Chitosan and imide-functional Fe₃O₄ nanoparticles to prepare new xanthene based poly(ether-imide) nanocomposites

Abstract

In this work a new series of multifunctional nanocomposites were synthesized based on a soluble poly(ether-imide). The poly(ether-imide) was successfully synthesized via direct polycondensation reaction. A methyl rich diamine containing xanthene group as a new monomer was synthesized and used for polymerization. The xanthene based poly(ether-imide) was characterized using size exclusion chromatography (SEC) and nuclear magnetic resonance (NMR). Fe₃O₄ nanoparticles were imide-functionalized by a dianhydride, chitosan and phenylalanine. The functional Fe₃O₄ nanoparticles were incorporated into the new synthesized poly(ether-imide). Effects of the functionalized Fe₃O₄ nanoparticles on the thermal and combustion properties of the corresponding nanocomposites were investigated. Thermogravimetric analysis (TGA) results indicated that the thermal stability and char yields of the nanocomposites were enhanced compared to the neat poly(ether-imide). Microscale combustion calorimetry (MCC) revealed that the synthesized poly(ether-imide) had low flammability. The incorporation of functional Fe₃O₄ nanoparticles could further improve the combustion properties of poly(ether-imide). The high interaction between poly(ether-imide) and functional Fe₃O₄, the presence of an imide group and high hydroxyl content of the functional Fe₃O₄ nanoparticles seem to be responsible for the improvement of the thermal and combustion properties. Furthermore, the presence of methyl, ether and bulky xanthene groups in the polyimide backbone decreased the glass transition temperature (T_g) and increased the solubility in organic solvents. These properties will be useful for processing and new applications of poly(ether-imide).

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1. Introduction

Aromatic polyimides (PI) are considered to be high-performance materials which make them useful for advanced technologies. PI could be an appropriate choice for the replacement of traditional materials including metals and ceramics in new technological applications.^1,2 PI offer a wide range of properties including transparency, non-flammability, outstanding strength-to-weight ratios, thermal resistance, good barrier, and solvent resistant properties.^3–7 However, poor solubility in common organic solvents and very high transition temperatures are the limiting factors for the processing and application of these materials. Recent basic and applied studies have focused on increasing their processability and solubility in order to enlarge the scope of the technological applications of these materials. It was known that the solubility of polymers often increased when flexible bonds such as [–O–, –SO₂–, –CH₂–], polar constituents and bulky pendent groups are incorporated into the polymer backbone by changing the intermolecular interactions and crystallinity.^8–12

Xanthenes are known as nature based compounds and are interesting for their utility as leuco-dyes,¹³ pH-sensitive fluorescent materials for the visualization of biomolecules¹⁴ and in laser technologies¹⁵ due to their outstanding spectroscopic properties. The introduction of xanthene group and its derivatives into polymer in order to improve the solubility and thermal stability has attracted several research efforts.^16–18 It has been proved that the introduction of rigid heteroaromatic xanthene group as a bulky pendent group to the polymer backbone could endow the polymer with new properties such as photoluminescence, electroluminescence and liquid crystallinity. Xanthene group can also decrease the negative effects resulting from the introduction of flexible linkages such as aliphatic and ether groups in the aromatic polyimide backbone. Therefore, polymers containing xanthene groups are expected to have good solubility, easy processability, and excellent thermal properties.^19–21

High-performance polymers, including PI, are ideal candidates for matrices in nanocomposite materials due to their unique properties such as high glass transition temperature (T_g), stiffness, strength, toughness, high chemical resistance, conductivity, thermo oxidative stability, outstanding flame resistance, low dielectric constant, stable properties at high temperatures, high thermal stability, long storage periods and recyclability.^22,23 The preparation of polymer nanocomposite based on magnetic nanoparticles has attracted different applications. These nanocomposites represent a class of functional materials which may have some potential applications in batteries, electromagnetic interference shielding, electro magneto rheological fluids, electrochemical display devices and microwave absorbing materials.^24–26 The main challenge in polymer nanocomposites is the prevention of nanoparticle aggregation. It was found that the preparation of well dispersed nanocomposites and the prevention of nanoparticle agglomeration in a polymer matrix were difficult due to their high specific surface area. However, researchers still have to face how to avoid the poor dispersions of Fe₃O₄ nanoparticles in polymeric matrix caused by the weak interfacial interactions between filler and polymer. Thus, developing polymer/Fe₃O₄ nanocomposites with excellent Fe₃O₄ dispersion in the polymer matrix has been an essential goal.²⁷ The surface modification of the nanoparticles can improve the interactions between the nanoparticles and the polymer matrix.^28,29

In this paper, we deal with a new poly(ether-imide) nanocomposite that is a soluble multifunctional polyimide (PIX: polyimide xanthene) reinforced with chitosan–imide-functional Fe₃O₄ nanoparticles ((Fe₃O₄)SiO₂@Ch–Im: chitosan–imide-functional magnetic nanoparticles). The new polyimide with ether, methyl and bulky xanthene groups could have high potential application due to multifunctionality and ease of processability. However, chitosan–imide-functional Fe₃O₄ nanoparticles as a new reinforcement are a potential candidate to improve properties and endow some new applications. The thermal stability and combustion behaviour of the PIX/functional Fe₃O₄ nanocomposite were studied by TGA and microscale combustion calorimeter (MCC).

2. Experimental

2.1 Materials

2,5-Dimethylphenol (2,5-xylenol), terephthalaldehyde, 4-toluene sulfonic acid monohydrate (p-TSA), pyromellitic anhydride, 1-fluoro-4-nitrobenzene, 2-naphthol 3,3′,4,4′-benzophenonetetracarboxylic dianhydride (BPD), [3-(2,3-epoxypropoxy)propyl]trimethoxysilane (EPO) and chitosan were purchased from Sigma-Aldrich. 2-Naphthol, FeCl₂·4H₂O, FeCl₃·6H₂O, palladium charcoal (Pd/C), N,N-dimethylformamide (DMF) and hydrazine hydrate from Merck were used without further purification.

2.2 Synthesis of 4-(4-((4-(14H-dibenzo[a,j]xanthen-14-yl)phenyl)(2,6-dimethyl-4-(p-tolyloxy)phenyl)methyl)-3,5-dimethylphenoxy)aniline

The diamine containing xanthene, methyl and ether groups was synthesized starting from 2-naphthol and terephthalaldehyde in four-step reactions according to the following procedure (Scheme 1).

Scheme 1 Synthesis route of the diamine.

2.2.1 Synthesis of 14-(4-benzaldehyde)-14H-dibenzo[a,j]-xanthene (BX). 2.44 g (20 mmol) of terephthalaldehyde, 6.48 g (45 mmol) of β-naphthol, 0.19 g (1 mmol) of p-TSA and 120 mL of 1,2-dichloroethane were stirred under N₂ atmosphere with a magnetic stirrer. The reaction mixture was heated for 24 h. Then, the organic solvent was evaporated and a mixture of ethanol–water (3 : 1) was added. Yield = 90%; melting point (M_p): 330–332 °C.

2.2.2 Synthesis of 4,4′-((4-(14H-dibenzo[a,j]xanthen-14-yl)phenyl)methylene)bis(2,5-dimethylphenol) (BXOH). A mixture of 2,5-dimethylphenol (4.27 g, 35 mmol), BX (5.79 g, 15 mmol) and pTSA (0.19 g, 1 mmol) was heated at 120 °C for 1 h under solvent-free conditions. When the reaction mixture became solid, it was cooled to ambient temperature and a solution of ethanol–water (2 : 1) was added. The suspension was stirred for 10 min and then the precipitate was filtered. Yield = 86%; M_p: 310–312 °C.

2.2.3 Synthesis of 14-(4-(bis(2,5-dimethyl-4-(4-nitrophenoxy)phenyl)methyl)phenyl)-14H-dibenzo[a,j]xanthene (BXNO₂). A mixture of the newly synthesized bis-phenol (BXOH) (7.34 g, 12 mmol), 4-fluoro nitrobenzene (4.24 g, 30 mmol), potassium carbonate (3.31 g, 24 mmol) and DMF (50 mL) were placed into a 100 mL round-bottomed flask and was heated to 130 °C for 24 h. Then, the mixture was cooled to room temperature and then washed with 200 mL of water. A yellow crude product was filtered and washed thoroughly with hot ethanol (yield = 89%; M_p = 312–314 °C).

2.2.4 Synthesis of 4,4′-((((4-(14H-dibenzo[a,j]xanthen-14-yl)phenyl)methylene)bis(2,5-dimethyl-4,1-phenylene))bis(oxy))dianiline (BXNH₂). A suspension of BXNO₂ (8.55 g, 10 mmol), palladium on carbon (Pd/C) 10% (0.2 g) and mixture of ethanol–THF (100 mL, 4 : 1) was prepared in a 250 mL round-bottomed flask. The mixture was heated and hydrazine monohydrate 80% (10 mL) in ethanol (15 mL) was added dropwise during 30 min through a dropping funnel. The mixture was heated to reflux for 2 h at 70 °C and then was filtered while hot to remove Pd/C. The solvent was evaporated under vacuum and the product was washed thoroughly with ethanol, and dried to recover the white solid product (Scheme 1). Yield = 91%; M_p = 182–184 °C.

2.3 Synthesis of PIX

To synthesize poly(amic acid) (PAA), 6.36 g (8 mmol) of BXNH₂ was put into a three-necked flask containing 25 mL of DMF at room temperature. BXNH₂ was completely dissolved, and then a solution of 1.74 g (8 mmol) of pyromellitic dianhydride in 10 mL of DMF was slowly added to the BXNH₂ solution. The solution was stirred at room temperature overnight to prepare PAA solution. Then the PAA solution was poured into a glass dish and heated with a programmed temperature at 70, 100, 150 and 200 °C for 8 h in a vacuum oven.

2.4 Synthesis of chitosan and imide-functional Fe₃O₄ nanoparticles (Fe₃O₄)SiO₂@Ch–Im

Fe₃O₄ nanoparticles were synthesized according to the literature.³⁰ Briefly, FeCl₂·4H₂O (2.39 g, 12 mmol) and FeCl₃·6H₂O (6.49, 24 mmol) were dissolved in deionized water (200 mL) under N₂ atmosphere with vigorous stirring using a mechanical stirrer. 25 mL of NH₃·H₂O (28 wt%) was added to the solution for 30 min and then was heated to 60 °C for 6 h. Then, the temperature was increased to 85 °C to vaporize residual ammonia. The nanoparticles were separated by a magnet and washed with deionized water.

2.4.1 Preparation of silica-coated Fe₃O₄ ((Fe₃O₄)SiO₂). The (Fe₃O₄)SiO₂ nanoparticles were prepared according to the Stöber method.³¹ Briefly, Fe₃O₄ nanoparticles (0.5 g) were homogeneously dispersed in a mixture of 40 mL of ethanol, 6 mL of deionized water, and 1 mL of NH₃·H₂O (28 wt%), followed by the addition of 1 mL of tetraethylorthosilicate (TEOS). The mixture was stirred at room temperature for 10 h and then the (Fe₃O₄)SiO₂ was collected using an external magnet.

2.4.2 Synthesis of (Fe₃O₄)SiO₂@Ch–Im. A mixture of 1 mmol BPD and 1 mmol of phenylaniline was dissolved in DMF and stirred for 2 h under nitrogen atmosphere. Then, 1 g chitosan was added to the solution and stirred overnight. After that the mixture was heated to 120 °C for 6 h. The reaction mixture was cooled down and 2 mmol of EPO was added to the mixture and stirred for 2 h under nitrogen atmosphere. 2 g of (Fe₃O₄)SiO₂ nanoparticles were dispersed in 5 mL DMF and added to the mixture. The mixture was stirred at 80 °C for 24 h. (Fe₃O₄)SiO₂@Ch–Im was magnetically isolated by an external magnet and washed with methanol and ethanol (Scheme 2).

Scheme 2 Preparation of (Fe₃O₄)SiO₂@Ch–Im.

2.5 Preparation of poly(ether-imide)/functional Fe₃O₄ nanocomposites (PIXN)

The poly(ether-imide)/functional Fe₃O₄ nanocomposites were prepared using the solvent-casting method. To prepare PIXN with 1 and 3 wt% of (Fe₃O₄)SiO₂@Ch–Im loading (PIXN 1 and PIXN 3), PIX was dissolved in DMF at 120 °C in a three-necked flask with mechanical stirring and ultrasonic irradiation to form a homogeneous solution. Then, (Fe₃O₄)SiO₂@Ch–Im was dispersed in DMF and added into the PIX solution. The mixture was stirred and sonicated subsequently for 1 day. The blended mixture was cast onto glass plates and dried at 40 °C in an oven for 24 h to form the nanocomposites (Scheme 3).

Scheme 3 Preparation of PIXN.

2.6 Characterization

Fourier transform infrared (FTIR) spectra were obtained using a Perkin-Elmer RXI spectrometer in the range of 400–4000 cm⁻¹ with a resolution of 2 cm⁻¹. ¹H NMR measurements were performed using a Bruker 300 MHz spectrometer. Molar mass (mass-average (M_w) and number-average (M_n) molar mass) were determined thanks to size exclusion chromatography (SEC) using Agilent Series 1100 (Agilent) system equipped with a pump, degasser and differential refractive index (RI) detector. UV-Vis spectra were recorded at 25 °C in the 190–790 nm spectral regions with a Perkin Elmer Lambda 15 spectrophotometer in DMF using cell lengths of 1 cm. The morphology of the nanocomposites was studied using transmission electron microscopy (TEM) with microscope LEO 912. Thermogravimetric analysis (TGA) was undertaken using a TA Instruments TGAQ5000 in the range between room temperature and 800 °C at a heating rate of 10 °C min⁻¹ in nitrogen atmospheres. Differential scanning calorimetry (DSC) (TA instrument Q1000) was used to study the thermal degradation properties. Three cycles of heating–cooling–heating with heating/cooling rate of 10 °C min⁻¹ were used. Combustion properties of the samples were studied by microscale combustion calorimeter (MCC, FTT) and repeated three times for each sample.

3. Results and discussions

3.1 Synthesis of the diamine

The new methyl rich diamine containing xanthene ring and ether group was prepared starting from 2-naphthol and terephthalaldehyde via a four-step reaction sequence, as outlined in Scheme 1. An equivalent of terephthalaldehyde with two equivalents of 2-naphthol in the presence of p-TSA as a catalyst were used to synthesize BX via one-pot condensation reaction. Then, the new methyl rich bisphenol containing xanthene ring (BXOH) was synthesized by a chemical synthesis pathway from 2,5-dimethylphenol and BX in the presence of p-TSA under solvent-free conditions. After recrystallization of BXOH in a mixture of ethanol–water, it was used in the next step of the reaction. In a mixture of DMF and K₂CO₃, BXOH was reacted with 1-fluoro-4-nitrobenzene to obtain BXNO₂. Hydrazine monohydrate and palladium on charcoal (10%) was used for reduction of BXNO₂ to BXNH₂ as a novel multifunctional diamine.

The ¹H NMR spectrum of BXNH₂ displays the characteristic resonance of methyl groups at 1.86 and 1.87, NH₂ amine groups at 4.87 ppm and C–H center at 5.31 ppm. The aromatic protons appeared in rang between 6.23 and 8.68 ppm (Fig. 1).

Fig. 1 ¹H NMR spectrum of BXNH₂.

The ¹³C NMR spectrum of BXNH₂ in dimethyl sulfoxide (DMSO-d6) is presented in Fig. 2. The four signals of aliphatic carbons appear in the region of 16–47 ppm. The carbon of the aromatic rings appears in the downfield (114–154 ppm) which confirms its chemical structure.

Fig. 2 ¹³C NMR spectrum of BXNH₂.

3.2 Polymer synthesis and characterization

In line with our previous works to introduce flexible, aliphatic and side chains group with different functionality for improving the processability and decreasing T_g of aromatic polyamides and polyimides without loss of their important properties such as thermal stability, in this work, the new poly(ether-imide) containing xanthene ring along with ether groups and high methyl content was synthesized and characterized. Xanthene rings in the side chain of polymer could be a successful approach to improve the solubility of the polymers.

Xanthene, ether and aliphatic groups in the PIX chains improved solubility of the resulting poly(ether-imide). The polymer was dissolved in polar organic solvents such as DMSO, DMF at room temperature and was insoluble in protic solvents such as methanol, ethanol and water.

The chemical structure of newly synthesized PIX was confirmed by means of ¹H-NMR spectrum (Fig. 3). In the ¹H-NMR spectrum the resonance of methyl protons appeared in the range of 1.82–1.93 ppm. The aromatic protons and an aliphatic proton related to CH xanthene appeared in the range of 6.70–8.81 ppm. The presence of resonance signals related to BXNH₂ and pyromellitic dianhydride in ¹H NMR spectrum confirmed the chemical structure of PIX.

Fig. 3 ¹H NMR spectrum of PIX.

The SEC measurement of the synthesized PIX exhibited number-average molar mass (M̄_n) and mass-average molar mass (M̄_w) of ca. 10 100 and 25 200 g mol⁻¹, respectively, according to poly(vinylpyridine) (PVP) standard. The polydispersity index was 2.5.

3.3 Synthesis of (Fe₃O₄)SiO₂@Ch–Im

The multifunctional Fe₃O₄ nanoparticles ((Fe₃O₄)SiO₂@Ch–Im) were synthesized via four steps (Scheme 2). The FTIR spectra of the (Fe₃O₄)SiO₂ and (Fe₃O₄)SiO₂@Ch–Im are shown in Fig. 4. The chitosan and imide coated onto (Fe₃O₄)SiO₂ by coupling agent EPO can be clearly observed thanks to the strong absorption bands at 1776 cm⁻¹ (C=O antisymmetric, imide), 1713 cm⁻¹ (C=O symmetric, imide) and the bands at around 2931 and 2860 cm⁻¹ which are attributed to the aliphatic C–H bonds in chitosan, phenylalanine and EPO. The absorption band at 1386 cm⁻¹ (C–N, imide) showed the presence of the imide heterocycle in the Fe₃O₄ nanoparticle. The absorption bands at 1100 and 558 cm⁻¹ correspond to the Si–O–Si and Fe–O bonds. The strong OH band (3413 cm⁻¹) assigned to the stretching vibrations of Fe–OH groups absorbed on the surface of (Fe₃O₄)SiO₂ and the OH of chitosan. These results revealed that the macromolecular chains of chitosan and imide are chemically linked on the surfaces of (Fe₃O₄)SiO₂ in a coupling manner, and that (Fe₃O₄)SiO₂@Ch–Im was formed.

Fig. 4 FTIR spectra of (Fe₃O₄)SiO₂ and (Fe₃O₄)SiO₂@Ch–Im.

UV-Vis was used to evaluate the formation of Fe₃O₄ and (Fe₃O₄)SiO₂@Ch–Im nanoparticles in suspension. As shown in Fig. 5, observed absorbance peaks at around 315, 358 and 400 nm can be attributed to [Fe²⁺]t₂g → [Fe²⁺]e, [Fe³⁺]eg → [Fe²⁺]t₂ and [Fe²⁺]t₂g → [Fe²⁺]t₂ transitions of Fe₃O₄ nanoparticles respectively, according to the W. F. J. Fontijn et al. assignments.³² (Fe₃O₄)SiO₂@Ch–Im nanoparticle showed an obvious absorption peak appeared at 329 nm, which can be attributed to π–π* transitions of the benzenoid rings, indicating the successful functionalization on the surface of Fe₃O₄ nanoparticles. Furthermore, the weak bound at around 639 nm correspond to the [Fe²⁺]t₂g → [Fe²⁺]eg transition.

Fig. 5 UV-Vis spectra of Fe₃O₄ and (Fe₃O₄)SiO₂@Ch–Im.

3.4 Properties of the nanocomposites

3.4.1 TEM images. The TEM image of the (Fe₃O₄)SiO₂@Ch–Im is shown in Fig. 6. The TEM analysis suggests that the (Fe₃O₄)SiO₂@Ch–Im nanoparticles are almost monocrystalline with spherical shapes. The chitosan and imide grafted onto the Fe₃O₄ produced a resultant (Fe₃O₄)SiO₂@Ch–Im which exhibited a size in the range of 25–30 nm. In order to evaluate the dispersion of (Fe₃O₄)SiO₂@Ch–Im nanoparticles in the PIX matrix, the PIXN 3 was observed via TEM, as shown in Fig. 6b. The dispersion of (Fe₃O₄)SiO₂@Ch–Im was good in PIX matrix.

Fig. 6 TEM images of (a) (Fe₃O₄)SiO₂@Ch–Im and (b) PIXN 3.

3.4.2 Thermal properties. The TGA thermograms of PIX and PIX/(Fe₃O₄)SiO₂@Ch–Im nanocomposites along with derivative thermogravimetry (DTG) curves in nitrogen atmosphere are shown in Fig. 7. The TGA data including temperatures at which 5% (T₅), 10% (T₁₀) degradation occur and the char residue at 800 °C are reported in Table 1. It was seen that the neat PIX exhibited T₅ and T₁₀ of about 430 °C and 454 °C, respectively, which could lead us to conclude that the neat PIX can be classified as a thermally stable polymer. PIX also showed a main decomposition step with maximum decomposition temperatures at 471 °C. The char yield of PIX was about 47%. The data confirmed that PIX containing xanthene ring, ether and methyl groups in its backbone possessed good thermal stability and a high char residue. By incorporation of 1 wt% of (Fe₃O₄)SiO₂@Ch–Im to the PIX matrix, the T₅ and T₁₀ were increased, reaching 438 and 456 °C values, respectively. These parameters did not show significant change by increasing the (Fe₃O₄)SiO₂@Ch–Im content to 3 wt%. The DTG curves showed similar maximum decomposition temperatures (T_max) to those of the samples. A small shoulder was observed after the main decomposition step of PIX at about 722 °C in PIX, while (Fe₃O₄)SiO₂@Ch–Im deleted this step of degradation in both PIXN 1 and PIXN 2. This can be attributed to the barrier role of (Fe₃O₄)SiO₂@Ch–Im which maximizes the heat insulation and minimizes the permeability of volatile degradation char at high temperature and formation a good protected char. The char yield at 800 °C of PIXN 1 and PIXN 3 were higher than that of the neat PIX and with increasing loading amount the char residue were also increased. There was a strong interaction in (Fe₃O₄)SiO₂@Ch–Im with the PIX matrix, which leads to an increase in thermal properties as compared to the neat PIX. This improvement is due to the presence of Fe₃O₄ nanoparticles, which take steps as barriers to maximize the permeability of volatile degradation and heat insulation. Moreover, due to the high compatibility and hydrogen bonds of (Fe₃O₄)SiO₂@Ch–Im and PIX, a physical barrier effect can be found, which slows down the diffusion of pyrolysis products. As a result, PIX/(Fe₃O₄)SiO₂@Ch–Im nanocomposites could exhibit superior thermal properties.

Fig. 7 TGA and DTG thermograms of PIX and the nanocomposites.

Table 1 Thermal properties of PIX and the nanocomposite

Samples	T5^a (°C)	T10^a (°C)	Char yield^b	Tg^c
a Temperature at which 5% and 10% weight loss was recorded by TGA at a heating rate of 10 °C min^−1.b Weight percentage residue of the material after TGA analysis at a maximum temperature of 800 °C.c Glass transition temperature was recorded at a heating rate of 10 °C min^−1 in a nitrogen atmosphere.
PIX	430	454	47	178
PIXN 1	438	456	50	180
PIXN 3	439	456	51	188

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The glass transition temperatures values of the neat PIX and the nanocomposites were investigated by DSC. The incorporation of xanthene side group, ether and methyl units into the PIX back-bone resulted an aromatic polyimide with low T_g of 178 °C as compared to conventional aromatic polyimide. A slight increase in T_g for PIXN 1 and PIXN 3 was observed. Dispersion and interaction of nanoparticles with polymer matrix controlled T_g. This increase could be explained by high interaction between (Fe₃O₄)SiO₂@Ch–Im and the PIX matrix which restricts the chains movements.

3.4.3 Combustion properties by MCC. MCC is a simple method for investigating the combustion properties of small amounts of materials (2–4 mg). The parameters measured include heat release rate (HRR) (calculated from the oxygen depletion measurements), heat release capacity (HRC) (obtained by dividing the sum of the peak HRR by the heating rate) and the total heat release (THR) (given by integrating the HRR curve).

The HRR curves of PIX and its nanocomposites are presented in Fig. 8 and the corresponding combustion data are reported in Table 2. The peak heat release rate (pHRR) value of PIX was 203 W g⁻¹. It was observed that the pHRR value decreases to 197 W g⁻¹, with loading 1 wt% (Fe₃O₄)SiO₂@Ch–Im, indicating an improvement in the combustion data of PIX. The pHRR value of PIXN 3 was 141 W g⁻¹, which showed a further improvement as compared with PIXN 1. HRC is also an important parameter used to predict and evaluate flame behaviour. Table 2 shows HRC value of 267 J g⁻¹ K⁻¹ for PIX, while the (Fe₃O₄)SiO₂@Ch–Im nanocomposites show lower HRC values. Total heat release (THR) calculated from the area under the HRR curve is another important parameter. THR value of PIX was 14 kJ g⁻¹ and this parameter reached 13.3 kJ g⁻¹ in PIXN 1. By increasing the (Fe₃O₄)SiO₂@Ch–Im content, the THR value decreased to 10 kJ g⁻¹. As compared to the neat PIX, 26% improvements in the THR value was obtained with an increase in the (Fe₃O₄)SiO₂@Ch–Im loading to 3 wt%. It can be concluded that the most efficient results were achieved with loading of 3 wt% of (Fe₃O₄)SiO₂@Ch–Im in the PIX matrix. It is well known that the condensed phase actions could not be detected in MCC because of small amounts of materials. Therefore, it is possible that PIXN 3 sample represents the better flame retardancy properties in cone calorimeter test, in addition to its action in gas phase which has been detected in MCC.

Fig. 8 HRR curves of PIX and the nanocomposites.

Table 2 MCC data of PIX and its corresponding nanocomposites

Samples	pHRR (W g^−1)	HRC (J g^−1 K^−1)	THR (kJ g^−1)	Tmax
PIX	203	267	14	478
PIXN 1	192	187	13	489
PIXN 3	141	143	10	485

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4. Conclusion

In this investigation, the new poly(ether-imide) bearing a xanthene ring pendent group along with ether and methyl groups has been synthesized. Thanks to the incorporation of the functionalities, flexibility and solubility of PIX were improved. It also increased the inter-chain spacing or free volume, thus decreased glass transition temperature. A new functional Fe₃O₄ with chitosan and imide groups was synthesized and incorporated into the poly(ether-imide) matrix. The functionalized Fe₃O₄ have been successfully prepared by surface modification with chitosan, the dianhydride, phenylalanine and EPO. The introduction of these groups increased interaction between the nanoparticle and the poly(ether-imide) matrix. (Fe₃O₄)SiO₂@Ch–Im could form strong hydrogen bonds with the PIX matrix and the results revealed showed good properties enhancement. TGA results showed that PIX has high thermal stability and char residue and incorporation of (Fe₃O₄)SiO₂@Ch–Im improved the thermal properties of PIX. From MCC data, it has been found that (Fe₃O₄)SiO₂@Ch–Im has a positive effect on improving the combustion data of PIX, the decrease in peak heat release rate (pHRR) and the total heat release (THR) of the PIXN nanocomposites. It was concluded that the best results, thermal and flammability, were obtained when 3 wt% of (Fe₃O₄)SiO₂@Ch–Im was incorporated to the PIX matrix. Thus these nanocomposites classified as a new processable high performance materials.

Acknowledgements

This work was financially supported by grants from INSF (Iran National Science Foundation) (Grant No. 930823).

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