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Thermal characterization of HCN polymers by TG–MS, TG, DTA and DSC methods
Polymer Degradation and Stability 96 (2011) 943e948
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Polymer Degradation and Stability journal homepage: www.elsevier.com/locate/polydegstab
Thermal characterization of HCN polymers by TGeMS, TG, DTA and DSC methods José L. de la Fuente a, *, Marta Ruiz-Bermejo b, César Menor-Salván b, Susana Osuna-Esteban b a
Instituto Nacional de Técnica Aeroespacial “Esteban Terradas” (INTA), Carretera Torrejón-Ajalvir, Km. 4, E-28850 Torrejón de Ardoz, Madrid, Spain Centro de Astrobiología [Consejo Superior de Investigaciones Científicas-Instituto Nacional de Técnica Aeroespacial “Esteban Terradas” (CSIC-INTA)], Carretera Torrejón-Ajalvir, Km. 4, E-28850 Torrejón de Ardoz, Madrid, Spain
b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 17 December 2010 Received in revised form 17 January 2011 Accepted 28 January 2011 Available online 22 February 2011
This paper presents a thermogravimetry (TG) study of hydrogen cyanide polymers, synthesized from the reaction of equimolar aqueous solutions of sodium cyanide and ammonium chloride. Differential thermal analysis (DTA) and differential scanning calorimetry (DSC) were also used to evaluate the thermal behaviour of these black polymers, which play an important role in prebiotic chemistry. A coupled TGemass spectrometer (MS) system allowed us to analyze the principal volatile thermal decomposition and fragmentation products of the isolated HCN polymers under dynamic conditions and an inert atmosphere. After dehydration, a multi-step decomposition occurred in this particular polymeric system, due to the release of ammonia, hydrogen cyanide (depolymerization reaction), isocyanic acid (or cyanic acid) and formamide; these two latter species allow us identify bond connectivities. Finally, data collected from TG experiments in an oxidative atmosphere showed significant differences at higher temperatures, above 400 C. According to these results, the different techniques of thermal analysis here applied have demonstrated to be an adequate methodology for the study and characterization of this complex macromolecular system, whose structure remains controversial even today. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: HCN polymers Coupled TGeMS Thermal decomposition DSC DTG
1. Introduction Hydrogen cyanide (HCN) is a ubiquitous molecule in the universe. HCN, whether as gas, a pure liquid, or in solution, polymerizes easily over a wide range of temperature and pressure in the presence of amounts of a base catalyst to yield a dark complex solid, known as “HCN polymers” [1e3]. In addition, recently HCN polymers have been synthesized from the thermal decomposition of formamide [4]. The HCN polymers may be among the organic macromolecules most readily formed within the solar system. The HCN polymers could be the major component of the dark matter observed on many bodies of the outer solar system including asteroids, moons, planets and, especially, comets [5,6]. It has been also proposed that the reddish haze (tholin) present in the atmosphere of Titan could be analogous to HCN polymers [7]. Besides the interest in this polymeric system from a prebiotic perspective, the HCN polymers can be used as a starting material for the synthesis of carbon nitride materials, expected to have unique mechanical properties such as wear, resistance coating and hardness [8e18]. However, certain structural aspects of these HCN
* Corresponding author. Tel.: þ34 91 5206841; fax: þ34 91 5206611. E-mail address: [email protected] (J.L. de la Fuente). 0141-3910/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2011.01.033
polymers remain controversial, despite extensive efforts over the last 50 years. Part of this is due to the various analytical techniques different authors have used, part of it is due to the manner in which the polymers were synthesized. Thermogravimetry (TG) is one of the most commonly used thermal analysis techniques for the characterization of both inorganic and organic materials, including polymers. It provides quantitative results regarding the weight loss of a sample as a function of temperature or time. Moreover, TG measurements give basic information about the thermal properties of the material and its composition. The derivative thermogravimetry (DTG) can be used to investigate the differences between thermograms. In recent years, multi-purpose thermal analysis coupled with evolved gas analyzers has become very popular, as it can be used to carry out further analysis of evolved gases during TG measurements; producing details of thermal decomposition processes, which in turn facilitate estimation of sample structure and composition. Regarding instrumentation, a great number of gas detectors or analyzers have been utilized in evolved gas analysis (EGA), being the most important analyzer Fourier transform infrared (FTIR) spectrometers, and pre-eminently mass spectrometers (MS). Both methods can be used to record spectra repetitively, thereby producing a time-dependent record of the composition of the gas phase, from which EGA curves can be constructed for selected species [19e22].
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In the present work, a systematic thermal degradation study stability of the HCN polymers has been carried out both in argon (thermal stability) and in oxygen (thermal-oxidative stability) atmosphere. A coupled TGeMS system was used to analysis the principal species evolved during dynamic thermal decomposition or fragmentation processes for the HCN polymers synthesized. In addition, differential thermal analysis (DTA) and differential scanning calorimetry (DSC) were also employed to evaluate different thermal features of these black polymers. The results obtained from this thermal analysis provide useful information in drawing a correlation between their properties and chemical structure, and to understand their decomposition mechanism. 2. Experimental section Three HCN polymer samples were synthesized using different reaction times. The polymers were prepared from solutions of equimolar amounts of ammonium chloride (NH4Cl) and sodium cyanide (NaCN) in pure water (MilliQ grade) at 1 M concentration, in a similar way to the one described by Borquez et al. [23]. The solutions were left standing for three, ten days, and one month without stirring. All solutions were heated at 38 C during the reaction time for comparative purposes with respect to our previous works about the formation of tholins under CH4 atmosphere [24e26]. Samples were filtered using glass fibre filters after reactions, and washed with distillate water (4) collecting black solids which were dried under reduced pressure. The yields in this work were calculated from the initial amount of carbon contained in the initial NaCN. The NH4Cl and NaCN were obtained from SigmaeAldrich. Elemental C, H and N analyses were performed using a LECO CHNS-932 elemental analyzer. For the elemental O analyses a LECO VTF-900 analyzer was used. Thermogravimetry (TG), derivative thermogravimetry (DTG) and differential thermal analysis (DTA) measurements were preformed with a simultaneous thermal analyzer model Q600 of TA Instruments. Non-isothermal experiments were performed in the temperature range from 25 to 1000 C and at a heating rate of 10 C/min. The average sample weight was w10 mg, and the argon and oxygen flow rate was 100 ml/min. A coupled TGeMS system was used to make an analysis of principal species evolved during the dynamic thermal decomposition of fragmentation processes of all the polymers samples; with an electron-impact quadruple mass-selective detector, model Thermostar QMS200 M3. A differential scanning calorimetry (DSC; Perkin Elmer DSC/TA7DX, PC series with liquid nitrogen for low temperatures) was also used. The temperature and heat flow were calibrated with common standards, such as indium. Samples (w5 mg) were scanned at 10 C/min under dry nitrogen (100 ml/min).
Table 1 Experimental conditions, yields (calculated from the initial amount of carbon containing in the initial NaCN) and elemental analysis for the series of HCN black polymers synthesized. Polymer sample
Reaction time (days)
Yield (%)
C (%)
H (%)
N (%)
O (%)
Empirical formula
1 2 3
3 10 30
25 27 33
36.5 36.5 36.2
4.4 4.4 3.9
40.8 41.9 39.6
17.4 17.2 19.2
C3H4N3O C3H4N3O C5H6N4O2
3.1. Thermogravimetric and DSC analysis The TG thermograms of HCN polymers were recorded under dynamic conditions from room temperature to 1000 C at a heating rate of 10 C/min and under argon atmosphere. Representative thermogravimetric curves are shown in Fig. 1a. As can be seen, the thermal degradation of the different samples shows practically an identical behaviour, which demonstrates the high degree of similarity between them; in spite of the fact that they were obtained with different reaction times. From this figure, it is evident that the thermal degradation of all samples can be divided into three stages: drying stage (<150 C), main pyrolysis stage (150e500 C) and carbonization (>500 C). The first one, between 25 and 150 C, involves a mass loss of around 9 wt%, and corresponds to the vaporization of moisture, to the desorption of water and to the possible emission of volatile organic compound. This initial step of weight loss at low temperatures indicates the hydrophilicity of the HCN polymers. The degradation starts around 150 C and distributes over a broader temperature range. The second stage, between 150 and 500 C, corresponds with
3. Results and discussion The yield found in the polymerization reactions as well as the results of the elemental analysis for the “HCN polymers” obtained under the experimental conditions described above are shown in Table 1. Our results are comparable to those reported by Eastman et al. [27] for short reaction times (1e10 days). For longer reaction times (30 days) a black, insoluble solid richer in oxygen was obtained. However, a longer reaction time does not have a significant influence on the yield of these insoluble products (29 4%). The analytical and spectroscopic data, elemental analysis, FTIR, and 13C CP-MAS-NMR, of the HCN polymers synthesized are very similar, indicating that basically the same macrostructure is present in all of them (see Figs. 1 and 2 in reference [24]). These spectra greatly resemble those reported in the literature for HCN polymers prepared under different experimental conditions [5,28e30].
Fig. 1. (a) TG and (b) DTG curves for HCN polymer samples synthesized. Heating rate was 10 C/min and the argon atmosphere.
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peak at 80 is attributed to the evaporation of absorbed water, and the rest of the observed peaks, at 265, 675 and 885 C, reflect decomposition. The DSC thermograms of the samples run in nitrogen atmosphere, where the first endothermic peaks are clearly observed at 90 and 255 C, are in good agreement with the DTA and DTG peaks found (the small differences are due to the technique considered). 3.2. Mass spectrometric thermal analysis
Fig. 2. DTA curve for an HCN polymer sample. Heating rate was 10 C/min and the argon atmosphere.
a mass loss of about 25 wt%. This broad stage may therefore be rationalized to be a composite of two thermal events as described below. The final stage occurs between approximately 500 and 1000 C. This last step is the major thermal decomposition stage, and the samples lost a maximum of ca. 50% of weight. At 1000 C, the char residue is another characteristic of these HCN polymers, being equal to 13, 17 and 15% for the samples 1e3, respectively (Table 1). The first derivatives of thermograms were calculated to highlight the inflection points that indicate thermal transitions. These DTG curves are shown in Fig. 1b, where a deconvolution of one of these curves into an individual Gaussian peak has been made assuming a linear background over the temperature range of the fitting. The temperatures of the DTG maxima with the corresponding rates of weight loss, dW/dT, for the three samples are collected in Table 2. Fig. 1b illustrates more clearly the small differences in the thermal decomposition behaviour of the three samples. The first DTG peak appears at 80 C, probably resulted from the desertion of water and/or organic volatile material as it was mentioned before. At the second stage, a double peak observed, the first one around 260 C and the last one appears between 334 and 371 C. A more significant change due to thermal degradation takes place from the third peak, and continues with a main degradation at the last stage, with a peak of maximum loss weight between 651 and 668 C, and finally one last shoulder around 810 C. The three samples follow globally the same stages of pyrolysis. However, some small differences appear, thus for the sample 1, obtained to the lowest reaction time, its first three maxima decomposition rates take place at slightly lower temperatures than the polymer samples prepared with longer reaction times. The DTA thermogram under argon of HCN polymer samples under study exhibits different stages according with the previous DTG analysis, with a very broad endotherm between 300 and 950 C, as is shown in Fig. 2. The first and well defined endothermic
MS coupled with TG system has been used to study volatile species of thermal decomposition and fragmentation processes for all HCN polymer samples obtained. Dynamic measurements were carried out in argon atmosphere. Fig. 3 (as an example) presents the ion current for m/z detected in the MS versus temperature for the sample 2 (identical results were obtained for the other samples). In this inert atmosphere major signals are observed in the range 50e550 C with an MS peak for H2Oþ (with m/z ¼ 18) that appeared at 85 C (these data coincide with loss observed on TG curves and DSC data). The second major signal corresponds with m/z ¼ 17, which is attributed to OHþ or/and ammonia NHþ 3 , with maximum rates of forming centred at 90 C (probably for OHþ), and a following broad peak (extended between 155 and 530 C) centred around 280 C. This second broad MS peak presents the same profile than the third major signal for m/z ¼ 16 (NHþ 2 ). In addition to the mentioned products, H2O and NH3, other significant components are released from the polymer such as: hydrogen cyanide, isocyanic acid HeN]C]O (or cyanic acid HeOeCN), and formamide HCONH2. The HCN evolution (thermal depolymerization) with m/z ¼ 26 for CNþ and 27 for HCNþ, starts at 120 C and takes place over all the temperature range under study. The shape of the main peaks of mass spectrometry resembles those of DTG curves. These profiles are very similar to those ion currents corresponding to Cþ and CHþ with m/z ¼ 12 and 13, respectively. On the other hand, the release of gases contributing to m/z ¼ 43, associated to isocyanic acid (or cyanic acid), begins at 150 C, and it also happens on a large range of temperature with a plateau form. The profiles for m/z ¼ 42 (NCOþ) and m/z ¼ 30 (NOþ) show the same tendencies due to their ion fragmentation. The release of formamide is followed with the profile of m/z ¼ 45 and 44 (CONHþ 2 ), where two peaks around 255 and 400 C can be observed. Finally, at temperatures above 550 C, the gaseous species can be mainly due to the tar cracking of the HCN polymer main chain, observing TGeMS profiles for the m/z ¼ 51 and 52 and probably with ion fragmentations producing a signal on the profiles of m/ z ¼ 24, 13 and 12. At these high temperatures the cleavage of the polymer chain can originate, for example, the formation of NCeC^CHþ (m/z ¼ 51) and NCeCH]CHeþ (m/z ¼ 52) type structures. Fig. 4 compares the ion current curve for the main thermal decomposition gases for an HCN polymer sample under study, with a signal at m/z ¼ 17, 18, 27, 43 and 45. As is shown in this figure, the intensity order of MS signal is m/z ¼ 18 (8.5$109) > 17 (2$109) > 27 (5$1010) > 43 (15$1012) > 45 (<1012). This indicates the majority of thermal decomposable components in the HCN polymers can release H2O, ammonia, HCN, isocyanic acid and
Table 2 Characteristic temperatures for the thermal decomposition of synthesized HCN black polymers in argon atmosphere, DTG maxima with the corresponding rates of weight loss, dW/dT. Polymer sample Tmax1 ( C) dW1/dT (wt%/ C) Tmax2 ( C) dW2/dT (wt%/ C) Tmax3 ( C) dW3/dT (wt%/ C) Tmax4 ( C) dW4/dT (wt%/ C) Tmax5 ( C) dW5/dT (wt%/ C) 1 2 3
68.1 81.0 78.1
0.101 0.087 0.093
252 260 259
0.103 0.085 0.083
334 368 371
0.093 0.084 0.082
651 653 668
0.148 0.126 0.133
818 807 815
0.0102 0.094 0.089
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Fig. 3. Ion intensity curves for an HCN polymer sample, heating at 10 C/min in argon atmosphere.
formamide. These results are in agreement with the scarce data of the literature concerning the thermal decomposition of the system under study [7,31,32]. On one hand, the recent work of Cataldo and co-worker [31], where these authors analyze HCN polymers obtained from thermal decomposition of formamide, and they find in their pyrolysis a large amount of gaseous HCN in a wide range of temperatures together with ammonia and isocyanic acid. On the other hand, the pioneering study of Ferris et al. where the major pyrolytic reaction products were identified as CO2, H2O, HCN, CH3CN, HCONH2, and pyridine [32].
The hydrophilic character of these HCN polymers demonstrated in this paper is also a feature of other well-known polymers, as for example polyamides based on cyanoimidazoles as has been welldescribed by Thurber and Rasmussen [33]. This fact is very relevant, so that the last studies about the structure of this polymeric system, performed Mamajanov and Herzfeld and based on solid state NMR, propose a polyaminoimidazole as the main chain for HCN polymers [30]. On the other hand, the pyrolytic formation of some of these products, such as formamide and isocyanic acid, is consistent with the presence of amide groups in these HCN polymers. The presence
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Fig. 4. (a) DTG curve and (b) MS signal comparison of m/z ¼ 17, 18, 27, 42 and 44 for an HCN polymer sample, heating at 10 C/min in argon atmosphere.
of secondary amide groups has been established by solid state NMR in HCN polymers obtained from equimolar amounts of H13CN and HC15N by Matthews and co-workers [34,35]. It should be mentioned that the intensity of m/z signal in this technique is related to the amount of species, but not decisive. It is also determined by different ionized energies of the distinct gas species. Therefore, the quantitative comparison of MS signals of different samples and components is not performed in this paper. On the other hand, it is also important to note that pyrolysis of these HCN polymers releases gaseous hydrogen cyanide, isocyanic acid and formamide in a wide temperature range. However, some of these molecules (as well as other oligomers from them) could not be thermal decomposition products, they can simply be side products, formed during the polymerization reaction, and trapped on the polymeric matrix. Thus for example, urea could be one of these side products, however it is thermally unstable, forming ammonia and cyanic acid (or isocyanic acid) in a simple decomposition reaction [36,37]. 3.3. Effect of oxygen on the thermal decomposition The effect of the oxygen on the thermal degradation of the HCN polymers was also analyzed. The HCN polymers thermal-oxidative degradation has a different behaviour than in argon degradation. Fig. 5a and b shows the TG and DTG curves, respectively, for an HCN polymer sample degraded in argon and oxygen environments from room temperature to 1000 C. A detailed analysis of the curves allow us to conclude that the two first degradation stages, until temperature around 400 C, are not apparently affected by the type of environment under study. Below this temperature the oxygen notably decreases the stability of the HCN polymers. The first DTG peak, caused by the presence of water and/or volatile compounds is shifted to lower temperatures, and now appears around 50 C in the thermo-oxidative degradation. At the second stage, where two separated DTG maxima under argon flux can be registered, it is unaffected by the change of the atmosphere. This fact suggests that these peaks, which correspond to the main pyrolysis stage, do not arise from the thermally weak structures sensitive to oxidation.
Fig. 5. (a) TG and (b) DTG curves for an HCN polymer sample. Heating rate was 10 C/ min and the oxygen atmosphere.
However, it is noticeable the great difference observed from ca. 400 C, where the HCN polymers present higher stability in argon than in an oxygen atmosphere at higher temperatures. Besides, the TG analyses of all HCN polymers under oxidative atmosphere provide no residues. Thus, it is very illustrative the clear change at the third decomposition stage, which is now quite narrow, between 400 and 600 C (versus 500 and 1000 C in argon). In an inert atmosphere, the DTG maximum is observed around 660 C, and in this case it appears at lower temperatures, 540 C. In addition, the second of DTG peaks, which appear as a shoulder around 815 C in argon, it is not observed in the presence of oxygen. The dW/dT values of this peak significantly increase under oxygen atmosphere, as this figure clearly shows. In this range of temperatures decompositionecarbonization reactions normally take place. Thus the presence of oxygen diminishes the stability of the macrostructure of the HCN polymer, and leads to intense thermo-oxidation processes. This means that the oxygen accelerated the mass loss of this polymer through oxidation. 4. Conclusions Various samples of HCN polymers were synthesized from the reaction of equimolar amounts of NaCN and NH4Cl in water, and characterized by FTIR and solid state NMR for a study of their thermal properties using different analytical techniques such as TG, DTA, DSC and TGeMS. The TG curves showed that the mass loss percentages for the decomposition were practically the same for each sample; therefore their thermal stability is not essentially influenced by the reaction time used in their syntheses. The thermal degradation of
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all the samples was divided into three stages: drying stage (<150 C), main pyrolysis stage (150e500 C) and carbonization (>500 C). The first one with a mass loss of w9 wt%, indicated the high hydrophilicity of the HCN polymers. The endothermic decomposition that began at about 150 C is characterized by a continuous release of HCN (with the maxima of ion currents associated with the maxima of loss weight observed on DTG curves) and isocyanic acid (or cyanic acid) up to temperature of 900 C. Together these species, also release ammonia and formamide, however, the maxima of elimination of these volatile species takes place at lower temperature, in the range 150e450 C. The pyrolytic formation of some of these products, such as formamide and isocyanic acid, is consistent with the presence of amide groups in these HCN polymers. Also MS peak for ion fragments with m/ þ þ þ z ¼ 12 (Cþ), 13 (CHþ), 16 (NHþ 2 ), 24 (C2 ), 26 (CN ), 30 (NO ), 42 þ þ (NCO ), 44 (H2NCO ), 51 and 52 were detected for the investigated polymers. DTA and DSC measurements revealed the exclusive endothermic nature for all the thermal decomposition processes. The oxidative decomposition of our HCN polymers showed the same stages, in the same manner as in argon. The first peak is slightly displaced to lower temperature, as well as the pyrolytic reactions that take place at the third degradation stage. This last stage is narrowed, and the corresponding rate of weight loss is considerably enhanced. In this final high temperature region, oxygen reacts readily with the char residue, resulting in rapid char oxidation. To our knowledge this is the first reported application of TGeMS technique to explore the thermal stability of this complex, heterogeneous and particular macromolecular system. These polymers have been suggested to be the key starting point for the origin of protein/nucleic acid based life, and therefore could be the critical link connecting cosmochemistry and biochemistry, their study being a topic that continues to be investigated in many research centres. It is believed that the results shown in this paper represent a non-negligible advancement in the structural characterization of HCN polymers and in the understanding of their thermal decomposition mechanism. Acknowledgements The authors have used the research facilities of Centro de Astrobiología (CAB) and have been supported by Instituto Nacional de Técnica Aeroespacial “Esteban Terradas” (INTA) and the project AYA2009-13920-C02-01 of the Ministerio de Ciencia e Innovación (Spain). We thank R. Rojas and S. Veintemillas from ICMM (Instituto de Ciencia de Materiales de Madrid. CSIC) for their useful comments. References [1] Minard RD, Yang W, Varma P, Nelson J, Matthews CN. Heteropolypeptides from poly-a-cyanoglycine and hydrogen cyamide: a model for the origin of proteins. Science 1975;90:387e9. [2] Ferris JP, Hagan WJ. HCN and chemical evolution: the possible role of cyano compounds in prebiotic synthesis. Tetrahedron 1984;40:1093e120. [3] Matthews CN, Minard RD. Hydrogen cyanide polymer, comets and the origin of life. Faraday Discuss 2006;133:393e401. [4] Cataldo F, Patane G, Compagnini G. Synthesis of HCN polymer from thermal decomposition of formamide. J Macromol Sci A Pure Appl Chem 2009;46: 1039e48. [5] Quirico E, Montagnac G, Lees V, McMillan PF, Szopa C, Cernogora G, et al. New experimental constraints on the composition and structure of tholins. Icarus 2008;198:218e31. [6] Matthews CN, Minard RD. Hydrogen cyanide polymers connect cosmochemistry and biochemistry. Organic matter in space. In: Proceedings IAU Symposium No. 251. 2008. p. 453e457.
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Thermal characterization of HCN polymers by TGeMS, TG, DTA and DSC methods José L. de la Fuente a, *, Marta Ruiz-Bermejo b, César Menor-Salván b, Susana Osuna-Esteban b a
Instituto Nacional de Técnica Aeroespacial “Esteban Terradas” (INTA), Carretera Torrejón-Ajalvir, Km. 4, E-28850 Torrejón de Ardoz, Madrid, Spain Centro de Astrobiología [Consejo Superior de Investigaciones Científicas-Instituto Nacional de Técnica Aeroespacial “Esteban Terradas” (CSIC-INTA)], Carretera Torrejón-Ajalvir, Km. 4, E-28850 Torrejón de Ardoz, Madrid, Spain
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a b s t r a c t
Article history: Received 17 December 2010 Received in revised form 17 January 2011 Accepted 28 January 2011 Available online 22 February 2011
This paper presents a thermogravimetry (TG) study of hydrogen cyanide polymers, synthesized from the reaction of equimolar aqueous solutions of sodium cyanide and ammonium chloride. Differential thermal analysis (DTA) and differential scanning calorimetry (DSC) were also used to evaluate the thermal behaviour of these black polymers, which play an important role in prebiotic chemistry. A coupled TGemass spectrometer (MS) system allowed us to analyze the principal volatile thermal decomposition and fragmentation products of the isolated HCN polymers under dynamic conditions and an inert atmosphere. After dehydration, a multi-step decomposition occurred in this particular polymeric system, due to the release of ammonia, hydrogen cyanide (depolymerization reaction), isocyanic acid (or cyanic acid) and formamide; these two latter species allow us identify bond connectivities. Finally, data collected from TG experiments in an oxidative atmosphere showed significant differences at higher temperatures, above 400 C. According to these results, the different techniques of thermal analysis here applied have demonstrated to be an adequate methodology for the study and characterization of this complex macromolecular system, whose structure remains controversial even today. Ó 2011 Elsevier Ltd. All rights reserved.
Keywords: HCN polymers Coupled TGeMS Thermal decomposition DSC DTG
1. Introduction Hydrogen cyanide (HCN) is a ubiquitous molecule in the universe. HCN, whether as gas, a pure liquid, or in solution, polymerizes easily over a wide range of temperature and pressure in the presence of amounts of a base catalyst to yield a dark complex solid, known as “HCN polymers” [1e3]. In addition, recently HCN polymers have been synthesized from the thermal decomposition of formamide [4]. The HCN polymers may be among the organic macromolecules most readily formed within the solar system. The HCN polymers could be the major component of the dark matter observed on many bodies of the outer solar system including asteroids, moons, planets and, especially, comets [5,6]. It has been also proposed that the reddish haze (tholin) present in the atmosphere of Titan could be analogous to HCN polymers [7]. Besides the interest in this polymeric system from a prebiotic perspective, the HCN polymers can be used as a starting material for the synthesis of carbon nitride materials, expected to have unique mechanical properties such as wear, resistance coating and hardness [8e18]. However, certain structural aspects of these HCN
* Corresponding author. Tel.: þ34 91 5206841; fax: þ34 91 5206611. E-mail address: [email protected] (J.L. de la Fuente). 0141-3910/$ e see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.polymdegradstab.2011.01.033
polymers remain controversial, despite extensive efforts over the last 50 years. Part of this is due to the various analytical techniques different authors have used, part of it is due to the manner in which the polymers were synthesized. Thermogravimetry (TG) is one of the most commonly used thermal analysis techniques for the characterization of both inorganic and organic materials, including polymers. It provides quantitative results regarding the weight loss of a sample as a function of temperature or time. Moreover, TG measurements give basic information about the thermal properties of the material and its composition. The derivative thermogravimetry (DTG) can be used to investigate the differences between thermograms. In recent years, multi-purpose thermal analysis coupled with evolved gas analyzers has become very popular, as it can be used to carry out further analysis of evolved gases during TG measurements; producing details of thermal decomposition processes, which in turn facilitate estimation of sample structure and composition. Regarding instrumentation, a great number of gas detectors or analyzers have been utilized in evolved gas analysis (EGA), being the most important analyzer Fourier transform infrared (FTIR) spectrometers, and pre-eminently mass spectrometers (MS). Both methods can be used to record spectra repetitively, thereby producing a time-dependent record of the composition of the gas phase, from which EGA curves can be constructed for selected species [19e22].
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In the present work, a systematic thermal degradation study stability of the HCN polymers has been carried out both in argon (thermal stability) and in oxygen (thermal-oxidative stability) atmosphere. A coupled TGeMS system was used to analysis the principal species evolved during dynamic thermal decomposition or fragmentation processes for the HCN polymers synthesized. In addition, differential thermal analysis (DTA) and differential scanning calorimetry (DSC) were also employed to evaluate different thermal features of these black polymers. The results obtained from this thermal analysis provide useful information in drawing a correlation between their properties and chemical structure, and to understand their decomposition mechanism. 2. Experimental section Three HCN polymer samples were synthesized using different reaction times. The polymers were prepared from solutions of equimolar amounts of ammonium chloride (NH4Cl) and sodium cyanide (NaCN) in pure water (MilliQ grade) at 1 M concentration, in a similar way to the one described by Borquez et al. [23]. The solutions were left standing for three, ten days, and one month without stirring. All solutions were heated at 38 C during the reaction time for comparative purposes with respect to our previous works about the formation of tholins under CH4 atmosphere [24e26]. Samples were filtered using glass fibre filters after reactions, and washed with distillate water (4) collecting black solids which were dried under reduced pressure. The yields in this work were calculated from the initial amount of carbon contained in the initial NaCN. The NH4Cl and NaCN were obtained from SigmaeAldrich. Elemental C, H and N analyses were performed using a LECO CHNS-932 elemental analyzer. For the elemental O analyses a LECO VTF-900 analyzer was used. Thermogravimetry (TG), derivative thermogravimetry (DTG) and differential thermal analysis (DTA) measurements were preformed with a simultaneous thermal analyzer model Q600 of TA Instruments. Non-isothermal experiments were performed in the temperature range from 25 to 1000 C and at a heating rate of 10 C/min. The average sample weight was w10 mg, and the argon and oxygen flow rate was 100 ml/min. A coupled TGeMS system was used to make an analysis of principal species evolved during the dynamic thermal decomposition of fragmentation processes of all the polymers samples; with an electron-impact quadruple mass-selective detector, model Thermostar QMS200 M3. A differential scanning calorimetry (DSC; Perkin Elmer DSC/TA7DX, PC series with liquid nitrogen for low temperatures) was also used. The temperature and heat flow were calibrated with common standards, such as indium. Samples (w5 mg) were scanned at 10 C/min under dry nitrogen (100 ml/min).
Table 1 Experimental conditions, yields (calculated from the initial amount of carbon containing in the initial NaCN) and elemental analysis for the series of HCN black polymers synthesized. Polymer sample
Reaction time (days)
Yield (%)
C (%)
H (%)
N (%)
O (%)
Empirical formula
1 2 3
3 10 30
25 27 33
36.5 36.5 36.2
4.4 4.4 3.9
40.8 41.9 39.6
17.4 17.2 19.2
C3H4N3O C3H4N3O C5H6N4O2
3.1. Thermogravimetric and DSC analysis The TG thermograms of HCN polymers were recorded under dynamic conditions from room temperature to 1000 C at a heating rate of 10 C/min and under argon atmosphere. Representative thermogravimetric curves are shown in Fig. 1a. As can be seen, the thermal degradation of the different samples shows practically an identical behaviour, which demonstrates the high degree of similarity between them; in spite of the fact that they were obtained with different reaction times. From this figure, it is evident that the thermal degradation of all samples can be divided into three stages: drying stage (<150 C), main pyrolysis stage (150e500 C) and carbonization (>500 C). The first one, between 25 and 150 C, involves a mass loss of around 9 wt%, and corresponds to the vaporization of moisture, to the desorption of water and to the possible emission of volatile organic compound. This initial step of weight loss at low temperatures indicates the hydrophilicity of the HCN polymers. The degradation starts around 150 C and distributes over a broader temperature range. The second stage, between 150 and 500 C, corresponds with
3. Results and discussion The yield found in the polymerization reactions as well as the results of the elemental analysis for the “HCN polymers” obtained under the experimental conditions described above are shown in Table 1. Our results are comparable to those reported by Eastman et al. [27] for short reaction times (1e10 days). For longer reaction times (30 days) a black, insoluble solid richer in oxygen was obtained. However, a longer reaction time does not have a significant influence on the yield of these insoluble products (29 4%). The analytical and spectroscopic data, elemental analysis, FTIR, and 13C CP-MAS-NMR, of the HCN polymers synthesized are very similar, indicating that basically the same macrostructure is present in all of them (see Figs. 1 and 2 in reference [24]). These spectra greatly resemble those reported in the literature for HCN polymers prepared under different experimental conditions [5,28e30].
Fig. 1. (a) TG and (b) DTG curves for HCN polymer samples synthesized. Heating rate was 10 C/min and the argon atmosphere.
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peak at 80 is attributed to the evaporation of absorbed water, and the rest of the observed peaks, at 265, 675 and 885 C, reflect decomposition. The DSC thermograms of the samples run in nitrogen atmosphere, where the first endothermic peaks are clearly observed at 90 and 255 C, are in good agreement with the DTA and DTG peaks found (the small differences are due to the technique considered). 3.2. Mass spectrometric thermal analysis
Fig. 2. DTA curve for an HCN polymer sample. Heating rate was 10 C/min and the argon atmosphere.
a mass loss of about 25 wt%. This broad stage may therefore be rationalized to be a composite of two thermal events as described below. The final stage occurs between approximately 500 and 1000 C. This last step is the major thermal decomposition stage, and the samples lost a maximum of ca. 50% of weight. At 1000 C, the char residue is another characteristic of these HCN polymers, being equal to 13, 17 and 15% for the samples 1e3, respectively (Table 1). The first derivatives of thermograms were calculated to highlight the inflection points that indicate thermal transitions. These DTG curves are shown in Fig. 1b, where a deconvolution of one of these curves into an individual Gaussian peak has been made assuming a linear background over the temperature range of the fitting. The temperatures of the DTG maxima with the corresponding rates of weight loss, dW/dT, for the three samples are collected in Table 2. Fig. 1b illustrates more clearly the small differences in the thermal decomposition behaviour of the three samples. The first DTG peak appears at 80 C, probably resulted from the desertion of water and/or organic volatile material as it was mentioned before. At the second stage, a double peak observed, the first one around 260 C and the last one appears between 334 and 371 C. A more significant change due to thermal degradation takes place from the third peak, and continues with a main degradation at the last stage, with a peak of maximum loss weight between 651 and 668 C, and finally one last shoulder around 810 C. The three samples follow globally the same stages of pyrolysis. However, some small differences appear, thus for the sample 1, obtained to the lowest reaction time, its first three maxima decomposition rates take place at slightly lower temperatures than the polymer samples prepared with longer reaction times. The DTA thermogram under argon of HCN polymer samples under study exhibits different stages according with the previous DTG analysis, with a very broad endotherm between 300 and 950 C, as is shown in Fig. 2. The first and well defined endothermic
MS coupled with TG system has been used to study volatile species of thermal decomposition and fragmentation processes for all HCN polymer samples obtained. Dynamic measurements were carried out in argon atmosphere. Fig. 3 (as an example) presents the ion current for m/z detected in the MS versus temperature for the sample 2 (identical results were obtained for the other samples). In this inert atmosphere major signals are observed in the range 50e550 C with an MS peak for H2Oþ (with m/z ¼ 18) that appeared at 85 C (these data coincide with loss observed on TG curves and DSC data). The second major signal corresponds with m/z ¼ 17, which is attributed to OHþ or/and ammonia NHþ 3 , with maximum rates of forming centred at 90 C (probably for OHþ), and a following broad peak (extended between 155 and 530 C) centred around 280 C. This second broad MS peak presents the same profile than the third major signal for m/z ¼ 16 (NHþ 2 ). In addition to the mentioned products, H2O and NH3, other significant components are released from the polymer such as: hydrogen cyanide, isocyanic acid HeN]C]O (or cyanic acid HeOeCN), and formamide HCONH2. The HCN evolution (thermal depolymerization) with m/z ¼ 26 for CNþ and 27 for HCNþ, starts at 120 C and takes place over all the temperature range under study. The shape of the main peaks of mass spectrometry resembles those of DTG curves. These profiles are very similar to those ion currents corresponding to Cþ and CHþ with m/z ¼ 12 and 13, respectively. On the other hand, the release of gases contributing to m/z ¼ 43, associated to isocyanic acid (or cyanic acid), begins at 150 C, and it also happens on a large range of temperature with a plateau form. The profiles for m/z ¼ 42 (NCOþ) and m/z ¼ 30 (NOþ) show the same tendencies due to their ion fragmentation. The release of formamide is followed with the profile of m/z ¼ 45 and 44 (CONHþ 2 ), where two peaks around 255 and 400 C can be observed. Finally, at temperatures above 550 C, the gaseous species can be mainly due to the tar cracking of the HCN polymer main chain, observing TGeMS profiles for the m/z ¼ 51 and 52 and probably with ion fragmentations producing a signal on the profiles of m/ z ¼ 24, 13 and 12. At these high temperatures the cleavage of the polymer chain can originate, for example, the formation of NCeC^CHþ (m/z ¼ 51) and NCeCH]CHeþ (m/z ¼ 52) type structures. Fig. 4 compares the ion current curve for the main thermal decomposition gases for an HCN polymer sample under study, with a signal at m/z ¼ 17, 18, 27, 43 and 45. As is shown in this figure, the intensity order of MS signal is m/z ¼ 18 (8.5$109) > 17 (2$109) > 27 (5$1010) > 43 (15$1012) > 45 (<1012). This indicates the majority of thermal decomposable components in the HCN polymers can release H2O, ammonia, HCN, isocyanic acid and
Table 2 Characteristic temperatures for the thermal decomposition of synthesized HCN black polymers in argon atmosphere, DTG maxima with the corresponding rates of weight loss, dW/dT. Polymer sample Tmax1 ( C) dW1/dT (wt%/ C) Tmax2 ( C) dW2/dT (wt%/ C) Tmax3 ( C) dW3/dT (wt%/ C) Tmax4 ( C) dW4/dT (wt%/ C) Tmax5 ( C) dW5/dT (wt%/ C) 1 2 3
68.1 81.0 78.1
0.101 0.087 0.093
252 260 259
0.103 0.085 0.083
334 368 371
0.093 0.084 0.082
651 653 668
0.148 0.126 0.133
818 807 815
0.0102 0.094 0.089
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Fig. 3. Ion intensity curves for an HCN polymer sample, heating at 10 C/min in argon atmosphere.
formamide. These results are in agreement with the scarce data of the literature concerning the thermal decomposition of the system under study [7,31,32]. On one hand, the recent work of Cataldo and co-worker [31], where these authors analyze HCN polymers obtained from thermal decomposition of formamide, and they find in their pyrolysis a large amount of gaseous HCN in a wide range of temperatures together with ammonia and isocyanic acid. On the other hand, the pioneering study of Ferris et al. where the major pyrolytic reaction products were identified as CO2, H2O, HCN, CH3CN, HCONH2, and pyridine [32].
The hydrophilic character of these HCN polymers demonstrated in this paper is also a feature of other well-known polymers, as for example polyamides based on cyanoimidazoles as has been welldescribed by Thurber and Rasmussen [33]. This fact is very relevant, so that the last studies about the structure of this polymeric system, performed Mamajanov and Herzfeld and based on solid state NMR, propose a polyaminoimidazole as the main chain for HCN polymers [30]. On the other hand, the pyrolytic formation of some of these products, such as formamide and isocyanic acid, is consistent with the presence of amide groups in these HCN polymers. The presence
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Fig. 4. (a) DTG curve and (b) MS signal comparison of m/z ¼ 17, 18, 27, 42 and 44 for an HCN polymer sample, heating at 10 C/min in argon atmosphere.
of secondary amide groups has been established by solid state NMR in HCN polymers obtained from equimolar amounts of H13CN and HC15N by Matthews and co-workers [34,35]. It should be mentioned that the intensity of m/z signal in this technique is related to the amount of species, but not decisive. It is also determined by different ionized energies of the distinct gas species. Therefore, the quantitative comparison of MS signals of different samples and components is not performed in this paper. On the other hand, it is also important to note that pyrolysis of these HCN polymers releases gaseous hydrogen cyanide, isocyanic acid and formamide in a wide temperature range. However, some of these molecules (as well as other oligomers from them) could not be thermal decomposition products, they can simply be side products, formed during the polymerization reaction, and trapped on the polymeric matrix. Thus for example, urea could be one of these side products, however it is thermally unstable, forming ammonia and cyanic acid (or isocyanic acid) in a simple decomposition reaction [36,37]. 3.3. Effect of oxygen on the thermal decomposition The effect of the oxygen on the thermal degradation of the HCN polymers was also analyzed. The HCN polymers thermal-oxidative degradation has a different behaviour than in argon degradation. Fig. 5a and b shows the TG and DTG curves, respectively, for an HCN polymer sample degraded in argon and oxygen environments from room temperature to 1000 C. A detailed analysis of the curves allow us to conclude that the two first degradation stages, until temperature around 400 C, are not apparently affected by the type of environment under study. Below this temperature the oxygen notably decreases the stability of the HCN polymers. The first DTG peak, caused by the presence of water and/or volatile compounds is shifted to lower temperatures, and now appears around 50 C in the thermo-oxidative degradation. At the second stage, where two separated DTG maxima under argon flux can be registered, it is unaffected by the change of the atmosphere. This fact suggests that these peaks, which correspond to the main pyrolysis stage, do not arise from the thermally weak structures sensitive to oxidation.
Fig. 5. (a) TG and (b) DTG curves for an HCN polymer sample. Heating rate was 10 C/ min and the oxygen atmosphere.
However, it is noticeable the great difference observed from ca. 400 C, where the HCN polymers present higher stability in argon than in an oxygen atmosphere at higher temperatures. Besides, the TG analyses of all HCN polymers under oxidative atmosphere provide no residues. Thus, it is very illustrative the clear change at the third decomposition stage, which is now quite narrow, between 400 and 600 C (versus 500 and 1000 C in argon). In an inert atmosphere, the DTG maximum is observed around 660 C, and in this case it appears at lower temperatures, 540 C. In addition, the second of DTG peaks, which appear as a shoulder around 815 C in argon, it is not observed in the presence of oxygen. The dW/dT values of this peak significantly increase under oxygen atmosphere, as this figure clearly shows. In this range of temperatures decompositionecarbonization reactions normally take place. Thus the presence of oxygen diminishes the stability of the macrostructure of the HCN polymer, and leads to intense thermo-oxidation processes. This means that the oxygen accelerated the mass loss of this polymer through oxidation. 4. Conclusions Various samples of HCN polymers were synthesized from the reaction of equimolar amounts of NaCN and NH4Cl in water, and characterized by FTIR and solid state NMR for a study of their thermal properties using different analytical techniques such as TG, DTA, DSC and TGeMS. The TG curves showed that the mass loss percentages for the decomposition were practically the same for each sample; therefore their thermal stability is not essentially influenced by the reaction time used in their syntheses. The thermal degradation of
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all the samples was divided into three stages: drying stage (<150 C), main pyrolysis stage (150e500 C) and carbonization (>500 C). The first one with a mass loss of w9 wt%, indicated the high hydrophilicity of the HCN polymers. The endothermic decomposition that began at about 150 C is characterized by a continuous release of HCN (with the maxima of ion currents associated with the maxima of loss weight observed on DTG curves) and isocyanic acid (or cyanic acid) up to temperature of 900 C. Together these species, also release ammonia and formamide, however, the maxima of elimination of these volatile species takes place at lower temperature, in the range 150e450 C. The pyrolytic formation of some of these products, such as formamide and isocyanic acid, is consistent with the presence of amide groups in these HCN polymers. Also MS peak for ion fragments with m/ þ þ þ z ¼ 12 (Cþ), 13 (CHþ), 16 (NHþ 2 ), 24 (C2 ), 26 (CN ), 30 (NO ), 42 þ þ (NCO ), 44 (H2NCO ), 51 and 52 were detected for the investigated polymers. DTA and DSC measurements revealed the exclusive endothermic nature for all the thermal decomposition processes. The oxidative decomposition of our HCN polymers showed the same stages, in the same manner as in argon. The first peak is slightly displaced to lower temperature, as well as the pyrolytic reactions that take place at the third degradation stage. This last stage is narrowed, and the corresponding rate of weight loss is considerably enhanced. In this final high temperature region, oxygen reacts readily with the char residue, resulting in rapid char oxidation. To our knowledge this is the first reported application of TGeMS technique to explore the thermal stability of this complex, heterogeneous and particular macromolecular system. These polymers have been suggested to be the key starting point for the origin of protein/nucleic acid based life, and therefore could be the critical link connecting cosmochemistry and biochemistry, their study being a topic that continues to be investigated in many research centres. It is believed that the results shown in this paper represent a non-negligible advancement in the structural characterization of HCN polymers and in the understanding of their thermal decomposition mechanism. Acknowledgements The authors have used the research facilities of Centro de Astrobiología (CAB) and have been supported by Instituto Nacional de Técnica Aeroespacial “Esteban Terradas” (INTA) and the project AYA2009-13920-C02-01 of the Ministerio de Ciencia e Innovación (Spain). We thank R. Rojas and S. Veintemillas from ICMM (Instituto de Ciencia de Materiales de Madrid. CSIC) for their useful comments. References [1] Minard RD, Yang W, Varma P, Nelson J, Matthews CN. Heteropolypeptides from poly-a-cyanoglycine and hydrogen cyamide: a model for the origin of proteins. Science 1975;90:387e9. [2] Ferris JP, Hagan WJ. HCN and chemical evolution: the possible role of cyano compounds in prebiotic synthesis. Tetrahedron 1984;40:1093e120. [3] Matthews CN, Minard RD. Hydrogen cyanide polymer, comets and the origin of life. Faraday Discuss 2006;133:393e401. [4] Cataldo F, Patane G, Compagnini G. Synthesis of HCN polymer from thermal decomposition of formamide. J Macromol Sci A Pure Appl Chem 2009;46: 1039e48. [5] Quirico E, Montagnac G, Lees V, McMillan PF, Szopa C, Cernogora G, et al. New experimental constraints on the composition and structure of tholins. Icarus 2008;198:218e31. [6] Matthews CN, Minard RD. Hydrogen cyanide polymers connect cosmochemistry and biochemistry. Organic matter in space. In: Proceedings IAU Symposium No. 251. 2008. p. 453e457.
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