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Thermal degradation of electrical conductivity of polyacrylic acid doped polyaniline: effect of molecular weight of the dopants
Synthetic Metals 138 (2003) 429–440
Thermal degradation of electrical conductivity of polyacrylic acid doped polyaniline: effect of molecular weight of the dopants Xuehong Lua,*, Chiang Yang Tana, Jianwei Xub, Chaobin Heb a
School of Materials Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore b Institute of Materials Research & Engineering, 3 Research Link, Singapore 117602, Singapore Received 10 July 2002; received in revised form 30 September 2002; accepted 13 October 2002
Abstract This paper describes the effect of molecular weight of dopants on thermal degradation behaviour of electrical conductivity of polyacrylic acid (PAA) doped polyaniline (PANI). Two PAA with weight-average molecular weight (Mw) of 5000 and 250,000, respectively, were used as dopants to synthesise PANI–PAA complexes by in situ oxidative polymerisation. PANI doped with the low Mw PAA has lower electrical conductivity, which is attributed to its smaller size of crystal islands due to the end group effect. When annealed at 180 8C, within a period of 2 h the conductivity of PANI doped with the high Mw PAA decreases continuously with the annealing time following s ¼ s0 exp½ðta =tÞ1=2 law, while the one doped with the low Mw PAA obeys this law only within 1 h of annealing. This indicates that the decrease of conducting island size is responsible for the thermal degradation of the conductivity of the complexes in a certain period and the length of this period depends on Mw of the dopants. TGA study shows that at 180 8C weight loss rates of the low and high Mw PAA doped PANI are about the same, which implies that the faster reduction of the conductivity in the low Mw PAA doped PANI is mainly due to its smaller initial crystal size. FTIR study shows that annealing leads to de-doping, but it is less pronounced in the high Mw PAA doped PANI. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Polyaniline; Polyacrylic acid; Electrical conductivity; Thermal degradation
1. Introduction Protonated polyaniline (PANI) is an important class of intrinsic conductive polymers because of their excellent stability in air. A number of researches have shown that protonated PANI has a heterogeneous structure in which metallic-like crystalline ‘‘islands’’ are separated by less conducting, amorphous ‘‘sea’’, and the electronic transport is governed by 3D hopping between the crystalline regions [1]. Electrical conductivity of protonated PANI degrades after thermal treatment at elevated temperatures. The conductivity degradation has been attributed to the contraction of the conducting islands [2], which may be caused by crosslinking reaction of PANI [3], degradation [4], evaporation or sublimation of the dopants and residual solvents [5] and segregation of the dopants from PANI [6]. It has been proposed that use of polymeric acids to replace small molecule acids as the dopants may enhance the thermal stability of protonated PANI because the evaporation of the dopants may be restricted and the tendency of * Corresponding author. Tel.: þ65-790-4585; fax: þ65-790-9081. E-mail address: [email protected] (X. Lu).
phase segregation weakened because the long chain nature of the dopants. Polyacrylic acid (PAA) doped PANI has been studied as one of such approaches. It was reported that PANI–PAA complexes have a moderate electrical conductivity of about 102 to 103 s/cm and the thermal degradation of the conductivity is indeed slowed down compared to PANI doped with small molecule acids [7,8]. However, to our knowledge there is no report on the formation of crystalline islands in chemically synthesised PANI–PAA complexes so that it is not clear if the decrease of conducting island size is responsible for the thermal degradation of electrical conductivity observed in this system. Furthermore, based on the scenario described earlier it seems that longer PAA chains would lead to less thermal degradation of electrical conductivity of the PANI–PAA complexes. While the dopant chain length may also affect the size of the crystal islands in the PANI–PAA complexes, if they do exist, so that its effect on electrical conductivity and its thermal degradation behaviour may not be straightforward. To address the above issues, in this work, two polyacrylic acids with different molecular weight were used as dopants to synthesise PANI–PAA complexes by in situ oxidative polymerisation. The effect of the molecular weight of PAA on the
0379-6779/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 2 ) 0 0 4 7 1 - X
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electrical conductivity of the complexes and their thermal degradation behaviours are studied in relation to the crystalline structures. The study provided some important information to understand the major factors governing the thermal degradation of electrical conductivity of the PANI–PAA complexes.
2. Experimental 2.1. Materials
where I, Vand t are current (A) set, voltage (V) measured and thickness of the disc. Since the diameter to thickness ratio of the discs is about 20, a correction factor 4.4364, instead of p/ ln 2, was used in the equation. 2.4. Wide angle X-ray scattering (WAXS) WAXS measurements on the polymer discs were carried out on a Shimadzu XRD-6000 X-ray diffractometer, using Ni-filtered Cu Ka radiation, at room temperature with a scan speed of 18/min from 10 to 458 2y.
Emeraldine base (PANI–Base) was obtained from Aldrich. Polyacrylic acid (PAA) doped polyaniline (PANI) was prepared by oxidative polymerisation of aniline with ammonium peroxydisulfate in the presence of PAA in aqueous solution according to the procedure for preparing PANI–poly(methacrylic acid) complexes [9]. The molar ratio of aniline to PAA was 0.3:0.7, 0.5:0.5 and 0.7:0.3, respectively. Aniline was supplied by Lancaster, and ammonium peroxydisulfate and PAA by Aldrich. The weightaverage molecular weight of PAA was 5000 (in the form of 50 wt.% solution in water) and 250,000 (in the form of 35 wt.% solution in water), which are named as PAA5K and PAA250K, respectively, and the corresponding complexes are named as PANI–PAA5K and PANI–PAA250K, respectively, in this paper. PANI–Base was dried in a vacuum oven at 50 8C for 4 h and the complexes at room temperature for 48 h prior to sample preparation. The polymer powders were then compressed into discs of diameter of 13 mm and thickness of about 0.65 mm under a pressure of 12 t at room temperature.
2.5. Thermogravimetric analysis (TGA)
2.2. Thermal treatment
3. Results and discussion
The polymer discs were heat-treated in air. The heat treatment consists of raising the temperature of the samples in an oven from room temperature to 180 8C at 10 8C/min and holding the samples at 180 8C for 0.5, 1.0, 1.5 and 2.0 h, respectively, and then cooling the samples back to room temperature in a desiccator.
3.1. Formation of crystalline islands in the PANI–PAA complexes
2.3. Conductivity measurement A four-point probe system (Signatone SP4-62.5-85-TC) with a Keithley 220 programmable current source and Keithley 2000 digital multimeter was used to measure the electrical conductivity of the polymer discs. For each material with the same thermal history, four discs were tested and the conductivity of each disc was measured four times at different positions of the disc. The average of 16 measurements was taken as the conductivity of each sample set. The formulae used for calculating conductivity is as follows: 1 V Conductivity; s ¼ 4:4364 t I
The TGA experiments were carried out on fraction of the disc samples using a TGA2850 (TA Instrument) thermogravimetric analyser. Both air and nitrogen were used as purge gas, respectively, in the experiments. Temperature scans were conducted from 25 to 600 8C at a heating rate of 10 8C/min. The samples were also heated from room temperature to 180 8C at a constant rate of 10 8C/min and then subjected to isothermal degradation at 180 8C over a period of 2 h for the purpose of simulating the annealing process in oven. 2.6. Fourier transformed infrared (FT-IR) spectroscopy Infrared spectra were recorded on a FT-IR spectrometer (Perkin-Elmer System 2000) to identify chemical structure changes of the complexes due to the heat treatment. The KBr technique was used to prepare FT-IR test samples.
The first report on WAXS study of PAA doped PANI was from Chen and Lee, whose PANI–PAA complexes were made by blending of emeraldine base with PAA (Mw ¼ 250; 000) in 1-methyl-2-pyrrolidone (NMP) solution and the end products contain about 10–18% NMP [10]. They found that no pronounced diffraction peaks in the WAXS patterns of their PAA/PANI blends at all the dopant concentrations studied and claimed that this is due to the large size of the PAA chains which makes an ordered packing of PANI chains difficult. On the contrary, our PANI–PAA complexes made by in situ polymerisation have several obvious diffraction peaks. As shown in Fig. 1, the X-ray diffraction pattern of the reference material, PANI–Base, is dominated by an amorphous hallo with weak peaks at about 19.7 and 25.48 2y. When PANI is doped with PAA the diffraction pattern is significantly different from the above. The major changes are that a new peak appears at around 15.58 2y, and the peaks at 19.7 and 25.48 2y become more
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Fig. 1. X-ray diffraction patterns of PANI–Base and PANI–PPA250K before heat treatment.
obvious. The X-ray diffraction patterns of the PANI–PAA complexes are somewhat similar to that of EBI–ESI reported by Pouget et al., so that this new peak may be assigned as (0 1 0) peak [11]. The changes can be explained by the enhancement of the long-range ordering because the chains become straighter after the doping. The WAXS pattern given in Fig. 1 is only an example. Similar patterns were observed for both PAA5K and PAA250K doped PANI and at all the dopant concentrations. This indicates that the PANI–PAA complexes can form ordered packing structures despite the long chain characteristic of the dopants. The structural difference between our PANI–PAA complexes and Chen and Lee’s PANI–PAA blends is caused by the difference in material preparation method [10]. By in situ polymerisation, the doping happens spontaneously with the
polymerisation so that PANI chains may grow along PAA chains to form relatively straight, rigid PANI–PAA strands which favours ordered packing. While by solution blending, both PAA and PANI chains have some conformational freedom in the NMP solution so that they can hardly be parallel to each other. Doping happens in a more or less random fashion among many different chains, which frustrate ordered inter-chain packing. 3.2. Effect of molecular weight of PAA on electrical conductivity of the complexes Fig. 2 shows the dependence of electrical conductivity of the PANI–PAA complexes on PAA molar concentration. The maximum conductivity was found at the PAA molar
Fig. 2. Electrical conductivity of (&) PANI–PAA5K and (&) PANI–PPA250K complexes as a function of PAA molar concentration.
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concentration of about 50 mol%, which is consistent with other reports [10]. The conductivity value of PANI– PAA250K at the PAA concentration of 50 mol% is one magnitude higher than the corresponding PANI–PAA blend of Chen and Lee’s [10]. This is obviously due to the more ordered structures of the complexes made by in situ polymerisation. In the whole range of composition, PANI–PAA250K shows higher electrical conductivity than PANI–PAA5K, which means that a higher molecular weight of the dopants leads to a higher electrical conductivity of the complexes. This fact may be related to the short length of PAA5K chains that restrict the size of the crystal islands in the complexes due to the end group effect. The weight-average molecular weight of PAA5K is 5000 which is corresponding to a degree of polymerisation of about 35 if we assume the polydispersity of the PAA is 2. The length of a PAA5K chain is, ˚ , which is just long enough to therefore, only about 80 A about seven doping sites on an emeraldine base chain. In this case, the growth of the crystals would be affected significantly by the presence of the end groups. While PAA250K chains are 50 times longer than PAA5K chains so that the crystal size is not restricted by the PAA chain length. Fig. 3(a)–(c) gives the comparison of WAXS patterns between PANI–PAA5K and PANI–PAA250. It shows that at all the three PAA concentrations (0 1 0) peak of PANI– PAA250K is slightly sharper than that of PANI–PAA5K. This indicates that the crystal size of PANI–PAA250K is indeed slightly larger than that of PANI–PAA5K. It is striking to see that at the optimum dopant concentration, i.e. 50 mol% PAA, the molecular weight of PAA only affects the conductivity slightly, while at higher and lower PAA concentrations, the molecular weight of PAA has a much larger impact on the conductivity of the complexes. This may be also due to the end group effect of PAA. The end groups of PAA chains would certainly take part in the amorphous shells around the conducting islands. At the PAA concentration of 30 and 70 mol% the dimension of the amorphous shell is already relatively large so that a further increase in the amorphous zone dimension may cause a sharp decrease of the conductivity. 3.3. Thermal degradation of electrical conductivity of the complexes Since at PAA concentration of 50 mol% the maximum electrical conductivity is achieved for this system, the following discussion on thermal degradation of electrical conductivity will be restricted to the PANI–PAA complexes at this optimum concentration. The focus is the effect of molecular weight of PAA dopants. Before heat treatment, the conductivity of PANI– PAA250K and PANI–PAA5K are 8:5 103 and 8:4 103 s/cm, respectively, in contrast to 1 108 s/cm of PANI–Base. After annealing at 180 8C, the conductivity of the complexes is reduced with the increase of the annealing
time as shown in Fig. 4. After annealing for 2 h, the conductivity of PANI–PAA250K and PANI–PAA5K are reduced by about 66 and 99%, respectively. Obviously, as the molecular weight of PAA increases, the thermal degradation of electrical conductivity of the complexes slows down. For PANI doped with small molecule acids, two models have been studied in relation to their thermal ageing behaviours [2]. The first one assumes a process in which oxygen diffuses into the sample, reacts with the polymers and the incorporation of oxygen into the cross-linked fibrils disrupts the conduction pathways. Based on this model, the relative decrease of conductivity should be proportional to the square root of the ageing time, as expressed in Eq. (1): ðs0 sÞ / ta1=2 s
(1)
where s0 is the initial value of conductivity, ta the ageing time, and s the conductivity at ageing time ta. The second model suggests that the conductive polymers have a granular metal structure with the size of the conducting grains decreasing with the ageing time. Based on this model Eq. (2) should be obeyed. 1=2 ta s ¼ s0 exp t
(2)
where s0 is the initial value of conductivity, ta the ageing time, s the conductivity at ageing time ta and t a constant. In Fig. 5(a) and (b) (s0 s)/s and ln (s/s0) of PANI– PAA5K and PANI–PAA250K were plotted as a function of the square root of the heat treatment time, respectively. From Fig. 5, we can see that the Eq. (1) is not obeyed by either PANI–PAA250K or PANI–PAA5K, while PANI–PAA250K follows the Eq. (2), i.e. the s ¼ s0 exp½ðta =tÞ1=2 law. In fact, within 1.0 h of annealing time the behaviour of PANI–PAA5K also follows Eq. (2) fairly well, although at longer annealing time there is a large deviation. This suggests that the decrease of conducting island size is responsible for the thermal degradation of electrical conductivity observed in this system within a certain period of heat treatment time. The length of this period is affected by the molecular weight of PAA dopants. Fig. 6(a)–(c) show the change of X-ray diffraction patterns of PANI–Base, PANI–PAA5K and PANI–PAA250K, respectively, with annealing time. After 2 h of annealing, the X-ray diffraction pattern of PANI–Base is hardly changed. While for PANI–PAA5K, the decrease of peak sharpness with the increase of annealing time is fairly significant. The trend closely resembles Wolter et al.’s WAXS results on HCl-pronotated PANI which shows progressive evolution of the structure from ES-I phase toward an ‘‘EB-I-like’’ phase [12]. In Fig. 6, the peak at 15.58 2y becomes hardly noticeable when the annealing time is longer than 1 h. This implies that the crystal islands shrink quickly in the first 1 h of annealing and after 1 h the crystal zone size is so small that leads to the deviation from the s ¼ s0 exp½ðta =tÞ1=2 law. For PANI–PAA250K the change of peak sharpness with the
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Fig. 3. X-ray diffraction patterns of PANI–PAA5K and PANI–PAA250K before heat treatment at PAA concentration of: (a) 30 mol%; (b) 50 mol%; and (c) 70 mol%.
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Fig. 4. Electrical conductivity of (&) PANI–PAA5K and (&) PANI–PAA250K as a function of heat treatment time, ta, at 180 8C in air.
annealing time is not very obvious. Only a slight broadening of the (0 1 0) peak is noticeable at the end of 2-h annealing. This coincides with the fact that its conductivity degrades more slowly than PANI–PAA5K. 3.4. TGA study The thermal stability of the polymers was investigated using TGA temperature scan. Fig. 7(a) and (b) illustrates the weight (wt.%) and the derivative (wt.%/8C) of PANI–Base, PAA5K and PAA250K as a function of temperature, respectively. In Fig. 7(b) on the derivative weight curve of PANI–Base a strong peak at about 440 8C, which commences at about 300 8C, is attributed to the decomposition of PANI backbones, and weak derivative weight peaks between 180 and 280 8C are attributed to the loss of small molecules such as solvents and impurities [13]. In contrast to PANI–Base, a strong derivative weight peak commences at about 180 and 190 8C for PAA5K and PAA250K, respectively, as shown in Fig. 7(b). From Fig. 7(a), we can see that the weight loss of both PAA5K and PAA250K in the temperature range of 180–280 8C are about 20%. This heavy weight loss may be associated with chain scission reaction of PAA polymer chains while the second derivative weight peak at about 420 8C is corresponding to degradation of the PAA polymer chains into monomers. An important conclusion drawn from Fig. 7 is that PAA5K chains start to break down at 180 8C, which is the annealing temperature employed in this study, while PAA250K chains are relatively stable at this temperature. This conclusion was verified by isothermal TGA experiments on the three base materials at 180 8C, as shown in Fig. 8. For PANI–Base, the weight loss increases very slowly with the isothermal time, while for both PAA5K and PAA250K there is a weight loss of about 10% within the first 20 min, i.e. the temperature ramping up period, which may
be attributed to evaporation of moisture, monomers and small molecule impurities. After the initial stage the PAA5K curve is much steeper than the PAA250K curve indicating that PAA250K is more thermally stable. To investigate the thermal stability of the complexes, isothermal TGA tests were also carried out on PANI–PAA5K and PANI–PAA250K at 180 8C for 2 h. The experimental results are compared with the isothermal TGA curves of the complexes constructed by proportional addition of the TGA curves of the three base materials, i.e. PANI–Base, PAA5K and PAA250K, as shown in Fig. 9. It can be seen that the experimental curves show larger weight loss than the calculated ones at a certain isothermal time, and the difference is mainly in the initial stage and more pronounced for PANI– PAA250K. This indicates that the existence of PAA chains in the system may disturb the polymerisation reaction so that more monomers or low oligomers were left in the system and subsequently evaporated out during the TGA tests. The higher the molecular weight of PAA the severer the disruption. It is interesting to see that the experimental and calculated curves for PANI–PAA250K are nearly parallel to each other after the initial stage, while in the case of PANI–PAA5K the calculated curve is much steeper than the experimental one. In fact, in the complexes, PAA5K and PAA250K degrades at about the same rate as the two experimental curves are nearly parallel to each other. This implies that the complexion actually helps to slow down the degradation of PAA5K. The faster reduction of the conductivity of PANI–PAA5K complex is, therefore, not because that PAA5K chains break down faster than PAA250K chains at the annealing temperature. As mentioned earlier, it may be related to the smaller initial size of the crystal islands in PANI–PAA5K, whose further shrinking may cause a sharp decrease of the conductivity. Similar isothermal TGA experiments were also conducted using air as the purge gas. The relation between weight loss
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Fig. 5. (a) Relative decrease of electrical conductivity of (&) PANI–PAA5K and (&) PANI–PAA250K; and (b) logarithmic plot of relative electrical conductivity of (&) PANI–PAA5K and (&) PANI–PAA250K as a function of the square root of the heat treatment time, ta, at 180 8C in air. The PAA concentration is 50 mol%.
and isothermal time observed in this case is fairly similar to that observed in nitrogen. This indicates that oxidation is not the major cause for degradation of PAA and PANI–PAA complexes. 3.5. FT-IR study FT-IR spectra obtained from PAA250K, PAA5K, PANI– Base, PANI–PAA250K and PANI–PAA5K in the wave-number range of 500–2000 cm1 are shown in Fig. 10. The presence of quinoid band (1585–1595 cm1) and benzenoid
band (1485–1500 cm1) in PANI–Base, PANI–PAA5K and PANI–PAA250K complexes indicates the emeraldine state of PANI. A moderately weak band at around 1144 cm1 in PANI–Base spectrum, referred as electronic-like band by MacDiarmid and co-workers [14], was considered as the delocalisation of electrons and was thus related to its electrical conductivity. This peak broadens and its intensity increases as compared to the quinoid band after doping, which is responsible for the conductivity increase observed. The peaks in the PAA250K spectrum in general is broader and less defined than those in the PAA5K spectrum, which is
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Fig. 6. X-ray diffraction patterns of: (a) PANI–Base, (b) PANI–PAA5K, and (c) PANI–PAA250K after the heat treatment at 180 8C in air for different times. The PAA concentration is 50 mol%.
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Fig. 7. TGA curves showing: (a) weight; and (b) derivative weight of PANI–Base, PAA5K and PAA250K as a function of temperature. The temperature scans were conducted from 25 to 600 8C at the heating rate of 10 8C/min with nitrogen as purge gas. (^) PANI–Base; (&) PAA5K; (~) PAA250K.
Fig. 8. TGA curves showing weight of PANI–Base, PAA5K and PAA250K as a function of isothermal time at 180 8C in nitrogen. The symbol (- -) represents the temperature profile. (^) PANI–Base; (&) PAA5K; (~) PAA250K.
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Fig. 9. TGA curves showing weight of PANI–PAA5K and PANI–PAA250K complexes as a function of isothermal time in nitrogen. (- -) Represents the temperature profile. The symbols (&) and (~) represent TGA curves obtained from PANI–PAA5K and PANI–PAA250K complexes. The symbol (&) represents PANI–PAA5K curve calculated by proportionally addition of the TGA curves of pure PANI–Base and PAA5K. The symbol (~) represents PANI– PAA250K curve calculated by proportionally addition of the TGA curves of pure PANI–Base and PAA250K.
due to the much higher molecular weight of PAA250K. However, many peaks in PANI–PAA250K spectrum are well defined but can hardly be seen in PANI–PAA5K spectrum due to overlapping, especially in the range of 1400–1800 cm1. For example, the bands at 1405 and 1715 cm1 represent the C–O stretch and C=O bond of carboxylic acid group of PAA, respectively, which appear in both PAA5K and PAA250K spectra and are fairly strong for PAA5K. For the complexes, the bands shift to slightly lower wave numbers because of the formation of the ionic bonds. For PANI–PAA250K these two peaks become relatively strong and sharp while for PANI–PAA5K they become weak and broad. The well-defined peaks in the PANI–PAA250K spectrum indicate that PAA chains in the PANI–PAA250K
complex are packed in a more ordered way despite their higher molecular weight. Fig. 11(a) shows the effect of heat treatment on the spectrum of PANI–Base. The spectra show a decrease in 1600/1500 cm1 peak intensity ratio with an increase in the annealing time. This indicates that cross-linking reaction in PANI–Base may have taken place during the annealing. For PANI–PAA5K and PANI–PAA250K complexes, after annealing the bands become broader and overlap together. This may be attributed to the decrease of the crystal zone size which renders both PANI and PAA chains more conformational freedom. Electronic-like band at 1144 cm1 becomes narrower after the annealing, which is responsible for the reduction of the conductivity. In addition, the C=O
Fig. 10. IR spectra of PAA250K, PAA5K, PANI–BASE, PANI–PAA250K and PANI–PAA5K before annealing.
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Fig. 11. FT-IR spectra of: (a) PANI–Base; (b) PANI–PAA5K; and (c) PANI–PAA250K after the heat treatment at 180 8C in air for different time. The PAA concentration is 50 mol%.
peak and the C–O stretch shift back to slightly higher wavenumbers, i.e. the positions for the free carboxylate acid groups, and for PANI–PAA250K the shift is obvious only at the end of 2-h annealing. This fact indicates that annealing leads to de-doping. The driven force for the de-doping could
be the incompatibility between PANI and PAA so that they tend to segregate from each other. While such phase segregation is indeed more difficult in the high molecular weight PAA doped complexes, as demonstrated by the FT-IR results.
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4. Conclusions 1. PANI doped with the low molecular weight PAA has lower electrical conductivity, which is attributed to its smaller size of crystal islands. 2. When annealed at 180 8C, within a period of 2 h the conductivity of PANI doped with the high Mw PAA decreases continuously with the annealing time following s ¼ s0 exp½ðta =tÞ1=2 law, while the one doped with the low Mw PAA obeys this law only within 1 h of annealing. This indicates that the decrease of conducting island size is responsible for the thermal degradation of the conductivity of the complexes in a certain period and the length of this period depends on Mw of the dopants. 3. TGA study shows that PAA5K degrades much faster than PAA250K at 180 8C. While in complexes their degradation rates are about the same after the initial stage. This implies that the faster reduction of the conductivity in the low molecular weight PAA doped PANI is mainly due to its smaller initial crystal size. 4. FT-IR study shows that annealing leads to de-doping, but it is less pronounced in PANI–PAA250 complexes.
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Thermal degradation of electrical conductivity of polyacrylic acid doped polyaniline: effect of molecular weight of the dopants Xuehong Lua,*, Chiang Yang Tana, Jianwei Xub, Chaobin Heb a
School of Materials Engineering, Nanyang Technological University, Nanyang Avenue, Singapore 639798, Singapore b Institute of Materials Research & Engineering, 3 Research Link, Singapore 117602, Singapore Received 10 July 2002; received in revised form 30 September 2002; accepted 13 October 2002
Abstract This paper describes the effect of molecular weight of dopants on thermal degradation behaviour of electrical conductivity of polyacrylic acid (PAA) doped polyaniline (PANI). Two PAA with weight-average molecular weight (Mw) of 5000 and 250,000, respectively, were used as dopants to synthesise PANI–PAA complexes by in situ oxidative polymerisation. PANI doped with the low Mw PAA has lower electrical conductivity, which is attributed to its smaller size of crystal islands due to the end group effect. When annealed at 180 8C, within a period of 2 h the conductivity of PANI doped with the high Mw PAA decreases continuously with the annealing time following s ¼ s0 exp½ðta =tÞ1=2 law, while the one doped with the low Mw PAA obeys this law only within 1 h of annealing. This indicates that the decrease of conducting island size is responsible for the thermal degradation of the conductivity of the complexes in a certain period and the length of this period depends on Mw of the dopants. TGA study shows that at 180 8C weight loss rates of the low and high Mw PAA doped PANI are about the same, which implies that the faster reduction of the conductivity in the low Mw PAA doped PANI is mainly due to its smaller initial crystal size. FTIR study shows that annealing leads to de-doping, but it is less pronounced in the high Mw PAA doped PANI. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Polyaniline; Polyacrylic acid; Electrical conductivity; Thermal degradation
1. Introduction Protonated polyaniline (PANI) is an important class of intrinsic conductive polymers because of their excellent stability in air. A number of researches have shown that protonated PANI has a heterogeneous structure in which metallic-like crystalline ‘‘islands’’ are separated by less conducting, amorphous ‘‘sea’’, and the electronic transport is governed by 3D hopping between the crystalline regions [1]. Electrical conductivity of protonated PANI degrades after thermal treatment at elevated temperatures. The conductivity degradation has been attributed to the contraction of the conducting islands [2], which may be caused by crosslinking reaction of PANI [3], degradation [4], evaporation or sublimation of the dopants and residual solvents [5] and segregation of the dopants from PANI [6]. It has been proposed that use of polymeric acids to replace small molecule acids as the dopants may enhance the thermal stability of protonated PANI because the evaporation of the dopants may be restricted and the tendency of * Corresponding author. Tel.: þ65-790-4585; fax: þ65-790-9081. E-mail address: [email protected] (X. Lu).
phase segregation weakened because the long chain nature of the dopants. Polyacrylic acid (PAA) doped PANI has been studied as one of such approaches. It was reported that PANI–PAA complexes have a moderate electrical conductivity of about 102 to 103 s/cm and the thermal degradation of the conductivity is indeed slowed down compared to PANI doped with small molecule acids [7,8]. However, to our knowledge there is no report on the formation of crystalline islands in chemically synthesised PANI–PAA complexes so that it is not clear if the decrease of conducting island size is responsible for the thermal degradation of electrical conductivity observed in this system. Furthermore, based on the scenario described earlier it seems that longer PAA chains would lead to less thermal degradation of electrical conductivity of the PANI–PAA complexes. While the dopant chain length may also affect the size of the crystal islands in the PANI–PAA complexes, if they do exist, so that its effect on electrical conductivity and its thermal degradation behaviour may not be straightforward. To address the above issues, in this work, two polyacrylic acids with different molecular weight were used as dopants to synthesise PANI–PAA complexes by in situ oxidative polymerisation. The effect of the molecular weight of PAA on the
0379-6779/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 9 - 6 7 7 9 ( 0 2 ) 0 0 4 7 1 - X
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electrical conductivity of the complexes and their thermal degradation behaviours are studied in relation to the crystalline structures. The study provided some important information to understand the major factors governing the thermal degradation of electrical conductivity of the PANI–PAA complexes.
2. Experimental 2.1. Materials
where I, Vand t are current (A) set, voltage (V) measured and thickness of the disc. Since the diameter to thickness ratio of the discs is about 20, a correction factor 4.4364, instead of p/ ln 2, was used in the equation. 2.4. Wide angle X-ray scattering (WAXS) WAXS measurements on the polymer discs were carried out on a Shimadzu XRD-6000 X-ray diffractometer, using Ni-filtered Cu Ka radiation, at room temperature with a scan speed of 18/min from 10 to 458 2y.
Emeraldine base (PANI–Base) was obtained from Aldrich. Polyacrylic acid (PAA) doped polyaniline (PANI) was prepared by oxidative polymerisation of aniline with ammonium peroxydisulfate in the presence of PAA in aqueous solution according to the procedure for preparing PANI–poly(methacrylic acid) complexes [9]. The molar ratio of aniline to PAA was 0.3:0.7, 0.5:0.5 and 0.7:0.3, respectively. Aniline was supplied by Lancaster, and ammonium peroxydisulfate and PAA by Aldrich. The weightaverage molecular weight of PAA was 5000 (in the form of 50 wt.% solution in water) and 250,000 (in the form of 35 wt.% solution in water), which are named as PAA5K and PAA250K, respectively, and the corresponding complexes are named as PANI–PAA5K and PANI–PAA250K, respectively, in this paper. PANI–Base was dried in a vacuum oven at 50 8C for 4 h and the complexes at room temperature for 48 h prior to sample preparation. The polymer powders were then compressed into discs of diameter of 13 mm and thickness of about 0.65 mm under a pressure of 12 t at room temperature.
2.5. Thermogravimetric analysis (TGA)
2.2. Thermal treatment
3. Results and discussion
The polymer discs were heat-treated in air. The heat treatment consists of raising the temperature of the samples in an oven from room temperature to 180 8C at 10 8C/min and holding the samples at 180 8C for 0.5, 1.0, 1.5 and 2.0 h, respectively, and then cooling the samples back to room temperature in a desiccator.
3.1. Formation of crystalline islands in the PANI–PAA complexes
2.3. Conductivity measurement A four-point probe system (Signatone SP4-62.5-85-TC) with a Keithley 220 programmable current source and Keithley 2000 digital multimeter was used to measure the electrical conductivity of the polymer discs. For each material with the same thermal history, four discs were tested and the conductivity of each disc was measured four times at different positions of the disc. The average of 16 measurements was taken as the conductivity of each sample set. The formulae used for calculating conductivity is as follows: 1 V Conductivity; s ¼ 4:4364 t I
The TGA experiments were carried out on fraction of the disc samples using a TGA2850 (TA Instrument) thermogravimetric analyser. Both air and nitrogen were used as purge gas, respectively, in the experiments. Temperature scans were conducted from 25 to 600 8C at a heating rate of 10 8C/min. The samples were also heated from room temperature to 180 8C at a constant rate of 10 8C/min and then subjected to isothermal degradation at 180 8C over a period of 2 h for the purpose of simulating the annealing process in oven. 2.6. Fourier transformed infrared (FT-IR) spectroscopy Infrared spectra were recorded on a FT-IR spectrometer (Perkin-Elmer System 2000) to identify chemical structure changes of the complexes due to the heat treatment. The KBr technique was used to prepare FT-IR test samples.
The first report on WAXS study of PAA doped PANI was from Chen and Lee, whose PANI–PAA complexes were made by blending of emeraldine base with PAA (Mw ¼ 250; 000) in 1-methyl-2-pyrrolidone (NMP) solution and the end products contain about 10–18% NMP [10]. They found that no pronounced diffraction peaks in the WAXS patterns of their PAA/PANI blends at all the dopant concentrations studied and claimed that this is due to the large size of the PAA chains which makes an ordered packing of PANI chains difficult. On the contrary, our PANI–PAA complexes made by in situ polymerisation have several obvious diffraction peaks. As shown in Fig. 1, the X-ray diffraction pattern of the reference material, PANI–Base, is dominated by an amorphous hallo with weak peaks at about 19.7 and 25.48 2y. When PANI is doped with PAA the diffraction pattern is significantly different from the above. The major changes are that a new peak appears at around 15.58 2y, and the peaks at 19.7 and 25.48 2y become more
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Fig. 1. X-ray diffraction patterns of PANI–Base and PANI–PPA250K before heat treatment.
obvious. The X-ray diffraction patterns of the PANI–PAA complexes are somewhat similar to that of EBI–ESI reported by Pouget et al., so that this new peak may be assigned as (0 1 0) peak [11]. The changes can be explained by the enhancement of the long-range ordering because the chains become straighter after the doping. The WAXS pattern given in Fig. 1 is only an example. Similar patterns were observed for both PAA5K and PAA250K doped PANI and at all the dopant concentrations. This indicates that the PANI–PAA complexes can form ordered packing structures despite the long chain characteristic of the dopants. The structural difference between our PANI–PAA complexes and Chen and Lee’s PANI–PAA blends is caused by the difference in material preparation method [10]. By in situ polymerisation, the doping happens spontaneously with the
polymerisation so that PANI chains may grow along PAA chains to form relatively straight, rigid PANI–PAA strands which favours ordered packing. While by solution blending, both PAA and PANI chains have some conformational freedom in the NMP solution so that they can hardly be parallel to each other. Doping happens in a more or less random fashion among many different chains, which frustrate ordered inter-chain packing. 3.2. Effect of molecular weight of PAA on electrical conductivity of the complexes Fig. 2 shows the dependence of electrical conductivity of the PANI–PAA complexes on PAA molar concentration. The maximum conductivity was found at the PAA molar
Fig. 2. Electrical conductivity of (&) PANI–PAA5K and (&) PANI–PPA250K complexes as a function of PAA molar concentration.
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concentration of about 50 mol%, which is consistent with other reports [10]. The conductivity value of PANI– PAA250K at the PAA concentration of 50 mol% is one magnitude higher than the corresponding PANI–PAA blend of Chen and Lee’s [10]. This is obviously due to the more ordered structures of the complexes made by in situ polymerisation. In the whole range of composition, PANI–PAA250K shows higher electrical conductivity than PANI–PAA5K, which means that a higher molecular weight of the dopants leads to a higher electrical conductivity of the complexes. This fact may be related to the short length of PAA5K chains that restrict the size of the crystal islands in the complexes due to the end group effect. The weight-average molecular weight of PAA5K is 5000 which is corresponding to a degree of polymerisation of about 35 if we assume the polydispersity of the PAA is 2. The length of a PAA5K chain is, ˚ , which is just long enough to therefore, only about 80 A about seven doping sites on an emeraldine base chain. In this case, the growth of the crystals would be affected significantly by the presence of the end groups. While PAA250K chains are 50 times longer than PAA5K chains so that the crystal size is not restricted by the PAA chain length. Fig. 3(a)–(c) gives the comparison of WAXS patterns between PANI–PAA5K and PANI–PAA250. It shows that at all the three PAA concentrations (0 1 0) peak of PANI– PAA250K is slightly sharper than that of PANI–PAA5K. This indicates that the crystal size of PANI–PAA250K is indeed slightly larger than that of PANI–PAA5K. It is striking to see that at the optimum dopant concentration, i.e. 50 mol% PAA, the molecular weight of PAA only affects the conductivity slightly, while at higher and lower PAA concentrations, the molecular weight of PAA has a much larger impact on the conductivity of the complexes. This may be also due to the end group effect of PAA. The end groups of PAA chains would certainly take part in the amorphous shells around the conducting islands. At the PAA concentration of 30 and 70 mol% the dimension of the amorphous shell is already relatively large so that a further increase in the amorphous zone dimension may cause a sharp decrease of the conductivity. 3.3. Thermal degradation of electrical conductivity of the complexes Since at PAA concentration of 50 mol% the maximum electrical conductivity is achieved for this system, the following discussion on thermal degradation of electrical conductivity will be restricted to the PANI–PAA complexes at this optimum concentration. The focus is the effect of molecular weight of PAA dopants. Before heat treatment, the conductivity of PANI– PAA250K and PANI–PAA5K are 8:5 103 and 8:4 103 s/cm, respectively, in contrast to 1 108 s/cm of PANI–Base. After annealing at 180 8C, the conductivity of the complexes is reduced with the increase of the annealing
time as shown in Fig. 4. After annealing for 2 h, the conductivity of PANI–PAA250K and PANI–PAA5K are reduced by about 66 and 99%, respectively. Obviously, as the molecular weight of PAA increases, the thermal degradation of electrical conductivity of the complexes slows down. For PANI doped with small molecule acids, two models have been studied in relation to their thermal ageing behaviours [2]. The first one assumes a process in which oxygen diffuses into the sample, reacts with the polymers and the incorporation of oxygen into the cross-linked fibrils disrupts the conduction pathways. Based on this model, the relative decrease of conductivity should be proportional to the square root of the ageing time, as expressed in Eq. (1): ðs0 sÞ / ta1=2 s
(1)
where s0 is the initial value of conductivity, ta the ageing time, and s the conductivity at ageing time ta. The second model suggests that the conductive polymers have a granular metal structure with the size of the conducting grains decreasing with the ageing time. Based on this model Eq. (2) should be obeyed. 1=2 ta s ¼ s0 exp t
(2)
where s0 is the initial value of conductivity, ta the ageing time, s the conductivity at ageing time ta and t a constant. In Fig. 5(a) and (b) (s0 s)/s and ln (s/s0) of PANI– PAA5K and PANI–PAA250K were plotted as a function of the square root of the heat treatment time, respectively. From Fig. 5, we can see that the Eq. (1) is not obeyed by either PANI–PAA250K or PANI–PAA5K, while PANI–PAA250K follows the Eq. (2), i.e. the s ¼ s0 exp½ðta =tÞ1=2 law. In fact, within 1.0 h of annealing time the behaviour of PANI–PAA5K also follows Eq. (2) fairly well, although at longer annealing time there is a large deviation. This suggests that the decrease of conducting island size is responsible for the thermal degradation of electrical conductivity observed in this system within a certain period of heat treatment time. The length of this period is affected by the molecular weight of PAA dopants. Fig. 6(a)–(c) show the change of X-ray diffraction patterns of PANI–Base, PANI–PAA5K and PANI–PAA250K, respectively, with annealing time. After 2 h of annealing, the X-ray diffraction pattern of PANI–Base is hardly changed. While for PANI–PAA5K, the decrease of peak sharpness with the increase of annealing time is fairly significant. The trend closely resembles Wolter et al.’s WAXS results on HCl-pronotated PANI which shows progressive evolution of the structure from ES-I phase toward an ‘‘EB-I-like’’ phase [12]. In Fig. 6, the peak at 15.58 2y becomes hardly noticeable when the annealing time is longer than 1 h. This implies that the crystal islands shrink quickly in the first 1 h of annealing and after 1 h the crystal zone size is so small that leads to the deviation from the s ¼ s0 exp½ðta =tÞ1=2 law. For PANI–PAA250K the change of peak sharpness with the
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Fig. 3. X-ray diffraction patterns of PANI–PAA5K and PANI–PAA250K before heat treatment at PAA concentration of: (a) 30 mol%; (b) 50 mol%; and (c) 70 mol%.
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Fig. 4. Electrical conductivity of (&) PANI–PAA5K and (&) PANI–PAA250K as a function of heat treatment time, ta, at 180 8C in air.
annealing time is not very obvious. Only a slight broadening of the (0 1 0) peak is noticeable at the end of 2-h annealing. This coincides with the fact that its conductivity degrades more slowly than PANI–PAA5K. 3.4. TGA study The thermal stability of the polymers was investigated using TGA temperature scan. Fig. 7(a) and (b) illustrates the weight (wt.%) and the derivative (wt.%/8C) of PANI–Base, PAA5K and PAA250K as a function of temperature, respectively. In Fig. 7(b) on the derivative weight curve of PANI–Base a strong peak at about 440 8C, which commences at about 300 8C, is attributed to the decomposition of PANI backbones, and weak derivative weight peaks between 180 and 280 8C are attributed to the loss of small molecules such as solvents and impurities [13]. In contrast to PANI–Base, a strong derivative weight peak commences at about 180 and 190 8C for PAA5K and PAA250K, respectively, as shown in Fig. 7(b). From Fig. 7(a), we can see that the weight loss of both PAA5K and PAA250K in the temperature range of 180–280 8C are about 20%. This heavy weight loss may be associated with chain scission reaction of PAA polymer chains while the second derivative weight peak at about 420 8C is corresponding to degradation of the PAA polymer chains into monomers. An important conclusion drawn from Fig. 7 is that PAA5K chains start to break down at 180 8C, which is the annealing temperature employed in this study, while PAA250K chains are relatively stable at this temperature. This conclusion was verified by isothermal TGA experiments on the three base materials at 180 8C, as shown in Fig. 8. For PANI–Base, the weight loss increases very slowly with the isothermal time, while for both PAA5K and PAA250K there is a weight loss of about 10% within the first 20 min, i.e. the temperature ramping up period, which may
be attributed to evaporation of moisture, monomers and small molecule impurities. After the initial stage the PAA5K curve is much steeper than the PAA250K curve indicating that PAA250K is more thermally stable. To investigate the thermal stability of the complexes, isothermal TGA tests were also carried out on PANI–PAA5K and PANI–PAA250K at 180 8C for 2 h. The experimental results are compared with the isothermal TGA curves of the complexes constructed by proportional addition of the TGA curves of the three base materials, i.e. PANI–Base, PAA5K and PAA250K, as shown in Fig. 9. It can be seen that the experimental curves show larger weight loss than the calculated ones at a certain isothermal time, and the difference is mainly in the initial stage and more pronounced for PANI– PAA250K. This indicates that the existence of PAA chains in the system may disturb the polymerisation reaction so that more monomers or low oligomers were left in the system and subsequently evaporated out during the TGA tests. The higher the molecular weight of PAA the severer the disruption. It is interesting to see that the experimental and calculated curves for PANI–PAA250K are nearly parallel to each other after the initial stage, while in the case of PANI–PAA5K the calculated curve is much steeper than the experimental one. In fact, in the complexes, PAA5K and PAA250K degrades at about the same rate as the two experimental curves are nearly parallel to each other. This implies that the complexion actually helps to slow down the degradation of PAA5K. The faster reduction of the conductivity of PANI–PAA5K complex is, therefore, not because that PAA5K chains break down faster than PAA250K chains at the annealing temperature. As mentioned earlier, it may be related to the smaller initial size of the crystal islands in PANI–PAA5K, whose further shrinking may cause a sharp decrease of the conductivity. Similar isothermal TGA experiments were also conducted using air as the purge gas. The relation between weight loss
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Fig. 5. (a) Relative decrease of electrical conductivity of (&) PANI–PAA5K and (&) PANI–PAA250K; and (b) logarithmic plot of relative electrical conductivity of (&) PANI–PAA5K and (&) PANI–PAA250K as a function of the square root of the heat treatment time, ta, at 180 8C in air. The PAA concentration is 50 mol%.
and isothermal time observed in this case is fairly similar to that observed in nitrogen. This indicates that oxidation is not the major cause for degradation of PAA and PANI–PAA complexes. 3.5. FT-IR study FT-IR spectra obtained from PAA250K, PAA5K, PANI– Base, PANI–PAA250K and PANI–PAA5K in the wave-number range of 500–2000 cm1 are shown in Fig. 10. The presence of quinoid band (1585–1595 cm1) and benzenoid
band (1485–1500 cm1) in PANI–Base, PANI–PAA5K and PANI–PAA250K complexes indicates the emeraldine state of PANI. A moderately weak band at around 1144 cm1 in PANI–Base spectrum, referred as electronic-like band by MacDiarmid and co-workers [14], was considered as the delocalisation of electrons and was thus related to its electrical conductivity. This peak broadens and its intensity increases as compared to the quinoid band after doping, which is responsible for the conductivity increase observed. The peaks in the PAA250K spectrum in general is broader and less defined than those in the PAA5K spectrum, which is
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Fig. 6. X-ray diffraction patterns of: (a) PANI–Base, (b) PANI–PAA5K, and (c) PANI–PAA250K after the heat treatment at 180 8C in air for different times. The PAA concentration is 50 mol%.
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Fig. 7. TGA curves showing: (a) weight; and (b) derivative weight of PANI–Base, PAA5K and PAA250K as a function of temperature. The temperature scans were conducted from 25 to 600 8C at the heating rate of 10 8C/min with nitrogen as purge gas. (^) PANI–Base; (&) PAA5K; (~) PAA250K.
Fig. 8. TGA curves showing weight of PANI–Base, PAA5K and PAA250K as a function of isothermal time at 180 8C in nitrogen. The symbol (- -) represents the temperature profile. (^) PANI–Base; (&) PAA5K; (~) PAA250K.
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Fig. 9. TGA curves showing weight of PANI–PAA5K and PANI–PAA250K complexes as a function of isothermal time in nitrogen. (- -) Represents the temperature profile. The symbols (&) and (~) represent TGA curves obtained from PANI–PAA5K and PANI–PAA250K complexes. The symbol (&) represents PANI–PAA5K curve calculated by proportionally addition of the TGA curves of pure PANI–Base and PAA5K. The symbol (~) represents PANI– PAA250K curve calculated by proportionally addition of the TGA curves of pure PANI–Base and PAA250K.
due to the much higher molecular weight of PAA250K. However, many peaks in PANI–PAA250K spectrum are well defined but can hardly be seen in PANI–PAA5K spectrum due to overlapping, especially in the range of 1400–1800 cm1. For example, the bands at 1405 and 1715 cm1 represent the C–O stretch and C=O bond of carboxylic acid group of PAA, respectively, which appear in both PAA5K and PAA250K spectra and are fairly strong for PAA5K. For the complexes, the bands shift to slightly lower wave numbers because of the formation of the ionic bonds. For PANI–PAA250K these two peaks become relatively strong and sharp while for PANI–PAA5K they become weak and broad. The well-defined peaks in the PANI–PAA250K spectrum indicate that PAA chains in the PANI–PAA250K
complex are packed in a more ordered way despite their higher molecular weight. Fig. 11(a) shows the effect of heat treatment on the spectrum of PANI–Base. The spectra show a decrease in 1600/1500 cm1 peak intensity ratio with an increase in the annealing time. This indicates that cross-linking reaction in PANI–Base may have taken place during the annealing. For PANI–PAA5K and PANI–PAA250K complexes, after annealing the bands become broader and overlap together. This may be attributed to the decrease of the crystal zone size which renders both PANI and PAA chains more conformational freedom. Electronic-like band at 1144 cm1 becomes narrower after the annealing, which is responsible for the reduction of the conductivity. In addition, the C=O
Fig. 10. IR spectra of PAA250K, PAA5K, PANI–BASE, PANI–PAA250K and PANI–PAA5K before annealing.
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Fig. 11. FT-IR spectra of: (a) PANI–Base; (b) PANI–PAA5K; and (c) PANI–PAA250K after the heat treatment at 180 8C in air for different time. The PAA concentration is 50 mol%.
peak and the C–O stretch shift back to slightly higher wavenumbers, i.e. the positions for the free carboxylate acid groups, and for PANI–PAA250K the shift is obvious only at the end of 2-h annealing. This fact indicates that annealing leads to de-doping. The driven force for the de-doping could
be the incompatibility between PANI and PAA so that they tend to segregate from each other. While such phase segregation is indeed more difficult in the high molecular weight PAA doped complexes, as demonstrated by the FT-IR results.
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4. Conclusions 1. PANI doped with the low molecular weight PAA has lower electrical conductivity, which is attributed to its smaller size of crystal islands. 2. When annealed at 180 8C, within a period of 2 h the conductivity of PANI doped with the high Mw PAA decreases continuously with the annealing time following s ¼ s0 exp½ðta =tÞ1=2 law, while the one doped with the low Mw PAA obeys this law only within 1 h of annealing. This indicates that the decrease of conducting island size is responsible for the thermal degradation of the conductivity of the complexes in a certain period and the length of this period depends on Mw of the dopants. 3. TGA study shows that PAA5K degrades much faster than PAA250K at 180 8C. While in complexes their degradation rates are about the same after the initial stage. This implies that the faster reduction of the conductivity in the low molecular weight PAA doped PANI is mainly due to its smaller initial crystal size. 4. FT-IR study shows that annealing leads to de-doping, but it is less pronounced in PANI–PAA250 complexes.
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