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Abstract

In this study, the thermally stable poly(amide–imide) (PAI)/cupric oxide (CuO) nanocomposites (NCs) containing 4, 6 and 8 wt% of CuO nanoparticles (NPs) were prepared by ultrasonic technique. At first, the optically active PAI was synthesized from the polymerization of a chiral monomer, N-trimellitylimido-l-valine, with 4,4′-methylenebis(3-chloro-2,6-diethylaniline) in a green medium using molten tetra-n-butylammonium bromide and triphenyl phosphite. Then, in order to prevent agglomeration of NPs and to obtain high compatibility between the NPs and the polymer matrix, the surface of the CuO NPs was successfully modified using a bioactive diacid containing l-valine amino acid. Finally, PAI/CuO NCs were produced via sonochemical reaction. The resulting NCs were characterized by means of Fourier transform infrared spectra, x-ray diffraction, thermogravimetric analysis (TGA), field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy techniques. FE-SEM results showed that the CuO NPs were homogeneously dispersed in the PAI matrix without agglomeration for NC 4 wt%. However, with increasing in the percentage of CuO NPs, some aggregations were observed. The TGA curves indicated that the obtained PAI/CuO NCs are thermally more stable than the pure PAI. The shielding ability of modified CuO NPs and PAI/CuO NCs was also investigated by ultraviolet–visible spectroscopy.

Keywords

Modified CuO nanoparticles optically active polymer nanocomposites ultrasonic technique thermal stability

Introduction

Nanocomposite (NC) is an interesting class of composites containing two or more phases, where at least one of the dimensions of the filler materials is in the nanoscale. In the last 20 years, the research on polymeric NCs (PNCs) has been extremely extended. PNCs have an enormous potential for many various applications, such as automobiles, sporting goods, packaging and electrical applications.¹ These materials have exhibited significant improvement in tunable physical properties (e.g., optical, mechanical, electronic and magnetic properties), permeability, thermal stability, flame retardancy and chemical resistance. These superior properties are owing to nanometre size characteristic of fillers, so various kinds of additives are necessary to be added to the pure polymers.^2
–4

Cupric oxide (CuO) nanoparticles (NPs) have been widely studied because of their unique properties such as catalytic and chelating effect,⁵ special electrical and optical properties⁶ and antioxidant and antimicrobial behavior.⁷ This metal oxide has broad applications in the electrochemical cells, gas sensors and lubricant.^8

–11 CuO NPs are added to different materials for the preparation of a wide range of nanostructured composites.^12

–18

The properties of the particle-reinforced PNCs intensely depend upon the dispersion ability of nanomaterials in the polymer matrix,¹⁹ but NPs due to their high surface energy tend to strongly agglomerate.²⁰ In order to achieve an improved compatibility between the NPs and polymer matrix, the use of different coupling agents containing hydroxyl (OH) and carboxyl groups for surface modification of NPs were suggested.^21
–23 The existence of these organic functional groups on the surface of the NPs leads to the formation of chemical and physical interactions with the polymer matrix.²⁴

In recent years, the utilization of ionic liquids as solvents in green chemistry has attracted wide attention because of its considerable properties, such as extensive-ranging liquid, high polarity, solvating ability of organic, inorganic and polymer materials, non-flammability, low volatility, exceptional chemical and thermal resistance.^25,26

Through the past decade, the research on biodegradable polymer-based NCs has been particularly increased.^27,28 Biodegradable polymers have been employed in the industrial and biomedical segments.²⁹ Aromatic polyimides are a kind of polymer materials with high performance. However, their applications are reduced owing to their high melting temperatures and insoluble nature in common organic solvents. To overcome these problems, some structural modifications often become necessary. For example, synthesis of poly(amide–imides) (PAIs) was caused balance between excellent thermal stability and processability.³⁰ PAIs are widely utilized for different applications due to its high-temperature performance, solvent resistance and special mechanical and thermal properties.^31,32

Fabrication and characterization of PAI-based NCs reinforced with modified NPs have been performed in our previous works.^33

–36 In the present study, first, the biodegradable PAI³⁷ was prepared by polycondensation reaction. Then, CuO NPs were modified using a chiral diacid (DA) coupling agent to increase the dispersion ability of the NPs into the polymeric matrix. Finally, NCs reinforced with CuO NPs were synthesized and characterized by various techniques, such as Fourier transform infrared spectra (FT-IR), X-ray diffraction (XRD), thermogravimetric analysis (TGA), field emission scanning electron microscopy (FE-SEM) and transmission electron microscopy (TEM). The optical properties of the obtained NCs were studied using ultraviolet–visible (UV–vis) spectroscopy.

Experimental

Materials

Nanosized CuO powder was purchased from Nanosabz Co. (Tehran, Iran), with average particle size <50 nm and specific surface area >80 m² g⁻¹. Solvents and chemicals were obtained from Aldrich Chemical Co. (Milwaukee, Wisconsin, USA), Fluka Chemical Co. (Buchs, Switzerland), Riedel-deHaen AG (Seelze, Germany) and Merck Chemical Co. (Darmstadt, Germany).

Characterization techniques

FT-IR spectra were recorded on a Jasco-680 spectrophotometer (Tokyo, Japan) at a resolution of 4 cm⁻ ¹. The samples were mixed with potassium bromide powder, ground and compressed into a pellet. They were scanned at wave number in the range of 400–4000 cm⁻¹. Proton nuclear magnetic resonance (¹H NMR, 500 MHz) spectra were recorded in N,N-dimethylsulfoxide-d ₆ solution by a Bruker Avance 500 instrument (Germany). The thermal stability of NCs was examined by thermal analyser (model STA503, BÄHR thermoanalyse GmbH, Hüllhorst, Germany) from 25 to 800°C at a heating rate of 20°C min⁻¹ under nitrogen atmosphere. The XRD pattern of the synthesized samples was studied by an x-ray diffractometer (Philips Xpert MPD, Karlsruhe, Germany), with copper (Cu) K_α radiation (λ = 1.540 Å) over Bragg angles ranging from 10° to 80° at the speed of 0.05° min⁻¹. The operating voltage and current were maintained at 40 kV and 35 mA, respectively. The morphology of the prepared NCs was investigated using a transmission electron microscope (model CM 120; Philips, Germany) under an acceleration voltage of 100 kV and field emission scanning electron microscope (model S-4160; HITACHI, Tokyo, Japan). The preparation of NCs was carried out by MISONIX (Raleigh, North Carolina, USA) ultrasonic liquid processors, XL-2000 SERIES (USA), with frequency wave of 2.25 × 10⁴ Hz. The absorption spectra of prepared NCs were measured using a UV/vis/near infrared spectrophotometer (model V-570; JASCO, Tokyo, Japan), with solid pellets of the samples in the spectral range between 200 and 800 nm.

Synthesis of N-trimellitylimido-l-valine monomer and polymer

N-Trimellitylimido-l-valine DA (3) and PAI (5) were fabricated according to our previous works by Mallakpour et al.³⁷ (Figure 1).

Figure 1.

Syntheses of DA and PAI. DA: diacid; PAI: poly(amide–imide).

Surface modification of CuO NPs with N-trimellitylimido-l-valine

In order to modify the surface of NPs, 0.10 g of CuO nanopowder and 15 wt% of N-trimellitylimido-l-valine as modifier were mixed in 15 mL of methanol. The mixture of NPs and coupling agent was strongly stirred for 24 h at room temperature. Then the resulting suspension was ultrasonicated for 30 min. After filtration, the precipitate was washed with methanol for removal of the unreacted DA. Finally, the prepared NPs were dried at 60°C for 24 h (Figure 2).

Figure 2.

Preparation of PAI/CuO NCs. PAI: poly(amide–imide); CuO: cupric oxide; NCs: nanocomposites.

Fabrication of PAI/CuO NCs

The preparation process of NCs is as follows. The samples were produced by mixing different amounts of modified CuO NPs (4, 6 and 8 wt%) to the 0.10 g of synthesized PAI in 20 mL of absolute ethanol. Solution was dispersed by stirring for 5 h at room temperature. Then, the NPs/PAI mixture was ultrasonically stirred for 30 min. At last, ethanol was removed and the pale brown solid was dried under vacuum for 2 h at 80°C (Figure 2).

Result and discussion

Syntheses of chiral bioactive DA and biodegradable polymer

N-Trimellitylimido-l-valine (3) as monomer and modifier was prepared via the condensation reaction of an equimolar combination of trimellitic anhydride (1) and l-valine amino acid (2) in acetic acid solution. Biodegradable and functional amino acid-based PAI was synthesized by direct polycondensation reaction of DA with 4,4′-methylenebis(3-chloro-2,6-diethylaniline) (MCDEA) in tetra-n-butylammonium bromide (TBAB) as solvent and triphenyl phosphite (TPP) as a condensing agent. The structure of this polymer was confirmed as PAI by means of FT-IR spectroscopy. The peak around 3395 cm⁻¹ shows the presence of the NH stretching band of the amide group. The absorption band appeared at 1721 cm⁻¹ is related to the carbonyl stretching of imide group and the C=O stretching band of amide group appeared at 1671 cm⁻¹ (Figure 4). In the ¹H NMR spectrum of prepared PAI, appearances of the peak at 4.60 ppm indicated the presence of the chiral carbon proton in the main chain of the polymer. The resonance of aromatic protons appeared in the range of 6.75–7.29 ppm. The peaks at 8.11–8.50 ppm are related to the other aromatic protons.³⁷

Preparation and FT-IR characterization of PAI/CuO NCs

It has been reported that the CuO NPs which were mixed with polymer matrix improved the thermal conductivity, thermal stability and other properties of the obtained NCs.^3,5,13 In order to avoid the formation of agglomerated NPs and to increase their dispersion in the matrix of polymer, it is essential to modify the surface of NPs.²⁰ Organic molecules containing two or more functional groups, such as dicarboxylic acids are appropriate coupling agents for surface modification of NPs. The modifiers were joined to the surface of the NPs via the coordination bond between the metal ions and the carboxylate group.³⁸ In this study, the surface of CuO NPs was treated with a bioactive DA using the sonochemical reaction. The modified NPs are attached to the PAI chains through H bonding between –OH groups and amidic–NH. The details of preparation of PAI/CuO NCs are shown in Figure 2.

Figure 3 shows the FT-IR spectra of pure CuO NPs (a), N-trimellitylimido-l-valine (b) and modified NPs (c). In Figure 3(a), appearance of the peak at 475 cm⁻¹ is attributed to the metal–oxygen (Cu–O) stretching. In the spectrum of modified CuO with N-trimellitylimido-l-valine (Figure 3(c)), characteristic peaks of CuO have been observed in the range of 500–400 cm⁻¹. Also there are two peaks at 1623 and 1715 cm⁻¹, which might be assigned to the interactions of carboxylic ion with the metal ion. These observations confirmed that N-trimellitylimido-l-valine was successfully attached to the surface of the CuO NPs through the coordination bond. Figure 4 shows FT-IR spectra of pure PAI and PAI/CuO NCs. The important peaks of PAI are characterized as follows: the bands corresponding to aromatic C–H stretching appeared at 2934 cm⁻¹ and CH₂ stretching at 2875 cm⁻¹. The absorption band at 1378 cm⁻¹ was characteristic peaks for imide heterocycle (C–N). The absorption peaks at 1778, 1721 and 1671 cm⁻¹ are due to two overlapped carbonyl stretching (imide and amide’s C=O), respectively. A band at 3395 cm⁻¹is related to N–H stretching vibration. The addition of CuO NPs in the PAI matrix created the slight variations in the intensities of absorption bands and confirmed the presence of CuO NPs in it. As shown in Figure 4, all characteristics peaks of PAI were also observed for the NCs reinforced with inorganic CuO; however, some of the peaks shifted to lower wave numbers in comparison with the pure polymer. This shift could be due to the interaction between CuO NPs and the PAI chain.

Figure 3.

FT-IR spectra of unmodified CuO NPs (a), DA (b) and modified CuO NPs (c). FT-IR: Fourier transform infrared spectra; CuO: cupric oxide; NPs: nanoparticles; DA: diacid.

Figure 4.

FT-IR spectra of pure PAI (a), 4 wt% PAI/CuO NC (b), 6 wt% PAI/CuO NC (c) and 8 wt% PAI/CuO NC (d). FT-IR: Fourier transform infrared spectra; PAI: poly(amide–imide); CuO: cupric oxide; NC: nanocomposite.

TGA

Figure 5 shows the thermal degradation of the pure PAI and PAI with different weight percentage of CuO NPs. The curves of thermal decomposition of the neat PAI and PAI/CuO NCs have two-step degradation processes. In the first step, slight weight loss is associated with the removal of moisture and physisorbed water molecules and the second weight loss is related to the degradation process of PAI chain. However, the thermogravimetric curves of the PAI/CuO NCs are placed at a higher temperature than that of the pure PAI. The improvement in thermal stability after the reinforcement of PAI with modified NPs may proves a strong interaction between the modified NPs and the polymer matrix. Metallic compounds could have catalytic effect on degradation of the polymer matrix.³ Therefore, the existence of the pure CuO NPs in the NCs may facilitate the thermal decomposition of the PAI by serving as a catalyst. But the presence of a thin interfacial layer from DA inhibits the direct contact of the CuO NPs with the polymer matrix by passivating the particle surface and so can increase the thermal stability of the NCs. Thermal stabilities of the PAI and PAI/CuO NPs were investigated based on 5 and 10% weight loss (T ₅ and T ₁₀) and residue at 800°C (CR, char yield). Also the limiting oxygen index (LOI) for materials is calculated based on Van Krevelen and Hoftyzer equation.³⁹

Figure 5.

TGA thermograms of PAI and PAI/CuO NCs under argon atmosphere at heating rate of 20°C min⁻¹. TGA: thermogravimetric analysis; PAI: poly(amide–imide); CuO: cupric oxide; NCs: nanocomposites.

The results of thermal degradation are shown in Table 1.

Table 1.

Thermal properties of PAI and PAI/CuO NCs.

Samples	Decomposition temperature (°C)		CR^a(%)	LOI
Samples	T ₅	T ₁₀	CR^a(%)	LOI
PAI	334	386	49.7	37.4
PAI/CuO NC (4 wt%)	323	379	50.6	37.7
PAI/CuO NC (6 wt%)	342	381	50.8	37.8
PAI/CuO NC (8 wt%)	338	401	51.3	38.0

T ₅: temperature at which 5% weight loss was recorded by TGA at a heating rate of 20°C min⁻¹ under nitrogen atmosphere; T ₁₀: temperature at which 10% weight loss was recorded by TGA at a heating rate of 20°C min⁻¹ under nitrogen atmosphere; LOI: limiting oxygen index evaluating CR at 800°C; PAI: poly(amide–imide); CuO: cupric oxide; NCs: nanocomposites; TGA: thermogravimetric analysis; CR: char yield.

^aWeight percentage of material left undecomposed after TGA at a temperature of 800°C under nitrogen atmosphere.

L O I = 17.5 + 0.4 C R

X-ray diffraction analysis

Figure 6 illustrates the XRD results of the neat PAI (a), CuO NPs (b), modified CuO (c), 6 wt% PAI/CuO NC (d) and 8 wt% PAI/CuO NC (e). Figure 6(a) shows the structure of synthesized neat PAI is amorphous. In Figure 6(b), the diffraction peaks can be indexed to a monoclinic CuO NPs. The XRD patterns of NCs (Figure 6(d) and (e)) clearly indicated that the samples are crystalline and consequent, processing of creation of NCs did not change the crystalline phase of CuO NPs. The average crystalline size of the CuO NPs in the composites was estimated to be about 35–50 nm using the Debye–Scherrer formula from the (200) reflection.

Figure 6.

XRD patterns of neat PAI (a), CuO NPs (b), modified CuO NPs (c), 6% NC (d) and 8% NC (e). XRD: x-ray diffraction; PAI: poly(amide–imide); CuO: cupric oxide; NPs: nanoparticles; NC: nanocomposite.

Size and morphology of PAI/CuO NCs

The structures, morphologies and sizes of modified CuO NPs, pure PAI and PAI/CuO NCs are demonstrated by means of FE-SEM and TEM (Figures 7 and 8). FE-SEM micrographs indicate that for 4 wt% NCs (Figure 7(e) and (f)), the CuO NPs were uniformly distributed in the polymer matrix, but in the case of 6 and 8 wt% NCs (Figure 7(g) to (j)), some aggregation are recognized with further enhancing NPs concentration. TEM images of pure CuO NPs, modified CuO NPs and NCs with the filler loading 4 wt% of CuO NPs are shown in Figure 8(a) to (f). TEM micrographs proved the presence of rod-like nanosize particle (30–50 nm) in the polymer matrix (Figure 8(d) to (f)). Also these photographs show that the shape of NCs is beehive like (honeybee) and introducing PAI did not change the general shape of CuO NPs. TEM observations proposed a proper dispersion of NPs in the polymer matrix at low concentration.

Figure 7.

FE-SEM photographs of modified CuO ((a) and (b)), pure PAI ((c) and (d)), 4% PAI/CuO NC ((e) and (f)), 6% NC ((g) and (h)) and 8% NC ((i) and (j)). FE-SEM: field emission scanning electron microscopy; CuO: cupric oxide; PAI: poly(amide–imide); NC: nanocomposite.

Figure 8.

TEM micrographs of pure CuO (a), modified CuO ((b) and (c)) and PAI/CuO NC containing 4 wt% CuO NPs ((d) to (f)). TEM: transmission electron microscopy; CuO: cupric oxide; PAI: poly(amide–imide); NC: nanocomposite; NPs: nanoparticles.

Optical properties of NCs

The UV–vis absorption spectra of the pure PAI, modified CuO and NCs containing different amounts of CuO NPs are illustrated in Figure 9(a) to (e). The modified CuO with N-trimellitylimido-l-valine exhibits a broad absorption in UV region with spectral wavelength between 250 and 800 nm, which can be observed as the characteristic absorption of CuO. The spectra of PAI/CuO NCs clearly indicate that the UV absorption intensity enhances gradually with increasing the percentage of CuO NPs. Also the absorption band shifted to longer wavelength. The shifting of the absorption peaks towards longer wavelength could indicate the strong interactions between the PAI and CuO NPs.

Figure 9.

UV absorption spectra of the PAI/CuO NCs. UV: ultraviolet; PAI: poly(amide–imide); CuO: cupric oxide; NCs: nanocomposites.

Conclusions

In the present investigation, the PAI-based NCs reinforced with CuO NPs were prepared. At first, a thermally stable optically active PAI was synthesized via direct polycondensation of DA with MCDEA in the presence of TBAB and TPP. Secondly, in order to improve the dispersion ability of NPs in the polymer matrix, the surface modification of NPs was carried out using N-trimellitylimido-l-valine as coupling agent. DA was chemically deposited on the surface of NPs by ultrasonic irradiation. The existence of DA as modifier could cause a strong chemical linkage between two phases. Finally, PAI/CuO NCs containing different amounts of NPs (4, 6 and 8 wt%) were synthesized by the sonochemical method. Different analytical techniques were used to characterize the properties of synthesized NCs. The results obtained from FT-IR measurement indicated the possible incorporation of CuO in polymer matrix. PAI/CuO NCs showed enhanced thermal stability as compared with the pure polymer. The improvement in thermal stability of composites reinforced with modified NPs is strongly attributed to homogeneous dispersion of NPs and formation of chemical interactions between them as well as with the PAI matrix. The XRD patterns proved that presence of PAI did not change the crystalline phase of monoclinic CuO NPs. The structural morphology and size of NCs were determined by FE-SEM and TEM measurements. FE-SEM images indicated that in the NCs consist of 4 wt% CuO, the NPs are uniformly dispersed, but the higher concentration of filler led to agglomeration phenomena. TEM photographs illustrated that the metal oxide particles in the PAI matrix were found to be in the range of 30–50 nm.

Footnotes

Declaration of Conflicting Interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) disclosed receipt of the following financial support for the research, authorship, and/or publication of this article: The authors would like to acknowledge the help and financial support of the Research Affairs Division Isfahan University of Technology (IUT). Further financial support from National Elite Foundation (NEF) and Center of Excellency in Sensors and Green Chemistry Research (IUT) Iran Nanotechnology Initiative Council (INIC) is gratefully acknowledged.

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Application of chiral diacid N -trimellitylimido- l -valine for the surface modification of copper oxide as inorganic filler and preparation of poly(amide–imide)/cupric oxide nanocomposites

Abstract

Keywords

Introduction

Experimental

Materials

Characterization techniques

Synthesis of N-trimellitylimido-l-valine monomer and polymer

Surface modification of CuO NPs with N-trimellitylimido-l-valine

Fabrication of PAI/CuO NCs

Result and discussion

Syntheses of chiral bioactive DA and biodegradable polymer

Preparation and FT-IR characterization of PAI/CuO NCs

TGA

X-ray diffraction analysis

Size and morphology of PAI/CuO NCs

Optical properties of NCs

Conclusions

Footnotes

Declaration of Conflicting Interests

Funding

References