gauche” conformations di~appear.~ The present work was conceived for exploring by NMR the conformational arrangements accessible to the chain, whether static or averaged, and, consequently, for obtaining the knowledge of the structure of a sPP sample when subjected to a variable thermal and mechanical history.
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Abstract published in Advance ACS Abstracts, September 1,
1993.
0024-9297/93/2226-5782$04.00/0
Results and Discussion The following treatment will deal with a homogeneous series of specimens of sPP obtained by the same synthesis.
0 1993 American Chemical Society
Macromolecules, Vol. 26, No. 21, 1993
13C
NMR Characterization of sPP
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Table I. DSC and X-ray Diffraction Characterization of sPP Samples crystallinity ( % ) sample A
B C
X-ray
DSC"
T m ('0
0.39 0.58 0.25
0.42 0.58 0.23
143 155 140
a The evaluation is given referred to sample B based on the melting enthalpye2
The samples will be identified as follows. Sample A was obtained by precipitation with methanol from the polymerization medium (also called "native"). Sample B was obtained after purification and annealing of sample A. The polymer was hot washed with toluene, precipitated with methanol, dried under vacuum, melted, and annealed at 140 "C for 12 h. This sample is called the "annealed" sample. Sample C was obtained by quickly cooling from the melt and is also called the "quenched" sample. Sample D was obtained from sample C by drawing at 0 "C and is also called the "stretched" sample. The main part of the paper will be devoted to samples A, B, and C, since the solid-state NMR characterization of sample D has already been given in ref 4. However, the latter was useful for comparison. The calorimetric properties of samples A and B have been recently reported by some of the authors.2 The crystalline content and phase properties are presented in Table I. The Crystalline Phase. CPMAS spectra of the annealed sample B recorded at temperatures ranging from 293 to 360 K and at 2 ms5 of contact time are presented in Figure 1. A MAS spectrum recorded without CP and a long recycle time (100 s) is also reported. This spectrum should represent a relaxed spectrum, and, therefore, the proportions between the signals of different carbons in the different phases are close to quantitative. Five narrow signals are detected: two for methyls, one for methines, and finally two main singals for methylenes, far apart from one another. Their appearance and line width are not much influenced by the temperature. The broad or minor signals appearing in different proportions in the spectra will be discussed in a following paragraph. The chemical shifts (CSs), spin-lattice relaxation times 21' , and the assignments are listed in Table 11. In particular, the chemical shifts and the assignments of the main narrow peaks are reported and assigned to the helix crystal phase, as will be explained later. The rigid nature of this phase appears to be clearly indicated by the nearly complete lack of dependence of the chemical shifts and the relaxation times on the temperature and by the fact that the relaxation times themselves are of the order of some tens of a second for methylenes and methines. In addition, only the nuclei placed in a rigid phase can cross-polarize efficiently under the adopted experimental conditions. Isotactic PP shows for the a-and @-crystallinephases relaxation times of the same magnitude.6 As regards the methyls, the characteristic mobility of this group about the bond connected to the main chain lowers considerably the relaxation times even for rigid phases. (a) Chain Conformation. The methylene carbons are exceptionally sensitive to the polymer chain arrangement. Apart from minor chemicalshift effects (Tc averaged 21.4 21.5 1.1 T,averaged 28.8 29.1 29.2 0.2 To averaeed 48.1 48.1 0.2
