Synthesis and characterization of cardanol based fluorescent composite for optoelectronic and antimicrobial applications

Polymer 108 (2017) 449e461

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Synthesis and characterization of cardanol based fluorescent composite for optoelectronic and antimicrobial applications Selvaraj Vaithilingam a, *, Jayanthi K.P. a, b, Alagar Muthukaruppan c a

Nanotech Research Lab, Department of Chemistry, University College of Engineering Villupuram, (A Constituent College of Anna University, Chennai), Kakuppam, Villupuram 605 103, Tamilnadu, India b Department of Chemistry, D.M.I College of Engineering, Palanchur-Nazarethpet Post, Chennai 600123, India c Polymer Composite Lab, Department of Chemical Engineering, Anna University, Chennai 600 025, India

a r t i c l e i n f o

a b s t r a c t

Article history: Received 10 September 2016 Received in revised form 1 December 2016 Accepted 6 December 2016 Available online 8 December 2016

The present study attempted to prepare para phenylene diamine functionalized multiwalled carbon nanotube (f-MWCNT) incopoarated polymer of a new pyrene core containing cardanol based benzoxazine (PyCBz) composite (f-MWCNT-PPyCBz). For this, a new monomer of pyrene core containing cardanol based benzoxazine (PyCBz) was synthesized through solventless method and its structure was confirmed using Fourier transform infrared (FTIR) spectroscopy, nuclear magnetic resonance spectroscopy (1H & 13C) and MALDI mass. In addition, multiwalled carbon nanotube (MWCNT) was functionalized using para phenylene diamine that acted as effective filler for preparation the composite materials. The curing behavior and thermal stability of para phenylene diamine functionalised multiwalled carbon nanotube (MWCNT) incopoarated pyrene core containing cardanol based polybenzoxazine(PPyCBz) composites were studied by differential scanning calorimetry (DSC) and thermogravimetric analysis (TGA). Antimicrobial studies were also carried out using Streptococcus pyogens, Bacillus cereus, Staphylococcus aureus and Escherichia coli. In short, the obtained f-MWCNT/PPyCBz nanocomposites possess high dielectric constant, good antibacterial and photo luminescent properties, which may find optical, antimicrobial and high dielectric applications. © 2016 Elsevier Ltd. All rights reserved.

Keywords: Pyrene core cardanol based polybenzoxazine Cardanol MWCNT Dielectric properties Antimicrobial

1. Introduction Poly(benzoxazine) is one the important and advanced thermosetting polymer of phenolic resins, which attracts researchers and industrialist due to its unique properties like near-zero volume shrinkage, low surface free energies, high glass transition temperatures, highly hydrophobic, high resistance toward chemicals with very good physical and mechanical properties along with low dielectric constants [1e6]. These unique characteristics of polybenzoxazine make it a better candidate over traditional phenolic resins in the fields like electronics, aerospace, and other industries. Benzoxazine is a cyclic compound obtained through the condensation of phenol, diamines and paraformaldehyde. So, the poly(benzoxazine) is obtained by thermal polymerization of the monomers through heterocyclic ring opening without using any

* Corresponding author. E-mail addresses: [email protected], [email protected] (S. Vaithilingam). http://dx.doi.org/10.1016/j.polymer.2016.12.017 0032-3861/© 2016 Elsevier Ltd. All rights reserved.

[email protected],

catalyst and also without releasing any byproducts [7e9]. Since non renewable source like petrochemicals are used mainly for the synthesis of polymers, the future generation is alarming with a deficiency in the resources. Hence, the agricultural waste materials, a pollutant of environment were made used for the synthesis of polymers such as phenolic resins, polyesters, polyurethanes, etc. The vast sources of phenolic resins are distributed in wood, seeds, shell, cashew nut shell liquid (CNSL), lignin, palm oil and other plant-based resources. Further, the lactic acid and long chain alcohols are derived from corn starch, soya bean and castor oil that are usually thrown as agricultural or agro-based industrial waste. Hence, the synthesis and utilization of monomers from various renewable resources for polymer composites have been investigated in the recent years [10]. Generally, polymeric materials exhibit low thermal stability and poor electrical conductivity, which made them to be used as insulating material. But, there is a demand for polymeric materials with good electrical conductivity as well as high thermal stability to find various applications in electronic devices, capacitors, gas sensors and bipolar plates for polymer electrolyte fuel cells [11e13]. To

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achieve this, the conducting micro fillers were incorporated into polymers that improve the electrical conductivity of the resultant composites [14]. Further, the poly(benzoxazine) based nanocomposites containing various nanofillers such as MMT, silicates, titania, and nanomagnetite have been reported earlier [15e19]. The incorporation of carbon materials into polymer matrix will result increased conductivity of the resultant polymer composites. Carbon nanotubes (CNTs) are crystalline form of carbon with nanostructure, which possesses high aspect ratio and much higher surface area [20,21]. Because of their large dielectric constant, electrically conductive carbon nanotubes are attractive as fillers for high dielectric-constant nanocomposites. The incorporation of CNTs even in very small fraction in polymer results in the formation of high electrically conductive composites. Currently, many researchers were developed CNTs incorporated composites containing different polymer matrix such as polystyrene, epoxy [22], poly(methylmethacrylate) [23], poly(anline) [24], poly(pyrrole), poly(urethane) [25], poly(vinyl alcohol) [26], etc., for various potential applications. The unique features of CNTs such as optical, electronic, magnetic, chemical, mechanical, thermal and electrical properties enable the use of CNT in various applications like tissue engineering, bio-sensing, high dielectric engineering materials, drug delivery and antimicrobial activity at a very low concentration levels of CNT [27]. The dispersion of CNT is a major problem and also the aggregates of CNT may cause cell damage. To overcome this problem, functionalizations of CNT were done to increase the chemical compatibility, the antimicrobial activity and to decrease the toxicity [28]. The antimicrobial activity of CNT/poly(L-lysine) and CNT/ poly(L-glutamic acid) against E. coli and S. epidermidis microorganisms were reported [29,30]. The covalently attached epilsonpolylysine with CNT exhibited enhanced antimicrobial activities against E. coli, P. aeruginosa and S. aureus. In addition, silver-CNT composite also showed strong antibacterial activity against E. coli in the dark [31]. Introducing hetero-aromatic rings into the main chain of a synthetic polymer would impart certain properties such as the rigidity and polarizability to the hetero-aromatic polymers [32,33]. The introduction of flexible linkages, hetero-aromatic rings and bulky pendant pyrene group may reduce the crosslink density and causes a lowering of the Tg of the cured products [34]. Further, a pyrene pendant core pyridine based diamine with fluorescence property was reported [35]. Only a limited number of compounds containing 1,4-benzoxazine ring system have been reported for chemotherapeutic activity and still the research has to undergo to fulfill the requirement of chemotherapeutic applications [36,37]. Ofloxacin is one of the important antimicrobial agents possessing the 1,4-benzoxazine ring system in its structure. All these above observations prompted us to develop this heterocyclic polymer nanocomposite system consisting of pyrene core in cardanol based polybenzoxazine incorporated with amine functionalized MWCNT (f-MWCNT) for the dielectric, antimicrobial and photoluminescence properties related applications.

diamine was obtained from Sigma Aldrich. MWCNTs were obtained from the Applied Science and Innovation Pvt. Ltd, Pune, India. Cardanol was procured from Sathya cashew chemicals Pvt Ltd, Chennai. 2.1.1. Synthesis of 4-(1-pyrenyl)-2, 6-bis(4-nitrophenyl) pyridine (PyBNPP) A two-necked round-bottomed flask equipped with a magnetic stirrer bar, thermometer and condenser was charged with a mixture of 10 g (0.0434 mol) of pyrene-1-carboxaldehyde, 14.3 g (0.0868 mol) of 4-nitroacetophenone, 70 g of ammonium acetate and 200 ml of glacial acetic acid. The reaction mixture was refluxed under stirring for 7 h. Upon cooling, crystals were gradually separated out, which was filtered and washed with 70% acetic acid. The crude PyBNPP product was re-crystallized from ethanol (Scheme 1). The yield of the product is 91%. 2.1.2. Synthesis of 4-(1-pyrenyl)-2, 6-bis(4-aminophenyl)pyridine (PyBAPP) A mixture of 8.0 g of PyBNPP, 0.8 g of Pd/C (10%), 30 ml of hydrazine monohydrate and 50 ml of ethanol were placed in a flask. The reaction mixture was refluxed at 90
C for 24 h and then the ethanol was removed under reduce pressure. Finally, 20 ml of THF was added to the above reaction mixture as a solvent and filtered to remove Pd/C and then the THF was removed using rotation evaporator under reduced pressure. The resulted light yellow color solid (Scheme 1) was re-crystallized from THF/ethanol (10/1, v/v) solvent in twice and then dried under vacuum. The yield of the product was 77% [38]. 2.1.3. Synthesis of pyrene core cardanol benzoxazine (PyCBz) To prepare pyrene core containing cardanol based benzoxazine (PyCBz), 4-(1-pyrenyl)-2,6-bis(4-aminophenyl)pyridine (12.0 g, 0.0260 mol), cardanol (15.6 g, 0.052 mol) and paraformaldehyde (3.31 g, 0.1092 mol) are mixed together in a round bottom flask for 15 min until the powder is homogeneously mixed. The mixture was heated at 110
-120
C for 30 min along with stirring and a clear solution is obtained. The solution is then heated to 130
C for about 30 min to obtain the corresponding benzoxazine. The liquid is poured into an aluminum dish and then cooled to produce a yellow solid product (Scheme 2). 2.1.4. Preparation of amine functionalized carbon nanotubes (fMWCNT) The acid functionalized carbon nanotubes (1.0 g) were dispersed in excess SOCl2 and refluxed at 70
C for 24 h in order to convert the carboxylic acid groups into acyl chloride groups. The excess thionyl chloride was removed through distillation method. The residue is dispersed well in DMAc (200 ml). To this solution, para phenylenediamine (150 mg) and pyridine (5 ml) were added. The mixture is stirred at 120
C for 72 h under nitrogen atmosphere. The final product p-phenylenediamine functionalized MWCNT or amine functionalized MWCNT (f-MWCNT) is centrifuged, washed well with methanol and dried in vacuum (Scheme 3).

2. Experimental methods 2.1. Materials Pyrene-1-carboxaldehyde (99%) was purchased from Acros Organics India Ltd and used as received. N,N-dimethyl formamide (99%), ammonium acetate (98%), glacial acetic acid (99.5%), paraformaldehyde (98%), tetrahydrofuran (99.5%), ethanol (99.9%), diethyl ether (99.5%), H2SO4 (95%) and HNO3 (70%) were procured from SRL India. Hydrazine hydrate (80%), 4-nitroacetopheneone (96%) were purchased from Loba Chemie, India. Para phenylene

2.1.5. Preparation of amine functionalized multi walled carbon nanotubes incorporated polypyrenecore cardanol benzoxazine (fMWCNT/PPyCBz) composite The f-MWCNT/PPyCBz nanocomposite was prepared using dimethyl formamide (DMF) as a solvent in an inert atmosphere (Scheme 4). Typically, various weight percentages of f-MWCNT (0.5%, 1%,1.5% and 2%) were dispersed in 5 ml of DMF through sonication process. 2 g of benzoxazine monomer dissolved in 5 ml DMF was added gradually to the above dispersion and the resulting solutions were mixed using magnetic stirrer in a two-necked RB

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Scheme 1. Synthesis of 4-(1-pyrenyl)-2,6-bis(4-aminophenyl)pyridine (PyBAPP) through the formation of 4-(1-pyrenyl)-2,6-bis(4-nitrophenyl)pyridine (PyBNPP).

Scheme 2. Synthesis of pyrene core cardanol based benzoxazines (PyCBz).

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COCl

COOH

SOCl2

H2SO4 / HNO3

60 oC

70 oC COCl

COOH

NH2

MWCNT

120 oC

H2N

CO

NH

NH2

CO

NH

NH2

f-MWCNT Scheme 3. The schematic representation for the formation of amine functionalized multi walled carbon nanotubes.

Scheme 4. Schematic representation of amine functionalized multi walled carbon nanotubes incorporated polypyrenecore cardanol benzoxazine (f-MWCNT/PPyCBz) composite.

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453

Fig. 1. The optical images of (a) neat PPyCBz and (b)f-MWCNT/PPyCBz composite.

flask with nitrogen to remove moisture. The reactants were stirred continuously for 4 h at 80
C and sonicated for another 1 h. Finally, the solutions were poured into silane coated glass plate and cured at 120
C for 1 h, 160
C for 1 h, 180
C for 1 h, 200
C for 2 h, and 220
C for 2 h to get a semi transparent film (Fig. 1). 2.1.6. Minimum inhibitory concentration (MIC) The minimum inhibitory concentration of the prepared compound against the human pathogens were analyzed by resazurin reduction assay. 2.1.7. Method to prepare resazurin dye solution The resazurin dye solution was prepared by dissolving a 270 mg tablet in 40 mL of sterile distilled water. The vortex mixer instrument was used to form well-dissolved and homogenous solution of resazurin dye. 2.1.8. Preparation of the activity plates The 96 plates were prepared under aseptic conditions. The sterile plates were labelled and a volume of 200 mL of test compounds (1 mg/mL) in 5% (v/v) Di methyl sulfoxide was pipetted into the first row of the 96 wells plate. To all other remaining wells 100 mL of nutrient broth was added for the bacterial cells. The serial dilutions were performed using micropipette with sterile pipette tips such that each well had 100 mL of the test material in serially descending concentrations. To all these wells 10 mL of resazurin dye solution was added. A volume of 10 mL of bacterial suspension (5  106 cells/mL) was added to each well to achieve a concentration of 5  105 cells/mL. The commercial antibiotics streptomycin was used as positive controls in the assay plate. The plates were placed in an incubator at 37
C for 18e24 h. The color change was then observed visually. The color changes from blue to pink or colourless were recorded as reduction of dye by the viable bacteria. The lowest concentration at which no color change occurred was taken as the MIC value. 2.1.9. Characterization techniques FT-IR spectra were recorded by making KBr disc with sample on Perkin Elmer 6X FT IR spectrometer. The 1H NMR and 13C NMR analyses were carried out in CHCl3 and DMSO-recorded on a Bruker 400 spectrometer. MALDI MASS was recorded using Matrix Assisted Laser Desorption Time of Flight Mass Spectrometer. DSC analysis was carried out using a Netzsch DSC-200 at a heating rate of 10
C/min under a continuous flow of nitrogen. TGA analysis was carried out using TGA 2950 from TA Instrument. The dielectric constant (DC) of f-MWCNT/PPyCBz systems were determined with

the help of an impedance analyzer (Solartron impedance/gain phase analyzer 1260) using Pt electrode at 40
C in a frequency range of 1 MHz. Powder X-ray diffraction patterns (XRD) were recorded using a Rigaku Mini flex diffractometer with Cu-KR radiation. A JEOL JSM-6360 field emission scanning electron microscope was used for morphological studies. The phase morphology of f-MWCNT/PPyCBz nanocomposites was characterized by using a HR-TEM (JEM-3010, JEOL, Tokyo, Japan), operating at 80 kV with a measured point-to-point resolution of 0.23Nm. UV-visible spectral measurements were explored by using Jasco V-650 spectrophotometer. The photoluminescence spectra were analyzed using Jasco spectrofluorometer Model FP - 8300.

3. Results and discussions 3.1. FTIR studies 3.1.1. FTIR spectrum of pyrene core cardanol benzoxazine (PyCBz) The characteristic absorptions peaks appeared at 1249 cm1 and 1172 cm1 were assigned to asymmetric stretching of Ar-O-C and symmetric stretching of C-O-C in cardanol benzoxazine. The peaks appeared at 2931 cm1 and 2862 cm1 were due to C-H aliphatic stretching vibration. The peaks at 1612 cm1 and 1519 cm1

Fig. 2. FTIR spectrum of pyrene core cardanol benzoxazine (PyCBz).

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structure of the monomer (Fig. 2). 3.1.2. FTIR spectra of f-MWCNT/PPyCBz composites The cured f-MWCNT/PPyCBz hybrid nanocomposites were examined by the FT-IR and their results were shown in Fig. 3. By comparing FTIR spectrum of pyrene core cardanol benzoxazine monomer with neat poly(pyrene core cardanol benzoxazine), the disappearance of peak at 956 cm1 was noticed, which indicates the occurrence of oxazine ring opening polymerization of pyrene core cardanol benzoxazine. The absorption peaks at 2931 cm1 and 2862 cm1confirm the presence of CeH aliphatic groups. The absorption peak appeared at 1612 cm1 corresponds to aliphatic C¼ C stretching frequency. The absorption peak at 1442 cm1 is due to tetra substituted benzene ring which confirm the ring opening polymerization. The characteristic absorption peak associated with the CeNeC groups of PPyCBz appeared at 1365 cm1 3.2. NMR studies

Fig. 3. FTIR spectra of various weight percentage of f-MWCNT incorporated PPyCBz composites.

confirm the presence of 1,2,3-tri substituted benzene ring in cardanol benzoxazine [39]. Further, 1365 (CeN), 1249 (CeO) and 956 (NeCH2eO or NeCeO) stretching along with AreOeC confirm the

Fig. 4 shows the 1H NMR spectrum of pyrene core cardanol benzoxazine monomer. The peak observed at 0.85 ppm(a) is due to eCH3 protons. The peaks observed at 1.27 1.32and 1.56 ppm(b-d) are corresponding to aliphatic CH2 protons. The peak visualized at 2.09 ppm (e) indicates the protons conjugated to the double bonds (-CH2-CH¼). The peak noticed at 2.63 ppm (f) is attributed to -CH2Ar, the peak at 4.5 ppm(g) corresponds to eAr-CH2-N and the peak emerged at 5.5 ppm (h) is assigned to -CH¼CH- protons

Fig. 4. 1H NMR Spectrum of pyrene core cardanol benzoxazine (PyCBz) monomer.

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Fig. 5.

455

13

C NMR spectrum of pyrene core cardanol benzoxazine (PyCBz).

present in the monomer [40]. The peak appeared at 6.0 ppm (i) is consigned to eO-CH2N protons. The peaks visible between 6.54 and 7.06 ppm (j-m) are corresponding to aromatic protons. Further, the peaks appeared between 7.9 and 8.20 ppm (n-s) correspond to pyrene core protons. Further, structure of pyrene core cardanol benzoxazine (PyCBz) monomer was confirmed by 13C NMR study. Fig. 5 shows the 13C NMR spectrum of pyrene core cardanol benzoxazine (PyCBz) monomer. The peaks viewed from 14.01 ppm to 39.79 ppm confirm the presence of aliphatic carbon atoms(a-g). The peak observed at 60.42 ppm (h) shows the presence of ph-CH2-N carbon, and the peak at 90.98 ppm is assigned to e O-CH2-N carbon (i) and the peak appeared at which authenticate the oxazine ring formation. The peaks noticed from 125.01 ppm to 133.43 ppm (j-m) confirm the presence of pyrene core carbon atoms The peak appeared at 129.60(n) ppm shows the presence of aliphatic -C¼C- in the side chain of cardanol moiety of the newly synthesized pyrene core cardanol benzoxazine monomer. The peaks observed at 152.12 ppm and 157.19 ppm (o-q) shows the presence of aromatic carbon atoms. 3.3. MALDI-mass analysis The formation of cardanol based pyrene core benzoxazine monomer (PyCBz) is also confirmed by determining its molecular weight using MALDI Mass spectroscopy. MALDI-Mass is an imperative technique used to determine the higher molecular weight organic molecules. MALDI-Mass result (Fig. 6) substantiates the molecular weight of pyrene core cardanol benzoxazine monomer as 1113.78, which is in good agreement with the theoretically calculated molecular weight value. Thus, the formation of pyrene core cardanol based benzoxazine is proved by 1H NMR, 13C NMR,

MALDI Mass and FTIR analysis. 3.4. Differential scanning calorimetry (DSC) The various phenomena involved during the thermal heating of amine functionalized multi walled carbon nanotubes reinforced poly(pyrene core cardanol benzoxazine) composites were investigated in terms of glass transition temperature (Tg), melting, and curing temperature by using DSC analysis (Fig. 7). The neat PPyCBz has the Tg value of 148.7
C where as 2% f-MWCNTs filled PPyCBz shows Tg value of about 160.9
C. Thus, the value of glass transition temperature (Tg) is shifted to higher temperatures for the composites with different weight percentage of f-MWCNT reinforced PPyCBz when compared to that of neat PPyCBz material (Fig. 7). Generally, the introduction of f-MWCNT into poly(pyrene core cardanol benzoxazine) matrix may expect the possibility of enhancement of free volume, which in turn may expect to decrease the Tg value. At the same time, the steric-hindrance due to the reinforcement of bulky para phenylene diamine functionalized MWCNT may expected to increase the Tg of the resultant composites. Since there is good chemical compatibility between amine functionalized multi walled carbon nanotubes (f-MWCNT) and PPyCBz, the inter molecular movement of macromolecular chains will be restricted and this effect predominates over free volume, which results an increase in Tg values. 3.5. Thermogravimetric analysis (TGA) The thermal decomposition properties of neat PPyCBz and various weight percentages of f-MWCNT incorporated PPyCBz composites were studied using the TGA analysis with temperatures ranging from ambient temperature to 700
C at the heating rate of

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Fig. 8. TGA thermo-grams of neat PPyCBz and f-MWCNT/PPyCBz composites. Fig. 6. MALDI-MASS spectrum of pyrene core cardanol based benzoxazine (PyCBz) monomer.

10
C per minute. Fig. 8 shows thermogravimetric analysis (TGA) curve of neat PPyCBz and various weight percentages of f-MWCNT filled PPyCBz nanocomposites. The thermal decomposition stability increases with increasing weight percentages of f-MWCNT into PPyCBz matrix (Table 1). The enhanced thermal stabilities of fMWCNT/PPyCBz hybrid nanocomposites are due to the better interfacial interaction between f-MWCNT and PPyCBz material. Further, the amino group in f-MWCNT prevents aggregation and allows the chemical bonding formation between MWCNT and the polymer of PyCBz. So, this can lead to the stabilization of PPyCBz matrix and thus results enhancement in the thermal stability of fMWCNT/PPyCBz nanocomposites. The incorporation of f-MWCNT not only increases the thermal decomposition temperatures but also increase the char yields, which is due to the presence of stable graphitic structure of carbon nanotubes up to 500e600
C under thermal oxidative processes.

Fig. 7. DSC graphs of neat PPyCBZ and various weight percentages of f-MWCNT incorporated PPyCBz composites.

Further, the CNT can effectively act as physical barriers to hinder the transport of volatile decomposed products of f-MWCNT/PPyCBz nanocomposite during thermal decomposition process. So, the CNT layers exhibit a good barrier effect on the thermal degradation process, which is leading to the retardation in the weight-loss rate during the thermal degradation of f-MWCNT/PPyCBz nanocomposites. Hence, the achieved enhancement in the thermal stability of f-MWCNT/PPyCBz nanocomposites are desired properties, which can be utilized for future applications. 3.6. Dielectric properties The high dielectric polymer matrix composites received more attentions in recent years for various applications such as high charge-storage capacitors, electrostriction for artificial muscles, etc. The dielectric constants of the composites are generally increased either by reinforcing high dielectric constant fillers [41e43], or copolymerizing with conductive polymer [44e46]. So in the present investigation, the dielectric properties of the composite films have been studied as a weight percentage function of reinforcing f-MWCNT material. The dielectric constants were found to be increased with increasing the quantity of f-MWCNT in poly(pyrene cardanol benzoxazine) matrix. The difference in polarizations of PPyCBz matrix and f-MWCNT fillers may give rise to interfacial polarization. In addition, when f-MWCNT filled poly(pyrenecardanolbenzoxazine) matrix are placed in an electric field, polarization occurs due to the orientation of dipoles. So, the orientation of dipoles and interfacial polarization of the composite will mainly depend upon the concentration of fillers [47] and hence the incorporation of functionalized MWCNT into PyCBz polymer matrix improves the electrical properties of the resultant polymer composites. Further, the addition of f-MWCNT in PPyCBz matrix also influences on dielectric constant (Fig. 9). As the concentration of fMWCNT increases in the composite, the dielectric constant also increases which is due to strong interfacial adhesion between functionalized filler and poly(pyrenecardanolbenzoxazine). The absence of permanent dipole makes the polymer to have lowest value of dielectric constant. So, PPyCBz matrix is an insulating material with lower charge carrier. But the introduction of polar filler molecules like f-MWCNT increases the orientation and interfacial polarization resulting in high dielectric constant. Also the

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457

Table 1 Characteristic parameters from TGA, DSC and Dielectric Analysis. S.No.

Samples

Tg(
C)

T10(
C)

T20(
C)

T30(
C)

char yield % (800
C)

Dielectric at 313 K 1 MHz

1. 2. 3. 4. 5. 6.

Neat PCBz (39) Neat PPyCBz 0.5% f-MWCNT/PPyCBz 1.0% f-MWCNT/PPyCBz 1.5% f-MWCNT/PPyCBz 2.0% f-MWCNT/PPyCBz

90.1 148.7 150.3 153.7 156.4 160.9

348 378 388 390 394 413

378 418 423 427 432 448

405 443 448 449 457 473

12.24 30.08 31.88 34.91 37.80 39.28

3.52 3.60 4.29 5.38 7.20 9.87

Table 2 Minimum inhibitory concentration of test compounds by resazurin reduction assay. MIC mg/mL Compounds

Bacterial pathogens Streptococcus Bacillus Staphylococcus Escherichia pyogens cereus aureus coli

Neat PPyCBz (5a to 5d) 0.5% f-MWCNT/PPyCBz (1a to 1d) 1.0%f -MWCNT/PPyCBz (2a to 2d) 1.5% f -MWCNT/PPyCBz (3a to 3d) 2.0% f -MWCNT/PPyCBz (4a to 4d) Streptomycin

Fig. 9. Dielectric constant of a neat PPyCBz and different weight percentages of fMWCNT/PPyCBz composites.

internal plasticization effect of long alkyl side chain of cardanol may reduce the rigidity and thus increases the dielectric constant [48]. Further, as the concentration of filler like f-MWCNT increases, the conductivity is also increasing, which is due to introduction of more polar groups with high crystallinity. This makes the composite systems to cross the percolation threshold energy and thus

Fig. 10. Dielectric losses of neat PPyCBz and various weight percentages of f-MWCNT/ PPyCBz composites.

25 25

25 25

50 50

25 12.5

12.5

25

25

12.5

12.5

12.5

12.5

6.25

6.25

12.5

12.5

6.25

3.12

3.12

6.25

3.12

increases the conductivity. In addition, the increase in dielectric constant can be directly related to antimicrobial activity of the system and the dielectric measurements provide supporting evidence for the antibacterial improvement of the resulting f-MWCNT/ PPyCBz composites. Fig. 10 shows the dielectric loss of the nanocomposites and it indicates that the increasing weight percentage of f-MWCNT in PPyCBz matrix will decrease the value of dielectric loss. The factors like direct current (DC) conduction, space charge migration, and the movement of molecular dipoles contribute dielectric loss. So, the decrease in value of dielectric loss from neat PPyCBz to different weight percentage of f-MWCNT reinforced PPyCBz were noticed,

Fig. 11. XRD of f-MWCNT and various weight percentages of f-MWCNT/PPyCBz composites.

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Fig. 12. SEM micrograph of (a) Neat PPyCBz (b) 0.5% f-MWCNT/PPyCBz (c) 1% f-MWCNT/PPyCBz (d) 1.5% f-MWCNT/PPyCBz and (e) 2% f-MWCNT/PPyCBz nanocomposites.

which may be due to the controlled migration of space charges for the formation of insulating layer of CNT on f-MWCNT/PCBz composites. The incorporation of amine functionalized MWCNT in the PPyCBz matrix improves the dispersion as well as enhanced interfacial adhesion of the nanocomposites, which may further restrict the movement of the molecular dipoles. Thus, f-MWCNT/ PPyCBz nanocomposites are expected to possess very good dielectric behavior (Table 1) and are best suitable for high performance dielectric applications.

3.7. Antimicrobial properties 3.7.1. Minimum inhibitory concentration (MIC) The resazurin dye reduction assay was used to determine the minimum inhibition concentration (MIC) of the compound [49]. The enzyme oxido-reductase present inside the microbial cells converted the resazurin to resorufin, which is pink in color. When the color of the dye remains blue then it indicates that there is no activity on the viable cells. The change of resazurin solution color from blue to pink indicates that the microbial cells are viable. So, the sample compound (f-MWCNT/PPyCBz composite) was added to kill the bacterial cells during incubation. The antimicrobial activities of the compound were determined by the blue or purple color of dye in the respective wells. The pink color formation in the wells even after treating with test compounds or commercial drug indicates the presence of viable cells. Thus, the least dilution in which the color remains blue was taken as the MIC value of the respective compound. The array of 96 sterile plates resazurin reduction assay

for minimum inhibitory concentration test compounds is given as supporting information. In this regard, the synthesized compound is subjected to MIC studies against various microorganisms and its MIC values with respect to different microorganisms are given in Table 2. Both Gram positive and Gram negative bacteria are susceptible to the test f-MWCNT/PPyCBz compound. The results conclude that the test compound possesses good antibacterial activities and further research on these compounds is required to know its mode of actions. The toxicity of the compounds may be due to the electrostatic interaction between the microbial cell membrane and the composites. The MIC results conclude that incorporation of f-MWCNT into PPyCBz polymer matrix leads to increase in antimicrobial activity upto 1.5 wt percentage of f -MWCNT filled PPyCBz matrix. However, 2 wt percentage of f-MWCNT filled PPyCBz shows no significant activity against both gram positive and gram negative bacteria except Streptococcus pyogens (among the microorganism undergone the MIC test). The exact reason for such a enhancement is not yet to be unknown and research has to undergo for such predictions.

3.8. X-ray diffraction analysis X-ray diffraction (XRD) patterns of the neat PPyCBz material and various weight percentages of f-MWCNT incorporated f-MWCNT/ PPyCBz composites are shown in Fig. 11. The characteristic diffraction peaks corresponding to f-MWCNT were appeared at the angles of 2q ¼ 21.6
(d002) and 43.24
(d100) for f-MWCNT/PPyCBz

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Fig. 13. TEM micrographs of f-MWCNT/PPyCBz nanocomposite.

nanocomposites. Thus, this pattern of diffraction peaks confirms the reinforcement of f-MWCNT in the poly(pyrenecorecardanolbenzoxazine) matrix, which indicates the successful formation of nanocomposites.

3.9. Morphological properties of f-MWCNT/PPyCBz nanocomposites The surface morphologies of the neat PPyCBz and different compositions of f-MWCNT/PPyCBz hybrid systems were investigated by the SEM analysis (Fig. 12). Generally, two types of agglomerates are possible in the case of CNT/polymer composites. The primary type is at the time of synthesis of CNTs and the secondary agglomeration may be at the time of composite preparations. But the ultrasonification and amine functionalization of MWCNT reduces the agglomeration to a greater extent. The SEM pictures evidences the homogeneous dispersion and distribution of the filler material in the polymer matrix without any phase separation of PPyCBz matrix resulting a smooth and homogeneous surface morphology of f-MWCNT/PPyCBz composites, which has great influence over thermal and electrical properties. Further, the use of functionalized CNT increases the bonding and adhesion of CNT with the polymers due to enhanced interfacial interaction, which results in smooth and homogeneous distribution of filler in the matrix (SEM images). The distribution and the dispersion of f-MWCNT within the polymer nanocomposites after curing process were examined by TEM analysis. The images of f-MWCNT reinforced poly(pyrene core cardanol based benzeoxazine) nanocomposite is shown in Fig. 13. From Fig. 13, it was clear that f-MWCNTs are well distributed and

dispersed within the polypyrene core cardanol based benzoxazine matrix, which is due to the amino group functionalization of MWCNT and presence of pyrene moiety in polybenzoxazine unit along with cardanol moieties. 3.10. Optical properties 3.10.1. UV-Vis absorption spectra The UV-Vis absorption studies of the neat PPyCBz and different weight ratios of f-MWCNT/PPyCBz nanocomposites are given in Fig. 14. From Fig. 14, it was noticed that the neat PPyCBz displays one small peak at 238 nm and two prominent bands with unsymmetrical shape at 325 nm and 409 nm. The f-MWCNT incorporated PPyCBz nanocomposites show the bands at the higher absorption wavelength (448 nm) compared to the neat PPyCBz (409). The shift in the UV-Vis spectra of the f-MWCNT/PPyCBz nanocomposites indicate the chemical interactions and compatibility between f-MWCNT and PPyCBz matrix. Further, the f-MWCNT incorporated PPyCBz nanocomposites display no significant change in the peak observed at 325 nm. 3.10.2. Photoluminescence spectra The emission spectra were measured with an excitation wavelength of 425 nm for neat Poly(pyrenecorecardanolbenzoxazine) and various weight percentages of f-MWCNT/PPyCBz composites in THF solvent. Fig. 15 shows the emission spectra of neat PPyCBz and various weight percentages of f-MWCNT/PPyCBz composites in THF solvent. The newly synthesized polymer in THF solution emits fluorescence with a prominent fluorescence emission at 464 nm

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composites. 4. Conclusion In this study, a new pyrene core cardanol based benzoxazine monomer was synthesized by solventless method. Various weight percentages of amine functionalized f-MWCNT incorporated poly (pyrene core cardanol based benzoxazine) nanocomposites were successfully prepared by handy process. The Tg values of the fMWCNT/PPyCBz nanocomposites significantly increased with respect to increase in the weight percentage of f-MWCNT. The presence of para phenylene diamine favours good dispersion of MWCNT, which is evidenced from SEM and TEM images. The thermal stability of the nanocomposites were increased by the addition of f-MWCNTs with enhanced char yield. The nanocomposites also exhibit good antimicrobial activities against both gram positive and gram negative bacteria. The polymer and its composite also possess photoluminescence and absorption properties. Thus, the prepared f-MWCNT/PPyCBz composite may find wide application in electronic and optical field in addition to antimicrobial property. Fig. 14. UV-Vis absorption spectra of various weight percentages of f-MWCNT/PPyCBz composites.

Acknowledgments The authors like to thank DST/Nanomission, New Delhi, India for financial support to carry out this work and for the establishment of Nanotech Research Lab through grant No. SR/NM/NS-05/2011(G). References

Fig. 15. PL spectra of neat PPyCBz and various wt.% of f-MWCNT/PPyCBz composites in THF solvent at a excited wavelength of 425 nm.

and a shoulder peak at 535 nm. Similarly, the various weight percentages of f-MWCNT filled PPyCBz composites exhibit a major fluorescence emission at 486 nm along with one shoulder peak. From the fluorescence spectra (Fig. 15), it was noticed that the shift in the PL spectra of f-MWCNT/PPyCBz composites were observed when compared to neat PPyCBz matrix, which authenticate the chemical bonding interaction between f-MWCNT and PPyCBz matrix. Moreover, the presence of pyrene core along with cardanol moiety causes the fluorescence property to newly formed neat benzoxazine polymer. Further, the introduction of f-MWCNT into the Poly(pyrene corecardanol benzoxazine) matrix will shift the emission peak from at 464 nme486 nm. Thus, these newly developed Poly(pyrenecorecardanolbenzoxazine) and f-MWCNTPoly(pyrenecorecardanolbenzoxazine) nanocomposites may perhaps have the potential use in optoelectronic devices. Hence, this experiment provides evidence for the composites formations and information about optical properties of the neat polymer and

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