Determination of the Glass-Transition Temperature of a Coal Extract

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Energy & Fuels 2004, 18, 1648-1655

Determination of the Glass-Transition Temperature of a Coal Extract, Using a Polymer Blend Methodology Pakorn Opaprakasit# and Paul C. Painter* The Energy Institute, The Pennsylvania State University, University Park, Pennsylvania 16802 Received April 8, 2004. Revised Manuscript Received June 10, 2004

The glass-transition temperature (Tg) of pyridine-soluble extracts obtained from Upper Freeport (UPF) coal was investigated. Previous studies have suggested that such a transition, if it exists, might occur at high temperatures and is probably masked by other transition processes. In this study, we used differential scanning calorimetry (DSC) experiments to calculate that the glass transition of the UPF coal extract occurs at ∼194 °C. This Tg value was obtained using an extrapolation of observed Tg values of miscible blends of the coal extract and poly(4-vinylpyridine) (P4VPy). UPF extract/P4VPy blends were prepared as a series that had a UPF extract content of 10-85 wt %. DSC thermograms of all blends exhibited a single Tg value at a temperature higher than that of pure P4VPy. The Tg values of the blends were dependent on the composition of the coal extract. Using the Kwei equation, which describes the relationship between blend composition and Tg, the Tg value of the coal extract was calculated.

Introduction Coal is an amorphous, glassy solid, and it is both rigid and brittle. Most glassy solids are characterized by a glass-transition temperature (Tg), where the physical properties change dramatically from solidlike to either liquidlike or rubbery (the latter occurs if the nature of the material is polymeric). Accordingly, the detection of a glass transition in coal is of some interest and has been pursued by various groups over the years.1-5 Differential scanning calorimetry (DSC) is the technique that has been used most extensively in this type of work, and certain transitions have been observed at high temperatures (>300 °C). However, these high-temperature transitions are not associated with a reversible process and are accompanied by weight loss,3,6-8 suggesting that they are largely a result of chemical decomposition. In addition to these high-temperature changes, a transition near 100 °C has been observed in certain coals and assigned to a glass transition.9,10 However, * Author to whom correspondence should be addressed. Telephone: 814-865-5767. Fax: 814-865-2917. E-mail address: [email protected]. # Present address: Department of Common and Graduate Studies, Sirindhorn International Institute of Technology, Thammasat University, Bangkok 12121, Thailand. Telephone: +66 (2) 986-9009, Ext. 3317. E-mail address: [email protected]. (1) Lucht, L. M.; Peppas, N. A. AIP Conf. Proc. 1981, 70, 28. (2) Yun, Y. S.; Suuberg, E. M. Energy Fuels 1992, 6, 328-330. (3) Yun, Y. S.; Suuberg, E. M. Fuel 1993, 72, 1245-1254. (4) Iino, M.; Takanohashi, T.; Ohsuga, H.; Toda, K. Fuel 1988, 67, 1639-1647. (5) Takanohashi, T.; Iino, M.; Nishioka, M. Energy Fuels 1995, 9, 788-793. (6) Yun, Y. S.; Suuberg, E. M. Prepr. Pap.sAm. Chem. Soc., Fuel Chem. Div. 1991, 202, 87. (7) Yun, Y.; Suuberg, E. M. Energy Fuels 1992, 6, 328-330. (8) Suuberg, E. M.; Otake, Y.; Yun, Y.; Deevi, S. C. Energy Fuels 1993, 7, 384-392. (9) Mackinnon, A. J.; Antxustegi, M. M.; Hall, P. J. Fuel 1994, 73, 113-115. (10) Mackinnon, A. J.; Hall, P. J. Fuel 1992, 71, 974-975.

we have recently demonstrated that an irreversible endothermic transition occurs in this temperature range, as a result of water evaporation.11 In addition, thermomechanical measurements, which are very sensitive to the presence of a glass transition in synthetic polymers, have provided no evidence of a glass transition below temperatures where chemical reactions occur; however, a small change in dimensions was observed when coal was heated to >180 °C. Although the magnitude of this “softening process” is not comparable to that observed in the glass-transition process of a polymer such as polystyrene, for example, it is still intriguing. Given that coal is highly heterogeneous and cross-linked, compared to polymers such as polystyrene, it is possible that a relaxation of part of the sample is being observed. To determine a lower limit for the glass transition of coal, we decided to examine the relaxations in soluble coal extracts. Because of their (relatively) low molecular weight, these should have a glass transition; however, we were unable to observe such a transition in our previous work.11 This may be a consequence of the heterogeneity of this material, however, in that the transition may be too broad and weak to be observed. Accordingly, we decided to apply a new methodology to investigate the existence of a glass transition in coal extracts by blending them with a polymer whose Tg value is known. The DSC thermogram of the resulting polymer/coal extract blends is then recorded. For miscible mixtures, the Tg value of the coal component can then be calculated by applying equations that describe the relationship between blend composition and the observed Tg values. Experimental Section Upper Freeport (UPF) coal samples were supplied by Argonne National Laboratory. The samples were stored under (11) Opaprakasit, P.; Painter, P. Energy Fuels 2003, 17, 354-358.

10.1021/ef049913v CCC: $27.50 © 2004 American Chemical Society Published on Web 08/25/2004

Tg Determination of a Coal Extract

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Figure 1. Differential scanning calorimetry (DSC) thermograms of the Upper Freeport (UPF) pyridine-soluble materials (thermogram a), the residue (thermogram b), and the raw UPF coal (thermogram c). nitrogen and used as received. Poly(4-vinylpyridine), or P4VPy (molecular weight of 160 000), was obtained from Aldrich. A novolac-type phenolic resin was synthesized in this laboratory, using a condensation reaction, as described elsewhere.12,13 The pyridine extraction was performed using a standard Soxhlet method.14 After completion of the extraction, the soluble material and residue were collected and the extraction yield was recorded. These materials were then used to prepare coal extract/polymer blends. A series of binary blends using these materials and P4VPy were prepared by dissolving 0.2 g of the blend components in 20 mL of pyridine. The mixture was stirred for 2 days, and the solvent was then removed using a rotary evaporator. Finally, the remaining solid mixture was ground, using a WIG-L-BUG grinding machine, for 2 min before the thermal analysis experiments. A similar procedure was also applied in the preparation of novolac/P4VPy blends. DSC experiments were conducted (TA instruments model Q100). Rates of 10 and 100 °C/min were used in the heating and cooling cycles, respectively. The temperature and the enthalpy change were calibrated using an indium standard. Ten milligrams of sample was placed in an aluminum pan, and an empty pan was used as a reference. All DSC thermograms are shown in an endothermic mode. The heat flow scale is normalized by sample mass (W/g). Fourier transform infrared (FTIR) spectra were recorded on a Digilab model FTS6000 FTIR spectrometer at a resolution of 2 cm-1. All spectra were recorded using 100 scans. The samples were prepared by casting films on a KBr window. Special efforts were made to ensure that all the samples were thin enough to be within the absorption range where the Beer-Lambert law is obeyed. After the majority of the pyridine solvent had evaporated, the samples were placed under vacuum at room temperature for 2 days, to remove the residual solvent.

Results and Discussion The pyridine-extraction yield obtained from the UPF coal was 36%, on a dry-ash-free (daf) basis, and the total recovery was 97%. This result is in good agreement with (12) Opaprakasit, P.; Scaroni, A. W.; Painter, P. J. Mol. Struct. (THEOCHEM) 2001, 570, 25-35.

those previously reported in the literature.14-16 DSC thermograms of the coal, the extract, and the residue were recorded and are shown in Figure 1. The results show no evidence of a transition that can be associated with the glass-transition process. However, all three thermograms show a sudden break in the slope at a temperature of ∼190 °C. This is most probably a result of degradation or chemical reactions that are occurring in this temperature range. The magnitude of this change is so large, particularly for the extract, that it might mask other transitions, i.e., the glass transition, if it occurs within this temperature range. Also note that the thermograms of the soluble extract and the residue show an endothermic peak at ∼80-100 °C that disappears in subsequent scans. As noted previously, this peak corresponds to an evaporation of water that is associated with the organic material.3,11 In the pyridinesoluble material, this bound water is probably a result of the extraction process, where the materials are washed with an excess amount of water to remove the pyridine. Given that the water content in the raw UPF coal is very small (1.1%, as-received), it is therefore not surprising that the corresponding peak is relatively weak in the raw-coal thermogram. Glass-Transition Temperature (Tg) of Polymers Blends and Studies of P4VPy/Novolac Mixtures. Because of the observed domination of transitions associated with degradation and/or other chemical reactions in the thermograms of these materials, we decided to use an indirect method to investigate whether coal extracts have a Tg value that can be estimated. A coal extract/polymer blend methodology was used. Before examining the results obtained from these coal extract/ (13) Opaprakasit, P. Master’s Thesis, The Pennsylvania State University, University Park, PA, 1999. (14) Nishioka, M. Fuel 1991, 70, 1413-1419. (15) Painter, P. C.; Opaprakasit, P.; Scaroni, A. W. Energy Fuels 2000, 14, 1115-1118. (16) Opaprakasit, P.; Scaroni, A. W.; Painter, P. C. Energy Fuels 2002, 16, 543-551.

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Opaprakasit and Painter

Figure 2. DSC thermograms of neat poly(4-vinylpyridine) (P4VPy) (thermogram a), novolac/P4VPy 30/70 (thermogram b), novolac/ P4VPy 50/50 (thermogram c), and novolac resin (thermogram d).

polymer blends, however, it is useful to discuss the theory that describes the effect of diluents on the glass transition of polymers, which was originally developed by Fox.17 Most polymers do not form miscible mixtures; therefore, the thermograms of such immiscible blends display two Tg values, which are near the Tg values of the pure components. In blends that are miscible, a single Tg is observed, somewhere between those of the pure components. In systems with relatively weak intermolecular interactions, the composition dependence of the Tg value of the blend can usually be described by the Fox equation:

W1 W2 1 ) + Tg m Tg 1 Tg 2

(1)

Tgm is the glass-transition temperature of the polymer blend, whereas Tg1 and Tg2 are the glass-transition temperatures of the pure components 1 and 2, respectively, and W1 and W2 are the respective weight fractions of these components. There are many other equations describing the dependence of Tg on blend composition, the most general of which seem to be those derived by Couchman and co-workers,18-20 in that most other equations can be derived from these relationships, given certain assumptions. Although the simplest forms of all these equations describe the composition dependence of Tg well, significant deviations are observed for systems with strong specific interactions. Various modifications to Couchman’s equation have been proposed to account for these (17) Fox, T. G. Bull. Am. Phys. Soc. 1956, 1, 123. (18) Couchman, P. R. Macromolecules 1978, 11, 1156-1161. (19) Couchman, P. R. Polym. Eng. Sci. 1984, 24, 135. (20) Couchman, P. R.; Karasz, F. E. Macromolecules 1978, 11, 117119.

effects, one of the most widely used being that due to Kwei:21,22

Tgm )

X1Tg1 + X2Tg2 X1 + kX2

+ X1X2q′m

(2)

where q′m is a term that is dependent on the balance of interactions in the mixtures and k is a fitting parameter (usually, k ) 1). To demonstrate the feasibility of applying the blend approach to studies of coal extracts, we first examined DSC thermograms obtained from mixtures of a novolac and P4VPy. In our previous work,13,23 we have shown that phenolic resins synthesized from mixtures of phenol and dihydroxynaphthalene served as promising molecular models for the study of the structure and properties of coal. These co-polymer models were used to determine the effect of cross-link density on the swelling behavior and the extractability of coals. Here, we will use a similar but less-complex resin (novolac, which contains only phenolic groups linked together by methylene bridges). Because of the strong hydrogen bonds that form between the phenolic OH groups and the basic nitrogen of the pyridine group, novolacs that are not cross-linked form miscible mixtures with P4VPy.24,25 This is confirmed by the DSC results shown in Figure 2. The novolac and P4VPy have glass transitions at Tg ) 63 and 143 °C, respectively. When blends of 50/50 and 30/ (21) Kwei, T. K. J. Polym. Sci., Polym. Lett. Ed. 1984, 22, 307. (22) Kwei, T. K.; Pearce, E. M.; Pennacchia, J. R.; Charton, M. Macromolecules 1987, 20, 1174-1176. (23) Painter, P. C.; Opaprakasit, P.; Sobkowiak, M.; Scaroni, A. W. Prepr. Pap.sAm. Chem. Soc., Fuel Chem. Div. 1999, 218, 17. (24) Pennacchia, J. R.; Pearce, E. M.; Kwei, T. K.; Bulkin, B. J.; Chen, J. P. Macromolecules 1986, 19, 973-977. (25) Kim, H. I.; Pennacchia, J. R.; Kwei, T. K.; Pearce, E. M. Prepr. Pap.sAm. Chem. Soc., Polym. Div. 1991, 202, 113.



Figure 3. Observed glass-transition temperature (Tg) values of novolac/P4VPy blends, as a function of novolac composition, and the Tg values predicted by the Kwei equation (curve a) and the Fox equation (curve b).

70 wt % (novolac/P4VPy) compositions are formed, the DSC thermograms, also shown in Figure 2, display single glass transitions, located at 120 and 132 °C, respectively. The transitions are weaker and broader than those of the pure components; however, this is characteristic of the glass transitions of blends. Unsurprisingly, the Tg value of these blends deviates from the values predicted by the Fox equation, as shown in Figure 3, as a result of the strong intermolecular interactions between the two polymers. However, a good fit to the Kwei equation is obtained. Values of k and q′m of 1 and 120 were obtained from a least-squares fit. Kwei et al.24 found that the value of q′m is dependent on the substitution pattern in the phenolic resins, as well as the strength of intermolecular interactions, which is a point that we will revisit later. The miscibility of these blends was supported by FTIR studies. The infrared spectra of novolac in the OHstretching and OH-deformation region, P4VPy, and the 50/50 blend, are shown in Figures 4 and 5. The “free” and self-associated hydrogen-bonded OH-stretching modes of the novolac, shown in Figure 4, are located at 3508 and 3347 cm-1, respectively. Upon blending with P4VPy, the intensity of the former seems to decrease significantly, whereas the peak position of the hydrogenbonded OH group with the maximum intensity is shifted dramatically and is now centered under the CH stretching modes near 3000 cm-1. This is a result of the formation of very strong hydrogen bonds between phenolic hydroxyl groups and the electron-donating nitrogen of the pyridine groups of P4VPy. In addition, Figure 5 shows FTIR spectra in the region of the

spectrum where the OH-deformation mode of the hydroxyls and the ring-stretching modes of pyridine appear. The 1416 cm-1 band is a ring-stretching mode of the pyridine units, which shifts to 1419 cm-1 on blending. The corresponding OH-deformation mode of the novolac hydroxyls at 1361 cm-1 also shifts, to 1380 cm-1, when mixed with P4VPy. The former effect indicates an interaction involving the N atom of pyridine units that affects the ring-stretching vibrations, whereas the latter reflects the formation of strong hydrogen bonds, similar to what we observed in calixarene-liked compounds several years ago.12 These results demonstrate the formation of intermolecular hydrogen bonds between the OH and the pyridine units, which is responsible for miscibility and the deviation of the Tg values of the blends from the Fox equation. Differential Scanning Calorimetry and Fourier Transform Infrared Spectroscopy Results of Upper Freeport Coal Extract/Polymer Blends. We now turn our attention to the consideration of polymer/ coal extract mixtures. To use the Fox or Kwei equations to determine the Tg value of the coal extract (see below), we must first form miscible blends. The entropy of mixing two high-molecular-weight materials is negligibly small; therefore, miscibility usually is dependent on a balance between dispersion forces, which contribute a positive term to the free energy of mixing and, hence, are unfavorable, and specific interactions, which are generally favorable but dependent on a balance between self-association and interassociation (see Coleman et

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Figure 4. Fourier transform infrared (FTIR) spectra (OH-stretching mode region) of novolac (spectrum a), novolac/P4VPy 50/50 blend (spectrum b), and P4VPy (spectrum c).

Figure 5. FTIR spectra (OH-deformation region) of novolac (spectrum a), novolac/P4VPy 50/50 blend (spectrum b), and P4VPy (spectrum c).

al.26 and citations therein). To minimize unfavorable dispersion forces, blend components with closely matching solubility parameters should be chosen. Although the solubility parameter of this coal extract is unknown, a value of the order of 11 (cal/cm3)0.5 would seem to be reasonable,27 given the aromaticity and functional group (26) Coleman, M. M.; Graf, J. F.; Painter, P. C. Specific Interactions and the Miscibility of Polymer Blends: Practical Guides for Predicting and Designing Miscible Polymer Mixtures; Technomic Publishing Co.: Lancaster, PA, 1991. (27) Painter, P. C.; Graf, J. F.; Coleman, M. M. Energy Fuels 1990, 4, 379-384.

content of many mid-to-high-rank coals. Because P4VPy has a solubility parameter of 10.8 (cal/cm3)0.5 and can also strongly hydrogen bond to phenolic and acidic functional groups, we believe it would be capable of forming miscible mixtures with the UPF extract. A series of blends were prepared in the same manner as the novolac/P4VPy mixtures. These blends were then annealed at a temperature above the Tg value of P4VPy (180 °C) for 10 min, and DSC thermograms were recorded in the range of 35-210 °C. These thermograms, shown in Figure 6, indicate that all the blends



Figure 6. DSC thermograms of neat P4VPy (thermogram a) and the following blends: UPF extract/P4VPy 11 (thermogram b), UPF extract/P4VPy 20 (thermogram c), UPF extract/P4VPy 31 (thermogram d), UPF extract/P4VPy 40 (thermogram e), UPF extract/P4VPy 50 (thermogram f), UPF extract/P4VPy 60 (thermogram g), UPF extract/P4VPy 70 (thermogram h), UPF extract/P4VPy 74 (thermogram i), and UPF extract/ P4VPy 85 (thermogram j). Table 1. Glass-Transition Temperature (Tg) and Change in Specific Heat Capacity (∆Cp) of Upper Freeport Coal Extract/Poly(4-vinylpyridine) (UPF Extract/P4VPy) Blends sample

Tg (°C)

∆Cp (J g-1 °C-1)

P4VPy UPF extract/P4VPy 11 UPF extract /P4VPy 20 UPF extract /P4VPy 31 UPF extract /P4VPy 40 UPF extract /P4VPy 50 UPF extract /P4VPy 60 UPF extract /P4VPy 70 UPF extract /P4VPy 74 UPF extract /P4VPy 85 UPF extract

143 146 144 151 154 158 164 163 164 N/Aa N/Aa

0.40 0.36 0.32 0.38 0.27 0.23 0.20 0.12 0.22 N/Aa N/Aa

a

The transition is too broad and cannot be determined.

in this series have a single Tg value, which increases with the UPF extract composition, but stabilizes at a UPF extract composition of >60%, as summarized in Table 1. This suggests that the UPF extract is miscible with P4VPy at compositions of 164 °C (the highest temperature obtained from the blends). A “predicted” Tg value will be calculated later. If this observed transition is indeed a glass transition, then it must appear reproducibly in subsequent scans. Otherwise, it might simply be due to some chemical

reaction or an evaporation of low-molecular-weight materials.3,6-8,11 A possibility that is of great concern is the effect of residual water or, more likely, given the temperature of the transition, pyridine solvent strongly bound in the samples, even though the transition we observe here occurs at a temperature much higher than the boiling point of this solvent. Accordingly, we stored the samples after the first scan (35-210 °C) for 3 days in air at room temperature. The samples were then rescanned using the same experimental conditions. The results are shown in Figure 7, where the first- and second-run thermograms of UPF extract/P4VPy 10 and 30 are displayed. The results from the second experiments are similar to those obtained from the first run, although the (single) Tg value of each blend increases slightly. This is probably an annealing effect that is a result of heating the sample to 210 °C at the end of the first run. The most significant difference in the thermograms between the first and second scan is the appearance of a large endothermic peak at ∼90 °C. This is in good agreement with what we observed in our previous study11 and the results obtained from the coal extract and the residue that we discussed earlier. This endothermic peak is most probably associated with an evaporation of water absorbed while standing in air. However, we emphasize from these results that the observed transitions at ∼160 °C are reversible and consistent with a glass transition, implying that the pure extract transition occurs at a temperature of >164 °C. The Kwei and Fox equations can now be used to estimate the Tg value of the UPF extract. Unsurprisingly, the experimental data deviates from the composition dependence predicted by the Fox equation, as shown in Figure 8 (where a line has been drawn, using an assumed Tg value of the coal extract, to show the general shape of the curve). This is most probably a result of specific interactions in these mixtures. However, up to an extract content of 60%, the Kwei equation fits the data very well. Values of k ) 1 and q′m ) -40 were obtained by this least-squares fit. These parameters were then used in the calculation of the Tg value of the UPF extract, which was determined to be ∼194 °C. The values of k and q′m obtained are in very close agreement with those determined by Kwei et al. in studies of certain substituted phenolic resin blends.22,24 Note that this calculated Tg value falls in the range where weight loss is observed in thermograms (see Figure 1), possibly explaining why it cannot be observed experimentally. The infrared spectra of these UPF extract/P4VPy blends were also examined, to study the origin of specific interactions between these mixtures. Results similar to those observed in novolac/P4VPy blends were obtained. The spectra in the OH-stretching region of P4VPy, the coal extract, and the coal/P4VPy 50 and 60 blends are shown in Figure 9. On blending, the free OH-stretching mode of the coal extract (3536 cm-1) decreases in intensity. The hydrogen-bonded OH stretching mode of the pure extract at 3288 cm-1 shifts to lower wavenumber and is now centered under the CH stretching modes near 3000 cm-1. These results indicate mixing of the components and the formation of hydrogen bonds be-

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Figure 7. DSC thermograms from the first and second run after samples were stored at room temperature for 3 days: UPF extract/P4VPy 30, first scan (thermogram a); UPF extract/P4VPy 30, second scan (thermogram b); UPF extract/P4VPy 10, first scan (thermogram c); and UPF extract/P4VPy 10, second scan (thermogram d).

Figure 8. Plot of Tg of UPF extract/P4VPy blends, as a function of UPF extract composition: experimental value (curve a), predicted value from the Kwei equation (curve b), and predicted value from the Fox equation (curve c).

tween hydroxyl groups of the coal extract and the basic pyridine groups in P4VPy. However, the concentration of the OH groups in the UPF extract is much lower than in the novolac, accounting for the higher value of q′m obtained in the blends involving the latter material and the fact that phase separation apparently occurs at high

UPF concentrations. (At high UPF extract concentrations, the balance of thermodynamic forces presumably becomes unfavorable.) Takahashi et al.28 determined that the molecular weight of the NMP/CS2-soluble extract obtained from UPF coal is low (with a number and weight average of



Figure 9. FTIR spectra of UPF extract (spectrum a), UPF extract/P4VPy 60 (spectrum b), UPF extract/P4VPy 50 (spectrum c), and neat P4VPy (spectrum d).

∼1200 and 1800, respectively). It seems probable that the pyridine-soluble material would not differ significantly from this, so the calculated Tg value of the coal extract is surprisingly high. For example, the lowmolecular-weight novolac resin used in this study has a glass-transition temperature of Tg ) 63 °C. The Tg value of a material increases as the molecular weight, the strength of intermolecular interactions, and the chain stiffness each increase, as a result of the effect of these factors on free volume. Because the novolac resin has a much higher concentration of phenolic OH groups than the UPF extract, this implies that the latter is a much-stiffer and less-flexible structure and/or has strong intermolecular interactions whose nature remains to be determined. Two final observations: First, the Tg value of the parent coal, if it has one, should be >194 °C, just because of the effect of molecular weight, probably putting it in the range where chemical reactions occur. (One should keep in mind that, in highly cross-linked systems, there may be insufficient free volume for a glass transition to occur below a temperature where the network is degraded.) Second, it is not possible to use the blend methodology with the parent coal. For ther(28) Takahashi, K.; Norinaga, K.; Masui, Y.; Iino, M. Energy Fuels 2001, 15, 141-146. (29) Bastide, J.; Candau, S.; Leibler, L. Macromolecules 1981, 14, 719-726.

modynamic reasons, a high-molecular-weight polymer suspended in a solvent in contact with a swollen network is not capable of diffusing into and mixing with that network.29 Conclusion The existence of a glass-transition temperature (Tg) in coal has been a point of dispute. Because this material is highly heterogeneous and is a highly cross-linked network, if it does have a glass transition, it might only be at a temperature higher than its degradation temperature. Degradation or other chemical reactions would dominate differential scanning calorimetry (DSC) experimental results and mask any evidence of a glass transition. However, a study on lower-molecular weight coal extracts should provide a useful measurement of the lowest-possible Tg value of a coal. A polymer blend methodology provides an effective tool to examine the Tg value of the pyridine-soluble fraction of Upper Freeport coal. A Tg value of ∼190 °C was determined. Acknowledgment. The authors gratefully acknowledge the support of the Office of Chemical Sciences, U. S. Department of Energy, under Grant No. DE-FG0286ER13537. P.O. is a recipient of, and partially supported by, the Development and Promotion of Science and Technology Talent Project (DPST), Thailand. EF049913V

Determination of the Glass-Transition Temperature of a Coal Extract

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