Research Note pubs.acs.org/IECR
Conversion of Fluorine-Containing Ozone-Depleting and Greenhouse Gases to Valuable Polymers in a Nonthermal Plasma Eric M. Kennedy,*,† Sazal K. Kundu,† John C. Mackie,† Clovia I. Holdsworth,‡ Thomas S. Molloy,† Vaibhav V. Gaikwad,† and Bogdan Z. Dlugogorski† †
Process Safety and Environment Protection Research Group, School of Engineering, and ‡Discipline of Chemistry, School of Environmental and Life Sciences, The University of Newcastle, Callaghan, NSW 2308, Australia S Supporting Information *
ABSTRACT: A dielectric barrier discharge (DBD) nonthermal plasma was used to convert a range of fluorocarbons into useful polymeric products. Reactions were conducted at atmospheric pressure, in an argon bath gas and where methane was added as reactant. The bulk gas temperature was less than 150 °C and yielded polymers from a number of methane/fluorocarbon mixtures, including fluorocarbons such as halons, chlorofluorocarbons (CFCs), hydrochlorofluorocarbons (HCFCs), hydrofluorocarbons (HFCs), and perfluorocarbons (PFCs). The results of gel permeation chromatography (GPC) reveal that a potentially valuable polymer is synthesized, with a number average molecular weight of between 60 000 and 130 000 g mol−1 and a polydispersity index (PDI) of between 1.2 and 2.9, depending on the fluorochemical converted.
1. INTRODUCTION The widespread use of fluorochemicals has had a direct and measurable effect on the earth’s atmospheric temperature. Following over 50 years of use worldwide, it is estimated that the discharge of these chemicals into the atmosphere has led to an (average) increase of 0.4 °C in the earth’s temperature,1 and that synthetic greenhouse gases contribute over 14% to the overall effect of global warming.2 Among the most significant groups of synthetic greenhouse gases are hydrofluorocarbons, compounds that contain H, F, and C atoms. This group of compounds has been widely adopted as alternatives to CFCs and halons, as they do not contain Cl or Br atoms and hence have an ozone depletion potential of zero. Globally, HFC demand is projected to increase to almost 600 000 tonnes per year by 2012.3 Hydrofluorochemicals have a relatively high radiative forcing. For example, CHF3 (HFC-23), is estimated to have an atmospheric lifetime of 264 years, a radiative forcing value of +0.16 W m−2 ppbv−1 and a global warming potential (GWP) of 11 700, the second highest GWP among all known greenhouse gases.4 It has been reported that the concentration of CHF3 in the atmosphere is steadily increasing and it is predicted to continue to do so at the rate of 5% per year.5 In 2004, the cumulative emissions of HFCs, PFCs and SF5, in the United States alone, were equivalent to 140 million tonnes of CO2.6 Ironically, the success in reducing halon and CFC use, as a result of concern over the destruction of stratospheric ozone, has resulted in an escalation of the use of HFCs, which in turn has led to increasing concentrations of these chemicals in the earth’s atmosphere. The production and consumption of HFCs were not controlled by any international agreement until the ratification of the Kyoto Protocol, which aims at limiting the emission of greenhouse gases including HFCs, came into effect in February 2005.7 The implementation of the Kyoto Protocol, and the development of carbon credit schemes, will inevitably result in the stockpiling of vast quantities of HFCs for disposal. © 2012 American Chemical Society
This will further intensify research efforts aimed at developing technologies for treatment of HFCs as well as ozone-depleting compounds such as halons and CFCs. As fluorocarbons possess valuable C−F bonds, they are attractive as chemical feedstock for production of useful products. In response to this potential, we developed a thermal process to convert fluorocarbons to useful products, also based on the reaction of these compounds with methane.8−13 Catalysts have also been combined with thermal reactions with a similar aim.14 Typically, the application of nonthermal plasma reactors for converting fluorocarbons was based on destructing fluorocarbons.15−18 Researchers have applied various approaches for destruction of fluorocarbons. For example, Ricketts et al. investigated destruction of CFC-12 in air and in nitrogen with water in a dielectric pellet-bed reactor19 while Ogata et al. studied the effects of catalysts (Al2O3 and TiO2) for destruction of CClF3, CHClF2, CHF3, and CF4 in a surface discharge reactor.18 A limited number of researchers explored the use of nonthermal plasma reactors to convert fluorocarbons to economically valuable products. Wei et al., for example, used a DBD reactor at very low pressure and produced cross-linked polymer films from fluorocarbons.20 Vinogradov et al. also used a DBD reactor to convert fluorocarbons to polymer films at atmospheric pressure21 and Ingaki et al. used methane as a reactant with hexafluoropropene in a bell-jar plasma reactor. They obtained polymeric films from hexafluoropropene/ methane mixture and compared the properties of these films with polymer films formed from hexafluoropropene in the absence of methane.22 They have concluded that the addition Received: Revised: Accepted: Published: 11279
April 9, 2012 July 23, 2012 August 10, 2012 August 10, 2012 dx.doi.org/10.1021/ie300932z | Ind. Eng. Chem. Res. 2012, 51, 11279−11283
Industrial & Engineering Chemistry Research
Research Note
Figure 1. Schematic diagram of cylindrical double dielectric barrier discharge plasma experimental facility for polymerization of fluorocarbons with methane.
of methane has the benefit of producing a polymeric film with higher surface energies. The present investigation has been conducted in a DBD reactor, in which a nonthermal plasma can be generated at atmospheric pressure. Although DBDs are proven technology for ozone generation, they are now being used in numerous ways including the study of gas conversion chemistry. They can be built either in planar or cylindrical geometry; however, cylindrical shape ensures uniform treatment of reactant gases. In this research, we have introduced methane as a reactant with various fluorocarbons (substances are listed in the Experimental Section); see Figure 1 for a schematic diagram of the facility developed to study the reaction of fluorochemical gases with methane. For the investigated fluorocarbons, the novelty of our approach is to study their reaction with methane in a dielectric barrier discharge reactor aimed at converting waste fluorochemical gases into useful, non-cross-linked polymers.
The dimensions of the outer dielectric are 23 mm OD, 2 mm wall thickness, while the dimensions of inner dielectric are 10 mm OD and a 1 mm wall thickness. A copper shim (24 mm in length) wrapped around the outer tube works as ground electrode, while a spiral copper wire (30 mm in length) inside the inner tube acts as the high voltage electrode. A pair of aluminum flanges was employed to support the outer tube. These aluminum flanges, along with two PTFE fittings, constitute the inlet and outlet manifolds. The PTFE fittings support the inner dielectric, as well directing inlet and exit gas streams. The reactor assembly was operated in a dedicated fume cupboard. All fluorocarbons used in experiments are gaseous at room temperature and atmospheric pressure except CFC-11. To vaporize the CFC-11 feed, an indigenous heater, constructed from an insulated box fitted with a heating element and a temperature controller was employed as depicted in Figure 1. Each experiment is conducted for 90 min. While the experiment is in progress, analytical instruments (micro-GC, FTIR, GC, and GC−MS) are used to measure gas phase products. When the experiment is terminated, the reactor system is purged with argon, and subsequently the reactor tubes are rinsed with tetrahydrofuran solvent. The collected solution contains a polymeric mixture, which is subsequently characterized by GPC.
2. EXPERIMENTAL SECTION A similar experimental setup has been described in our recent publication.23 Briefly, a dielectric barrier discharge reactor constructed using alumina dielectrics (99.8% purity) has been employed in this research (Figure 1). The reactor is of cylindrical arrangement, with two concentric dielectric tubes. 11280
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Research Note
The volumetric flow rates of feed gases are controlled by mass flow controllers (Brooks). Methane (99.95%, Linde) and argon (99.999%, Coregas) have been used in all experiments. Fluorocarbons, tested in the experiments, are CFC-11 (>99%), CFC-12 (99.8%), HCFC-22 (>98%), HFC-23 (>98%), FC-31-10 (>98%), halon 1211 (98.7%), halon 1301 (98.5%), and a refrigerant mixture (recovered in Australia and contains 96% CFC-12, 3.3% HFC134a, 0.4% HCFC-22). Total flow rate used for all experiments is 100 cm3/min. The residence time (L/(F/ A),24 where L is the length of the electrode, F is the total volumetric feed rate, and A is the annular cross-sectional area bounded by the dielectric experiments) for all experiments was 2.95 s. An indigenously built power supply, with a variable voltage resonant converter topology, has been used in this study. The power supply can deliver a sinusoidal output of up to 20 kV (rms) at 21.5 kHz. An indigenous capacitive divider (3450:1) is used to measure the voltage developed across the dielectrics during experiments. This capacitive divider was calibrated against a commercially available high voltage probe (Tektronix P6015A, 1000:1). A current sensing resistor (50 ohm) is employed in series with the reactor to visualize instantaneous current. A thermocouple (J-type) was used to measure the wall temperature of the outer dielectric in plasma zone. An aliquot of zinc oxide based thermal transfer compound and a mica sheet were used to isolate the thermocouple electrically while maintaining thermal contact. Carbon containing feed and product gas species were quantitated by an in line micro-GC (Varian CP-4900) using thermal conductivity detectors. This micro-GC is equipped with molecular sieve 5A and PoraPLOT Q columns. For identification of gaseous species, a GC−MS (Shimadzu QP5000) equipped with AT-Q column was used. For quantitation, standard gases (Matheson Tri-Gas Inc.) were used where possible and relative molar response (RMR) factors, available in literature for many species or calculated from published correlations,25−27 were used for the remaining species. For quantitation of hydrogen, a gas chromatograph (Shimadzu GC-17A), equipped with a molecular sieve 13X column and a thermal conductivity detector was used. A standard gas (Matheson Tri-Gas Inc.) was used to calibrate this gas chromatograph. Analysis of acid gases was performed by a Perkin-Elmer Fourier transform infrared spectrometer (Spectrum 100). This FTIR is equipped with a very short path length (11.7 mm) acid-resistant gas cell and with KBr windows. The resolution, used for all scans, was 1.0 cm−1. The spectra were then processed (QASoft) to obtain absorption spectra and quantitated using external calibration. The external calibration was performed by producing acid gases in situ by a thermal reactor (from a fluorocarbon and methane), obtaining an infrared spectrum (reference spectrum) by passing the gas stream through the FTIR cell and finally through two caustic soda solution scrubbers for a known time to convert them into halide salts. An ion-chromatograph (Dionex 100) was employed to determine concentrations of anions of the caustic soda scrubber solution. These anionic standards were used to estimate the concentration of acid gases in the reference spectra. This calibration was used for the quantitation of acidic species in the sample FTIR spectrum.
A gel-permeation-chromatograph (GPC) (Shimadzu, Prominence) was used to quantitate the molecular weight of polymers. The GPC was equipped with refractive index (RI) detector and two Styragel columns (HR5E and HR3) operating at 40 °C. Linear polystyrene standards (Shodex) in the molecular weight range of 530 to 505 000 g mol−1 (Mn) were used for calibration. Data were analyzed by Shimadzu LCSolution 10A software.
3. RESULTS AND DISCUSSION 3.1. Formation of Non-cross-linked Polymers from Fluorocarbons. The reactions were undertaken in an argon bath gas, where oxygen and nitrogen were deliberately excluded. Additionally, the reactions were conducted at atmospheric pressure. Under these conditions, we obtained a mixture of polymers as solid product. The solid materials readily dissolve in tetrahydrofuran solvent. This is an indicative of the formation of non-cross-linked polymers. Polymers synthesized from plasma reactors are mostly categorized as cross-linked, and are insoluble in organic solvents. For fluorocarbons, Ingaki et al. reported formation of cross-linked polymers from hexafluoropropene and methane in a plasma reactor.22 Non-cross-linked polymers have several advantages over cross-linked polymers, not least of which their solubility in common solvents renders them straightforward to purification and categorization, and, from an application perspective, they can be reshaped by heating. 3.2. Conversion of Fluorocarbons and Methane. We screened a range of fluorochemical gases, including CFCs, HCFCs, HFCs, PFCs, and halons. The breakdown voltage varies from one fluorocarbon−methane mixture to another. The applied voltages, presented in Table 1, are slightly higher Table 1. Percentage Conversion of Reactants (Total Volumetric Flow Rate Is 100 cm3/min. Feed Gas Concentrations: Fluorocarbon, 1.25%; Methane, 1.25%; Remainder, Argon) Fluoro-chemical feed
applied voltage (kV, peak−peak)
% conversion, fluorocarbon
% conversion, methane
CCl3F (CFC11) CCl2F2 (CFC12) CHClF2 (HCFC-22) CHF3 (HFC23) C4F10 (FC-3-1 -10) CBrCIF2 (Halon 1211) CBrF3 (Halon 1301)
16.0
38
40
13.5
59
56
13.5
68
68
12.5
48
72
16.0
21
48
14.0
41
27
13.5
34
35
than the breakdown voltages for fluorocarbon−methane mixtures. The characteristics of the plasma formed are not the same for all fluorocarbon−methane combinations. Therefore, the conversion of fluorocarbons and the formation of product species vary. Table 1 shows the percentage conversion of fluorocarbons and methane. Data from micro-GC have been used to determine the percentage conversion of feed components. For multireactant systems, conversion of a reactant depends on the co-fed species. In the current investigation, the 11281
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conversion of methane varies depending on the fluorochemical added in the feed. Its conversions at 13.5 kV (peak−peak) with CBrF3, CCl2F2, and CHClF2 are 35%, 56%, and 68%, respectively (Table 1). The addition of methane to fluorocarbons obviates the production of F2 and Cl2 and facilitates the formation of HF and HCl. From a safety and process perspective, these acid gases can be removed much more readily from the stream compared to F2 and Cl2. 3.3. Formation of HF and Mass Balance. Existing technologies tend to convert fluorocarbons into acids and CO2, where the concentration of HF is extremely high, often up to 10% of the product gas stream. In the current plasma process, the amount of liberated HF is very low compared to these technologies, as the fluorine is a major component of the polymeric mixture formed in the plasma. The mixture of polymers, generated from the reaction of CFC-12 and methane, was analyzed and the composition was determined to be (in wt %): C, 27.51%; H, 1.82%; F, 25.88%; Cl, 44.92%. In mole ratio, C/H/F/Cl = 1.8:1.4:1.1:1.0. This result additionally shows that the solid molecules retain a significant amount of fluorine. On the basis of the solid composition analyzed and the quantitated data for gas phase species from GC, micro-GC, and FTIR, a mass balance has been conducted for the reaction of CFC-12 and methane. The overall mass balance has been found to exceed 96% of the feed reactants. Elemental mass balances have also been conducted and they are carbon, 96%; hydrogen, 95%; fluorine, 91%; and chlorine, 99% of their feed masses. The major gas phase products were hydrogen and acid gases (HCl and HF, the mole ratio of HCl/HF is around 3 for the reaction conditions noted); carbon-containing gaseous products were present only in small quantities. Among carbon containing gaseous products CH3Cl, CH2Cl2, and CHCl3 were dominant. Carbon−fluorine containing identified gaseous species are CHF3, C2H3F, CHClF2, and CF3CH2F. Of the products formed from this reaction, 52% (w/w) is solid products and 48% (w/w) is gas phase products (majority of gas phase products are acid gases) at the investigated reaction conditions. Both kinetic electrons and metastable argon play significant roles in the reaction mechanism. The formation of metastable argon atoms may be explained by the collision of neutral argon atoms with electrons:
e− + Ar → Ar* + e−
in Figure 2. As can be seen in Figure 2, the polymeric species, generated from the investigated fluorocarbons, are composed of
Figure 2. Typical GPC trace of polymeric species produced from the reaction of CFC-12 and methane in an argon bath gas.
essentially two distinguishable fractions. From GPC results, the number averaged molecular weight of the low molecular weight portion varies from 500 to 2500 g mol−1 (with respect to polystyrene standards) for the investigated reactants and reaction conditions. GPC analysis for the high molecular weight polymeric fractions is presented in Table 2. As can be seen in Table 2, the Table 2. Molecular Weights of Polymers Formed from Different Fluorocarbons (Total Volumetric Flow Rate Is 100 cm3/min. Feed Gas Concentrations: Fluorocarbon, 1.25%; Methane,1.25%; Remaining Bath Gas Was Argon) molecular weights (g mol−1, with respect to polystyrene standards)
reactants CCl3F and CH4 CCl2F2 and CH4 CHClF2 and CH4 CHF3 and CH4 C4F10 and CH4 CBrCIF2 and CH4 CBrF3 and CH4 CFC-Mix and CH4
(R01)
The collision of reactant molecules with kinetic electrons/ metastable argon atoms generates various molecular fragments. Some of these may be explained by the following reactions: Ar*/e− + CH4 → CH3 + H + Ar/e−
(R02)
Ar*/e− + CH3 → CH 2 + H + Ar/e−
(R03)
Ar* /e− + CCl 2F2 → CClF2 + Cl + Ar/e−
(R04)
−
Ar* /e + CClF2 → CF2 + Cl + Ar/e
−
number average molecular weight, Mn
weight average molecular weight, Mw
Polydispersity index (PDI)
16.0
58 000
159 000
2.7
13.5
79 000
154 000
2.0
13.5
127 000
153 000
1.2
12.5
120 000
146 000
1.2
16.0
68 000
152 000
2.2
14.0
57 000
168 000
2.9
13.5
67 000
159 000
2.4
14.0
90 000
152 000
1.7
high molecular weight fraction has a number average molecular weight range of 60 000 to 130 000 (with respect to polystyrene standards) with a PDI range of 1.2 to 2.9. Typically, plasma polymerization generates cross-linked polymers, and as such their lack of solubility makes determination of their molecular weights difficult. The range of molecular weights from the current investigation shows that formation of polymer products is in a generally favorable Mw distribution range, and the results suggest the formation of functional groups, and therefore their polymerization, can be controlled by manipulation of the applied voltage used to generate the polymer.
(R05)
Some molecular fragments are incorporated as functional groups within polymeric product species. Other molecular fragments react to form gas phase products. For example, the formation of CH3Cl may be explained by CH3 + Cl + (M) → CH3Cl + (M)
applied voltage (kV, peak−peak)
(R06)
3.4. Molecular Weights of Polymers. A typical gel permeation chromatography (GPC) trace has been presented 11282
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(7) Oram, D. E.; Sturges, W. T.; Penkett, S. A.; McCulloch, A.; Fraser, P. J. Growth of Fluoroform (CHF3, HFC-23) in the Background Atmosphere. Geophys. Res. Lett. 1998, 25, 35−38. (8) Yu, H.; Kennedy, E. M.; Uddin, A.; Sullivan, S. P.; Dlugogorski, B. Z. Experimental and Computational Studies of the Gas-Phase Reaction of Halon 1211 with Hydrogen. Environ. Sci. Technol. 2005, 39, 3020− 3028. (9) Kennedy, E. M.; Li, K.; Moghtaderi, B.; Dlugogorski, B. Z. A Process for Disposal of Halon 1301 (CBrF3). Chem. Eng. Commun. 1999, 176, 195−200. (10) Li, K.; Kennedy, E. M.; Moghtaderi, B.; Dlugogorski, B. Z. Experimental and Computational Studies on the Gas-Phase Reaction of CBrF3 with Hydrogen. Environ. Sci. Technol. 2000, 34, 584−590. (11) Tran, R.; Kennedy, E. M.; Dlugogorski, B. Z. Gas-Phase Reaction of Halon 1211 (CBrClF2) with Methane. Ind. Eng. Chem. Res. 2001, 40, 3139−3143. (12) Uddin, M. A.; Kennedy, E. M.; Yu, H.; Sakata, Y.; Dlugogorski, B. Z. Catalytic Process for the Conversion of Halon 1211 (CBrClF2) to Halon 1301 (CBrF3) and CFC 13 (CCIF3). Ind. Eng. Chem. Res. 2003, 42, 6000−6006. (13) Han, W.; Kennedy, E. M.; Mackie, J. C.; Dlugogorski, B. Z. Mechanistic Study of the Reaction of CHF3 with CH4. Chem. Eng. J. 2011, 166, 822−831. (14) Yu, H.; Kennedy, E. M.; Adesina, A. A.; Dlugogorski, B. Z. A Review of CFC and Halon Treatment TechnologiesThe Nature and Role of Catalysts. Catal. Surv. Asia 2006, 10, 40−54. (15) Chang, M. B.; Yu, S. J. An Atmospheric-Pressure Plasma Process for C2F6 Removal. Environ. Sci. Technol. 2001, 35, 1587−1592. (16) Futamura, S.; Einaga, H.; Zhang, A. Comparison of Reactor Performance in the Nonthermal Plasma Chemical Processing of Hazardous Air Pollutants. IEEE Trans. Ind. Appl. 2001, 37, 978−985. (17) Masuda, S.; Hosokawa, S.; Tu, X.; Wang, Z. Novel Plasma Chemical TechnologiesPPCP and SPCP for Control of Gaseous Pollutants and Air Toxics. J. Electrost. 1995, 34, 415−438. (18) Ogata, A.; Kim, H.; Futamura, S.; Kushiyama, S.; Mizuno, K. Effects of Catalysts and Additives on Fluorocarbon Removal with Surface Discharge Plasma. Appl. Catal. B 2004, 53, 175−180. (19) Ricketts, C. L.; Wallis, A. E.; Whitehead, J. C.; Zhang, K. A Mechanism for the Destruction of CFC-12 in a Nonthermal, Atmospheric Pressure Plasma. J. Phys. Chem. A 2004, 108, 8341−8345. (20) Wei, L.; Xiaodong, T.; Dongping, L.; Yanhong, L.; Zhiqing, F.; Baoxiang, C. Growth of Fluorocarbon Films by Low-Pressure Dielectric Barrier Discharge. Plasma Sci. Technol. 2008, 10, 74−77. (21) Vinogradov, I. P.; Dinkelmann, A.; Lunk, A. Measurement of the Absolute CF2 Concentration in a Dielectric Barrier Discharge Running in Argon/Fluorocarbon Mixtures. J. Phys. D 2004, 37, 3000−3007. (22) Inagaki, N.; Ohkubo, J. Plasma Polymerization of Hexafluoropropene/Methane Mixtures and Composite Membranes for Gas Separations. J. Membr. Sci. 1986, 27, 63−75. (23) Kundu, S. K.; Kennedy, E. M.; Gaikwad, V. V.; Molloy, T. S.; Dlugogorski, B. Z. Experimental Investigation of Alumina and Quartz as Dielectrics for a Cylindrical Double Dielectric Barrier Discharge Reactor in Argon Diluted Methane Plasma. Chem. Eng. J. 2012, 180, 178−189. (24) Wang, Q.; Yan, B.; Jin, Y.; Cheng, Y. Investigation of Dry Reforming of Methane in a Dielectric Barrier Discharge Reactor. Plasma Chem. Plasma Process. 2009, 29, 217−228. (25) Barry, E. F.; Rosie, D. M. Response Prediction of the Thermal Conductivity Detector with Light Carrier Gases. J. Chromatogr. A 1971, 59, 269−279. (26) Messner, A. E.; Rosie, D. M.; Argabright, P. A. Correlation of Thermal Conductivity Cell Response with Molecular Weight and Structure. Anal. Chem. 1959, 31, 230−233. (27) Height, M. J.; Kennedy, E. M.; Dlugogorski, B. Z. Thermal Conductivity Detection Relative Molar Response Factors for Halogenated Compounds. J. Chromatogr. A 1999, 841, 187−195.
4. CONCLUSION A preliminary investigation of a range of fluorocarbons mixed with methane in argon bath gas reacting in a dielectric barrier discharge shows non-cross-linked polymers from the methane fluorocarbon reaction. The addition of methane leads to the formation of HF and HCl and methane also contributes to the polymer backbone structure. The fluoropolymers produced are soluble in tetrahydrofuran solvent and expected to be of economical value. The development of processes which limit the discharge of greenhouse gases into the atmosphere, is a topic of intense international interest, especially those that mitigate CO2 emissions. The treatment of fluorine-containing greenhouse gases, especially where the cost of the treatment process can be offset or even alleviated by producing valuable products is attractive, as it provides incentive to users of these gases to responsibly manage their end-of-life disposal. Therefore, formation of non-cross-linked polymers from fluorine-containing greenhouse gases applying nonthermal plasma can be a new possibility for the treatment of fluorine-containing greenhouse gases.
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ASSOCIATED CONTENT
S Supporting Information *
List of fluorocarbons employed for this study. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The authors would like to thank the Australian Research Council for financial support of this project. Sazal K. Kundu and Vaibhav V. Gaikwad are indebted to the Australian Government and the University of Newcastle, Australia for postgraduate scholarships. Ms Azrinawati Mohd Zin, Ph.D. student, Discipline of Chemistry, the University of Newcastle deserves thanks for assisting GPC analysis. We also thank members of Microanalytical Unit, Research School of Chemistry, The Australian National University, for their assistance in elementary analysis of the polymeric mixture.
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REFERENCES
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