Effect of the polyimide structure and ZnO concentration on the morphology and characteristics of polyimide/ZnO nanohybrid films

Effect of the Polyimide Structure and ZnO

Concentration on the Morphology and

Characteristics of Polyimide/ZnO Nanohybrid Films

Institute of Materials Science & Engineering, National Chiao Tung University, 1001 Ta Hsueh Road, Hsin Chu, Taiwan 300, Republic of China

Fax: 886-3-5724727; E-mail: [email protected]

Received: August 6, 2004; Revised: November 12, 2004; Accepted: November 15, 2004; DOI: 10.1002/macp.200400326 Keywords: nanocomposites; polyimides; structure-property relations; thermal properties; ZnO

Introduction

Metal or semiconductor nanoparticles dispersed in poly-meric matrixes have been widely studied recently.[1 – 5] These nanocomposites exhibit interesting properties and could be widely applied in the microelectronic and opto-electronic industries. Polyimide (PI) is a promising matrix for these nanocomposites because of its good thermal stability and chemical resistance.[5 – 7] The mechanical, electrical, and optical properties of pure PI are further improved by incorporating inorganic materials.

Zinc oxide (ZnO) has attracted much attention because of its excellent physical properties, such as a wide band gap

(3.37 eV) at room temperature and a large exciton bonding energy (60 meV). It can be used in ultraviolet light-emitting diodes, transparent electrodes, and piezoelectric devices etc.[8,9] Many different methods have been reported to fabricate ZnO nanocrystals. Spanhel and Anderson[10] re-ported a simple sol-gel method to prepare quantum size ZnO particles. To obtain a sharp size distribution, the use of surfactant- or polymer-stabilized ZnO nanoparticles has been reported.[11 –13]

Several ZnO/organic composites have been reported, such as poly(ethylene glycol),[14]low density polyethylene,[15] poly(ethylene oxide),[16]Nylon-6, and poly(styrene butyl acrylate).[17]Our laboratory successfully prepared highly Summary:A series of polyimide/ZnO nanohybrid films with

different ZnO content were prepared from a rigid pyromel-litic dianhydride-4,40-diaminodiphenyl ether (PMDA-ODA) polyimide (PI) and a flexible 3,30,4,40 -benzophenonete-tracarboxylic acid dianhydride-4,40-diaminodiphenyl ether (BTDA-ODA) PI with ZnO nanoparticles (3–4 nm). Fourier-transform infrared (FT-IR) and X-ray photoelectron spectro-scopy (XPS) depict that the ZnO nanoparticles function as a physical cross-linking agent with PI through hydrogen bonding between the OH on the ZnO nanoparticles and the C O of the imide groups. ZnO nanoparticles in the rigid PMDA-ODA matrix cause a larger percentage decrease in the coefficient of linear thermal expansion (CTE) than in the flexible BTDA-ODA matrix. The BTDA-ODA/ZnO hybrid films have two transition peaks in dynamic mechanical tan d curves, but PMDA-ODA/ZnO hybrid films only have one transition peak. Thermogravimetric analysis reveals that ZnO decreases the thermal degradation temperature (Td) in both

hybrid films, but less so in PMDA-ODA/ZnO films. Trans-mission electron microscopy (TEM) images reveal that the rigid matrix induces larger particle size (30–40 nm) com-pared to the flexible matrix (10–15 nm).

Illustration of the interaction between ZnO nanoparticles and PI.

transparent and stable luminescent ZnO/poly(hydroxyethyl methacrylate) nanocomposites.[18] However, the inferior thermal stability of organic matrices may restrict a com-posite’s application in the electronic and optoelectronic industries. For electronic and optoelectronic applica-tions, high reliability usually requires a highly thermal stable polymer matrix. PI is a well-known thermally stable polymer. In this study, we chose two different PIs, 3,30,4, 40-benzophenonetetracarboxylic acid dianhydride-4,40 -diaminodiphenyl ether (BTDA/ODA) and pyromellitic dianhydride-4,40-diaminodiphenyl ether (PMDA/ODA), as host polymers to synthesize a series of PI/ZnO nano-composites with different ZnO content. The first PI is more flexible than the second PI. We intend to study the effect of PI characteristics on the size of ZnO in PI and the charac-teristics of the PI/ZnO nanohybrid films. First, quantum-dot ZnO nanoparticles with a modified surfactant are prepared. The ZnO is then introduced to the poly(amic acid) PI precursors, and finally, the composite is thermally imidized to form PI/ZnO nanohybrid films. The thermal, mechanical, and morphology characteristics of the hybrid films are investigated to understand the effect of the ZnO content and PI structure on these characteristics. In addition, the effects of polyimide structure on the size of ZnO and the structural variation of ZnO nanoparticles before and after thermal imidization are reported.

Experimental Part

Materials

Zinc acetate dihydrate (99.0%) from Showa, lithium hydroxide monohydrate (99.0%) from TEDIA, 3-(trimethoxysilyl)propyl

methacrylate (TPM, 98%) from Aldrich, absolute ethanol (99.5%) and dimethyl sulfoxide (DMSO) from Nasa, 4, 40-diaminodiphenyl ether (ODA, 98%), 3,30,4,40 -benzopheno-netetracarboxylic acid dianhydride (BTDA, 99%), and pyro-mellitic dianhydride (PMDA, 99%) from TCI, were used as received without further purification.

Synthesis of ZnO-TPM Nanoparticles

ZnO nanoparticle colloids with an average particle size of 3.2 nm were first prepared from zinc acetate dihydrate, lithium hydroxide monohydrate, and absolute ethanol according to the method of Spanhel and Anderson.[10]The ZnO nanoparticles produced were further stabilized by adding TPM. The TPM (molar ration of TPM to ZnO was 1:10) was diluted in 10 mL ethanol, and then added dropwise to the ZnO nanoparticle colloids under continuous stirring. The reaction proceeded for 12 h. The synthetic TPM-stabilized ZnO-nanoparticle solution was filtered through a 0.1 mm glass fiber filter. The nanopar-ticles were washed with heptane/ethanol several times. Finally, the solvent (ethanol) of the TPM-stabilized ZnO solution was replaced by the same amount of DMSO using rotary evapora-tion. The details of the synthesis route and characterization of the stabilized ZnO has been reported in our previous research paper.[18]

Preparation of the PI/ZnO Nanocomposities

The procedures for preparing poly(amic acid) and PI/ZnO nanocomposites are shown in Figure 1. The polycondensation was carried out in a flask by adding the diamine ODA (2.002 g, 0.01 mol) and the dianhydride BTDA (3.222 g, 0.01 mmol) in DMSO (30 mL) under a nitrogen stream at room temperature. BTDA was added to the solution in five portions. After the dissolution of all BTDA, the reaction mixture was further

stirred for 2 h at room temperature. Various amounts of the TPM-stabilized ZnO DMSO solution were added dropwise to the PAA solution and further stirred for 12 h. The modified and unmodified PAA solutions were cast on glass plates, and then step-heated at 100, 150, and 200 8C, for 1 h at each temperature, and finally at 300 8C for 2 h. The PI/ZnO nanohybrid films have an average thickness of 30–35 mm. The hybrid films of the PMDA series were prepared in a similar manner.

Measurements

Thermogravimetric analysis (TGA) was used to characterize the thermal stability of the polyimides with a TA Instruments TGA 2950 at a heating rate of 20 8C
min1_{from 30–900 8C}

under nitrogen. The coefficient of the linear thermal ex-pansion (CTE) was measured by thermomechanical analysis (TMA) with a TA Instruments TMA 2940 at heating rate of 10 8C
min1. The glass transition temperature (Tg)

measure-ment with dynamic mechanical experimeasure-ments was performed using a DuPont DMA Q800 Dynamic Mechanical Analyzer at 5 8C
min1_{and a frequency 1 Hz. Fourier-transform infrared}

(FT-IR) spectra were measured with a Nicolet Prote´ge´ 460. XPS spectra were obtained by using an ESCA PHI 1600 spectrometer working in the constant analyzer energy mode with a pass energy of 50 eVand Mg Ka(1 253.6 eV) radiation as

the excitation source. XPS analysis was done at room temper-ature and pressures below 1010Torr. The take-off angle used in the XPS measurements was 908. Transmission electron microscopy (TEM) imaging was performed using a JEOL-200FX transmission electron microscope.

Results and Discussion

Infrared spectra of BTDA/ODA pure PI and BTDA-ODA/ ZnO nanocomposites are shown in Figure 2(b)–(e). In Figure 2(c)–(e), all the hybrid films exhibit the charac-teristic absorption peaks of imide groups at 1 772, 1 719, and 1 380 cm1. The former two peaks are caused by

asymmetric and symmetric C O stretching and the last peak is associated with the C–N stretching. The intensity of the absorption band at 3 200–3 700 cm1increased with increasing ZnO content and this may be related to different states of hydrogen bonding. The characteristic imide peaks (C O) of these hybrid films are shifted towards lower wavenumbers compared to that of the pure PI (Figure 2b). Hydrogen-bond formation between the C O of the imide group and the OH group in ZnO nanoparticles can account for the phenomenon.[19 – 21]However, the Si–O–Si or Si–O absorption at 1 200–1 050 cm1in the TPM-stabilized ZnO nanoparticles (Figure 2a) was also found in the PI/ZnO hybrid films.

Figure 3 shows the XPS wide-scan spectra of the BTDA-ODA/ZnO-5 wt.-% hybrid film. It shows carbon, oxygen, and zinc characteristic peaks. This confirms that ZnO does appear in the PI hybrid film. But with the different sputtering time needed to detect ZnO at different depths, no Si signal was ever observed in the Si core-level spectra. That is, Si was not able to be detected on the surface and in the bulk of the hybrid film. Two possibilities can account for this: first, the TPM content on the surface of the hybrid films is too low to detect; and/or the surface TPM network decomposes during thermal imidization. In our previous study,[18]the 3-(trimethoxysilyl)propyl methacry-late groups of TPM yield a thin layer of an organic silica nanonetwork capping the ZnO nanoparticles by hydrolysis-condensation reactions.

Figure 4 shows thermogravimetric profiles of the un-modified ZnO nanoparticles and TPM-stabilized ZnO nanoparticles. Both samples show obvious weight loss at temperatures between 300 and 700 8C. This mass loss of the unmodified ZnO nanoparticle is attributed to desorption of acetate anions or acetic acid at the surface of the ZnO nanoparticle.[20]However, TPM-stabilized ZnO nanoparti-cles shows greater weight loss, 5 wt.-% at 300–700 8C, in comparison with unmodified ZnO nanoparticles. We further analyze the isothermal gravimetric profiles of both

Figure 2. FT-IR spectra of the BTDA-ODA/ZnO hybrid films.

Figure 3. XPS wide-scan spectrum of the BTDA-ODA/ZnO-5 wt.-% hybrid film.

samples at 300 8C for 2 h under similar conditions to the imidization. The data reveals that the TPM-stabilized ZnO nanoparticle show a 1 wt.-% larger loss than the unmodified ZnO nanoparticles. Clearly, part of the TPM network (about 20%) decomposes during the imidization at 300 8C for 2 h, but a significant portion of the TPM network stabilizer (about 80%) remains after the imidization.

Figure 5 shows core-level spectra of Zn 2p in the hybrid films with different Zn content. There are two Zn peaks identified; one at 1 019 eV corresponding to Zn 2p3/2, and the other at 1 042 eV is attributed to Zn 2p1/2. The peaks are slightly shifted toward lower energies compared to the spectra of ZnO bulk (1 022 and 1 046 eV corresponds to the Zn 2p2/3 and Zn 2p1/2, respectively).[9] This shift can

be attributed to the different chemical environment on the surface of ZnO nanoparticles[5,19]from that of the bulk. In the polymer hybrid, the organic polymer component and the adsorbed organics of low surface energy dominate the surface. This is particularly the case in smaller nanoparti-cles. This can cause a change in the binding energy of core electrons in Zn atoms.

According to the results of FT-IR spectroscopy, XPS, and TGA, the probable structure of the PI/ZnO nanocomposites can be inferred and is shown in Figure 6. The exposed OH groups on the ZnO nanoparticle surface can bond to the imide group (C O) of PI through interchain hydrogen bonding to form physical cross-linking. This creates a PI-ZnO interfacial domain with higher Tg.

The coefficient of thermal expansions (CTE) of the PI/ZnO nanohybrid film are listed in Table 1. Both series of the nanohybrid films show the CTE decreasing with in-creasing ZnO content. In general, the CTEs of the polymer are related to main chain free volume and relaxation.[2]For

Figure 4. Thermogravimetric profiles of the unmodified ZnO nanoparticle and TPM-stabilized ZnO nanoparticle. The inset shows the thermogravimetric profiles of both samples measured at 300 8C isothermally for 2 h.

Figure 5. Zn 2p core-level spectra of various BTDA-ODA/ZnO hybrid films with different ZnO content.

Figure 6. Illustration of the interaction between ZnO nanopar-ticles and PI.

Table 1. The coefficient of thermal expansion of pure PI and PI/ ZnO nanohybrid films.

ZnO content BTDA/ODA PMDA/ODA

wt.-% CTEa) _CTE decrement CTEa) _CTE decrement ppm
K1 % ppm
K1 % 0 42.5 – 32.5 – 1 40.6 4 25.5 21 3 35.7 16 17.3 47 5 30.8 12 –b) –

a)_{The CTE values are determined from 50 to 250 8C.} b)_{The hybrid film was too brittle to be measured.}

our case, ZnO nanoparticles dispersed in the PI matrix reduce the free volume of the molecular structure, and further restrict the segmental relaxation. In addition, the PMDA/ODA hybrid films show a larger CTE decrement than the BTDA/ODA hybrid films. The PMDA/ODA molecular structure is more rigid than BTDA/ODA and has denser molecular packing and less free volume. Even a small amount of 1 wt.-% ZnO nanoparticles doping signi-ficantly decreases the CTE of the PMDA-ODA/ZnO hybrid film with a decrement of 21%. The sample with 3 wt.-% of ZnO doping cause a CTE decrement up to 47%. On the other hand, the BTDA/ODA matrix contains a kink point because of the C O group in the dianhydride fragment. It disturbs chain packing and increases free volume. Hence, the percentage change in CTE in this series of hybrid film is much less than that of the corresponding PMDA-ODA/ZnO hybrid film.

The dynamic mechanical analyses of PI/ZnO hybrid films are shown in Figure 7 and 8. In Figure 7, the storage moduli of both PMDA-ODA/ZnO and BTDA-ODA/ZnO nanohybrid films increase with increasing ZnO content. In

addition, all the hybrid films have a larger storage modulus than pure PI in the test temperature range. The tan d curves of the all samples are shown in Figure 8. With increasing ZnO content, both series of hybrid films show higher transition temperatures and lower damping in comparison with the pure PI. This is attributed to the physical cross-linking through hydrogen bonding in the PI/ZnO hybrid film as shown in Figure 6. In addition, the series of BTDA/ ODA hybrid films show the tendency of two damping peaks (Figure 8a) with increasing ZnO content. This phenomenon can be further analyzed by deconvolution of tan d curves and the results are shown in Figure 9 and listed in Table 2. The tan d curve of BTDA-ODA/ZnO nanohybrid films can be resolved into two curves, tan d1 with Tg1 and tan d2

withTg2. Both Tg1and Tg2shift to higher temperature with

increasing ZnO content, and tan d1damping decreases as

well as tan d2increases with increasing ZnO content. As in

our previous report om PI/TiO2hybrid films,[6]the peaks

of tan d1and tan d2are related to the pure PI matrix and the

PI/inorganic interfacial domains, respectively. The virgin PMDA/ODA PI film is more rigid than the BTDA/ODA film

Figure 7. Dynamic mechanical storage moduli of (a) BTDA-ODA/ZnO, and (b) PMDA-ODA/ZnO hybrid films.

Figure 8. Dynamic mechanical tan d curves of the PI/ZnO hybrid films: (a) BTDA-ODA/ZnO, and (b) PMDA-ODA/ZnO.

and has a higher Tg. It gives the chance for the two tan

d peaks to overlap into one peak in the rigid PI/ZnO nanohybrid. Therefore, the series of PMDA/ODA hybrid films show only one tan d in Figure 8b.

Figure 10(a) shows the thermogravimetric profiles of the BTDA-ODA/ZnO nanohybrid films. The Tddecreases with

increasing ZnO content, and shifts from 579 (pure BTDA/

ODA) to 484 8C (5 wt.-% ZnO). The dramatic decrease in thermal stability of the hybrid films can be attributed to two reasons: metallic-compound-induced oxidation, which can oxidatively degrade PI films;[5]and the desorption of the organic molecules, which are adsorbed at the ZnO nanoparticles surfaces.[20]As shown in Figure 10(b), the PMDA-ODA/ZnO nanohybrid films show a similar phe-nomenon, but the Tddecrement is less significant, as shown

in Table 3. It seems that the rigid PMDA-ODA is more resistant to the ZnO oxidative degradation.

TEM images of TPM-stabilized ZnO nanoparticle, BTDA-ODA/ZnO-5 %, and PMDA-ODA/ZnO-5 wt.-% hybrid films are shown in Figure 11. The as-synthesized TPM-stabilized ZnO nanoparticle are spheres with a parti-cle size of 3–4 nm (Figure 11a). In Figure 11(b), the ZnO particles are of a uniform dispersion in the BTDA/ODA matrix, but the nanoparticles are bigger (10–15 nm) than the as-synthesized particles. Some are elongated. The change in size and shape can be attributed to the high tem-perature and long time of the imidization process. During the period of the thermal treatment, the silica network caps are partially decomposed and released from the surface of the ZnO nanoparticles. The less-protected particles can undergo crystal growth individually or can aggregation with

Figure 9. Deconvolution of tan d curves of BTDA-ODA/ZnO nanohybrid films.

Table 2. Glass transition temperatures (Tg’s) of pure PI and PI/

ZnO nanohybrid films determined by DMA measurement.

ZnO content 1st Transition 2nd Transition

wt.-% BTDA/ODA PMDA/ODA BTDA/ODA

Tg1 DTg1a) Tg1 DTg1 Tg DTgb) 8C 8C 8C 8C 8C 8C 0 288 – 362 – – – 1 310 þ22 405 þ43 360 þ50 3 327 þ39 420 þ58 382 þ55 5 343 þ55 –c) – 403 þ60 a)_DT

g1¼ Tg1of hybrid film Tg1of pure PI. b)_DT

g¼ Tg2 Tg1.

another growing particle resulting in change of particle size and shape.[18,20]The ZnO particles show notable aggrega-tion as well as irregular shape in the PMDA/ODA matrix, and the average particle size (30–40 nm) is much larger than the ZnO nanoparticles in the BTDA-ODA/ZnO hybrid film. The rigid structure of the PMDA-ODA causes ZnO

Figure 10. Dynamic thermogravimetric profiles of (a) BTDA-ODA/ZnO and (b) PMDA-BTDA-ODA/ZnO nanohybrid films.

Table 3. Thermogravimetric analysis of pure PI and PI/ZnO nanohybrid films.

ZnO content BTDA/ODA PMDA/ODA

wt.-% Tda) Tddecrement Td Tddecrement 8C 8C 8C 8C 0 579 – 575 – 1 520 59 553 22 3 491 88 505 70 5 484 95 487 88

a)_{5 wt.-% decomposition temperature measured under N} 2.

Figure 11. TEM images of (a) TPM-stabilized ZnO nanoparti-cles, (b) BTDA-ODA/ZnO-5 wt.-%, and (c) PMDA-ODA/ZnO-5 wt.-% hybrid films.

nanoparticle aggregation and crystal growth more readily than in BTDA-ODA, which has a kink structure and is more flexible.

Conclusion

The data from FT-IR spectroscopy, XPS, TGA, and DMA analyses infer that the morphology of PI/ZnO nanohybrid films arises from interchain hydrogen bonding between the ZnO nanoparticles and the PI matrix. The interchain hydrogen bonding, being a kind of physical cross-linking, exists in the PI-ZnO interfacial domain. Owing to this physical cross-linking, the storage modulus, CTE decre-ment, and Tgof the hybrid films can be effectively improved

in comparison with that of pure PI. The improvements on these characteristics of PMDA-ODA/ZnO nanohybrid films are much more significant than that of BTDA-ODA/ZnO nanohybrid films. The physical cross-linking structure also causes BTDA-ODA/ZnO nanohybrid films to have two Tg’s, one for the pure PI domain, the other higher Tgfor the

the PI-ZnO interfacial domain. PMDA-ODA/ZnO nano-hybrid films have only one Tgbecause the PMDA-ODA,

having a high Tg results in the two Tg’s to overlap and

become one. Although the Tdof the hybrid films are lower

than for pure PI, it is good enough for practical application. From TEM images, the ZnO particles show a uniform dispersion in the BTDA/ODA matrix, but most particles are bigger in size (10–15 nm) and some are elongated in comparison with the as-synthesized TPM-stabilized ZnO nanoparticles. This can be attributed to the high tempera-ture and long time taken for the imidization process. However, the aggregation, as well as irregular shape, of the ZnO nanoparticles are more notable in the PMDA-ODA/

ZnO hybrid film. The matrix structures significantly affect the morphology and characteristics of the PI/ZnO nano-hybrid films.

Acknowledgements: The authors gratefully acknowledge the National Science Council of the Republic of China NSC 92-2216-E-009-003 and the Lee and MTI Center in NCTU for financial support of this work.

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