Solid State Electrical Conductivity of Radical Polymers as a Function

Article pubs.acs.org/Macromolecules

Solid State Electrical Conductivity of Radical Polymers as a Function of Pendant Group Oxidation State Lizbeth Rostro, Si Hui Wong, and Bryan W. Boudouris* School of Chemical Engineering, Purdue University, 480 Stadium Mall Drive, West Lafayette, Indiana 47907, United States S Supporting Information *

ABSTRACT: We establish the relationship between pendant group chemical identity and the ability of a specific radical polymer, poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA), to transport charge in the solid state. Radical polymers (i.e., macromolecules with aliphatic carbon backbones and pendant groups containing stable radical moieties) have attracted much attention in organic electronic applications due to straightforward synthetic methods, easily tunable electronic properties, and relatively high-performance with respect to charge transport. Because charge transport can occur only through the pendant group of these completely amorphous radical polymers, controlling the precise chemical nature of these functional groups is of key import. Specifically, we have determined that the deprotection step, which converts the pendant group functionality through a simple oxidation reaction, can lead to four distinct chemical functionalities along the radical polymer, as monitored by a range of complementary spectroscopic techniques. Of these four functionalities, only two (i.e., the stable free radical and the corresponding oxoammonium cation) are able to contribute positively to the charge transport ability of the macromolecule. As such, manipulating the reaction conditions for this deprotection step, and monitoring closely the resultant chemical functionalities, is critical in tuning the electrical properties of radical polymers. However, if these parameters are controlled well, we are able to generate transparent, conducting thin films of pristine (i.e., not doped) nonconjugated radical polymers with electrical conductivities as high as (1.5 ± 0.3) × 10−5 S cm−1.

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with many of these polymers.21,22 As such, elucidating fully the nature of the charge transport processes in a class of macromolecules that are completely amorphous could aid in opening new pathways in the designs of organic electronic materials. Polymers containing stable pendant radical groups (i.e., not within the backbone of the macromolecules, as in the case of polyradicals23−25) have been investigated previously for solution-based devices (e.g., organic batteries) primarily.26−29 In these systems with aliphatic carbon−carbon backbones, the remarkably quick charge transport was aided by the presence of an electrolyte solution.30−33 Furthermore, these solution-based systems outlined clearly that charge contained within the radical polymers was transferred by an oxidation−reduction reaction with a large rate constant.29,30,34,35 Recently, our team and others have focused on expanding the charge transport ability of radical polymers to the solid state.36,37 However, due to the currently limited usage of these materials in the solid state, the fundamental question of how the chemical identity affects the charge transport ability of radical polymers has not been addressed in full. Here, we evaluate systematically the oxidation reaction that forms the radical moieties and its effect on the

INTRODUCTION Optoelectronically active macromolecules have been investigated extensively due to their potential impact across a wide range of electronic device platforms (e.g., organic field-effect transistors (OFETs), organic photovoltaic (OPV) devices).1−5 The majority of the interest in these macromolecules has been focused on π-conjugated systems, where a high degree of electron delocalization and a semicrystalline polymer microstructure serve as the basis for the charge-transporting medium.6−10 Many strides have been made to improve the charge transport ability in these classes of semiconducting and conducting polymers but some concerns still remain, and these concerns have prevented their utilization in widespread applications.11−13 Moreover, many synthetic challenges in obtaining large quantities of nearly pure (i.e., for electronic applications), highly crystalline polymers still prevail, which limits the performance of many organic electronic devices.14−16 Additionally, much effort has been placed on optimizing the processing of semicrystalline polymer systems and establishing how this processing affects the overall charge transport performance of these materials.17,18 These concerns can be attributed to the microstructural dependence of charge transport properties for semicrystalline polymers.19,20 Furthermore, despite all of this effort, the charge transport in πconjugated polymer-based devices may very well be limited by the relatively large amount of amorphous domains associated © 2014 American Chemical Society

Received: March 26, 2014 Revised: May 4, 2014 Published: May 30, 2014 3713

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PTMPM was synthesized as previously demonstrated. An example synthesis is detailed below. PTMPM Synthesis. Poly(2,2,6,6-tetramethyl-4-piperidyl methacrylate) (PTMPM-RAFT) was synthesized via a controlled, reversible addition−fragmentation chain transfer-mediated (RAFT-mediated) polymerization in a 250 mL reaction flask with a Teflon-coated magnetic stir bar. To the reaction flask 1 g of 2,2,6,6-tetramethyl-4piperidyl methacrylate (4.4 mmol), 0.018 mL of 2-phenyl-2propylbenzodithioate (0.074 mmol), 2 mg (1.2 μmol) of 2,2′azobis(2-methylpropionitrile) (AIBN), and 20 mL of anhydrous toluene were added. The reaction was heated to 75 °C and allowed to proceed overnight. The polymer was precipitated in hexanes and dried under reduced pressure. The RAFT terminus was removed by reacting PTMPM-RAFT with an excess of AIBN (75 mol equiv relative to the number of polymer chain end groups) in a 250 mL reaction flask with a Teflon-coated magnetic stir bar. 1 g of PMTPM-RAFT (Mn = 11.3 kg mol−1, 0.08 mmol of end groups) was added to the reaction flask with 1 g of AIBN (6 mmol) and 50 mL of anhydrous toluene. The reaction temperature was set to 75 °C, and the reaction was allowed to proceed overnight. The reaction was precipitated in hexanes and dried under reduced pressure. Oxidation of PTMPM to PTMA. PTMPM was oxidized through the reaction of the precursor polymer with mCPBA in anhydrous dichloromethane to produce the functional PTMA polymer. The conditions for all reactions were as follows. PTMPM (0.50 g, 0.045 mmol) was dissolved in 5 mL of anhydrous dichloromethane, and mCPBA (0.96 g, 5.6 mmol) was dissolved in 10 mL of anhydrous dichloromethane. The mCPBA solution was added dropwise to the PTMPM solution while stirring at room temperature in an inert atmosphere. The polymer was washed with an aqueous sodium carbonate solution (20%, by weight) three times and then precipitated in hexanes. The reaction was allowed to proceed for times ranging from 0.5 to 20 h. The same parent polymer, PTMPM, was used for all the oxidation times. For this polymer, the number-average molecular weight (Mn) was 11.3 kg mol−1, as determined by 1H NMR spectroscopy, and the polymer had a dispersity value (Đ) of 1.3, as measured using size exclusion chromatography (SEC) against polystyrene standards. General Methods. Size exclusion chromatography data were collected on a Hewlett-Packard 1260 Infinity series equipped with a Hewlett-Packard G1362A refractive index (RI) detector and equipped with three PLgel 5 μm MIXED-C columns. Polystyrene standards (Agilent Easi Cal) of molecular weights ranging from 1 to 200 kg mol−1 were utilized to calibrate the SEC. The mobile phase was tetrahydrofuran (THF) at 40 °C flowing at 1 mL min−1. Attenuated total internal reflectance−Fourier transform infrared spectroscopy (ATR-FTIR) spectra were attained using a Thermo-Nicolet Nexus FTIR. The data were collected in 36 scans (over the range 800 cm−1 ≤ ν ≤ 4500 cm−1) while the ATR-FTIR was under a nitrogen purge. The measurements were acquired on a diamond substrate using a deuterated triglycine sulfate (DTGS) KBr detector and a KBr beam splitter. Ultraviolet−visible (UV−vis) light absorption spectra were acquired using a Cary 60 spectrometer over a wavelength range of 300−800 nm with chloroform serving as a blank. A Bruker EMX-EPR was utilized for electron paramagnetic resonance (EPR) measurements of PTMA in toluene (2 mg of polymer per 1 mL of solvent) at room temperature. A Kratos Axis Ultra DLD imaging X-ray photoelectron spectrometer with a monochromatic Al Kα (1486.6 eV) was utilized for XPS measurement under high vacuum (1 × 10−9 Torr). The samples were fabricated and transferred via an oxygen-free environment. PTMA Thin Film Generation, Device Fabrication, and Device Testing. Solutions of 20 mg of PTMA (with varying lengths of oxidation times) dissolved in 1 mL of chloroform were created. The solutions were stirred at room temperature overnight. Tin-doped indium oxide (ITO) substrates were cleaned by sequential sonication in acetone, chloroform, and isopropyl alcohol for 10 min each. The solutions were spun-coat onto clean ITO substrates yielding transparent films of thickness of 200 nm. Inside of an inert atmosphere glovebox, 100 nm of gold was evaporated into patterned arrays using a

oxidation state of the functional pendant group of radical polymers. Poly(2,2,6,6-tetramethylpiperidinyloxy methacrylate) (PTMA) has been used as a model radical polymer for a variety of organic electronic applications; however, the effects of the oxidation reaction to functionalize the protected precursor polymer to form PTMA have not been studied in detail. For the first time, we demonstrate that the commonly used oxidation reaction that forms the stable radical can overoxidize the radical species to form the cation analogue and the hydroxylamine species (i.e., the alcohol). This overoxidation affects greatly the electrical properties of the nonconjugated conducting polymer. Here, we monitored the oxidation of poly(2,2,6,6-tetramethyl-4-piperidinyl methacrylate) (PTMPM) with m-chloroperbenzoic acid (mCPBA) to form PTMA, as mCPBA has been the primary oxidizing agent in the deprotection of PTMPM. However, it should be stressed that other promising oxidizing agents (e.g., NaWO4)38 have been introduced recently in order to form the stable radical pendant groups.39 The presence of the stable radicals was evident in the ultraviolet−visible (UV−vis) spectrum (with a maximum absorption at λ = 458 nm), as this peak is wellassociated with the radical-bearing species. Furthermore, the loss of radical moieties to cation sites was visible in the UV−vis as oxidation time increases. In addition, the radical density also was found to decrease for longer oxidation times, as monitored via electron paramagnetic resonance (EPR). Moreover, we apply attenuated total internal reflectance−Fourier transform infrared (ATR-FTIR) spectroscopy to confirm that the decrease in radical site population corresponds to an increase in the presence of oxoammonium cation sites (+NO) at longer oxidation reaction times. The counterion to the oxoammonium sites is determined to be the conjugate base of the mCPBA, as monitored by X-ray photoelectron spectroscopy (XPS). Additionally, XPS reveals the presence of three distinct nitrogen functionalities associated with the pendant radical site of the PTMA macromolecules (i.e., the unreacted functionality without a radical site, the radical species, and the corresponding cationic species). Critically, we establish that the chemical nature of these pendant groups affects the conductivity of the polymer films by almost an order of magnitude, and we determine that the conductivity of the radical polymer thin films can be tracked well with the chemical nature of the pendant groups. The maximum conductivity occurs after slight cation doping (at 3 h of oxidation time using our reaction conditions) and is found to be (1.5 ± 0.3) × 10−5 S cm−1. We reiterate that this is without the presence of intentional, external dopants, which are typically used to increase the conductivity of conjugated semiconducting polymers. These data indicate that moderate doping of the radical polymers could improve their ability to conduct charge in the solid state relative to pristine radical polymer materials (i.e., radical polymers with 100% radical group functionality) and provide a new design handle for these functional macromolecules.

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EXPERIMENTAL SECTION

Materials. All chemicals were used as received from Sigma-Aldrich unless otherwise noted. m-Chloroperbenzoic acid (mCPBA) was washed with water from ether and dried under reduced pressure. The monomer 2,2,6,6-tetramethyl-4-piperidyl methacrylate (TMPM) was purchased from TCI America and was used without purification. 3714

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shadow mask under reduced pressure. This ITO/PTMA/Au sandwich structure was selected due to the work functions of the metals and the transport level of the PTMA. Specifically, because the work functions of ITO and Au are very similar and match well with the transport level of PTMA (at 5.2 eV removed from vacuum), the devices showed ohmic behavior when the devices were placed in operation (Figure S1). Current−voltage measurements were acquired by sweeping voltages from −1 to +1 V and measuring the current between a 200 nm thick film of PTMA that was sandwiched between the ITO and gold contacts, from which the resistance was determined and the electrical conductivity was calculated (Figure S1). These measurements were performed using a Keithley 2400 sourcemeter and recorded using a LabView code. X-ray photoemission spectroscopy (XPS) analyses were conducted in an inert environment with PTMA thin films on ITO substrates. In these experiments, PTMA was spuncoat in a glovebox out of chloroform at a solution concentration of 20 mg mL−1. The binding energy of adventitious carbon was assumed to be 284.8 eV and was used for charge correction. Casa XPS software was utilized for all XPS data analyses. Film thicknesses were determined using KLA Tencor profilometer by scratching the film and measuring the height change.

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RESULTS AND DISCUSSION Beyond a very recent and impressive anionic polymerizationbased result that was able to synthesize PTMA with every pendant group containing a stable radical species,40 most synthetic protocols call for the creation of the polymer bearing protected pendant groups, poly(2,2,6,6-tetramethyl-4-piperidinyl methacrylate) (PTMPM), followed by the oxidation to the stable radical species (PTMA), as shown in Figure 1a.41,42 However, the effects of the oxidation reaction that creates the stable radicals on the pendant groups for this common procedure have not been elucidated fully. Specifically, we establish the effects of the oxidation reaction on the solid-state electrical conductivity of PTMA. As shown in Figure 1a, we used mCPBA to oxidize PTMPM to form PTMA and demonstrate that, after forming the stable radical site, there can be further oxidation to form the cation analogue (PTMA+) and the hydroxylamine analogue (PTMA-OH). The parent polymer, PTMPM, was synthesized as previously shown, via a RAFT-mediated polymerization scheme. From the same parent polymer, the oxidation reaction, using the same oxidation reagent solution, was performed in inert conditions at room temperature with an excess of mCPBA to produce the stable radical sites followed by a wash with an aqueous sodium carbonate solution. After the oxidation step, the UV−vis spectra of the PTMA polymers exhibited an increase in absorbance at shorter wavelengths, which corresponds to a shift in minimum absorption for longer oxidation times (Figure 1b). This change in absorption is attributed to a higher concentration of oxoammonium cations (PTMA+) in the pendant groups of the radical polymer. This assignment is rather straightforward to make, as the commercially available small molecule analogue of PTMA+, 4-acetomino-2,2,6,6-tetramethyl-1-oxopiperidinium tetrafluoroborate (4-acetylamino TEMPOnium), shows a remarkably similar absorption spectra (inset of Figure 1c). In addition, the growing presence of a second species is indicated by the isosbestic point at λ = 420 nm. Specifically, the increase in absorption at λ < 420 nm corresponds to the absorption of the cation species (Figure 1b). However, due to the electronic interaction between PTMA and PTMA+ pendant groups in solution, via reversible oxidation−reduction (redox reaction), the total absorption cannot be viewed as a simple linear summation of independent cation and radical spectra.

Figure 1. (a) Reaction scheme for the oxidation of PTMPM to form PTMA, PTMA+, and PTMA-OH. (b) UV−vis spectra of the functional radical polymer PTMA in chloroform at a solution concentration of 15 mg mL−1 for 0.5, 7, and 20 h of oxidation reaction time. The shift of the minimum from 367 to 390 nm is attributed to the increasing number of cations present in PTMA. Additionally, the isosbestic point at λ = 420 nm indicates a conversion between two species (i.e., a decreasing presence of PTMA and an increasing presence of PTMA+). (c) UV−vis absorption of 4acetylamino TEMPOnium, the cation small molecule analogue of PTMA+, at different solution concentrations. This molecule has a greater absorption than the TEMPO functionality at shorter wavelengths. This is in a manner similar to PTMA that has been oxidized for longer times, suggesting the presence of PTMA+ in these longer-reacted radical polymers.

Furthermore, the pendant groups in PTMA have demonstrated the ability to undergo a disproportionation reaction under acidic conditions to form the oxidized counterpart of PTMA (i.e., +NO) and the hydroxylamine pendant group (i.e., N−OH).43−45 Such a reaction results in the decreasing concentration of radicals in PTMA, which was monitored in solution directly via EPR. The signal intensity associated with the radical spin density decreases with longer oxidation reaction time, confirming that the radical population decreases as 3715

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oxidation time increases. Similar behavior is observed for the small molecule radical analogue, 4-hydroxy-TEMPO (TEMPOOH), which also exhibits a single peak in EPR at the same applied magnetic field (Figure S2). Moreover, the single peak in toluene for both TEMPO-OH and PTMA is indicative of the high concentration of radicals in solution.35 The radical density monitored via EPR suggests three oxidation stages: The first corresponds to shorter oxidation times (up to 1 h) where the pendant groups are being functionalized to the stable radical. After an hour the radical density is constant until the 3 h mark. At this point the pendant groups are being overoxidized to the cationic species, PTMA+, while continuing to form the stable radical from segments of the chain that are still protected (i.e., PTMPM). Following this mild doping stage, the radical density falls significantly and remains roughly constant for oxidation times up to 20 h (Figure 2). These results, in conjunction with the UV−vis results, suggest that the oxidation reaction proceeds to form other species after forming the stabilized radical.

Figure 2. EPR spectra for PTMA as a function of oxidation time. The absorption intensity, corresponding to the radical density, initially increases as oxidation time increases up to 3 h. After 3 h the radical density decreases significantly and then stabilizes. The reduction in intensity of the free radical signal from EPR as a function of oxidation time demonstrates that the radical density is decreasing with oxidation time, suggesting the overoxidation to a nonradical species. These data were acquired for a solution composed of 2 mg of PTMA in 1 mL of toluene.

Figure 3. (a) ATR-FTIR spectra of the small molecule analogues of the possible pendant group functionalities in PTMA: N−H, N−O•, and +NO. (b) ATR-FTIR spectra of PTMA as a function of oxidation time, demonstrating an increase in cations present with increasing oxidation reaction time. The peak at 1467 cm−1 is associated with N−O• while the peak at 1540 cm−1 is associated with +NO. All peaks are normalized to the peak of at ∼3000 cm−1, associated with the alkyl C−H chemical functionality.

The conversion of the radical functionality to that of the cationic species can be observed to increase in the solid state as well. That is, attenuated total internal reflectance−Fourier transform infrared (ATR-FTIR) spectroscopy and X-ray photoemission spectroscopy (XPS) data indicated that the conversion of the stable radical pendant group to that of the oxoammonium cation is not due to the presence of a solvent. Specifically, the ATR-FTIR signature of the cation species (i.e., + NO) is distinct from that of the radical species (i.e., N−O•), and this peak appears at 1540 cm−1, in good agreement with the small molecule salt analogue (Figure 3a). Importantly, many peak assignments for the wavenumber associated with the N−O• bond stretch have been made over a range of wavenumber values; however, none of the assignments appear at wavenumbers greater than 1500 cm−1.46 This is consistent with the signal from the small molecule radical analogue, TEMPO-OH (Figure 3a), as there is no peak present at wavenumbers from 1500 to 1600 cm−1. Furthermore, the peak intensity at 1540 cm−1 increases for longer oxidation reaction

times, prior to saturating in intensity, indicative of an increasing presence of a different chemical functionality. Additionally, the broad hydroxyl absorption signal of the protonated species (N−OH) is found to increase mildly for increasing oxidation times (Figure S3). In addition to confirming the presence of the same three nitrogen environments observed in the FTIR-ATR spectra, XPS (full spectra are shown in Figure S4) revealed also the presence of the balancing anion in PTMA+ at binding energies of 220 and 270 eV for the Cl2p and Cl2s functionalities, respectively (Figure 4a). Comparison of the relative peak areas for the chlorine and nitrogen signals allows for the quantification of the relative amounts of these two chemical entities (Figure 4b). 3716

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Figure 5. High-magnification XPS spectra of PTMA thin films, where, prior to film casting, the PTMA macromolecule had been oxidized for a rage of times: 0.5 h (lower), 7 h (middle), and 20 h (upper). These spectra indicate the presence of three distinct nitrogen chemical states: N−H (red), N−O• (blue), and +NO (magenta). The raw data are the black lines, and the color lines are associated with curve fits attributed to each peak in the raw data. These fit curves were used in the calculations regarding the relative concentrations of each species. As expected, the area associated with the radical and cation peaks increases with increasing oxidation reaction time.

Figure 4. (a) High-magnification XPS spectra of PTMA thin films, where, prior to film casting, the PTMA macromolecule had been oxidized for a rage of times: 0.5 h (lower), 7 h (middle), and 20 h (upper). The spectra demonstrate the growing presence of the anion balancing PTMA+, which is the conjugate base of the oxidizing acid, as oxidation time increases. (b) Atomic ratio of chlorine to the entire area nitrogen (black points) as a function of oxidation reaction time and the relative atomic ratio of the cation nitrogen to all the entire nitrogen area (blue points). Note that the concentration of the anion and the cation are in a relatively good agreement, at a 1:1 ratio as expected. The dashed lines are guides to the eye. The chemical structure of the conjugated base is the inset.

drastic manner, and the maximum in electrical conductivity occurs at a doping level of ∼2.5% cation sites. Furthermore, there are three regimes (Figure 6) that emerge in conductivity with respect to the oxidation reaction time to form PTMA from PTMPM. The first regime corresponds to shorter oxidation times where the polymer is being functionalized from the protected pendant group to the radical pendant groups. As such, as more pendant groups are available to participate actively in the charge transport mechanism the electrical conductivity increases. The second regime corresponds to intramolecular doping that occurs as the pendant groups are oxidized to PTMA+. The effects of the electron-poor pendant groups, PTMA+, are very similar to the addition of electronwithdrawing dopants to π-conjugated polymers, which are known to increase the charge transport ability.50,51 Consequently, as the concentration of PTMA+ increases the electrical conductivity triples to a value of (1.52 ± 0.3) × 10−5 S cm−1 at an oxidation time of 3 h. Interestingly, the cation concentration stays relatively constant after 3 h, as shown by XPS and EPR. This indicates that this regime allows for conversion of N−H functionalities to the stable radical functionality while a portion of the stable radicals are being converted to the oxoammonium cation as well. The last regime corresponds to long oxidation times where the electrical conductivity decreases significantly. The decrease in electrical conductivity is attributed to the protonation of the pendant groups (Figure S3). Once the pendant group has reached this chemical state, it can no longer participate in the oxidation− reduction reaction that transports charge, thus hindering the charge transport. As such, we attribute the decrease in electrical

The entire nitrogen signal is used for the analysis because it remains constant for all PTMA samples, as the nitrogen atom does not participate in the oxidation reaction (Figure 4b). Additionally, the nitrogen 1s peak exemplifies three distinct nitrogen chemical states (Figure 5). Each peak is indicative of different chemical functionalities on the pendant groups of PTMA. The peak at the lower binding energy (BE) of 399.8 eV is assigned to the protected nitrogen (i.e., N−H), and this is confirmed through acquisition of a similar XPS spectrum of the parent PTMPM polymer (Figure S5). Moreover, the binding energies of the oxidized pendant groups are expected to be higher as they are known to increase as the oxidation state increases.47−49 Consequently, the signals associated with the N−O• and +NO functionalities appear at 401.3 and 405.6 eV, respectively (Figure 4). Similarly, by comparing the different chemical states of nitrogen, the +NO concentration was found and compares well with the anion concentration (Figure 4b), in agreement with charge neutrality. Utilizing XPS, the amount of PTMA+ was quantified by both the presence of the +NO cation on the macromolecular chains and the chlorine anion of the small molecule, confirming that the radical sites can be oxidized further to form the oxoammonium cationic species and that this overoxidation is coupled necessarily to the presence of the conjugate base of mCPBA. Critically, the presence of PTMA+ sites affects the solid state electrical conductivity of the radical polymer thin films in a 3717

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affect greatly the solid-state charge transport and can be optimized for solid-state charge transport through simple chemical methodologies.

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ASSOCIATED CONTENT

S Supporting Information *

Representative current−voltage (I−V) curves showing the linearity of the raw data used to conductivity values, EPR data of the TEMPO-OH small molecule analogue, ATR-FTIR data for the protonated pendant groups, the XPS spectrum of a representative PTMA sample for the wide range of binding energies, and the full XPS spectrum for a PTMPM thin film. This material is available free of charge via the Internet at http://pubs.acs.org.

Figure 6. Electrical conductivity (left axis) as a function of oxidation time showing three regimes. Region 1 shows the functionalization of the pendant groups to the redox-active species occurring quickly for the first hour; thus, the electrical conductivity increases. Region 2 highlights the window where mild intramolecular doping effects of the overoxidized species (i.e., PTMA+) and the conductivity increases to a maximum average conductivity of (1.52 ± 0.3) × 10−5 S cm−1. Region 3 includes the section where the effects of the protonation of the pendant groups in PTMA to N−OH significantly lower the electrical conductivity due to the insulation nature of these species. The intensity corresponding to the radical density as determined via EPR (right axis) as a function of oxidation time follows the electrical conductivity very well; therefore, as the radical density increases (i.e., more charge transporting sites), the higher the electrical conductivity. The electrical conductivity was measured in devices of the geometry ITO/PTMA(200 nm)/Au. The symbols represent the average conductivity values, and the error bars represent the standard deviation across 10 devices. The dashed lines serve as guides to the eye.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (B.W.B.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We gratefully acknowledge financial support from the Air Force Office of Scientific Research through the Young Investigator Program (AFOSR YIP, Grant FA9550-12-1-0243, Program Manager: Dr. Charles Lee). L.R. appreciatively acknowledges the National Science Foundation for support through the Graduate Research Fellowship Program (Grant DGE13333468). S.H.W. thanks the National Science Foundation for partial support of her work through the Nanotechnology Undergraduate Education (NUE) in Engineering Program (Award Number 1242171, Program Manager: Dr. Mary Poats). We thank Dr. Dmitry Zelmyanov and the Surface Analysis Facility of the Birck Nanotechnology Center at Purdue University for assistance in the acquisition of the XPS data.

conductivity to the termination of redox active sites in PTMA for oxidation times exceeding 3 h.

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CONCLUSIONS As an emerging class of conducting polymers, radical polymers have shown significant promise as the solid state performance has been determined to be on par with commonly used pristine (i.e., not doped) conjugated polymers; importantly, we demonstrate that the oxidizing reaction that forms the stable radical pendant groups affects the solid state charge transport greatly. Here, we have quantified how the number of pendant radical groups and pendant cationic groups, which serve as intramolecular dopants, dictates the charge transport ability of these emerging functional macromolecules. The oxidation reaction was found to produce a mixture of four different chemical functionalities: the electrically active N−O• and +N O species and the insulating N−OH and N−H chemistries. The amounts of cationic and anionic elements are found to increase with increasing oxidation time up to 3 h, and then they remain roughly constant after 3 h until and up to 20 h of oxidation reaction time. Specifically, we find that there are three regimes corresponding to the electrical conductivity. The first regime corresponds to the functionalization of the polymer to the redox-active sites. The second regime corresponds to the increasing presence of PTMA+, which serves as intramolecular dopants; as such, the conductivity increases further to (1.52 ± 0.3) × 10−5 S cm−1. Lastly, the third regime corresponds to the termination of the redox-active sites through protonation of the pendant groups to N−OH. This demonstration of conducting, highly transparent thin films could reasonably lead to their utilization in critical future technologies. Therefore, the pendant group functionality in radical polymers is found to

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dx.doi.org/10.1021/ma500626t | Macromolecules 2014, 47, 3713−3719