J. Phys. Chem. 1988, 92, 5011-5019 expected to enhance the interactions with the lattice vibrations. Moreover, it appears generally that the librational levels are not as efficient in the scattering of thermal p h ~ n o n s . ~ ~Finally, J~ recent experiments and theoretical calculations also show that the guest molecules in clathrate hydrates play an unique role in enhancing the crystal anharmonicity leading to larger thermal expansion coefficients than in ordinary ice.55s56
Conclusions We have determined, from the analysis of the low-temperature thermal conductivity of THF hydrate, that the low-frequencyguest vibrations cause the clathrate hydrates to be thermal glasses, despite their regular crystalline structures. The interactions of these localized optical modes fall in the frequency region of the (53) Narayanamurti, V.;Seward, W. D.; Pohl, R. 0. Phys. Rev. 1966,147, 481. (54) See ref 1, Table X,p 177. (55) Tse, J. S.;McKinnon, W. R.; Marchi, M. J . Phys. Chem. 1987, 91, 4188. (56) Marchi, M.; Mountain, R. D. J . Chem. Phys. 1987, 86, 6454.
5011
acoustic lattice vibrational modes, providing an efficient process for three-phonon ~ c a t t e r i n g . ~This ~ mechanism for phonon scattering is analogous to that in doped crystals and crystalline polymers where small "imperfections" in the crystal lattice give rise to low-frequency localized optical modes that couple with the lattice vibrations and result in low thermal conductivity. At high temperatures (>150 K), all the localized excitations in clathrate hydrates are fully activated, therefore, appearing to give a constant (and relatively short) mean free path for phonon ~ c a t t e r i n g . ' ~ Glasses are usually thought of as materials without long-range positional correlations; in clathrate hydrates we have a glass within a crystalline matrix.
Acknowledgment. We thank M. Van Oort and P. Jessop for experimental assistance and N S E R C and Energy, Mines and Resources (Canada) for financial support. Registry No. THF hydrate, 18879-05-5. (57) Donovan, B.; Angress, J. F. Lattice Vibrations; Chapman and Hall: London, 1971; p 80.
Glassy Carbons from Poly(furfury1 alcohol) Copolymers: Structural Studies by High-Resolution Solid-state NMR Techniques Hellmut Eckert,*?+Yiannis A. Levendis,$ and Richard C. Flagant Department of Chemistry, University of California at Santa Barbara, Goleta, California 931 06, and Department of Environmental Engineering Science, California Institute of Technology, Pasadena, California 91 125 (Received: January 25, 1988)
The chemical structure of glassy carbon particles produced from poly(furfury1 alcohol) copolymers is studied by "C cross-polarization/magic-angle spinning (CP-MAS) NMR and high-speed 'H MAS NMR. In agreement with earlier proposals, I3C NMR spectra confirm the buildup of a highly unsaturated system at the expense of furan rings and aliphatic carbon atoms, and upon heating to 800 K this conversion is essentially complete. Successive carbonization by air oxidation or pyrolysis at temperatures up to 1600 K is reflected in a gradual decrease of the I3C chemical shift from ca. 130 to 115 ppm versus tetramethylsilane. 'H MAS NMR is used to detect and quantitate the amount of residual C-bonded hydrogen species at various stages of the carbonization process. In addition, these spectra show intense, narrow resonances due to sorbed H 2 0 molecules, which resonate over a wide range of chemical shifts (between 2.5 and -8 ppm versus tetramethylsilane). In analogy with effects observed by Tabony and co-workers for molecules adsorbed above the basal plane of graphite, the upfield shifts observed for water sorbed in the glassy carbons of the present study are attributed to the large susceptibility anisotropy of submicroscopicallyordered, turbostratic, or partially graphitized regions of the samples. The extent of this ordering is inversely correlated with the absolutecontent of residual C-bonded hydrogen species and depends mainly on the temperature of pyrolysis, whereas the oxygen content of the heating atmosphere and the composition of the initial polymeric material appear to be of secondary importance. The results suggest that sorbed H 2 0 molecules can function as sensitive NMR chemical shift probes for the initial stages of crystallization processes in glassy carbons.
Introduction Glassy carbon materials are produced by pyrolysis of a variety of highly cross-linked polymers.' As the result of their extensive network of micropores, these materials possess molecular sieve properties.* Interest for technological application of glassy carbons includes their use as catalyst supports and polymeric membranes. Their physical and chemical characteristics are largely independent of the chemical nature of the precursor materials, although the degree of porosity can be controlled by mixing certain pore-forming agents with the initial polymer, by varying the duration and temperature of the heat treatment, and by activation processes through partial ~xidation.~Poly(furfury1alcohol) (PFA) has been used extensively as a starting material for the production of glassy carbons, because of its high carbon yield4 and the utility of the resulting materials to function as model systems for studying the 'University of California at Santa Barbara. *California Institute of Technology.
0022-3654/88/2092-501 1$01.50/0
combustion of pulverized c0al.~9~ In view of this interest, knowledge of the structural properties and their change during pyrolysis, oxidation, and changes of composition is essential. A variety of techniques have been employed, including infrared s p e c t r o ~ c o p y , ' ~differential ~*~ thermal a n a l y ~ i s , dilatometry,8 ~-~ thermogravimetric a n a l y ~ i s , ' . mass ~ , ~ spectrometry,I0 gas chromatography," and traditional solid-state nuclear magnetic reso( I ) Fitzer, E.; Schafer, w.; Yamada, S. Carbon 1969, 7, 643. (2) Schmitt, J. L., Jr.; Walker, P. L., Jr. Carbon 1972, 10, 87. (3) Walker, P. L., Jr.; Oya, A.; Mahajan, 0. P. Carbon 1980, 18, 377.
(4) Senior, C. L.; Flagan, R. C. Inr. Symp. Combust., ZOth, The Combustion Institute, 1984, 921. (5) Levendis, Y.A,; Flagan, R. C. Combust. Sci. Technol. 1987.53, 117. (6) Fitzer, E.;Schafer, W. Carbon 1970, 8, 353. (7) Conky, R. T.;Metil, I. J . Appl. Polym. Sci. 1963, 7, 37. (8) Nakamura, H. H.; Atlas, L. M. Proc. Fourth Con$ Carbon 1960,625. (9) Dollimore, D.; Heal, G. R. Carbon 1967, 5, 65. (IO) Saxena, R.R.; Bragg, R. H. Carbon 1978, 16, 373.
0 1988 American Chemical Society
5012 The Journal ofPhysica1 Chemistry. Vol. 92. No. 17. 1988
nance ( N M R ) methods.lz The mechanism of carbonization is thought to involve rupture of the methylene bridges between 400 and 720 K, leading to furan ring-opening, CH, elimination, and buildup of condensed aromatic ring systems. Above 720 K elimination of carbon monoxide takes place, and at even higher temperatures significant amounts of hydrogen are released, resulting in a highly unsaturated hydrocarbon residue.4I2 The above mechanism is supported by gas chromatography studies, in which the same order in the evolution of gaseous products is retraced.” Important information on the molecular mobility of poly(furfuryl alcohol)-based materials carbonized a t temperatures between 330 and 1130 K was obtained from N M R line-shape and spin-lattice relaxation studies.I2 Heating a t temperatures up to 590 K was found to increase the molecular weight and the degree of cross-linking to the point where the structural rigidity suppresm any segmental motion of the polymer backbone. Significant hydrogen release was inferred to take place at temperatures above 860 K. However, due to limitations inherent to the techniques used in that study, no structural information could be obtained on a molecular level. In the present study we have applied modern solid-state high-resolution N M R techniques to obtain further insight into the process of graphitization. We present here the results of both I3C cross-polarization magic-angle spinning (CPMAS) and IH high-speed MAS N M R studies on the pyrolysis (650-1600 K) and oxidation (1300-1600 K) of poMfurfuryl alcohol) (PFA) and several copolymer materials. Basic Principles of High-Resolution ‘)c and ‘HNMR in the Solid State In contrast to the high resolution achievable in liquid-state NMR, polycrystalline and amorphous solid samples produce N M R spectra that are severely broadened by direct dipole-dipole interactions as well as the anisotropy (Le.. the orientational dependence) of the chemical shift. For an isolated pair of spins the internuclear dipole-dipole interaction will cause a splitting into two lines, the separation of which allows determination of the internuclear distance.14 In the more general case, however, larger ensembles of nuclear dipoles interact, resulting in a structureless broadening of the spectrum, and a concomitant loss of structural information. However, the effect of the magnetic dipole-dipole interactions (as well as the chemical shift anisotropy) can be removed by rapid spinning of the sample at the angle of 54.7O with respect to the static field direction.” This technique (magic-angle-spinning (MAS) NMR) forms the basis of most high-resolution N M R studies in solids. Spinning speeds required depend on the strength of the dipolar interactions to be timeaveraged; in the case of IH MAS NMR, speeds of 5-8 kHz are not atypical, but are frequently insufficient.16 For solid-state I3C high-resolution N M R studies the MAS N M R technique is usually combined with cross polarization and high-power dewupling from neighboring ‘H nuclear spins.17 The cross-polarization technique takes advantage of the large nuclear polarization of the abundant proton spin system, if the effective precession frequencies in the rotating frame of both types of nuclei are matched. This is achieved by spin-locking the ‘H magnetiand applying a radio-frezation in a radio-frequency field auency field B, ,- at the ”C frequency such that the Hartmann-
~.~ ~
~I~ ~~~
(16) Ycsi&ki. I. P.; Eck . 1988. 110, 1367. (17) SchHfer. 1.;Stejskal. E. 0.; Buchdahl, R.Mocromoleeuler 1975.8, 291. (18) Hartmann, S. R.: Hahn. E. L. Phys. Reo. 1962, 128, 2042. ,
r-
Eckert et al.
I
i
Figure 1. Electron micrograph of glassy carbon particles mndc from IX‘& poly(ethylenc glycol) and PFA cured at 800 K for I h (horizontal bar denotes 10 urn).
is fulfilled, where Y~ and yc are the gyromagnetic ratios of IH and ”C, respectively. At the end of this “contact period” (typically 1-5 ms), the proton radio-frequency field is left on to provide high-power decoupling during the period in which the ”C free induction decay (FID) is acquired. A modification of the above technique can be employed to discriminate between protonated and nonprotonated carbon atoms by taking advantage of the different strengths of their IH-”C dipolar interactions. The addition of a delay time of 60-100 FS after the cross-polarization period but before the decoupling and acquisition period results in a dephasing of all signals arising from protonated carbon atoms and, therefore, facilitates the selective observation of nonprotonated carbon Protonated carbons can also be selectively identified by measurements employing short contact times (100 ,LS or less), since the proton-carbon cross-relaxation times are greatly reduced due to the stronger dipolar coupling for these carbon nuclei. Experimental Section Sample Preparation and Characterization. The glassy carbon materials under investigation were synthesized from three basic ingredients, consisting of a carbon yielding binder, mixing agent, and a pore-forming agent. The binder was furfuryl alcohol partially polymerized with the aid of p-toluenesulfonic acid catalyst. Acetone was added to the polymer in the ratio of 2 to 1 by volume to lower the viscosity of the polymer and to facilitate mixing and subsequent atomization. Finally, various organic liquids or solids were dissolved or suspended in the polymeracetone mixture to serve as pore-forming agents. The mixtures were fed through a syringe pump to an acoustically excited aerosol generator and were subsequently sprayed inside an externally heated thermal reactor. The full description of this system and the thermal reactor is given elsewhere? Following this procedure equal sized droplets were generated, heated to a maximum temperature of 650 K i n an inert atmosphere to evaporate the solvent and form solid particles, and collected by sedimentation at the bottom of the reactor. The total residence time in the reactor was approximately 4 s. Pores, in addition to those of the polymer matrix, were generated by evaporation or decomposition of the (19) Alla, M.;Lippmaa, E. Chem. Phys. Lett. 1976. 37, 260
The Journal of Physical Chemistry, Vol. 92, No. 17, 1988 5013
Poly(furfury1 alcohol) Copolymers TABLE I: Properties of Glassy Carbon Materials
chem anal material PFACcured at 650 K PFA cured at 800 K PFA + 18% TAd cured at 650 K PFA + 17% TA cured at 800 K PFA + 50% TA cured at 800 K PFA + 8% TA cured at 800 K PFA + 9% PEGe cured at 800 K PFA + 18% PEG cured at 800 K PFA + 20% glydcured at 800 K PFA
+ 20% glyc + 3% PEG + 3% triton cured at 800 K
hydrogen speciesa 100% type I 12.8% type I1 87.2% type I 100% type I 15.9% type I1 84.1% type I 15.9% type I1 84.1% type I 14.0% type IIb 15.3% type I1 70.7% type I 17.2% type I1 82.8% type I 19.3% type I1 80.7% type I 18.7% type I1 81.3% type I 19.1% type I1 80.9% type I
8,,
ppm (*O.l) 5.9 2.1
burnout, %
%Hb
%C
0
5.0 3.45 (3.01)
73.7 85.8
h
0 0
5.5 2.2
0 0
h
h
2.5
0
h
-3.1 2.09
h
3.0 (2.51) 3.08 (2.59) 1.92 (1.36)
0
87.2 82.6 91.02
h
2.0
3.25 (2.69) 3.36 (2.71) 3.20 (2.60) 3.41 (2.76)
0
h
2.4
0
h
2.1
0
h
2.2
0
h
87.5 87.26 87.2 84.6
C-bonded hydrogen (in wt %) in parentheses. Poly(furfury1 alcohol). dTannic acid. e Poly(ethy1ene a Determined by NMR peak integration. glycol). /Glycerol. ZWeighted average of types I1 and IIb = -0.44. *Not determined. 0
u
50% TAN AC 1600 K Air
I
'@Ln
7
PFA PARTIALLY COMBUSTED AT 1600 K IN AIR FOR 3 sec
17%TAN AC 1600 K Air
