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kaolinite
Composites Science and Technology 70 (2010) 981–988
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Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Preparation and characterization of novel nanocomposites based on polyacrylonitrile/kaolinite Dewen Sun, Yanfeng Li *, Bo Zhang, Xiaobing Pan State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Institute of Biochemical Engineering & Environmental Technology, Lanzhou University, Lanzhou 730000, China
a r t i c l e
i n f o
Article history: Received 19 July 2009 Received in revised form 4 February 2010 Accepted 17 February 2010 Available online 20 February 2010 Keywords: A. Polymer–matrix composites A. Layered structures A. Nanoclays B. Thermal properties D. Scanning/transmission electron microscopy (STEM)
a b s t r a c t A novel intercalation nanocomposite based on polyacrylonitrile (PAN)/kaolinite was prepared by a simple in situ emulsion polymerization in the presence of organically modified layered silicates (kaolinite). The crude kaolinite (K0) was firstly modified by dimethylsulfoxide-methanol and potassium acetate-aqueous systems; acrylonitrile monomer was then intercalated into kaolinite by displacing potassium acetate from KAc-kaolinite (the kaolinite modified by KAc, marked as K2). The polymer/kaolinite composites were prepared by in situ emulsion polymerization, and characterized by means of X-ray diffraction, scanning electron microscope, transmission electron microscope and thermal gravimetric analysis. Experimental results indicate that the clay layers of kaolinite in PAN/K2 composites are well distributed and delaminated. The weight-average molecular weights were measured by small-angle X-ray scattering for all samples. The presence of clay results in an increase in molecular weights compared to pure PAN polymer due to the highly crosslinked structure. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction In recent years, polymer/layered silicate nanocomposites have attracted great interests, both in industry and in academic fields, since they frequently exhibit unexpected properties compared with virgin polymer or conventional micro- and macro-composites. The new and improved properties of polymer/layered silicate composites are derived from the two components of organic and inorganic materials and widely used in electronic, adhesives, and automotive fields. Engineers and researchers are focusing on polymer/silicate nanocomposites, expecting high stiffness, strength, flame-retarding, and gas barrier properties with small amount of silicate. In general, polymer/layered silicate nanocomposites are of three different types, namely intercalated nanocomposites, flocculated nanocomposites and exfoliated nanocomposites [1]. The Toyota group reported that nylon-6/silicate nanocomposites show enhanced modulus and strength without sacrificing other compensating properties such as impact resistance and so on [2]. Since then, many nanocomposites such as polystyrene, poly(ethylene oxide), poly(n-isopropylacrylamide), polycarbonate, urethane, imide, and poly(acrylic acid-sodium acrylate) [3–9] have been prepared. However, the investigations on polymer/silicate nanocomposites
* Corresponding author. Tel./fax: +86 931 8912113. E-mail address: [email protected] (Y. Li). 0266-3538/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2010.02.016
with homopolymers of PAN were rare. Only PAN/smectite and PAN/montmorillonite nanocomposites were synthesized by in situ emulsion polymerization [10]. Styrene-acrylonitrile copolymer (SAN)/MMT nanocomposites were synthesized by Noh and co-workers, and the basal space between the adjacent silicate layers became wider due to the intercalation morphology [11]. Kaolinite, with the 1:1-type layered structure has high crystallinity and unique structure: one side of the interlayer space is covered with hydroxyl groups of the Al2(OH)4 octahedral sheets and the other side is covered by oxygens of the SiO4 tetrahedron [12]. With the hydrogen-bonding between the layers, it has often been classified as non-expandable mineral, while only a limited number of polar organic compounds such as dimethylsulfoxide (DMSO), deuterated dimethylsulfoxide [13], formamide [14], N-methylformamide (NMF), dimethylformamide, acetamide, pyridine N-oxide, potassium acetate (KAc), methanol and octadecylamine [15–17] can be directly intercalated. In recent years, intercalation reactions of kaolinite have been extended by guest displacement method [16]. Thus, kaolinite intercalation compounds can be as precursors, and a new guest can be intercalated by displacing previously intercalated precursor species. KAc-kaolinite was prepared by the guest displacement reaction between KAc and DMSO via this approach, and the organic modified kaolinite was used as clay precursor to synthesize the polymer/kaolinite nanocomposites. In our previous paper on the preparation of poly(acrylic acid-sodium acrylate)/
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kaolinite nanocomposites [9], kaolinite precursor was synthesized by DMSO and KAc step by step, and this method was extended to PAN in this study: a simple and convenient way to obtain exfoliated PAN/kaolinite nanocomposites through in situ emulsion polymerization. 2. Experimental section 2.1. Materials and instrumentations The kaolinite used in this study was provided by China-kaolinite Company with a diameter of 5–10 lm, Suzhou. Hydrochloric acid (A.R) was from Beijing-Chemical Plant, Beijing; dimethylsulfoxide (DMSO, A.R), methanol (A.R), ethanol (A.R), potassium acetate (KAc, A.R), sodium lauryl benzenesulfate (DBS, A.R), N, N-dimethylformamide (DMF, A.R) used in this research were manufactured by Fuchen-Chemical reagent Factory, Tianjin and used as received. Acrylonitrile (AN, A.R) was purchased from Fuchen Chemical Company in Tianjin China also and potassium persulfate (KPS) of A.R grade, an initiator was recrystallized using deionized water. X-ray diffraction (XRD) analyses were carried out on an XRD6000 equipped vertical goniometer equipped with a curved graphitediffracted beam monochromator. The radiation applied was Cu Ka from a long fine-focus Cu tube, operating at 40 kV and 30 mA, the 2h from 2° to 80°. Fourier transform infrared spectroscopy (FTIR) was carried out in a Bomem Michelson FTIR Spectrophotometer, Model MB100. KBr discs were prepared after mixing the test
samples respectively with dry KBr. Analyses were performed in the wave number scale mode between 400 and 4000 cm1, with a resolution of 4 cm1 and approximately 50 scans. Transmission electron microscope (TEM) was performed using a JEOL 1010 equipped with a digital Bioscan (Gatan) image acquisition system. The morphological images of materials were evaluated by a Scanning electron microscope (SEM) with a model JEDL JSM-6330F and an Olympus DP10 Polarizing microscope. Thermal gravimetric analysis (TGA) was carried out with a Perkin–Elmer thermo-balance by heating from a room temperature to 800 °C with a heating rate of 10 °C/min under a N2 atmosphere. Differential scanning calorimetry (DSC) was performed on a DSC in the 50–200 °C range under N2 flow at the programmed heating rate of 10 °C/min1. The weight-average molecular weights of all the productions were determined by small-angle X-ray scattering (SAXS) in N, N-dimethylformamide (DMF). 2.2. Preparation of organic-kaolinite [3,18] The illustration of preparation of clay precursor and composites is shown in Scheme 1. Firstly, the kaolinite precursor was prepared by an intercalation reaction between DMSO and the crud kaolinite (marked as K0) in methanol: 2.0 g purified kaolinite was stirred with 20.0 mL DMSO solution containing 9% methanol (v/v) at 85 °C for 120 h, the suspending system was filtrated and washed by hot ethanol for three times in order to get rid of the excess DMSO. The washed kaolinite was dried under dynamic vacuum
Scheme 1. Preparation process for PAN/K2 composites.
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at 60 °C for 24 h, which was marked as K1. Secondly, organic-kaolinite was prepared by the guest displacement reaction between KAc and DMSO: 2.0 g K1 was stirred in 30 mL KAc/H2O (5 mol/L) solution at 50 °C for at least 10 h, the mixture was then purified by deionized water for three times when it was filtrated; the product was dried at 80 °C for 24 h under dynamic vacuum, and marked as K2.
2.3. Synthesis of polyacrylonitrile (PAN)/kaolinite nanocomposites Polymerizations were carried out in the following way with various amounts of kaolinite. 1.0 g K2 was dispersed by magnetic stirrer in 10 mL AN for 1 h, then the mixture was sonicated for 5 min. After stirred at room temperature for several hours, the suspending system of AN monomer and K2 was charged into a 250 mL three-neck reactor equipped with a mechanic stirrer, a reflux condenser, a nitrogen inlet containing 125 mL deionized water in which 0.3 g DBS was diluted as surface-active agent. Under a N2 environment the suspending system was dispersed by mechanic stirrer for 30 min at room temperature. Then, the temperature of the reactor was raised to 65 °C naturally, and 20 mL of aqueous initiator solution (KPS = 1 wt.%) was injected into the reactor via a glass syringe. Initial polymerization was performed at 65 °C for 1 h. The polymerization time was checked after the initiator injected to remove external factors affecting the polymerization, such as the increasing rate of temperature from room temperature to react temperature and the dispersing efficiency before the initiator was injected. With K2, white particles were generated within 10 min after the initiator introduced. In the absence of K2, white particles were formed in about 1 h. After the initial polymerization was completed, 10 mL of AN was fed into the reactor with a syringe pump at the rate of 0.20 mL/min. After feeding monomer was completed, the polymerization was continued at the same temperature for additional 4 h. The reaction was terminated by addition of 50 mL of aluminum sulfate (15 wt.%). The reaction system was filtered through 4–9 lm opening mesh and washed with distilled water for three times to remove the excess AN monomer and reactive surfactant, and then the products were dried under a high vacuum at 50 °C for 48 h. The dried nanocomposites cake was molded for the next process and marked as PAN/K2. To prepare a composite membrane, 5.0 g dried nanocomposites were dissolved into 100 mL dimethylformamide by magnetic stirrer at 60 °C, and then the solution was slowly poured into a glass dish. The filled dish was placed on the leveled plate, and then was dried in a fume hood at 50 °C. Finally, the residual solvent in the composite membrane was fully removed by evacuation at 100 °C for 24 h. To determine when the exfoliation occurred during the polymerization, 20 mL reactant was collected from the reactor at fixed time intervals. Each sample was dried at room temperature and molded in the same way for the XRD measurement.
Fig. 1. XRD patterns of (a) K0, (b) K1, (c) K2.
IR ¼ Iið001Þ
Ikð001Þ þ Iið001Þ 100%
ð1Þ
The basal spacing (d001) of K1 expands from 0.72 nm (2h = 12.33°) to 1.12 nm (2h = 7.90°), which is an increase of 0.40 nm. After treated with KAc in water, the basal spacing expands to 1.43 nm (2h = 6.18°) from 1.12 nm, corresponds to a 0.71 nm expansion compared to the original kaolinite. For comparison, in this research, the intercalation ability of kaolinite and precursors was tested. All of the characteristic diffraction patterns of crude kaolinite (Fig. 2a) occurred although the composite only contains 5 wt.% crude clay. The result supports that the monomer has not been filled into kaolinite in former intercalation process. The 0 0 1 planes’ intensity of K1 (Fig. 2b) was shut down in PAN/K1 composite in which the clay content was 10 wt.% while no 0 0 1 pattern occurs in PAN/K2 curve (Fig. 2c) at 2h = 6.18°, which shows that K2 possessing the best intercalation property and K1 has ecumenical intercalation ability. This result supports that the modifying treatment of kaolinite with DMSO and KAc was effective. The intercalation rates of K2 were not 100% so that low intensity peak remains at 0.72 nm on trace (c). All these profiles for polymer composites are complex and consist of several overlapping peaks, which indicates that the micro-morphology is trend to form crystal at short distance while the whole composites system is amorphous.
3. Results and discussion 3.1. X-ray diffraction analysis The X-ray diffraction patterns of kaolinite and precursors with DMSO and KAc (Fig. 1.) show that the intercalated rate of kaolinite was to the level of 94% for DMSO/CH3OH while 94.34% for KAc/ H2O. The intercalation rates (IR) given as percentage were calculated as described in Eq. (1), where Ii(001) was the peak intensity observed for intercalate and Ik(001) was the peak intensity observed for kaolinite.
Fig. 2. XRD patterns of (a) PAN/K0 5 wt.%K0, (b) PAN/K1 10 wt.%K1, (c) PAN/K2 10 wt.%K2, intercalation time is 72 h.
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The former analysis supports that it is easy to fill monomer into K2, so K2 was used as precursor to synthesis polymer/clay composites. The best intercalate reaction of monomer with K2 was established, firstly. K2’s d-spacing is 1.43 nm, after has been reacted with AN monomer less than 72 h, which still occurred at 2h = 6.18° (Fig. 3b and c) still, while the intensity is weaker when the reaction time is longer. It means that AN molecules can enlarge the basal space. When monomer (AN) was reacted with K2 for 72 h, the basal space pattern of K2 at 2h = 6.18° disappeared (Fig. 3d), which indicates that kaolinite was exfoliated completely in composites. To get fully exfoliated polymer/clay composites, AN monomer has to be reacted with K2 for at least 72 h. The variation of basal space of PAN/K2 loads 10 wt.% clay with time during polymerization was investigated. After 20 min of polymerization, the characteristic peak of K2 becomes smaller (Fig. 4c), and no pristine PAN peak occurs at 2h = 18°. After 30 min of polymerization, coupled with the appearance of characteristic peak of PAN, the peak at 2h = 6.18° (Fig. 4d) is smaller compared with curve c. As the polymerization going on, the peak of K2 disappears gradually, at least the peak is not observable. It infers that K2 layers were exfoliated beginning at 20 min after the initiation of polymerization. Fig. 5 illustrates the X-ray diffraction patterns of nanocomposites with different clay contents. The crude kaolinite peak in the sample with 5 wt.% K2 (Fig. 5b) is the smallest than other samples with 7, 10, 15 wt.% K2 (Fig. 5c–e), because the intercalation rates of K0 with DMSO, K1 with KAc were not 100%, a small quantity of crude kaolinite existed in K2 and was not intercalated with AN monomer, while no K2’ characteristic peak appears in Fig. 5b–d, which means that all of the K2 in PAN/K2 composites have been exfoliated when the clay content below 10 wt.%. The result supports that K2 was intercalated by AN monomer successfully while it is impossible to synthesize PAN/kaolinite intercalation composite with K0 under the same conditions, the preparation of K2 was effective for the compounding of kaolinite. The weak basal space peak of K2 (Fig. 5e) beginning to peep in the sample with 15% K2 indicates that with the loading of kaolinite increases the intercalation ratio decrease, and the crystallized clay appeared.
Fig. 4. XRD patterns of PAN/K2 composites with 10 wt.% K2 sample at fixed time intervals during polymerization (a) 0 min, (b) 10 min, (c) 20 min, (d) 30 min, (e) 72 h.
Fig. 5. XRD patterns of (a) PAN, (b) PAN/K2 5 wt.%K2, (c) PAN/K2 7 wt.%K2, (d) PAN/ K2 10 wt.%K2, (e) PAN/K2 15 wt.%K2, intercalation time is 72 h.
3.2. FT-IR of kaolinite and polymer/clay composites
Fig. 3. XRD patterns of (a) PAN, (b) PAN/K2 5 wt.% intercalation time is 24 h, (c) PAN/K2 5 wt.% intercalation time is 48 h, (d) PAN/K2 5 wt.% intercalation time is72 h.
Kaolinite has characteristic O–H stretching vibrations at 3696, 3669 and 3654 cm1 (Fig. 6a) attributed to the inner-surface hydroxyl groups and the intensity and location of these bands are usually sensitive to intercalation of organic molecules. Another characteristic band at 3622 cm1 is attributed to the stretching of the inner hydroxyls and is usually not affected by intercalation [3,19]. After being treated with DMSO, additional bands at 3663 and 905 cm1 are observed while the bands of 3669 and 3654 cm1 disappeared (Fig. 6b), the intensity of 3696 cm1 decreases while 3622 cm1 remains. The band of 3663 cm1
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Fig. 6. FT-IR spectra of (a) K0, (b) K1, (c) K2.
appeared and the bands of 3669 and 3654 cm1 disappeared are attributed to the inner-surface hydroxyl which is hydrogen bonded to the DMSO, 905 cm1 is the hydroxyl deformation of the innersurface hydroxyl groups that are hydrogen bonded to the –S = O group of the DMSO. The bands of 3023 and 2937 cm1 are attributed to in-plane bending vibration and out of plane vibration of C–H bond. Upon the intercalation of kaolinite with KAc, the significant band at 3663 cm1 vanished, two bands of 3698 and 3610 cm1 (Fig. 6c) were observed. The reappearance of outer-surface hydroxyl band at 3698 cm1 indicates that the DMSO molecules linked on outer surface have been washed out in KAc solution, the released outer-surface hydroxyl of K2 has not been bonded with KAc. The appearance of band at 3610 cm1 supports that KAc molecular was intercalated into the clay layers successfully.
The spectrum of PAN/K2 composites (Fig. 7b) contains characteristic absorbance bands of pristine polymer and kaolinite. C–H stretching at 2940 cm1 and C–N stretching at 2245 cm1 are characteristic absorbance bands of PAN. Absorbance peaks from O-H stretching at about 3696 cm1 and 3621 cm1 of outer-surface hydroxyl groups and inner hydroxyl groups respectively, Si–O stretching at about 1034 cm1, Al–O stretching at 692 cm1, and
Table 1 The weight-average measurements.
molecular
weight
from
small-angle
Nanocomposites
PAN/K2
The content of K2 (%) Weight-average molecular weight (105)
0 2.69
Fig. 7. FT-IR spectra of (a) PAN, (b) PAN/K2 20 wt.%K2, intercalation time is 72 h.
1 62.8
X-ray
2 137
5 30.2
scattering
10 6.55
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Fig. 8. TEM image of (a) PAN water system, (b) PAN/K2 20 wt.%K2 water system, (c) PAN/K2 20 wt.%K2 DMF solution, intercalation time is 72 h.
Fig. 9. SEM of (a) PAN coating, (b–d) PAN/K2 coating 20 wt.%K2, intercalation time is 72 h.
Si–O bending at 542 cm1 confirm the presence of kaolinite in the composites and polymers bond to inner-surface hydroxyl groups. 3.3. Molecular weight of solvent extractable nanocomposites The weight-average molecular weight is calculated from Eq. (2):
M W ¼ 1=ðKC=Rh ÞC¼0;h¼0
ð2Þ
where K ¼ 2p2 n20 ðdn=dcÞ2 =N A k40 . The value of dn/dc of polyacrylonitrile was 0.084 mL/g. in DMF at 25 °C [20].The weight-average molecular weights of the polymers which have been prepared were determined after removal of the clay, and the results are shown in Table 1. For the nanocomposites, the highest molecular weight was observed for PAN/K2 (2% clay), followed by PAN/K2 (1% clay), PAN/K2 (5% clay), PAN/K2 (10% clay), and pure PAN which gives the lowest. The weight-average molecular weights are invariable higher, which is probably due to the formation of highly branched or star polymers with the presence of clay (silicates work as multi-functional crosslinkers). Thus, the fast growth of molecular weights observed in nanocomposites is attributed to PAN chains
Fig. 10. TGA curves under nitrogen flow of (a) PAN, (b) PAN/K0, 10 wt.% K0, (c) PAN/ K1, 10 wt.% K1, (d) PAN/K2, 10 wt.% K2.
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Fig. 11. DSC thermograms of nanocomposites: (a) pure PAN, (b) PAN/K0, 10 wt.% K0, (c) PAN/K1, 10 wt.% K1, (d) PAN/K2, 10 wt.% K2, (e) PAN/K2, 20 wt.% K2.
intercalated into the clay galleries or the formation of hydrogen bondings between PAN chains and the surface hydroxyls of clay layers. The polymer chains are shortened with the content of clay zincreased, because of the free radical transfer or termination on clay surface increased. The crosslinking between polymer chains and clay layers might predominate when the content of clay lower than 2%, while the free radical transfer or termination predominate with the increase of clay. 3.4. Morphology of the nanocomposites The TEM images of pristine PAN (Fig. 8a) and PAN/K2 (Fig. 8b) show that the scale of PAN emulsion ball is about 5 lm, the addition of clay does not affect the diameter of polymer balls. After being dissolved in DMF, the intercalation and exfoliated cases can be clearly observed (Fig. 8c). Exfoliated clay particle appears as dark strips only several nano-meters. It indicates that layers of kaolinite in PAN/K2 composites are well distributed and delaminated, and the exfoliated morphology of kaolinite is confirmed. SEM was used to study the morphology of polymer/clay material also. SEM images in Fig. 9 show that no granule shape in PAN pure polymer membrane that is different to the latter three pictures of composites. After the polymerization, K2 was dispersed on the background of PAN in the form of white particle size from 1 to 1000 nm. As shown in former TEM picture, much of the clay layers are well exfoliated and distributed. 3.5. Thermal properties of the nanocomposites In general, major weight losses are observed in the range of 200–250 °C for PAN materials, which may be correspond to the structure decomposition of the polymers. The range of 155– 200 °C observed in Fig. 10 may be the weight losses for the surface-active agent, which was not cleared. Pure polymer (began at 238 °C, Fig. 10a) has the lowest thermal stability temperature while PAN/K2 (with 10 wt.% clay, Fig. 10d, 268 °C) possess the highest one compared with PAN/K0 (with 10 wt.% clay, Fig. 10b, 247 °C) and PAN/K1 (with 10 wt.% clay, Fig. 10c, 255 °C) composites with
the same clay content, which indicates that K2 is the most effective clay in promoting PAN’s thermal stability. The glass transition behavior of pure PAN and nanocomposites are described in Fig. 11 by DSC thermogram. The pure polymer exhibits (Fig. 11a) an endotherm approximately at 145.7 °C and the enhancement of Tg transition of the nanocomposites are found. The Tg of PAN/K2 (10 wt.%) is the highest than the others, which indicated that the inorganic materials prevented the segmental motion of polymer chains to transfer the glass transition upward. The transition temperatures are not clearly found in the DSC thermogram of nanocomposites (Fig. 11b–e) and are ascribed to the confinement of the intercalated polymer chains within the silicate galleries that prevents the segmental motions of the polymer chains also.
4. Conclusions The potassium acetate intercalated into kaolinite was substituted by acrylonitrile monomer, and polyacrylonitrile/kaolinite composites were prepared by in situ emulsion polymerization of acrylonitrile-intercalating kaolinite. The micro-morphology of the resulting nanocomposites displays both a form of crystal at short distance and whole composites system is amorphous. The kaolinite in composites is well exfoliated and the clay layers are distributed in the form of particle size from 1 to 1000 nm on the background of PAN. The weight-average molecular weights of the polymers are higher in the presence of the clay, which is probably a result of increasing interaction between clay and polymer chains. The exfoliated clay layers in polymer/kaolinite composite exhibited the enhanced thermal stability when compared with pure polyacrylonitrile.
Acknowledgement We thank the financial support from the key research project of Gansu Province (2GS064-A52-036-02).
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Contents lists available at ScienceDirect
Composites Science and Technology journal homepage: www.elsevier.com/locate/compscitech
Preparation and characterization of novel nanocomposites based on polyacrylonitrile/kaolinite Dewen Sun, Yanfeng Li *, Bo Zhang, Xiaobing Pan State Key Laboratory of Applied Organic Chemistry, College of Chemistry and Chemical Engineering, Institute of Biochemical Engineering & Environmental Technology, Lanzhou University, Lanzhou 730000, China
a r t i c l e
i n f o
Article history: Received 19 July 2009 Received in revised form 4 February 2010 Accepted 17 February 2010 Available online 20 February 2010 Keywords: A. Polymer–matrix composites A. Layered structures A. Nanoclays B. Thermal properties D. Scanning/transmission electron microscopy (STEM)
a b s t r a c t A novel intercalation nanocomposite based on polyacrylonitrile (PAN)/kaolinite was prepared by a simple in situ emulsion polymerization in the presence of organically modified layered silicates (kaolinite). The crude kaolinite (K0) was firstly modified by dimethylsulfoxide-methanol and potassium acetate-aqueous systems; acrylonitrile monomer was then intercalated into kaolinite by displacing potassium acetate from KAc-kaolinite (the kaolinite modified by KAc, marked as K2). The polymer/kaolinite composites were prepared by in situ emulsion polymerization, and characterized by means of X-ray diffraction, scanning electron microscope, transmission electron microscope and thermal gravimetric analysis. Experimental results indicate that the clay layers of kaolinite in PAN/K2 composites are well distributed and delaminated. The weight-average molecular weights were measured by small-angle X-ray scattering for all samples. The presence of clay results in an increase in molecular weights compared to pure PAN polymer due to the highly crosslinked structure. Ó 2010 Elsevier Ltd. All rights reserved.
1. Introduction In recent years, polymer/layered silicate nanocomposites have attracted great interests, both in industry and in academic fields, since they frequently exhibit unexpected properties compared with virgin polymer or conventional micro- and macro-composites. The new and improved properties of polymer/layered silicate composites are derived from the two components of organic and inorganic materials and widely used in electronic, adhesives, and automotive fields. Engineers and researchers are focusing on polymer/silicate nanocomposites, expecting high stiffness, strength, flame-retarding, and gas barrier properties with small amount of silicate. In general, polymer/layered silicate nanocomposites are of three different types, namely intercalated nanocomposites, flocculated nanocomposites and exfoliated nanocomposites [1]. The Toyota group reported that nylon-6/silicate nanocomposites show enhanced modulus and strength without sacrificing other compensating properties such as impact resistance and so on [2]. Since then, many nanocomposites such as polystyrene, poly(ethylene oxide), poly(n-isopropylacrylamide), polycarbonate, urethane, imide, and poly(acrylic acid-sodium acrylate) [3–9] have been prepared. However, the investigations on polymer/silicate nanocomposites
* Corresponding author. Tel./fax: +86 931 8912113. E-mail address: [email protected] (Y. Li). 0266-3538/$ - see front matter Ó 2010 Elsevier Ltd. All rights reserved. doi:10.1016/j.compscitech.2010.02.016
with homopolymers of PAN were rare. Only PAN/smectite and PAN/montmorillonite nanocomposites were synthesized by in situ emulsion polymerization [10]. Styrene-acrylonitrile copolymer (SAN)/MMT nanocomposites were synthesized by Noh and co-workers, and the basal space between the adjacent silicate layers became wider due to the intercalation morphology [11]. Kaolinite, with the 1:1-type layered structure has high crystallinity and unique structure: one side of the interlayer space is covered with hydroxyl groups of the Al2(OH)4 octahedral sheets and the other side is covered by oxygens of the SiO4 tetrahedron [12]. With the hydrogen-bonding between the layers, it has often been classified as non-expandable mineral, while only a limited number of polar organic compounds such as dimethylsulfoxide (DMSO), deuterated dimethylsulfoxide [13], formamide [14], N-methylformamide (NMF), dimethylformamide, acetamide, pyridine N-oxide, potassium acetate (KAc), methanol and octadecylamine [15–17] can be directly intercalated. In recent years, intercalation reactions of kaolinite have been extended by guest displacement method [16]. Thus, kaolinite intercalation compounds can be as precursors, and a new guest can be intercalated by displacing previously intercalated precursor species. KAc-kaolinite was prepared by the guest displacement reaction between KAc and DMSO via this approach, and the organic modified kaolinite was used as clay precursor to synthesize the polymer/kaolinite nanocomposites. In our previous paper on the preparation of poly(acrylic acid-sodium acrylate)/
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kaolinite nanocomposites [9], kaolinite precursor was synthesized by DMSO and KAc step by step, and this method was extended to PAN in this study: a simple and convenient way to obtain exfoliated PAN/kaolinite nanocomposites through in situ emulsion polymerization. 2. Experimental section 2.1. Materials and instrumentations The kaolinite used in this study was provided by China-kaolinite Company with a diameter of 5–10 lm, Suzhou. Hydrochloric acid (A.R) was from Beijing-Chemical Plant, Beijing; dimethylsulfoxide (DMSO, A.R), methanol (A.R), ethanol (A.R), potassium acetate (KAc, A.R), sodium lauryl benzenesulfate (DBS, A.R), N, N-dimethylformamide (DMF, A.R) used in this research were manufactured by Fuchen-Chemical reagent Factory, Tianjin and used as received. Acrylonitrile (AN, A.R) was purchased from Fuchen Chemical Company in Tianjin China also and potassium persulfate (KPS) of A.R grade, an initiator was recrystallized using deionized water. X-ray diffraction (XRD) analyses were carried out on an XRD6000 equipped vertical goniometer equipped with a curved graphitediffracted beam monochromator. The radiation applied was Cu Ka from a long fine-focus Cu tube, operating at 40 kV and 30 mA, the 2h from 2° to 80°. Fourier transform infrared spectroscopy (FTIR) was carried out in a Bomem Michelson FTIR Spectrophotometer, Model MB100. KBr discs were prepared after mixing the test
samples respectively with dry KBr. Analyses were performed in the wave number scale mode between 400 and 4000 cm1, with a resolution of 4 cm1 and approximately 50 scans. Transmission electron microscope (TEM) was performed using a JEOL 1010 equipped with a digital Bioscan (Gatan) image acquisition system. The morphological images of materials were evaluated by a Scanning electron microscope (SEM) with a model JEDL JSM-6330F and an Olympus DP10 Polarizing microscope. Thermal gravimetric analysis (TGA) was carried out with a Perkin–Elmer thermo-balance by heating from a room temperature to 800 °C with a heating rate of 10 °C/min under a N2 atmosphere. Differential scanning calorimetry (DSC) was performed on a DSC in the 50–200 °C range under N2 flow at the programmed heating rate of 10 °C/min1. The weight-average molecular weights of all the productions were determined by small-angle X-ray scattering (SAXS) in N, N-dimethylformamide (DMF). 2.2. Preparation of organic-kaolinite [3,18] The illustration of preparation of clay precursor and composites is shown in Scheme 1. Firstly, the kaolinite precursor was prepared by an intercalation reaction between DMSO and the crud kaolinite (marked as K0) in methanol: 2.0 g purified kaolinite was stirred with 20.0 mL DMSO solution containing 9% methanol (v/v) at 85 °C for 120 h, the suspending system was filtrated and washed by hot ethanol for three times in order to get rid of the excess DMSO. The washed kaolinite was dried under dynamic vacuum
Scheme 1. Preparation process for PAN/K2 composites.
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at 60 °C for 24 h, which was marked as K1. Secondly, organic-kaolinite was prepared by the guest displacement reaction between KAc and DMSO: 2.0 g K1 was stirred in 30 mL KAc/H2O (5 mol/L) solution at 50 °C for at least 10 h, the mixture was then purified by deionized water for three times when it was filtrated; the product was dried at 80 °C for 24 h under dynamic vacuum, and marked as K2.
2.3. Synthesis of polyacrylonitrile (PAN)/kaolinite nanocomposites Polymerizations were carried out in the following way with various amounts of kaolinite. 1.0 g K2 was dispersed by magnetic stirrer in 10 mL AN for 1 h, then the mixture was sonicated for 5 min. After stirred at room temperature for several hours, the suspending system of AN monomer and K2 was charged into a 250 mL three-neck reactor equipped with a mechanic stirrer, a reflux condenser, a nitrogen inlet containing 125 mL deionized water in which 0.3 g DBS was diluted as surface-active agent. Under a N2 environment the suspending system was dispersed by mechanic stirrer for 30 min at room temperature. Then, the temperature of the reactor was raised to 65 °C naturally, and 20 mL of aqueous initiator solution (KPS = 1 wt.%) was injected into the reactor via a glass syringe. Initial polymerization was performed at 65 °C for 1 h. The polymerization time was checked after the initiator injected to remove external factors affecting the polymerization, such as the increasing rate of temperature from room temperature to react temperature and the dispersing efficiency before the initiator was injected. With K2, white particles were generated within 10 min after the initiator introduced. In the absence of K2, white particles were formed in about 1 h. After the initial polymerization was completed, 10 mL of AN was fed into the reactor with a syringe pump at the rate of 0.20 mL/min. After feeding monomer was completed, the polymerization was continued at the same temperature for additional 4 h. The reaction was terminated by addition of 50 mL of aluminum sulfate (15 wt.%). The reaction system was filtered through 4–9 lm opening mesh and washed with distilled water for three times to remove the excess AN monomer and reactive surfactant, and then the products were dried under a high vacuum at 50 °C for 48 h. The dried nanocomposites cake was molded for the next process and marked as PAN/K2. To prepare a composite membrane, 5.0 g dried nanocomposites were dissolved into 100 mL dimethylformamide by magnetic stirrer at 60 °C, and then the solution was slowly poured into a glass dish. The filled dish was placed on the leveled plate, and then was dried in a fume hood at 50 °C. Finally, the residual solvent in the composite membrane was fully removed by evacuation at 100 °C for 24 h. To determine when the exfoliation occurred during the polymerization, 20 mL reactant was collected from the reactor at fixed time intervals. Each sample was dried at room temperature and molded in the same way for the XRD measurement.
Fig. 1. XRD patterns of (a) K0, (b) K1, (c) K2.
IR ¼ Iið001Þ
Ikð001Þ þ Iið001Þ 100%
ð1Þ
The basal spacing (d001) of K1 expands from 0.72 nm (2h = 12.33°) to 1.12 nm (2h = 7.90°), which is an increase of 0.40 nm. After treated with KAc in water, the basal spacing expands to 1.43 nm (2h = 6.18°) from 1.12 nm, corresponds to a 0.71 nm expansion compared to the original kaolinite. For comparison, in this research, the intercalation ability of kaolinite and precursors was tested. All of the characteristic diffraction patterns of crude kaolinite (Fig. 2a) occurred although the composite only contains 5 wt.% crude clay. The result supports that the monomer has not been filled into kaolinite in former intercalation process. The 0 0 1 planes’ intensity of K1 (Fig. 2b) was shut down in PAN/K1 composite in which the clay content was 10 wt.% while no 0 0 1 pattern occurs in PAN/K2 curve (Fig. 2c) at 2h = 6.18°, which shows that K2 possessing the best intercalation property and K1 has ecumenical intercalation ability. This result supports that the modifying treatment of kaolinite with DMSO and KAc was effective. The intercalation rates of K2 were not 100% so that low intensity peak remains at 0.72 nm on trace (c). All these profiles for polymer composites are complex and consist of several overlapping peaks, which indicates that the micro-morphology is trend to form crystal at short distance while the whole composites system is amorphous.
3. Results and discussion 3.1. X-ray diffraction analysis The X-ray diffraction patterns of kaolinite and precursors with DMSO and KAc (Fig. 1.) show that the intercalated rate of kaolinite was to the level of 94% for DMSO/CH3OH while 94.34% for KAc/ H2O. The intercalation rates (IR) given as percentage were calculated as described in Eq. (1), where Ii(001) was the peak intensity observed for intercalate and Ik(001) was the peak intensity observed for kaolinite.
Fig. 2. XRD patterns of (a) PAN/K0 5 wt.%K0, (b) PAN/K1 10 wt.%K1, (c) PAN/K2 10 wt.%K2, intercalation time is 72 h.
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The former analysis supports that it is easy to fill monomer into K2, so K2 was used as precursor to synthesis polymer/clay composites. The best intercalate reaction of monomer with K2 was established, firstly. K2’s d-spacing is 1.43 nm, after has been reacted with AN monomer less than 72 h, which still occurred at 2h = 6.18° (Fig. 3b and c) still, while the intensity is weaker when the reaction time is longer. It means that AN molecules can enlarge the basal space. When monomer (AN) was reacted with K2 for 72 h, the basal space pattern of K2 at 2h = 6.18° disappeared (Fig. 3d), which indicates that kaolinite was exfoliated completely in composites. To get fully exfoliated polymer/clay composites, AN monomer has to be reacted with K2 for at least 72 h. The variation of basal space of PAN/K2 loads 10 wt.% clay with time during polymerization was investigated. After 20 min of polymerization, the characteristic peak of K2 becomes smaller (Fig. 4c), and no pristine PAN peak occurs at 2h = 18°. After 30 min of polymerization, coupled with the appearance of characteristic peak of PAN, the peak at 2h = 6.18° (Fig. 4d) is smaller compared with curve c. As the polymerization going on, the peak of K2 disappears gradually, at least the peak is not observable. It infers that K2 layers were exfoliated beginning at 20 min after the initiation of polymerization. Fig. 5 illustrates the X-ray diffraction patterns of nanocomposites with different clay contents. The crude kaolinite peak in the sample with 5 wt.% K2 (Fig. 5b) is the smallest than other samples with 7, 10, 15 wt.% K2 (Fig. 5c–e), because the intercalation rates of K0 with DMSO, K1 with KAc were not 100%, a small quantity of crude kaolinite existed in K2 and was not intercalated with AN monomer, while no K2’ characteristic peak appears in Fig. 5b–d, which means that all of the K2 in PAN/K2 composites have been exfoliated when the clay content below 10 wt.%. The result supports that K2 was intercalated by AN monomer successfully while it is impossible to synthesize PAN/kaolinite intercalation composite with K0 under the same conditions, the preparation of K2 was effective for the compounding of kaolinite. The weak basal space peak of K2 (Fig. 5e) beginning to peep in the sample with 15% K2 indicates that with the loading of kaolinite increases the intercalation ratio decrease, and the crystallized clay appeared.
Fig. 4. XRD patterns of PAN/K2 composites with 10 wt.% K2 sample at fixed time intervals during polymerization (a) 0 min, (b) 10 min, (c) 20 min, (d) 30 min, (e) 72 h.
Fig. 5. XRD patterns of (a) PAN, (b) PAN/K2 5 wt.%K2, (c) PAN/K2 7 wt.%K2, (d) PAN/ K2 10 wt.%K2, (e) PAN/K2 15 wt.%K2, intercalation time is 72 h.
3.2. FT-IR of kaolinite and polymer/clay composites
Fig. 3. XRD patterns of (a) PAN, (b) PAN/K2 5 wt.% intercalation time is 24 h, (c) PAN/K2 5 wt.% intercalation time is 48 h, (d) PAN/K2 5 wt.% intercalation time is72 h.
Kaolinite has characteristic O–H stretching vibrations at 3696, 3669 and 3654 cm1 (Fig. 6a) attributed to the inner-surface hydroxyl groups and the intensity and location of these bands are usually sensitive to intercalation of organic molecules. Another characteristic band at 3622 cm1 is attributed to the stretching of the inner hydroxyls and is usually not affected by intercalation [3,19]. After being treated with DMSO, additional bands at 3663 and 905 cm1 are observed while the bands of 3669 and 3654 cm1 disappeared (Fig. 6b), the intensity of 3696 cm1 decreases while 3622 cm1 remains. The band of 3663 cm1
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Fig. 6. FT-IR spectra of (a) K0, (b) K1, (c) K2.
appeared and the bands of 3669 and 3654 cm1 disappeared are attributed to the inner-surface hydroxyl which is hydrogen bonded to the DMSO, 905 cm1 is the hydroxyl deformation of the innersurface hydroxyl groups that are hydrogen bonded to the –S = O group of the DMSO. The bands of 3023 and 2937 cm1 are attributed to in-plane bending vibration and out of plane vibration of C–H bond. Upon the intercalation of kaolinite with KAc, the significant band at 3663 cm1 vanished, two bands of 3698 and 3610 cm1 (Fig. 6c) were observed. The reappearance of outer-surface hydroxyl band at 3698 cm1 indicates that the DMSO molecules linked on outer surface have been washed out in KAc solution, the released outer-surface hydroxyl of K2 has not been bonded with KAc. The appearance of band at 3610 cm1 supports that KAc molecular was intercalated into the clay layers successfully.
The spectrum of PAN/K2 composites (Fig. 7b) contains characteristic absorbance bands of pristine polymer and kaolinite. C–H stretching at 2940 cm1 and C–N stretching at 2245 cm1 are characteristic absorbance bands of PAN. Absorbance peaks from O-H stretching at about 3696 cm1 and 3621 cm1 of outer-surface hydroxyl groups and inner hydroxyl groups respectively, Si–O stretching at about 1034 cm1, Al–O stretching at 692 cm1, and
Table 1 The weight-average measurements.
molecular
weight
from
small-angle
Nanocomposites
PAN/K2
The content of K2 (%) Weight-average molecular weight (105)
0 2.69
Fig. 7. FT-IR spectra of (a) PAN, (b) PAN/K2 20 wt.%K2, intercalation time is 72 h.
1 62.8
X-ray
2 137
5 30.2
scattering
10 6.55
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Fig. 8. TEM image of (a) PAN water system, (b) PAN/K2 20 wt.%K2 water system, (c) PAN/K2 20 wt.%K2 DMF solution, intercalation time is 72 h.
Fig. 9. SEM of (a) PAN coating, (b–d) PAN/K2 coating 20 wt.%K2, intercalation time is 72 h.
Si–O bending at 542 cm1 confirm the presence of kaolinite in the composites and polymers bond to inner-surface hydroxyl groups. 3.3. Molecular weight of solvent extractable nanocomposites The weight-average molecular weight is calculated from Eq. (2):
M W ¼ 1=ðKC=Rh ÞC¼0;h¼0
ð2Þ
where K ¼ 2p2 n20 ðdn=dcÞ2 =N A k40 . The value of dn/dc of polyacrylonitrile was 0.084 mL/g. in DMF at 25 °C [20].The weight-average molecular weights of the polymers which have been prepared were determined after removal of the clay, and the results are shown in Table 1. For the nanocomposites, the highest molecular weight was observed for PAN/K2 (2% clay), followed by PAN/K2 (1% clay), PAN/K2 (5% clay), PAN/K2 (10% clay), and pure PAN which gives the lowest. The weight-average molecular weights are invariable higher, which is probably due to the formation of highly branched or star polymers with the presence of clay (silicates work as multi-functional crosslinkers). Thus, the fast growth of molecular weights observed in nanocomposites is attributed to PAN chains
Fig. 10. TGA curves under nitrogen flow of (a) PAN, (b) PAN/K0, 10 wt.% K0, (c) PAN/ K1, 10 wt.% K1, (d) PAN/K2, 10 wt.% K2.
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Fig. 11. DSC thermograms of nanocomposites: (a) pure PAN, (b) PAN/K0, 10 wt.% K0, (c) PAN/K1, 10 wt.% K1, (d) PAN/K2, 10 wt.% K2, (e) PAN/K2, 20 wt.% K2.
intercalated into the clay galleries or the formation of hydrogen bondings between PAN chains and the surface hydroxyls of clay layers. The polymer chains are shortened with the content of clay zincreased, because of the free radical transfer or termination on clay surface increased. The crosslinking between polymer chains and clay layers might predominate when the content of clay lower than 2%, while the free radical transfer or termination predominate with the increase of clay. 3.4. Morphology of the nanocomposites The TEM images of pristine PAN (Fig. 8a) and PAN/K2 (Fig. 8b) show that the scale of PAN emulsion ball is about 5 lm, the addition of clay does not affect the diameter of polymer balls. After being dissolved in DMF, the intercalation and exfoliated cases can be clearly observed (Fig. 8c). Exfoliated clay particle appears as dark strips only several nano-meters. It indicates that layers of kaolinite in PAN/K2 composites are well distributed and delaminated, and the exfoliated morphology of kaolinite is confirmed. SEM was used to study the morphology of polymer/clay material also. SEM images in Fig. 9 show that no granule shape in PAN pure polymer membrane that is different to the latter three pictures of composites. After the polymerization, K2 was dispersed on the background of PAN in the form of white particle size from 1 to 1000 nm. As shown in former TEM picture, much of the clay layers are well exfoliated and distributed. 3.5. Thermal properties of the nanocomposites In general, major weight losses are observed in the range of 200–250 °C for PAN materials, which may be correspond to the structure decomposition of the polymers. The range of 155– 200 °C observed in Fig. 10 may be the weight losses for the surface-active agent, which was not cleared. Pure polymer (began at 238 °C, Fig. 10a) has the lowest thermal stability temperature while PAN/K2 (with 10 wt.% clay, Fig. 10d, 268 °C) possess the highest one compared with PAN/K0 (with 10 wt.% clay, Fig. 10b, 247 °C) and PAN/K1 (with 10 wt.% clay, Fig. 10c, 255 °C) composites with
the same clay content, which indicates that K2 is the most effective clay in promoting PAN’s thermal stability. The glass transition behavior of pure PAN and nanocomposites are described in Fig. 11 by DSC thermogram. The pure polymer exhibits (Fig. 11a) an endotherm approximately at 145.7 °C and the enhancement of Tg transition of the nanocomposites are found. The Tg of PAN/K2 (10 wt.%) is the highest than the others, which indicated that the inorganic materials prevented the segmental motion of polymer chains to transfer the glass transition upward. The transition temperatures are not clearly found in the DSC thermogram of nanocomposites (Fig. 11b–e) and are ascribed to the confinement of the intercalated polymer chains within the silicate galleries that prevents the segmental motions of the polymer chains also.
4. Conclusions The potassium acetate intercalated into kaolinite was substituted by acrylonitrile monomer, and polyacrylonitrile/kaolinite composites were prepared by in situ emulsion polymerization of acrylonitrile-intercalating kaolinite. The micro-morphology of the resulting nanocomposites displays both a form of crystal at short distance and whole composites system is amorphous. The kaolinite in composites is well exfoliated and the clay layers are distributed in the form of particle size from 1 to 1000 nm on the background of PAN. The weight-average molecular weights of the polymers are higher in the presence of the clay, which is probably a result of increasing interaction between clay and polymer chains. The exfoliated clay layers in polymer/kaolinite composite exhibited the enhanced thermal stability when compared with pure polyacrylonitrile.
Acknowledgement We thank the financial support from the key research project of Gansu Province (2GS064-A52-036-02).
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