Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 2021–2028

Polyimide/kaolinite composite films: Synthesis and characterization ofmechanical, thermal and waterproof properties

Xiumei Qiu a, Hongquan Wang a, Chunyu Zhou a, Dan Li a, Yi Liu b, Chunjie Yan a,*a Engineering Research Center of Nano-Geomaterials of Ministry of Education, China University of Geosciences, Wuhan 430074, Chinab Faculty of Material Science and Chemistry, China University of Geosciences, Wuhan 430074, China

A R T I C L E I N F O

Article history:

Received 7 October 2013

Received in revised form 17 January 2014

Accepted 19 January 2014

Available online 8 February 2014

Keywords:

Polyimide

Kaolinite

Mechanical property

Thermal stability

Waterproof property

A B S T R A C T

Polyimide (PI)/kaolinite composites were successfully synthesized via in situ polymerization using

kaolinite–potassium acetate intercalation compound (KKAc) as the intermediate. XRD patterns, FTIR

spectrum and SEM images revealed that polyimide have partially intercalated into the layers of kaolinite

by replacing KAc molecules. The mechanical, thermal and optical properties of the composite films were

characterized by universal tester, TGA, and UV–vis spectrometer. It was found that the introduction of

modified kaolinite led to significant increase in tensile strength and elongation at break of PI matrix

when KKAc content was no more than 5 wt%. Electron microscopy characterization indicated that some

plate-like and clusters-like structure appeared in perpendicular to the fracture surface of composite

films after tensile tests other than in pure PI film, which meant stronger fracture resistance existing in

the composite films. The introduction of modified kaolinite also resulted in improved thermal stability,

slight decrease in transmittance and marked decrease in water uptake ratio at low clay content (�5 wt%).

Therefore, the polyimide/kaolinite composites have potential application in microelectronic devices.

� 2014 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V. All rights reserved.

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Journal of the Taiwan Institute of Chemical Engineers

jou r nal h o mep age: w ww.els evier . co m/lo c ate / j t i c e

1. Introduction

Recently, extensive research has been devoted to developorganic–inorganic composites owing to their good thermalstability, excellent mechanical strength, dielectric insulation,and chemical resistance, such as composite membranes [1,2].Particularly, lots of clay particles are continued to be interestingfiller materials into polymers in developing cost-effective highperformance materials, because the inclusion of clay alters thestructural, thermal and mechanical properties of the compositesand makes the composites have both the advantages of organic andinorganic materials [3–8].

Aromatic polyimide (PI) has been well-known for its excellentdielectric properties, mechanical properties, thermal stability,physical and chemical properties [9], and widely applied for films,microelectronic devices, adhesives, aerospace engineering, andmembrane [10]. Various combinations of polyimide with inorganicfillers including silica, layered silicate, graphene, alumina havebeen reported, showing that outstanding mechanical and physicalproperties can be achieved when proper contents of fine fillers areincorporated. Especially, the layered silicate clay particles, such as

* Corresponding author. Tel.: +86 027 67885098; fax: +86 027 67885098.

E-mail address: chjyan2005@126.com (C. Yan).

http://dx.doi.org/10.1016/j.jtice.2014.01.012

1876-1070/� 2014 Taiwan Institute of Chemical Engineers. Published by Elsevier B.V.

montmorillonite or mica, when the layers of clay are organic,exfoliated or intercalated [11–13], they can be well dispersed andmuch more effective to reinforce the performance of polymer. Thecomposites which prepared from polyimide and layered clay bysolution dispersion technique or in situ polymerization have beenfound to display novel properties [14–16]. It was noticed that theintroduction of well dispersed clay layers such as montmorillonite(MMT) into a polyimide matrix has been proved to be extremelyeffective in the improvement of mechanical, thermal and barrierproperties of the polyimide matrix [17,18]. The same as MMT,kaolinite is one of the most ubiquitous layered clay minerals in theearth, and it is a dioctahedral 1:1 phyllosilicate formed bysuperposition of silicon tetrahedral sheets and aluminum octahe-dral sheets. But the adjacent layers of kaolinite are held together byvan der Waals forces and hydrogen bonds to form a distinct spacebetween the layers, which is different from that of MMT and resultsin the low reactivity of kaolinite for intercalation of organicpolymer [19]. Therefore, the modification of kaolinite is necessaryfor preparing polymer/kaolinite composites, for example, theintercalation of small molecule. Nowadays, some novel polymer/kaolinite nanocomposites have been prepared by using variousintermediates such as kaolinite–urea, kaolinite–dimethylsulfoxide(DMSO), kaolinite–potassium acetate (KAc), and so on [20–22].Kaolin is readily available and cost-effective mineral clay and hasbeen used to reinforce the polymers [23]. To our best knowledge,

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X. Qiu et al. / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 2021–20282022

rare references about polyimide/kaolinite composite materialshave been reported before.

In this work, polyimide/kaolinite composite films have beenprepared via in situ polymerization, which featured the virtue oflow cost to improve the performance of polyimide. The kaoliniteparticles were intercalated by KAc firstly. The relationship betweenstructure and properties of composite films was well described.The effect of clay content on the mechanical, thermal, optical andwaterproof properties of PI/kaolinite composites were systemati-cally investigated through various techniques and for comparing,the properties of pure PI were also tested.

2. Experimental and methods

2.1. Materials

Kaolin particles used in this study were purchased fromGuangdong (China) and received as finely divided white powdersof high purity. The chemical composition of this kaolin waspresented in Table 1. Pyromellitic dianhydride (PMDA) and 4,40-oxydianiline (ODA) of chemical reagent grade were supplied byTianjin Bodi Chemical Co. (Tianjin, China). N,N-Dimethylacetamide(DMAc) of analytical grade was obtained from SinopharmChemical Reagent Co. Ltd. (China). Potassium acetate (KAc) wasprovided by Tianjin Chemical Reagent Co. Ltd. (Tianjin, China). Allagents were dried or distilled before use.

2.2. Preparation of kaolinite–KAc compound (KKAc)

Kaolinite–KAc compound (KKAc) was prepared from kaoliniteand KAc through dry grinding: 5 g of kaolinite containing 5 wt%moisture and 1.5 g of KAc were placed in an agate mortar andthoroughly grinded for 40 min at room temperature. After that,KKAc was obtained by dried the mixture in a vacuum oven at 95 8Cfor 12 h.

2.3. Preparation of pure PI film and PI/kaolinite composite films

The PI/kaolinite composite films were prepared via an in situ

polymerization. Different content of KKAc (0 wt%, 1 wt%, 3 wt%,5 wt%, 7 wt%, 9 wt% of two monomers content) were thoroughlydispersed in 45 mL DMAc with the help of vigorous stirring in a100 mL three-necked flask. To this solution, 3.00 g of ODA wasadded and then 3.32 g of PMDA was slowly placed into the mixedsolution. The mixture was stirred vigorously for about 30 h toobtain a yellow, viscous, translucent mixed poly(amide acid)/claysolution. The solution was cast onto a glass plate for naturallydrying to remove air-bubble, and then were thermally treated at60, 100, and 250 8C for 2 h separately, and 300 8C for 20 min in ahigh temperature oven heating. Finally, pure PI and PI/kaolinitecomposite films were obtained.

2.4. Measurement and characterization

X-ray diffraction (XRD) was recorded on a D8-Focus typeX-ray powder diffractometer (XRD, D8-FOCUS, Broker AXS

Table 1Chemical composition of raw kaolinite.

Component Content (%) Component Content (%)

SiO2 48.28 K2O 0.010

Al2O3 36.03 TiO2 0.70

TFe2O3 1.01 P2O5 0.20

MgO 0.033 MnO 0.003

CaO 0.13 LOI 13.60

Na2O 0.030 Total 100.026

Germany) with Cu Ka radiation. The data were collected from3 to 50 8 2u with a scan rate of 0.5 8/min. The morphologies ofsurface and cross-section of samples were observed by fieldemission scanning electron microscopy (FE-SEM, SU8010,Hitachi Japan) with an acceleration voltage of 1.0 kV, 3.0 kV,5.0 kV and 15.0 kV, respectively. A Malvern 2000 particle sizeanalyzer (Britain) was used to analyze the particle sizedistribution (PSD) of kaolin and the kaolin powder was dispersedin deionized water and vibrated 10 min before PSD analysis.Fourier Transform infrared spectroscopy (FTIR) was conductedon a Perkin-Elmer 983 FT-IR spectrometer between 4000 and400 cm�1 at 25 8C. The mechanical properties of samples weremeasured with a GP-TS2000S/10 K universal testing machine at aloading rate of 5 mm/min and a clip distance of 20 mm. The testfilms were cut into dumbbell of 50 mm � 5 mm � 0.05 mm.Thermal stabilities of films were examined by thermogravimetricanalysis (NETZSCH STA 409 PG/PC instrument, Germany),employing a heating rate of 10 8C/min from 30 to 800 8C and anitrogen flow rate of 60 mL/min. Water absorption of all filmswas measured by soaking a certain weight of sample (dimensionof 20 mm � 30 mm) in 25 mL water at 25 8C for 1 h, then 100 8Cfor 1 h and weighed. Water uptake ratios of samples werecalculated by formula (1).

Water uptake ratio ð%Þ ¼ 100ðW2 � W1ÞW1

(1)

Here, W1 and W2 are the initial dry and final wet matter weightsof films, respectively. An UV-1801 spectrophotometer wasperformed between 350 and 800 nm to measure the transmittanceof films with the same dimension of 10 mm � 20 mm andthickness of 0.05 mm.

3. Results and discussions

3.1. Characterization of kaolinite and KKAc

The particle size distribution and morphology of raw kaoliniteare shown in Fig. 1. It exhibits fine particles and narrow particlesize distribution, and the particles content of 0–1 mm is 100 wt%.The inserted FE-SEM image reveals that the particles of kaoliniteare in the range of microns and exhibit smoother surface withpseudo-hexagonal edge.

Kaolinite–KAc compound (KKAc) was prepared as an interme-diate by dry grinding from kaolinite and KAc firstly. FTIR spectra ofraw kaolinite and KKAc are demonstrated in Fig. 2. The peaksaround 1000–1050 cm�1 and 469 cm�1 are the characteristicabsorption bands of the Si–O from the silicate in kaolinite (seeFig. 2a). Three characteristic bands at 3694, 3664, and 3649 cm�1

for inner-surface hydroxyl stretching are found in raw kaolinite,

Fig. 1. Particle size distribution and FE-SEM micrograph (inserted image) of raw

kaolinite.

Fig. 2. FTIR spectra of (a) kaolinite, (b) KKAc, (c) PI/kaolinite composite film and (d)

pure PI film.

Fig. 3. XRD patterns of (a) kaolinite, (b) KKAc, (c) pure PI film and (d)–(g) PI/kaolinite

composite films containing 3, 5, 7, 9 wt% KKAc.

X. Qiu et al. / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 2021–2028 2023

which shift to 3692, 3662, and 3646 cm�1 in KKAc sample (Fig. 2b)due to the influence of interlamellar modifications [24,25].Compare KKAc with raw kaolinite (see Fig. 2b and a), twoadditional bands at 1418 and 1604 cm�1 which attributed to thevibration peaks of symmetric and antisymmetric stretch vibrationsof CH3COO– have respectively appeared in the spectra of KKAcsample [26]. The coupling of C–O vibration and O–H deformationare marked by the new band at 1346 cm�1. The band at 3606 cm�1

provides the interaction between the inner surface O–H ofkaolinite and the hydrogen bond of CH3COO–. In addition, XRDpatterns of raw kaolinite and KKAc are illustrated in Fig. 3a and b.The characteristic maxima of raw kaolinite is observed at2u = 12.368 (very sharp, intense, and narrow), which correspondsto the basal spacing of kaolinite (0.71 nm). After intercalation ofKAc, as expected, the XRD pattern of raw kaolinite is dramaticallymodified. The peak at 2u = 12.368 of original kaolinite, assigned asthe first basal peak, d001, greatly shifts to small reflection angles(2u = 6.238) during the intercalation, which is produced by thepresence of intercalated KAc (1.41 nm) [27,28]. It can be concludedthat KAc molecules have been intercalated into the layers ofkaolinite with an intercalated ratio of 90% which calculatedaccording to the reports [29,30].

3.2. Distribution of kaolinite in PI matrix

The PI/kaolinite composites were prepared on the basis of thetwo-step synthetic method of pure polyimide [31]. The solutionsystem exhibited homogeneous indicating good dispersion ofkaolinite particles in polymer solution. This phenomenon wassimilar to PI/MMT composites prepared by Li et al. [32].

Fig. 3c–g illustrates the XRD patterns of PI/kaolinite compo-sites with different kaolinite contents (0–9 wt%). On the patternsof pure PI in the range of 2u = 12–288 a broad centered at about188 occurs (Fig. 3c). The significance of this broad band is thatpure PI is amorphous. It is obvious that the peak at 2u = 6.238corresponding to the intercalation of KKAc disappears completelyin composite films (Fig. 3d–g). Furthermore, when KKAc contentis not exceeded 5 wt%, the intensity of characteristic diffractionpeak (d001) decreases sharply in Fig. 3d and e. The obviousdecrease in the basal spacing indicates that the polyimide chainshave intercalated into the layers of kaolinite by replacing KAcmolecules, which results in almost disappearing of the interlayerspacing (d001) of kaolinite. Fig. 3d and e exhibits that mostkaolinite layers are delaminated, whereas some layers retaintheir basal spacing [33]. The results above show that the PI/kaolinite composites are successfully prepared. The layers ofkaolinite have dispersed into polyimide matrix and lost theiroriginal structure resulting from the intercalation of polyimide.But when KKAc content exceeds 5 wt%, diffraction peak (d001)increases sharply in intensity due to the aggregation of kaoliniteparticles (see Fig. 3f and g).

Fig. 2c and d exhibits the FTIR spectra of PI/kaolinite compositefilm containing 5 wt% KKAc and pure PI film. The presence ofcharacteristic group frequencies of polyimide and kaolinite can beobserved. A few characteristic absorption bands at 1378 cm�1 (C–N–C stretching, imide II), 1720 cm�1 (asymmetric C–O stretching,imide I), 1777 cm�1 (symmetric C–O stretching, imide I),3071 cm�1 (N–H stretching of R-NH3), and 3475 cm�1 (polymericO–H stretching) of PMDA/ODA polyimide are indicated in Fig. 2d.For PI/kaolinite composite film containing 5 wt% KKAc (see Fig. 2c),characteristic frequencies of polyimide and kaolinite are stillpresent, but the peaks at 3694, 3664 and 3649 cm�1 assigned toinner-surface hydroxyl stretching bands of raw kaolinite nearlydisappear, which is resulted from the intercalation of polyimidechains into layers of kaolinite [34]. Meanwhile, according to FTIRspectra of KKAc, the peak at 3606 cm�1 for CH3COO– hasdisappeared, confirming that the polyimide molecules havesubstituted KAc molecules and intercalated into the interlayerof kaolinite. A possible mechanism of preparing PI/kaolinitecomposite films has been illustrated in Fig. 4. Larger layer spacingof KKAc particles resulted from the intercalation of KAc molecule isbeneficial for polyimide intercalating into the layer of kaolinite bysubstituting KAc molecules [13].

The surface morphologies of pure PI and PI/kaolinite compositefilms are measured by FE-SEM, as shown in Fig. 5. Rough andporous surface of Pure PI is exhibited in Fig. 5a, which isdetrimental to its performance. When the content of KKAc is lessthan 5 wt%, kaolinite particles are homogeneously dispersed inpolyimide matrix and the introduction of kaolinite make thesurface of PI/kaolinite composite films smoother than pure PI film,as exhibited in Fig. 5b–d. But when the introduced content of KKAcexceeds 5 wt%, lots of concave convex and holes have appear due tothe aggregation of kaolinite particles, as shown in Fig. 5e and f,which has quite a serious effect on the properties of compositefilms. Kaolinite particles play an important role in polyimidematrix, so some differences may be found in the properties ofcomposite films. Therefore, the mechanical and thermal property,water absorption property, and transparency of all films arethoroughly investigated below.

Fig. 4. Schematic representation of preparation of PI/kaolinite composite films.

Fig. 5. FE-SEM images of the surface of (a) pure PI, (b) PI/1 wt%KKAc, (c) PI/3 wt%KKAc, (d) PI/5 wt%KKAc, (e) PI/7 wt%KKAc and (f) PI/9 wt%KKAc.

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Fig. 6. Tensile strength and elongation at break of PI/kaolinite composite films with

different KKAc content.

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3.3. Mechanical properties of PI/kaolinite composites

Polyimide films are usually used in the areas of microelectron-ics, high temperature and aviation aerospace. An excellentmechanical property is critical characteristics for polyimide films.Fig. 6 presents the relationship between KKAc content andmechanical properties of the composite films. Apparently, theincorporation of 1 wt% KKAc in polyimide results in the increasedtensile strength and elongation at break of composites films,

Fig. 7. FE-SEM images of fresh cross-sections of (a) pure PI, (b) PI/1 wt%KKAc, (

comparing to those of pure PI film. Further increasing the KKAccontent, the tensile strength of composite films increases from87.21 MPa for pure PI film to 141.23 MPa for PI/kaolinitecomposite film containing 5 wt% KKAc. It is also observed thatthe elongation at break of composite film containing 5 wt% KKAc(39.59%) is obviously higher than that of pure PI film (10.24%).Upon the incorporation of 5 wt% KKAc, however, both the tensilestrength and elongation at break of composite film decrease. Thiscan be explained by the oriented growth of polyimide chains in theinterlayer of kaolinite, as shown in Fig. 4, and the layer structure ofkaolinite limit the motion of their chains. When KKAc content isless than 5 wt%, good dispersion of clay particles in polyimidematrix leads to an efficient stress transfer and avoids excessivelyrapid growth of micro-cracks [35]. There is an obvious orientationeffect for polyimide molecules chains. Consequently, the tensilestrength and elongation at break of composite film are significantlyimproved. However, when the KKAc content exceeds 5 wt%,kaolinite particles are partially reunited and unevenly dispersed,and the polyimide molecules partly stretch out of the layer ofkaolinite. So the tensile strength and elongation at break of thecomposite films decrease. Ye et al. [36] revealed that the clayparticle acted as a barrier to avoid the motion of polymermolecules, and the data of their mechanical properties indicatedthe same rules as our research.

Fig. 7 shows the FE-SEM micrographs of a cross-section of purePI and PI/kaolinite composite films containing 1–9 wt% KKAc aftertensile tests. Fig. 7a exhibits the fracture surface of pure PI film isextremely smooth, but some plate-like and clusters-like structureare found in perpendicular to the fracture surface for PI/kaolinite

c) PI/3 wt%KKAc, (d) PI/5 wt%KKAc, (e) PI/7 wt%KKAc and (f) PI/9 wt%KKAc.

Fig. 8. TGA curves of kaolinite, pure PI and PI/kaolinite composite films.

Table 2The thermal, waterproof and optical properties of pure PI and PI/kaolinite composite films.

MMT content (wt%) 0 1 3 5 7 9

T5 (8C)a 506.5 533.2 565.3 576.6 579.0 580.4

T10 (8C)b 531.5 559.2 582.5 591.6 596.5 597.0

Water uptake rate (%)c 0.88 0.84 0.83 0.78 0.77 0.77

Water uptake rate (%)d 1.52 1.15 1.06 0.96 0.96 0.95

Onset/80% wavelength (nm)e 453/566 439/660 456/745 445/795 462/800 465/–

a Temperature at 5% weight loss from TGA measurement, scan rate: 10 8C/min, N2 protection.b Temperature at 10% weight loss from TGA measurement, scan rate: 10 8C/min, N2 protection.c The sample was soaked in water at 25 8C for 1 h.d The sample was soaked in water at 25 8C for 1 h, 100 8C for 1 h.e Transmission onset and 80% transmittance wavelength from UV–vis spectra.

X. Qiu et al. / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 2021–20282026

composite films (Fig. 7b–f) [37]. It indicates that a fractureresistance exists in the composites, which is in a microstructureconsisting of clay and polymer matrices because of the presence ofthe possible interaction between kaolinite and PI matrix. Itobserves that the roughness of the fracture surface is increasedwith increasing KKAc content and reaches the largest when KKAccontent is 5 wt%. The results of mechanical testing reveal that theintroduction of kaolinite particles can reinforce the mechanicalproperties of PI matrix at low clay content.

3.4. Thermal properties of PI/kaolinite composites

The TGA curves of pure PI and PI/kaolinite composite films withdifferent clay content are shown in Fig. 8. With increasing oftemperature the curves of all films are nearly the same until about400 8C. The thermal stability of composite films is improved with theincreasing of KKAc content and becomes constant when the contentof KKAc exceeds 5 wt%. Table 2 lists the thermal property data of PI/kaolinite composites with various KKAc contents. The decomposi-tion temperature at 5% and 10% weight loss are increased with

Fig. 9. The cross-sectional morphology of (a) PI/3 wt%KKAc, (b) PI/5 wt%KKAc, (c) PI/7 wt%KKAc and (d) PI/9 wt%KKAc.

Fig. 10. UV–vis spectra of pure PI and PI/kaolinite composite films with various

KKAc content.

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increasing KKAc content. For example, the T5 and T10 measured byTGA are improved from 506.5 and 531.5 8C for pure PI film to 576.6and 591.6 8C for PI/kaolinite film containing 5 wt% KKAc. Thisphenomenon is linked by the fact that the nanoscale compoundingof the clay particles can improve the thermal stability of polymer/clay nanocomposites [38,39]. Kaolinite possesses high thermalstability, its layer act as an insulator to avoid the polyimide chainscontacting with oxygen and limit the continuous decomposition ofthe PI matrix. Fig. 9 illustrates the cross-sectional morphology of PI/kaolinite composite films contained 3 wt%, 5 wt%, 7 wt%, 9 wt% KKAcat high magnification. Two different forms, polyimide matrix andkaolin particles coexist in PI/kaolinite composite films. At low clayloading (3 wt%), few discrete kaolin particles are visible in polyimidematrix, but the amount of exfoliated clay is not enough to enhancethe thermal stability. When increasing the clay concentration(5 wt%), much more exfoliated clay is formed, more easily andeffectively and consequently promotes the thermal stability ofthe composites. At even higher clay loading level (up to 9 wt%), theintercalated clay structure is the dominant population, and themorphology of the composite probably does not allow formaintaining a higher thermal stability. So, the introduction ofkaolinite particles (�5 wt%) can significantly improve the thermalstability of PI matrix.

3.5. Water absorption of PI/kaolinite composites

Moisture or water absorbed in a polymer matrix can lead to awide range of effects, including debonding at filler–matrixinterfaces, structural damage, and chemical degradation of thematrix during long-term exposure to water [40]. In response to theeffects of water in polymers, their application properties can besignificantly affected. It is well known that polyimides absorbmoisture or water. Although the moisture absorption characteris-tic of polyimide films is widely used in sensor applications [41], thepresence of moisture in the films affects the electrical properties[42] in some other applications. Water absorption is dependent onthe chemical structure of the polyimide control or composite films,temperature and duration of exposure [43].

In general, the water absorption of PI is very high because thewater molecules can easily diffuse into and form hydrogen bondsalong polymer chains. Kamal et al. [44] found that the diffusion ofwater molecule became different for a composite that containedlaminated filler. Table 2 lists the water uptake ratios of control filmand composite films with different KKAc content. It is noted thatthe water uptake ratios of composite films are lower than those ofpure PI film and significantly decrease with increasing KKAccontent until 5 wt%. Introduction of kaolinite leads to an obviousdecrease in water absorption of PI/kaolinite composite films. Forinstance, the water uptake ratio is reduced from 0.88% (25 8C) and1.52% (100 8C) for pure PI film to 0.78% (25 8C) and 0.96% (100 8C)for PI/kaolinite composite film containing 5 wt% KKAc. Because thelayers of kaolinite act as a barrier to water diffusion into thepolyimide film and separate the polyimide and water molecules[45]. Better clay dispersions make composite films possess lowerwater absorption [46].

3.6. Optical properties of PI/kaolinite composites

Optical appearance is an acceptable and convenient method injudging the dispersion of inorganic phase in polymer matrix. If thepolymer/inorganic composite are transparent, the inorganic phasecan be well dispersed in the polymer matrix [47]. The variation oftransparency values with wavelength for PI and PI/kaolinitecomposite films is shown in Fig. 10a. It is found that the compositefilms lose their transparency when the clay content furtherincreases. Two quantities, the transmission onset and the 80%

transmission wavelength are usually used to evaluate thetransparency of these films [48,49], as listed in Table 2. Theyrange from 453 to 465 nm for the onset wavelength, and from 566to 800 nm for the 80% transmission wavelength. The transmittanceof all films at 780 nm is given in Fig. 10b. With KKAc contentincreases from 1 to 7 wt% in polyimide matrix, the transmittance ofcomposite films slightly decreases. But when KKAc content is morethan 7 wt%, the transparency of films rapidly decreased. This is anevidence for a poor dispersion of KKAc particles in polyimidematrix, and this phenomenon is consistent with the results of SEM(Figs. 5f and 9d).

4. Conclusions

A series of composite films that consist of polyimide andmodified layered kaolinite clay have been successfully preparedvia in situ intercalation polymerization. Kaolinite–potassiumacetate (KKAc) was used as an intermediate in this system. Thestructures, morphologies and properties of PI/kaolinite compositefilms were systematically described. Results of XRD, FTIR, and SEMrevealed that polyimide chains have partially intercalated into thelayers of kaolinite by replacing KAc molecules and the kaoliniteparticles showed well dispersion in polyimide matrix at low claycontent (�5 wt%). The composite films exhibited more excellentperformances than those of pure PI films by means of universaltester, TGA, and UV–vis spectrometer. The tensile strength,elongation at break and thermal stability of the composite filmswere significantly improved until KKAc content exceeded 5 wt%.The absorption behavior of PI/kaolinite films suggested adramatically better waterproof property than pure PI film. Withthe increasing of KKAc content in polyimide matrix, the transmit-tance of the PI/kaolinite composite films slightly decreased.

Acknowledgments

This work was supported by the Fundamental Research Fundsfor the Central Universities (CUGL100407) and the NationalNatural Science Foundation of China (50903077, 20872044), theNatural Science Foundation of Hubei Province (2009CDB163,2008CDZ065).

References

[1] Jana S, Purkait MK, Mohanty K. Clay supported polyvinyl acetate coatedcomposite membrane by modified dip coating method: application for thepurification of lysozyme from chicken egg white. J Membr Sci 2011;382:243–51.

X. Qiu et al. / Journal of the Taiwan Institute of Chemical Engineers 45 (2014) 2021–20282028

[2] Jana S, Saikia A, Purkait MK, Mohanty K. Chitosan based ceramic ultrafiltrationmembrane: preparation, characterization and application to remove Hg(II)and As(III) using polymer enhanced ultrafiltration. Chem Eng J 2011;170:209–19.

[3] Yu YH, Lin CY, Yeh JM, Lin WH. Preparation and properties of poly(vinylalcohol)-clay nanocomposite materials. Polymer 2003;44:3553–60.

[4] Ray SS, Okamoto M. Polymer/layered silicate nanocomposites: a review frompreparation to processing. Prog Polym Sci 2003;28:1539–41.

[5] Deshmane C, Yuan Q, Misra RDK. High strength-toughness combination ofmelt intercalated nanoclay-reinforced thermoplastic olefins. Mater Sci Eng AStruct 2007;460(461):277–87.

[6] Turhan Y, Dogan M, Alkan M. Poly(vinyl chloride)/kaolinite nanocomposites:characterization and thermal and optical properties. Ind Eng Chem Res 2010;49:1503–13.

[7] Baniasadi HA, Ramazani SA, Javan Nikkhah S. Investigation of in situ preparedpolypropylene/clay nanocomposites properties and comparing to melt blend-ing method. Mater Des 2010;31:76–84.

[8] Suin S, Maiti S, Shrivastava NK, Khatua BB. Mechanically improved andoptically transparent polycarbonate/clay nanocomposites using phosphoniummodified organoclay. Mater Des 2014;54:553–63.

[9] Brandao LS, Mendes LC, Medeiros ME, Sirelli L, Dias ML. Thermal and mechan-ical properties of poly(ethylene terephthalate)/lamellar zirconium phosphatenanocomposites. J Appl Polym Sci 2006;102:3868–76.

[10] Elbokl TA, Detellier C. Intercalation of cyclic imides in kaolinite. J ColloidInterface Sci 2008;323:338–48.

[11] Li YF, Zhang B, Pan XB. Preparation and characterization of PMMA–kaoliniteintercalation composites. Compos Sci Technol 2008;68:1954–61.

[12] Cheng S, Shen DZ, Zhu XS, Tian XG, Zhou DY, Fan LJ. Preparation of nonwovenpolyimide/silica hybrid nanofiberous fabrics by combining electrospinningand controlled in situ sol–gel techniques. Eur Polym J 2009;45:2767–78.

[13] Shi YJ, Mu LW, Feng X, Lu XH. The tribological behavior of nanometer andmicrometer TiO2 particle-filled polytetrafluoroethylene/polyimide. Mater De-sign 2011;32:964–70.

[14] Tyan HL, Liu YC, Wei KH. Enhancement of imidization of poly(amic acid)through forming poly(amic acid)/organoclay nanocomposites. Polymer1999;40:4877–86.

[15] Leu CM, Wu ZW, Wei KH. Synthesis and properties of covalently bondedlayered silicates/polyimide (BTDA-ODA) nanocomposites. Chem Mater2002;14:3016–21.

[16] Park JS, Chang JH. Colorless polyimide nanocomposite films with pristine clay:thermal behavior, mechanical property, morphology, and optical transparen-cy. Polym Eng Sci 2009;49:1357–65.

[17] TyanHL, LeuCM WeiKH. Effect of reactivity of organics-modified montmoril-lonite on the thermal and mechanical properties of montmorillonite/poly-imide nanocomposites. Chem Mater 2001;13:222–6.

[18] Liang ZM, Yin J, Wu JH, Qiu ZX, He FF. Polyimide/montmorillonite nanocom-posites with photolithographic properties. Eur Polym J 2004;40:307–14.

[19] Itagaki T, Komori Y, Sugahara Y, Kuroda K. Synthesis of a kaolinite–poly(b-alanine) intercalation compound. J Mater Chem 2001;11:3291–5.

[20] Valaskova M, Rieder M, Matejka V, Capkova P, Slıva A. Exfoliation/delamina-tion of kaolinite by low-temperature washing of kaolinite–urea intercalates.Appl Clay Sci 2007;35:108–18.

[21] Lapides I, Yariv S. Thermo-X-ray-diffraction analysis of dimethylsulfoxied–kaolinite intercalation complexes. J Therm Anal Calorim 2009;97:19–25.

[22] Mako E, Rutkai G, Kristof T. Simulation-assisted evidence for the existence oftwo stable kaolinite/potassium acetate intercalate complexes. J Colloid Inter-face Sci 2010;349:442–5.

[23] Zhao SF, Qiu SC, Zheng YY, Cheng L, Guo Y. Synthesis and characterization ofkaolin with polystyrene via in-situ polymerization and their application onpolypropylene. Mater Design 2011;32:957–63.

[24] Cheng HF, Liu QF, Zhang JH, Yang J, Frost RL. Delamination of kaolinite–potassium acetate intercalates by ball-milling. J Colloid Interface Sci2010;348:355–9.

[25] Cheng HF, Liu QF, Yang J, Zhang Q, Frost RL. Thermal behavior and decomposi-tion of kaolinite–potassium acetate intercalation composite. ThermochimActa 2010;503(504):16–20.

[26] Wang WJ, Yuan YQ, Wang JL. The mineralogical characteristics of kaolinite andpreparation techniques of its Intercalated compound. Beijing: GeologicalPress; 2008.

[27] Cheng HF, Liu QF, Yang J, Du XM, Frost RL. Influencing factors on kaolinite–potassium acetate intercalation complexes. Appl Clay Sci 2010;50:476–80.

[28] Zhang B, Li YF, Pan XB, Jia X, Wang XL. Intercalation of acrylic acid and sodiumacrylate into kaolinite and their in situ polymerization. J Phys Chem Solids2007;68:135–42.

[29] Wang BX, Zhao XP. The influence of intercalation rate and degree of substitu-tion on the electrorheological activity of a novel ternary intercalated nano-composite. J Solid State Chem 2006;179:949–54.

[30] Zhu XY, Chen JC, Chen JY, Lei XR, Yan CJ. Urea intercalation compoundproduction in industrial scale for paper coating. Chem Ind Chem Eng Q2013. http://dx.doi.org/10.2298/CICEQ121025007Z.

[31] Nah C, Han SH, Lee JH, Lee MH, Lim SD, Rhee JM. Intercalation behavior ofpolyimide/organoclay nanocomposites during thermal imidization. Compo-sites Part B Eng 2004;35:125–31.

[32] Li GY, Zhang QZ, Yang WH. Synthesis and characterization of polyimide/montmorillonite nanocomposites. J Shandong Univ 2003;38:105–8.

[33] Zhao XP, Wang BX, Li J. Synthesis and electrorheological activity of a modifiedkaolinite/carboxymethyl starch hybrid nanocomposite. J Appl Polym Sci2008;108:2833–9.

[34] Borogl MS, Boz I, Gurkaynak MA. Structural characterization of silica modifiedpolyimide membranes. Polym Adv Technol 2006;17:6–11.

[35] Jena KK, Raju KVSN, Narayan R, Rout TK. Sodium montmorillonite clay loadednovel organic–inorganic hybrid composites: synthesis and characterization.Prog Org Coat 2012;75:33–7.

[36] Ye YS, Yen YC, Cheng CC, Syu YJ, Huang YJ, Chang FC. Polytriazole/claynanocomposites synthesized using in situ polymerization and click chemistry.Polymer 2010;51:430–6.

[37] Tang JC, Lin GL, Yang HC, Jiang GJ, Chen-Yang YW. Polyimide–silica nano-composites exhibiting low thermal expansion coefficient and water absorp-tion from surface-modified silica. J Appl Polym Sci 2007;104:4096–105.

[38] Jin HS, Chang JH. Colorless polyimide nanocomposite films: thermomechani-cal properties, morphology, and optical transparency. J Appl Polym Sci2008;107:109–17.

[39] Sun DW, Li YF, Zhang B, Pan XB. Preparation and characterization of novelnanocomposites based on polyacrylonitrile/kaolinite. Compos Sci Technol2010;70:981–8.

[40] Xu SY, Dillard DA. Environmental aging effects on thermal and mechanicalproperties of electrically conductive adhesives. J Adhesion 2003;79:699–723(25).

[41] Lim DI, Cha JR, Gong MS. Preparation of flexible resistive micro-humiditysensors and their humidity-sensing properties. Sens Actuators B Chem2013;183:574–82.

[42] Yasufuku S, Tcdoki M. Dielectric and thermoanalytical behavior of moistureand water in aromatic polyamide and polyimide films. In: Conference Recordof the 1994 IEEE International Symposium on Elecmcal Insulation. 1994. p.

197–200.[43] Niyogi S, Maiti S, Adhikari B. Chemical durability of polyimide blend films.

Polym Degrad Stabil 2000;68:459–64.[44] Kamai MR, Garmabi H, Hozhabr S, Arghyeris L. The development of laminar

morphology during extrusion of polymer blends. Polym Eng Sci 1995;35:41–51.

[45] Magaraphan R, Lilayuthalert W, Sirivat A, Schwank JW. Preparation, structure,properties and thermal behavior of rigid-rod polyimide/montmorillonitenanocomposites. Compos Sci Technol 2001;61:1253–64.

[46] Tsay SY, Chen BK, Chen CP. Synthesis and properties of polyimide (containingnaphthalene) nanocomposites with organo-modified montmorillonites. J ApplPolym Sci 2006;99:2966–72.

[47] Liaw WC, Chen KP. Preparation and characterization of poly(imide siloxane)(PIS)/titania (TiO2) hybrid nanocomposites by sol-gel processes. Eur Polym J2007;43:2265–78.

[48] Li F, Ge JJ, Honigfort PS, Fang S, Chen JC, Harris FW, et al. Dianhydridearchitectural effects on the relaxation behaviors and thermal and opticalproperties of organo-soluble aromatic polyimide films. Polymer 1999;40:4987–5002.

[49] Li F, Fang S, Ge JJ, Honigfort PS, Chen JC, Harris FW, et al. Diamine architectureeffects on glass transitions, relaxation processes and other material propertiesin organo-soluble aromatic polyimide films. Polymer 1999;40:4571–83.