Multi-Scale Assembly of Polythiophene-Surfactant Supramolecular

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Multi-Scale Assembly of Polythiophene-Surfactant Supramolecular Complexes for Charge Transport Anisotropy David Bilger,† Amrita Sarkar,‡ Cameron Danesh,† Manesh Gopinadhan,§ Gregory Braggin,† Jose Figueroa,† Thanh Vy Pham,† Danielle Chun,† Yashas Rao,† Chinedum O. Osuji,§ Morgan Stefik,‡ and Shanju Zhang*,† †

Department of Chemistry and Biochemistry, California Polytechnic State University, San Luis Obispo, California 93407, United States ‡ Department of Chemistry and Biochemistry, University of South Carolina, Columbia, South Carolina 29208, United States § Department of Chemical and Environmental Engineering, Yale University, New Haven, Connecticut 06511, United States S Supporting Information *

ABSTRACT: Multiscale assembly of poly(3-alkylthiophene)s complexed with various alkyl-chain surfactant architectures has been investigated in dilute and concentrated solutions by means of ultraviolet−visible absorption and fluorescence spectroscopy, polarized optical microscopy, small-angle X-ray scattering, and four-point probe conductivity measurements. Supramolecular complexation occurs via ionic interactions between poly(3-alkylthiophene)s electrolytes and ionic surfactants. In dilute solutions, the supramolecular complex undergoes a coil-to-rod conformational transition as evidenced by a time-dependent chromism. Spectroscopic studies on transition kinetics reveal an inverse first-order rate law. While surfactant architectures significantly affect the persistence length of the complexes, the inverse first-order rate law is maintained. When concentrated above a critical value, the supramolecular complex exhibits an isotropic-to-liquid crystalline transition yielding hexagonally ordered microstructures. The liquid crystalline phase boundaries are largely dependent on polymer and surfactant architectures. The correlations between the intrinsic rigidity of conjugated polymers, optoelectronic properties, and liquid crystalline formation are presented. The dried films made from the sheared liquid crystalline solutions inherit liquid crystalline monodomains and display four times faster charge transport along the backbone alignment direction than the perpendicular direction.

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INTRODUCTION Conjugated polymers are semiconducting materials that have demonstrated promise for emerging applications in flexible optoelectronic devices, including polymer light-emitting diodes (PLEDs), organic photovoltaics (OPVs), and organic fieldeffect transistors.1 In particular, conjugated polymers display intriguing anisotropic optoelectronic properties because of onedimensional (1D) p-orbital overlaps along the backbone.2−5 The capacity to control macroscopic alignment and electronic structure is a key component to utilize such unique anisotropic properties.6 In other words, the polymers are capable of producing thin films with an enhanced molecular order to support charge transport. These properties could be achieved through the appropriate implementation of molecular modification and control of the materials microstructure, crystallization, and electronic coupling.7 Monte Carlo simulations have been successful in describing charge mobility as a function of positional and electronic disorder.7 Broadening of the electronic density of states, often represented by a Gaussian distribution, is achieved through disordered local conforma© XXXX American Chemical Society

tions, structural defects, and random orientation of polar groups.7 This effect results from nonlinear arrangements of the polymer backbone, allowing for decreased p-orbital overlap and hindrance of charge mobility through decreased electronic couplings.2 Currently, various techniques including mechanical rubbing, Langmuir−Blodgett technique, nanoimprinting, prepatterned substrates, and matrix-assisted alignment have been reported to achieve macroscopic alignment of conjugated polymers.8−10 Recently, columnar, smectic, and nematic liquid crystals (LCs) have been widely accepted as a viable method to achieve improved charge transport in optoelectronic devices.7 This confidence is derived from the ability of LCs to spontaneously organize into large-area thin films containing highly ordered domains.11 Conjugated polymers designed to exhibit lyotropic LC characteristics can achieve alignment of the backbone at Received: November 8, 2016 Revised: December 26, 2016

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DOI: 10.1021/acs.macromol.6b02416 Macromolecules XXXX, XXX, XXX−XXX

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(potassium-6-hexanoatethiophene)-2,5-diyl] (P3K6T, average Mw = 60 kg/mol, Rieke Metals Inc.), poly[3-(potassium-7-heptanoatethiophene)-2,5-diyl] (P3K7T, average Mw = 60 kg/mol, Rieke Metals Inc.), centyltrimethylammonium bromide (Sigma-Aldrich) including its various alkyl chain derivatives with the side-chain length equal to 10, 12, and 14 carbons, and dihexadecyl dimethylammonium bromide (Sigma-Aldrich) were used as received in preparation of stock hydrogels (0.10 M). Solutions were prepared in nanopure water (Milli-Q, 18 MΩ) through dropwise addition of surfactant to polymer at a 1:1 molar ratio after heating of the individual solutions for 1 h (50 °C) or until fluid. The supramolecular complexes were then microcentrifuged (18-Centrifuge Biotechnical Services Inc.) for 30 min at a speed of 10,000 rpm, resulting in stable hydrogels. Chemical structures of polymers and surfactants are shown in Figure 1.

macroscopic scales, enabling charge conduction along the conjugated backbone.2 Importantly, external fields such as mechanical shearing, electric, and magnetic fields can further induce uniform LC monodomains under appropriate conditions.12 On the basis of Onsager’s rigid-rod model, formation of the nematic LC phase is driven through the increase of the aspect ratio in rod-shaped particle solutions.13 The exclusion of solvent volume due to packing between adjacent rigid-rods leads to an increase and decrease in positional and orientational entropy, respectively. As a result, particles of a more anisotropic nature form nematic LC phases at a lower concentration. Therefore, the promotion of high-aspect-ratio rod-like microstructures is a facile way to achieve LC conjugated polymers.14,15 Poly(3-alkylthiophenes) (P3AT’s) are a commonly studied system due to their low band gap and impressive charge carrier mobility.16−18 Influence of these properties has been shown to arise from solvent and side-chain interactions, producing solidstates with high crystallinity stemming from alignment of polymer backbones. One such example, poly(3-hexylthiophene) (P3HT), has been shown to produce highly crystalline nanowires in poor organic solvents;19−21 a morphological characteristic known to improve device performance in optoelectronics such as OPV’s.22−25 Conversely, poly[3(potassium-6-hexanoatethiophene)-2,5-diyl] (P3K6T), a water-soluble P3HT derivative, maintains a disordered structure when constructed from aqueous solutions.26 In order to mitigate P3K6T’s amorphous qualities, the addition of counterionic surfactants has been employed previously to enhance P3K6T crystallinity.27 Columbic interactions between the surfactants and P3K6T carboxylate moieties facilitate the formation of a supramolecular complex in aqueous solution, which has been shown to induce a coil-to-rod transition of intramolecular origin.27 This transition could trigger highly ordered columnar LC formation in concentrated solutions under appropriate conditions.28 Moreover, surfactant complexation yields an alternation of optoelectronic properties29−31 and enhances processing of conjugated polymers.32 In this work, we report on the multiscale assembly of polythiophene-surfactant supramolecular complexes in dilute and concentrated solutions. The effect of surfactant alkyl-chain length and architecture on the dynamic process of the coil-torod transition is spectroscopically studied in dilute solutions. When concentrated, aqueous solutions of supramolecular complexes undergo isotropic-to-LC phase transitions. The consequence of surfactant alkyl-chain characteristics are revealed to have an influence on the LC phase boundary. By applying a mechanical shear force across still-wet LC films, long-range molecular alignment is achieved and maintained through the drying process. A significant anisotropy of the electrical conductivity in the aligned film is obtained. The correlations between the intrinsic rigidity, optoelectronic properties and LC formation are addressed. We believe the development of these methodologies promotes the structural memory of the LC order from solutions to the solid state, allowing for a potential route to the production of high performance optoelectronic devices.

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Figure 1. (a) Chemical structure of polythiophene P3KnT with n = 4, 5, 6, or 7, (b) single-tailed surfactant (STSn) with n = 10, 12, 14, or 16, (c) double-tailed surfactant (DTSn) with n = 16, and (d) illustration of the supramolecular complexation of P3KnT with single and doubletailed surfactants. Characterization Methods. Fourier transform infrared (FTIR) spectra of supramolecular complexes were collected on a Nicolet iS10 FT-IR spectrometer with an attenuated total reflection (ATR) mode at a resolution of 4 cm−1 of 1000 scans. Spectroscopic measurements of diluted solutions were obtained on a Cary-Win UV−vis spectrophotometer and a Jasco FP-6500 spectrofluorometer. Temperature control was set to 25 °C and the samples were closed from ambient air atmosphere. All dilutions were made with nanopure water (Milli-Q, 18 MΩ). Typically, a 1.0 mM stock solution of the hydrogel was prepared and heated to 50 °C in a temperature controlled water bath. The sample was then transferred from the water bath and further diluted into a cuvette, which was preset at 25 °C. The LC textures of concentrated solutions were evaluated on a Leica DM2500P polarized optical microscope (POM). A Leica ICC50 HD video camera was used to capture the images. To study the effect of mechanical shearing on polymer alignment, a thin layer of the LC solution was casted onto a clean glass slide and subsequently sheared at a high coating speed. Electrical conductivity measurements were performed on a Keithley Instrument 2400 SourceMeter in a four-point probe configuration. At least three voltage sweeps were run with the average of these sweeps being taken as the slope of their respective I−V curves. Small-angle X-ray scattering (SAXS) of dilute solutions was performed on a pinhole collimated Rigaku instrument (SMAX3000) with a 1.54 Å Cu Kα radiation source. The scattering data was recorded using a 2D multiwire electronic area gas detector. The instrument was calibrated using silver behenate with a d-spacing of 58.38 Å. The scattering patterns were obtained over a period of 5 h of X-ray exposure. The samples were sealed in Kapton capillary tubes with an acrylic adhesive. Samples were heated in situ by a custom-built temperature controller to within 0.1 °C. SAXS experiments of concentrated solutions were conducted using a Linkam Scientific Instrument HFS350X-GI hot stage in a SAXSLab Ganesha. A Xenocs GeniX3D microfocus source was used with a Cu target to generate a monochromic beam with a 1.54 Å wavelength. Calibration was performed using National institution of Standard and

EXPERIMENTAL SECTION

Materials. Regioregular (82−90% head-to-tail) poly[3-(potassium4-butanoatethiophene)-2,5-diyl] (P3K4T, average Mw = 14 kg/mol, Rieke Metals Inc.), poly[3-(potassium-5-penanoatethiophene)-2,5diyl] (P3K5T, average Mw = 35 kg/mol, Rieke Metals Inc.), poly[3B


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Figure 2. UV−vis spectra of the diluted solutions of P3KnTs of varying alkyl-chain length (n = 4, 5, 6, and 7) (a) before and (b) after complexation with STS16 over 90 min. The concentration of solutions is 0.25 mM.

Figure 3. UV−vis spectra of (a) P3K6T complexed in 1:1 molar ratios with various single tailed surfactants STSn and (b) with various ratios of STS16: DTS16. The concentration of solutions is 0.25 mM and the aging time is 90 min. Technology (NIST) reference material, 640c silicon powder with a peak position at 2θ = 28.44° where 2θ is the total scattering angle. A Pilatus 300 K detector (Dectris) was used to collect the twodimensional (2D) scattering patterns. 2D images were azimuthally integrated to one-dimensional (1D) data of intensity (I) versus scattering vector q. The samples were placed between mica or Kapton windows (mica thickness of mica ∼20 μm, from Molmex Scientific Inc.) with a rubber O-ring positioned carefully. All data were acquired after equilibration for 30 min at each temperature followed by a 1.0 h measurement with an X-ray flux of ∼21.4 M photons/s incident upon the sample.

P3K4T and P3K5T reveal narrowing of peak widths and significant blue-shifting, respectively. The resulting Gaussianlike absorption peaks represent disruption of π−π stacking interactions due to surfactant complexation. In contrast to P3K4T and P3K5T, P3K7T displays no hysochromic shifts, but instead gains a more sharply defined absorbance peak at λmax = 560 nm and a vibronic structure at λ = 600 nm. Interestingly, P3K6T initially demonstrates a broad absorption peak at λmax = 430 nm. Over the course of 90 min, gradual red-shifting occurs resulting in a new absorption at λmax = 550 nm, accompanied by a strong vibronic structure at λ = 590 nm (see below). This time-dependent chromism is consistent with the previous report of the 1:1 molar P3K6T:STS16 supramolecular complex.27 Figure 3 shows the UV−vis absorption spectra of the supramolecular complexes of P3K6T with the quaternary ammonium surfactants containing different alkyl-chain lengths and architectures at 1:1 molar ratios. The ensuing trend from the addition of varying alkyl-chain length surfactants (Figure 3a) shows that increasing and decreasing alkyl-chain lengths promote bathochromic and hypsochromic shifts, respectively. The apparent vibronic structures present in red-shifted peaks of P3K6T:STS14 and P3K6T:STS16 are indicative of rod-like conformations.27,34,35 It has been recognized that regioregular P3AT’s adopt ordered rod-like structures through anticoplanar arrangements,36 increasing the effective conjugation length of the polythiophene backbone and minimizing charge trapping.37 Conversely, rotational defects between adjacent repeat units are attributed to disordered coil-like states, inducing broad hypsochromic peaks in the UV−visible spectra.36 These defects lead to heightened torsional angles between thiophene rings, resulting in perturbations of the effective backbone conjugation.36 By decreasing the conjugation length, the band gap

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RESULTS AND DISCUSSION Diluted Solutions. Supramolecular complexes were prepared using P3KnT’s and single-tailed or double-tailed quaternary ammonium surfactants (STSn or DTSn) at 1:1 molar ratios, resulting in stable hydrogels. The FT-IR spectra were received for P3KnT’s before and after adding surfactants to verify the complexation (Supporting Information, Figure S1). Significant changes in the vibrational energies of methylene bending bands (2800−3000 cm−1) as well as carboxylate stretching bands (1300−1600 cm−1) suggest completion of the complexation reaction.27 Slight blue shifts in methylene vibrations and sizable reductions of peak halfwidths are ascribed to the improved packing of aliphatic chains in the supramolecular complex. Figure 2 displays the UV−vis absorbance spectra of pure P3KnT’s before and after complexation with STS16. The UV− vis spectra of pure P3KnT’s (Figure 2a) show the absorption maximum at λmax = 550 nm due to the aggregation of polymer chains.27 An exception is P3K4T which exhibits a broad absorption peak at λmax = 420 nm due to the hindered aggregation,33 yielding the coupled blue-shift and peak broadness. After complexation with surfactants (Figure 2b), C


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Figure 4. Time-dependent chromism of diluted solutions of the supramolecular complexes at 25 °C. (a) UV−vis and (b) fluorescence spectra of P3K6T-STS16; (c) UV−vis and (d) fluorescence spectra of P3K6T-9:1 STS16: DTS16. The concentration of the solutions is 0.25 mM.

Figure 5. Kinetics of the coil-to-rod transitions of diluted solutions of supramolecular complexes at 25 °C. (a) Time-dependent absorbance at λ = 590 nm against aging time of P3K6T-STS16, (b) the initial rate versus polymer concentration of P3K6T-STS16, (c) time-dependent absorbance at λ = 590 nm against aging time of P3K6T-9:1 STS16:DTS16, and (d) the initial rate versus polymer concentration of P3K6T-9:1 STS16:DTS16.

incorporation of STS16 and DTS16 surfactants at overall 1:1 molar ratios into the P3K6T backbone are shown in Figure 3b. Gradual blue-shifts of λmax’s in the UV−vis spectra result from the increased loading of DTS16. This is attributed to the incorporation of double hydrocarbon tails, which produces high grafting density and enhanced bulky steric effects between polythiophene and surfactant alkyl-chains.27 It is believed that the enhanced hydrophobicity of DTS16 alkyl-chains promotes coiled-like structures to minimize interfacial area with the

energy between the HOMO and LUMO molecular orbitals is increased and the wavelength is reduced. As such, the incorporation of long alkyl-chains inhibits free rotation of the P3KnT backbone, promoting planarized rod-like conformations and minimal interchain aggregations.38 To further demonstrate this effect, the outcome of increased grafting density was studied by incorporating double-tailed quaternary ammonium surfactant (DTS16) onto the backbone of P3K6T-STS16 complexes. The spectra resulting from D


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supramolecular complexes, respectively. This inverse first-order rate law is in good agreement with the reported data on the poly(3-alkylthiophene),27,39 indicating that the mechanism of the coil-to-rod transition in conjugated polymers comes from an intramolecular origin. In other words, the coil-to-rod transition is a single chain conformational change39 and no significant intermolecular H- or J-aggregation occurs in the diluted solutions of supramolecular complexes within a time scale of this study. It should be noted that such an intramolecular mechanism of the coil-to-rod transition is also supported by in situ fluorescence data (Figure 4, parts b and d), which shows no significant photoquenching during the bathchromic shift. Figure 6 shows SAXS data of P3K6T complexed with the mixture of STS16 and DTS16 surfactants. The typical scenario of

surrounding medium.27 As these steric effects are not distributed uniformly throughout the polythiophene backbone, kinks and bends can form between the thiophene rings, leading to reductions in torsional angles and promotions of coiled-like states.27 At a low loading of DTS16 these kinks are minimal, and therefore strong red-shifts and a time-dependent chromism are observed in the P3K6T-9:1STS16:DTS16 supramolecular complex (see below). Conversely, increased loading of DTS16 results in a greater number of kinks, inhibiting the coil-to-rod transition and exhibiting hypsochromic shifts in the UV−vis spectra. No time-dependent chromisms are observed for P3K6T-8:2STS16:DTS16 and P3K6T-7:3STS16:DTS16 supramolecular complexes. To understand the physical origin of the time dependent chromism, P3K6T-STS16 and P3K6T-9:1 STS 16 : DTS 16 supramolecular complexes were chosen to further study the dynamic process of the coil-to-rod conformational transition. Figure 4 shows time-dependent spectroscopic properties of newly diluted solutions of supramolecular complexes. Over the course of 90 min, the UV−vis spectra of both complexes demonstrate gradual red shifting’s of the absorption maximum at room temperature from λmax = 430 nm to λmax = 550 nm (Figure 4, parts a and c). The pronounced isosbestic points are shown in both supramolecular complexes, which are indicative of two existing phases in solutions: a coil phase identified by λmax = 430 nm and a rod phase indicated by λmax = 550 nm.21,27,39,40 The strong vibronic structure at λ = 590 nm are apparent in the rod-like conformations of both supramolecular complexes.27 To provide additional support for the abovementioned conversion, the emission spectra of these supramolecular complexes were collected through fluorescence spectroscopy (Figure 4, parts b and d). No substantial photoquenching is observed and the emission maximum gradually shifts to higher wavelengths over time for both complexes. This phenomenon is consistent with the observed bathochromic shifts of the corresponding absorbance spectra. The Stokes shift, which is defined as the wavelength difference between the emission and absorption maxima, provides information about vibrational relaxations of excited state fluorophores. In this regard, the Stokes shift of the supramolecular complexes is minimized from 136 nm (coil-like state) to 70 nm (rod-like state), supporting minimized charge trapping due to polymer backbone extensions.38,31,41 To gain further insight into intramolecular versus intermolecular origin of the coil-to-rod transition, UV−vis absorption spectroscopy was utilized to determine transition kinetics. To this end, the method of initial rates was employed to determine the rate law and reaction order.42 Figure 5 shows the absorbance at λ = 590 nm vs aging time and the initial rate vs concentration of the P3K6T-STS16 and P3K6T-9:1 STS16:DTS16 supramolecular complexes. It has been recognized that the absorbance at λ = 590 nm of poly(3-alkylthiophene)s is associated with the amount of rod-like structures, and the initial slope of absorbance at λ = 590 nm against aging time can be used as an initial rate of the coil-to-rod transition.21,27,40 Intriguingly, both supramolecular complexes demonstrate that the rate of coil-to-rod transition increases with declining concentration (Figure 5). In theory, the scaling relationship between the transition rate and concentration is constructed as R ∝ Cn, where R is the transition rate, C is the concentration, and n is an exponent denoting the reaction order. In this regard, the initial rate scales with concentration as R ∝ C−1.05±0.12 and R ∝ C−1.04±0.05 for P3K6T-STS16 and P3K6T-9:1STS16:DTS16

Figure 6. Room temperature SAXS profiles of aqueous solutions of P3K6T supramolecular complexes (∼0.60 wt %) with different STS16:DTS16 ratios and of pure P3K6T (∼0.20 wt %).

supramolecular complexes includes scattering intensity I(q) vs vector q that approximates the functions, I(q) ∼ q−2 at low q and I(q) ∼ q−1 at high q, with the point of the transition between two functions varying with the ratio of STS16:DTS16. Fitting both regimes with a power law enables the determination of these transition points. It is agreed upon that the fractal dimensions of scattering objects relate to the slopes of the log−log plot of I(q) vs q and the cutoff for the slope change shifts to lower q as the conjugation length increases.43 At the small size scale (high q), the slope −1 corresponds to the rod-like structures. At the large size scale (low q), the slope −2 indicates the coil-like features.44−47 To this end, the degree of intrinsic rigidity of rod-like structures can be evaluated by the persistence length (Lp) that is calculated from the crossover region (q*) between two regimes, 6 where q* × Lp = π .46−49 Table 1 lists the q* and persistence length Lp values of the supramolecular complexes at different temperatures. The values of persistance length of P3K6T-STS16 at 20 and 70 °C are 78.6 and 61.2 Å, respectively. They correspond to rod-like and coil-like structures, respectively.27 This is much longer than the persistence length (30 Å) of the coil-like P3HT in dichlorobenzene.46 At 20 °C, additions of 15% DTS16 and 30% DTS16 result in 3.0% and 31% reduction of the persistence length, respectively, as compared to the 0% E


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of peaks is rather equivocal where alternative morphologies such as, e.g., those from cubic aspect 8 cannot be excluded based upon SAXS alone. Figure 8b shows the temperature dependent SAXS profile during heating. Clearly, the long-range symmetry is retained through the heating experiment up to 110 °C. At 140 °C, the √3q* peak disappears and the first scattering peak shifts to a lower q value of 0.202 Å−1. Moreover, a small portion of the original peak persists at q = 0.218 Å−1. While nematic and lamellar LC phases are predominant in the conjugated polymer solutions,14,15 the hexagonal LC phase suggested in this work may be attributed to the long surfactant side chains and molecular weight polydispersity of P3K6T.53 In addition, the hexagonal LC phases in conjugated polymer solutions can also be stabilized by geometrical frustrations.54 Recently, the formation of the hexagonal LC phase has been reported in a sulfonated polythiophene complexed with a surfactant, centyltrimethylammonium bromide (CTAB).28 Figure 9 illustrates the LC phase diagrams of supramolecular complexes at room temperature, which indicate the correlations between the surfactant chain length/architecture and critical concentration of LC ordering. With an increase of the alkyl chain length of single-tailed surfactants (STS), the critical concentration of LC ordering decreases linearly in the studied system (Figure 9a), demonstrating consistency with greater rod-like structures as revealed by absorption spectroscopy (Figure 3a). In contrary, addition of the double-tailed surfactant (DTS16) results in an irregular growth of the critical concentration (Figure 9b). This phenomenon may be attributed to nonuniform steric effects along the polythiophene backbone as addressed by absorption spectroscopy (Figure 3b). The observations of the phase boundaries of LC ordering against the surfactant length and architecture are also in good agreement with the intrinsic rigidity of rod like structures (Table 1). Lyotropic LC formation of supramolecular complexes provides a facile way to fabricate large-area alignment of thin films for utilization of anisotropic properties of conjugated polymers in various emerging energy applications.7 Under mechanical shearing, the uniform LC monodomains were received (Supporting Information, Figure S2). After complete drying, LC alignment was maintained in the thin film. To this end, the electrical conductivities were measured at room temperature in a four-point probe configuration. Figure 10 displays illustration of conductivity measurements and I−V curves of P3K6T-STS16 films parallel and perpendicular to the shear direction. The anisotropy in electrical conductivity is

Table 1. Calculated Persistence Lengths for P3K6TSTS16:DTS16 Supramolecular Complexes STS16:DTS16

temperature (°C)

crossover q* (Å−1)

persistence length Lp (Å)

10:0 8.5:1.5 7:3 10:0 8.5:1.5 7:3

20 20 20 70 70 70

0.0243 0.0251 0.035 0.0312 0.0325 0.035

78.6 76.1 54.6 61.2 58.8 54.6

DTS16 complex. This observation indicates the persistence length of conjugated polymers is closely associated with their effective conjugation length. As such, there is a strong correlation between the intrinsic rigidity of conjugated polymers and their optoelectronic properties.38,41 At 70 °C, all complexes exhibit coil-like structures and the effect of DTS16 on the persistence length of supramolecular complexes is significantly reduced. It should be noted that the coil-to-rod transition is thermally reversible, which is consistent with the thermochromism of conjugated polymers.27,39 Concentrated Solutions. A consequence of conjugated backbone extensions after complexation with surfactants is the increase of aspect ratios of the rod-like structures. From a theoretical standpoint, such high-aspect-ratio rod-like structures will trigger the LC formation in concentrated solutions.50 The correlation between the intrinsic rigidity of conjugated polymers and the critical concentration of LC ordering can be codified through phase-boundary diagrams. In this regard, polarized optical microscopy (POM) was employed to estimate the phase transition boundaries. Figure 7a shows a typical optical image of 51 wt % aqueous solution of the P3K6T-STS16 supramolecular complex. The strong birefringence of the solution implies the formation of a lyotropic LC phase. Upon rotation of the crossed polarizers, the dark and bright LC domains change alternatively. Interestingly, a banded texture was observed after mechanical shearing as shown in Figure 7b. In this regard, the polymer backbone is perpendicular to the bands and thus parallel to the shear direction.28,51 Figure 8a shows SAXS data from 51 wt % LC aqueous solutions of P3K6T-STS16 supramolecular complex. The scattering peaks occur at q-ratios consistent with 1, √3, 2, and √7. This interpretation leads to a d-spacing of 26.8 Å. The scattering pattern is consistent with the expected hexagonal symmetry from a hexagonal or columnar LC phase.28,52 However, morphology interpretation from a limited number

Figure 7. Optical images of 51 wt % aqueous solution of the P3K6T-STS16 supramolecular complex under crossed polarizers. (a) Without mechanical shearing and (b) with shearing. The arrow in part b indicates the shear direction. F


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Figure 8. (a) Room temperature SAXS pattern and (b) temperature dependent in situ SAXS profile of the lyotropic gel of the P3K6T-STS16 supramolecular complex. The concentration of the gel is ∼51 wt %.

Figure 9. Room-temperature liquid crystalline phase diagrams P3K6T supramolecular complexes of (a) varying chain lengths of single tailed surfactants STSn and (b) varying double tailed surfactant DTS16 loading. Dashed lines guide the eyes only.

Figure 10. Room-temperature measurements of anisotropic conductivity. (a) Illustration of electrical conductivity measurements and (b) current vs voltage curves of sheared P3K6T-STS16 LC films after complete drying parallel and perpendicular to the shear direction.

solutions. In diluted solutions, the supramolecular complexes undergo coil-to-rod transitions of an intramolecular mechanism. Spectroscopic examination of the dynamic process reveals a universal inverse first-order rate law of the transition. The surfactant length and architecture effectively tune the persistence length and the effective conjugation length. As such, the intrinsic rigidity of conjugated polymers is pivotal in forming rod-like structures. In concentrated solutions, the supramolecular complexes exhibit ordered liquid crystalline phases above the critical concentration. SAXS measurements are suggestive of hexagonally packed rod-like polymer chains. The combination of mechanical shearing with liquid crystallinity results in aligned thin films at macroscopic scales, which shows four times faster charge transport along the backbone alignment direction than the perpendicular direction. As the

clearly shown. Careful calculations show that the electrical conductivities of the aligned films parallel and perpendicular to the shear direction are 8 × 10−4 and 2 × 10−4 S/m, respectively. As the conjugated backbone is parallel to the shear direction,28 our results imply that the intrachain charge transport along the conjugated backbone is much more efficient than the interchain charge hopping through π−π stacking. As such, LC formation via supramolecular complexation promotes backbone alignment of conjugated polymer by minimizing the interchain charge transport.

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CONCLUSIONS In summary, we have reported assembling behaviors of conjugated polymers complexed with various surfactants at different length scales in both diluted and concentrated G


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Macromolecules

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alignment capacity of conjugated polymers is pivotal in determining the device performance, our work may provide a facile way to direct backbone alignment of conjugated polymers via supramolecular complexation for high performance electronic devices.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.macromol.6b02416. FTIR data and optical images (PDF)

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +1 805 756 2591. Fax +1 805 756 5500. E-mail: [email protected] (S.Z.). ORCID

Chinedum O. Osuji: 0000-0003-0261-3065 Shanju Zhang: 0000-0002-2556-6073 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is primarily supported by the American Chemical Society Petroleum Research Fund (53970-UR7). S.Z. acknowledges financial support from the National Science Foundation (CMMI-1345138, CBET-1510207). This work made use of the South Carolina SAXS Collaborative, supported by the NSF Major Research Instrumentation program (DMR-1428620).

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REFERENCES

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Multi-Scale Assembly of Polythiophene-Surfactant Supramolecular

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