Engineering polyamide materials: s-triazine framework with specialized bulky side chains for advanced applications

The focus of this study is on the synthesis of organic fluorescent and thermally stable 9 polyamides using an s-triazine frame. Coumarin and n-phenyl anthranilic acid have been utilized as bulky pendent groups in the synthesis of the monomer, resulting in polyamides with two groups that enhance stability and fluorescence. The synthesized polyamides have 12 been characterized using a variety of techniques. The thermal stability of the polyamides has been studied using thermogravimetric analysis. These polyamides offer appealing features such as fluorescence and enhanced thermal stability, making them significant for a 15 wide range of applications, including biosensors, clean energy technologies, and explosive sensing.


INTRODUCTION
Polyamides are an important type of polymer that is widely utilized in various industries for their exceptional mechanical qualities, high glass transition temperatures, and thermal and chemical durability [1 -5].The demand for heat-resistant materials has led to extensive research on thermally stable polymer systems, with aromatic polyamides garnering global attention for their unique properties [6].Recently, there has been a growing focus on developing polyamides with enhanced properties, leading to the incorporation of s-triazineheterocyclic compounds and derivatives into polyamides.These compounds, such as the six-membered heterocyclic compound s-triazine, have exceptional heat resistance and stability [7 -10].Heterocyclic polyamides containing the s-triazine moiety have been widely applied in various fields, including construction, transportation, consumer products, industrial machinery, aircraft, electronics, packaging, and more [11 -12].
Newly developed fluorescent polyamides boast a unique blend of high-performance features achieved by bonding between s-triazine and coumarin rings [13 -15].Coumarin was selected as the photosensitive group due to its proven ability to cause a simple [2 + 2] photodimerization of reactive C=C double bonds in various polymer thin films when exposed to wavelengths above 300 nm [16 -20].Numerous fields have found uses for these fluorescent-thermally stable polyamides, including biological imaging [22], explosive sensing, pH/temperature monitoring, clean energy technologies [21], ultra-sensitive molecular diagnosis [22], and novel light-emitting nanodevices [23].Among other applications, they can be used in fluorescent imaging, drug delivery, fluorescent chemosensors, smart polymer machines, biological detection schemes, and fluorescent molecular thermometers.
Many research groups have synthesized and studied polymers from coumarin and striazine.Three-armed star-shaped polymer has been synthesized by using the ATRP method from the chlorine ends of the initiator containing triazole coumarin groups.The resulting polymers are used as materials in electronic applications [24].Feng et al., [25] have synthesized three s-triazine-based functional monomers with thermopolymerizable propargyl-ether units.These monomers may be thermally cured to generate cross-linked networks.The synthesized polymers demonstrated good thermal stability and are used in variable fields.
The various experimental evidence provides strong support for the current work, which focuses on synthesizing and characterizing a new type of thermally stable polyamides.These polyamides boast exceptionally high performance and fluorescent properties, thanks to the incorporation of the s-triazine ring and coumarin ring as the primary moiety.The current study involves the synthesis and characterization of fluorescent-thermally stable polyamides, using 7-hydroxy 4-methyl coumarin, n-phenyl anthranilic acid, cyanuric chloride, and several commercial diamines.

EXPERIMENTAL
In the preparation of the solutions, freshly distilled water was employed.Chemicals such as sodium bicarbonate, sodium hydroxide, thionyl chloride, and cetrimide were used directly.
Polyamide synthesis is given in a schematic manner in which monomer synthesis is explained by steps 1 -3 and then polyamide synthesis is described by step 4.

Step-1: Synthesis of 2-(7-hydroxy 4-methyl coumarino)-4,6-dichloro-s-triazine [CT]:
Sodium bicarbonate (10.6 g) was dissolved in 100 mL of distilled water and then cooled to a temperature range of 0 -5 °C.In a separate step, a slurry of cyanuric chloride (18.44 g, 0.1 mol) in 60 mL acetone was created.The sodium bicarbonate solution was added to the cyanuric chloride slurry in a 250 mL three-necked flask equipped with a mechanical stirrer while maintaining a temperature range of 0 -5 °C.Next, a solution of 7-hydroxy 4-methyl coumarin (17.60 g, 0.1 mol) was added to the cooled cyanuric chloride slurry, and the mixture was stirred for 2 h at the same temperature range.The resulting off-white product was filtered, recrystallized, and dried in vacuum desiccators, yielding 78% of the product with a melting point (M.P) of 120 °C.

RESULTS AND DISCUSSION
Properties of polyamides: Polyamides that contain coumarin and n-phenyl anthranilic acid, along with various diamines, exhibit dark colors.Table 1 displays the colors of these polyamides, which correspond to the specific diamines used.Generally, polyamides based on coumarin and n-phenyl anthranilic acid are dark in color.However, those containing ethylene diamine, o-phenylene diamine, and p-phenylene diamine are dark, while those containing diaminodiphenyl methane and diaminodiphenyl sulfone are grey.Solubility: The solubility of various polyamides which were formed from coumarin and nphenyl anthranilic acid as a main moiety has been examined in different solvents at both room temperature and 50 °C, and the results are presented in Table 2. Interestingly, the polyamides were found to be insoluble in aliphatic chlorinated solvents such as chloroform and carbon tetrachloride, as well as in halogenated and non-halogenated aromatic solvents like chlorobenzene and benzene.However, the polyamides exhibited partial solubility in solvents such as acetone, methanol, ethanol, THF, n-butanol, isopropyl alcohol, and ether, while showing full solubility in dimethyl formamide, dimethyl sulfoxide, and ethyl acetate.
Please note that the solubility of each polyamide in a respective solvent is indicated by the symbols ++, ± ±, and ˗ ˗ in Table 2.The ++ symbol indicates the polyamide is completely soluble in the respective solvent and the -symbol indicates insolubility and the ± ± symbol illustrates polyamide is partly soluble in the respective solvent.Viscosity: Vasava et al., [26] have synthesized s-triazine-based polymers and evaluated Intrinsic, reduced, and inherent viscosities along with Huggins and Kraemer's constants for 1% solution.In which, inherent viscosity is in the range of 0.397 -0.541 g/dL.In the present investigation, to measure the viscosity of polyamides, solutions were prepared in DMF and filtered through a G-3 sintered glass funnel before measuring flow time.The solvent's flow time (to) was 72 seconds, and an Ubbelohde suspended level viscometer was used to measure viscosity for diluted solutions.NC-11 to NC-15 polyamides were examined at various concentrations, and relative viscosities (ηrel), specific (ηsp) viscosities, reduced viscosities, and inherent viscosities were calculated.The results were tabulated in Table 3, and it was found that NC-12, which contains diamino diphenyl methane as diamine, had the highest solution viscosity.The range of inherent viscosity was 0.417 -0.50 g/dL.Intrinsic viscosity followed the sequence NC-15 < NC-14 < NC-13 < NC-11 < NC-12.Polyamides that contain bulky aromatic side chains showed high intrinsic as well as inherent viscosity.
Huggins's and Kraemer's plots were used to obtain intrinsic viscosity for each of the polyamides [NC], and the results for intrinsic, reduced, and inherent viscosities, along with Huggin's and Kraemer's constants for 1% solution, are shown in Fig. 1.IR spectra: The analysis of infrared absorption frequencies is a crucial aspect of the study of polyamides.In this regard, Table 4 reveals the details of infrared absorption frequencies of NC-11, NC-12, NC-14, and NC-15 for various functional groups.Analyzing the IR spectrum, it has been found that polyamides showed strong or moderately strong absorption at frequencies of 835 -839 cm -1 and 1487 -1491 cm -1 , which can be predicted to out-ofplane and in-plane vibrations of the s-triazine ring, respectively.Additionally, the vibration of the amide group is seen at 3310 -3312 cm -1 , while C-N stretching vibration is assigned to 1272 -1275 cm -1 .Furthermore, Ar-O-Ar asymmetric and symmetric vibrations appear at 1134 -1136 cm -1 and 1102-1104 cm -1 , respectively, with Ar (C=C) stretching vibration at 1662 -1665 cm -1 and C-O-C (sym) stretching vibration at 1059-1072 cm -1 .Fig. 2 illustrates the IR study of polyamide NC-13, which reveals C-N stretching vibration at 1275 cm -1 , >C=O stretching vibration at 1665 cm -1 , Ar-O-Ar asymmetric at 1179 cm -1 , and Ar-O-Ar symmetric at 1136 cm -1 .Notably, out-of-plane and in-plane vibrations of the s-triazine ring appear at 839.08 cm -1 and 1491 cm -1 , respectively, while N-H stretching vibration of the -CONH group is seen at 3310 cm -1 , and C-H stretching vibration of CH3 appears at 2923 cm -1 .Thermogravimetric analysis: Thermal degradation characteristics and kinetic parameters of polyamides based on coumarin and n-phenyl anthranilic acid have been evaluated through thermogravimetric analysis.A qualitative assessment of the thermal stability of some of the polyamides has been attempted based on the visual thermograms obtained, with T0 and T10 serving as some of the main criteria for thermal stability.The heat stability of polyamides is directly proportional to the values of T0 and T10.TGA curves have been obtained for polyamides at a scan rate of 10 °C/min.Table 6 describes the thermal characteristics of NC-11 to NC-15, while Table 6 provides an evaluation of the activation energy Ea for all synthesized polyamides using the Broido method and Horowitz-Metzger method from the TGA graph.The thermal properties of NC-14 were examined through TGA under a nitrogen atmosphere, with a heating rate of 100 °C/min.The 10% weight loss temperature and 50% weight loss temperature of the aromatic polyamide in nitrogen were recorded at 267 °C and 567 °C, respectively.Fig. 4 shows the TGA thermogram of NC-14 polyamide, which exhibited high thermal stability.°C, respectively.In line, it is concluded that in the present research, synthesized polyamides showed good thermal stability, which will be applicable in various fields such as automotive components, aerospace applications, etc.

Solvent
Linear deterioration analysis was conducted on the experimental data to obtain the values of apparent activation energy corresponding to the degradation steps involved.The calculated energy of activation (Ea) values for all polyamides [NC] are presented in Table 7.
An examination of the energy of activation Ea reveals that both the Broido method and Horowitz-Metzger method yield analogous values for Ea.Fluorescence spectra: Polyamides NC-14 were synthesized using 7-hydroxy 4-methyl coumarin, n-phenyl anthranilic acid, and diamino diphenyl methane as an aromatic diamine.
Fig. 5 showcases their fluorescence spectrum.To obtain the emission spectra, polyamide NC-14 was dissolved in DMSO-D6 and excited at 300 nm.NC-14 showed broad emission between 350 -500 nm, with the emission peak located at 400 nm.The resulting polyamide showed the emission peak in the visible region therefore NC-14 polyamide is a fluorescent polyamide.

CONCLUSION
A new type of polyamide has been successfully synthesized through polymerization at an elevated temperature.This process involved the polycondensation of monomers that contained an s-triazine ring with n-phenyl anthranilic acid and 7-hydroxy 4 methyl coumarin with various types of diamines.

Table 2 .
Relative solubility of polyamides in polar and non-polar solvents[NC]

Table 6 .
Thermal characteristics of polyamides NC-11 to NC-15 for 10% weight loss and