Apparent Activation Energies for Molecular Motions in Solid Asphalt

Aug 31, 2006 - The hydrogen spin−lattice relaxation time in the rotating frame was measured for three whole asphalts at temperatures ranging from 20...
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Energy & Fuels 2006, 20, 2181-2188

2181

Apparent Activation Energies for Molecular Motions in Solid Asphalt† Daniel A. Netzel* Western Research Institute, 365 North 9th Street, Laramie, Wyoming ReceiVed April 24, 2006. ReVised Manuscript ReceiVed July 10, 2006

The hydrogen spin-lattice relaxation time in the rotating frame was measured for three whole asphalts at temperatures ranging from 20 to -45 °C. These data were used to calculate the apparent activation energies for the molecular motion of the aromatic and aliphatic components found in asphalt. The measured activation energies ranged from 8.8 to 9.8 kJ/mol for the aliphatic components in the three asphalts and were attributed to rapid methyl rotation of the terminal and branched methyl groups on the long carbon chainlength alkanes. The apparent activation energies measured for the molecular motion of the aromatic components in the three asphalts ranged from 6.5 to 7.2 kJ/mol. The low barrier to molecular motion observed for the aromatic constituents can be explained by two mechanisms. The first mechanism is spin-diffusion interaction of the aromatic ring hydrogens with the aliphatic hydrogens of the rapidly rotating methyl substituents on aromatic rings. The second mechanism is the in-plane rotation of relatively small polycondensed aromatic molecules and torsional oscillations of pendant phenyl groups as guest molecules within a nanopore matrix of rigidamorphous aliphatic components.

Introduction Asphalt is classified as a viscoelastic material. It is composed of several thousand different molecules. Typically, the carbon content ranges from 80 to 86 wt %, and the hydrogen content, from 10 to 12 wt %. Heteroatom content makes up the remaining weight percentage. The relative distribution of carbon atoms is ∼30% aromatic and ∼70% aliphatic. The majority of aromatic carbons are associated with polycondensed heteronuclear aromatic clusters. However, some single phenyl ring compounds with alkane substituents are possible. Structurally, the aliphatic carbons are associated with normal, branched, and cyclic alkanes. These alkane structural units can either be attached to a condensed aromatic cluster, or they can exist independently (e.g., paraffin waxes). The molecular interactions (hydrogen bonding, π-π interactions, etc.) in a complex molecular system such as asphalt are extensive. At any given temperature, these interactions will tend to control the molecular motions of the constituents in asphalt. The various types of molecular motions encountered in asphalt are shown in Figure 1. These motions include, but are not limited to: (1) rotation of terminal, branched, and alkyl-aromatic substituted methyl groups, (2) rotation of pendant phenyl groups, (3) in-plane rotation of polycondensed aromatic rings, and (4) segmental motions of methylene carbons of various chainlength in normal and branched alkanes. Translational and rotational † Disclaimer: The mention of specific brand names does not imply endorsement by Western Research Institute or the Federal Highway Administration. The contents of this paper reflect only the views of the author, who is responsible for the facts and accuracy of the data presented. This document is disseminated under the sponsorship of the Department of Transportation in the interest of information exchange. The United States Government assumes no liability for its contents or use thereof. The contents do not necessarily reflect the official views or policies of the Federal Highway Administration. This paper does not constitute a standard, specification, or regulation. * Retired. Mailing address: 2526 Sky View Lane, Laramie, WY 82070. E-mail address: [email protected].

Figure 1. Schematic representation of some of the possible types of molecular motions in aromatic and aliphatic components in asphalt.

motions of the whole molecule within the asphalt matrix at room temperature and below are not expected to occur at any appreciable rate. Molecular motion associated with asphalt molecular components is one of the more fundamentally important properties of asphalt. Motions in the midfrequency range of 10-100 kHz

10.1021/ef0601768 CCC: $33.50 © 2006 American Chemical Society Published on Web 08/31/2006

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are characteristic of the relatively long-range motions of alkane main-chain carbons and restricted motions of aromatic carbons below the glass-transition temperature. These motions can strongly influence the macroscopic rheological behavior in terms of mechanical loss1 and impact strength.2 Low-temperature cracking and high-temperature rutting (deformation) of road asphalt can be attributed to the degree of intra- (rotational and segmental) and inter- (translational) molecular motions, respectively. Because composition, molecular structure, intermolecular interactions, and, thus, molecular motions in asphalt vary for asphaltic material derived from different sources, the cracking and rutting characteristics will also differ. Thus, to achieve good road performance at all temperatures, an understanding of the molecular dynamics (motion) and energetics (activation energies) as it relates to the rheological properties of asphalt is of paramount importance. Netzel et al.3 have shown that the mass percent of mobile aliphatic carbons in asphalt can be correlated to many of the rheological properties of asphalt. These authors also have shown that the mass percent of mobile aliphatic carbon is inversely related to the “fracture temperature”. The fracture temperature is that temperature at which asphalt begins to crack as a result of increased tensile stresses due to a decrease in specific volume induced in asphalt as the temperature decreases. Recently, the fracture temperature was shown to be directly related to the rigid (nonmobile) crystalline methylene carbon content in asphalt.4 Of the many spectroscopic techniques, NMR is most often used to study the molecular structure and dynamics of carbonaceous materials. Solution-state NMR is the preferred method for quantification of carbon types because of the high resolution that can be achieved. However, solid-state NMR can be more advantageous than solution-state NMR because carbon types in different phase structures (crystalline versus amorphous) can be observed and quantified.4,5 In addition, solid-state NMR techniques can be easily used to study changes in molecular motions as a function of temperature for a solid material such as asphalt. Several investigations using NMR to study the molecular dynamics throughout the service temperature range for asphalt and asphalt-related materials have been reported.5-9 Gu¨lsu¨n10 investigated the hydrogen-1 (1H) and carbon-13 (13C) spinlattice relaxation times of asphalt suspensions and found that the correlation time is dependent on concentration as well as temperature. With increasing concentration, the molecular diffusion slows down as a consequence of increased friction (1) Havens, J. R.; Koenig, J. L. Appl. Spectrosc. 1983, 37 (3), 226. (2) Schaefer, J.; Stejskal, E. O.; Buchdahl, R. Macromolecules 1977, 10, 384. (3) Netzel, D. A.; Miknis, F. P.; Wallace, J. C., Jr.; Butcher, C. H.; Thomas, K. P. In Handbook of Asphalt Science and Technology; Usmani, A., Ed.; Marcel Dekker: New York, 1997; Chapter 2. (4) Michon, L. C.; Netzel, D. A.; Turner, T. F.; Martin, D.; Planche, J.-P. Energy Fuels 1999, 13 (3), 602. (5) Netzel, D. A. Transp. Res. Rec. 1998, 1638, 23. (6) Kohno, T.; Yokono, T.; Sanada, Y.; Harrel, J. W., Jr. Fuel 1985, 64, 1329. (7) Miknis, F. P.; Netzel, D. A. In Magnetic Resonance in Colloid and Interface Science; Resing, H. A., Wade, C. G., Eds.; American Chemical Society, Symposium Series No. 34; American Chemical Society: Washington, D.C., 1976; p 182. (8) VanderHart, D. L, Manders, W. F.; SHRP-A-335, Binder Characterization and EValuation by Nuclear Magnetic Resonance Spectroscopy; Strategic Highway Research Program, National Research Council: Washington, D.C., 1993. (9) Netzel, D. A. WRI/FHWA Annual Technical Report; Western Research Institute: Laramie, WY, Nov 1, 1994-May 15, 1995. (10) Gu¨lsu¨n, Z. Fuel 1987, 66, 1449.

Netzel

among the soluble molecules. VanderHart et al.11 studied the structural inhomogeneity and physical aging of asphalts by solidstate NMR. Using 1H line shapes and NMR spin-echo data, they observed no detectable changes in the molecular mobility during aging. Also, the presence of an aggregate did not change the average molecular mobility of the asphalt cement at ambient temperature. These authors, using 1H spin-diffusion data as a probe for molecular size, showed that there was no evidence for a micellar or gel structure in asphalts at ambient temperatures. In addition, VanderHart12 has shown that the molecular motions in some asphalt materials are in the same frequency range as those of glassy polymers and suggested that these asphalt materials will have reduced impact strengths and be prone to low-temperature cracking. Kohno et al.6 measured the hydrogen spin-lattice relaxation times, TH1 , and the hydrogen H spin-lattice relaxation time in the rotating frame, T1F , as a function of temperature for 10 pitch samples. They found that H the T1F minimum occurred at approximately the same temperature as the softening point and concluded that the softening phenomenon of pitch can be detected and studied at the H measurement. molecular level by NMR T1F To more fully understand the importance of molecular motion on the asphalt rheological properties as it relates to lowtemperature cracking, the NMR spin-lattice relaxation time in H the rotating frame, T1F , was measured as a function of temperature for three widely different asphalts. From these data, activation energies were determined and compared to activation energies of rigid polymers and guest molecules in a solid host matrix. Experimental Section Asphalt Samples. Three Strategic Highway Research Program (SHRP) asphalts (AAA-1, AAB-1, and AAM-1) were obtained from the Material Research Library (MRL). The elemental composition, molecular weight, and chemical class composition for these asphalt samples can be found in Netzel et al.3 Nuclear Magnetic Resonance (NMR). A Chemagnetics CMX 100/200 solid-state NMR spectrometer operating at a 13C frequency of 25 MHz was used for 1H f 13C cross-polarization with magicangle spinning (CP/MAS) experiments. Asphalt samples were heated to a temperature between 100 and 170 °C. Using a vacuum syringe, the liquid asphalt was pulled into a 5-mm i.d. Teflon tubing. After cooling, a 10 mm section was cut and cool with liquid nitrogen vapor. The Teflon was removed, and the solid asphalt cylinder was inserted into a 7.5 mm zirconia pencil rotor assembly and capped. These samples remained at room temperature in the rotor for 2 months to 1 year before conducting the low-temperature CP/MAS experiments. Temperatures of 20 °C and below were obtained using an electric refrigeration FTS systems XR series Air-Jet sample cooler. The cooled, ultra-dry air from the cooler was transferred to the NMR probe via an insulated 2.4-m transfer line. The probe temperature was controlled within 0.1 °C. Parameters for the variable contact time CP/MAS experiments included a pulse width of 5 µ (90°), a pulse delay of 1 s, variable contact times from 0.02 to 6 ms, a sweep width of 20 kHz, a free induction decay size of 512 points, a rotor spinning rate of 4.5 kHz, and 1800-3000 acquisitions. The 13C signal intensity was obtained from the total integrated areas of the aromatic and aliphatic regions of the spectra.

Theoretical Consideration The extent of the overall molecular motions, such as rotational tumbling and translational diffusion, and the motions of specific (11) VanderHart, D. L.; Manders, W. F.; Campbell, G. C. Prepr. Am. Chem. Soc. DiV. Petrol. Chem. 1990, 35 (3), 445. (12) VanderHart, D. L. SHRP A-002C Final Report; Strategic Highway Research Program, National Research Council: Washington, D.C., Feb 1991; p 139.

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Figure 2. Nonlinear least-squares regression analysis of relaxation times 〈TCH〉CH3, 〈TCH〉CH2, and 〈TH1F〉 for the aliphatic carbons in asphalt AAA-1 from variable contact time 13C CP/MAS spectral data at -45 °C.

organic functional groups, such as the segmental motions of methylene carbon chains and methyl rotation in asphalt, influence the NMR relaxation times. Thus, by measuring the relaxation time constant for specific carbon types or functional groups as a function of temperature, a description and magnitude of the type of motion can be obtained. Because motions depend on the size, molecular conformation, and weak and strong associative interactions of the molecules, a measure of the relaxation time can also, in specific cases, provide information about these properties. The hydrogen spin-lattice relaxation H time in the rotating frame, T1F , is often used to probe the molecular motions in the midfrequency range. Figure 2 shows a plot of the 13C NMR signal intensity as a function of the 1H f 13C contact time for aliphatic carbons in asphalt AAA-1 at -45 °C. The initial rise in the carbon signal is due to the fast rate of cross-polarization (1/TCH) of the carbon atoms by hydrogens followed by a decrease in the carbon signal due to the relaxation of the hydrogens to their ground state. Generally, a two-exponential equation as reported by Garroway et al.13 is used for fitting the growth and decay of the 13C signal for a simple system (eq 1).

Mt ) Moλ-1[1 - e-λt/TCH]e-t/T1F H

(1)

H where λ ≡ 1 - (TCH/T1F ); Mt ) NMR signal intensity at contact time, t; Mo ) Hartmann-Hahn maximum 13C signal H ) intensity; TCH ) cross-polarization relaxation time; T1F hydrogen spin-lattice relaxation time in the rotating frame; and t ) contact time. However, for a complex system such as asphalt that contains many carbon types, it is necessary to modify eq 1 to include two cross-polarization time constants to fit the experimental data at low temperatures. The modification of eq 1 is shown in eq 2.

Mt ) Mo[xλ1-1(1 - e-λt/〈TCH〉S) + (1 - x)λ2-1(1 - e-λ2t/〈TCH〉L)]e-t/〈T1F〉 (2) H

H H where λ1 ≡ 1 - (〈TCH〉S/〈T1F 〉) and λ2 ≡ 1 - (〈TCH〉L/〈T1F 〉); Mt ) NMR signal intensity at contact time, t; Mo ) Hartmann-

(13) Garroway, A. N.; Moniz, W. B.; Resing, H. A. In Carbon-13 NMR in Polymer Science; Pasika, W. M., Ed.; ACS Symposium Series 103:67; American Chemical Society: Washington, D.C., 1979; Chapter 4.

Hahn maximum 13C signal intensity; x ) fraction of protonated aromatic carbons or methylene carbons; 1 - x ) fraction of nonprotonated aromatic carbons or methyl carbons; t ) contact time; 〈TCH〉S ) cross-polarization relaxation time for an ensemble of protonated aromatic carbons or an ensemble of methylene carbons (S ) p or CH2); 〈TCH〉L ) cross-polarization relaxation time for an ensemble of nonprotonated aromatic carbons or an H ensemble of methyl carbons (L ) np or CH3); and 〈T1F 〉 ) hydrogen spin-lattice relaxation time in the rotating frame for an ensemble of aromatic or aliphatic hydrogens. A nonlinear least squares regression analysis was used to H obtain the x, TCH, and T1F parameter values. This equation was used to fit the data for the carbons in both the aliphatic and aromatic regions of the asphalt spectrum. The individual components for the initial rapid rise of the carbon signal intensity due to cross-polarization of the methylene and methyl carbons and the decrease of the carbon signal as a result of the relaxation of the hydrogens to their ground state is also shown in Figure 2. It can be seen that the rate of cross-polarization of the methylene carbons is much greater than that of the methyl carbons. The slower rate of cross-polarization of the methyl carbons is because the rapidly rotating methyl group reduces the efficiency of the cross-polarization interaction. In the aromatic region, the two cross-polarization times result from very different cross-polarization rates for the protonated and nonprotonated aromatic carbons. A plot of the 13C signal intensity of the aromatic carbons versus the contact time is very similar to Figure 2 and, therefore, is not shown. Results and Discussion Hydrogen Spin-Lattice Relaxation Time in the Rotating Frame for Asphalt. Table 1 gives the ensemble average values H H (symbolized by 〈T1F 〉) obtained at various temperatures of T1F for the aromatic and aliphatic hydrogens for three asphalts. The data, with the exception of the 20 °C data for asphalts AAA-1 and AAM-1, were obtained using eq 2 with a single-exponential H fit for 〈T1F 〉. At temperatures above 0 °C, the CP/MAS data H 〉 values exist for aliphatic hydrogens in suggest that two 〈T1F asphalts AAA-1 and AAM-1. The data for asphalt AAB-1 can H best be fitted with only one 〈T1F 〉 value at temperatures above 0 °C. Figure 3 shows a semilogarithmic plot of the 13C signal intensity versus the contact time for asphalt AAA-1 at 20 °C. The dashed line in the figure represents the fit of the data H 〉 assuming all aliphatic hydrogens relax with a single 〈T1F value, which is clearly not the case. Equation 2 was modified (see eq 3) to include the assumption that two different environmental domains exist for the aliphatic hydrogens in the H H1 H2 asphalt, each with a different 〈T1F 〉 values (T1F and T1F ).

Mt ) Mo[xλ1-1(1 - e-λ1t/〈TCH〉S) + (1 - x)λ2-1(1 - e-λ2t/〈TCH〉L)]ye-t/〈T1F 〉 + (1 - y)e-t/〈T1F 〉 (3) H1

H2

where all symbols are the same and those defined in eq 2 except for y; y ) fraction of aliphatic hydrogens in environment 1, and 1 - y ) fraction of aliphatic hydrogens in environment 2. Using eq 3, a reasonably good fit of the data (solid line) is obtained, as shown in Figure 3. Thus, for asphalt AAA-1 and possibly for asphalt AAM-1 at temperatures above the glass H1 H2 and T1F values suggest that the aliphatic transition, the two T1F hydrogens exist in two separate domains: that is, two amorphous phases with different mobilities.14,15 H The measured 〈T1F 〉 values are comparable to the literature values for glassy amorphous polymers [poly(phenylene oxide)/

2184 Energy & Fuels, Vol. 20, No. 5, 2006

Netzel

Table 1. Hydrogen Spin-Lattice Relaxation Time in the Rotating Frame (ms)a for Aromatic (〈TH1F〉Ar) and Aliphatic (〈TH1F〉Al) Hydrogens in Asphalts AAA-1, AAB-1, and AAM-1 at Various Temperatures asphalt AAA-1 temp, °C

〈TH1F〉Ar

20 0 -10 -20 -30 -45

2.67 ( 3.40 ( 0.24 3.84 ( 0.51 3.98 ( 0.32 4.58 ( 0.75 5.92 ( 0.95

0.34b

asphalt AAB-1 〈TH1F〉Al

( 0.09 2.00 ( 0.07 2.41 ( 0.13 2.89 ( 0.10 3.67 ( 0.14 4.57 ( 0.19

1.51c,d

〈TH1F〉Ar 2.47 ( 0.27 2.29 ( 0.26 3.54 ( 0.56 3.46 ( 0.18 4.51 ( 0.29 6.19 ( 1.80

asphalt AAM-1 〈TH1F〉Al

〈TH1F〉Ar

〈TH1F〉Al

( 0.05 1.34 ( 00 2.38 ( 0.10 2.90 ( 0.05 3.42 ( 0.14 4.30 ( 0.21

2.98 ( 0.27 3.53 ( 0.30 4.55 ( 0.43 4.90 ( 0.47 6.28 ( 0.87 7.43 ( 1.30

2.01c,d ( 0.05 2.69 ( 0.07 3.30 ( 0.07 4.11 ( 0.16 4.90 ( 0.14 5.60 ( 0.20

1.56c

Using eq 2. b Standard deviation. c Using eq 1. d Using eq 3, the two 〈TH1F〉 values for asphalt AAA-1 are the following: 〈TH1F1〉 ) 1.07 and 〈TH1F2〉 ) 5.12 ms. For asphalt AAM-1, 〈TH1F1〉 ) 0.42 and 〈TH1F2〉 ) 2.34 ms. a

Figure 3. Carbon-13 CP/MAS NMR signal intensity as a function of contact time for asphalt AAA-1 at 20 °C: (2) data; (- - -) singleexponential fit for 〈TH1F〉; (s) two-exponential fit for 〈TH1F〉.

polystyrene (6.7 ms); polystyrene (5.6 ms)]16 and solid carbonaceous fossil fuel materials {pitch [aromatic hydrogens (2.78 ms); aliphatic hydrogens (2.02 ms)], kerogen [aromatic hydrogens (5.6 ms)], and Powhatan #5 coal [aromatic hydrogens(3.8 ms); aliphatic hydrogens (4.4 ms)]}17 For the three asphalts, H H 〉Ar and aliphatic 〈T1F 〉Al values increase with the aromatic 〈T1F decreasing temperature, indicating a decrease in molecular motion and, therefore, an increase in the rigidity of the asphalt matrix as expected. In general, the aromatic and aliphatic H 〈T1F 〉Ar,Al values for the asphalts can be ranked as follows: H H [〈T1F 〉Ar,Al(AAM-1)] > [〈T1F 〉Ar,Al(AAB-1)] = H 〉Ar,Al(AAA-1)] [〈T1F

Thus, the aromatic and aliphatic components in asphalt AAM-1 have less molecular motion than those in asphalts AAA-1 and AAB-1. That is, asphalt AAM-1 is more rigid than either asphalt AAA-1 or AAB-1 at any given temperature. This ranking is supported by the many rheological properties of these asphalts.18 Inspection of the data in Table 1 shows that the aromatic H 〉Ar values for the three asphalts are greater than the 〈T1F (14) Vigier, G.; Tatibouet, J.; Benatmane, A.; Vassoille, R. Colloid Polym. Sci. 1992, 270, 1182. (15) Veeman, W. S.; Menger, E. M.; Ritchey, W.; de Boer, E. Macromolecules 1979, 12 (5), 924. (16) High Resolution NMR Spectroscopy of Synthetic Polymers in Bulk; Komoroski, R. A., Ed.; Methods in Stereochemical Analysis; VCH Publishers: Deerfield Beach, FL, 1986; Vol. 7. (17) Axelson, D. E. Solid State Nuclear Magnetic Resonance of Fossil Fuels: An Experimental Approach; Multiscience Publications Limited, Montreal, Quebec, Canada, 1985. (18) Materials Reference Library Report on Asphalt Properties; Strategic Highway Research Program, National Research Council: Washington, D.C., 1992.

H 〉Al values at all temperatures. Because larger aliphatic 〈T1F H values of T1F typically indicate less intramolecular mobility, one would infer that the midfrequency motions for the aromatic condensed rings are less than the methylene segmental and methyl rotation motions at any given temperature. This argument has been used to explain the molecular motions of aromatic and aliphatic components in whole coals and maceral concenH trates19-21 in which the ensemble average aromatic 〈T1F 〉Ar H values were also found to be greater than the 〈T1F〉Al values of aliphatic components. However, more recent relaxation data on coals by Wind et al.22 disagree with the previously published H 〉Ar data. These authors showed that the aromatic hydrogen 〈T1F H values in coal are less than the 〈T1F〉Al values for the aliphatic hydrogen. This conclusion is in accordance with results by Jurkiewicz et al.23 Wind et al.22 suggest that the differences observed are due to the occurrence of nonprotonated carbons with large TCH values that comprise a major fraction of the coal aromatic carbons and that the interplay between the mechanisms for the cross-polarization relaxation and the 1H spin-lattice relaxation in the rotating frame leads to complicated results. Carbon-Hydrogen Cross-Polarization Relaxation Time for Asphalt. Molecular motions that are very slow can also be studied via cross-polarization relaxation time (TCH) measurements using the variable contact time technique. This technique H simultaneously gives both the TCH and T1F relaxation time constants. The rate constant, 1/TCH, at which various carbon types are polarized is proportional to the strength of the C-H dipole interaction as determined by molecular motions and the interatomic C-H distances. Intramolecular cross-polarization can also occur but with considerable attenuation because of the larger intramolecular distances between the carbons of one molecule and the hydrogens of another molecule. Aromatic Carbons. The average carbon-hydrogen crosspolarization relaxation times were determined for the protonated 〈TCH〉p and nonprotonated 〈TCH〉np aromatic carbons in asphalts AAA-1, AAB-1, and AAM-1 at temperatures ranging from 20 to -45 °C. The values of 〈TCH〉p and 〈TCH〉np calculated using eq 2 are given in Table 2. The 〈TCH〉np values for the nonprotonated aromatic carbons show considerable scatter and large standard deviations at the temperatures for all three asphalts. The large standard deviations observed are comparable to the aromatic TCH standard deviation values reported by Alemany et al.24 for model compounds. These authors suggest

(19) Sullivan, M. J.; Maciel, G. E. Anal. Chem. 1982, 54 (9), 1615. (20) Dudley, R. L.; Fyfe, C. A. Fuel 1982, 61, 651. (21) Botto, R. E.; Wilson, R.; Winans, R. E. Energy Fuels 1987, 1, 173. (22) Wind, R. A.; Maciel, G. E.; Botto, R. E. Quantitation in 13C NMR Spectroscopy of Carbonaceous Solids. In Magnetic Resonance of Carbonaceous Solids; Botto, R. E., Sanada, Y., Eds.; Advances in Chemistry Series 229; American Chemical Society: Washington, D.C., 1993. (23) Jurkiewicz, A.; Wind, R. A.; Maciel, G. E. Fuel 1990, 69, 830. (24) Alemany, L. B.; Grant, D. M.; Pugmire, R. J.; Alger, T. D.; Zilm K. W. J. Am. Chem. Soc. 1983, 105, 2142.

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Energy & Fuels, Vol. 20, No. 5, 2006 2185

Table 2. Cross-Polarization Times (µs)a for Protonated (〈TCH〉p) and Nonprotonated (〈TCH〉np) Aromatic Carbons in Asphalts AAA-1, AAB-1, and AAM-1 at Various Temperatures asphalt AAA-1

a

asphalt AAB-1

asphalt AAM-1

temp, °C

〈TCH〉p

〈TCH〉np

〈TCH〉p

〈TCH〉np

〈TCH〉p

〈TCH〉np

20 0 -10 -20 -30 -45

48.3 ( 41.3b 40.5 ( 17.6 39.2 ( 15.2 33.3 ( 9.8 43.6 ( 11.1 40.7 ( 11.3

289 ( 183 275 ( 84 436 ( 154 436 ( 78 715 ( 205 586 ( 155

48.2 ( 29.9 42.7 ( 12.7 23.9 ( 14.7 35.8 ( 6.5 40.3 ( 10.4 87.7 ( 66.4

359 ( 144 595 ( 158 420 ( 144 510 ( 58 423 ( 59 458 ( 403

43.8 ( 16.8 27.1 ( 8.6 49.8 ( 13.3 45.6 ( 13.1 57.6 ( 22.9 65.4 ( 21.0

449 ( 114 501 ( 80 530 ( 103 535 ( 93.7 486 ( 124 551 ( 167

Using eq 2. b Standard deviation.

Table 3. Cross-Polarization Times (µs)a for Methylene (〈TCH〉CH2) and Methyl (〈TCH〉CH3) Carbons in Asphalts AAA-1, AAB-1, and AAM-1 at Various Temperatures asphalt AAA-1 temp, °C 20 0 -10 -20 -30 -45 a

〈TCH〉CH2 ( 41.4 ( 53.8 37.4 ( 10.8 38.3 ( 5.1 36.5 ( 3.0 34.3 ( 3.5

62.9b

6.0c

〈TCH〉CH3 ( 6.0 74.8 ( 184 174 ( 209 222 ( 109 312 ( 127 271 ( 101

62.9b

asphalt AAB-1 〈TCH〉CH2

〈TCH〉CH3

( 3.5 54.6 ( 3.9 40.9 ( 7.4 40.8 ( 3.8 42.6 ( 6.6 47.5 ( 6.3

69.4b

( 3.5 d 221 ( 174 179 ( 59 231 ( 170 258 ( 325

69.4b

asphalt AAM-1 〈TCH〉CH2

〈TCH〉CH3

( 2.9 46.4 ( 12.8 48.2 ( 6.2 45.4 ( 8.6 44.2 ( 3.6 41.0 ( 5.5

70.6b ( 2.9 143 ( 106 179 ( 102 193 ( 129 247 ( 84 187 ( 99

70.6b

Using eq 2. b Using eq 1. c Standard deviation. d Unreasonable value.

that their computed uncertainties are meaningless because the H magnitude of TCH and T1F are similar. The values of 〈TCH〉p for the three asphalts are comparable to those for the aromatic carbons in rigid polymers (12-33 µs)17 but much shorter than those reported for model compounds (119-218 µs).24,25 However, the 〈TCH〉np values measured for the three asphalts are in the same range as those reported for model compounds (334-596 µs),17 coals (371-437 µs),26 and polymers (89-345 µs).17 There are three types of nonprotonated aromatic carbons: aromatic carbons that have attached alkane groups, heteroatom substituents, and aromatic carbons that are at the bridgehead of condensed aromatic rings. These three types of nonprotonated aromatic carbons have different 〈TCH〉np values depending on the proximity of hydrogen to the carbon atoms. Alemany et al.25 reported that for model compounds the TCH values for nonprotonated aromatic carbons with alkane substituents range from 452 to 793 µs, whereas, for carbon atoms far removed from any hydrogens, the TCH values range from 1 to 3 ms.24 The observed values of 〈TCH〉np are the weighted average of the cross-polarization rates for the three types of nonprotonated aromatic carbons in the asphalts. The low values of 〈TCH〉np at near ambient temperature for asphalt AAA-1 suggest a relatively high number of hydrogens in the proximity of the nonprotonated aromatic carbon atoms. At lower temperatures, changes in the molecular packing of the aromatic ring structure may be occurring, reducing the number of hydrogens in close proximity to the nonprotonated carbons and resulting in a larger value of 〈TCH〉np. For the relatively rigid asphalts AAB-1 and AAM-1, less change in the molecular packing of the aromatic clusters occurs at lower temperatures, thus resulting in less change in the values of 〈TCH〉np. Aliphatic Carbons. The TCH values of the aliphatic carbons in the three asphalts were also measured as a function the temperature ranging from 20 to -45 °C and are listed in Table 3. The TCH values determined result from the near static, low(25) Alemany, L. B.; Grant, D. M.; Pugmire, R. J.; Alger, T. D.; Zilm, K. W. J. Am. Chem. Soc. 1983, 105, 2133. (26) Magnetic Resonance of Carbonaceous Solids; Botto, R. E., Sanada, Y., Eds.; Advances in Chemistry Series 229; American Chemical Society: Washington, D.C., 1993.

frequency segmental methylene and methyl rotational motions. Neglecting the 20 °C data, the 〈TCH〉CH2 values range from 34.3 to 54.6 µs and the 〈TCH〉CH3 values range from 143 to 312 µs. These values are comparable to values reported in the literature for methylene and methyl carbons in model compounds.25 However, in rigid polymers, the TCH values for the main-chain methine and methylene carbons were reported to range from 12 to 32 µs and those of the methyl carbons from 21 to 67 µs.17 The TCH values for all aliphatic carbons in oil shale kerogen ranged from 70 to 90 µs and for coal with a very rigid structure ∼25 µs.17 At low temperatures, the 〈TCH〉CH2 values for the three asphalts suggest that any segmental motion of the main-chain methylene carbons remains nearly constant. That is, the methylene carbons in the main chain are structurally fixed and generate near-static frequencies as the carbons oscillate about a fixed point. The values of 〈TCH〉CH3 for the rapidly rotating methyl carbons are considerably higher than the 〈TCH〉CH2 values at low temperatures. The near-static frequencies associated with rapidly rotating methyl groups at low temperatures are independent of the temperature. Temperatures below -100 °C are necessary to stop the rotational motions of the methyl carbons. At these temperatures, the 〈TCH〉CH3 would have values comparable to or less than those measured for methyl carbons in rigid polymers (2167 µs).17 The cross-polarization relaxation times for the aromatic and aliphatic carbons should be important to the impact strength of the asphalts.27 Unfortunately, the impact strength of asphalt as a function of temperature is not known. Apparent Activation Energies for Molecular Motions of Aromatic Components in Asphalt. The Arrhenius plots of the H aromatic relaxation rate 1/〈T1F 〉Ar versus 1/T for the three asphalts are shown in Figure 4 along with the calculated activation energy values for the ensemble aromatic ring motions. The activation energies (6.5-8.2 kJ/mol) show a small increase from asphalts AAA-1 to AAM-1. Thus, the aromatic ring motions in asphalt AAA-1 have a lower barrier to motion than those of asphalt AAB-1, which, in turn, have a lower barrier to (27) Schaefer, J.; Stejskal, E. O.; Buchdahl, R. Macromolecules 1977, 10, 384.

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Figure 4. Arrhenius plots for the hydrogen spin-lattice relaxation rate in the rotating frame for the aromatic and aliphatic hydrogens in asphalts AAA-1, AAB-1, and AAM-1.

motion than those of asphalt AAM-1. This is in agreement with the fact that asphalt AAA-1 has both a lower glass-transition temperature28 and a lower number of aromatic condensed rings per average molecule29 than either AAB-1 or AAM-1 and, thus, more molecular ring motions. The activation energies calculated for aromatic ring motions in the three asphalts are low and not in agreement with the high values (24-50 kJ/mol) reported in the literature for aromatic (phenyl) ring motions in polymers in the solid state30-35 or for crystalline polynuclear aromatic compounds.36-41 In polymers, the phenyl rings are within the backbone of the polymer structure or pendant. The phenyl rings can be either rotating freely (180° π flip) or hindered (torsional oscillations with angles less than 180°). Whether a phenyl ring undergoes a 180° ring flip is attributed to the conformation space available at that ring. A relatively low phenyl rotational barrier of 13 kJ/mol was reported for a dissolved polycarbonate polymer,42 and a theoretical calculated activation energy of 6 kJ/mol was reported for a restricted cooperative phenyl ring rotation in a phenylene polymer.43 (28) Western Research Institute. Fundamental Properties of Asphalts and Modified Asphalts, Volume I: InterpretiVe Report; FHWA-RD-99-212, U. S. Department of Transportation, Federal Highway Administration: McLean, VA, 2001; p 151. (29) SHRP-A-335, Binder Characterization and EValuation by Nuclear Magnetic Resonance Spectroscopy, Strategic Highway Research Program, National Research Council: Washington, D.C., 1993. (30) O’Gara, J. F.; Jones, A. A.; Hung, C.-C.; Inglefield, P. T. Macromolecules 1985, 18, 1117. (31) Tanabe, Y. J. Polym. Sci. Polym. Phys. Ed. 1985, 23, 601. (32) Larsen, D. W.; Corey, J. Y. J. Am. Chem. Soc. 1977, 99, 1740. (33) Andrew, E. R.; Eades, R. G. Proc. Phys. Soc., Ser. 1953, A218, 537. (34) Suwelack, D.; Rothwell, W. P.; Waugh, J. S. J. Chem. Phys. 1980, 73 (6), 2559. (35) Eisenberg, A.; Cayrol, B. J. Polymer Sci., Part C 1971, 35, 129. (36) Andrew, E. R. J. Chem. Phys. 1950, 18 (5), 607. (37) Fyfe, C. A.; Gilson, D. F. R.; Thompson, K. H. Chem. Phys. Lett. 1970, 5 (4), 215. (38) Fyfe, C. A.; Dunell, B. A.; Ripmeester, J. Can. J. Chem. 1971, 49, 3332. (39) Rushworth, F. A. Letters to the Editor. J. Chem. Phys. 1952, 20, 920. (40) Sanford, W. E.; Kupferschmidt, G. J.; Fyfe, C. A.; Boyd, R. K.; Ripmeester, J. A. Can. J. Chem. 1980, 58, 906. (41) Fyfe, C. A.; Kupferschmidt, G. J. Mol. Cryst. Liq. Cryst. 1975, 28, 179. (42) Jones, A. A.; Bisceglia, M. Macromolecules 1979, 12 (6), 1136.

However, phenyl rings in an asphalt differ from those in most polymers in that they are thought to be random condensed polynuclear aromatic clusters linked through H-bonding and π-π interactions. Phenyl rings pendant to a parent alkane molecule through a single covalent bond are also likely to be present. Presumably, large polynuclear aromatic structures would have little translational or full rotational motions at temperatures near and below the glass-transition temperature of asphalt. Therefore, it is reasonable to assume that the energy barrier for full rotation of the whole condensed aromatic cluster and for hindered oscillations within the cluster in asphalt would be at least equal to the activation energies (24-50 kJ/mol) for phenyl rotation and ring flexing as reported for solid polymeric systems. But, this was not observed. Fyfe et al.37 studied the molecular motion in solid crystalline pyrene (a polynuclear aromatic compound with four rings) and found the activation energy to be 57.7 kJ/mol for the rotation of the pyrene molecule about the axis perpendicular to the molecular plane (in-plane rotation). Fyfe et al.38 also investigated the molecular motion in solid coronene, a highly condensed polynuclear aromatic compound, and determined the activation energy of the in-plane rotation to be 24.7 kJ/mol. For a much smaller polynuclear aromatic compound, acenaphthalene, two distinct molecular in-plane rotational motions in the solid state with activation energies of 14.2 and 18.9 kJ/mol were reported.40 The low activation energies measured for the phenyl ring motions in the three asphalts (6.5-8.2 kJ/mol) over a temperature range from 20 to -45 °C suggest that some type of rapid rotating motion of the small condensed aromatic structure or pendant phenyl group exists in the solid state. On an NMR time scale, it may be that the small aromatic molecules in asphalt at low temperatures are very liquidlike but without translational motion, that is, oscillating or rotating rapidly in a spatially fixed position. At room temperature and above, an activation energy of 6.4 kJ/mol (21-72 °C) for the rotational diffusion of asphalt molecules dissolved (40% w/w) in carbon disulfide was reported by Gu¨lsu¨n.44 Another explanation for the low activation energies for the motions of the aromatic structure in the solid state is based on (43) Chen, C. L.; Lee, C. L.; Chen, H. L.; Shih, J. H. Macromolecules 1994, 27, 7872. (44) Gu¨lsu¨n, Z. Fuel 1987, 66, 1449.

Apparent ActiVation Energies in Solid Asphalt

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1H

spin-diffusion between the aromatic hydrogens and the aliphatic hydrogens of methyl substituents on the aromatic ring.21 Thus, because of 1H spin-diffusion transfer, the low activation energies measured for the aromatic hydrogen components in the asphalts may be directly influenced by the attached rapid methyl group rotation. The barriers to methyl rotation for methyl groups attached to an aromatic ring have been reported for o-xylene (5.8 kJ/mol), hemimellitine (6.06 kJ/mol), and isodurene (6.48 kJ/mol).45 Apparent Activation Energies for Methylene Segmental and Methyl Rotational Motions of Aliphatic Components in Asphalt. The Arrhenius plots of the aliphatic relaxation rate H 1/〈T1F 〉 versus 1/T for the three asphalts over the temperature range from 20 to -45 °C are also given in Figure 4. The activation energies (8.8-9.8 kJ/mol) for the molecular motion of the aliphatic components in asphalt measure the combined average barrier to segmental motions of the methylene carbons and rotational motions of the various types of methyl groups (terminal and branched hydrocarbons and as an aromatic substituent). The measured activation energies for the three asphalts are slightly lower than the reported values for alkane polymers and neat normal and branched alkanes in the liquid and solid state. Highly methyl-branched long chain hydrocarbons (C26-C32) in the viscous state have activation energies ranging from 26 to 37 kJ/mol.46 However, for nonviscous liquids, Agishev47 reported an activation energy of 12.1 kJ/mol for normal paraffins hexane, dodecane, and octadecane in CCl4, and Woessner et al.48 reported a value of 17.0 kJ/mol for neat n-dodecane. The activation energies for normal alkanes (C6-C40) in the solid state were reported to be 10.9 kJ/mol and were ascribed to the 3-fold reorientation of the methyl group.49 The methylene hydrogens of the alkanes were assumed to be essentially immobile, and their spin relaxation was ascribed to spindiffusion to the methyl hydrogens. Douglas and Jones50 also measured the activation energies for n-alkanes ranging from C4 to C94. The high-temperature (∼70 °C) activation energy of 92 kJ/mol for these compounds was ascribed to chain rotation, while the low-temperature (-190 °C) activation energy of 10.9 kJ/mol was attributed to methyl-group rotation via spindiffusion. Two activation energies were also reported for uncrosslinked polypropylene.51 An activation energy of 16.2 kJ/ mol at high temperatures (∼112 °C) was attributed to dynamic movement of polypropylene segments. The low-temperature (approximately -63 °C) activation energy of 7.1 kJ/mol was attributed to methyl-group rotation. It should be noted that the polypropylene polymer has a glass-transition temperature range (-20 to -5 °C) similar to that of the three asphalts. On the basis of the data in the literature for the activation energies associated with n-alkanes, the activation energies for the motions of the aliphatic components of the asphalts at temperatures above and below the glass-transition temperature are attributed mainly to methyl rotation of the various types of methyl groups present. The activation energies for the segmental motion of the methylene carbons may be contributing partly to (45) Mann, B. E. Prog. NMR Spectrosc. 1977, 11 (2), 95. (46) Cutnell, J. D.; Schisla, R. M.; Hammann, W. C. J. Phys. Chem. 1973, 77 (9), 1134. (47) Agishev, A. Sh. Zh. Eksp. Teor. Fiz. 1964, 46, 3; SoV. Phys. JETP 1964, 19, 1. (48) Woessner, D. E.; Snowden, B. S., Jr.; McKay, R. A.; Strom, E. T. J. Magn. Reson. 1969, 1, 105. (49) Anderson, J. E.; Slichter, W. P. J. Phys. Chem. 1965, 69 (9), 3099. (50) Douglas, D. C.; Jones, G. P. J. Chem. Phys. 1966, 45 (3), 956. (51) Jurkiewicz, A.; Pislewski, N.; Kunert, K. A. J. Macromol. Sci.Chem. 1982, A18 (4), 511.

Table 4. Activation Energies for Glass-Forming Systemsa and Asphalts molecule cyclohexane 10% cyclohexane 10% cyclohexane 10% cyclohexane 5% cyclohexane 10% cyclohexane 20% cyclohexane 10% cyclohexane benzene 10% benzene 10% benzene 10% benzene a

matrix (20-230 K)

Ea* (kJ/mol)

crystalline o-terphenyl glass benzophenone glass cis-decalin glass o-terphenyl glass o-terphenyl glass o-terphenyl glass zeolite (1.3 × 10-9 m) crystalline o-terphenyl glass benzophenone glass cis-decaline glass

40 9.6 9.5 9.9 8.5 9.6 7.7 6.2 16 6.7 7.3 7.1

asphalt (228-298 K)

Ea (kJ/mol)

AAA-1 (Al)b AAB-1 (Al) AAM-1 (Al)

9.8 8.8 9.2

AAA-1 (Ar)c AAB-1 (Ar) AAM-1 (Ar)

6.5 7.7 8.2

Reference 52. b Aliphatic. c Aromatic.

the measured activation energy but cannot be differentiated because of 1H spin-diffusion of the methylene hydrogens being controlled by rapid methyl-group rotation. Motions of Guest Molecules in Solid Glasses. Many asymmetric organic compounds bound by van der Waals forces form glass phases in both the pure state and mixtures at low temperatures. A nearly universal feature of glass-forming host molecules with small concentrations of guest molecules is that the guest molecule has high intramolecular mobility even at extremely low temperatures. That is, the guest molecules are nearly liquidlike within a nanopore structure. Mu¨ller-Warmuth and Otte52 studied the low-temperature molecular mobility of benzene and cyclohexane as guest molecules in several glass matrixes made of simple organic molecules (Table 4). They reported activation energies ranging from 6.7 to 7.3 kJ/mol for the in-plane rotation of benzene (10 wt %) in o-terphenyl, benzophenone, and cis-decaline glasses, whereas the activation energy for the in-plane motion of neat crystalline benzene was reported by Andrew and Eades33,53 to be considerably higher (16 kJ/mol). The activation energies for cyclohexane in the three types of glass were found to range from 9.5 to 9.9 kJ/mol, whereas crystalline cyclohexane has an activation energy of 40 kJ/mol.54,55 Mu¨ller-Warmuth and Otte52 explained the observed data on the basis of holes in the network of the glass-forming substance, which are filled with one or more mobile guest molecules. By way of confirmation, these authors measured the activation energy for the molecular motions of cyclohexane in a zeolite with cavities of equal diameters (1.3 × 10-9 m) at low temperature. The value found was 6.2 kJ/mol. Nanopore Structural Model of Asphalt. The activation energies for the molecular motion of aromatic components in the asphalts are remarkably close to the activation energies of small aromatics molecules in a glass matrix (see Table 4). On the basis of the rheological properties of asphalts at low temperature, asphalts can be considered to be glasslike. Therefore, it is conceivable that the low activation energies observed for the aromatic components are indicative of relatively fast molecular reorientation of single phenyl rings and small polynuclear aromatic molecules as guests within a frozen nanopore matrix of rigid-amorphous aliphatic components. (52) Mu¨ller-Warmuth, W.; Otte, W. J. Chem. Phys. 1980, 72 (3), 1749. (53) Andrew, E. R.; Eades, R. G. Proc. Phys. Soc., London, Sect, A 1953, 66, 415. (54) Andrew, E. R.; Eades, R. G. Proc. Phys. Soc., London, Sect. A 1952, 65, 371. (55) Andrew, E. R.; Eades, R. G. Proc. Phys. Soc., London, Sect. A 1953, 216, 398.

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The results of this investigation do not support the traditional colloidal dispersion models proposed for the structure of asphalt. Various models assume that the aromatic molecules are strongly associated to form of large asphaltene moieties colloidally dispersed in a solvent phase (maltenes) and peptized by polar materials called resins. It does, however, support the NMR results obtained by VanderHart and Manders.8 These authors investigated the molecular mobility and size of the components in eight SHRP asphalts using solid-state 1H NMR line-shape analysis and spin-diffusion techniques. They concluded that mobility heterogeneity exists in asphalt on a scale of 2 or 3 molecular diameters. The different colloidal/micellar models and their limitations to explain the rheological properties of asphalt, as well as the more recently proposed microstructural model for asphalt, can be found in ref 56. Conclusions The apparent activation energies for molecular motions were determined for the ensemble of aromatic and aliphatic compo(56) SHRP-A-367, Binder Characterization and EValuation; Strategic Highway Research Program, National Research Council: Washington, D.C., 1993; Vol. 1.

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nents in three asphalts. The barrier to molecular motions for the aromatics components was found to be lower than expected. Two mechanisms are proposed to explain the results. The first mechanism is spin-diffusion of the aromatic ring hydrogens to the aliphatic hydrogens of the rapidly rotating methyl substituents. The second mechanism is the relatively fast inplane rotation of relatively small aromatic molecules as guest molecules within a nanopore matrix of rigid-amorphous aliphatic components. The activation energies for the motions of aliphatic moieties in the asphalts were found to be of the same magnitude reported for solid aliphatic-type polymers and for both solid normal and branched alkanes. Thus, by analogy, the measured activation energies are associated with rapid methyl rotation of the terminal and branched methyl groups of the long carbon chain length alkanes. Acknowledgment. The author expresses his appreciation to Jackie Greaser for typing the manuscript and to the Federal Highway Administration for their support of this program under FHWA Contract No. DTFH61-92C-00170. EF0601768