Tuning the Morphology in the Nanoscale of NH4CN Polymers Synthesized by Microwave Radiation: A Comparative Study

Abstract

A systematic study is presented to explore the NH4CN polymerization induced by microwave (MW) radiation, keeping in mind the recent growing interest in these polymers in material science. Thus, a first approach through two series, varying the reaction times and the temperatures between 130 and 205 °C, was conducted. As a relevant outcome, using particular reaction conditions, polymer conversions similar to those obtained by means of conventional thermal methods were achieved, with the advantage of a very significant reduction of the reaction times. The structural properties of the end products were evaluated using compositional data, spectroscopic measurements, simultaneous thermal analysis (STA), X-ray diffraction (XRD), and scanning electron microscopy (SEM). As a result, based on the principal component analysis (PCA) from the main experimental results collected, practically only the crystallographic features and the morphologies in the nanoscale were affected by the MW-driven polymerization conditions with respect to those obtained by classical syntheses. Therefore, MW radiation allows us to tune the morphology, size and shape of the particles from the bidimensional C=N networks which are characteristic of the NH4CN polymers by an easy, fast, low-cost and green-solvent production. These new insights make these macromolecular systems attractive for exploration in current soft-matter science.

Multivariate Analysis Applied to Microwave-Driven Cyanide Polymerization: A Statistical View of a Complex System

“…In any case, comparatively, for all the time ranges studied here, the MWR leads to lower yields for insoluble NH 4 CN polymers at any temperature than CTH, except for the two unexpected points indicated at 67 min. This general result is in agreement with the data previously reported, which indicated that an increase in the temperature leads to lower conversion values [ 35 , 36 ] and that the MWR improves the oxidation and hydrolysis processes of cyanide and the intermediate products during the polymerization reactions, decreasing the overall yield for the insoluble macromolecular fractions [ 30 , 37 ]. Therefore, an important effect of the MWR is to reduce the hydrothermal production of insoluble cyanide polymers.…”

“…Cyanide polymerization assisted by MWR is described using three different temperatures, 170, 190 and 205 °C. These temperatures were chosen on the basis of a previous report about microwave-driven cyanide polymerization [ 37 ]. In addition, syntheses at 80 °C using CTH have been considered representative control experiments to determine the effect of the MWR.…”

“…The NH 4 CN polymers synthesized in this work were obtained following the methodology detailed in [ 10 , 37 ] using a Biotage Initiator + microwave reactor purchased from Biotage (Uppsala, Sweden). The initial equimolar concentrations of NaCN and NH 4 Cl were always 1 M. The synthesis of the control polymers prepared at 80 °C is described in [ 29 ].…”

“…The reaction time and temperature are the main factors in the tunable characteristics of HCN-derived polymers [ 25 , 29 , 34 , 35 , 36 ]. Recently, microwave radiation (MWR) has been shown to significantly reduce the reaction time of NH 4 CN polymerization, producing cyanide polymers with spectroscopic and chemical compositional properties similar to the spectroscopic and chemical compositional properties of polymers synthesized using conventional thermal heating (CTH) but with morphological properties, size and shape that are very different [ 37 ]. The notable reduction in the reaction time and the generation of new nanoparticles and nanofibers of HCN-derived polymers [ 10 , 37 ], which have not previously been reported for these macromolecular systems, make microwave-driven cyanide polymerization highly attractive for comprehensive exploration.…”

“…As a result, the better fit to a kinetic model for cyanide polymerization is directly dependent on the working temperature. However, none of the kinetic models or kinetic approaches mentioned above were suitable when NH 4 CN polymerization was assisted by MWR, for two main reasons: (i) the use of MWR renders it impossible to acquire conversion values for reaction times less than 2 min due to the necessary thermal heating ramp to reach the desired temperatures and, therefore, the reaction kinetics cannot be properly studied by thermogravimetric methods; and (ii) conversion values, chemical composition and parameters calculated from the Fourier-transform infrared (FTIR) spectra showed fluctuating and apparently random behavior along the reaction time [ 10 , 37 ], which was not previously observed in these systems when CTH was used [ 29 , 35 , 36 ]. In fact, the use of the MWR seems to make it more difficult to predict the evolution of the cyanide polymerization system over time and, consequently, a direct and guided modification of the desired properties of the reaction products is not obvious.…”