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UV–vis analysis
Synthetic Metals 159 (2009) 666–674
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Synthetic Metals journal homepage: www.elsevier.com/locate/synmet
Investigation of polyaniline processibility using GPC/UV–vis analysis Dali Yang ∗ , Wen Lu 1 , Russell Goering, Benjamin R. Mattes Santa Fe Science and Technology, Inc., Santa Fe, NM 87505, USA
a r t i c l e
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Article history: Received 15 February 2008 Received in revised form 23 September 2008 Accepted 17 December 2008 Available online 10 February 2009 Keywords: Polyaniline (PANI) Emeraldine base (EB) Molecular weight Gel permeation chromatography (GPC) UV–vis spectroscopy Thermal stability
a b s t r a c t In this work, we characterized the molecular weight and UV–vis spectra of polyaniline (PANI) simultaneously using a gel permeation chromatography (GPC) system equipped with a photodiode array (PDA) detector. Therefore, we constructed correlations between weight average molecular weight (Mw ) values and UV–vis spectral features such as the ratio of quinoid units/benzenoid units (Q/B ratio), and the B peak positions for the continuum of discrete chromatographic fractions of PANI emeraldine base (EB). The correlation is further verified by electrochemical techniques. Using this correlation, we analyzed newly synthesized PANIs and investigated how the synthesis and post-treatment conditions impact the quality, uniformity, and processibility of the PANIs. The study also reveals that the portions with low molecular weights of PANI are more sensitive to the process conditions than those with high molecular weights. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Due to its simple non-redox doping/dedoping chemistry based on the acid/base reaction, polyaniline (PANI) emeraldine base (EB) is one of the most frequently studied conducting polymers [1,2]. The final properties of PANI, oxidization state, molecular weight (MW), polydispersity, etc., are determined by synthesis conditions, such as oxidant/aniline molar ratio, acidic dopant (inorganic or organic), acid concentration, oxidant addition rate, temperature, mixing, and post-treatment [3–8]. Consequently, these properties critically determine the processibility and physical properties in the final form of PANI [9–11]. Typically, polymers that are synthesized by free-radical polymerization, such as PANI, have a broad molecular weight distribution [12]. Therefore, those polymers are composed of hundreds to thousands of chains with different MWs that result in a characteristic MW distribution. If the discrete fractions of PANI possess different oxidation states besides their different MWs, they will change the intrinsic properties of PANI. Over the past decades, numerous research groups used UV–vis or UV–vis–NIR spectroscopy to study the properties of PANI solutions because appreciable changes in the optical absorption are associated with
∗ Corresponding author. Current address: MST-7, MS E549, Los Alamos National Laboratory, Los Alamos, NM 87544, USA. Tel.: +1 505 665 4054; fax: +1 505 667 8109. E-mail address: [email protected] (D. Yang). 1 Current address: ADA Technologies, Inc., 8100 Shaffer Parkway, Suite 130, Littleton, CO 80127, USA. 0379-6779/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2008.12.013
changes in the chain conformation, oxidation state, solution concentration, and degree of EB protonation [1,13–21]. However, most of the optical spectra of the PANI solutions presented in the literature are the composite spectra from different MW fractions of PANI. At the same timeline, some efforts were also devoted to accurately characterize the MW of PANI [22–31]. Little studies were reported towards determining the MW together with characterizing the oxidization state of the fractionated PANI EB [25,28]. Recently, Kolla et al. reported that they determined the absolute molecular weight of PANI in different oxidation states using a three angle light scattering instrument equipped with a 785 nm laser [30]. Although the work of Kolla et al. suggests that the PANI MW determined by gel permeation chromatography (GPC) method is over estimated 2–30% using polystyrene as standards, the overestimation decreases to 2% when the aniline repeat units reached 800. Hence, Kolla et al. suggest that the narrowly distributed polystyrene standards can be used as reliable standards in the GPC analysis. In 2007, Zengin et al. reported to determine PANI MW using membrane osmometry method which was a primary method and did not rely on the standards [32]. Interestingly, the PANI MW obtained from the membrane osmometry is even higher than that by GPC, which indicates that the PANI MW determined using GPC is underestimated rather than overestimated. GPC is commonly used for the characterization of polymer MW [9]. Although it is a secondary method that relies on polymer standards, the analytic method is mature and reproducible. It is thought that the hydrodynamic volumes of polystyrene in solvent is different from that of PANI, resulting in the difference between the relative MW of PANI to polystyrene and the absolute MW of
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PANI. It is especially true when the PANI MW is low (n < 200) [30]. However, since polystyrene is very stable in NMP/LiBF4 , solution aging does not appreciably affect its retention time. Although the PANI MW is a relative number, the results can be reliably reproduced. Many types of detectors are used in the GPC study, but three detectors—differential refractometer (also called refractive index), light scattering, and UV–vis adsorption photometer are frequently used in the PANI study. While the first two detectors are concentration-based detectors, the UV–vis detector is a structureselective detector [10,19,33,34]. If the UV–vis spectrum of the discrete fractions of PANI can be analyzed simultaneously, the spectral characteristics of fractionated PANI along with the MW can be obtained from the GPC analysis. The GPC system with a photodiode array (PDA) detector was used to characterize water soluble PANI and showed that the UV–vis spectra changed as a function of MW. The lower MW species have a spectral characteristic of a lower conductivity material (no free carrier tail) [28]. In our preliminary work [25], we also combined the GPC and UV–vis spectroscopy to analysis PANI samples and demonstrated that this analytical method could be used to study the properties of PANI in depth. In this work, we systematically conducted the GPC/UV–vis spectroscopic analysis for several PANIs which were synthesized at different temperatures and/or posttreated at different conditions. The study shows that the UV–vis spectra of PANI EBs are not only a function of oxidization state [16], but also a function of MW. So far, we have not found any systematic studies in the literature describing the effect of MW on the UV–vis spectra of PANI EB. In this work, we also confirm that the synthesis and post-treatment conditions significantly change the properties of PANIs.
2. Experimental 2.1. Polyaniline synthesis Low-, medium-, and high-MW EB powders (Mw > 20k g/mol) were synthesized in SFST by the chemical oxidation of aniline with sodium persulphate in 45% (w/w) phosphoric acid solutions for sample 1 (S1), sample 2 (S2), sample 3 (S3) and sample 4 (S4) at 20, 10, 0 and −10 ◦ C, respectively, and in 60% (w/w) phosphoric acid solutions for sample 5 (S5) at −35 ◦ C at a scale of 400 g PANI. The mole ratio of aniline to the oxidant was controlled ∼0.8. As the synthesis temperature was lowered, the reaction rate became slower. Therefore it was necessary to increase the oxidant addition time, and hence the total reaction time varied to suit the rate of reaction at a particular temperature. The oxidant was added to the monomer solution dropwise using a peristaltic pump over a long period of time (up to 40 h). Once the oxidant addition had finished, the reaction needed some time (up to 5 h) to complete. Since the aniline polymerization reaction is exothermic, to ensure the reproducibility between different batches, it is necessary to control bath temperature so that the temperature raise is no larger than a couple of degrees. Hence, the reaction mixture remained in a stirrable and fluid state throughout the entire reaction to adequately heat dissipation. Upon completion, the reaction mixture was filtered and washed with water. The filtered powder was deprotonated with ammonium hydroxide solution (pH 11), washed with water, and finally with methanol (ACS grade, 95% purity) to yield the EB powder. The powders were dried in a vacuum oven at elevated temperature for overnight. The yield for the PANI synthesized at temperature above 0 ◦ C is >65% while the yield for the EB synthesized at temperature below 0 ◦ C is typically >85% [35]. Neste EB (NEB) was purchased from Neste Oy company (Helsinki, Finland). Three low-MW EB powders (labeled as A1, A2 and A3,
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respectively) were purchased from Aldrich, and were used as received. To study the effect of drying conditions on the EB processibility, a batch of 400 g EB was synthesized at −35 ◦ C in 60% (w/w) H3 PO4 aqueous solution. During the synthesis, the reactants were well mixed to ensure a constant temperature (−35 ± 0.5 ◦ C). After being deprotonated and rinsed with water, the EB was thoroughly washed with isopropanol (ACS grade, 99% purity) to remove water, oligomers, and some impurities. After the PANI EB was dried on a Büchner funnel under vacuum several days, we prepared five samples to dry at different conditions. DV1 and DV2 samples were dried in a vacuum oven for more than 1 day at room temperature and 100 ◦ C, respectively. D1–D3 samples were dried at different temperatures in a furnace oven for 5 h. During the whole process, the ovens were covered with alumina foil to avoid light exposure. 2.2. GPC study A Waters GPC system (Alliance 2690) with a photodiode array (PDA) (PDA 996) and a refractive index (RI 2410) detectors was used to measure the MW distribution and MW of the PANI EB powders. The GPC columns (4.6 mm i.d. × 300 mm)—a Styagel® HR 4E and a 5E were used in series. The column temperature was controlled at 60 ◦ C. The flowrate of the eluent was 0.35 ml/min, and the injection volume was 50 l. Polystyrene standards (Polymer Laboratories) with 10 narrowly distributed MWs, were used to calibrate the columns. The UV–vis spectrum was collected at a 0.36 nm resolution while the GPC chromatogram was collected at 1 s time interval. The calculated MWs were relative to the polystyrene standards. Ultra-pure NMP (99.0+% HPLC grade) containing <0.01 M LiBF4 was used as an eluent as well as a solvent to prepare the PANI EB solutions [26]. Since LiBF4 has a poor solubility in NMP (less than 0.01 M), to prevent pressure fluctuation in the GPC system, the solvent/eluent was filtered through a 0.45 m PTFE filter prior to use. The 0.02 mass% of PANI powders were added into NMP/LiBF4 solvent and were equilibrated overnight at ambient condition prior to the MW determination. All of the polymer solutions were filtered through a 0.45 m PTFE syringe filter before they were automatically injected into the GPC columns. The PANI MW and MW distribution were analyzed using a Waters Millennium [32] software. 2.3. Thin film preparation EB powders were dissolved into NMP to obtain a 1 mass% solution and allowed to equilibrate a couple of days under ambient conditions and protected from light. The filtered EB solutions were spin-coated onto indium–tin-oxide (ITO) glass electrodes (0.7 cm × 1.7 cm) at 6000 rpm for 5 min at room temperature. The thickness of the resulting EB films was usually <0.1 m. Prior to electrochemical measurements, these EB/ITO electrodes were dried under dynamic vacuum overnight at room temperature. 2.4. Electrochemical study Electrochemical measurements of the EB/ITO electrodes were carried out in an electrochemical cell consisting of the EB/ITO electrode as the working electrode, a Pt wire as the counter electrode, and an Ag/AgCl (3 M NaCl) as the reference electrode. Zero-current chronopotentiograms were recorded immediately once the 1 M HCl electrolyte was added into the cell. The open circuit potential (OCP) of the PANI thin film was obtained at the steady state after 1 h. Subsequently, cyclic voltammograms (CV) were recorded for the PANI
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thin film in the same electrochemical cell. A stable CV was obtained after continuous potential cycling for a few cycles. 3. Background To better understand latter materials in the paper, we need to discuss the effects of oxidation state on the UV–vis spectra of PANI. PANI exists in a number of unique structures, and is characterized by the oxidation state (i.e., the ratio of amine to imine nitrogens), and the extent of protonation. The oxidation state of the polymer increases with the value of y (0 ≤ y ≤ 1) (see Fig. 1). The value of y is the molar fraction of oxidized units in PANI structure. The three well-defined oxidation states are the fully oxidized pernigraniline base (PNB) (y = 1), the fully reduced leucoemeraldine base (LEB) (y = 0), and the half oxidized EB (y = 0.5) [13]. The spectra of EB/NMP solutions consist of two classic peaks [14,36–38]. The first peak at ∼630 nm (Q peak) is assigned to the excitation of an electron from the highest occupied molecular orbital (HOMO, b ) of the benzenoid rings to the lowest unoccupied molecular orbital (LUMO, q ) of the quinoid rings [36]. The absorption of Q peak reflects both intra- and inter-chain interaction; therefore its absorption intensity and wavelength change with the EB chain configuration, which is affected by the solubility, additive, concentration, oxidization level, MW etc. [20,21,26,39]. The second peak at ∼330 nm (B peak) is due to → * transition associated with the electrons in the benzene rings, and mainly a function of intra-chain interaction [1,20,40]; therefore the solution properties have a less impact on its intensity and wavelength. The intensity ratio of Q peak to B peak is called Q/B ratio, and proportional to the ratio of quinoid units to benzenoid units in PANI. In the UV–vis spectrum of the EB/NMP solution (<1 mass%), the typical reported value of Q/B ratio is 0.86 ± 0.05 [14,21,31,40,41]. Fig. 2 presents the UV–vis spectral features of PANI at different oxidized levels, which is constructed using the experimental data presented by Albuquerque et al. [16]. As EB is reduced toward LEB, the value of Q/B ratio approaches 0, and B peak gradually red shifts from 335 to 347 nm; Q peak also red shifts from 634 to 645 nm at the initial stage, then level off. On the contrary, as EB is oxidized toward PNB, the value of Q/B ratio again is reduced to 0.69, but thereafter stays constant at ∼0.69; while Q peak gradually blue shifts from 634 to 571 nm, B peak stays at ∼335 nm [14,16]. Therefore, from the changes in the Q/B ratio and the shifts of Q or B peak in the UV–vis spectra, we may identify the oxidization state of PANI. It has also been reported that when different lithium salts are added into the EB/NMP solutions, changes in the Q peak wavelength and Q/B ratio reflect the differences in the ionic interactions [15,26,42,43]. For example, when Cl− is added to the EB solution, Cl− can sometimes cause a red shift of Q peak all the way up from 630 to 675 nm [15]. Furthermore, the interaction between ions and EB changes the chain conformation of EB, which cause a large increase in the value Q/B ratio (up to 0.92), which is attributed to the interaction between Li+ and the imine site of EB via a “pseudo-doping”
process [44]. This “pseudo-doping” reduces the inter-/intra-chain interaction and enhance the solubility of EB, and thus enhance the Q/B ratio and cause the red shift on Q peak [45]. In this work, LiBF4 was added to deaggregate the EB chains. Therefore, the peak wavelengths and Q/B ratio in the UV–vis spectra of the EB/NMP/LiBF4 solutions will slightly depart from the values reported in the literature for the EB/NMP solutions [21,40,46]. Nevertheless, Fig. 2 is still a valuable reference to estimate the oxidization state from the UV–vis spectra of the studied PANIs. 4. Results and discussion 4.1. Effect of molecular weight on UV/vis spectra of PANI in NMP Feng et al. [47] reported that when the EB oligomer (dissolved in NMP) changes in size from tetramer to hexadecamer, the value of Q/B ratio changes from 0.33 to 0.52. At the same time, B peak red shifts from 320 to 328 nm and the Q peak red shifts from 588 to 610 nm, respectively. However, until now, we have not found any systematic study on the effect of the EB MW on its UV–vis spectrum as the polymer chain length increases from oligomer to polymer. We therefore characterized several EBs which weight average MW (Mw ) varies from 3000 to 285,000 g/mol. Table 1 summarizes the Mw , number average MW (Mn ), and polydispersity index (PDI) of these samples. As the synthesis temperature decreases, both Mw and Mn of EB increase, and the PDI increases as well. The similar trend was also reported by Zengin et al. [32]. Among six SFST EB samples, the synthesis temperature were well controlled within ±<2 ◦ C of the desired temperature except for S3. During the synthesis process of S3, the bath temperature was over shut about 2–3 ◦ C above the desired temperature at the beginning of the synthesis, which may be part of reasons why the PDI of S3 is unusually high. In Table 1, we also include the GPC results of other four outside samples. The detailed synthesis conditions are unknown. To ensure fewer defect and no chlorine in PANI, we synthesized it in H3 PO4 but not in HCl. Some studies show that if the polymerization is carried out in an acid with a large amount of LiCl present, especially if the acid is HCl, significant ring chlorination occurs (typically 1% by weight of the base polymer) [48]. For some applications, it is desirable to eliminate this chlorine and any other impurities/defects that may occur by this route [35]. SFST invented several new polymerization processes, including using H3 PO4 to produce high MW PANI without much defect and chlorine. Some batches of PANI have been reproduced for more than 10 times. Their MW and PDI were also very reproducible. Therefore, the high PDI of these PANIs is not caused by the poor controlled synthesis, but is inherently associated with the H3 PO4 system. Its PDI ranges typically between 5 and 10 depending on the PANI MW. The UV–vis spectra of these EBs in Fig. 3, collected at the peaks of the GPC chromatograms, suggest that when Mw of EB is greater than 30,000 g/mol, both Q/B ratio and the wavelength of Q peak remain constant. For the EB samples with Mw < 30k g/mol, as the MW increases, both B and Q peaks undergo a red shift while the Q/B Table 1 Summary of calculated molecular weights of EB samples (NA - not available).
Fig. 1. Repeat unit of PANI in its different oxidation states.
Label
Manufacture
Synthesis temperature (◦ C)
A1 A2 A3 S1 S2 NEB S3 S4 S5
Aldrich Aldrich Aldrich SFST SFST Neste Oy SFST SFST SFST
NA NA NA 20 10 NA 0 −10 −35
Mw (g/mol) 3,020 11,980 15,200 21,900 27,100 79,100 130,500 143,400 285,300
Mn (g/mol) 1,550 3,270 4,170 8,480 8,780 13,900 17,400 27,500 42,200
PDI 1.9 3.7 3.6 2.6 3.1 5.7 7.5 5.9 6.8
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Fig. 2. Correlations between oxidation state and Q (or B) peak (a) and between oxidation state and Q/B ratio (b) (plots are based on the data of UV–vis spectra of 1 mass% PANI at the different oxidized levels in NMP at ambient conditions [16].
ratio increases from 0.48 to 0.89. When the EB Mw is greater than 15k g/mol, the B peak wavelength remains constant with increasing Mw , but Q peak continuously shows a red shift with Mw until the value of Mw reaches ∼30k g/mol. Fig. 4 displays the correlations of Q/B ratio vs. Mw , and the wavelength of B(Q) peaks vs. Mw . In Fig. 4, we also include the Q/B ratios and wavelength of B(Q) peaks of the EB oligomers obtained from the Feng’s article [47]. These UV–vis spectral features illustrate the dependence of the spectral characteristics on the MW. It is worthy noted that when we constructed these curves, we assumed that all of these synthesized EB samples were exactly in the EB oxidation state (y = 0.5), which may not be the case. Furthermore, compared to the UV–vis spectra for the EB/NMP/LiBF4 solution to the EB/NMP solution, one should expect a slightly higher Q/B ratio than ∼0.86 and a higher Q peak wavelength than 630 nm of the former. All of these factors might be responsible for the discrepancy between our curves and the curve constructed from the literature data. In summary, when the MW of PANI is less than 30k g/mol, both MW and oxidization state impact the UV–vis spectral characteristics of the EB/NMP solutions. When the MW is greater than 30k g/mol, oxidization state of PANI is the major factor in determining its spectral features. With these findings, we would like to construct the correlations between MW and UV–vis spectral features from an analytic EB, and use the correlations to study how
synthesizing and post-treatment conditions change EB properties. To cover a wide range of MW, we conducted the following study using S3, NEB, S4 and S5, which have medium and high MWs. 4.2. Electrochemistry study of PANI samples Before we construct the correlations, we need to determine which PANI sample has the oxidation state mostly close to the analytic EB oxidization state (y = 0.5) among these four samples. Huang et al. [1] determined the analytic EB state to be approximate 0.4 V (vs. SCE) in 1 M HCl for both chemically and electrochemically synthesized PANIs. Considering the fact that the standard potential of the reference electrode (Ag/AgCl) employed in the present work is lower than that of the SCE [49], the EB state would appear at a slightly higher potential when Ag/AgCl is used. Based on the cyclic voltammogram (CV) of these PANI samples, we determined the EB state to be at ∼0.46 V (vs. Ag/AgCl) in 1 M HCl (see insert plot in Fig. 5). A steady-state open circuit potentials (OCPs) can be used to determine the oxidization state of PANI samples. The zero-current chronopotentiograms of these four PANI/ITO electrodes obtained in 1 M HCl are shown in Fig. 5. Since the OCP of the analytical EB should be ∼0.46 V (vs. Ag/AgCl), a lower or higher OCP than this value suggests that the oxidization state of PANI is more reduced or more
Fig. 3. Effect of MW on the UV–vis spectra of EB/NMP/LiBF4 . Low MW EB samples are shown in (a) and medium and high MW EB samples are shown in (b) (UV–vis spectra were collected from the EB solutions at the GPC peak).
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Fig. 4. Curves of Q/B ratio vs. Mw (a) and B (or Q) wavelength vs. Mw (b) for the EB/NMP solutions (dotted lines (- - -) represent curves constructed from the literature values of EB oligomers in NMP [47] and solid lines (—) represent curves constructed from the UV-vis spectra of the EB/NMP/LiBF4 solutions).
oxidized than that of the analytical EB, correspondingly. Hence, the presence of higher oxidation state than EB was clearly observed for the S3 film (OCP = 0.57 V). By contrast, the lower OCP value of S5 film (0.42 V) indicates that this sample contains more reduced form than the EB state. Although the OCP values of NEB and S4 films are very close to one another, NEB seems to have the characteristics of an analytic EB. The comparison of the CVs between NEB and S4 (see insert plot in Fig. 5) also suggests the red-ox reversibility of NEB seems better than that of S4.
4.3. Simultaneous UV–vis/GPC analysis of newly synthesized PANI
Fig. 5. Zero-current chronopotentiograms of EB sample coated ITO glass electrodes obtained in 1 M HCl. The insert plot is the cyclic voltammogram of the thin films of S4 and NEB coated ITO glass electrode (obtained in 1 M HCl at the scan rate of 20 mV/s at 23 ◦ C).
Fig. 6 shows the combination of UV–vis spectra/GPC chromatograms of a 0.02 mass% NEB/NMP/LiBF4 solution. In this 3D-plot, correlations between retention time, wavelength, and UV–vis absorption intensity are revealed. For example, the GPC results can be extracted at the strong absorption (e.g. ∼330 nm or ∼630 nm) while the UV–vis spectra of a narrow fraction can be obtained at any retention times. The absorption intensity vs. retention time reflects the changes in the concentration of each fraction. With these 3D-plots, we could construct the correlations of Mw vs. UV–vis spectral features for the fractioned PANI.
Fig. 6. Illustration of a 3D-plot with UV–vis spectra/GPC result obtained from the 0.02 mass% NEB/NMP/LiBF4 solution.
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Fig. 7. Plots of Q/B ratio vs. Mw for four PANI powders dissolved into NMP/LiBF4 .
Fig. 7 illustrates the correlation of Mw vs. Q/B ratio of four PANI samples. Their GPC results are already summarized in Table 1. As discussed in the previous sections, the zero-current chronopotentiograms of these PANI suggest that characteristics of NEB are almost the same as the analytic EB. Therefore, the correlation of NEB can be used as a reference for the analytic EB. Interestingly, the Q/B ratio of NEB increases as the Mw increases, and reaches this highest value, and then starts to decrease slightly, which may be due to the decreases in the EB solubility as its MW gets larger and larger [31]. Actually, the other three curves also show similar behavior. However, the large differences are observed for the PANI fractions with low MWs, which be discussed in great details in the following sections. The lowest Q/B ratio of S5 within a wide range of MW suggests its most reduced form among these PANI samples, which is consistence with the lowest OCP value (0.42 V). On the other hand, the fairly constant Q/B ratios of the fractioned S3 vs. Mw distinguishes its correlation from the other three. The highest OCP of S3 (0.57 V) suggests that it contains more oxidized form than the other three samples, and thus more quinoid units, which may result in a high Q/B ratio than NEB in the low MW range. The difference in the correlation between Q/B ratio and MW of NEB and S4 is more obvious than what we observed in their OCP measurement. The lower Q/B ratio of S4 may suggest some reduced form in S4. Listing these four samples in the order from the reduced to the oxidized form, we have the following order:
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Fig. 8. The GPC chromatograms of 0.02 mass% of four PANIs dissolved in NMP/LiBF4 —the highest absorbance of S5 suggests the best solubility (chromatograms were collected at ∼330 nm wavelength, column temperature was set at 60 ◦ C).
be better than that of EB. This may explain why S3 also dissolved better than NEB. Since S4 is slightly reduced than NEB, the slightly better solubility of S4 than NEB is expected. It is noteworthy that the concentration of PANI can potentially affect the Q/B ratio [20,21,31]. However, in this study, the PANI concentration is very low (<0.02 mass%). The addition of LiBF4 in PANI solution is sufficient to block the interaction between PANI chains. The single peak of GPC chromatograms are obtained for all of the samples. The GPC column temperature was also controlled at 60 ◦ C, which is very close to the theta temperature in PANI/NMP system reported by Zengin et al. [32]. Therefore, the interaction between polymer chains would be minimized. Both UV–vis spectral features and the retention time were not appreciably affected by PANI concentration (up to 0.05 mass%) and solution ages. The changes in the Q/B ratio here were mainly due to the oxidization state. Fig. 9 describes the correlations between the wavelength of B peak and the Mw for these PANI samples. Again, the main differences among them are observed at the low MW portions. According to the discussion in Fig. 2, one knows that when PANI changes its oxidation state from EB toward LEB, B peak shows its characteristically red shift, which provides additional evidence of more reduced
S5 (some reduced) > S4 (slightly reduced) > NEB (EB) > S3 (some oxidized) When we tested the solubility of these four PANI powders, their solubility in NMP/LiBF4 decreased in the order of S5 > S3 > S4 > NEB (see Fig. 8). The poor solubility of EB in organic solvents is mainly due to the hydrogen bonding between imine nitrogen and amine hydrogen atoms [19,33,50]. Since the structure of LEB does not contain the imine nitrogens (see Fig. 1) to form this H-bond interaction, although there are some weak interactions between amine hydrogen and amine nitrogen, the solubility of LEB is much better than that of EB [51]. One can logically predict that the solubility of PANI increases as its oxidation state changes incrementally from EB to LEB. This may explain why S5 had the best solubility. Similarly, as the oxidization state of PANI changes toward the PNB state, due to the decreased amine sites, the possibility of H-bond interactions between amine hydrogen and imine nitrogen also decreases. Hence, the solubility of PANI with a higher oxidization state than EB should
Fig. 9. The correlations between B peak vs. Mw for four PANIs dissolved in NMP/LiBF4 .
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Table 2 Summary of drying conditions and GPC results for five PANI samples. Sample label DV1 DV2 D1 D2 D3
Drying temperature (◦ C) 23 (vacuum) 100 (vacuum) 60 (furnace oven) 80 (furnace oven) 100 (furnace oven)
Drying time (h) >24 >24 5 5 5
Mn (g/mol) 43,100 27,800 43,300 46,600 32,500
Mw (g/mol) 271,200 350,700 257,800 263,700 181,900
PDI 6.3 12.6 5.9 5.7 5.6
form in S5 than that in the other three PANIs. Similarly, B peaks of the low MW portion of S4 also show a red shift, which also suggests some portions containing slightly reduced form. On the contrary, the changes in B peak of S3 with Mw is different from both S4 and S5, this may be attributed to its slightly oxidized form at the low MW portions. In principle, the correlation between Q peak wavelength and Mw can also be constructed to study the difference among these PANI samples. However, besides the MW and the oxidization state, additives in the solution and PANI concentration also influence its shift, which needs more caution for this application. In summary, when we synthesized these PANI EB, we expected that the MW increases with decreasing synthesized temperature. However, the simultaneous GPC/UV–vis analysis reveals the different properties of these PANI besides their MW. Besides temperature, other conditions (oxidant addition rate, mixing, batch temperature control, post-treatments, etc.) also result in the different products, and thus change the uniformity and processibility of synthesized PANIs. 4.4. Drying effect From the GPC characterization of many early batches of PANI synthesized in SFST, we noticed that their GPC results and processibilities vary from a batch to another even when some of them were synthesized at the same synthesis conditions. Several research groups reported that heat treatment would induce cross-linkage between EB molecules in NMP solvent [52–54]. We suspect that the similar phenomena might occur when the PANI EB was posttreated differently. Furthermore, using the above methodology, we may obtain more insights on the structural and MW changes during the drying process besides the poor solubility. In this study, we investigated five PANI samples which were synthesized in the same batch, but dried at different conditions. Their GPC results together with the post-treatment conditions are summarized in Table 2. Fig. 10(a) and (b) illustrate that the correlations of Q/B ratio vs. Mw and B peak vs. Mw for these samples, respectively. After drying,
the appreciable changes in both Q/B ratio and B peak wavelength are observed. DV1 sample, vacuum dried at room temperature (∼23 ◦ C), is the closest one to NEB among these five samples. Although DV2 was dried under vacuum oven whereas D1, D2 and D3 were dried in the furnace oven, the changes in the UV spectra of these four samples suggests that all of them were reduced from its starting PANI sample (DV1). As the drying temperature was increased and drying time was prolonged, the decrease in Q/B ratio and the increases in B peak wavelength become more obvious. After drying for 5 h, the most changes occur in the low MW portions. As the drying temperature increases from 60 to 100 ◦ C, the higher MW portions starts to changes as well. Similarly, when the DV2 was dried at 100 ◦ C for more than 24 h, the chemical reduction took place in the entire MW range. The cross-linked PANI EB also shows the reduced characteristics due to the lose of quinoid units [52]. Therefore, the large changes on Q/B ratio and red shift on B peaks of over heated samples suggest that the cross-linkage among the EB chains occurs in the drying stage. Actually, even all of the solutions were started from 0.02 mass% of PANI, the low intensity of their GPC chromatograms in Fig. 11 suggests that the solubility of D3 and DV2 is largely decreased. An appreciable aggregation is observed in the GPC chromatogram of DV2, which may be caused by the cross-linked materials with a much larger MW than non-cross-linked PANI. As some aggregates were filtrated from the solution, the PANI concentration would be lower than that of the solution without aggregated PANI. Comparing to DV1, D1–D3 have the lower MW and PDI (see Table 2). These results may suggest that some low MW portions of PANI interacted with high MW portions during the drying treatment, and cause aggregations. These aggregates could not be dissolved into the solutions, and thus were removed in the filtration step, which may cause the low absorption intensity of D1, D2, and D3. On the other hand, although both D3 and DV2 were dried at 100 ◦ C, DV2 gave much larger Mw and the PDI than those of D3. We believe that besides the drying time, the drying under vacuum and at ambient pressure also contributes the difference. Drying in the furnace oven, the removing rate of residual solvent is much slower than that under vacuum. This slow evaporation of the solvent may enhance the similar annealing effect that was reported in the EB/NMP solutions [52–54]. Although some of aggregates was removed from the filtration step, and resulting in the lowest absorption intensity of DV2, some of aggregates were passed through the filter and gave a distortion of the GPC chromatogram. In summary, all of these results support that the high drying temperature can chemically change the PANI EB oxidization
Fig. 10. Drying effect on the Q/B ratio vs. Mw (a) and the B peak wavelength vs. Mw (b) of five PANI samples dried at different conditions.
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Acknowledgements This material is based upon work supported by the Defense Advanced Research Projects Agency Defense Science Office. DARPA Order No. G874/00 is issued by DARPA/CMD under contract #MDA972-99-C-0004, for which the authors are thankful. Special thanks to Dr. Ian Norris for his useful comments. References
Fig. 11. The effect of drying conditions on the GPC chromatograms of PANI samples (the refractive index detector was used to collect the chromatograms, column was heated at 60 ◦ C).
state during the post-treatment step. Even drying under vacuum, high drying temperature can change the properties of EB. The other important finding is that PANIs with low MWs seem to be more vulnerable than PANIs with high MWs to the synthesizing and post-treatment conditions. Recently, more and more research efforts have been dedicated to the investigation of nano-form PANI. Their MWs are, typically, less than 50k g/mol. Therefore, extra caution should be paid when one synthesizes and post-treats the nano-formed PANI EB to preserve its intrinsic properties. 5. Conclusions By separating the superimposed UV–vis spectra, and correlating the UV–vis spectra with each narrow fraction of PANI, we can more precisely determine the oxidation state of PANI samples with different weight average molecular weights (Mw ). Combining the GPC with the UV–vis spectroscopic analysis of the fractionated PANI, we can evaluate the quality and processibility of PANI more in depth. This methodology can also be used to aid in analysis and improvement of a PANI synthesis and post-treatment procedures. From the correlation between UV–vis spectral characteristics and the molecular weight, we may tune the synthesis conditions to modify the oxidation state of the final product. Furthermore, we have constructed a correlation between Mw and UV–vis spectral characteristics for the analytic EB, which is also very important for determining the oxidation state of fractioned PANI with a MW less than 30k/g. This study also reveals that the fractioned PANI EB with low MWs is more vulnerable to the process conditions than those with high MWs. Recently, more and more research efforts have been dedicated to the nano-form PANI synthesis. Most of these PANI have low molecular weight. Therefore, extra caution (e.g. temperature control) must be paid not only in the synthesis step but also in the post-treatment. The low drying temperature (<60 ◦ C) under vacuum is critical to preserve the EB nature. Moreover, using zero-current potentiometry, we have verified the oxidation states of the PANI samples. Excellent correlation between the OCPs and UV–vis characteristics of these PANI samples suggests that the electrochemical approach can also provide a rapid qualitative estimation of the relative degree of the reduced and the oxidized forms present in PANI.
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Investigation of polyaniline processibility using GPC/UV–vis analysis Dali Yang ∗ , Wen Lu 1 , Russell Goering, Benjamin R. Mattes Santa Fe Science and Technology, Inc., Santa Fe, NM 87505, USA
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Article history: Received 15 February 2008 Received in revised form 23 September 2008 Accepted 17 December 2008 Available online 10 February 2009 Keywords: Polyaniline (PANI) Emeraldine base (EB) Molecular weight Gel permeation chromatography (GPC) UV–vis spectroscopy Thermal stability
a b s t r a c t In this work, we characterized the molecular weight and UV–vis spectra of polyaniline (PANI) simultaneously using a gel permeation chromatography (GPC) system equipped with a photodiode array (PDA) detector. Therefore, we constructed correlations between weight average molecular weight (Mw ) values and UV–vis spectral features such as the ratio of quinoid units/benzenoid units (Q/B ratio), and the B peak positions for the continuum of discrete chromatographic fractions of PANI emeraldine base (EB). The correlation is further verified by electrochemical techniques. Using this correlation, we analyzed newly synthesized PANIs and investigated how the synthesis and post-treatment conditions impact the quality, uniformity, and processibility of the PANIs. The study also reveals that the portions with low molecular weights of PANI are more sensitive to the process conditions than those with high molecular weights. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Due to its simple non-redox doping/dedoping chemistry based on the acid/base reaction, polyaniline (PANI) emeraldine base (EB) is one of the most frequently studied conducting polymers [1,2]. The final properties of PANI, oxidization state, molecular weight (MW), polydispersity, etc., are determined by synthesis conditions, such as oxidant/aniline molar ratio, acidic dopant (inorganic or organic), acid concentration, oxidant addition rate, temperature, mixing, and post-treatment [3–8]. Consequently, these properties critically determine the processibility and physical properties in the final form of PANI [9–11]. Typically, polymers that are synthesized by free-radical polymerization, such as PANI, have a broad molecular weight distribution [12]. Therefore, those polymers are composed of hundreds to thousands of chains with different MWs that result in a characteristic MW distribution. If the discrete fractions of PANI possess different oxidation states besides their different MWs, they will change the intrinsic properties of PANI. Over the past decades, numerous research groups used UV–vis or UV–vis–NIR spectroscopy to study the properties of PANI solutions because appreciable changes in the optical absorption are associated with
∗ Corresponding author. Current address: MST-7, MS E549, Los Alamos National Laboratory, Los Alamos, NM 87544, USA. Tel.: +1 505 665 4054; fax: +1 505 667 8109. E-mail address: [email protected] (D. Yang). 1 Current address: ADA Technologies, Inc., 8100 Shaffer Parkway, Suite 130, Littleton, CO 80127, USA. 0379-6779/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.synthmet.2008.12.013
changes in the chain conformation, oxidation state, solution concentration, and degree of EB protonation [1,13–21]. However, most of the optical spectra of the PANI solutions presented in the literature are the composite spectra from different MW fractions of PANI. At the same timeline, some efforts were also devoted to accurately characterize the MW of PANI [22–31]. Little studies were reported towards determining the MW together with characterizing the oxidization state of the fractionated PANI EB [25,28]. Recently, Kolla et al. reported that they determined the absolute molecular weight of PANI in different oxidation states using a three angle light scattering instrument equipped with a 785 nm laser [30]. Although the work of Kolla et al. suggests that the PANI MW determined by gel permeation chromatography (GPC) method is over estimated 2–30% using polystyrene as standards, the overestimation decreases to 2% when the aniline repeat units reached 800. Hence, Kolla et al. suggest that the narrowly distributed polystyrene standards can be used as reliable standards in the GPC analysis. In 2007, Zengin et al. reported to determine PANI MW using membrane osmometry method which was a primary method and did not rely on the standards [32]. Interestingly, the PANI MW obtained from the membrane osmometry is even higher than that by GPC, which indicates that the PANI MW determined using GPC is underestimated rather than overestimated. GPC is commonly used for the characterization of polymer MW [9]. Although it is a secondary method that relies on polymer standards, the analytic method is mature and reproducible. It is thought that the hydrodynamic volumes of polystyrene in solvent is different from that of PANI, resulting in the difference between the relative MW of PANI to polystyrene and the absolute MW of
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PANI. It is especially true when the PANI MW is low (n < 200) [30]. However, since polystyrene is very stable in NMP/LiBF4 , solution aging does not appreciably affect its retention time. Although the PANI MW is a relative number, the results can be reliably reproduced. Many types of detectors are used in the GPC study, but three detectors—differential refractometer (also called refractive index), light scattering, and UV–vis adsorption photometer are frequently used in the PANI study. While the first two detectors are concentration-based detectors, the UV–vis detector is a structureselective detector [10,19,33,34]. If the UV–vis spectrum of the discrete fractions of PANI can be analyzed simultaneously, the spectral characteristics of fractionated PANI along with the MW can be obtained from the GPC analysis. The GPC system with a photodiode array (PDA) detector was used to characterize water soluble PANI and showed that the UV–vis spectra changed as a function of MW. The lower MW species have a spectral characteristic of a lower conductivity material (no free carrier tail) [28]. In our preliminary work [25], we also combined the GPC and UV–vis spectroscopy to analysis PANI samples and demonstrated that this analytical method could be used to study the properties of PANI in depth. In this work, we systematically conducted the GPC/UV–vis spectroscopic analysis for several PANIs which were synthesized at different temperatures and/or posttreated at different conditions. The study shows that the UV–vis spectra of PANI EBs are not only a function of oxidization state [16], but also a function of MW. So far, we have not found any systematic studies in the literature describing the effect of MW on the UV–vis spectra of PANI EB. In this work, we also confirm that the synthesis and post-treatment conditions significantly change the properties of PANIs.
2. Experimental 2.1. Polyaniline synthesis Low-, medium-, and high-MW EB powders (Mw > 20k g/mol) were synthesized in SFST by the chemical oxidation of aniline with sodium persulphate in 45% (w/w) phosphoric acid solutions for sample 1 (S1), sample 2 (S2), sample 3 (S3) and sample 4 (S4) at 20, 10, 0 and −10 ◦ C, respectively, and in 60% (w/w) phosphoric acid solutions for sample 5 (S5) at −35 ◦ C at a scale of 400 g PANI. The mole ratio of aniline to the oxidant was controlled ∼0.8. As the synthesis temperature was lowered, the reaction rate became slower. Therefore it was necessary to increase the oxidant addition time, and hence the total reaction time varied to suit the rate of reaction at a particular temperature. The oxidant was added to the monomer solution dropwise using a peristaltic pump over a long period of time (up to 40 h). Once the oxidant addition had finished, the reaction needed some time (up to 5 h) to complete. Since the aniline polymerization reaction is exothermic, to ensure the reproducibility between different batches, it is necessary to control bath temperature so that the temperature raise is no larger than a couple of degrees. Hence, the reaction mixture remained in a stirrable and fluid state throughout the entire reaction to adequately heat dissipation. Upon completion, the reaction mixture was filtered and washed with water. The filtered powder was deprotonated with ammonium hydroxide solution (pH 11), washed with water, and finally with methanol (ACS grade, 95% purity) to yield the EB powder. The powders were dried in a vacuum oven at elevated temperature for overnight. The yield for the PANI synthesized at temperature above 0 ◦ C is >65% while the yield for the EB synthesized at temperature below 0 ◦ C is typically >85% [35]. Neste EB (NEB) was purchased from Neste Oy company (Helsinki, Finland). Three low-MW EB powders (labeled as A1, A2 and A3,
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respectively) were purchased from Aldrich, and were used as received. To study the effect of drying conditions on the EB processibility, a batch of 400 g EB was synthesized at −35 ◦ C in 60% (w/w) H3 PO4 aqueous solution. During the synthesis, the reactants were well mixed to ensure a constant temperature (−35 ± 0.5 ◦ C). After being deprotonated and rinsed with water, the EB was thoroughly washed with isopropanol (ACS grade, 99% purity) to remove water, oligomers, and some impurities. After the PANI EB was dried on a Büchner funnel under vacuum several days, we prepared five samples to dry at different conditions. DV1 and DV2 samples were dried in a vacuum oven for more than 1 day at room temperature and 100 ◦ C, respectively. D1–D3 samples were dried at different temperatures in a furnace oven for 5 h. During the whole process, the ovens were covered with alumina foil to avoid light exposure. 2.2. GPC study A Waters GPC system (Alliance 2690) with a photodiode array (PDA) (PDA 996) and a refractive index (RI 2410) detectors was used to measure the MW distribution and MW of the PANI EB powders. The GPC columns (4.6 mm i.d. × 300 mm)—a Styagel® HR 4E and a 5E were used in series. The column temperature was controlled at 60 ◦ C. The flowrate of the eluent was 0.35 ml/min, and the injection volume was 50 l. Polystyrene standards (Polymer Laboratories) with 10 narrowly distributed MWs, were used to calibrate the columns. The UV–vis spectrum was collected at a 0.36 nm resolution while the GPC chromatogram was collected at 1 s time interval. The calculated MWs were relative to the polystyrene standards. Ultra-pure NMP (99.0+% HPLC grade) containing <0.01 M LiBF4 was used as an eluent as well as a solvent to prepare the PANI EB solutions [26]. Since LiBF4 has a poor solubility in NMP (less than 0.01 M), to prevent pressure fluctuation in the GPC system, the solvent/eluent was filtered through a 0.45 m PTFE filter prior to use. The 0.02 mass% of PANI powders were added into NMP/LiBF4 solvent and were equilibrated overnight at ambient condition prior to the MW determination. All of the polymer solutions were filtered through a 0.45 m PTFE syringe filter before they were automatically injected into the GPC columns. The PANI MW and MW distribution were analyzed using a Waters Millennium [32] software. 2.3. Thin film preparation EB powders were dissolved into NMP to obtain a 1 mass% solution and allowed to equilibrate a couple of days under ambient conditions and protected from light. The filtered EB solutions were spin-coated onto indium–tin-oxide (ITO) glass electrodes (0.7 cm × 1.7 cm) at 6000 rpm for 5 min at room temperature. The thickness of the resulting EB films was usually <0.1 m. Prior to electrochemical measurements, these EB/ITO electrodes were dried under dynamic vacuum overnight at room temperature. 2.4. Electrochemical study Electrochemical measurements of the EB/ITO electrodes were carried out in an electrochemical cell consisting of the EB/ITO electrode as the working electrode, a Pt wire as the counter electrode, and an Ag/AgCl (3 M NaCl) as the reference electrode. Zero-current chronopotentiograms were recorded immediately once the 1 M HCl electrolyte was added into the cell. The open circuit potential (OCP) of the PANI thin film was obtained at the steady state after 1 h. Subsequently, cyclic voltammograms (CV) were recorded for the PANI
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thin film in the same electrochemical cell. A stable CV was obtained after continuous potential cycling for a few cycles. 3. Background To better understand latter materials in the paper, we need to discuss the effects of oxidation state on the UV–vis spectra of PANI. PANI exists in a number of unique structures, and is characterized by the oxidation state (i.e., the ratio of amine to imine nitrogens), and the extent of protonation. The oxidation state of the polymer increases with the value of y (0 ≤ y ≤ 1) (see Fig. 1). The value of y is the molar fraction of oxidized units in PANI structure. The three well-defined oxidation states are the fully oxidized pernigraniline base (PNB) (y = 1), the fully reduced leucoemeraldine base (LEB) (y = 0), and the half oxidized EB (y = 0.5) [13]. The spectra of EB/NMP solutions consist of two classic peaks [14,36–38]. The first peak at ∼630 nm (Q peak) is assigned to the excitation of an electron from the highest occupied molecular orbital (HOMO, b ) of the benzenoid rings to the lowest unoccupied molecular orbital (LUMO, q ) of the quinoid rings [36]. The absorption of Q peak reflects both intra- and inter-chain interaction; therefore its absorption intensity and wavelength change with the EB chain configuration, which is affected by the solubility, additive, concentration, oxidization level, MW etc. [20,21,26,39]. The second peak at ∼330 nm (B peak) is due to → * transition associated with the electrons in the benzene rings, and mainly a function of intra-chain interaction [1,20,40]; therefore the solution properties have a less impact on its intensity and wavelength. The intensity ratio of Q peak to B peak is called Q/B ratio, and proportional to the ratio of quinoid units to benzenoid units in PANI. In the UV–vis spectrum of the EB/NMP solution (<1 mass%), the typical reported value of Q/B ratio is 0.86 ± 0.05 [14,21,31,40,41]. Fig. 2 presents the UV–vis spectral features of PANI at different oxidized levels, which is constructed using the experimental data presented by Albuquerque et al. [16]. As EB is reduced toward LEB, the value of Q/B ratio approaches 0, and B peak gradually red shifts from 335 to 347 nm; Q peak also red shifts from 634 to 645 nm at the initial stage, then level off. On the contrary, as EB is oxidized toward PNB, the value of Q/B ratio again is reduced to 0.69, but thereafter stays constant at ∼0.69; while Q peak gradually blue shifts from 634 to 571 nm, B peak stays at ∼335 nm [14,16]. Therefore, from the changes in the Q/B ratio and the shifts of Q or B peak in the UV–vis spectra, we may identify the oxidization state of PANI. It has also been reported that when different lithium salts are added into the EB/NMP solutions, changes in the Q peak wavelength and Q/B ratio reflect the differences in the ionic interactions [15,26,42,43]. For example, when Cl− is added to the EB solution, Cl− can sometimes cause a red shift of Q peak all the way up from 630 to 675 nm [15]. Furthermore, the interaction between ions and EB changes the chain conformation of EB, which cause a large increase in the value Q/B ratio (up to 0.92), which is attributed to the interaction between Li+ and the imine site of EB via a “pseudo-doping”
process [44]. This “pseudo-doping” reduces the inter-/intra-chain interaction and enhance the solubility of EB, and thus enhance the Q/B ratio and cause the red shift on Q peak [45]. In this work, LiBF4 was added to deaggregate the EB chains. Therefore, the peak wavelengths and Q/B ratio in the UV–vis spectra of the EB/NMP/LiBF4 solutions will slightly depart from the values reported in the literature for the EB/NMP solutions [21,40,46]. Nevertheless, Fig. 2 is still a valuable reference to estimate the oxidization state from the UV–vis spectra of the studied PANIs. 4. Results and discussion 4.1. Effect of molecular weight on UV/vis spectra of PANI in NMP Feng et al. [47] reported that when the EB oligomer (dissolved in NMP) changes in size from tetramer to hexadecamer, the value of Q/B ratio changes from 0.33 to 0.52. At the same time, B peak red shifts from 320 to 328 nm and the Q peak red shifts from 588 to 610 nm, respectively. However, until now, we have not found any systematic study on the effect of the EB MW on its UV–vis spectrum as the polymer chain length increases from oligomer to polymer. We therefore characterized several EBs which weight average MW (Mw ) varies from 3000 to 285,000 g/mol. Table 1 summarizes the Mw , number average MW (Mn ), and polydispersity index (PDI) of these samples. As the synthesis temperature decreases, both Mw and Mn of EB increase, and the PDI increases as well. The similar trend was also reported by Zengin et al. [32]. Among six SFST EB samples, the synthesis temperature were well controlled within ±<2 ◦ C of the desired temperature except for S3. During the synthesis process of S3, the bath temperature was over shut about 2–3 ◦ C above the desired temperature at the beginning of the synthesis, which may be part of reasons why the PDI of S3 is unusually high. In Table 1, we also include the GPC results of other four outside samples. The detailed synthesis conditions are unknown. To ensure fewer defect and no chlorine in PANI, we synthesized it in H3 PO4 but not in HCl. Some studies show that if the polymerization is carried out in an acid with a large amount of LiCl present, especially if the acid is HCl, significant ring chlorination occurs (typically 1% by weight of the base polymer) [48]. For some applications, it is desirable to eliminate this chlorine and any other impurities/defects that may occur by this route [35]. SFST invented several new polymerization processes, including using H3 PO4 to produce high MW PANI without much defect and chlorine. Some batches of PANI have been reproduced for more than 10 times. Their MW and PDI were also very reproducible. Therefore, the high PDI of these PANIs is not caused by the poor controlled synthesis, but is inherently associated with the H3 PO4 system. Its PDI ranges typically between 5 and 10 depending on the PANI MW. The UV–vis spectra of these EBs in Fig. 3, collected at the peaks of the GPC chromatograms, suggest that when Mw of EB is greater than 30,000 g/mol, both Q/B ratio and the wavelength of Q peak remain constant. For the EB samples with Mw < 30k g/mol, as the MW increases, both B and Q peaks undergo a red shift while the Q/B Table 1 Summary of calculated molecular weights of EB samples (NA - not available).
Fig. 1. Repeat unit of PANI in its different oxidation states.
Label
Manufacture
Synthesis temperature (◦ C)
A1 A2 A3 S1 S2 NEB S3 S4 S5
Aldrich Aldrich Aldrich SFST SFST Neste Oy SFST SFST SFST
NA NA NA 20 10 NA 0 −10 −35
Mw (g/mol) 3,020 11,980 15,200 21,900 27,100 79,100 130,500 143,400 285,300
Mn (g/mol) 1,550 3,270 4,170 8,480 8,780 13,900 17,400 27,500 42,200
PDI 1.9 3.7 3.6 2.6 3.1 5.7 7.5 5.9 6.8
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Fig. 2. Correlations between oxidation state and Q (or B) peak (a) and between oxidation state and Q/B ratio (b) (plots are based on the data of UV–vis spectra of 1 mass% PANI at the different oxidized levels in NMP at ambient conditions [16].
ratio increases from 0.48 to 0.89. When the EB Mw is greater than 15k g/mol, the B peak wavelength remains constant with increasing Mw , but Q peak continuously shows a red shift with Mw until the value of Mw reaches ∼30k g/mol. Fig. 4 displays the correlations of Q/B ratio vs. Mw , and the wavelength of B(Q) peaks vs. Mw . In Fig. 4, we also include the Q/B ratios and wavelength of B(Q) peaks of the EB oligomers obtained from the Feng’s article [47]. These UV–vis spectral features illustrate the dependence of the spectral characteristics on the MW. It is worthy noted that when we constructed these curves, we assumed that all of these synthesized EB samples were exactly in the EB oxidation state (y = 0.5), which may not be the case. Furthermore, compared to the UV–vis spectra for the EB/NMP/LiBF4 solution to the EB/NMP solution, one should expect a slightly higher Q/B ratio than ∼0.86 and a higher Q peak wavelength than 630 nm of the former. All of these factors might be responsible for the discrepancy between our curves and the curve constructed from the literature data. In summary, when the MW of PANI is less than 30k g/mol, both MW and oxidization state impact the UV–vis spectral characteristics of the EB/NMP solutions. When the MW is greater than 30k g/mol, oxidization state of PANI is the major factor in determining its spectral features. With these findings, we would like to construct the correlations between MW and UV–vis spectral features from an analytic EB, and use the correlations to study how
synthesizing and post-treatment conditions change EB properties. To cover a wide range of MW, we conducted the following study using S3, NEB, S4 and S5, which have medium and high MWs. 4.2. Electrochemistry study of PANI samples Before we construct the correlations, we need to determine which PANI sample has the oxidation state mostly close to the analytic EB oxidization state (y = 0.5) among these four samples. Huang et al. [1] determined the analytic EB state to be approximate 0.4 V (vs. SCE) in 1 M HCl for both chemically and electrochemically synthesized PANIs. Considering the fact that the standard potential of the reference electrode (Ag/AgCl) employed in the present work is lower than that of the SCE [49], the EB state would appear at a slightly higher potential when Ag/AgCl is used. Based on the cyclic voltammogram (CV) of these PANI samples, we determined the EB state to be at ∼0.46 V (vs. Ag/AgCl) in 1 M HCl (see insert plot in Fig. 5). A steady-state open circuit potentials (OCPs) can be used to determine the oxidization state of PANI samples. The zero-current chronopotentiograms of these four PANI/ITO electrodes obtained in 1 M HCl are shown in Fig. 5. Since the OCP of the analytical EB should be ∼0.46 V (vs. Ag/AgCl), a lower or higher OCP than this value suggests that the oxidization state of PANI is more reduced or more
Fig. 3. Effect of MW on the UV–vis spectra of EB/NMP/LiBF4 . Low MW EB samples are shown in (a) and medium and high MW EB samples are shown in (b) (UV–vis spectra were collected from the EB solutions at the GPC peak).
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Fig. 4. Curves of Q/B ratio vs. Mw (a) and B (or Q) wavelength vs. Mw (b) for the EB/NMP solutions (dotted lines (- - -) represent curves constructed from the literature values of EB oligomers in NMP [47] and solid lines (—) represent curves constructed from the UV-vis spectra of the EB/NMP/LiBF4 solutions).
oxidized than that of the analytical EB, correspondingly. Hence, the presence of higher oxidation state than EB was clearly observed for the S3 film (OCP = 0.57 V). By contrast, the lower OCP value of S5 film (0.42 V) indicates that this sample contains more reduced form than the EB state. Although the OCP values of NEB and S4 films are very close to one another, NEB seems to have the characteristics of an analytic EB. The comparison of the CVs between NEB and S4 (see insert plot in Fig. 5) also suggests the red-ox reversibility of NEB seems better than that of S4.
4.3. Simultaneous UV–vis/GPC analysis of newly synthesized PANI
Fig. 5. Zero-current chronopotentiograms of EB sample coated ITO glass electrodes obtained in 1 M HCl. The insert plot is the cyclic voltammogram of the thin films of S4 and NEB coated ITO glass electrode (obtained in 1 M HCl at the scan rate of 20 mV/s at 23 ◦ C).
Fig. 6 shows the combination of UV–vis spectra/GPC chromatograms of a 0.02 mass% NEB/NMP/LiBF4 solution. In this 3D-plot, correlations between retention time, wavelength, and UV–vis absorption intensity are revealed. For example, the GPC results can be extracted at the strong absorption (e.g. ∼330 nm or ∼630 nm) while the UV–vis spectra of a narrow fraction can be obtained at any retention times. The absorption intensity vs. retention time reflects the changes in the concentration of each fraction. With these 3D-plots, we could construct the correlations of Mw vs. UV–vis spectral features for the fractioned PANI.
Fig. 6. Illustration of a 3D-plot with UV–vis spectra/GPC result obtained from the 0.02 mass% NEB/NMP/LiBF4 solution.
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Fig. 7. Plots of Q/B ratio vs. Mw for four PANI powders dissolved into NMP/LiBF4 .
Fig. 7 illustrates the correlation of Mw vs. Q/B ratio of four PANI samples. Their GPC results are already summarized in Table 1. As discussed in the previous sections, the zero-current chronopotentiograms of these PANI suggest that characteristics of NEB are almost the same as the analytic EB. Therefore, the correlation of NEB can be used as a reference for the analytic EB. Interestingly, the Q/B ratio of NEB increases as the Mw increases, and reaches this highest value, and then starts to decrease slightly, which may be due to the decreases in the EB solubility as its MW gets larger and larger [31]. Actually, the other three curves also show similar behavior. However, the large differences are observed for the PANI fractions with low MWs, which be discussed in great details in the following sections. The lowest Q/B ratio of S5 within a wide range of MW suggests its most reduced form among these PANI samples, which is consistence with the lowest OCP value (0.42 V). On the other hand, the fairly constant Q/B ratios of the fractioned S3 vs. Mw distinguishes its correlation from the other three. The highest OCP of S3 (0.57 V) suggests that it contains more oxidized form than the other three samples, and thus more quinoid units, which may result in a high Q/B ratio than NEB in the low MW range. The difference in the correlation between Q/B ratio and MW of NEB and S4 is more obvious than what we observed in their OCP measurement. The lower Q/B ratio of S4 may suggest some reduced form in S4. Listing these four samples in the order from the reduced to the oxidized form, we have the following order:
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Fig. 8. The GPC chromatograms of 0.02 mass% of four PANIs dissolved in NMP/LiBF4 —the highest absorbance of S5 suggests the best solubility (chromatograms were collected at ∼330 nm wavelength, column temperature was set at 60 ◦ C).
be better than that of EB. This may explain why S3 also dissolved better than NEB. Since S4 is slightly reduced than NEB, the slightly better solubility of S4 than NEB is expected. It is noteworthy that the concentration of PANI can potentially affect the Q/B ratio [20,21,31]. However, in this study, the PANI concentration is very low (<0.02 mass%). The addition of LiBF4 in PANI solution is sufficient to block the interaction between PANI chains. The single peak of GPC chromatograms are obtained for all of the samples. The GPC column temperature was also controlled at 60 ◦ C, which is very close to the theta temperature in PANI/NMP system reported by Zengin et al. [32]. Therefore, the interaction between polymer chains would be minimized. Both UV–vis spectral features and the retention time were not appreciably affected by PANI concentration (up to 0.05 mass%) and solution ages. The changes in the Q/B ratio here were mainly due to the oxidization state. Fig. 9 describes the correlations between the wavelength of B peak and the Mw for these PANI samples. Again, the main differences among them are observed at the low MW portions. According to the discussion in Fig. 2, one knows that when PANI changes its oxidation state from EB toward LEB, B peak shows its characteristically red shift, which provides additional evidence of more reduced
S5 (some reduced) > S4 (slightly reduced) > NEB (EB) > S3 (some oxidized) When we tested the solubility of these four PANI powders, their solubility in NMP/LiBF4 decreased in the order of S5 > S3 > S4 > NEB (see Fig. 8). The poor solubility of EB in organic solvents is mainly due to the hydrogen bonding between imine nitrogen and amine hydrogen atoms [19,33,50]. Since the structure of LEB does not contain the imine nitrogens (see Fig. 1) to form this H-bond interaction, although there are some weak interactions between amine hydrogen and amine nitrogen, the solubility of LEB is much better than that of EB [51]. One can logically predict that the solubility of PANI increases as its oxidation state changes incrementally from EB to LEB. This may explain why S5 had the best solubility. Similarly, as the oxidization state of PANI changes toward the PNB state, due to the decreased amine sites, the possibility of H-bond interactions between amine hydrogen and imine nitrogen also decreases. Hence, the solubility of PANI with a higher oxidization state than EB should
Fig. 9. The correlations between B peak vs. Mw for four PANIs dissolved in NMP/LiBF4 .
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Table 2 Summary of drying conditions and GPC results for five PANI samples. Sample label DV1 DV2 D1 D2 D3
Drying temperature (◦ C) 23 (vacuum) 100 (vacuum) 60 (furnace oven) 80 (furnace oven) 100 (furnace oven)
Drying time (h) >24 >24 5 5 5
Mn (g/mol) 43,100 27,800 43,300 46,600 32,500
Mw (g/mol) 271,200 350,700 257,800 263,700 181,900
PDI 6.3 12.6 5.9 5.7 5.6
form in S5 than that in the other three PANIs. Similarly, B peaks of the low MW portion of S4 also show a red shift, which also suggests some portions containing slightly reduced form. On the contrary, the changes in B peak of S3 with Mw is different from both S4 and S5, this may be attributed to its slightly oxidized form at the low MW portions. In principle, the correlation between Q peak wavelength and Mw can also be constructed to study the difference among these PANI samples. However, besides the MW and the oxidization state, additives in the solution and PANI concentration also influence its shift, which needs more caution for this application. In summary, when we synthesized these PANI EB, we expected that the MW increases with decreasing synthesized temperature. However, the simultaneous GPC/UV–vis analysis reveals the different properties of these PANI besides their MW. Besides temperature, other conditions (oxidant addition rate, mixing, batch temperature control, post-treatments, etc.) also result in the different products, and thus change the uniformity and processibility of synthesized PANIs. 4.4. Drying effect From the GPC characterization of many early batches of PANI synthesized in SFST, we noticed that their GPC results and processibilities vary from a batch to another even when some of them were synthesized at the same synthesis conditions. Several research groups reported that heat treatment would induce cross-linkage between EB molecules in NMP solvent [52–54]. We suspect that the similar phenomena might occur when the PANI EB was posttreated differently. Furthermore, using the above methodology, we may obtain more insights on the structural and MW changes during the drying process besides the poor solubility. In this study, we investigated five PANI samples which were synthesized in the same batch, but dried at different conditions. Their GPC results together with the post-treatment conditions are summarized in Table 2. Fig. 10(a) and (b) illustrate that the correlations of Q/B ratio vs. Mw and B peak vs. Mw for these samples, respectively. After drying,
the appreciable changes in both Q/B ratio and B peak wavelength are observed. DV1 sample, vacuum dried at room temperature (∼23 ◦ C), is the closest one to NEB among these five samples. Although DV2 was dried under vacuum oven whereas D1, D2 and D3 were dried in the furnace oven, the changes in the UV spectra of these four samples suggests that all of them were reduced from its starting PANI sample (DV1). As the drying temperature was increased and drying time was prolonged, the decrease in Q/B ratio and the increases in B peak wavelength become more obvious. After drying for 5 h, the most changes occur in the low MW portions. As the drying temperature increases from 60 to 100 ◦ C, the higher MW portions starts to changes as well. Similarly, when the DV2 was dried at 100 ◦ C for more than 24 h, the chemical reduction took place in the entire MW range. The cross-linked PANI EB also shows the reduced characteristics due to the lose of quinoid units [52]. Therefore, the large changes on Q/B ratio and red shift on B peaks of over heated samples suggest that the cross-linkage among the EB chains occurs in the drying stage. Actually, even all of the solutions were started from 0.02 mass% of PANI, the low intensity of their GPC chromatograms in Fig. 11 suggests that the solubility of D3 and DV2 is largely decreased. An appreciable aggregation is observed in the GPC chromatogram of DV2, which may be caused by the cross-linked materials with a much larger MW than non-cross-linked PANI. As some aggregates were filtrated from the solution, the PANI concentration would be lower than that of the solution without aggregated PANI. Comparing to DV1, D1–D3 have the lower MW and PDI (see Table 2). These results may suggest that some low MW portions of PANI interacted with high MW portions during the drying treatment, and cause aggregations. These aggregates could not be dissolved into the solutions, and thus were removed in the filtration step, which may cause the low absorption intensity of D1, D2, and D3. On the other hand, although both D3 and DV2 were dried at 100 ◦ C, DV2 gave much larger Mw and the PDI than those of D3. We believe that besides the drying time, the drying under vacuum and at ambient pressure also contributes the difference. Drying in the furnace oven, the removing rate of residual solvent is much slower than that under vacuum. This slow evaporation of the solvent may enhance the similar annealing effect that was reported in the EB/NMP solutions [52–54]. Although some of aggregates was removed from the filtration step, and resulting in the lowest absorption intensity of DV2, some of aggregates were passed through the filter and gave a distortion of the GPC chromatogram. In summary, all of these results support that the high drying temperature can chemically change the PANI EB oxidization
Fig. 10. Drying effect on the Q/B ratio vs. Mw (a) and the B peak wavelength vs. Mw (b) of five PANI samples dried at different conditions.
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Acknowledgements This material is based upon work supported by the Defense Advanced Research Projects Agency Defense Science Office. DARPA Order No. G874/00 is issued by DARPA/CMD under contract #MDA972-99-C-0004, for which the authors are thankful. Special thanks to Dr. Ian Norris for his useful comments. References
Fig. 11. The effect of drying conditions on the GPC chromatograms of PANI samples (the refractive index detector was used to collect the chromatograms, column was heated at 60 ◦ C).
state during the post-treatment step. Even drying under vacuum, high drying temperature can change the properties of EB. The other important finding is that PANIs with low MWs seem to be more vulnerable than PANIs with high MWs to the synthesizing and post-treatment conditions. Recently, more and more research efforts have been dedicated to the investigation of nano-form PANI. Their MWs are, typically, less than 50k g/mol. Therefore, extra caution should be paid when one synthesizes and post-treats the nano-formed PANI EB to preserve its intrinsic properties. 5. Conclusions By separating the superimposed UV–vis spectra, and correlating the UV–vis spectra with each narrow fraction of PANI, we can more precisely determine the oxidation state of PANI samples with different weight average molecular weights (Mw ). Combining the GPC with the UV–vis spectroscopic analysis of the fractionated PANI, we can evaluate the quality and processibility of PANI more in depth. This methodology can also be used to aid in analysis and improvement of a PANI synthesis and post-treatment procedures. From the correlation between UV–vis spectral characteristics and the molecular weight, we may tune the synthesis conditions to modify the oxidation state of the final product. Furthermore, we have constructed a correlation between Mw and UV–vis spectral characteristics for the analytic EB, which is also very important for determining the oxidation state of fractioned PANI with a MW less than 30k/g. This study also reveals that the fractioned PANI EB with low MWs is more vulnerable to the process conditions than those with high MWs. Recently, more and more research efforts have been dedicated to the nano-form PANI synthesis. Most of these PANI have low molecular weight. Therefore, extra caution (e.g. temperature control) must be paid not only in the synthesis step but also in the post-treatment. The low drying temperature (<60 ◦ C) under vacuum is critical to preserve the EB nature. Moreover, using zero-current potentiometry, we have verified the oxidation states of the PANI samples. Excellent correlation between the OCPs and UV–vis characteristics of these PANI samples suggests that the electrochemical approach can also provide a rapid qualitative estimation of the relative degree of the reduced and the oxidized forms present in PANI.
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