Materials Research Bulletin 46 (2011) 1679–1685

Synthesis, characterization, UV and dielectric properties of hexagonaldisklike ZnO particles embedded in polyimides

Sema Vural a, Suleyman Koytepe a, Turgay Seckin a,*, Ibrahim Adıguzel b

a Inonu University, Chemistry Department, 44280 Malatya, Turkeyb Inonu University, Physics Department, 44280 Malatya, Turkey

A R T I C L E I N F O

Article history:

Received 17 February 2010

Received in revised form 6 May 2011

Accepted 27 May 2011

Available online 23 June 2011

Keywords:

A. Composites

A. Oxides

B. Dielectric properties

A B S T R A C T

A series of novel ZnO/polyimide composite films with different ZnO contents was prepared through

incorporation hexagonal disklike ZnO particles into poly(amic acid) of the pre polymer of the polyimide.

The hexagonal disklike ZnO particles with a diameter of 300–500 nm were synthesized from zinc acetate

and NaOH in water with citric acid. The prepared zinc oxide–polyimide composites were characterized

for their structure, morphology, and thermal behavior employing Fourier transform infrared

spectroscopy, scanning electron micrograph, X-ray diffraction and thermal analysis techniques.

Thermal analyses show that the ZnO particles were successfully incorporated into the polymer matrix

and these ZnO/polymer composites have a good thermal stability. Scanning electron microscopy studies

indicate the ZnO particles were uniformly dispersed in the polymer and they remained at the original

size (300–500 nm) before immobilization. All composite films with ZnO particle contents from 1 to

5 wt% show good transparency in the visible region and luminescent properties.

� 2011 Elsevier Ltd. All rights reserved.

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Materials Research Bulletin

jo u rn al h om ep age: ww w.els evier .c o m/lo c ate /mat res b u

1. Introduction

Metal oxide particles have been the subject of intense scientificand technological activities due to their interesting size dependentphysicochemical and optoelectronic properties and consequentlyexciting application potential [1]. However, the dispersion ofinorganic metal oxide particles into an organic polymer to formpolymer composites has drawn enormous attention in recent years[1]. Combining the properties of the polymer matrix and theinorganic filler creates a new, economic way to obtain desired high-performance materials [2]. A variety of polymer/inorganic fillercomposites that offer attractive mechanical, thermal, optical andelectric properties have been investigated extensively [3–7]. It isknown that the composite properties can also change with thedispersion state, geometric shape, and surface quality of the fillerparticles as well as the particle size. For instance, Sumita et al. [8]studied the effect of carbon black dispersion in polymer blends onthe electrical conduction properties of the composite, and thedependence of electrical properties on the shape and distribution ofthe filler particles was reported by Flandin et al. [9]. Low scale fillersare different from bulk materials and conventional micron-sizefillers due to their small size and corresponding increase in surfacearea [8–11]. It is expected that the addition of metal oxide particles

* Corresponding author at: Inonu University, Chemistry Department, Campus

Street, 44280 Malatya, TR, Turkey. Tel.: +90 4223410756; fax: +90 4223410037.

E-mail address: tseckin@inonu.edu.tr (T. Seckin).

0025-5408/$ – see front matter � 2011 Elsevier Ltd. All rights reserved.

doi:10.1016/j.materresbull.2011.05.036

into polymers would lead to unprecedented ability to control theelectrical properties of filled polymers [12,13]. Especially, there hasbeen interest in applying zinc oxide (ZnO) particles to a shortwavelength photonic device based on its wide band-gap electricalproperty and exciton Bohr radius in the range of 1.4–3.5 nm [14].ZnO is a commercially important material used in paints, protectivecoatings for metals, rubber proceeding and sunscreens because it isabundant and nontoxic. In the past decade, ZnO structures rangefrom, but not limited to, quantum dots, nanorods, nanowires,nanocombs, to tetrapods [15,16]. These nanostructures withdifferent morphologies are typically produced using variousprocesses such as polyol method [17], thermal decomposition[18], thermal evaporation [19] and other soft chemical solutionmethods [20]. ZnO nanowires and nanoparticles can possess novelelectronic and optical properties with uses as room temperatureutltraviolet (UV) lasers, field-effect transistors, photodetectors, gassensors and solar cells [21–27].

The use of polyimides as a printed circuit board substratematerial or dielectric material is well known. The advantage ofpolyimides include high temperature stability, low moistureabsorption, and outstanding chemical resistance even at elevatedtemperatures. Additionally, the dielectric constant varies remark-ably little over a wide range of temperatures and frequencies. Thus,polyimides and composites of polyimides are uniquely suited forhigh-speed digital and high frequency printed circuit boards.However, reduced crosstalk makes possible higher circuit densities.In addition, they are distinguished from other high-performancepolymers by the solubility of their poly(amic acid) precursor form,

S. Vural et al. / Materials Research Bulletin 46 (2011) 1679–16851680

which can be cast into uniform films and quantitatively converted topolyimide [28–30]. Owing to all of these properties, polyimides asorganic matrix has recently been used to prepare polyimidecomposites with improved mechanical, thermal and other proper-ties by incorporation of inorganic components [31–38].

In this study, we have prepared a series of ZnO/polyimidecomposite optical films with good thermal stability. The ZnOparticles without aggregation were uniformly dispersed in thepolymer matrix, which contributed to the high transparency of thecomposites. The disklike ZnO particles and the ZnO/polymercomposites were characterized using X-ray, Fourier transforminfrared (FTIR) spectroscopy, scanning electron microscopy (SEM),TGA and UV–Visible and photoluminescence spectroscopy. Inaddition, other aim of this work was to study the dielectricconstant of ZnO composites with ZnO content in the films. Moreimportantly, we intend to explore the possibility of incorporatingZnO particles through dispersion into the polyimide network toachieve the polyimide hybrid with lower dielectric constant (low-k). The lowest dielectric constant achieved of the ZnO-PI material(PI-10P) is 2.74 by incorporating 5 wt% ZnO (pure PI, k = 3.22). Inaddition, composite films with high ZnO content (5 wt%) are able toabsorb UV irradiation below 300 nm, indicating that thesecomposite films exhibit good UV screening effects.

2. Experimental

2.1. Materials

Chemicals of high purity were obtained from various commer-cial sources, which consisted of pyromellitic dianhydride (PMDA;Aldrich), Zn(CH3COO)2 (Merck), NaOH (Acros), 2,4-diamino-1,3,5-triazine (DTA, Acros), and N-methyl-2-pyrrolidone (NMP; Aldrich).NMP was purified by distillation under reduced pressure overcalcium hydride and stored over 4 A molecular sieves. Otherorganic solvents were purified by vacuum distillation. PMDA wasdehydrated by drying under a vacuum at 170 8C for 24 h. The otherreagents were used as received.

2.2. Measurements

The samples were characterized by X-ray diffraction (XRD) forthe crystal structure, average particle size and the concentration ofimpurity compounds present. Rigaku Rad B-Dmax II powder X-raydiffractometer was used for X-ray diffraction patterns of thesesamples. The 2u values were taken from 208 to 1108 with a step sizeof 0.048 using Cu Ka radiation of wavelength 2.2897 A.

Infrared spectra were recorded as KBr pellets in the range 4000–400 cm�1 on an ATI UNICAM systems 2000 Fourier transformspectrometer. Differential scanning calorimetry (DSC), differentialthermal analysis (DTA) and thermogravimetry (TG) were per-formed with Shimadzu DSC-60, DTA-50 and TGA-50 thermalanalyzers, respectively.

Chemical composition analysis by EDAX was performed withRonteck xflash detector analyzer associated to a scanning electronmicroscope (SEM, Leo-Evo 40xVP). Incident electron beamenergies from 3 to 30 keV had been used. In all cases, the beamwas at normal incidence to the sample surface and the measure-ment time was 100 s. All the EDAX spectra were corrected by usingthe ZAF correction, which takes into account the influence of thematrix material on the obtained spectra.

Dielectric constants were measured by Agilent Technologies4294A Precision Impedance analyzer at 1–1000 kHz. The diameterof the electrode and the film was 20 mm [39]. To ensure goodelectrical contact between the electrodes and the polyimide film,prior to measurement the films were sputter-coated for 20 s onboth sides with a silver layer of around 0.05 mm. The dielectric

constants of the film were determined over the frequency range of100–107 Hz at room temperature.

UV–Vis and photoluminescence spectra of the ZnO particleswere measured at room temperature under carbon tetrachloridedispersion on a Perkin Elmer UV–Vis and Photoluminescencespectrometers.

2.3. Synthesis of hexagonal disklike ZnO

The hydrothermal process was carried out to product hexagonaldisklike ZnO particles [40]. In a typical synthesis: 0.5 gZn(CH3COO)2 was put into 100 mL water under stirring. After10 min stirring, 0.5 g citric acid was added into the solutions. Afterthe dissolution of citric acid, 20 mL 2 M NaOH solution wasintroduced into the aqueous solution, resulting in a clear aqueoussolution (pH value was equal to 13). And then the solution wastransferred into Teflon lined stainless steel autoclave, which wassealed and maintained at 160 8C for 20 h. After the reactioncompleted, the resulted white solid product was centrifugalized,washed with distilled water and ethanol to remove the ionspossibly remaining in the final product, and finally dried at 60 8C inair.

2.4. Synthesis of polyimides

Polyimides were synthesized by reacting DTA with PMDA asshown in Scheme 1. The reactor was purged with dry nitrogen for30 min. 2,4-Diamino-1,3,5-triazine (DTA) (0.01 mol) and NMP(30 mL) was charged into the reactor and stirred until all diaminewas dissolved in NMP. PMDA 2.18 g (0.01 mol) was charged inseveral portions and stirred at room temperature for 2–3 h to obtaina viscous poly(amic acid) (PAA) solution. Xylene (15 mL) was addedand the mixture was heated to 170–180 8C at reflux for 12 h until thewater was azeotropically distilled off via a Dean–Stark trap. Heatingwas continued to distill off the residual xylene. After completion ofpolymerization, reaction mixture was kept at 175 8C for 3 h, theresulting viscous solution was poured into a large excess ofmethanol and filtered. The precipitated polymer was washedseveral times with water and methanol, and then the polymerobtained was dried at 100 8C for 12 h in vacuum. Polymerizationyield was 85%. The inherent viscosity of PI was 0.85 dL/g measured ata concentration of 0.5 g/dL with NMP as the solvent at 30 8C.

2.5. Preparation of the ZnO/PI composite films

PI/ZnO composites were prepared with different weightpercentages of hexagonal disklike zinc oxide (1, 3, and 5 wt%)(Scheme 2.). The details of the method as follows: 0.01 mol of 2,4-diamino-1,3,5-triazine (DTA) and 0.01 mol of dianhydride in 30 mLNMP gave viscous gel of poly(amic acid), which is used for theexperiment. Different weight percent of zinc oxide (1, 3, and 5 wt%)were sonicated for an hour in poly(amic acid) solution and thesuspension was stirred for 2 h at room temperature under the flowof nitrogen followed by successively heating at 80, 120, 150, 200,and 300 8C each for 1 h to reflux for thermal imidization. Theproduct was isolated by filtration. It was washed with distilledwater three times, than washed with methanol and finally dried at110 8C for 8 h. The polyimide/ZnO composite films with differentZnO contents were obtained.

3. Results and discussion

3.1. Characterization of the hexagonal disklike ZnO

Hexagonal disklike morphologies of ZnO structures areobtained in citric acid assisted hydrothermal process. The

Scheme 1. Synthesis of the polyimide (DTA-PI).

S. Vural et al. / Materials Research Bulletin 46 (2011) 1679–1685 1681

hexagonal disklike ZnO with hexagonal (wurtzite) structure werecharacterized by X-ray powder diffraction (XRD), scanningelectron microscopy (SEM), UV–Vis absorption and FTIR spectros-copy techniques.

FTIR spectra of hexagonal disklike ZnO particle is shown inFig. 1(a). There is a broad band with very low intensity at3493 cm�1 corresponding to the vibration mode of water OHgroup indicating the presence of small amount of water adsorbedon the ZnO particle surface. The band at 1628 cm�1 is due to theOH bending of water. A strong band at 490 cm�1 is attributed tothe Zn–O stretching band. Fig. 1(b) shows the absorption

Scheme 2. Synthesis of the ZnO/polyimide composites.

spectrum of hexagonal disklike ZnO particles in ethanol solution.The ZnO particle spectrum exhibits an obvious blue-shiftexcitation band at around 320 nm compared with that of bulkZnO (373 nm).

Figure 2 shows DTA and TGA thermograms of hexagonaldisklike ZnO materials. The XRD pattern of hexagonal disklike ZnOparticles also confirms this result (Fig. 3(a)). All diffraction peakscorresponding to (1 0 0), (1 0 1), (1 0 2), (1 1 0), (1 0 3) and (1 1 2)planes are in agreement with the typical wurtzite structure of bulkZnO. The typical SEM images of the above-mentioned ZnOstructures are shown in Fig. 3. Large quantities of hexagonal

Fig. 1. FTIR (a) and UV (b) spectrum of hexagonal disklike ZnO.

Fig. 2. DTA and TGA thermograms of hexagonal disklike ZnO.

S. Vural et al. / Materials Research Bulletin 46 (2011) 1679–16851682

disklike ZnO structures are observed from Fig. 3(b). An individualhexagonal disklike ZnO structure with the diameter of severalmicrometers and several hundred nanometers in thickness isobserved from the magnified SEM image shown in Fig. 3(c).

3.2. Characterization of the hexagonal disklike ZnO/PI composites

Fig. 4 shows the FTIR spectra of pure polyimide and ZnO/polymer composite films with different contents of ZnO. All thecomposite films exhibit the characteristic absorption peaks of C55Ostretching in imide groups (1778 and 1726 cm�1) and C–Nstretching in imide groups (1374 cm�1). The absorption peaks ofthe polymer matrix at 1778 and 1726 cm�1 cover the characteristicvibrations of ZnO particle surface. However, it is also found that theabsorption bands of ZnO surface gradually increase in intensitywith increasing ZnO particle content, compared with the intensityof the absorption bands of imide groups at 725 cm�1. This indicatesthat an increasing amount of ZnO is immobilized into the polymermatrix.

Fig. 3. X-ray spectrum (a) and SEM images

Fig. 5 shows the DTA thermograms of the ZnO/polymercomposite films with different ZnO contents. Two small and onelarge exothermic peak was found at 420 8C, 500 8C and 580 8C,respectively. The first and the second peak were accompanied bythe crystallization of the ZnO particle in polymeric structure [40].The last peak may be by the combustion of polyimide. The thermalstability of the polyimide and PI-hexagonal disklike ZnO compo-sites was evaluated by DTA. The thermal measurements werecarried out under air atmosphere at a heating rate of 10 8C/min. Itcan be seen that the decomposition temperature increase withincreasing ZnO content in the ZnO/polymer composite. We canreasonably conclude that interaction of hydrogen bond or othercoordinate bonds between ZnO inorganic network formed andpolymeric chains, restrict the thermal action of macromolecules,increasing the rigidity of macromolecular chain and enhancing theenergy needed by polymeric chain movement and breakage [25].Usually, the properties of the polymer composites containinginorganic spherical particles depend on many factors, such as thesize and dispersion of the particles, the interaction between theparticles and the polymer chains, and the properties of the polymermatrix and the particles. It has been shown in the literature thatZnO/PI composites may have different thermal properties depend-ing on the preparation methods. Direct blending of nanosized ZnOwith polymer does not show a significant improvement in thermalstability [41]. On the other hand, ZnO/PI composites preparedthrough in situ polymerization tend to exhibit much higherthermal stability than the neat polymer [42]. Clearly, the gooddispersion of ZnO particles in PI leads to the higher thermalstability of the composites.

Fig. 6 shows the TGA profiles of the ZnO/polymer compositefilms with different ZnO contents. The initial decompositiontemperatures of the composite films with different ZnO contents(1–5 wt%) are about 450–500 8C. These values are related to thedecomposition of pristine polymer matrix. The bare ZnO particles(Fig. 2) have lower thermal stability than the polymer matrix, sothe incorporation of ZnO particles can partially improve the

(b) and (c) of hexagonal disklike ZnO.

Fig. 5. DTA thermograms of the PI-hexagonal disklike ZnO composites.

Fig. 4. FTIR spectra of the PI and hexagonal disklike ZnO/PI composites.

Fig. 7. DSC thermograms of the hexagonal disklike ZnO/PI composites.

S. Vural et al. / Materials Research Bulletin 46 (2011) 1679–1685 1683

thermal decomposition rate of the composites, but the initialdecomposition temperature does increase with increasing ZnOcontent in the ZnO/polymer composite. In addition, the bare ZnOnanoparticles show obvious weight loss between 200 8C. Thecomposite film with ZnO contents of 1–5 wt% have high thermalstability and they increase with increasing ZnO content in thecomposite, indicating that the ZnO particles were successfullyincorporated into the polymer matrices.

Fig. 6. TGA thermograms of the hexagonal disklike ZnO/PI composites. (a) Pure

polyimide, (b) 1% ZnO, (c) 3% ZnO, and (d) 5% ZnO.

The glass transition temperature is an important property of adielectric film. Exceeding Tg the polymer may cause a largedecrease in Young’s modulus and typically results in a shift in thedielectric properties. Hence, a polymer with Tg greater or equal tothe highest processing temperature is desirable. All compositeswere subjected to DSC measurements for the purpose of examiningmiscibility. Fig. 7 shows the DSC thermograms of all compositesexhibiting only one Tg from all composition. A single Tg stronglyimplies that all these composites are homogenous. Fig. 7 shows thedependence of the Tg on the composition of these composites,increasing the ZnO content results in Tg increase than averagevalues. The introduction of nano-ZnO particles slightly increasesthe Tg values of the ZnO/PI composites. It seems that the interactionof ZnO with the highly polar matrix polymers can strongintermolecular interactions between the polymer chains [43].

Fig. 8 shows the SEM photographs of the fracture surface ofcomposite films. It can be clearly seen that the particles (ZnO) withuniformly in the polymer matrix for the hybrid films with 1 wt% ofZnO. However, the similar particles cannot be clearly observed inthe hybrid sample with high ZnO content (5 wt%). This result is ingood agreement with that of the transmittance spectra of the ZnO/polymer composite films with low ZnO content. The efficiency ofparticles in improving the properties of the polymer material isprimarily determined by the degree of dispersion in the matrix.These nanostructured ZnO in the polymer can change the thermaland optical properties of the composite. The size of the ZnOparticles in the PI matrix corresponds to that of primary particles,except a very few agglomerates [44]. This may be that the ZnOcontent is very high in polymer and aggregation of ZnO particles inthe composites.

The XRD characterization results are depicted in Fig. 9. The XRDpeaks, which correspond to the (1 0 0), (0 0 2), (1 0 1), (1 1 0) and(1 0 3) basal diffraction of wurtzite ZnO phase of ZnO particles. Thepolyimide (PI) film prepared from a PAA containing 5% of ZnOshowed the most distinct peaks indicating the existence of ZnOcrystal structure in the polyimide film. It was confirmed that ZnOdissolved in PAA solution was successfully converted to ZnOparticles by thermal decomposition.

3.3. UV properties of the ZnO/PI composites

Fig. 10(i) is the UV–Vis absorption spectra of hexagonal disklikeZnO dispersed in polyimide. It shows a strong absorption a peakappears at the range of 350–200 nm. The room-temperaturephotoluminescence spectrum (PL) of the sample shown inFig. 10(ii) was measured using a 300 nm Xe lamp as the excitationlight source. Peak in Fig. 10(ii) is a strong and broad emission in theyellow-green part (540 nm) of the visible spectrum, attributed tooxygen vacancies or surface states.

Fig. 8. SEM image of the DTA-PI (a) and hexagonal disklike ZnO/PI composites 1% (b), 3% (c), and 5% (d).

Fig. 9. X-ray spectrum of the DTA-PI and hexagonal disklike ZnO/PI composites.

Fig. 10. UV (i) and room-temperature photoluminescence spectrum (ii) of

S. Vural et al. / Materials Research Bulletin 46 (2011) 1679–16851684

3.4. Dielectric characteristics of ZnO/polyimide composites

Lower dielectric constant is one of the most desirable propertiesfor next generation electronic devices. Fig. 11 gives the dielectricconstant of hexagonal disklike ZnO/PI composites vs. frequency.Hexagonal disklike ZnO/PI composites have lower dielectricconstant than the neat PI at all tested frequencies. In addition,they show a nearly constant dielectric constant over a broad rangeof frequencies. The reduction in dielectric constants can beexplained in terms of the reduction in the freedom of orientationof the ZnO particles and main-chain flexibility of the PI matrix. Thelatter can be attributed to the presence of hexagonal disklike ZnOparticles preventing the main-chain flexibility of PI molecules.Therefore, the extent of dielectric constant reduction depends onhow well ZnO is dispersed in the PI matrix. The increase of theamount of ZnO particles in hybrid composition results higherreduction in dielectric constants.

the hexagonal disklike ZnO/PI composites 1% (a), 3% (b), and 5% (c).

Fig. 11. Dielectric properties of the PI and hexagonal disklike ZnO/PI composites

(%1, %3, and %5).

S. Vural et al. / Materials Research Bulletin 46 (2011) 1679–1685 1685

4. Conclusion

In summary, hexagonal disklike ZnO have been successfullyprepared by hydrothermal synthesis using Zn(CH3COO)2 precursorwith citric acid. The ZnO/PI composite films were prepared frompoly(amic acid) and a hexagonal disklike ZnO. The ZnO particlesfrom decomposed ZnO distributes homogeneously in polyimidematrix. The strong interaction between ZnO particles andpolyimide matrix brings obvious effect on the thermal transitionbehavior and mechanical properties of composites, although thethermal stability decreases. Thus, to improve the properties ofcomposites, it is important to control the nanophase uniformdispersion in polymer matrix as well as the interaction of differenthybrid components. Photoluminescence (PL) spectrum at room-temperature shows a UV emission at 400 nm, which is likelyrelated to the emission of high concentration excitons.

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