3.1. MW Radiation Effect in the Production of NH4CN Polymers
In order to explore the effect of the MW radiation in the aqueous polymerization of NH
4CN, two set of experiments were carried out. The first one used equivalent reaction times (polymers
1–5,
Table 1); the second one fixed the reaction time at 67 min (polymers
2,
6–9,
Table 1). For the first series of syntheses (series 1), the reaction times were chosen with a comparative proposal in a relationship with the NH
4CN polymer obtained at 80 °C using conventional heating and a reaction time of 144 h (control polymer) [
1]. Based on the manufacturer specification of the MW reactor, 6 days under a classical thermal treatment is equivalent to the short reaction times using MW radiation indicated in
Table 1 (series 1). On the other hand, the fixed time for the second series (series 2) was based on the fact that, in order to probe a polymerization time shorter than 309 min (polymer
1) but longer than those explored previously in the NH
4CN polymerization assisted by MW at 180 °C, with a reaction time of up to 30 min [
13]. Thus, the reaction time for polymer
2 was chosen for polymeric series 2.
The conversion degrees, α (%), reached for each polymer are displayed in
Figure 2a, and they were calculated as is described in [
1] by means of gravimetric measurements of the gel fraction and the insoluble product. The use of conventional heating demonstrated that the increase in the temperature leads to a decrease in the yield of the NH
4CN polymers [
14]. For the control polymer, the conversion was about ≈38%, while this value decreased to ≈17% when the polymerizations are conducted at 180 °C using MW radiation [
13]. Note that the higher conversion was found in series 1, around 25%, at the lowest temperature under study 130 °C, and that the data of series 1 is in agreement with these previous results. However, a slight increase in the conversion for the polymerizations was observed at temperatures higher than 180 °C. For series 2, at 130 °C, a decrease of practically five times in the polymerization time produced a relevant and unexpected increase in conversion. On the contrary, an increase in the reaction time at temperatures of 170 and 205 °C causes an increase in the yield of insoluble polymer. The conversion values for polymers
6,
7 and
9 was really unexpected, reaching yields similar to those obtained by means of a traditional heating process [
1,
14]. This behavior of the polymerization in the presence of MW radiation seems to indicate significant differences with respect to the kinetic approach reported using conventional heating, in which a clear limit conversion was observed after a concrete reaction time for a fixed temperature [
4]. Further works are in progress to study in detail the kinetic behavior of the MW-driven polymerization of cyanide.
In addition, the pathways proposed to explain the formation of NH
4CN polymers in aqueous media suggest collateral reactions during the course of this precipitation polymerization, such as deaminations, denitrogenations, dehydrocyanations, hydrolysis and oxidations. These last processes are responsible for the incorporation of oxygen in the macrostructures [
4], which would lead to a gain of weight considering the total initial mass input in the system (the initial weight of NaCN plus NH
4Cl).
Figure 2b shows the weight balance found in both series, considering the reaction conditions shown in
Table 1, based on the following formula: Weight Balance (%) = [(weight NaCN + weight NH
4Cl) − (gel fraction weight + freeze-dry sol content)]/(weight NaCN + weight NH
4Cl)]·100. In any case, no direct relationship could be found between the conversion values and the weight balance, as can be observed when comparing
Figure 2a,b. In series 1, an increasing weight loss was found as the temperature increased; however, in series 2, when the polymerization time was kept constant, a dissimilar behavior was observed. In this case, at low temperatures, the weight loss, around 5%, was maintained throughout these polymerizations. However, the MW reactions at 170, 190 and 205 °C occurred with substantial weight gain, close to 15% or higher. The increase of weight might be related with a minor loss of nitrogen and/or to a higher increase of mass by oxygenation, but the results of the elemental analysis were inconsistent with this proposal (see below), which suggests that complex secondary processes have no reflex directly in the generation of the gel fraction macrostructures.
The elemental analysis data displayed in
Figure 2c indicates that the C percentage did not present significant differences among the two series under study, and it was about 41%. The same behavior was observed for the %O, at around 17%. Only a slight decrease in the nitrogen content was observed in series 2 with the increase of the temperature, from 41 to 37%. This result was also reflexed in the C/H and C/N molar relationship for this series, when the reaction time was fixed, going from 1.18 to 1.31 for this last ratio (
Figure 2d). Therefore, it seems that the increase in the temperature favors the deamination processes during the polymerization based on the lower %N and %H. This fact was in agreement with previous results [
13]. In contrast, for series 1, no significant variations were found in the molar relationships with the increasing of the polymerization temperature.
As a result, it can be concluded that a temperature of 170 °C seems to be a key parameter in these polymerizations under MW radiation, and also that low polymerization temperatures, such as 130 °C, present a singular behavior. However, the compositional data do not give a clear response to the high weight gain observed in series 2.
At this point, it was necessary to point out that the lower yields for the insoluble polymers may be due to the hydrothermal conditions achieved by MW radiation, which favors the oxidation, hydrolysis and decomposition processes of the NaCN and/or other intermediate reaction products, as in
Scheme 1, decreasing the availability of them, and therefore reducing the final amount of insoluble polymers collected. Note that all of the reactions were prepared under ambient conditions of pressure and moisture, and neither oxygen or CO
2 were removed from the reaction vessel or the solvent.
The left part of
Scheme 1 shows the possible processes which can decrease the available amount of cyanide to polymerize. Atmospheric oxygen can lead to the partial oxidation of cyanide at elevated temperatures, generating cyanate. CO
2 can react with cyanide to produce carbonate and HCN. In addition, the hydrolysis of the cyanide solutions under heating can produce formiate and ammonium. Under the conditions considered herein, the initial cyanide solutions have a pH of 9.2, and a ~50% of HCN was present in the solutions, which can be hydrolyzed to give formic acid and ammonia. The generation of volatile compounds such as HCOOH, NH
3 and even HCN may explain the negative weight balances observed in the NH
4CN polymerization above studied. However, the increase observed for the production of insoluble polymers
6,
7 and
9 might be due to the recycling of the delivered HCN and NH
3, generated first as by-products, that finally were implicated in new processes of oligomerization/polymerization, as a closed system was used in the described reactions. On the other hand, the positive weight balances can be due to the formation of no volatile oxygenated molecules such as carbonate and cyanate, as this takes place at high temperatures within series 2. Besides this, in order to explore whether the ambient conditions have a significant influence in these plausible side reactions, where the atmospheric oxygen and the CO
2 were implicated, two additional reactions were carried out. Polymers
3 and
7 were again synthetized, but using an inert atmosphere of nitrogen and carefully removing the CO
2 and O
2 of the water by an N
2 stream. The following conversion values were obtained (syntheses were made twice) α(%) = 7.7 ± 0.1 and α(%) = 3.8 ± 0.3 for the analogous polymers
3 and
7, respectively. Furthermore, in both cases a loss of mass was observed in the total weight balance. Note that the effect of the absence of air in the conversion obtained was outstandingly significant for the analogous polymer
7. Therefore, the lack of air in the reaction environment does not seem to prevent the side reactions in the cyanide polymerization assisted by MW radiation; on the contrary, the yield of the reaction was remarkably lower. Due to the great impact of the air in the microwave-cyanide polymerizations, new works will be developed to obtain a full understanding of this factor in these highly complex polymerization reactions.
In addition, we might take into consideration that other hydrolysis and/or oxidations of some the intermediate products during the polymerization process finally lead to the lack of formation of the insoluble polymers, as is the case for diaminomaleonitrile (DAMN). DAMN, the tetramer of the HCN, was considered to be a main intermediate in the generation of the HCN polymers (
Scheme 1) (see, e.g., [
4]). However, this compound can be oxidized to diiminosuccinonitrile in water, and this is finally decomposed into oxalic acid and urea (upper part of the
Scheme 1). Furthermore, DAMN in an aqueous medium can be transformed into formamide, glycine and aminomanolic acid (the bottom part the
Scheme 1), as was revealed by Ferris and Edelson [
20].
The higher yields (~75%) in the hydrothermal polymerizations of DAMN than were the obtained in analogous NH
4CN polymerizations (35–40%) using conventional heating for the production of black insoluble HCN-derived polymers [
1], and the significant diminution of these yields in the MW-driven polymerization of DAMN (~30%) (a further work is in progress; no data are shown here) allows us to reaffirm the following points: (i) the DAMN was a main intermediate product cyanide polymerization, as similar—although not identical—final products can be obtained in both ways, i.e., the cyanide and DAMN polymerizations produced resemble—but are not identical—to HCN-derived polymers [
1]. The possible polymerization of cyanide by other different DAMN pathways through its dimer and/or its trimer, aminomalononitrile (AMN), for the generation of the suggested C=N networks of the HCN-derived polymers must also be taken in consideration (
Scheme 2); (ii) the higher yields obtained for the DAMN polymers seem to indicate that the cyanide undergoes several processes in water, which prevent the generation of DAMN and the subsequent polymerization of this oligomer. Therefore, it seems that the cyanide polymerization via the DAMN pathway was the main process.
As a summary of the above, one can say that: (i) the lack of air in the cyanide polymerization assisted by MW radiation does not seem to inhibit the side reactions which prevent the generation of DAMN, and (ii) the decomposition processes of the DAMN are related to the presence of water. The increase of the temperate increases the ratio of DAMN decomposition in water environments. Both aspects are the main focus of other works in progress.
The FTIR spectra of both series closely resembled those reported from the NH
4CN polymerizations under a classical heating and also assisted by MW at 180 °C (
Figure 3a,b) [
1,
4,
13,
14]. In these previous works, analyses of the features for these spectra were discussed in detail. The greatest difference between the spectral features of series 1 and 2 was the relative intensity of the bands related with the nitrile groups (-C≡N), around 2200 cm
−1. This difference was clearly observed when the EOR values (the extension of the reaction, EOR = [I
1645/(I
1645 + I
2200)] 100) [
22] were represented (
Figure 3c). For series 1, this value decreased with the temperature, and by the contrary, for series 2 the behavior was the opposite. This quantitative spectroscopic parameter was directly related to the progress of the polymerization reaction under traditional heating conditions, i.e., the increase of the monomer conversion along the reaction time gives noticeably higher EOR values [
22]. However, there was no observed clear relationship between the EOR and the conversion degree, especially in series 2 (
Figure 3d, upper part). This behavior was also observed from the NH
4CN polymers synthetized at 180 °C by MW [
13].
Scheme 2.
General pathways for the generation of HCN-derived polymers in water. In this scheme is shown the oligomer formation as far as the tetramer DAMN, as well as proposed macromolecular structures from the dimer (red color), the trimer (AMN) (black color) and DAMN (green color) according to recent works [
1,
3,
4,
23]. Note that the incorporation of oxygen in some of the structures for the suggested macromolecular systems was due to hydrolytic processes (blue color).
Scheme 2.
General pathways for the generation of HCN-derived polymers in water. In this scheme is shown the oligomer formation as far as the tetramer DAMN, as well as proposed macromolecular structures from the dimer (red color), the trimer (AMN) (black color) and DAMN (green color) according to recent works [
1,
3,
4,
23]. Note that the incorporation of oxygen in some of the structures for the suggested macromolecular systems was due to hydrolytic processes (blue color).
In addition, the calculation of the EOC (extension of the conjugation, EOC = [I
1645/(I
1645 + I
3330)]·100) [
1,
13] indicated no significant differences between series 1 and 2, with the exception of the values determined at 170 °C (
Figure 3c). Likewise, there was no clear trend found when these values were confronted with the conversion. Again, these spectroscopic data revealed that 170 °C shows a remarkable singularity with respect to the rest of the temperatures under study.
Simultaneous thermal analysis (STA) for representative samples of polymers
1–9 was carried out (
Figure 4), and some parameters obtained from these measurements are listed in
Table 1. All of the samples showed the same amount of moisture, at about 10%; however, while the NH
4CN polymers from series 1 presented a similar content of char residue, 19–22%, the samples from series 2 exhibited an increase of the char from 13 to 25% with the increase of the temperature. Polymer
9 was the sample that presented the highest percentage of char, and by the contrary, polymer
6 registered the lowest value. In general, a greater amount of char was related with highly cross-linking macrostructures and rigid chains; in particular, NH
4CN polymers were associated with the presence of cross-linking oxygenated groups, such as inter- and intramolecular amide bonds [
4]. The C/H, C/N, C/O and N/O molar ratios were 0.9, 1.2, 4.1 and 3.5, respectively, for polymer
6; and 1.0, 1.3, 3.5 and 2.7, respectively, for polymer
9. Thus, it seems that the higher thermal stability of sample
9 was related with greater oxygenated macrostructures, and with a minor amount of nitrogenized groups, but not with a more conjugated structure, considering the C/H values and the EOC data. Therefore, it might be proposed that this stability may be due to the presence of oxygenated cross-linking groups.
On the other hand, the DTG curves showed similar profiles to other NH
4CN polymers synthetized at 80 °C [
1]. Five characteristic peaks were observed: (i) at ~70 °C (water desorption); (ii) at 220–260 °C and at ~400 °C (thermal break of the weakest bonds); and (iii) at ~670–680 °C and 820–840 °C (high thermal decomposition, carbonization stage). Only a slight difference was found for polymer
5 with an additional degradation peak at 477 °C, and the decrease of the relative peak intensity at ~260 °C in series 2 with the increase of the temperature. In addition, the DSC curves were really dissimilar to those registered for NH
4CN polymers synthetized at lower temperatures [
1,
4], proving that the thermal analysis techniques provide excellent fingerprints to distinguish NH
4CN polymers with very similar FTIR spectra [
4]. For both series of experiments, the first endothermic peak, at 75–85 °C, can be related to the loss of the absorbed water in the polymeric matrix, and the rest of peaks can be related to degradative processes.
Some morphological aspects, such as the hydrodynamic diameter, were measured for representative polymers
1–9 (Z-average,
Table 1). Considering this parameter, for series 1, the particle size was decreased with the increase of the temperature, reaching nanoparticles with a diameter of ~280 nm and with relatively low PdI = 0.22–0.25. On the contrary, in series 2, although an apparent decrease in the molecular size may be related with the increase of the temperature, the PdI values were higher than those in series 1, indicating a higher degree of heterogeneity. Indeed, the data from sample
9 were not properly registered due to a presumable aggregation of the particles.
The XRD pattern profiles for NH
4CN polymers
1–9 have the same look as those previously reported for analogous polymers synthetized using a conventional hydrothermal procedure or MW radiation [
13,
14] (
Figure 5a,b). It was expected that an increase in the temperature would lead to a more ordered macrostructure [
13,
14]. This is the unique observed peak (2θ ~27°) related to graphitic-like two-dimensional (2-D) structures, such as layered g-C
3N
4 [
16,
24], would be higher and narrower at the higher polymerization temperatures. However, this prediction was not valid for the range of temperatures under study when analyzing the data of the crystallinity of the different samples collected in
Table 1. There was no clear relationship between the order level of the macrostructures with the increase of the temperature or the reaction time. All the samples exhibited an average crystallinity values of 65%, with the exception of polymers prepared at 130 °C, with values higher and lower values than this; showing a particular behavior of the cyanide polymerization at this temperature at least related with the internal order of the macrostructure.
It was clearly shown above that the heating by MW radiation has an unexpected and significant influence in the behavior of the cyanide polymerization, providing results which were not previously observed with conventional heating, i.e., there was not a proportional direct relationship between the reaction time and the conversion degree (%) and EOC [
14,
22], or temperature and crystallinity [
14], for example. However, the final reaction products present resemble the spectroscopic and thermal characteristics of analogous HCN-derived polymers synthetized using conventional heating; for the size of the particles and the higher order of the macrostructures, this was the main difference found between the conventional heating against MW radiation. No nanoparticles were identified in NH
4CN polymers synthetized using conventional heating (the Z-average and PdI could not be measured; the data are not shown; however, see [
1] for SEM images); at the lower temperature herein considered, the greater size of particles were observed. In addition, at 130 °C and 170 °C, particular features were observed with respect to the syntheses carried out at the other temperatures. The global and proper interpretation of all of these results is not simple and trivial. Thus, multivariate analysis could be an excellent analytical tool to reach a better and more objective interpretation of the data presented above, and could help us to choose preferential conditions to tune the nanoscale of the NH
4CN polymeric particles. In this way, a PCA was carried out to reach a first full overview of the MW radiation effect in the NH
4CN polymerization behavior (
Figure 6). For this analysis, the total data for the nine polymers (
Table 1) were considered. The conversion degree, the balance weight, and all of the molar ratios were calculated; the EOR, the EOC, the crystallinity, the char content, the moisture percentage, the Z-average and the PdI were taken in consideration, explaining 99% of the variance. The first axis shows the highest positive correlation among a wide number of variables, with the exception of EOC and the balance weight. The right-hand portion of the first axis was predominantly occupied by EOR, moisture, monomer conversion, C/O molar ratio, Pdl, Z-average and N/O molar ratio; the left-hand portion was occupied by crystallinity (%) and the C/N molar ratio. The second axis indicates the highest positive correlation among char (%) and the C/H molar ratio. By contrast, the EOC and balance weight exhibited the highest negative correlation with this axis. From these statistical results, one can say that the there is a direct relationship between the conversion degree and the EOR values, as was expected, although a clear relationship was not observed in
Figure 3d. In addition, a higher conversion degree was directly related to less-oxidized structures; interestingly, this minor oxidation degree was directly related with a higher size of the particle and a higher polydispersity degree. On the other hand, the percentage of char (%) was strongly correlated with the molar C/H ratio, indicating that the more conjugated structures are the more thermally stable, but apparently independent of the %O in the structure. A higher internal order based on the percentage of crystallinity is directly related with the C/N ratio and with the char (%), i.e., the more carbonaceous and more conjugated structures are the more crystalline and also the more thermally stable, but the Z-average and crystallinity (%) are independent variables. Therefore, if the MW radiation is considered to obtain nanoparticles of NH
4CN polymers, as under conventional heating these are not formed and MW notably reduces the reaction times, it seems that reaction conditions which lead preferentially to the oxidization of structures but are highly conjugated should be considered.
With respect to the polymeric samples, PCA analysis indicated groupings of the NH4CN polymers into well-differentiated sets: (i) one corresponding to series 1 (polymers 2, 3, 4 and 5), with the exception of polymer 1; (ii) another one with polymer 7 and 8; (iii) and polymers 1, 6 and 9 isolated from the rest, indicating a particular behavior, as was expected especially for 1 and 6, as they were the polymers synthetized at 130 °C, although polymer 1 was next to the group of series 1, in agreement with a relative resemblance with the samples of this group. In relation to the first group, the clear grouping of polymers 2, 3, 4 and 5 indicated that, statistically, the choice of the equivalent reaction times for different temperatures according to the instruction of the manufacturer of the MW reactor, and considering the experimental results indicated above, gives NH4CN polymers with similar characteristics. The second group, formed by polymers 7 and 8, showed higher char (%), C/H and C/N molar ratios and crystallinity (%), indicating that these polymers are thermally stables carbonaceous structures. For the first, the polymers 6 and 9, and to a lesser extent 1, showed a higher EOR, moisture (%), α (%), C/O molar ratio, Pdl, Z-average and N/O molar, i.e., they presented the polymeric particles with the highest size and lowest oxygen content. Therefore, the lowest and the highest temperature here studied do not seem adequate for the development of nanoparticles.
These well-defined groupings by similar and resemblance features revealed by the PCA analysis, in a certain way, could also be observed for the shape differences found by SEM (
Figure 7). Thus, polymers
2,
3,
4 and
5 presented rice-shaped nanoparticles and other stacking oval particles together with isolated nanofibers. Polymers
7 and
8 showed clear groupings of nanofibers together with other particles with other shapes (please see
Figures S1–S9 from the supplementary information for details), whereas polymer
9 was estranged from this set in the PCA analysis, showing isolated nanofibers and undefined-shape particles. Polymers
1 and
6, synthetized at the lowest temperature, presented a particular behavior. Polymer
1 displayed similar rice-shaped nanoparticles and staking oval particles to the rest of the polymers from series 1, but also spherical particles similar to those identified in the control polymer [
1]. The particles observed in polymer
6 were similar to the particles found for series 1, although the staking oval particles were not identified. These SEM images also seemed to indicate that 130 °C and 205 °C are not optimal temperatures to produce nanoparticles from cyanide, which is in agreement with the PCA results.
3.2. Structural Comparative Study between NH4CN Polymers
In order to complete this study about the effect of MW radiation on cyanide polymerization, a detailed structural comparison between the control polymer and polymer 3, as a representative sample of series 1, was carried out. The previous results for this series indicated that polymers 2, 3, 4 and 5 are very similar, as was expected considering the equivalence of the reaction times; they resembled polymer 1, but were at least morphologically very different to the control polymer. Thus, this section can help us to obtain a comprehensive knowledge about the MW heating role in the polymerization of the NH4CN when equivalent polymerization times are considered on the spectroscopic and thermal properties, as the morphological differences clearly showed above.
The comparison of the data from the control polymer and from polymer
3 indicated that the conversion degree decreases notably when the NH
4CN polymerization is assisted by MW radiation, as it was explained above, but no significant elemental composition variations were observed. For the control polymer, the elemental compositional data were %C 41.4 ± 0.3, %H 3.8 ± 0.2, %N 40.1 ± 0.4 and %O 14.7 ± 0.7, and for polymer
3 they were %C 40.3 ± 0.6, %H 3.5 ± 0.1, %N 39.3 ± 0.8, %O 16.7 ± 1.5 (taking into consideration at least three samples synthetized independently). The subtraction of the normalized FTIR spectra of both samples does not indicate significant differences among them, except for a few low-intensity features (
Figure 8a). Some of these bands can be related with the resonances found when the corresponding
13C NMR spectra were subtracted (
Figure 8b). The FTIR band centered at 2163 cm
−1 can be related with the resonance at 115 ppm assigned to nitrile groups, the band at 1720 cm
−1 with the resonance at 151 ppm associated with carbonyl groups, and the bands at 3615 and 3495 cm
−1 with the signal at 51 ppm related to hydroxyl groups. However, the relative intensity of these FTIR bands and resonances seems to point to there being no great differences between the two polymers.
The XRD analyses showed the same diffraction, but in the case of the polymer
3, this peak was higher and narrower, indicating a more ordered structure (
Figure 8c). In addition, the analysis of the second derivative of their corresponding TG curves showed a very resemble thermal behavior (
Figure 8d) which would indicate similar macrostructures, as both polymers present the same thermal behavior. Only the decomposition step at 278 °C may be more noticeable for the polymer
3. This thermal decomposition step would be related, based on the TG-MS curves (data no shown), with the fragment
m/
z 44 which can be assigned to the loss of CO
2 and/or HC(=NH)NH
2 or HCONH- (a detailed discussion of the TG-MS results is out of the scope of the present work, and it will be given in a further paper). The slightly higher delivery of CO
2 or HCONH- for polymer
3 is in good agreement with the spectroscopic data and elemental compositional data indicated above, indicating a higher content of oxygen in polymer
3.
In the light of these results, the control polymer and the polymer
3 seem to resemble one another, except for very little differences related with the amount of oxygenated functional groups. Thus, detailed XPS analyses of these two samples were made in order to provide further information about them.
Figure 9 shows the core-level spectra of the C (1s), N (1s) and O (1s) peaks, as well as their deconvolutions on different components of the control polymer and polymer
3 samples.
A deconvolution study of the C (1 s) peak showed three components for both cases: the first component at 285.1 eV (binding energy) was attributed to the C adventitious, C–H and C–C group; the second component at 286.7 eV corresponded to C-N, C-O, C=N and amide groups; whereas the third component was observed at 288.5 eV, and was assigned to the C=O and nitriles groups. Both samples showed similar carbon components, and the ratio between the components was also comparable; polymer 3 showed a slight increase of 10% for the first component and a decrease of 10% for the second. Thus, the resemblance between spectra C (1 s) seems to indicate very similar macromolecular structures for polymer 3 and the control polymer. The N(1 s) peaks of both samples were resolved into two components, the first one at 398.9 eV being assigned to -CONH2 and imines (-N=C<), which were predominant in the control polymer sample, and the second one at 400.0 eV corresponding to -CONH- groups, amides and nitriles; both nitrogen components showed a similar percentage for polymer 3’s case. Regarding the O (1 s) peak, we fitted the experimental data points using three components. The first component occurred at 530.9 eV, which was possibly assigned to the carboxylate group and to the amide group (-CONH-), which was predominant in the control polymer’s case; the second one appeared at 532.0 eV, but it was mainly a contribution from contamination during the sample preparation in air instead of under UHV conditions; the main component for the polymer 3 sample, and finally a third component at 533.7 eV assigned to C=O and COOH groups, were similar in both cases. Therefore, this comparison study did not show large differences between both samples. The overlapping of several functional groups at similar binding energies did not help us to make an unambiguous assignment for the complex functional group mixture present in the NH4CN synthetized polymer’s structure. Nevertheless, the carbon and oxygen components related to the adventitious are more intense for polymer 3, whereas for the control polymer, the carboxylate and C=N functional groups seems to be the principal component of the analysis.
As a result, taking into account the comparative results between the control polymer and polymer
3, the more significant effects of the NH
4CN polymerization assisted by MW radiation are the decrease on the conversion degree and variations in the textural and morphological properties of the final products (as was indicated above, please see the SI of [
1] for the look of the control polymer particles; these ones were not nanoparticles). The MW radiation leads to the generation of nanoparticles and/or nanofibers of cyanide polymers in minor yields, but with similar compositional/structural characteristics and the same thermal properties with respect to those microparticles produced under conventional heating conditions and with a more ordered macrostructure. Thus, nanoparticles/nanofibers can be obtained using MW radiation by aqueous cyanide polymerization. The size, shape and polydispersity of these particles seems to be directly related with the reaction time and with the temperature. In order to deepen this result, in the next section, analyses of the morphology of a series of NH
4CN polymers synthetized at 170 °C and distinct reaction times are given.