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inorganic polyurethane hybrids
European Polymer Journal 45 (2009) 387–397
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Macromolecular Nanotechnology
Synthesis and properties of conducting organic/inorganic polyurethane hybrids Tzong-Liu Wang a,*, Chien-Hsin Yang a, Yeong-Tarng Shieh a, An-Chi Yeh b a
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 9 June 2008 Received in revised form 13 October 2008 Accepted 14 November 2008 Available online 21 November 2008
A series of polyurethane/polyaniline/silica organic/inorganic hybrids were synthesized via the conventional polyurethane (PU) prepolymer technique. Amine-endcapped polyaniline (PANI) with low molecular weight and higher solubility was firstly synthesized. This PANI oligomer was then used together with nano-silica bearing silanol groups as chain extenders to prepare the conducting polyurethane hybrids. The polyurethane hybrids were designated as PU-xPANI-ySiO2 (x + y = 1). For comparison, the urethane-aniline block copolymer and the PU/silica hybrid were designated as PU-PANI and PU-SiO2, respectively. The structures of PU-PANI, PU-SiO2 and conducting polyurethane hybrids were confirmed by FT-IR, solid-state 13C, and 29Si NMR spectra. In nano-silica containing organic/inorganic conducting polyurethane hybrids, UV–vis spectra revealed the maximum absorption bands similar to that of PU-PANI. X-ray diffraction patterns indicated that these samples are typical of semicrystalline/amorphous materials. SEM image of PU-0.5PANI-0.5SiO2 showed that PANI was dispersed homogeneously and interconnected continuously in the insulating PUsilica matrix. TGA results of the polymer hybrids exhibited higher thermal stabilities and lower decomposition rates than that of PU-PANI both in nitrogen and air. Differential scanning calorimetry (DSC) studies indicated that the polyurethane hybrids had higher glasstransition temperatures (Tg) with the increase of PANI, but lower than that of PU-PANI. Stress–strain curves for all of the polyurethane hybrids showed the elastomeric behavior of typical polyurethanes. The surface resistivity values of all hybrids were about 108 1010 X/sq. and might meet the requirement of the anti-electrostatic materials. Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Organic/inorganic hybrids Polyurethane prepolymer Nano-silica Block copolymer Surface resistivity
1. Introduction
tivity and potential application in electronic devices [1,2]. The general formula for ideal PANI materials in their base forms consists in three (–C6H4–NH–) benzenoid units and one (–N@C6H4@N–) quinoid unit [1].
Polyaniline (PANI) is a conducting polymer which has been extensively studied due to its relatively high conduc-
NH
NH
N y
* Corresponding author. Tel.: +886 7 5919278; fax: +886 7 5919277. E-mail address: [email protected] (T.-L. Wang). 0014-3057/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2008.11.020
N 1-y n
The y value accounts for the oxidation state of the polymer. Leucoemeraldine base (LEB), emeraldine base (EB) and pernigraniline base (PNB) correspond to the
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Department of Chemical and Materials Engineering, National University of Kaohsiung, No. 700, Kaohsiung University Road, Nan-Tzu District, Kaohsiung 811, Taiwan, ROC b Department of Chemical and Materials Engineering, Cheng Shiu University, Kaohsiung County 833, Taiwan, ROC
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completely reduced (y = 1), half oxidized (y = 0.5) and completely oxidized (y = 0) states, respectively. Whereas the three bases are insulating materials, base protonation leads to the corresponding salts. Emeraldine salt (ES) is the only one exhibiting high electric conductivity and has consequently received considerable interest. The increase in electronic conductivity that occurs after the transition emeraldine base (EB) to emeraldine salt (ES), involves a doping mechanism different from those commonly observed with conducting polymers [3–5]. Unfortunately, the poor processability of PANI and its inadequate mechanical properties limit its commercial applications. In order to overcome these problems, numerous methods have been studied [6,7]. A conducting polymer blend with PANI and conventional polymer can exhibit good mechanical properties associated with interesting electrical properties [8–14]. However, blending has very little effect on the environmental stability of the conducting polymer itself. One of the major limitations of the blending is that it almost always produces a highly heterogeneous two-phase morphology. Therefore, it may be more effective to prepare a conducting polymer with improved processibility and mechanical property by the chemical synthesis approach. The combination of PANI segments with blocks of another polymer has been recently reported, such as the grafting of polyethylene glycol [15] or polyacrylic acid [16] onto a PANI backbone, and a block copolymer consisting of polyurethane and PANI oligomer [17,18], etc. The use of a thermoplastic elastomer with conducting polymers is very attractive due to the combination of mechanical properties and a processability which does not require vulcanization. Thermoplastic polyurethane (PU) elastomer is one of the most versatile products in the group of engineering thermoplastics with excellent physical properties, chemical resistance, abrasion resistance as well as its ease of processing. Typically, thermoplastic PU elastomers (i.e., segmented copolyurethanes) are block copolymers of the (A–B)n type, consisting of alternating soft and hard-segments. The soft-segment is typically a long chain polyol which is a polyester-, polyether-, or polyalkyl-diol with a molecular weight between 500 and 5000. The hard-segment generally consists of an aromatic diisocyanate chain extended with a low molecular weight diol or diamine (i.e., chain extender). The unusual properties of these copolymers are directly related to their two-phase structures. The two-phase feature of the original reaction mixture is due to thermodynamic incompatibility of the different block polymer segments. On the other hand, conductive composites of PANI and inorganic compounds have been synthesized in order to get new materials with modified properties [19–21]. Among those inorganic materials, silica (SiO2) has received great attention because of its unique properties and wide applications [22,23]. Recently, a conductive hybrid composed of PANI dispersed in a polyurethane-silica matrix via the sol-gel process was also reported [24]. In past several years, polyurethane-related composites have been extensively studied in our laboratory; herein we present a simple methodology to synthesize a kind of elastomeric PU/PANI/silica conductive hybrids using the conventional polyurethane prepolymer technique. To meet the
above-mentioned requirements, both amine-endcapped PANI and nano-silica bearing silanol groups were used as chain extenders. Hence, this kind of conductive hybrid consists of one type of urethane-aniline block copolymer homogeneously dispersed in the matrix of urethane-silica polymer network. In this work, a series of elastomeric polyurethane hybrids with different mole ratios of PANI and silica were synthesized and their properties were also investigated. 2. Experimental 2.1. Materials 4,40 -Methylene bis(4-phenyl isocyanate) (MDI, Aldrich), 1-methyl-2-pyrrolidinone (NMP, FERAK), aniline (ANI, TCI), p-phenylenediamine (p-PDA, TCI) were distilled under reduced pressure. Ammonium persulfate (APS), dodecylbenzene sulfonic acid (DBSA), hydrochloric acid (HCl) and ammonia solution were all from Union Chemical Works Ltd. and used without further purification. The silica nanoparticles (Mw = 60.09 g/mol) bearing surface silanol groups (18.7 mmol OH/g) were purchased from Desunnano Co., Ltd.; the purity is higher than 99% and the polydispersity is 1.20. The surface area and particle size are 640 m2/g and 10 nm, respectively. Polytetramethylene ether glycol (PTMG, Aldrich) with molecular weight (Mn = 1000) was degassed under vacuum at 55 °C at 600 Pa (4.5 mm Hg) for 3 h to remove any absorbed water. 2.2. Preparation of amine-endcapped polyaniline (PANI) chain extender The polymerization of aniline was carried out as follows. Ammonia persulfate (APS) and aniline, mixed with 1 M HCl aqueous solution respectively, were individually kept in a refrigerator overnight. To control the molar mass, a suitable p-phenylene diamine (p-PDA) was mixed with the aniline solution. To the aniline solution, APS aqueous solution was added dropwise and the mixture was kept at 0 5 °C for 2 h with continuous stirring. The resulting polyaniline was isolated by filtration and dedoped by stirring in 0.1 M aqueous solution of ammonia for 24 h followed by filtration. The filtered cake was dried in a vacuum oven for 3 days and ground into powder by mortar and pestle. The yield was about 78.5%. Number average molecular weight (Mn) determined by gel permeation chromatography (GPC) was found to be 1849 g/mol. 2.3. Synthesis of the polyurethane prepolymer Isocyanate terminated PU prepolymer was prepared by reacting 0.02 mol of MDI and 0.01 mol of PTMG in 70 ml NMP at 80–85 °C for 3 h. This solution was then chain extended with 0.01 mol of chain extender solution in the following step. 2.4. Preparation of PU/PANI/silica conductive hybrids by sequential chain extension reaction Two chain extender solutions were prepared by mixing x mole fraction of amine-endcapped PANI and y mole
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with the residual isocyanate-endcapped polyols. In a similar manner, the reaction was carried out at 70 °C for 1 h to obtain the resultant PU/PANI/silica hybrid. The molar ratio of the amine-endcapped PANI relative to the SiO2–OH was varied to control the content of urethane-aniline block copolymer in the PU/PANI/silica hybrid but keeping the total amount equal to that of PU prepolymers. Samples were labeled according to the following notation: PU-xPANI-
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fraction of nano-silica bearing surface silanol groups (SiO2– OH) in NMP, respectively, where x + y = 1. To the above stirred prepolymer solution, a stoichiometric amount of SiO2–OH chain extender solution was added with a slow stream of nitrogen purge. The reaction was continued for 1 h at 70 °C to obtain the polyurethane-silica network. Next, the calculated amount of PANI chain extender solution was added to the above resulting solution to react
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Scheme 1. Synthetic route for the preparation of PU/PANI/silica polymer hybrids.
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ySiO2. Seven samples were prepared, where x is equal to 0, 0.1, 0.3, 0.5, 0.7, 0.9, and 1.0, respectively. These samples were then doped with dodecylbenzenesulfonic acid (DBSA) except PU–SiO2. The procedure for the sample preparation is illustrated in Scheme 1.
the polyurethane prepolymer was prepared via the conventional prepolymer technique for typical polyurethane synthesis. Following this step, a series of PU/PANI/silica conductive hybrids were obtained by means of sequential polymerization of polyurethane prepolymer with various stoichiometric amounts of silica nanoparticles (bearing surface silanol groups) and the PANI oligomer. These polyurethane hybrids consist of the urethane-aniline block copolymer dispersed in a polyurethane-silica network as shown in Scheme 1. The synthetic data and GPC results for all PU/PANI/silica polymer hybrids are listed in Tables 1 and 2, respectively. In the synthesis of PANI oligomers, it has been indicated that method of feeding aniline has a noticeable effect on the yield due to the reaction being exothermic [25]. In the case of feeding aniline in one portion or several portions, rapid heat production would result in lower molecular weights and yields. Therefore, the reactions were always carried out by continuous dropping of aniline in an ice-water bath below room temperature. The resulting amine-endcapped polyaniline displayed number average molecular weights of up to 1849 g/mol at a yield of 78.5%. Since it has been found that the oligomeric PANI with 7–8 aniline units can reach the conductivity level of conventional PANI [26], we decided to use the oligomeric PANI with number average molecular weight of 1849 g/mol as our conductive chain extender. For typical polyurethanes, the unusual properties of these copolymers are directly related to their two-phase structures. The twophase feature is due to thermodynamic incompatibility of the soft and hard-segment. In this study, both amine-endcapped PANI and nano-silica bearing silanol groups were used as chain extenders and became part of the hard-segment. Since silica particles bearing multi-silanol groups on the surface reacted with the PU prepolymer, a polyurethane-silica network was obtained. After this reaction, the residual isocyanate groups reacted with the aminecapped PANI to form the PU-PANI copolymers. Therefore,
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2.5. Characterization Infrared spectra of the samples were obtained with a Bio-Rad FTS 165 FTIR spectrometer over the frequency range of 4000–400 cm1 at a resolution of 4 cm1. Solidstate 13C and 29Si NMR spectra were recorded by means of polarization transfer from 1H nuclei and magic angle spinning (CP-MAS) on a Bruker Avance 400 FT-NMR spectrometer with tetramethylsilane (TMS) as the internal standard. The spinning rate was 5 kHz. Number average (Mn), weight average (Mw) molecular weights and molecular weight distribution (MWDs) (Mw=Mn) were determined by gel permeation chromatography (GPC) using Waters liquid chromatograph equipped with a 410 RI detector. NMP was used as the eluent. Ten standard polystyrenes were used for the calibration. Wide-angle X-ray diffractograms (WAXD) were obtained on a Rigaku Geiger Flex D-Max III, using Ni-filtered CuKa radiation (40 kV, 15 mA). The morphologies of the conductive films were examined with a JEOL 5610 scanning electron microscope (SEM). Thermogravimetric analysis (TGA) experiments of the PU/PANI/silica hybrids were carried out on films placed in a platinum sample pan with a TA Instruments SDT-2960 analyzer. The samples were heated from 50 to 800 °C under nitrogen and air atmosphere respectively, at the heating rate of 20 °C/min. Differential scanning calorimetry (DSC) thermograms from 150 to 200 °C were obtained with a TA Instruments modulated DSC 2920 analyzer at a heating rate of 5 °C/min under a dry nitrogen purge. Stress–strain data of the elastomeric films were obtained using a Universal Testing Machine (Shimadzu AGS-500A Series) with a 10 kg load cell and film grips. The crosshead speed was 50 mm/min. Surface resistivity measurements were carried out by a surface resistivity instrument (CHY-24CR LCR Meter).
Table 2 GPC results of PU/PANI/silica polymer hybrids.
3. Results and discussion 3.1. Preparation of PU/PANI/silica polymer hybrids The PU/PANI/silica conductive composites were synthesized by three sequential reactions. The first reaction was to synthesize polyaniline (PANI) oligomer powders. Then
Sample
Mn 103 (g/mol)
Mw 103 (g/mol)
PDI (Mw=Mn)
PU-SiO2 PU-PANI PU-0.1PANI-0.9SiO2 PU-0.3PANI-0.7SiO2 PU-0.5PANI-0.5SiO2 PU-0.7PANI-0.3SiO2 PU-0.9PANI-0.1SiO2
42.0 194.9 138.9 143.4 111.3 146.9 118.3
125.8 439.8 433.7 324.3 236.0 308.8 413.0
3.0 2.3 3.1 2.3 2.1 2.1 3.5
Table 1 Synthetic data of PU/PANI/silica polymer hybrids. Sample
MDI (mmol)
PTMG (mmol)
PANI (mmol)
PANI (wt%)
SiO2 (mmol)
SiO2 (wt%)
PU-SiO2 PU-PANI PU-0.1PANI-0.9SiO2 PU-0.3PANI-0.7SiO2 PU-0.5PANI-0.5SiO2 PU-0.7PANI-0.3SiO2 PU-0.9PANI-0.1SiO2
7.62 7.62 7.62 7.62 7.62 7.62 7.62
3.81 3.81 3.81 3.81 3.81 3.81 3.81
0 3.81 0.38 1.14 1.91 2.67 3.43
0 55.20 10.62 26.41 37.74 46.06 52.51
3.81 0 3.43 2.67 1.91 1.14 0.38
3.44 0 2.79 1.80 1.09 0.57 0.17
T.-L. Wang et al. / European Polymer Journal 45 (2009) 387–397
urethane hybrid, the stronger the hydrogen-bonded carbonyl group is observed. Clearly, the incorporated PANI molecules in the hybrids have groups being able to induce hydrogen bonding and can alter the ratio of free vs. Hbonded urethane carbonyl groups of polyurethane.
PU-0.9PANI-0.1SiO2 PU-0.7PANI-0.3SiO2
3.3. Solid-state
PU-0.5PANI-0.5SiO2 PU-0.3PANI-0.7SiO2 PU-0.1PANI-0.9SiO2 PU-SiO2
3500
3000
2500
2000 1500 -1 Wavenumber (cm )
1000
Fig. 1. FT-IR spectra of PU/PANI/silica polymer hybrids.
the PU/PANI/silica hybrids were prepared. However, no urea groups were present in the polyurethane-silica network, while the urethane-aniline block copolymers have lots of urea groups in the hard-segments. More urea groups in the PU/PANI/silica hybrids would result in a higher degree of phase mixing between hard and soft-segments due to the interaction of urea groups of PU-PANI with ether oxygens of PTMG soft-segments. Consequently, physical properties of the resultant PU/PANI/silica hybrids were then affected and varied with the mole ratio of PANI/silica. 3.2. IR characterization The IR spectra of PU-PANI, PU-SiO2 and the PU/PANI/silica polyurethane hybrids are shown in Fig. 1. Since the polyurethane hybrids consist of urethane-aniline block copolymer and polyurethane-silica network, the characteristic peaks of polyurethane, PANI, urethane-aniline copolymer and PU-silica hybrids should be present in the spectra. In all of the spectra, the strong peak around 3350 cm1 comes from the stretching vibration of N–H bond in the urethane segments. The absorption peak around 1700– 1735 cm1 is due to the stretching vibration of C@O bond of the urethane, and the peak at about 1600 cm1 corresponds to C@C stretching peak in the benzene ring. Two peaks at 1012 and 1510 cm1 arise from symmetric and asymmetric stretching vibration of N–C–N in the urethane-aniline copolymers, corresponding to the reactions of the –NCO groups with the –NH2 groups of the PANI. The benzenoid and quinoid absorption peaks of the doped PANI appear at 1501 and 1587 cm1, respectively. Two small peaks at 1006 and 1034 cm1 are ascribed to the –SO3 H groups after doped with DBSA. The broader absorption around 1100 cm1 accompanied by a shoulder at ca. 1080 cm1 can be attributed to the C–O–C and Si–O–C stretching vibrations, respectively. Combining the above results, it is concluded that the PU/PANI/silica conductive composites have been successfully prepared. Furthermore, the absorptions at 1734 and 1707 cm1 are assigned to non-bonded and hydrogen-bonded urethane carbonyl C@O stretching, respectively. It is noteworthy from the spectra that the higher the PANI content is contained in the poly-
13
C NMR
Fig. 2 shows the 13C CP/MAS NMR spectra of PANI, urethane–aniline copolymer (PU-PANI), PU-silica network and a representative PU/PANI/silica hybrid (PU-0.5PANI0.5SiO2). As shown in Fig. 2a, the repeat unit of emeraldine base form of PANI was confirmed by solid-state 13C NMR to consist of a benzenoid–quinoid alternating structure. The assignment of all different carbons is consistent with those previously published results [17,27,28]. As seen from Fig. 2b, the 13C NMR spectrum of the PU–PANI appears merely as a superposition of the individual spectra of the component polymers, i.e., PANI and PU prepolymer. Two strong and sharp peaks centered at ca. 27.2 and 71.1 ppm are ascribed to the PTMG soft-segment carbons. The peak at 154 ppm is attributed to the resonance of urethane carbonyl. The resonance at 40 ppm is ascribed to –CH2– groups of MDI. These assignments are in agreement with those published values [17,29]. However, it is broader and weaker in comparison with that of neat polyurethane. According to the results of Wang et al. [17], this can be attributed to that the resonance of –CH2– in MDI is restricted by the neighboring PANI oligomers. In addition, the aromatic region ranging from 115 to 140 ppm has contributions from all of the PANI carbons and from the aromatic rings of the PU prepolymer. However, because the chemical shifts of resonance peaks in this region are affected by the disturbance of electron clouds from individual components [17], the peaks presenting in the spectrum are broader as compared to those of neat polyurethanes. In this region, the peak at 119 and 129 ppm are assigned to the benzenoid protonated ring carbons of PANI and the protonated aromatic MDI carbons, while the peak at 136 ppm is associated with the quinoid protonated ring carbons of PANI and quaternary MDI ring carbons [17,27–29]. Next, from the spectrum of PU-SiO2 (Fig. 2c), it also shows a spectrum similar to that of neat polyurethane. As compared to that of PU-PANI, weaker peaks in the aromatic region are observed, while the peak at 40 ppm shows a little stronger. It is clearly that these differences in peak intensity result from the absence of PANI segments in this polymer hybrid. On the other hand, if the silica and PANI were incorporated into the polymer hybrid simultaneously, a much different spectrum would be observed. As seen from Fig. 2d, the peak at 40 ppm is not as discernible as that of PUSiO2. It is probably arising from that the resonance of –CH2– in MDI unit of PU-silica network is disturbed by the trapped PU-PANI copolymer. Some peaks in the regions of aromatic and urethane carbonyl are almost invisible, which is also ascribed to the electron clouds disturbance arising from the trapped PU-PANI. 3.4. Solid-state
29
Si CP-MAS NMR
Fig. 3 shows the solid-state 29Si CP-MAS NMR spectroscopy of the neat silica particles and the polyure-
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Transmittance (%)
PU-PANI
4000
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392
Fig. 2. Solid-state
13
C NMR of (a) PANI, (b)PU-PANI, (c) PU-SiO2, and (d) PU-0.5PANI-0.5SiO2. Asterisks (*) denote spinning side bands.
thane propolymer chain extended with SiO2–OH nanoparticles. In the spectrum of neat silica nanoparticles (Fig. 3a), the chemical shifts of the Q2, Q3, and Q4 sil-
icon nuclei of bulk silica are observed as a trimodal signal at 91.2, 100.1, and 108.5 ppm, respectively. Here, Qn indicates the SiO unit with formula of
Fig. 3. Solid-state
29
393
Si NMR of (a) SiO2 and (b) PU-SiO2.
Si(OSi)n(OH)4n [30–34]. The major constituents were Q3 and Q4 species for the silica nanoparticles, and Q2 was barely detected. In the spectrum of PU-SiO2 (Fig. 3b), the chemical shifts of Q4 silicon nuclei of bulk silica are observed at 108.7 ppm. From the spectrum, it is obvious that Q2 and Q3 are almost invisible, indicating almost all of the surface silanol groups of silica particles have reacted with NCO groups of polyurethane prepolymers. 3.5. UV–vis absorption spectra To investigate the conductive behavior of the conducting polyurethane hybrids, UV–vis is a powerful characterization tool. Since the PU/silica hybrids have no effect on the resonance absorptions of urethane-aniline copolymers, all of the absorption peaks shown in Fig. 4 have the same characteristic wavelength. As seen in Fig. 4a, the peak around 280 nm is related to the absorption in the benzene ring of PU and DBSA. Two characteristic absorption bands at 330 and 570 nm are attributed to the p–p* transition of the benzenoid rings and the n– p* transition of the quinoid rings, respectively. Moreover, for all conductive polyurethane hybrids, the absorption intensity around 570 nm increases with the increasing content of doped PANI, as depicted by Fig. 4b. Our UV– vis results are consistent with the earlier conclusion of the successful synthesis of hybrids, as discussed in the IR section.
3.6. X-ray diffraction Fig. 5 illustrates the X-ray diffraction patterns of PU/ PANI/silica polymer hybrids. For both PU-PANI and PUSiO2 materials, the patterns show broad peaks around 2h = 20.0°. Furthermore, all PU/PANI/silica polymer hybrids show similar XRD peak characteristics. The X-ray patterns for these samples indicate that the samples are typical of semicrystalline/amorphous materials. In particular, all PANI-containing hybrids have a sharp reflection in the small angle region at ca. 2h = 2.0 3.5°. They are attributed to the layered structures whereby the alkyl side chains arising from the DBSA dopant function as spacers between the stacks of the PANI main chains [35–40]. 3.7. SEM The SEM images of PU-PANI, PU-SiO2 and a representative example of PU/PANI/silica (PU-0.5PANI-0.5SiO2) polymer hybrids are shown in Fig. 6. For the PU-PANI copolymer, a smooth and grayish surface is observed (Fig. 6a). In contrast, a whitish and uniformly dispersed polymer network is present in the PU-SiO2 hybrid (Fig. 6b). Therefore, when PANI and silica nanoparticles were incorporated into the polyurethane hybrid (e.g., PU-0.5PANI-0.5SiO2), the SEM image looks like a combination of the above two pictures. It can be seen clearly that PU-PANI is dispersed homogeneously and interconnected continuously in the insulating PU-silica matrix, as demonstrated in Fig. 6c.
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3.8. Thermogravimetric analysis of polyurethane hybrids
PU-0.1PANI-0.9SiO2 PU-0.3PANI-0.7SiO2 PU-0.5PANI-0.5SiO2 PU-0.7PANI-0.3SiO2
Absorption
PU-0.9PANI-0.1SiO2
200
300
400
500
600
700
800
Wavelength (nm) PU-0.1PANI-0.9SiO2 PU-0.5PANI-0.5SiO2 PU-0.7PANI-0.3SiO2
Absorption
PU-0.9PANI-0.1SiO2
400
500
600
700
800
Wavelength (nm) Fig. 4. UV–vis spectra of (a) PU/PANI/silica conductive hybrids and (b) UV–vis spectra of PU/PANI/silica conductive hybrids in a wavelength window between 400 and 800 nm.
PU-SiO2
Relative Intensity
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PU-0.3PANI-0.7SiO2
PU-0.1PANI-0.9SiO2 PU-0.3PANI-0.7SiO2 PU-0.5PANI-0.5SiO2 PU-0.7PANI-0.3SiO2 PU-0.9PANI-0.1SiO2 PU-PANI 10
20
30
40
50
60
2θ (degrees) Fig. 5. X-ray diffraction patterns of PU/PANI/silica polymer hybrids.
The thermal stability for the polyurethane hybrids in nitrogen is illustrated in Fig. 7. As seen in the figure, for all of the polymer samples, the degradation thermograms are similar to those of urethane-aniline copolymers except those of PU-SiO2 hybrid and neat PU (PU-EDA). All TGA curves of PU/PANI/silica polymer hybrids show two stages of degradation while only one stage of degradation is present for PU-SiO2 and PU-EDA. In view of the onset temperature of degradation observed, the thermal stabilities of all hybrids decrease with the increase of PANI content. As compared to the TGA curve of PU-0.1PANI-0.9SiO2, it is surprising that the thermal stability of PU-SiO2 is much higher. Actually, the PU-SiO2 hybrid show a similar thermogram as the typical polyurethane, indicating the temperature stability of PU-SiO2 is as stable as that of typical polyurethane. As seen from Table 1, the weight percent of SiO2 in the PU-SiO2 is only 3.44%, while that of PANI in the PU-PANI is 55.20%. Therefore, it is clearly that the presence of PANI segment results in the rapid degradation of the PU/PANI/silica polymer hybrids. There is a vast literature on the thermal stability of PANI blends, hybrids, etc [41–48]. Based on their findings, some conclusions are presented as follows: (i) water, solvent and free DBSA are released below 220 °C, (ii) degradation and evaporation of the bounded dopants occur around 250 330 °C, and (iii) degradation of PANI backbones starts at around 330 °C. Consequently, it can be concluded from Fig. 7 that there is no excess DBSA left due to degradation of all PANI-containing hybrids starting at a temperature of ca. 200 °C. On the other hand, for typical polyurethanes, it has been indicated that the initial degradation occurs in the hard-segment while the apparent weight loss is correlated with the soft-segment [49]. In the sequential polymerization of polyurethane hybrids, the PANI segments end-capped with amine groups and silica nanoparticles bearing surface silanol groups (SiO2–OH) are both used as the chain extender and function as the hard-segment. Since the PU-SiO2 hybrid is more stable than all of the other samples, it is postulated that the silica hard-segment is more stable than the PANI segment. In addition, the dopant DBSA is a small organic molecule compared to the PU/PANI/silica polymer hybrids. Therefore, for the first step degradation, based on the aforementioned conclusions, it is probable that the initial degradation is caused by the combination of removal of the dopant DBSA and degradation of the MDI and PANI hard-segment. After the start of degradation, the rapid weight loss in the first step is due to the PTMG soft-segment and continuing degradation of the MDI and PANI hard-segment. However, if the hard-segment is more stable, e.g., the SiO2–OH hard-segment is more stable than the doped PANI segment; the degradation temperature of soft-segment is higher as illustrated by the thermogram of PUSiO2. This can be attributed to the mutual stabilization effect of the hard and soft-segment, which has been reported previously by us [50]. In the second step (up to 500 °C) degradation, a slow and somewhat gradual weight loss profile is noticed for all of the polyurethane hybrids,
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PU-PANI PU-0.9PANI-0.1SiO2 PU-0.7PANI-0.3SiO2 PU-0.5PANI-0.5SiO2 PU-0.3PANI-0.7SiO2 PU-0.1PANI-0.9SiO2 PU-SiO2 PU-EDA
100
Weight(%)
80
60
40
20
0 0
100
200
300 400 500 Temperature(oC)
600
700
800
PU-PANI PU-0.9PANI-0.1SiO2 PU-0.7PANI-0.3SiO2 PU-0.5PANI-0.5SiO2 PU-0.3PANI-0.7SiO2 PU-0.1PANI-0.9SiO2 PU-SiO2 PU-EDA
100
Weight(%)
80
60
40
20
0 0
100
200
300
400
500
600
700
800
Temperature(oC) Fig. 8. TGA curves of PU/PANI/silica polymer hybrids under air atmosphere.
Fig. 6. SEM micrographs of (a) PU-PANI, (b) PU-SiO2, and (c) PU-0.5PANI0.5SiO2.
which is ascribed to the continuing degradation of the remaining PANI segments since the weight loss (%) decreases with the decrease of PANI content. In addition, it is found that the char residue over 500 °C increases with the PANI content. It is apparent that the higher content of char residue is caused by the higher content of PANI in the hybrid, i.e., part of the PANI hard-segments has left as a residue. In contrast, because both SiO2–OH and EDA hard-segment are more thermally stable than
the doped PANI hard-segment, the thermograms of both PU-SiO2 and PU-EDA show higher onset temperatures of degradation and only one stage of degradation is observed. When the TGA experiments were carried out in air, the thermograms display a three-stage of degradation, as depicted in Fig. 8. In a similar trend, the first degradation step for all PANI-containing polymer hybrids can be attributed to the initial degradation of MDI and PANI hard-segment accompanied by removal of the dopant DBSA, followed with the rapid degradation and oxidation reactions of PTMG soft-segment and partially degradation of the MDI and PANI hard-segment. Since it has been stated that oxidation reactions of the hard-segment occur in a later stage than those of the soft-segment, the second degradation step is probably associated with the complete degradation of PTMG soft-segment and oxidation reactions along with continuing degradation of the MDI and PANI hard-segment [49,50]. Finally, the residue of the PANI left in the second step is totally degraded in the third stage. Furthermore, at high temperatures, the neat PU and all PU hybrids show less weight loss in air because the oxidation of polymer backbones has caused weight gain [48].
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Fig. 7. TGA curves of PU/PANI/silica polymer hybrids under nitrogen atmosphere.
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5.0 4.5 4.0
Stress(Mpa)
3.5 3.0 2.5 PU-PANI PU-0.9PANI-0.1SiO2
2.0
PU-0.7PANI-0.3SiO2
1.5
PU-0.5PANI-0.5SiO2 PU-0.3PANI-0.7SiO2
1.0
PU-0.1PANI-0.9SiO2
0.5
PU-SiO2
0.0
0
50
100 150 200 250 300 350 400 450 Strain(%)
Fig. 10. Stress–strain curves of PU/PANI/silica polymer hybrids.
Fig. 9. DSC curves of PU/PANI/silica polymer hybrids. (a) PU-SiO2, (b) PU-0.1PANI-0.9SiO2, (c) PU-0.3PANI-0.7SiO2, (d) PU-0.5PANI-0.5SiO2, (e) PU-0.7PANI-0.3SiO2, (f) PU-0.9PANI-0.1SiO2, (g) PU-PANI.
3.9. Differential scanning calorimetry Fig. 9 and Table 3 demonstrate the effect of PANI content on the phase separation of the polyurethane hybrids. The picture and data presented in Fig. 9 and Table 3 show that the soft-segment Tg of all hybrids increases with the increase of PANI content. It is apparent that the lower soft-segment Tg arises from the higher degree of phase separation which is due to the incorporation of higher mole ratio of inorganic silica particles in the hard-segment. The increase of incompatibility between the soft-segment and hard-segment results in the higher degree of phase separation and lower soft-segment Tg. Conversely, if the PANI content is higher, phase mixing will increase due to the increase of hydrogen bonding arising from the amine groups of PANI hard-segment with the ether groups of the PTMG soft-segment.
The stress–strain curves for all of the polyurethane hybrids show the elastomeric behavior of typical polyurethanes. It can be seen clearly that the higher the PANI content the stronger the tensile strength, as illustrated in Fig. 10. On the contrary, for all of the samples, the extensibility is lower if the PANI content is higher. Therefore, as compared to the neat PU-Silica hybrid, the PANI-containing polyurethane hybrids are more rigid and brittle. The stronger tensile strengths are probably due to the presence of urea groups in these hybrids, while the higher extensibility is owing to that only urethane groups are contained in the neat PU-Silica hybrid. In addition, the more brittle behavior of the PANI-containing polyurethane hybrids is attributed to more phase mixing present in these samples, while the higher extensibility of PU-SiO2 is ascribed to a higher degree of phase separation in this material. 3.11. Surface resistivity The effect of PANI content on the surface resistivity of the polyurethane hybrids is depicted in Fig. 11. For the neat (a) PU-0.1PANI-0.9SiO2 (b) PU-0.3PANI-0.7SiO2 (c) PU-0.5PANI-0.5SiO2
1E11
(d) PU-0.7PANI-0.3SiO2 (e) PU-0.9PANI-0.1SiO2 (f ) PU-PANI
Ohm/sq
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3.10. Tensile properties
Table 3 DSC results of PU/PANI/silica polymer hybrids. Sample
Tg (°C)
PU-SiO2 PU-0.1PANI-0.9SiO2 PU-0.3PANI-0.7SiO2 PU-0.5PANI-0.5SiO2 PU-0.7PANI-0.3SiO2 PU-0.9PANI-0.1SiO2 PU-PANI
52.7 50.0 47.2 41.1 33.7 26.8 25.6
1E10
1E9
1E8
(a)
(b)
(c)
(d) Sample
(e)
(f)
Fig. 11. Surface resistivity values of PU/PANI/silica conductive hybrids.
PU-silica hybrids, the surface resistivity value is in the order of 1012 X/sq., while the value for the PU-PANI is around 3.2 108 X/sq. However, for the rest of PANI-containing polymer hybrids, the surface resistivity value decreases as the PANI content increases. Their values are around 109 1012 X/sq. and might meet the requirement of the anti-electrostatic materials. As seen in the figure, the surface resistivity values for PANI content that ranging from 0.3 M ratio to 0.9 M ratio (based on one mole chain extender) show little variation. Therefore, the percolation threshold of PANI for these hybrids may be between 0.1 mol (10.62 wt%) and 0.3 mol (26.41 wt%). It is probable that the urethane-aniline copolymers containing PANI segments higher than 10.62 wt% can form an interconnected phase in the insulting PU-silica matrix as shown in Scheme 1. 4. Conclusions The PU/PANI/silica conductive hybrids were successfully synthesized by three sequential reactions and confirmed by FT-IR, solid-state 13C, and 29Si NMR spectra. UV–vis absorption spectra for all conductive hybrids showed the same characteristic wavelength as that of PU-PANI copolymer. In addition, the absorption intensity around 580–630 nm increased with the doped PANI content. SEM images for the conductive hybrids displayed that PANI was dispersed homogeneously and interconnected continuously in the insulating PU-silica matrix. It is noteworthy that the degree of phase separation plays an important role in the thermal stability, glass-transition temperature and tensile properties of the polyurethane hybrids. When inorganic nanosilica particles were incorporated into the polymer hybrids, higher thermal stability, Tg, and extensibility were obtained due to the higher degree of phase separation. From the surface resistivity measurements, the percolation threshold of PANI was attained at a weight ratio of PANI content ranging from 10.62 to 26.41 wt% and the conductive hybrids could be used as antistatic materials. References [1] MacDiarmid AG, Epstein AJ. Faraday Discuss Chem Soc 1989;88:317. [2] Kaneko M, Nakamura H. J Chem Soc Chem Commun 1985:1441. [3] Huang WS, Humphrey BD, MacDiarmid AG. J Chem Soc Faraday Trans 1 1986;82:2385. [4] Cao Y, Smith P, Heeger A. Synth Met 1992;48(1):91. [5] Genie‘s EM, Boyle A, Lapkowski M, Tsintavis C. Synth Met 1990;36:139. [6] Anand J, Palaniappan S, Sathyanarayana DN. Prog Polym Sci 1998;23:993.
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[7] De Paoli M-A. In: Nalwa HS, editor. Handbook of organic conductive molecules and polymers, vol. 2. New York: John Wiley & Sons Ltd; 1997 [chapter 18]. [8] Jousseaume V, Morsli M, Bonnet A, Tesson O, Lefrant S. J Appl Polym Sci 1998;67:1205. [9] Segal E, Haba Y, Narkis M, Siegmann A. J Polym Sci B Polym Phys 2001;39:611. [10] Abraham D, Bharathi A, Subramanyam SV. Polymer 1996;37:5295. [11] Yang J, Zhao C, Cui D, Hou J, Wan M, Xu M. J Appl Polym Sci 1995;56:831. [12] Lee WJ, Kim YJ, Kaang S. Synth Met 2000;113:237. [13] Frisch HL, Ma J, Song H, Frisch KC, Mengnjoh PC, Molla AH. J Appl Polym Sci 2001;80:893. [14] Jeon BH, Kim S, Choi MH, Chung I. Synth Met 1999;104:1160. [15] Wang P, Tan KL, Zhang F, Kang ET, Neoh KG. Chem Mater 2001;13:581. [16] Chen Y, Kang ET, Neoh KG, Tan KL. Polymer 2000;41:3279. [17] Wang YZ, Hsu YC, Wu RR, Kao HM. Synth Met 2003;132:151. [18] Liu JY, Yang CC, Wang YZ. J Appl Polym Sci 2007;103:3803. [19] Granot E, Basnar B, Cheglakov Z, Katz E, Willner I. Electroanalysis 2006;18:26. [20] Granot E, Katz E, Basnar B, Willner I. Chem Mater 2005;17:4600. [21] Lee CF, Tsai HH, Wang LY, Chen CF, Chiu WY. J Polym Sci A Polym Chem 2005;43:342. [22] Liu P, Liu WM, Xue QJ. Mater Chem Phys 2004;87:109. [23] Dutta K, De SK. Phys Lett A 2007;361:141. [24] Luo J, Wang X, Li J, Zhao X, Wang F. Polymer 2007;48:4368. [25] Xie HQ, Ma YM, Feng DS. Eur Polym J 2000;36:2201. [26] Wei Y, Yeh JM, Wang J, Jia X, Yang C, Jin D. Polym Mater Sci Eng 1996;74:202. [27] Kaplan S, Conwell EM, Richter AF, MacDiarmid AG. J Am Chem Soc 1988;110:7647. [28] Hjertberg T, Salaneck WR, Lundstrom I, Somasiri NLD, MacDiarmid AG. J Polym Sci Polym Lett 1985;23:503. [29] Okamoto DT, Cooper SL, Root TW. Macromolecules 1992;25: 1068. [30] Marzouk S, Rachdi F, Fourati M, Bouaziz J. Colloids Surf A Physicochem Eng Aspects 2004;234:109. [31] Chevalier Y, Grillet AC, Rahmi MI, Liere C, Masure M, Hemery P, et al. Mater Sci Eng C 2002;21:143. [32] Xie XQ, Ranade SV, DiBenedetto AT. Polymer 1999;40:6297. [33] Casanovas J, Illas F, Pacchioni G. Chem Phys Lett 2000;326:523. [34] Wang TL, Ou CC, Yang CH. J Appl Polym Sci 2008;109:3421. [35] Taka T, Laakso J, Levon K. Solid State Commun 1994;92(5):393. [36] Zheng WY, Wang RH, Levon K. Macromol Chem Phys 1995; 196:2443. [37] Kim S, Ko JM, Chung JI. Polym Adv Tech 1996;7:599. [38] Han MG, Cho SK, Oh SG, Im SS. Synth Met 2002;126:53. [39] Jia W, Segal E, Kornemandel D, Lamhot Y, Narkis M, Siegmann A. Synth Met 2002;128:115. [40] Pan W, Yang SL, Li G, Jiang MJ. Eur Polym J 2005;41:2. [41] Chan HSO, Ho PKH, Tan MM, Tan KL, Tan BTG, Lim YK. Synth Met 1989;31:95. [42] Han MG, Im SS. J Appl Polym Sci 1999;71:2169. [43] Paul RK, Pillai CKS. J Appl Polym Sci 2002;84:1438. [44] Chen CH. J Polym Res 2002;9:195. [45] Jeevananda T, Siddaramaiah. Polym Eng Sci 2003;43(5):1138. [46] Schmidt V, Domenech SC, Soldi MS, Pinheiro EA, Sildi V. Polym Degrad Stab 2004;83:519. [47] Li X, Wang G, Li X. Surf Coat Tech 2005;197:56. [48] Lu X, Xu J, Wong L. Synth Met 2006;156:117. [49] Ferguson I, Petrovic Z. Eur Polym J 1976;12:177. [50] Wang TL, Hsieh TH. Polym Degrad Stab 1997;55:95.
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T.-L. Wang et al. / European Polymer Journal 45 (2009) 387–397
Contents lists available at ScienceDirect
European Polymer Journal journal homepage: www.elsevier.com/locate/europolj
Macromolecular Nanotechnology
Synthesis and properties of conducting organic/inorganic polyurethane hybrids Tzong-Liu Wang a,*, Chien-Hsin Yang a, Yeong-Tarng Shieh a, An-Chi Yeh b a
a r t i c l e
i n f o
a b s t r a c t
Article history: Received 9 June 2008 Received in revised form 13 October 2008 Accepted 14 November 2008 Available online 21 November 2008
A series of polyurethane/polyaniline/silica organic/inorganic hybrids were synthesized via the conventional polyurethane (PU) prepolymer technique. Amine-endcapped polyaniline (PANI) with low molecular weight and higher solubility was firstly synthesized. This PANI oligomer was then used together with nano-silica bearing silanol groups as chain extenders to prepare the conducting polyurethane hybrids. The polyurethane hybrids were designated as PU-xPANI-ySiO2 (x + y = 1). For comparison, the urethane-aniline block copolymer and the PU/silica hybrid were designated as PU-PANI and PU-SiO2, respectively. The structures of PU-PANI, PU-SiO2 and conducting polyurethane hybrids were confirmed by FT-IR, solid-state 13C, and 29Si NMR spectra. In nano-silica containing organic/inorganic conducting polyurethane hybrids, UV–vis spectra revealed the maximum absorption bands similar to that of PU-PANI. X-ray diffraction patterns indicated that these samples are typical of semicrystalline/amorphous materials. SEM image of PU-0.5PANI-0.5SiO2 showed that PANI was dispersed homogeneously and interconnected continuously in the insulating PUsilica matrix. TGA results of the polymer hybrids exhibited higher thermal stabilities and lower decomposition rates than that of PU-PANI both in nitrogen and air. Differential scanning calorimetry (DSC) studies indicated that the polyurethane hybrids had higher glasstransition temperatures (Tg) with the increase of PANI, but lower than that of PU-PANI. Stress–strain curves for all of the polyurethane hybrids showed the elastomeric behavior of typical polyurethanes. The surface resistivity values of all hybrids were about 108 1010 X/sq. and might meet the requirement of the anti-electrostatic materials. Ó 2008 Elsevier Ltd. All rights reserved.
Keywords: Organic/inorganic hybrids Polyurethane prepolymer Nano-silica Block copolymer Surface resistivity
1. Introduction
tivity and potential application in electronic devices [1,2]. The general formula for ideal PANI materials in their base forms consists in three (–C6H4–NH–) benzenoid units and one (–N@C6H4@N–) quinoid unit [1].
Polyaniline (PANI) is a conducting polymer which has been extensively studied due to its relatively high conduc-
NH
NH
N y
* Corresponding author. Tel.: +886 7 5919278; fax: +886 7 5919277. E-mail address: [email protected] (T.-L. Wang). 0014-3057/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.eurpolymj.2008.11.020
N 1-y n
The y value accounts for the oxidation state of the polymer. Leucoemeraldine base (LEB), emeraldine base (EB) and pernigraniline base (PNB) correspond to the
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Department of Chemical and Materials Engineering, National University of Kaohsiung, No. 700, Kaohsiung University Road, Nan-Tzu District, Kaohsiung 811, Taiwan, ROC b Department of Chemical and Materials Engineering, Cheng Shiu University, Kaohsiung County 833, Taiwan, ROC
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completely reduced (y = 1), half oxidized (y = 0.5) and completely oxidized (y = 0) states, respectively. Whereas the three bases are insulating materials, base protonation leads to the corresponding salts. Emeraldine salt (ES) is the only one exhibiting high electric conductivity and has consequently received considerable interest. The increase in electronic conductivity that occurs after the transition emeraldine base (EB) to emeraldine salt (ES), involves a doping mechanism different from those commonly observed with conducting polymers [3–5]. Unfortunately, the poor processability of PANI and its inadequate mechanical properties limit its commercial applications. In order to overcome these problems, numerous methods have been studied [6,7]. A conducting polymer blend with PANI and conventional polymer can exhibit good mechanical properties associated with interesting electrical properties [8–14]. However, blending has very little effect on the environmental stability of the conducting polymer itself. One of the major limitations of the blending is that it almost always produces a highly heterogeneous two-phase morphology. Therefore, it may be more effective to prepare a conducting polymer with improved processibility and mechanical property by the chemical synthesis approach. The combination of PANI segments with blocks of another polymer has been recently reported, such as the grafting of polyethylene glycol [15] or polyacrylic acid [16] onto a PANI backbone, and a block copolymer consisting of polyurethane and PANI oligomer [17,18], etc. The use of a thermoplastic elastomer with conducting polymers is very attractive due to the combination of mechanical properties and a processability which does not require vulcanization. Thermoplastic polyurethane (PU) elastomer is one of the most versatile products in the group of engineering thermoplastics with excellent physical properties, chemical resistance, abrasion resistance as well as its ease of processing. Typically, thermoplastic PU elastomers (i.e., segmented copolyurethanes) are block copolymers of the (A–B)n type, consisting of alternating soft and hard-segments. The soft-segment is typically a long chain polyol which is a polyester-, polyether-, or polyalkyl-diol with a molecular weight between 500 and 5000. The hard-segment generally consists of an aromatic diisocyanate chain extended with a low molecular weight diol or diamine (i.e., chain extender). The unusual properties of these copolymers are directly related to their two-phase structures. The two-phase feature of the original reaction mixture is due to thermodynamic incompatibility of the different block polymer segments. On the other hand, conductive composites of PANI and inorganic compounds have been synthesized in order to get new materials with modified properties [19–21]. Among those inorganic materials, silica (SiO2) has received great attention because of its unique properties and wide applications [22,23]. Recently, a conductive hybrid composed of PANI dispersed in a polyurethane-silica matrix via the sol-gel process was also reported [24]. In past several years, polyurethane-related composites have been extensively studied in our laboratory; herein we present a simple methodology to synthesize a kind of elastomeric PU/PANI/silica conductive hybrids using the conventional polyurethane prepolymer technique. To meet the
above-mentioned requirements, both amine-endcapped PANI and nano-silica bearing silanol groups were used as chain extenders. Hence, this kind of conductive hybrid consists of one type of urethane-aniline block copolymer homogeneously dispersed in the matrix of urethane-silica polymer network. In this work, a series of elastomeric polyurethane hybrids with different mole ratios of PANI and silica were synthesized and their properties were also investigated. 2. Experimental 2.1. Materials 4,40 -Methylene bis(4-phenyl isocyanate) (MDI, Aldrich), 1-methyl-2-pyrrolidinone (NMP, FERAK), aniline (ANI, TCI), p-phenylenediamine (p-PDA, TCI) were distilled under reduced pressure. Ammonium persulfate (APS), dodecylbenzene sulfonic acid (DBSA), hydrochloric acid (HCl) and ammonia solution were all from Union Chemical Works Ltd. and used without further purification. The silica nanoparticles (Mw = 60.09 g/mol) bearing surface silanol groups (18.7 mmol OH/g) were purchased from Desunnano Co., Ltd.; the purity is higher than 99% and the polydispersity is 1.20. The surface area and particle size are 640 m2/g and 10 nm, respectively. Polytetramethylene ether glycol (PTMG, Aldrich) with molecular weight (Mn = 1000) was degassed under vacuum at 55 °C at 600 Pa (4.5 mm Hg) for 3 h to remove any absorbed water. 2.2. Preparation of amine-endcapped polyaniline (PANI) chain extender The polymerization of aniline was carried out as follows. Ammonia persulfate (APS) and aniline, mixed with 1 M HCl aqueous solution respectively, were individually kept in a refrigerator overnight. To control the molar mass, a suitable p-phenylene diamine (p-PDA) was mixed with the aniline solution. To the aniline solution, APS aqueous solution was added dropwise and the mixture was kept at 0 5 °C for 2 h with continuous stirring. The resulting polyaniline was isolated by filtration and dedoped by stirring in 0.1 M aqueous solution of ammonia for 24 h followed by filtration. The filtered cake was dried in a vacuum oven for 3 days and ground into powder by mortar and pestle. The yield was about 78.5%. Number average molecular weight (Mn) determined by gel permeation chromatography (GPC) was found to be 1849 g/mol. 2.3. Synthesis of the polyurethane prepolymer Isocyanate terminated PU prepolymer was prepared by reacting 0.02 mol of MDI and 0.01 mol of PTMG in 70 ml NMP at 80–85 °C for 3 h. This solution was then chain extended with 0.01 mol of chain extender solution in the following step. 2.4. Preparation of PU/PANI/silica conductive hybrids by sequential chain extension reaction Two chain extender solutions were prepared by mixing x mole fraction of amine-endcapped PANI and y mole
T.-L. Wang et al. / European Polymer Journal 45 (2009) 387–397
with the residual isocyanate-endcapped polyols. In a similar manner, the reaction was carried out at 70 °C for 1 h to obtain the resultant PU/PANI/silica hybrid. The molar ratio of the amine-endcapped PANI relative to the SiO2–OH was varied to control the content of urethane-aniline block copolymer in the PU/PANI/silica hybrid but keeping the total amount equal to that of PU prepolymers. Samples were labeled according to the following notation: PU-xPANI-
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fraction of nano-silica bearing surface silanol groups (SiO2– OH) in NMP, respectively, where x + y = 1. To the above stirred prepolymer solution, a stoichiometric amount of SiO2–OH chain extender solution was added with a slow stream of nitrogen purge. The reaction was continued for 1 h at 70 °C to obtain the polyurethane-silica network. Next, the calculated amount of PANI chain extender solution was added to the above resulting solution to react
389
Scheme 1. Synthetic route for the preparation of PU/PANI/silica polymer hybrids.
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ySiO2. Seven samples were prepared, where x is equal to 0, 0.1, 0.3, 0.5, 0.7, 0.9, and 1.0, respectively. These samples were then doped with dodecylbenzenesulfonic acid (DBSA) except PU–SiO2. The procedure for the sample preparation is illustrated in Scheme 1.
the polyurethane prepolymer was prepared via the conventional prepolymer technique for typical polyurethane synthesis. Following this step, a series of PU/PANI/silica conductive hybrids were obtained by means of sequential polymerization of polyurethane prepolymer with various stoichiometric amounts of silica nanoparticles (bearing surface silanol groups) and the PANI oligomer. These polyurethane hybrids consist of the urethane-aniline block copolymer dispersed in a polyurethane-silica network as shown in Scheme 1. The synthetic data and GPC results for all PU/PANI/silica polymer hybrids are listed in Tables 1 and 2, respectively. In the synthesis of PANI oligomers, it has been indicated that method of feeding aniline has a noticeable effect on the yield due to the reaction being exothermic [25]. In the case of feeding aniline in one portion or several portions, rapid heat production would result in lower molecular weights and yields. Therefore, the reactions were always carried out by continuous dropping of aniline in an ice-water bath below room temperature. The resulting amine-endcapped polyaniline displayed number average molecular weights of up to 1849 g/mol at a yield of 78.5%. Since it has been found that the oligomeric PANI with 7–8 aniline units can reach the conductivity level of conventional PANI [26], we decided to use the oligomeric PANI with number average molecular weight of 1849 g/mol as our conductive chain extender. For typical polyurethanes, the unusual properties of these copolymers are directly related to their two-phase structures. The twophase feature is due to thermodynamic incompatibility of the soft and hard-segment. In this study, both amine-endcapped PANI and nano-silica bearing silanol groups were used as chain extenders and became part of the hard-segment. Since silica particles bearing multi-silanol groups on the surface reacted with the PU prepolymer, a polyurethane-silica network was obtained. After this reaction, the residual isocyanate groups reacted with the aminecapped PANI to form the PU-PANI copolymers. Therefore,
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2.5. Characterization Infrared spectra of the samples were obtained with a Bio-Rad FTS 165 FTIR spectrometer over the frequency range of 4000–400 cm1 at a resolution of 4 cm1. Solidstate 13C and 29Si NMR spectra were recorded by means of polarization transfer from 1H nuclei and magic angle spinning (CP-MAS) on a Bruker Avance 400 FT-NMR spectrometer with tetramethylsilane (TMS) as the internal standard. The spinning rate was 5 kHz. Number average (Mn), weight average (Mw) molecular weights and molecular weight distribution (MWDs) (Mw=Mn) were determined by gel permeation chromatography (GPC) using Waters liquid chromatograph equipped with a 410 RI detector. NMP was used as the eluent. Ten standard polystyrenes were used for the calibration. Wide-angle X-ray diffractograms (WAXD) were obtained on a Rigaku Geiger Flex D-Max III, using Ni-filtered CuKa radiation (40 kV, 15 mA). The morphologies of the conductive films were examined with a JEOL 5610 scanning electron microscope (SEM). Thermogravimetric analysis (TGA) experiments of the PU/PANI/silica hybrids were carried out on films placed in a platinum sample pan with a TA Instruments SDT-2960 analyzer. The samples were heated from 50 to 800 °C under nitrogen and air atmosphere respectively, at the heating rate of 20 °C/min. Differential scanning calorimetry (DSC) thermograms from 150 to 200 °C were obtained with a TA Instruments modulated DSC 2920 analyzer at a heating rate of 5 °C/min under a dry nitrogen purge. Stress–strain data of the elastomeric films were obtained using a Universal Testing Machine (Shimadzu AGS-500A Series) with a 10 kg load cell and film grips. The crosshead speed was 50 mm/min. Surface resistivity measurements were carried out by a surface resistivity instrument (CHY-24CR LCR Meter).
Table 2 GPC results of PU/PANI/silica polymer hybrids.
3. Results and discussion 3.1. Preparation of PU/PANI/silica polymer hybrids The PU/PANI/silica conductive composites were synthesized by three sequential reactions. The first reaction was to synthesize polyaniline (PANI) oligomer powders. Then
Sample
Mn 103 (g/mol)
Mw 103 (g/mol)
PDI (Mw=Mn)
PU-SiO2 PU-PANI PU-0.1PANI-0.9SiO2 PU-0.3PANI-0.7SiO2 PU-0.5PANI-0.5SiO2 PU-0.7PANI-0.3SiO2 PU-0.9PANI-0.1SiO2
42.0 194.9 138.9 143.4 111.3 146.9 118.3
125.8 439.8 433.7 324.3 236.0 308.8 413.0
3.0 2.3 3.1 2.3 2.1 2.1 3.5
Table 1 Synthetic data of PU/PANI/silica polymer hybrids. Sample
MDI (mmol)
PTMG (mmol)
PANI (mmol)
PANI (wt%)
SiO2 (mmol)
SiO2 (wt%)
PU-SiO2 PU-PANI PU-0.1PANI-0.9SiO2 PU-0.3PANI-0.7SiO2 PU-0.5PANI-0.5SiO2 PU-0.7PANI-0.3SiO2 PU-0.9PANI-0.1SiO2
7.62 7.62 7.62 7.62 7.62 7.62 7.62
3.81 3.81 3.81 3.81 3.81 3.81 3.81
0 3.81 0.38 1.14 1.91 2.67 3.43
0 55.20 10.62 26.41 37.74 46.06 52.51
3.81 0 3.43 2.67 1.91 1.14 0.38
3.44 0 2.79 1.80 1.09 0.57 0.17
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urethane hybrid, the stronger the hydrogen-bonded carbonyl group is observed. Clearly, the incorporated PANI molecules in the hybrids have groups being able to induce hydrogen bonding and can alter the ratio of free vs. Hbonded urethane carbonyl groups of polyurethane.
PU-0.9PANI-0.1SiO2 PU-0.7PANI-0.3SiO2
3.3. Solid-state
PU-0.5PANI-0.5SiO2 PU-0.3PANI-0.7SiO2 PU-0.1PANI-0.9SiO2 PU-SiO2
3500
3000
2500
2000 1500 -1 Wavenumber (cm )
1000
Fig. 1. FT-IR spectra of PU/PANI/silica polymer hybrids.
the PU/PANI/silica hybrids were prepared. However, no urea groups were present in the polyurethane-silica network, while the urethane-aniline block copolymers have lots of urea groups in the hard-segments. More urea groups in the PU/PANI/silica hybrids would result in a higher degree of phase mixing between hard and soft-segments due to the interaction of urea groups of PU-PANI with ether oxygens of PTMG soft-segments. Consequently, physical properties of the resultant PU/PANI/silica hybrids were then affected and varied with the mole ratio of PANI/silica. 3.2. IR characterization The IR spectra of PU-PANI, PU-SiO2 and the PU/PANI/silica polyurethane hybrids are shown in Fig. 1. Since the polyurethane hybrids consist of urethane-aniline block copolymer and polyurethane-silica network, the characteristic peaks of polyurethane, PANI, urethane-aniline copolymer and PU-silica hybrids should be present in the spectra. In all of the spectra, the strong peak around 3350 cm1 comes from the stretching vibration of N–H bond in the urethane segments. The absorption peak around 1700– 1735 cm1 is due to the stretching vibration of C@O bond of the urethane, and the peak at about 1600 cm1 corresponds to C@C stretching peak in the benzene ring. Two peaks at 1012 and 1510 cm1 arise from symmetric and asymmetric stretching vibration of N–C–N in the urethane-aniline copolymers, corresponding to the reactions of the –NCO groups with the –NH2 groups of the PANI. The benzenoid and quinoid absorption peaks of the doped PANI appear at 1501 and 1587 cm1, respectively. Two small peaks at 1006 and 1034 cm1 are ascribed to the –SO3 H groups after doped with DBSA. The broader absorption around 1100 cm1 accompanied by a shoulder at ca. 1080 cm1 can be attributed to the C–O–C and Si–O–C stretching vibrations, respectively. Combining the above results, it is concluded that the PU/PANI/silica conductive composites have been successfully prepared. Furthermore, the absorptions at 1734 and 1707 cm1 are assigned to non-bonded and hydrogen-bonded urethane carbonyl C@O stretching, respectively. It is noteworthy from the spectra that the higher the PANI content is contained in the poly-
13
C NMR
Fig. 2 shows the 13C CP/MAS NMR spectra of PANI, urethane–aniline copolymer (PU-PANI), PU-silica network and a representative PU/PANI/silica hybrid (PU-0.5PANI0.5SiO2). As shown in Fig. 2a, the repeat unit of emeraldine base form of PANI was confirmed by solid-state 13C NMR to consist of a benzenoid–quinoid alternating structure. The assignment of all different carbons is consistent with those previously published results [17,27,28]. As seen from Fig. 2b, the 13C NMR spectrum of the PU–PANI appears merely as a superposition of the individual spectra of the component polymers, i.e., PANI and PU prepolymer. Two strong and sharp peaks centered at ca. 27.2 and 71.1 ppm are ascribed to the PTMG soft-segment carbons. The peak at 154 ppm is attributed to the resonance of urethane carbonyl. The resonance at 40 ppm is ascribed to –CH2– groups of MDI. These assignments are in agreement with those published values [17,29]. However, it is broader and weaker in comparison with that of neat polyurethane. According to the results of Wang et al. [17], this can be attributed to that the resonance of –CH2– in MDI is restricted by the neighboring PANI oligomers. In addition, the aromatic region ranging from 115 to 140 ppm has contributions from all of the PANI carbons and from the aromatic rings of the PU prepolymer. However, because the chemical shifts of resonance peaks in this region are affected by the disturbance of electron clouds from individual components [17], the peaks presenting in the spectrum are broader as compared to those of neat polyurethanes. In this region, the peak at 119 and 129 ppm are assigned to the benzenoid protonated ring carbons of PANI and the protonated aromatic MDI carbons, while the peak at 136 ppm is associated with the quinoid protonated ring carbons of PANI and quaternary MDI ring carbons [17,27–29]. Next, from the spectrum of PU-SiO2 (Fig. 2c), it also shows a spectrum similar to that of neat polyurethane. As compared to that of PU-PANI, weaker peaks in the aromatic region are observed, while the peak at 40 ppm shows a little stronger. It is clearly that these differences in peak intensity result from the absence of PANI segments in this polymer hybrid. On the other hand, if the silica and PANI were incorporated into the polymer hybrid simultaneously, a much different spectrum would be observed. As seen from Fig. 2d, the peak at 40 ppm is not as discernible as that of PUSiO2. It is probably arising from that the resonance of –CH2– in MDI unit of PU-silica network is disturbed by the trapped PU-PANI copolymer. Some peaks in the regions of aromatic and urethane carbonyl are almost invisible, which is also ascribed to the electron clouds disturbance arising from the trapped PU-PANI. 3.4. Solid-state
29
Si CP-MAS NMR
Fig. 3 shows the solid-state 29Si CP-MAS NMR spectroscopy of the neat silica particles and the polyure-
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Transmittance (%)
PU-PANI
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Fig. 2. Solid-state
13
C NMR of (a) PANI, (b)PU-PANI, (c) PU-SiO2, and (d) PU-0.5PANI-0.5SiO2. Asterisks (*) denote spinning side bands.
thane propolymer chain extended with SiO2–OH nanoparticles. In the spectrum of neat silica nanoparticles (Fig. 3a), the chemical shifts of the Q2, Q3, and Q4 sil-
icon nuclei of bulk silica are observed as a trimodal signal at 91.2, 100.1, and 108.5 ppm, respectively. Here, Qn indicates the SiO unit with formula of
Fig. 3. Solid-state
29
393
Si NMR of (a) SiO2 and (b) PU-SiO2.
Si(OSi)n(OH)4n [30–34]. The major constituents were Q3 and Q4 species for the silica nanoparticles, and Q2 was barely detected. In the spectrum of PU-SiO2 (Fig. 3b), the chemical shifts of Q4 silicon nuclei of bulk silica are observed at 108.7 ppm. From the spectrum, it is obvious that Q2 and Q3 are almost invisible, indicating almost all of the surface silanol groups of silica particles have reacted with NCO groups of polyurethane prepolymers. 3.5. UV–vis absorption spectra To investigate the conductive behavior of the conducting polyurethane hybrids, UV–vis is a powerful characterization tool. Since the PU/silica hybrids have no effect on the resonance absorptions of urethane-aniline copolymers, all of the absorption peaks shown in Fig. 4 have the same characteristic wavelength. As seen in Fig. 4a, the peak around 280 nm is related to the absorption in the benzene ring of PU and DBSA. Two characteristic absorption bands at 330 and 570 nm are attributed to the p–p* transition of the benzenoid rings and the n– p* transition of the quinoid rings, respectively. Moreover, for all conductive polyurethane hybrids, the absorption intensity around 570 nm increases with the increasing content of doped PANI, as depicted by Fig. 4b. Our UV– vis results are consistent with the earlier conclusion of the successful synthesis of hybrids, as discussed in the IR section.
3.6. X-ray diffraction Fig. 5 illustrates the X-ray diffraction patterns of PU/ PANI/silica polymer hybrids. For both PU-PANI and PUSiO2 materials, the patterns show broad peaks around 2h = 20.0°. Furthermore, all PU/PANI/silica polymer hybrids show similar XRD peak characteristics. The X-ray patterns for these samples indicate that the samples are typical of semicrystalline/amorphous materials. In particular, all PANI-containing hybrids have a sharp reflection in the small angle region at ca. 2h = 2.0 3.5°. They are attributed to the layered structures whereby the alkyl side chains arising from the DBSA dopant function as spacers between the stacks of the PANI main chains [35–40]. 3.7. SEM The SEM images of PU-PANI, PU-SiO2 and a representative example of PU/PANI/silica (PU-0.5PANI-0.5SiO2) polymer hybrids are shown in Fig. 6. For the PU-PANI copolymer, a smooth and grayish surface is observed (Fig. 6a). In contrast, a whitish and uniformly dispersed polymer network is present in the PU-SiO2 hybrid (Fig. 6b). Therefore, when PANI and silica nanoparticles were incorporated into the polyurethane hybrid (e.g., PU-0.5PANI-0.5SiO2), the SEM image looks like a combination of the above two pictures. It can be seen clearly that PU-PANI is dispersed homogeneously and interconnected continuously in the insulating PU-silica matrix, as demonstrated in Fig. 6c.
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3.8. Thermogravimetric analysis of polyurethane hybrids
PU-0.1PANI-0.9SiO2 PU-0.3PANI-0.7SiO2 PU-0.5PANI-0.5SiO2 PU-0.7PANI-0.3SiO2
Absorption
PU-0.9PANI-0.1SiO2
200
300
400
500
600
700
800
Wavelength (nm) PU-0.1PANI-0.9SiO2 PU-0.5PANI-0.5SiO2 PU-0.7PANI-0.3SiO2
Absorption
PU-0.9PANI-0.1SiO2
400
500
600
700
800
Wavelength (nm) Fig. 4. UV–vis spectra of (a) PU/PANI/silica conductive hybrids and (b) UV–vis spectra of PU/PANI/silica conductive hybrids in a wavelength window between 400 and 800 nm.
PU-SiO2
Relative Intensity
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PU-0.3PANI-0.7SiO2
PU-0.1PANI-0.9SiO2 PU-0.3PANI-0.7SiO2 PU-0.5PANI-0.5SiO2 PU-0.7PANI-0.3SiO2 PU-0.9PANI-0.1SiO2 PU-PANI 10
20
30
40
50
60
2θ (degrees) Fig. 5. X-ray diffraction patterns of PU/PANI/silica polymer hybrids.
The thermal stability for the polyurethane hybrids in nitrogen is illustrated in Fig. 7. As seen in the figure, for all of the polymer samples, the degradation thermograms are similar to those of urethane-aniline copolymers except those of PU-SiO2 hybrid and neat PU (PU-EDA). All TGA curves of PU/PANI/silica polymer hybrids show two stages of degradation while only one stage of degradation is present for PU-SiO2 and PU-EDA. In view of the onset temperature of degradation observed, the thermal stabilities of all hybrids decrease with the increase of PANI content. As compared to the TGA curve of PU-0.1PANI-0.9SiO2, it is surprising that the thermal stability of PU-SiO2 is much higher. Actually, the PU-SiO2 hybrid show a similar thermogram as the typical polyurethane, indicating the temperature stability of PU-SiO2 is as stable as that of typical polyurethane. As seen from Table 1, the weight percent of SiO2 in the PU-SiO2 is only 3.44%, while that of PANI in the PU-PANI is 55.20%. Therefore, it is clearly that the presence of PANI segment results in the rapid degradation of the PU/PANI/silica polymer hybrids. There is a vast literature on the thermal stability of PANI blends, hybrids, etc [41–48]. Based on their findings, some conclusions are presented as follows: (i) water, solvent and free DBSA are released below 220 °C, (ii) degradation and evaporation of the bounded dopants occur around 250 330 °C, and (iii) degradation of PANI backbones starts at around 330 °C. Consequently, it can be concluded from Fig. 7 that there is no excess DBSA left due to degradation of all PANI-containing hybrids starting at a temperature of ca. 200 °C. On the other hand, for typical polyurethanes, it has been indicated that the initial degradation occurs in the hard-segment while the apparent weight loss is correlated with the soft-segment [49]. In the sequential polymerization of polyurethane hybrids, the PANI segments end-capped with amine groups and silica nanoparticles bearing surface silanol groups (SiO2–OH) are both used as the chain extender and function as the hard-segment. Since the PU-SiO2 hybrid is more stable than all of the other samples, it is postulated that the silica hard-segment is more stable than the PANI segment. In addition, the dopant DBSA is a small organic molecule compared to the PU/PANI/silica polymer hybrids. Therefore, for the first step degradation, based on the aforementioned conclusions, it is probable that the initial degradation is caused by the combination of removal of the dopant DBSA and degradation of the MDI and PANI hard-segment. After the start of degradation, the rapid weight loss in the first step is due to the PTMG soft-segment and continuing degradation of the MDI and PANI hard-segment. However, if the hard-segment is more stable, e.g., the SiO2–OH hard-segment is more stable than the doped PANI segment; the degradation temperature of soft-segment is higher as illustrated by the thermogram of PUSiO2. This can be attributed to the mutual stabilization effect of the hard and soft-segment, which has been reported previously by us [50]. In the second step (up to 500 °C) degradation, a slow and somewhat gradual weight loss profile is noticed for all of the polyurethane hybrids,
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PU-PANI PU-0.9PANI-0.1SiO2 PU-0.7PANI-0.3SiO2 PU-0.5PANI-0.5SiO2 PU-0.3PANI-0.7SiO2 PU-0.1PANI-0.9SiO2 PU-SiO2 PU-EDA
100
Weight(%)
80
60
40
20
0 0
100
200
300 400 500 Temperature(oC)
600
700
800
PU-PANI PU-0.9PANI-0.1SiO2 PU-0.7PANI-0.3SiO2 PU-0.5PANI-0.5SiO2 PU-0.3PANI-0.7SiO2 PU-0.1PANI-0.9SiO2 PU-SiO2 PU-EDA
100
Weight(%)
80
60
40
20
0 0
100
200
300
400
500
600
700
800
Temperature(oC) Fig. 8. TGA curves of PU/PANI/silica polymer hybrids under air atmosphere.
Fig. 6. SEM micrographs of (a) PU-PANI, (b) PU-SiO2, and (c) PU-0.5PANI0.5SiO2.
which is ascribed to the continuing degradation of the remaining PANI segments since the weight loss (%) decreases with the decrease of PANI content. In addition, it is found that the char residue over 500 °C increases with the PANI content. It is apparent that the higher content of char residue is caused by the higher content of PANI in the hybrid, i.e., part of the PANI hard-segments has left as a residue. In contrast, because both SiO2–OH and EDA hard-segment are more thermally stable than
the doped PANI hard-segment, the thermograms of both PU-SiO2 and PU-EDA show higher onset temperatures of degradation and only one stage of degradation is observed. When the TGA experiments were carried out in air, the thermograms display a three-stage of degradation, as depicted in Fig. 8. In a similar trend, the first degradation step for all PANI-containing polymer hybrids can be attributed to the initial degradation of MDI and PANI hard-segment accompanied by removal of the dopant DBSA, followed with the rapid degradation and oxidation reactions of PTMG soft-segment and partially degradation of the MDI and PANI hard-segment. Since it has been stated that oxidation reactions of the hard-segment occur in a later stage than those of the soft-segment, the second degradation step is probably associated with the complete degradation of PTMG soft-segment and oxidation reactions along with continuing degradation of the MDI and PANI hard-segment [49,50]. Finally, the residue of the PANI left in the second step is totally degraded in the third stage. Furthermore, at high temperatures, the neat PU and all PU hybrids show less weight loss in air because the oxidation of polymer backbones has caused weight gain [48].
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Fig. 7. TGA curves of PU/PANI/silica polymer hybrids under nitrogen atmosphere.
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5.0 4.5 4.0
Stress(Mpa)
3.5 3.0 2.5 PU-PANI PU-0.9PANI-0.1SiO2
2.0
PU-0.7PANI-0.3SiO2
1.5
PU-0.5PANI-0.5SiO2 PU-0.3PANI-0.7SiO2
1.0
PU-0.1PANI-0.9SiO2
0.5
PU-SiO2
0.0
0
50
100 150 200 250 300 350 400 450 Strain(%)
Fig. 10. Stress–strain curves of PU/PANI/silica polymer hybrids.
Fig. 9. DSC curves of PU/PANI/silica polymer hybrids. (a) PU-SiO2, (b) PU-0.1PANI-0.9SiO2, (c) PU-0.3PANI-0.7SiO2, (d) PU-0.5PANI-0.5SiO2, (e) PU-0.7PANI-0.3SiO2, (f) PU-0.9PANI-0.1SiO2, (g) PU-PANI.
3.9. Differential scanning calorimetry Fig. 9 and Table 3 demonstrate the effect of PANI content on the phase separation of the polyurethane hybrids. The picture and data presented in Fig. 9 and Table 3 show that the soft-segment Tg of all hybrids increases with the increase of PANI content. It is apparent that the lower soft-segment Tg arises from the higher degree of phase separation which is due to the incorporation of higher mole ratio of inorganic silica particles in the hard-segment. The increase of incompatibility between the soft-segment and hard-segment results in the higher degree of phase separation and lower soft-segment Tg. Conversely, if the PANI content is higher, phase mixing will increase due to the increase of hydrogen bonding arising from the amine groups of PANI hard-segment with the ether groups of the PTMG soft-segment.
The stress–strain curves for all of the polyurethane hybrids show the elastomeric behavior of typical polyurethanes. It can be seen clearly that the higher the PANI content the stronger the tensile strength, as illustrated in Fig. 10. On the contrary, for all of the samples, the extensibility is lower if the PANI content is higher. Therefore, as compared to the neat PU-Silica hybrid, the PANI-containing polyurethane hybrids are more rigid and brittle. The stronger tensile strengths are probably due to the presence of urea groups in these hybrids, while the higher extensibility is owing to that only urethane groups are contained in the neat PU-Silica hybrid. In addition, the more brittle behavior of the PANI-containing polyurethane hybrids is attributed to more phase mixing present in these samples, while the higher extensibility of PU-SiO2 is ascribed to a higher degree of phase separation in this material. 3.11. Surface resistivity The effect of PANI content on the surface resistivity of the polyurethane hybrids is depicted in Fig. 11. For the neat (a) PU-0.1PANI-0.9SiO2 (b) PU-0.3PANI-0.7SiO2 (c) PU-0.5PANI-0.5SiO2
1E11
(d) PU-0.7PANI-0.3SiO2 (e) PU-0.9PANI-0.1SiO2 (f ) PU-PANI
Ohm/sq
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3.10. Tensile properties
Table 3 DSC results of PU/PANI/silica polymer hybrids. Sample
Tg (°C)
PU-SiO2 PU-0.1PANI-0.9SiO2 PU-0.3PANI-0.7SiO2 PU-0.5PANI-0.5SiO2 PU-0.7PANI-0.3SiO2 PU-0.9PANI-0.1SiO2 PU-PANI
52.7 50.0 47.2 41.1 33.7 26.8 25.6
1E10
1E9
1E8
(a)
(b)
(c)
(d) Sample
(e)
(f)
Fig. 11. Surface resistivity values of PU/PANI/silica conductive hybrids.
PU-silica hybrids, the surface resistivity value is in the order of 1012 X/sq., while the value for the PU-PANI is around 3.2 108 X/sq. However, for the rest of PANI-containing polymer hybrids, the surface resistivity value decreases as the PANI content increases. Their values are around 109 1012 X/sq. and might meet the requirement of the anti-electrostatic materials. As seen in the figure, the surface resistivity values for PANI content that ranging from 0.3 M ratio to 0.9 M ratio (based on one mole chain extender) show little variation. Therefore, the percolation threshold of PANI for these hybrids may be between 0.1 mol (10.62 wt%) and 0.3 mol (26.41 wt%). It is probable that the urethane-aniline copolymers containing PANI segments higher than 10.62 wt% can form an interconnected phase in the insulting PU-silica matrix as shown in Scheme 1. 4. Conclusions The PU/PANI/silica conductive hybrids were successfully synthesized by three sequential reactions and confirmed by FT-IR, solid-state 13C, and 29Si NMR spectra. UV–vis absorption spectra for all conductive hybrids showed the same characteristic wavelength as that of PU-PANI copolymer. In addition, the absorption intensity around 580–630 nm increased with the doped PANI content. SEM images for the conductive hybrids displayed that PANI was dispersed homogeneously and interconnected continuously in the insulating PU-silica matrix. It is noteworthy that the degree of phase separation plays an important role in the thermal stability, glass-transition temperature and tensile properties of the polyurethane hybrids. When inorganic nanosilica particles were incorporated into the polymer hybrids, higher thermal stability, Tg, and extensibility were obtained due to the higher degree of phase separation. From the surface resistivity measurements, the percolation threshold of PANI was attained at a weight ratio of PANI content ranging from 10.62 to 26.41 wt% and the conductive hybrids could be used as antistatic materials. References [1] MacDiarmid AG, Epstein AJ. Faraday Discuss Chem Soc 1989;88:317. [2] Kaneko M, Nakamura H. J Chem Soc Chem Commun 1985:1441. [3] Huang WS, Humphrey BD, MacDiarmid AG. J Chem Soc Faraday Trans 1 1986;82:2385. [4] Cao Y, Smith P, Heeger A. Synth Met 1992;48(1):91. [5] Genie‘s EM, Boyle A, Lapkowski M, Tsintavis C. Synth Met 1990;36:139. [6] Anand J, Palaniappan S, Sathyanarayana DN. Prog Polym Sci 1998;23:993.
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