J. Phys. Chem. 1991, 95, 10156-10157
10156
The N 1s core level spectra of the films after the second reprotonation cycle are shown in Figure 6c,d. The imine/amine ratios of the base films remain substantially higher than that of the pristine base, and the BE separation between these two components further increases slightly, being 1.30 and 1.50 eV, respectively, with HCI and HC104 as the protonating acid. When the base samples are reprotonated for the third time, the C1 2p spectra of the films (Figure 7) show distinct differences from those after the first reprotonation (Figure 3) regardless of the type of acid used. The CI 2p spectra in Figure 7 cannot be properly curve fitted, assuming only the species C1-, Cl*, and -Cl or C104- and C104* at the BE’S discussed earlier. The number of chemical states of chlorine appears to have increased, indicating some changes in the intrinsic structure of the polyaniline chains. This phenomenon is not observed with the powder samples, and the CI 2p core level spectra of these samples after the third reprotonation are not substantially different from those after the first reprotonation. The chemical states of the chlorine in the bulk environment are not determined and may show variations from those on the surface. Such variations may also be manifested differently in powders and films due to the difference in specific surface areas. In spite of the changes observed in the CI 2p core
level spectra of the films, the N+/N ratios are reduced by about only 10%from the values obtained in the first reprotonation cycle and the u’s of these films are also not significantly lower than the values obtained after the first cycle. Conclusion
X-ray photoelectron spectroscopy analysis of emeraldine base films cast from NMP solutions and reprotonated by HClO, has established that, in addition to the imine units, a substantial proportion of the amine units can be protonated. Deprotonation of the salt films reveals structural changes which are dependent on the extent of reprotonation and the acid used in the process. These modifications drastically reduce the solubility of the base films but do not significantly affect the subsequent reprotonation process and the conductivity of the resulting salts. In the third reprotonation cycle of these films, changes in the chemical state of the anions become apparent. In the case of powder samples, the reprotonation/deprotonation process appears to be reversible over the three cycles tested with no apparent change in the structure of the polyaniline. Registry No. HCI, 7647-01-0; HC104, 7601-90-3; aniline (homopolymer), 25233-30-1.
Measurements of ‘H Spin Diffusion and Cross Polarization Rates in Annealed (Monoclinic) and Quenched (Smectic) Isotactic Polypropylene Hiroshi Tanaka* Macromolecular Research Laboratory, Faculty of Engineering, Yamagata University, Yonezawa, Yamagata, 992 Japan
and Shiro Maeda Department of Applied Chemistry and Biotechnology, Faculty of Engineering, Fukui University, Fukui, 910 Japan (Received: June 12, 1991)
Measurements of the relaxation time constants characterizing proton spin diffusion, Td, and cross polarization, TCH,have been made on annealed (monoclinic) and quenched (smectic) isotactic polypropylene. Td for methyl protons, Td(CH3), is longer than Tadfor methylene and methine protons, Td(CH,) and Td(CH), respectively, and Td(CH2) is longer than Td(CH). Td for the quenched sample is slightly longer than that for the annealed sample. The difference, however, is relatively small. There is a definite difference in TCHof methylene, TcH(CH,), methine, TcH(CH), and methyl carbons, Tc,(CH,). The order of TCH for the three carbons is TcH(CH3)> TcH(CH) > TcH(CH2). There is no substantial difference i n TCH for both quenched and annealed samples.
Introduction
It is well-known that the proton spin diffusion occurs in entire portions of the sample, but with a limit of diffusion length.l.2 In a plot of intensity vs time for the measurement of proton spinlattice relaxation time, T I ,of semicrystalline polymers consisting of crystalline and amorphous regions, the signal intensity often decays exponentially, indicating that the proton spin diffusion is effectively operating. Recently, Wu et aL3 have reported the two-stage feature of cross relaxation in the depolarization experiment for glycine and terephthalic acid and derived the following equation for the depolarized 13Cmagnetization of the I3CH group during high-speed spinning at the magic angle:
( 1 ) McCall,
D.W . Acc. Chem. Res. 1971, 4 , 223.
(2) McBrierty, V . J. Faraday Discuss. Chem. Soc. 1979, 68, 78 (3) Wu,X.;Zhang, S.; Wu,X . Phys. Rev. E 1988, 37,9827.
0022-3654/91/2095-10156$02.50/0
where M,,is the depolarized 13Cmagnetization for ”CH group, R is the spin diffusion rate among protons, and t is the time of depolarization. If we assume that the above equation is applicable to semicrystalline polymers, we can obtain information on spin diffusion from the first term of the equation and on cross depolarization from the second term. In this paper we describe some preliminary results of a relaxation time constant characterizing proton spin diffusion, Td, defined as Td = l / R , for methylene, methine, and methyl protons, where R is the spin diffusion rate among protons in the above equation. In addition, an estimation of a cross depolarization time constant, TcH, for methylene, methine, and methyl carbons was also carried out using above equation. The results obtained are informative and will serve as a basis for more detailed work. Exoerimental Section
The polymer used in this study was isotactic polypropylene extracted with boiling n-heptane. The molecular weight char@ 1991 American Chemical Society
The Journal of Physical Chemistry, Vol. 95, No. 24, 1991 10157
Isotactic Polypropylene 9@x decoupling
Figure 1. Pulse sequence for measurements of Td and Tc-. In this experiment CT, and 1, were 1 ms and 0.5 ms, respectively. TABLE I: Relaxation Time Constant Characterizing Spin Diffusion, TJ
Td3PS
sample quenched an nea I ed
CH,
CH
CHa
206 205
186
232 220
174
acteristics after extraction was M, = 24.4 X lo4 and M, = 7.6 X 104. A quenched sample was prepared by compression molding of the polymer at 230 OC for 5 min followed by quenching in ice water. An annealed sample was prepared by annealing the quenched sample in poly(ethy1ene glycol) at 153 "C for 1 h followed by quenching in ice water. The crystal form and crystallinity, X,, of the quenched and annealed samples are smectic, X, = 38%, and monoclinic, X , = 72%, respectively. 13C signals were obtained using a JEOL Model NM-SH60S operating at 15.04 MHz for 13Cby the conventional spin-locked CP/MAS method with a magic angle rotation speed of -2.5 kHz. In this work the cross depolarization behavior was investigated by using the pulse sequence reported by Wu et al.? in which double contact time (CT, and CT2) was adopted to observe the cross depolarization process. CT, and t, (shown in Figure 1) were 1 ms and 0.5 ms, respectively, and the repetition time was 3 s. The proton decoupling was carried out with radio frequency field strengths of 60 kHz, and the proton decoupling frequency was chosen to maximize the peak height of methylene carbons. For the measurements of Tsdand TCH 640 transients were accumulated for each spectrum. All measurements were made at ambient temperature.
Results and Discussion Parts a-c of Figure 2 show the semilog plots of the intensity vs CT2(7in Figure 1) for methylene, methine, and methyl carbons in the annealed sample, respectively. An initial rapid drop followed by a slow decrease was observed for the methylene and methine carbons. Even for the methyl carbons, for which the two-stage feature is usually not ob~erved,~ the signal intensity decays in two stages, though the initial decrease is relatively slow compared to that of methylene and methine carbons. From a slope of a straight line in the slow-decrease region, Td was obtained as shown in the figure. All the Td values for the quenched and annealed samples are listed in Table I. The order of Td for three kinds of protons is Td(CH3) > Td(CH2) > Td(CH). The longest value of Td(CH3) indicates the slower spin diffusion rate in the rotating methyl protons. Although there is some scatter in these data (*5%), it seems that T,(CH2) is longer than Td(CH). There is experimental evidence that in isotactic polypropylene the C P spectrum obtained by cross polarization (CP) showed mainly associated carbon nuclei in the rigid regions4 In that experiment the CP spectrum was investigated as a function of temperature and compared with a spectrum obtained by a conventional single-pulse method without CP. The spectrum without C P varies largely with increasing temperature, indicating that the carbon nuclei in the amorphous regions also contribute to the spectrum, while the spectrum obtained by the C P method does (4) Maeda, S.; Terao, T.;Saika, A. Proceedings of the 18th NMR Symposium, Osaka, 1979; p 49.
T.d-220
O''
100
200
pS 300 t,
ps
Figure 2. Cross depolarization behavior of methylene, methine, and methyl carbons in annealed sample. Plots of signal intensity I (arbitrary unit) vs time: (a) methylene carbons ( 0 ) ;(b) methine carbons ( 0 ) ;(c) methyl carbons (0). Plot of signal intensity difference I (arbitrary unit)
vs t2: (d) methine carbons ( 0 ) .
TABLE II: Time Constant Characterizing Cross Polarization, Tcw
quenched annealed
19 18
26 26
49 44
not change substantially at least up to 130 OC. These results indicate that at least in isotactic polypropylene the CP/MAS spectrum is associated with the carbon nuclei only in the rigid regions. This leads to the nearly same value in Td for both quenched and annealed samples. If we assume that the information on the transfer of magnetization between protons and carbons can be obtained by the analysis of the cross depolarization, we can obtain TCH value from the analysis of the second term of the Wu's equation. Strictly speaking, as the second term consists of the product of exp(-3Rt/2) and exp(-t2/2T2*), it is difficult to obtain T2values (here T2implies TCH)from a slope of a straight line in a semilog plot of intensity vs t 2 as the exp(-3Rt/2) is not a constant. For the very initial stage of time, however, the values of -3Rt/2 = -3t/2Td do not change much. For example, if we adopt the value of 200 MS for Td (see Table I), the values of 3t/2Td are 0.075, 0.15, and 0.3 for t = 10,20, and 40 ps, respectively. Corresponding values of exp(-3t/2Tsd) for these times are ca. 0.93,0.86, and 0.74, respectively. These values are in the range of ca. 0.84 f 0.09 ( 1 l%), and to a very rough approximation, we can consider exp(-3t/2Td) as a constant for the very initial range of time. Figure 2d was obtained on the assumption mentioned above. Good linearity was observed for r2 plots and TCHwas calculated from the slope of this line. In Table I1 are included TCH for methylene, methine, and methyl carbons in both quenched and annealed samples. The longest value of TcH(CH3) indicates weaker dipolar interaction between protons and carbons, and the shorter value for TcH(CH2) than that for TcH(CH) can be attributed to the number of protons bonded to methylene carbons. The difference between annealed and quenched samples was not observed as is similar to the case of Td. Registry No. Isotactic polypropylene, 25085-53-4.
