*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 NH4CN 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−<sup>1</sup> can be related with the resonance at 115 ppm assigned to nitrile groups, the band at 1720 cm−<sup>1</sup> with the resonance at 151 ppm associated with carbonyl groups, and the bands at 3615 and 3495 cm−<sup>1</sup> 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.

**Figure 8.** Comparative spectroscopic and thermal studies between polymer **3** and a control polymer synthetized using conventional heating. (**a**) Substation of the FTIR spectra; (**b**) substation of the 13C NMR spectra; (**c**) XRD patters; (**d**) second derivative of the TG curves.

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 CO2 and/or HC(=NH)NH2 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 CO2 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.

**Figure 9.** XPS photoemission spectra of the C (1 s), N (1 s) and O (1 s) core level peaks, and the deconvolution of a control polymer and polymer **3**.

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 NH4CN 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 NH4CN polymers synthetized at 170 ◦C and distinct reaction times are given.

### *3.3. Polymeric Particles' Morphology Variations along the Reaction Time*

A relative study of samples synthetized at 170 ◦C was completed by SEM using different reaction times, from 5 to 120 min. Representative images of these new polymeric series are shown in Figure 10 (for more details, please see Figures S11–S17 in the supplementary information section). We focused on this temperature based on the data reported in the first part of this work, as greater yields were obtained at this temperature; 130 ◦C and 205 ◦C were ruled out to explore the production of nanoparticles/nanofibers based on the PCA results, and also due to the easier dispersion of the polymer synthetized at 170 ◦C in EtOH to prepare the samples for the SEM measurements compared to those synthetized at higher temperatures, i.e., 190 ◦C and 205 ◦C. Note that the values of the PdI and Z-average increase with the increase of the temperature and with the reaction time (Table 1), leading finally to molecular aggregates.

All of the samples from the 170 ◦C series present isolated long nanofibers, with the exception of polymer **7** (reaction time ~ 67 min), which showed a clear grouping of long nanofibers (Figure 7 and Figure S7). Other shapes observed were spherical/oval particles, irregular and planar stacking, rice-shaped nanoparticles and short nanofiber networks. In all of the samples studied, on general lines, there was a dominant morphology against others depending on the reaction time, as is qualitatively summarized in Table 2. Interestingly, it is the generation of short nanofibers at 36 and 52 min which was not observed previously for this type of polymer. On the other hand, the production of riceshaped nanoparticles was specially improved at 105 min, and was practically the only shape observed. Thus, to highlight, short nanofibers, long nanofibers and nanoparticles were obtained at 170 ◦C with reaction times of 36, 67 and 105 min, respectively. Therefore, it seems that there is a clear effect of the reaction time on the shape and size of the NH4CN polymers synthetized in the presence of MW radiation.

**Figure 10.** SEM images for representative samples from NH4CN polymers synthetized at 170 ◦C. Note the variation of the morphology with the reaction time indicated in each picture.

**Table 2.** Qualitative summary for the observed shapes based on the SEM images from the NH4CN polymers synthetized at 170 ◦C (please see the images collected in the supplementary information). – = not detected; + = detected but very minority and poorly representative; ++ and +++ = observed; ++++ = main shape observed.


Based on the PCA results discussed above, the more oxidized and conjugated macrostructures derived from the MW-driven cyanide polymerization would be nano-sized. This statistical result could be experimentally proven. Thus, Figure 11 shows the evolution in the composition along the time of the cyanide polymers synthetized at 170 ◦C. It can clearly be seen that the two samples with a greater content in oxygen, in this case those obtained at 36 and 52 min, present short nanofiber networks, with these being apparently the smallest nanoparticles observed. It is also interesting that the sample obtained at 105 min, with the highest content in oxygen, showed the tiniest particles observed in all of the cases here studied. On the contrary, the sample prepared using a reaction time of 67 min, polymer **7**, presented the lower content in oxygen, and in this case long nanofiber networks were found. Therefore, taking into account these results, it seems possible to tune

the morphologies of the NH4CN polymers to obtain mainly short or long nanofibers or nanoparticles for the development of new families of polymeric materials. However, the lack of a direct relationship between the conversion degree, elemental composition and reaction time, as was observed in Figure 11, encourages us to carry out comprehensive studies about the cyanide polymerization promoted by MW radiation.

**Figure 11.** Evolution of the polymerization of NH4CN with the time at 170 ◦C. (**a**) Variation of the conversion degree, α (%), and the elemental composition of the cyanide polymers; (**b**) variation of the molar relationships.

#### **4. Conclusions and Outlooks**

This is the first systematic study regarding the MW-assisted polymerization of cyanide, addressing a wide range of temperatures and reaction times. As the main results, the MW radiation has a notable influence on the yields of the insoluble polymers obtained. This fact could be due to the likely increase of the decomposition processes for the cyanide, and of its main oligomer, the DAMN, hindering the polymerization pathways proposed for the production of the extended C=N polymeric networks, as postulated for the HCNderived polymers. However, using equivalent reaction times, polymers with very similar compositional, spectroscopic and thermal properties were obtained. In addition, note that for particular reaction conditions at higher temperatures, the conversions achieved were similar to those using traditional heating systems. Moreover, it is highly informative that the MW radiation allows the generation of HCN-derived polymeric particles at the nanoscale which were not observed previously under any experimental condition using conventional methods; this technique could be successful for the tuning of their morphological properties, and for extension to obtain a new promising family of nanomaterials, taking into account the recent potential revealed by these polymeric systems. Due to the unexpected behavior of the cyanide MW polymerization, the multivariate analysis has turned out to be a successful tool to obtain a global interpretation of the results obtained herein and beyond to help us to find appropriate reaction conditions which lead to the generation of materials with concrete structural and morphological properties.

Being a very fast, robust, easy, low cost and green-solvent process, and the possibility to tune properly the properties of the final products makes the aqueous microwave-driven cyanide polymerization a highly attractive and promising methodology for the generation of new multifunctional materials. However, due to the apparently random behavior of the system along the reaction time, mainly due to the experimental variables to tune the properties of the polymers, it is necessary to carry out exhaustive synthetic and structural studies, and to examine the results under the light of statistical approaches to develop properly cyanide-based materials.

**Supplementary Materials:** The following are available online at https://www.mdpi.com/article/10.3 390/polym14010057/s1. Figures S1–S17: SEM images of all of the cyanide polymers synthetized herein. **Author Contributions:** C.P.-F., validation, formal analysis and investigation; P.V., formal analysis and resources; E.G.-T., formal analysis and writing—original draft preparation; E.M.-M., formal analysis, resources, writing—original draft preparation and funding acquisition; J.L.d.l.F., writing—review and editing, and supervision; M.R.-B., conceptualization, methodology, formal analysis, writing—original draft preparation, writing—review and editing. All authors have read and agreed to the published version of the manuscript.

**Funding:** This research was funded by the projects PID2019-104205GB-C21, PID2019-107442RBC32 and PID2019-104205GB-C22 from the Spanish Ministerio de Ciencia e Innovación, and by the Spanish State Research Agency, project MDM-2017-0737 Centro de Astrobiología (CSIC-INTA), Unidad de Excelencia María de Maeztu. C.P.-F. was supported by a research training grant from the Spanish Ministerio de Ciencia e Innovación, PRE2018-085781.

**Institutional Review Board Statement:** Not applicable.

**Informed Consent Statement:** Not applicable.

**Data Availability Statement:** Not applicable.

**Acknowledgments:** C.P.-F., E.G.-T., M.R.-B. and E.M.-M. used the research facilities of the Centro de Astrobiología (CAB), and were supported by the Instituto Nacional de Técnica Aeroespacial "Esteban Terradas" (INTA). Additionally, the authors are grateful to M<sup>a</sup> Teresa Fernández for performing the FTIR spectra and the XRD measurements, the "Servicio de Análisis Térmico" of ICMM (CSIC, Spain), and also to the "Servicio de Resonancia Magnética Nuclear" of ICMM (CSIC, Spain).

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.

### **References**

