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Aug 13, 2012 - Mahesh P. Bhatt, Prakash Sista, Jing Hao, Nadia Hundt, Michael C. Biewer, and ... P3HT and PBLG indicating that the crystalline nature ...
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Electronic Properties-Morphology Correlation of a Rod−Rod Semiconducting Liquid Crystalline Block Copolymer Containing Poly(3-hexylthiophene) Mahesh P. Bhatt, Prakash Sista, Jing Hao, Nadia Hundt, Michael C. Biewer, and Mihaela C. Stefan* Department of Chemistry, University of Texas at Dallas, 800 West Campbell Road, Richardson, Texas 75080, United States S Supporting Information *

ABSTRACT: The influence of the solvent and annealing temperature on the field-effect mobilities and morphologies of poly(3-hexylthiophene)-b-poly(γ-benzyl-L-glutamate) (P3HT-bPBLG) rod−rod diblock copolymer has been investigated. Thin film X-ray diffraction studies show peaks originating from both P3HT and PBLG indicating that the crystalline nature of both the blocks is conserved after the formation of the block copolymer. It has been observed that the field-effect mobilities of the diblock copolymer are independent of the annealing temperatures for thin films deposited from both 1,2,4trichlorobenzene and chloroform solvents. The correlation between the field-effect mobility and morphology indicates that the P3HT block self-assembles at the surface SiO2 dielectric.

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INTRODUCTION Block copolymers are attractive because they combine the properties of different blocks to realize the advantages of both polymers. While numerous rod−coil block copolymers have been reported in the past few decades,1−3 synthesis of rod−rod block copolymers still remains a relatively unexplored area.4 There are mainly two types of polymers that display rigid rod like structures: liquid crystalline polymers, which have α-helical secondary structure, or π-conjugated polymers. Presently, there are few reports of rod−rod copolymers. For example, poly(Lleucine)-poly(L-lysine) diblock copolymer has been synthesized by Deming and co-workers, and it is reported to form large vesicular aggregates in water.5 Poly(γ-benzyl-L-glutamate)-bpolyisocyanide has been synthesized by Cornelissen and the copolymer assembled upon drying into ordered layers of hollow capsules.4,6 The synthesis and the formation of various nanostructured assemblies of polyfluorene-b-poly(γ-benzyl-Lglutamate) has been reported by Jenekhe and Kong.7,8 These nanostructured assemblies were generated upon changing the diblock composition and secondary structures of the polypeptide block.7,8 Recently, poly(3-hexylthiophene)-b-poly(γ-benzyl-L-glutamate) (P3HT-b-PBLG) rod−rod diblock copolymer has been reported by both our group and Bielawski’s group by using different synthetic routes.9,10 This polymer is the first reported rod−rod diblock copolymer containing semiconducting regioregular poly(3-hexylthiophene) (P3HT).9,10 Regioregular P3HT is a widely used semiconducting polymer in organic electronic applications, such as solar cells and thin film transistors.11 While various rod−coil diblock copolymers containing P3HT have been synthesized and their opto© 2012 American Chemical Society

electronic properties have been correlated with their compositions and morphologies, combining a rigid liquid crystalline block with P3HT could affect the optoelectronic properties since the degree of order of the material is expected to be affected by the supramolecular self-assembly of the liquid crystalline block. Although many biomolecules have been widely used in selfassembly studies due to their inherent ability to organize into well-defined nanostructures, only recently have bioinspired approaches found further applications in the self-assembly of πconjugated systems.12,13 By combining the liquid crystalline polypeptide PBLG and the semiconducting P3HT blocks into the same polymer backbone, new properties could be potentially obtained. This diblock copolymer may have potential biomedical applications including biosensors or scaffolds for tissue engineering. In our previous paper, we have reported the synthesis of P3HT-b-PBLG by using an amine terminated-P3HT macroinitiator for the ring-opening polymerization of N-carboxyanhydride (NCA) to generate the PBLG block.9 UV−vis data showed the effective conjugation length of P3HT was not affected by the presence of the PBLG block, and we have found that the morphology of the copolymer was dependent on annealing conditions and the solvent used for the film casting.9 In this paper, we report a more systematic study of the solvent and annealing effect on the charge carrier mobility of Received: April 27, 2012 Revised: August 10, 2012 Published: August 13, 2012 12762

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The film thickness was measured using a Veeco Dektak VIII profilometer. Fourier Transform Infrared (FT-IR) Measurements. FT-IR spectra were recorded on a Nicolet 380 FT-IR spectrometer. The measurements were performed on a polymer powder on a diamond crystal substrate. FT-IR spectra were also recorded on a Nicolet 4700 FT-IR spectrometer which allowed the recording of the FT-IR spectra on thin films. The copolymer film was deposited in air by spin coating a 0.25 mg/mL polymer solution (chloroform solvent) onto a gold (100 nm) coated glass substrate. The thickness of the polymer film was 38 nm as measured via a Veeco Dektak VIII profilometer. The spectra were recorded at room temperature. The FT-IR spectra were collected on nonannealed film and also on films annealed at 80, 120, and 150 °C. Differential Scanning Calorimetry (DSC). A Q100 differential scanning calorimeter from TA Instruments with an RCA cooling system was used to study thermal properties of block copolymers. Hermetic aluminum pans were filled with polymer sample (4.3 mg). The polymer sample was subjected to heat−cool−heat cycle to erase thermal memory. Data were recorded by heating the sample from 20 to 350 °C at the rate of 20 °C/min.

rod−rod P3HT-b-PBLG diblock copolymer in organic fieldeffect transistors (OFETs) and their correlation to morphology. Polymer films were drop-casted on OFET devices from chloroform and 1,2,4-trichlorobenzene (TCB) solvents. The field effect mobilities of the polymer were measured in four different conditions, that is, without annealing and annealing at various temperatures. The measured field-effect mobilities were correlated with the surface morphologies of the P3HT-b-PBLG diblock copolymer investigated using tapping mode atomic force microscopy (TMAFM). X-ray diffraction studies were also carried out on films cast from both solvents to study solvent effect on the diffraction pattern of the copolymer.

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EXPERIMENTAL SECTION

The synthesis of poly(3-hexylthiophene)-b-poly(γ-benzyl-L-glutamate) was performed according to a previously published procedure.9 Field-Effect Mobility Measurements. Field-effect mobility measurements of the synthesized block copolymers were performed on thin-film transistors with a common bottom-gate, bottom-contact configuration. Highly doped, n-type silicon wafers with a resistivity of 0.001−0.003 Ω cm were used as substrates. Thermal oxide (SiO2) was thermally grown at 1000 °C on silicon substrate to obtain a 200 nm thickness. Chromium metal (5 nm) followed by 100 nm of gold was deposited by E-beam evaporation as source-drain contacts. The source-drain pads were formed by photolithographically patterning the metal layer. The SiO2 on the back side of the wafer was etched with buffered oxide etchant (BOE from JT Baker) to generate the common bottom-gate. The resulting transistors had a channel width of 475 μm and channel lengths varying from 2 to 80 μm. The measured capacitance density of the SiO2 dielectric was 17 nF/cm2. Prior to copolymer deposition, the substrates were cleaned by UV/ozone for 10 min. The devices were then cleaned in air with water, methanol, hexane, chloroform, and dried with nitrogen flow followed by vacuum for 30 min at 80 °C. The copolymer films were deposited in air by drop casting 10 μL of a 1.0 mg/mL solution in distilled chloroform and allowed to dry in a Petri dish saturated with chloroform. The films casted from 1,2,4-trichlorobenzene solvent were allowed to dry in air until all the solvent evaporated. The devices were then further annealed for 30 min according to the conditions required for each of the devices prior to measurements. Annealing temperatures of 80, 120, and 150 °C were used in this study. A Keithley 4200-SCS semiconductor characterization system was used to probe the devices. The probe station used for electrical characterization was a Cascade Microtech Model Summit Microchamber. When measuring current− voltage curves and transfer curves, VGS was scanned from +20 to −100 V. All the measurements were performed at room temperature in air. Tapping Mode Atomic Force Microscopy (TMAFM). TMAFM investigation of thin film morphology was carried out using a Nanoscope IV-Multimode Veeco instrument, equipped with an E-type vertical engage scanner. TMAFM measurements were performed on the OFET devices in the channel region. The AFM images were collected at room temperature in air using silicon cantilevers with nominal spring constant of 42 N/m and nominal resonance frequency of 320 kHz (standard silicon TESP probes with aluminum coating on backside). A typical value of AFM detector signal corresponding to an rms cantilever oscillation amplitude was equal to ∼1−2 V, and the images were acquired at 0.5 Hz scan frequency in 5 × 5 μm2 scan areas. X-ray Diffraction (XRD) Studies. X-ray diffraction patterns were obtained on a RIGAKU Ultima III diffractometer. Samples were subjected to Cu Kα radiation (λ ∼ 1.5406 Å) and scanned from 1 to 40 degrees (2θ) at 0.04° intervals at a rate of 1°/min. A silicon wafer with 200 nm of thermally deposited SiO2 (22 mm × 22 mm) was used as the sample substrate. The copolymer films obtained from chloroform were deposited in air by drop casting a 5.0 mg/mL polymer solution and allowed to dry in a Petri dish saturated with chloroform. While the films deposited from TCB were allowed to dry in air. The devices were then further annealed for 30 min at 120 °C.

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RESULTS AND DISCUSSION The synthesis and characterization of the rod−rod block copolymer of poly(3-hexylthiophene)-b-poly(γ-benzyl-L-glutamate) (P3HT-b-PBLG) has been published by our group (Figure 1).9 The diblock copolymer used in this study had a

Figure 1. Poly(3-hexylthiophene)-b-poly(γ-benzyl-L -glutamate) (P3HT-b-PBLG) rod−rod diblock copolymer (Mn = 16 525 g/mol, PDI = 1.44; 43 mol % poly(γ-benzyl-L-glutamate) determined by 1H NMR).

molecular weight of Mn = 16 525 g/mol and contained ∼43 mol % poly(γ-benzyl-L-glutamate) as determined by 1H NMR analysis. PBLG block is an insulating block, and it is expected to influence the opto-electronic properties of the block copolymer. Variations in processing conditions can alter the surface morphology of the block copolymer and hence alter the fieldeffect mobility of the polymer. Chloroform and TCB were selected as solvents for the P3HT-b-PBLG diblock copolymer because they are good solvents for both P3HT and PBLG. Additionally, chloroform and TCB are helicogenic solvents (helix promoting) for the PBLG liquid crystalline block.14 Bottom-gate bottom-contact OFETs were fabricated, and the P3HT-b-PBLG diblock copolymer solution either in chloroform or TCB was deposited on the devices by drop-casting (Figure 2). The OFET devices were annealed under vacuum at three different temperatures (80, 120, and 150 °C) after the polymer deposition. The plots of source-drain current (IDS) versus source-drain voltage (VDS) are shown in Figures 3−6 and the Supporting Information. The charge carrier mobilities were extracted from a plot of IDS1/2 vs VGS using the following equation: IDS = 12763

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displayed in Figures 3−6. The output characteristics show a slow charge induction with a near linear increase of the drain current with source-drain voltage bias (from 20 to −100 V). Average mobilities and standard deviation were calculated from the field-effect mobility data obtained for four different channel lengths (Table 1 and Supporting Information Tables S1−S8). We attempted to correlate the field-effect mobilities with the surface morphology of the P3HT-b-PBLG films. The TMAFM analysis of the films was performed in the channel region of the OFET devices. The 3D-TMAFM images of the films deposited from TCB indicate a substantial change in morphology (Figure 7). For the nonannealed film, densely packed ribbonlike domains were observed (Figure 7a). The slow evaporation rate of TCB most likely promotes the formation of nanoribbons which may be helical aggregates of the rigid PBLG block. There is an incremental change in the length of the nanoribbons upon annealing at 80 and 120 °C. This change in the length of nanoribbons indicates the formation of larger domains of selfassembled PBLG upon annealing. No evidence of formation of nanofibrils of P3HT was observed even after the polymer films were annealed at 120 °C (Figure 7c). It is well-known that P3HT with nanofibrillar morphology displays reasonably high field-effect mobility (>0.01 cm2/(V s)).18−29 The lack of the nanofibrillar morphology specific to P3HT may be due to the fact that TMAFM, being a surface probing technique, can only map the surface of the polymer film. Moreover, the P3HT block may self-assemble close to the SiO2 surface, enabling the observation of the morphology of PBLG block at the surface of the film. Due to the self-assembly of P3HT at the SiO2 dielectric surface, one can expect to maintain a reasonable field-effect mobility for the diblock polymer films. Annealing the films at 150 °C, which is above the transition temperature for PBLG, seems to decrease the length of the nanoribbons and make the films more granular in nature (Figure 7d), yet does not dramatically affect the mobility. A more thorough investigation of the nature of self-assembly of the P3HT-bPBLG diblock copolymer at the SiO2 dielectric surface is currently under investigation in our group. The TMAFM images of the films deposited from chloroform also display a substantial change in morphology (Figure 8). The nonannealed film displays nanoribbons of PBLG block, similar to the TMAFM image obtained from TCB solvent. However,

Figure 2. Device structure of the bottom-gate bottom-contact OFET used for the field-effect mobility measurements.

where W is the channel width, L is the channel length (Figure 2), Ci is the capacitance per unit area of the dielectric, and VT is the threshold voltage.9 It has been demonstrated that PBLG adopts a helical conformation with seven residues per two turns.15 On annealing at 120 °C, PBLG homopolymers display a first order transition from 7:2 to an 18:5 α-helical structure.9,16,17 The transition temperature was shown to vary with the molecular weight of the PBLG homopolymers, with the lower molecular weight fractions displaying a lower transition temperature.16,17 The field-effect mobilities of P3HT-b-PBLG diblock copolymer were measured for films deposited from both chloroform and TCB. It was demonstrated in our earlier publication that the field-effect mobility of the diblock copolymer is 1 order of magnitude lower as compared to the P3HT precursor.9 This relatively low reduction in the fieldeffect mobility of the diblock copolymer containing 43% insulating block suggests that the P3HT block self-assembles at the surface of SiO2 where the charge transport occurs in fieldeffect transistors. The field-effect mobilities of the P3HT-bPBLG films obtained from chloroform and TCB appear to be unaffected by the annealing temperature (Table 1). The films formed from both chloroform and TCB displayed field effect mobilities of ∼6 × 10−4 cm2/(V s). The nonvariation of the field effect mobility indicates that there is no significant change in the self-assembly of the diblock copolymer film by varying the solvent and annealing conditions. The output characteristics and transfer curves of the OFETs with P3HT-b-PBLG diblock copolymer films deposited from chloroform and TCB nonannealed and annealed at 120 °C are

Figure 3. Current−voltage characteristics of poly(3-hexylthiophene)-b-poly(γ-benzyl-L-glutamate) (P3HT-b-PBLG) film casted from chloroform on OFET nonannealed: output curves at different gate voltages (left) and transfer curve at VDS= −100 V (μ = 6.8 × 10−4 cm2/(V s), VT = 22 V, Ion/Ioff = 1.0 × 102). (Slope and intercept calculated from 0 to −60 V.) 12764

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Table 1. Summary of Field-Effect Mobility Data Obtained for Poly(3-hexylthiophene)-b-Poly(γ-benzyl-L-glutamate) (P3HT-bPBLG) at Different Annealing Conditions for Films Deposited from Chloroform and 1,2,4-Trichlorobenzene (TCB) Solvents solvent TCB TCB TCB TCB CHCl3 CHCl3 CHCl3 CHCl3

annealing temperature (°C) nonannealed 80 120 150 nonannealed 80 120 150

mobility (cm2/(V s))a 5.24 5.02 6.32 5.04 6.80 7.90 6.50 4.13

× × × × × × × ×

−4

10 10−4 10−4 10−4 10−4 10−4 10−4 10−4

Ion/Ioffa 5 1 1.5 4 1 1 1.3 3

× × × × × × × ×

VT (V)a 1

10 101 102 102 102 102 102 102

26.2 36.6 23.3 18.5 22.0 29.4 28.0 23.0

average mobility (cm2/(V s))b 4.66 5.11 5.49 5.21 5.93 6.64 5.66 4.74

× × × × × × × ×

−4

10 10−4 10−4 10−4 10−4 10−4 10−4 10−4

SDb 1.17 1.18 1.03 9.75 1.02 1.44 7.02 4.56

× × × × × × × ×

10−4 10−4 10−4 10−5 10−4 10−4 10−5 10−5

Field-effect mobilities estimated on a OFET device with the channel length L = 20 μm; the VT and on/off ratios are measured on the same device. The average mobilities and standard deviation were calculated from measurements of four different channel length (out of 6, 10, 20, 40, and 80 μm). a b

Figure 4. Current−voltage characteristics of poly(3-hexylthiophene)-b-poly(γ-benzyl-L-glutamate) (P3HT-b-PBLG) film casted from chloroform on OFET annealed at 120 °C: output curves at different gate voltages (left) and transfer curve at VDS = −100 V (μ = 6.5 × 10−4 cm2/(V s), VT = 28 V, Ion/Ioff = 1.3 × 102). (Slope and intercept calculated from 0 to −40 V.)

Figure 5. Current−voltage characteristics of poly(3-hexylthiophene)-b-poly(γ-benzyl-L-glutamate) (P3HT-b-PBLG) film casted from 1,2,4trichlorobenzene on OFET nonannealed: output curves at different gate voltages (left) and transfer curve at VDS= −100 V(μ = 5.24 × 10−4 cm2/(V s), VT = 26.2 V, Ion/Ioff = 5 × 101). (Slope and intercept calculated from 20 to −40 V.)

the length of the nanoribbon is smaller for the films deposited from chloroform as compared to the films deposited from TCB. On annealing the film to 80 °C, there is little change in the morphology of the film (Figure 8b). However, the film annealed at 120 °C displays longer nanoribbons as compared to the films annealed at 80 °C (Figure 8c). On further annealing to 150 °C, the length of the nanoribbons decreases and the morphology becomes more granular (Figure 8d). The trend in the change in morphology of the films obtained from chloroform is similar to the trend obtained for the films obtained from TCB. However, the length of nanoribbons

obtained in the films casted from TCB are longer than those in chloroform. The presence of longer nanoribbons for films deposited from TCB may be attributed to the slower evaporation rate of TCB which most likely enables the diblock copolymer to have more time to self-assemble. The possibility of TCB being a better helicogenic solvent (helix supporting solvent) for the PBLG block may also play a role in the formation of the longer nanoribbons. For P3HT-b-PBLG films deposited from both chloroform and TCB, the change in the length of nanoribbons at different annealing temperatures does not influence the field-effect 12765

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Figure 6. Current−voltage characteristics of poly(3-hexylthiophene)-b-poly(γ-benzyl-L-glutamate) (P3HT-b-PBLG) film casted from 1,2,4trichlorobenzene on OFET annealed at 120 °C: output curves at different gate voltages (left) and transfer curve at VDS = −100 V (μ = 6.32 × 10−4 cm2/(V s), VT = 23.3 V, Ion/Ioff = 2 × 102). (Slope and intercept calculated from 20 to −40 V.)

Figure 7. 3-D TMAFM images of poly(3-hexylthiophene)-b-poly(γ-benzyl-L-glutamate) (P3HT-b-PBLG) measured in the channel between the source-drain electrodes on a field-effect transistor at different annealing conditions: (a) nonannealed (rms = 8.43 nm), (b) 80 °C (rms = 9.79 nm), (c) 120 °C (rms = 9.83 nm), (d) 150 °C (rms = 8.26 nm). The polymer was dissolved in 1,2,4-trichlorobenzene (TCB) at 1 mg/mL concentration and drop-casted on the substrate.

mobility of the polymer films. This observation supports the argument that the P3HT block may self-assemble closer to the dielectric (where the charge transport occurs in field-effect transistors) whereas the PBLG block self-assembles near the surface of the polymer film as observed by TMAFM imaging. This behavior is different from that observed for rod−coil block copolymers containing P3HT, for which the morphology is controlled by the rod P3HT block and display nanofibrillar morphology similar to P3HT homopolymer.30−35 In contrast, the morphology for the rod−rod block copolymer P3HT-bPBLG appears to be dependent on the solvent used for casting of the films.

The presence of the amide I, amide II, and amide V bands at 1648 cm−1 (CO stretching of the amide group), 1544 cm−1 (N−H bending), and 608 cm−1 (N−H out of plane bending) in the IR spectrum of the block copolymer obtained from powder FT-IR indicates that the PBLG block adopts α-helical conformation (Figure 9).36−40 The lack of a peak at 1630 cm−1 demonstrates the absence of β-sheet formation. The peak at 1728 cm−1 is due to the CO stretching in the ester of the side chain of the PBLG block.37,39 and the peak at 1454 cm−1 is corresponds to the CC stretching of the thiophene ring of P3HT block. 12766

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Figure 8. 3-D TMAFM images of poly(3-hexylthiophene)-b-poly(γ-benzyl-L-glutamate) (P3HT-b-PBLG) measured in the channel between the source-drain electrodes on a field-effect transistor at different annealing conditions: (a) nonannealed (rms = 5.75 nm), (b) 80 °C (rms = 6.31 nm), (c) 120 °C (rms = 9.12 nm), (d) 150 °C (rms = 7.87 nm). The polymer was dissolved in chloroform at 1 mg/mL concentration and drop-casted on the substrate.

corresponding to the (100) lamellar stacking of P3HT41−44 and a peak at 2θ = 7.0° (d = 12.6 Å) corresponding to the hexagonal packing of PBLG rods.45,46 The presence of peaks from both P3HT and PBLG indicate that the crystalline nature of each of the individual blocks is conserved after the formation of the block copolymer. The conservation of the crystalline nature of the P3HT block explains the reasonably good fieldeffect mobility of P3HT-b-PBLG diblock copolymer in comparison to the field-effect mobility of the P3HT precursor.9 The film deposited from chloroform shows higher order reflections of the P3HT stacking ((200) and (300)) 2θ = 10.9° and 2θ = 16.3° corresponding to d-spacings of 8.1 and 5.4 Å, respectively.41−44 The peaks corresponding to the angles 2θ = 7.0°, 12.5°, 14.5°, and 21.9° (d-spacing of 12.6, 7.1, 6.1, and 4.0 Å, respectively) are due to the lattice planes of (110), (200), and (300) corresponding to a triclinic cell in a PBLG molecule.36,45 The appearance of reflections both from P3HT and PBLG indicates the microphase separation of the diblock copolymer.47,48 XRD patterns of the annealed films (red trace, Figure 10) are similar to those of the nonannealed films (black trace, Figure 10), and this is most probably due to the rigid nature of both blocks preventing any significant gain in order. XRD is a bulk technique where the X-rays penetrate through the sample and hence no significant change in order was observed on annealing the films. The change in surface morphology on annealing as observed in the AFM images may be due to the fact that AFM is a surface technique and changes in the surface morphology can be monitored by AFM. The XRD patterns of film obtained using TCB (Figure 10, right), however, are slightly different from that of film casted from chloroform. The film deposited from TCB displays only

Figure 9. FT-IR spectrum of poly(3-hexylthiophene)-b-poly(γ-benzylL-glutamate) (P3HT-b-PBLG) (Mn = 16 525 g/mol; PDI = 1.44; 43 mol % PBLG).

We recorded the FT-IR spectra of polymer films deposited on gold coated glass before and after annealing at 80, 120, and 150 °C (Figure S18, Supporting Information). Similar IR spectra were observed for both nonannealed and annealed films, indicating that there are no major structural changes observed upon annealing (Figure S18, Supporting Information). Thin film XRD measurements were performed for the P3HT-b-PBLG block copolymer films deposited from both chloroform (Figure 10, left) and TCB (Figure 10, right), and they indicate the presence of a peak at 2θ = 5.3° (d = 16.7 Å) 12767

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Figure 10. Thin film XRD patterns of poly(3-hexylthiophene)-b-poly(γ-benzyl-L-glutamate) (P3HT-b-PBLG) on SiO2 substrate deposited from chloroform (left) and 1,2,4 trichlorobenzene (right): nonannealed film (black trace) and annealed film at 120 °C (red trace).

second order transition at 61 °C which is due to the glass transition temperature of P3HT block. The reported first order transition at 120 °C due to the transition from 7:2 to an 18:5 αhelical structure of the PBLG block was not observed in the DSC thermogram of the P3HT-b-PBLG diblock copolymer.9,16,17 This thermal transition is most likely masked by the presence of P3HT block.

(100), (200) peaks corresponding to P3HT and (300) peaks corresponding to PBLG. The other peaks at 2θ = 16.3° 12.5°, and 14.5° which are present in the pattern of the film casted from chloroform (Figure 10, left) are not observed in the film casted from TCB (Figure 10, right). This may be due to the variation in the film thickness in the films deposited from chloroform and TCB. The film deposited from chloroform had a thickness of 14.9 ± 2.3 μm, while that deposited from TCB had a thickness of 0.5 ± 0.08 μm. This variation in film thickness can be attributed to the evaporation rate of the solvent. As chloroform evaporates faster when it is drop-casted, the solvent evaporates from the side and creates a thick polymer film at the center of the drop. By contrast, TCB evaporates slowly and thus allows the polymer film to form evenly on the substrate. The peaks observed at 2θ = 2° and 4° (Figure 10, right) are due to the SiO2 substrate. The XRD pattern of a blank substrate supports this argument (Figure S15, Supporting Information). The peaks from the substrate are observed in the XRD patterns of films deposited from TCB and are due to their lower thickness. DSC thermogram of P3HT-b-PBLG diblock copolymer is shown in Figure 11. The thermogram displays two endothermic peaks at 210 and 325 °C which are due to the melting of P3HT and PBLG blocks, respectively. An expansion of the DSC thermogram (Figure S19, Supporting Information) shows a

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CONCLUSIONS

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ASSOCIATED CONTENT

Solvent and annealing effects on the field-effect mobility of poly(3-hexylthiophene)-b-poly(γ-benzyl glutamate) (P3HT-bPBLG) rod−rod diblock copolymer in field-effect transistors were investigated. The field-effect mobilities of the diblock copolymer remained unchanged upon variations in solvent and annealing conditions. TMAFM analysis revealed the formation of nanoribbons for nonannealed films of diblock copolymer deposited from both chloroform and 1,2,4-trichlorobenzene. The length of nanoribbons increased when the films were annealed at 80 and 120 °C. The films obtained from TCB displayed longer nanoribbons as compared to the films obtained from chloroform due to the slower evaporation rate of TCB which allows a longer time for the self-assembly process. On further annealing to 150 °C, the nanoribbons break in length and the films become more granular. The presence of 43% of insulating PBLG block and the nondependence of the field-effect mobility on the film morphology indicate that the P3HT block most likely self-assembles near the SiO2 dielectric surface. XRD analysis indicates that the microphase separation of the diblock copolymer was not affected by annealing.

S Supporting Information *

Procedure for the synthesis of the diblock copolymer, phase and height TMAFM images, output curves and transfer curves of OFET measurements, XRD pattern of the blank silicon dioxide substrate, and FT-IR spectra of diblock polymer. This material is available free of charge via the Internet at http:// pubs.acs.org.

Figure 11. DSC thermogram of poly(3-hexylthiophene)-b-poly(γbenzyl-L-glutamate) (P3HT-b-PBLG). 12768

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Corresponding Author

*Phone: (972) 883-6581. Fax: (972) 883-2925. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS M.C.S. gratefully acknowledges financial support from the Welch Foundation (AT 1740). REFERENCES

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