Article pubs.acs.org/JPCB
Dynamic Interactions between Poly(3-hexylthiophene) and SingleWalled Carbon Nanotubes in Marginal Solvent Yanqi Luo, Franceska A. Santos, Taylor W. Wagner, Eric Tsoi, and Shanju Zhang* Department of Chemistry and Biochemistry, California Polytechnic State University, San Luis Obispo, California 93407, United States S Supporting Information *
ABSTRACT: Interfacial interactions between conjugated polymers and carbon nanotubes are pivotal in determining the device performance of nanotube-based polymer electronic devices. Here, we report on interfacial structures and crystallization kinetics of poly(3-hexylthiophene) (P3HT) in the presence of single-walled carbon nanotubes (SWNTs) in anisole by means of transmission electron microscope (TEM) and ultraviolet−visible (UV−vis) absorption spectroscopy. Confined on SWNT surfaces, the P3HT forms nanofibril crystals perpendicular to the long axis of SWNTs. The equilibrium dissolution temperature of the P3HT crystals in anisole is determined to be 381 ± 10 K according to the Hoffman−Weeks extrapolation approach. Upon cooling, the polymer solution spontaneously undergoes a time-dependent chromism. Various kinetics factors such as crystallization temperature, concentration, and SWNT loading have been investigated. It is found that the growth rate (G) of the crystals scales with concentration (C) as G ∝ C1.70±0.16. The Avrami model is utilized to analyze the nucleation mechanism and the Avrami exponents vary between 1.0 and 1.3. The Lauritzen−Hoffman theory is applied to study the chain-folding process. The fold surface free energy is calculated to be (5.28−11.9) × 10−2 J m−2. It is evident that the addition of 0.30 wt % SWNTs reduces the fold surface free energy by 55.6%.
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exfoliation of individual CNTs,19,20 separation of semiconducting CNTs and metallic CNTs,14,21 etc. Recently, it has been recognized that the interfacial interactions between the conjugated polymer and CNTs are of utmost importance for effectively enhancing the device performance.13,22,23 In this regard, the CNTs serve as an orientation template to organize conjugated polymer chains in an ordered pattern at the interface via π−π stacking interactions. Classical molecular dynamic simulations predict that at equilibrium the conjugated backbones have a tendency to align themselves along the long axis of CNTs to maximum π−π stacking, resulting in an elongated conjugation length and fast interfacial electron transfer.24−27 Experimentally, various kinetically trapped polymer conformations have been observed using highresolution imaging tools, including metastable circumstance and helix structures.28−30 On the other hand, the CNTs can also act as a heterogeneous nucleation agent for polymer crystallization.31−33 In general, the conjugated polymers employed in the photoactive layer of the PV cells are semicrystalline materials. In a marginal solvent, the conjugated polymers will crystallize into fibril-like nanostructures, which have been utilized to effectively enhance the PV device performance.34−36 During dilute solution crystallization, the conjugated polymer chains undergo a complex process
INTRODUCTION Polymer-based photovoltaic (PV) cells are an emerging solar technology in which the solar cells can be processed from solution by low-cost printing/coating processes in a roll-to-roll fashion like graphic films.1,3 The most efficient polymer-based PV cells to date are on the base of the bulk heterojunction (BHJ) in which the conjugated polymer as an electron donor and fullerene as an electron acceptor phase separate to form bicontinuous interpenetration networks in a bulk volume.4−6 Currently, the power conversion efficiency (PCE) of polymerbased PV cells is about 7%,1,3−5,7 far less than 23.2% of the theoretical limit.8 To enhance the PCE of the PV devices, it has been suggested to replace fullerene by carbon nanotubes (CNTs).9−13 The CNTs are a one-dimensional (1-D) nanostructured allotropic form of carbon and possess high carrier mobility and unique ballistic conduction pathways. Interestingly, the band gap of CNTs can be tuned over a wide range by controlling the diameter and chirality. It is recently found that semiconducting CNTs form favorable type II heterojunctions in the photoactive layer.14−16 Despite such great potential, polymer PV cells using CNTs as acceptor materials have shown the limited device performance, and the PCE is typically below 1%.9,10,12 Only recently, the dramatic improvement has been achieved in boron-doped CNTs with 4.1% PCE.17 During the past years, intense efforts have been made toward high efficiency polymer PV cells using CNTs as acceptor materials. It includes ultrapurification of CNTs,11,18 full © 2014 American Chemical Society
Received: March 29, 2014 Revised: May 14, 2014 Published: May 23, 2014 6038
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including coil-to-rod transition, chain folding, and rod−rod aggregation.32,37−40 To this end, the dual role of CNTs in the solution will allow the conjugated polymer to behave differently at the interface in terms of backbone conformation, chain folding, crystallinity, and so forth. It is expected that polymer morphology and microstructure on the CNT surfaces play a crucial role in the production of high efficiency PV cells.13 However, the knowledge about dynamic interactions between the conjugated polymer and CNTs in solution is limited. In this work, we report on interfacial structures and solution crystallization kinetics of poly(3-hexylthiophene) (P3HT) in the presence of single-walled carbon nanotubes (SWNTs) by means of transmission electron microscopy (TEM) and ultraviolet−visible (UV−vis) absorption spectroscopy. P3HT is a benchmark material serving as an electron donor in the PV cells while SWNTs have huge surface-to-volume areas and are considered as an ideal electron acceptor to replace fullerene in the PV cells. In a marginal solvent of anisole, P3HT nanofibril crystals grow perpendicular to SWNT surfaces with uniform size dimension. Addition of only 0.30 wt % SWNTs significantly speeds up the solution crystallization and remarkably decreases the fold surface free energy.
Figure 1. (a, b) Bright-field TEM images of P3HT nanofibril crystals in the presence of SWNTs. (c) Schematic diagram of P3HT nanofibril crystals perpendicular to the long axis of SWNTs.
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mimicking hybrid shish-kebab nanostructures that have been widely observed in the CNT-induced polymer nanocrystals.31,41−44 This observation is also in good agreement with the recent report on P3HT−SWNT nanohybrids.44 The length of P3HT nanofibrils and their attachment density on SWNTs are dependent on crystallization temperature, nanotube loading, and polymer concentration. The long axis of the nanofibrils corresponds to the direction of π−π stacking, and therefore, the conjugated backbone is aligned parallel to the long axis of SWNTs (Figure 1c).37,44 Such a strong nucleating ability of SWNTs is presumably correlated to the epitaxial interactions between the P3HT backbone and SWNT surfaces.37 The resulting ordered nanohybrids represent nanoscale BHJs with efficient interfacial electron transfer and fast ambipolar charge transport. The polymer solar cells with a photoactive layer containing such ordered P3HT−SWNT nanohybrids have demonstrated much enhanced device performance.45 Solution Crystallization Kinetics. The kinetics of dilute solution crystallization of P3HT with SWNTs in anisole has been studied using in situ UV−vis spectroscopy. As comparison, the solution crystallization kinetics of the control P3HT in anisole under the same conditions has also been investigated (Supporting Information Figures 1 and 2). To determine the appropriate isothermal crystallization temperature (Tc), the temperature-dependent chromism of P3HT in anisole containing 0.10 wt % SWNTs has been investigated as shown in Figure 2a. At high temperature of 323 K, the P3HT solution exhibits an absorption maximum at λ = 450 nm, indicating a coil-like conformation of the dissolved polymer chains.46 With decreasing temperature, the absorbance at λ = 450 nm decrease significantly while the low-energy absorption bands at λ = 560 nm and λ = 600 nm appear and their absorption intensities gradually increase. These new vibronic bands are attributed to the rodlike conformation and electronic interactions in the P3HT crystals.47 The coil-to-rod transition during solution crystallization is further evidenced by the presence of a distinct isosbestic point at λ = 480 nm. As the absorbance at λ = 600 nm is directly proportional to the crystallinity of P3HT crystals,48 the crystallization evolution of P3HT in the presence of SWNTs can thus be determined by in situ monitoring of the absorbance at λ = 600 nm. In this work, the Tc window has
EXPERIMENTAL SECTION Electronic grade poly(3-hexylthiophene) (P3HT) with regioregular of 91−94% head-to-tail (average molecular weight is 50 000−70 000, Rieke Metals) and single-walled carbon nanotubes (SWCNTs) (the purity is 99 wt %, Cheap Tubes Inc.) were used as received. The nanotube dispersion with concentration of 0.025 mg/mL was prepared by dissolving nanotubes in odichlorobenzene (o-DCB) with an aid of P3HT at a 1:2 ratio of SWNT:P3HT under bath ultrasonication for 2 h. The 1.25 mg/ mL stock solution of P3HT in anisole was prepared in a hot oil bath at 80 °C and then diluted into different concentration solutions for the study of crystallization kinetics. Typically, the P3HT solution was mixed with a certain amount of SWNT dispersion at 80 °C for at least 5 min to erase thermal history of the sample, and then the mixture was quickly cooled at ∼180 °C/min to the preset isothermal crystallization temperature. Transmission electron microscope (TEM) images were collected on an EFI Tecnai G2 sphera microscope at an accelerating voltage of 120 kV. The TEM samples were prepared by drop-casting the dilute solution of P3HT with SWNTs to the holey carbon film on 300 mesh copper grids. The samples were examined without staining or shadowing under TEM. The UV−vis absorption spectra were received in solutions on a Jasco V-550 UV−vis spectrophotometer equipped with temperature-control and magnetic stirring features. The sample chamber was insulated from surroundings during the data collection. The UV−vis absorbance at a wavelength of λ ∼ 600 nm was monitored over time during the isothermal solution crystallization.
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RESULTS AND DISCUSSION
Interfacial Morphology. We employed TEM to directly visualize interfacial morphology and structures of P3HT on SWNT surfaces after the complete solution crystallization. Figures 1a and 1b show typical bright-field images of the ordered P3HT nanofibril crystals grown in anisole in the presence of SWNTs at room temperature. It is evident that the P3HT crystalline nanofibrils with an ∼20 nm diameter are preferably oriented perpendicular to the long axis of SWNTs, 6039
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Figure 2. (a) Thermochromism of P3HT with 0.10 wt % SWNTs in anisole and (b) heating (red) and cooling (blue) curves of absorbance at λ = 600 nm versus temperature. The P3HT concentration is 0.020 mg/mL. The heating and cooling rates in (b) are 2.0 K/min.
studying of the absorbance at λ = 600 nm over time as shown in Figure 4b. It displays that the slope of the absorbance versus time plot decreases constantly, and thus the growth rate of the P3HT crystals decreases over time. When the dissolved polymer is exhausted, the absorbance reaches a plateau and the crystals stop growing. Figure 5 shows the absorbance at λ = 600 nm over time plots of solution crystallization of P3HT in anisole with various SWNT loading under different crystallization temperature. To study the kinetic behavior of polymer crystallization, the method of initial rates was used to evaluate the growth rate.46 To this end, the initial slope of absorbance versus time plots was extracted. The data are summarized in Figure 6. As expected, decreasing crystallization temperature results in the coil-to-rod transition to minimize the total free energy and thereafter accelerates the crystallization process of P3HT. At the same crystallization temperature, adding SWNTs to P3HT solution significantly increases the growth rate. This phenomenon is well consistent with nucleation and growth processes.51 The SWNT-prompted crystallization behavior demonstrates that SWNTs effectively act as a heterogeneous nucleation agent for P3HT crystallization. With increasing SWNT loading, the growth rate increases more or less linearly. This tendency is most apparent at the high-temperature end. This phenomenon is also indicative of no significant aggregations of SWNTs in solution. Theoretically, the concentration (C) dependence of the growth rate (G) at constant temperature is expressed by eq 150
been selected in the range from 323 K down to 292 K. It should be noted that the thermochromism of P3HT solution containing SWNTs is reversible with the presence of the ratedependent hysteresis, as shown in Figure 2b. The cooling and heating curves correspond to the crystallization and dissolution processes, respectively. Thus, the dissolution temperature (Td) during heating can be determined by UV−vis spectroscopy. To evaluate the dissolution temperature at equilibrium (T0d) of the pure P3HT with 100% crystallinity in anisole, the Hoffman−Weeks method was used.49 After the complete isothermal crystallization at the preset crystallization temperature (Tc), the P3HT samples were heated up to dissolve the produced crystals. The dissolution process was then monitored by UV−vis spectroscopy to obtain the dissolution temperature (Td). The data were plotted as Td versus Tc as shown in Figure 3. The diagonal line in Figure 3 represents the function of Td =
G = kC α Figure 3. Dissolution temperature (Td) of P3HT in anisole as a function of isothermal crystallization temperature (Tc). The data are linearly extrapolated to the line of Td = Tc to obtain T0d at the intersection point. The concentration of P3HT is 0.020 mg/mL.
(1)
where k represents the rate constant and α is the reaction order. The latter is solely associated with the reaction mechanism. To evaluate the value of α, the plots of ln G versus ln C were obtained as shown in Figure 7. The data points of the control P3HT (Figure 7a) and P3HT with SWNTs (Figure 7b−d) fall onto the best fit straight lines, and the slopes of the straight lines reveal the reaction order α. The data are summarized in Table 1. It is found that α = 1.53−1.69 for the control P3HT and α = 1.57−1.86 for P3HT with SWNTs. Our data indicate that the addition of SWNTs changes little on the reaction order α and thus the crystallization mechanism. According to the secondary nucleation theory, the rod−rod aggregation is predominant over the coil−rod transition and chain-folding during dilute solution crystallization of P3HT.32 This discovery is consistent with our earlier reported work on ZnO nanowires induced P3HT crystallization in anisole.32 The slight increase in the value of the reaction order α by adding SWNTs may reflect
Tc. The data of Td versus Tc were then linearly extrapolated to an intersection, T0d, on the diagonal line. In this work, the value of T0d of the pure P3HT in anisole was calculated to be 381 ± 10 K. This value is broadly consistent with 371 K of the recent literature report on P3HT crystallization in anisole.32 The big error bar on the fit may be attributed to the small Tc window in this work.50 Figure 4a shows the time-dependent chromism of P3HT with 0.10 wt % SWNTs upon cooling the hot polymer solution to 298 K. With aging time, the absorbance at λ = 450 nm decreases while the absorbance at λ = 600 nm steadily increases, displaying SWNT-induced crystallization of P3HT in anisole. The crystallization kinetics was then evaluated by in situ 6040
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Figure 4. (a) Time-dependent chromism of P3HT with 0.10 wt % SWNTs in anisole at 298 K. (b) Absorbance at λ = 600 nm versus crystallization time. The concentration of P3HT is 0.010 mg/mL.
Figure 5. Kinetics of isothermal solution crystallization of P3HT with various loading of SWNTs at different crystallization temperature: (a) 298, (b) 296, (c) 294, and (d) 292 K. The P3HT concentration is 0.020 mg/mL.
1 − X t = exp( −kt n)
(2)
where Xt is the relative crystallinity at time t, k represents the Avrami rate constant, and n denotes the Avrami exponent whose value depends on the nucleation mechanism. As the crystallinity is proportional to the UV−vis absorbance at λ = 600 nm,48 the value of Xt is determined by eq 3 Xt =
At − A0 A max − A 0
(3)
where A0, Amax, and At are respectively the initial absorbance, the maximum absorbance, and the absorbance at time t. To simplify eq 2, the double-logarithmic linear form is obtained as shown in eq 4
Figure 6. Effect of SWNT loading on the growth rate of P3HT crystals in anisole. The P3HT concentration is 0.020 mg/mL.
ln[− ln(1 − X t )] = ln k + n ln t
(4)
Figure 8 displays the plots of ln[−ln(1 − Xt)] versus ln t for control P3HT (Figure 8a) and P3HT with various SWNT loading at different crystallization temperature (Figure 8b−d). The experimental data fall onto the best fit straight lines. At the high relative crystallinity, the data slightly deviate from the straight lines, indicative of the existence of secondary crystallization.53 The values of the Avrami exponent n and the Avrami rate constant k were then calculated from the slope
strong rod−rod aggregations due to the template effect of SWNTs. Nucleation. Based on the polymer crystallization theory, the growth rate of the crystals (G) is proportional to the nucleation rate (S) as G ∝ S.52 To elucidate the nucleation geometry and nature of growth of the nuclei, the classical Avrami theory is applied,2 as shown in eq 2 6041
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Figure 7. Plots of ln G versus ln [P3HT] at different crystallization temperature for P3HT with various loading of SWNTs: (a) control P3HT, (b) 0.10 wt %, (c) 0.20 wt %, and (d) 0.30 wt %.
Table 1. Data of Crystallization Kinetics of P3HT with Various Loading of SWNTs in Anisole at Different Tc from the Rate Law and the Avrami Model samples control P3HT
0.10 wt % SWNT
0.20 wt % SWNT
0.30 wt % SWNT
Tc [K]
initial rate [s−1] × 104
reaction order α
Avrami exponent n
Avrami constant k × 103
t1/2,exp [s]
t1/2,cal [s]
292 294 296 298 292 294 296 298 292 294 296 298 292 294 296 298
22.9 10.9 4.72 2.77 25.1 11.4 6.42 4.52 26.8 17.3 9.03 6.79 25.8 16.6 10.7 10.3
1.61 1.64 1.53 1.69 1.62 1.72 1.68 1.84 1.59 1.57 1.68 1.81 1.69 1.68 1.86 1.81
1.31 1.27 1.19 1.10 1.27 1.27 1.20 1.11 1.21 1.20 1.02 1.01 1.19 1.13 1.03 1.03
5.46 3.38 2.69 3.09 7.45 3.38 2.80 3.52 9.76 6.10 8.83 7.52 11.3 9.66 10.6 8.92
35.0 68.5 111.0 140.0 34.5 65.0 101.0 123.0 34.0 53.0 75.0 88.0 33.0 46.5 61.0 71.0
40.3 66.1 106 137 35.5 66.1 98.8 117 33.9 51.6 72.1 88.1 31.8 43.9 57.9 68.5
loading, the value of Avrami exponent n is sensitive to the crystallization temperature Tc, which is evidenced by the smaller n value at higher Tc. This observation is consistent with the literature report.57 The crystallization half-life t1/2 is defined as the time at which the extent of crystallization is complete 50%. Thus, the t1/2 values were evaluated experimentally when Xt1/2 = 0.5. The data are denoted by t1/2,exp in Table 1. From the values of the Avrami exponent n and the Avrami rate constant k, the t1/2 values can also be calculated using eq 52
and intercept of the straight lines, respectively. The data are also summarized in Table 1. It is found that the values of the Avrami exponent n vary from 1.10 to 1.31 for the control P3HT and from 1.01 to 1.27 for P3HT with SWNTs. The low values of the Avrami exponent in our work are in good agreement with the literature reports on the isothermal melt crystallization of the conjugated polymers.54−56 Therefore, the solution crystallization of P3HT in anisole undergoes one-dimensional (1D) nucleation with linear growth and addition of SWNTs in P3HT solution has little effect of the nucleation mechanism. Such nucleation mechanism is also supported by our observations on the fibril-like crystals and a mixed fractional order rate law. The slight decrease in the value of the Avrami exponent n by adding SWNTs is attributed to geometric confinement of polymer chains at the interface due to the orientation template effect of SWNTs. At constant SWNT
t1/2 =
⎛ ln 2 ⎞1/ n ⎜ ⎟ ⎝ k ⎠
(5)
The data are denoted by t1/2,cal in Table 1. It shows that the t1/2,exp and t1/2,cal are consistent with each other and both decrease with decreasing Tc. A lower crystallization temperature 6042
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Figure 8. Plots of ln(−ln(1 − Xt)) versus ln t for isothermal crystallization of P3HT with various loading of SWNTs: (a) control P3HT, (b) 0.10 wt %, (c) 0.20 wt %, and (d) 0.30 wt %. Polymer concentration is 0.020 mg/mL.
Figure 9. Plots of ln G versus 1/TΔT: (a) the control P3HT with different polymer concentration and (b) P3HT with various loading of SWNTs. The P3HT concentration in (b) is 0.020 mg/mL.
transition temperature), ΔT is the degree of supercooling that is measured from the equilibrium dissolution temperature T0d (where ΔT = T0d − Tc), and Kg is the nucleation parameter which is a function of the fold surface free energy. The first exponential term in eq 5 is associated with the diffusion process of the polymer chain segments in solution whereas the second exponential term represents the thermodynamic driving force of chain-folding during the nucleation process. In the dilute solution, polymer crystallization is mainly determined by absorbed polymers rather than dissolved polymers, and therefore, the second exponential term is predominant over the first one.32 In this case, the growth rate (G) of the crystals mainly depends on 1/TcΔT. Equation 6 is simplified and expressed by eq 7.
Tc will lead to a higher supercooling degree, which results in the faster formation of the nuclei for the crystallization. At constant temperature, addition of SWNTs also decreases the half-life and therefore increases the nucleation rate. This phenomenon is attributed to the fact that SWNTs serve as a heterogeneous nucleation agent for P3HT crystallization.44 Chain Folding. Chain folding is a major characteristic of semicrystalline polymers, and the fold surface free energy (σe) is determinative to chain-folding during the nucleation process.51 According to the Lauritzen−Hoffman theory of secondary nucleation during the chain-folding process,58 the growth rate is determined by eq 6 ⎛ −U * ⎞ ⎛ −K g ⎞ G = G0 exp⎜ ⎟ exp⎜ ⎟ ⎝ R(Tc − T∞) ⎠ ⎝ TcΔT ⎠
(6)
where G0 is the pre-exponential factor that includes all temperature-independent terms, U* denotes the activation energy for the transport of crystal units across the phase boundary, R is the universal gas constant, Tc represents the crystallization temperature, T∞ = Tg − 30 (where Tg is the glass
ln G =
−K g TcΔT
+ constant
(7)
The value of Kg represents the energy term required for the formation of nuclei of critical size and is evaluated by eq 8 6043
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4bσσeTd0 Kg = kBΔHf
interactions between P3HT and SWNTs.24−26,44 It is believed that the lowering of the fold surface free energy is one of the driving forces to accelerate P3HT crystallization in the solution containing SWNTs. This thermodynamic analysis is consistent with our observations on the crystallization kinetics from the reaction rate law and the nucleation mechanism from the Avrami model.
(8)
where b represents the thickness of the single molecular layer (stem) in the polymer crystal, σ and σe are respectively the lateral and fold surface free energies, T0d is the equilibrium dissolution temperature, kB is Boltzmann’s constant, and ΔHf denotes the heat of fusion per unit volume of the crystals. Figure 9a displays the plots of ln G versus 1/TcΔT of the control P3HT with different concentrations. The best-fit straight line was found for each sample. From the slopes of the straight lines, the value of Kg of the control P3HT was determined to be ∼1.20 × 106 K2. It is found that the Kg value is independent of the P3HT concentration in the range of 0.005−0.025 mg/mL. Therefore, the fold surface free energy σe remains constant in the dilute solution. However, addition of SWNTs to P3HT solution significantly decreases the Kg and thus the fold surface free energy σe. Figure 9b shows a plot of ln G versus 1/TcΔT of P3HT with various SWNT loading at P3HT concentration of 0.020 mg/mL. With increasing SWNT loading from 0.00 to 0.30 wt %, Kg decreases from 1.16 × 106 to 0.515 × 106 K2. It has been reported that for P3HT crystals b = 7.75 × 10−10 m, σ = 1.24 × 10−2 J m−2, and ΔHf = 1.096 × 108 J m−3.54 From these values and eq 7, the value of σσe was calculated, and thus the value of σe was determined for each sample. The data are listed in Table 2. The data of the σe versus
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CONCLUSIONS In summary, we have studied the kinetic behavior of isothermal solution crystallization of poly(3-hexylthiophene) in anisole in the presence of single-walled carbon nanotubes. Nanotubes serve as an orientation template and heterogeneous nucleation agent for polymer crystallization. The polymer forms crystalline nanofibrils perpendicular to the long axis of nanotubes. A detailed investigation on the crystallization kinetics shows that the system follows a complex rate law with a mixed fractional order. The secondary nucleation model displays predominant rod−rod aggregations. The Avrami analysis demonstrates that polymer crystallization occurs by one-dimensional (1D) heterogeneous nucleation with linear growth. The chain-folding process is analyzed using the Laurtzen−Hoffman theory. It is evident that the addition of 0.30 wt % SWNTs remarkably reduces the fold surface free energy. Our work addresses a very important fundamental issue concerning interfacial interactions between conjugated polymers and carbon nanotubes. The further investigations into the dynamic process of interfacial charge transfer due to interfacial interactions are required for the production of high efficiency nanotube-based polymer electronic devices. This is a topic of ongoing research work in our group, and significant progress will be reported in subsequent publications.
Table 2. Data of Thermodynamic Characteristics of P3HT Crystallization in the Presence of SWNTs from the Lauritzen−Hoffman Model material
Kg [K2] × 10−6
σσe [J2 m−4] × 103
σe [J m−2]
control P3HT P3HT/0.10 wt % SWNTs P3HT/0.20 wt % SWNTs P3HT/0.30 wt % SWNTs
1.16 0.925 0.768 0.515
1.48 1.17 0.976 0.655
0.119 0.0940 0.0787 0.0528
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ASSOCIATED CONTENT
S Supporting Information *
UV−vis spectra of crystallization of pure P3HT in anisole. This material is available free of charge via the Internet at http:// pubs.acs.org.
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SWNT loading are plotted in Figure 10. The linear dependence is found. The σe value of the control P3HT in anisole is ∼0.119 J/m2, which is consistent with our recent report on 0.103 J/m2 of P3HT in anisole.32 Addition of only 0.30 wt % SWNTs to P3HT solution remarkably lowers the fold surface free energy σe by 55.6%. Such a significant change in the fold surface free energy of P3HT is attributed to the effective interfacial
AUTHOR INFORMATION
Corresponding Author
*Tel +1 805 756 2591; Fax +1 805 756 5500; e-mail
[email protected] (S.Z.). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We thank financial support from the Cal Poly Extramural Funding Initiative, the National Science Foundation under CMMI-135138, and the American Chemistry Society Petroleum Research Fund under 53970-UR7. F.A.S. and T.W.W. acknowledge financial support from the Cal Poly Bill Moore Fellowship and Willian Frost Fellowship, respectively.
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REFERENCES
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Figure 10. Fold surface free energy versus SWNT loading for solution crystallization of P3HT in anisol. The P3HT concentration is 0.020 mg/mL. 6044
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