Determination of Solvent–Polymer and Polymer–Polymer Flory

Aug 15, 2013 - During film casting, the solvent volume fraction in these formulations, ..... appeared darker in color than the bright orange ϕo = 0.9...
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Determination of Solvent−Polymer and Polymer−Polymer Flory− Huggins Interaction Parameters for Poly(3-hexylthiophene) via Solvent Vapor Swelling Jillian A. Emerson,† Daniel T. W. Toolan,‡ Jonathan R. Howse,‡ Eric M. Furst,*,† and Thomas H. Epps, III*,† †

Center for Molecular and Engineering Thermodynamics, Department of Chemical and Biomolecular Engineering, University of Delaware, Newark, Delaware 19716, United States ‡ Department of Chemical and Biological Engineering, University of Sheffield, Sheffield S1 3JD, United Kingdom S Supporting Information *

ABSTRACT: We report the use of solvent vapor swelling of ultrathin polymer films to determine Flory−Huggins solvent− polymer and polymer−polymer interaction parameters (χi−j) for poly(3-hexylthiophene) (P3HT) and polystyrene (PS) over a wide solvent composition range. From the calculated interaction parameters, we constructed a polymer/polymer/solvent phase diagram that was validated experimentally. χtetrahydrofuran−P3HT (1.04 ± 0.04) and χCHCl3−P3HT (0.99 ± 0.01) were determined through swelling of ultrathin P3HT films. Similar experiments using PS films gave χtetrahydrofuran−PS = 0.41 ± 0.02 and χCHCl3−PS = 0.39 ± 0.01, consistent with literature values. As expected, these χi−j parameters indicated that P3HT is less compatible than PS with either solvent. From δPS (17.9 ± 0.2 MPa1/2) and δP3HT (14.8 ± 0.2 MPa1/2), determined through regular solution theory, we calculated χPS−P3HT = 0.48 ± 0.06 at 23 °C. The resulting phase diagram was validated by solution-based transmission measurements of PS/ P3HT blends in o-xylene. Although we focused on PS/P3HT blends in this work, this approach is easily adaptable to other polymer/polymer combinations of interest.

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generation of the requisite interfaces for charge separation.14 This enhanced phase separation and increased active layer efficiency are often achieved through thermal15−19 or solvent20−28 annealing of the polymer blends following casting.29,30 Unlike thermal annealing, solvent annealing may be practical for OPV devices with flexible substrates that are temperature sensitive,22 although solvent choice has an important role in the final device performance.23,24 Park et al. found that relatively poor solvents led to better efficiencies for P3HT/PCBM BHJs, whereas good solvents promoted excessive phase separation and hence nonideal nanoscale domain thicknesses.23 Despite the effect of annealing solvent choice on BHJ device efficiency, interactions between P3HT and common organic solvents are not well characterized. Polymer compatibility with solvents and other polymers can be quantified by the solvent−polymer or polymer−polymer Flory−Huggins interaction parameter (χi−j).31,32 Using Flory− Huggins theory, χi−j can be estimated from solubility parameter (δ) data.32 As solubility parameters for most organic solvents are readily available in the literature, this method can provide a

INTRODUCTION Since the mid-1980s, conducting poly(alkylthiophene)s (PAT)s have been studied for applications in organic electronics.1,2 The advantage of PATs comes largely from the solubilizing alkyl side chains that permit solution processing, therefore reducing manufacturing costs and widening the scope of available production methods to include continuous deposition techniques such as roll-to-roll processing.3 Poly(3-hexylthiophene) (P3HT) is one of the most commonly studied materials due to its relatively high field effect mobility, solubility in a variety of organic solvents, and crystallization into well-ordered structures.4 In thin film applications, P3HT is used as an electron donor material in organic light-emitting diodes, organic field-effect transistors, and organic photovoltaic (OPV) devices.5−11 The most common polymer OPV structure is the bulk heterojunction (BHJ).12 An idealized BHJ structure consists of interpenetrating, bicontinuous networks of an electron acceptor (such as (6,6)-phenyl-C61-butyric acid methyl ester [PCBM]) and a polymer electron donor (such as P3HT), with domain thicknesses between 4 nm and 20 nm.13 As-cast blends of P3HT and PCBM typically are well-mixed, which leads to nonideal performance, as phase separation of the P3HT and PCBM is required for the crystallization of P3HT and the © 2013 American Chemical Society

Received: March 21, 2013 Revised: July 2, 2013 Published: August 15, 2013 6533

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the films were accessed. Jaczewska et al. selected this range based on observations from previous studies on casting polymer blend films in humid environments, in which they noted droplet condensation on their films and lateral film swelling at higher solvent concentrations.42 However, it is important to access a wide composition range because the Flory−Huggins interaction parameter in solvent−polymer mixtures can vary widely with composition. To the authors’ knowledge, there have been no studies examining the concentration dependence of χi−P3HT over the wide composition range applicable to organic photovoltaic solution formulations. During film casting, the solvent volume fraction in these formulations, initially high (>0.90), is reduced primarily through evaporation, as the final film is formed, in which only residual amounts of solvent remain ( 0.36, χCHCl3−PS decreased with increasing polymer concentration.

decrease in Flory−Huggins interaction parameter with increasing polymer concentration was consistent with data reported in the literature.55,56 To quantify a single interaction parameter between PS and P3HT, only the swelling behavior for PS in CHCl3 in the low polymer volume fraction region (ΦP < 0.6) was used, in which χCHCl3−PS changed little with concentration. The Flory−Huggins solvent−polymer interaction parameters, determined from the slopes of the lines in Figure 3, are shown in Table 1. P3HT had similar solvent−polymer Flory− Huggins interaction parameters in THF and CHCl3, which was reasonable because the difference between the solubility parameters of THF and CHCl3 (δTHF = 19.5 MPa1/2, δCHCl3 Table 1. Solvent−Polymer Flory−Huggins Interaction Parametersa PS P3HT

THF

CHCl3

0.41 ± 0.02 1.04 ± 0.04

0.39 ± 0.01 0.99 ± 0.01

a

Solvent−polymer Flory−Huggins interaction parameters (χi−j) for PS and P3HT with THF and CHCl3, calculated from eq 1 and linear regression of the data in Figure 3. χCHCl3−PS was determined using the data for polymer volume fractions less than 0.6. The uncertainties are the standard error in the slope from the linear regressions in Figure 3. 6536

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and 10−2, which is smaller in magnitude than the enthalpic contribution given by the right-hand side in eq 3.58−60 We did not consider the entropic contribution to the polymer− polymer interaction parameter in this work because our polymer−polymer interaction parameter was on the order of 10−1 (see Table 2). The calculated solubility parameter for PS (17.9 ± 0.2 MPa1/2) was well within the range of values found in the literature (δPS = 17.4−19.3 MPa1/2), which was a direct result of the agreement of our χCHCl3−PS and χTHF−PS with those reported previously.32 Although there are few reports of P3HT solubility parameters found in the literature, the values range from 13 to 20 MPa1/2.33−37 Our calculated solubility parameter (14.8 ± 0.2 MPa1/2) fell within this reported range and was very similar to the value reported by Jaczewska based on Heffner’s light scattering data.37,38 From regular solution theory and the calculated solubility parameters, we determined that the polymer−polymer Flory− Huggins interaction parameter between PS and P3HT was 0.48 ± 0.06, which was in the vicinity of χPS−P3HT reported from Monte Carlo simulations (0.60).61 In classic phase-separating polymer blends, such as PS/poly(methyl methacrylate) or PS/ polybutadiene, the polymer−polymer Flory−Huggins interaction parameters are ∼0.01 and 0.03 to 0.06, respectively.62 Additionally, based on the degree of polymerization for PS and P3HT, χcritical for our blend is 0.015. Our calculated χPS−P3HT is significantly higher than χcritical, further indicating that blends of PS and P3HT should phase separate. To illustrate the expected phase behavior of blends of PS and P3HT, we created a polymer blend phase diagram (see Figure 4). As we are interested in the onset of phase separation behavior during thin film casting, we accounted for solvent compatibilization of the two polymers in the phase diagram. We selected o-xylene as the solvent, as it evaporated slowly, minimizing the change in concentration during transmission measurements on the polymer blend solutions. To construct a phase diagram for PS/P3HT blends in oxylene, we used the partial molar Gibbs free energy of each component (eqs 4a−4c), as described by Scott:63

= 19.0 MPa1/2) was small.32 Based on the interaction parameters, PS dissolves more readily in either solvent than P3HT, matching experimental observations. The solvent− polymer Flory−Huggins interaction parameters for PS from our method (χTHF−PS = 0.41 ± 0.02, χCHCl3−PS = 0.39 ± 0.01) were within the range of values reported in literature (χTHF−PS = 0.16−0.70, χCHCl3−PS = 0.17−0.52), supporting the validity our experimental setup and procedure.32,39,55,56 A high molecular weight PS (892 kg/mol) was used in these experiments, as films cast from a lower molecular weight PS dewet at higher CHCl3 concentrations (Φsolv > 0.30). However, a lower molecular weight PS (17 kg/mol), similar in molecular weight to the P3HT (17.4 kg/mol), was desired for our phase behavior studies between PS and P3HT. When comparing the swelling behavior (below Φsolv = 0.30) of the 17 kg/mol PS with the 892 kg/mol PS (see Figure S3), we noted minimal differences based on molecular weight, consistent with the literature.55 Thus, we applied χCHCl3−PS and χTHF−PS determined for the 892 kg/mol PS to the 17 kg/mol PS in the next section. Polymer−Polymer Interactions. Solubility parameters for PS and P3HT can be estimated from the Flory−Huggins interaction parameters in Table 1 using regular solution theory, as shown in eq 2. χi − j =

Vi (δi − δj)2 + 0.34 RT

(2)

χi−j is the solvent−polymer interaction parameter, Vi is the molar volume of the solvent, R is the gas constant, T is the absolute temperature in K, and δi and δj are the solubility parameters for the polymer and solvent, respectively. The first term in eq 2 comes from enthalpic contributions to χi−j, and 0.34 represents a correction factor for entropic contributions.32 Because P3HT often is blended with other materials, it is of interest to determine polymer−polymer Flory−Huggins interaction parameters for P3HT blend components. From the calculated solubility parameters shown in Table 2, we can Table 2. Polymer Solubility Parameters and Polymer− Polymer Interaction Parameter PS P3HT

δa [MPa1/2]

χPS−P3HTb

17.9 ± 0.2 14.8 ± 0.2

0.48 ± 0.06

⎛ ⎛ ΔG̅0 1 ⎞ 1 ⎞ =ln ϕo + ⎜1 − ⎟ϕ ⎟ϕPS + ⎜1 − RT mPS ⎠ mP3HT ⎠ P3HT ⎝ ⎝ + χo‐PS ϕPS2 + χo‐P3HT ϕP3HT 2 + (χo‐PS + χo‐P3HT

a

Solubility parameters (δ) were determined for PS and P3HT with THF and CHCl3 from regular solution theory and the solvent− polymer Flory−Huggins interaction parameters from Table 1. We report the average of the solubility parameter values found using both solvents. bThe polymer−polymer interaction parameter was calculated using eq 3. Error was determined via error propagation from the uncertainties listed in Table 1.

− χPS − P3HT )ϕPSϕP3HT ⎛ ΔG̅ PS mPS ⎞ = ln ϕPS + (1 − mPS)ϕo + ⎜1 − ⎟ϕ RT mP3HT ⎠ P3HT ⎝

+ mPS[χo‐PS ϕo 2 + χPS − P3HT ϕP3HT 2 + (χo‐PS + χPS − P3HT − χo‐P3HT )ϕ0ϕP3HT]

determine a polymer−polymer interaction parameter for PS and P3HT (according to eq 3, also from regular solution theory):57 χPS − P3HT =

V0 (δ PS − δ P3HT)2 RT

(4a)

(4b)

⎛ ⎞ ΔG̅ P3HT m = ln ϕP3HT + (1 − mP3HT)ϕo + ⎜1 − P3HT ⎟ RT mPS ⎠ ⎝ ϕPS + mP3HT[χo‐P3HT ϕo 2 + χPS − P3HT ϕPS2

(3)

in which χPS−P3HT is the polymer−polymer interaction parameter, V0 is the geometric mean of the polymer segment molar volumes, and δPS and δP3HT are the solubility parameters for PS and P3HT. For polymer−polymer interaction parameters, the entropic contribution is usually between 10−6

+ (χo‐P3HT + χPS − P3HT − χo‐PS )ϕϕ ] o PS

(4c)

ΔG̅ i is the partial molar Gibbs free energy of component i, R is the universal gas constant, T is the temperature in K, ϕi is the volume fraction of component i with respect to total solution 6537

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Figure 5. Relative transmission of 1:1 blends of PS:P3HT in o-xylene vs volume percentage of o-xylene. The transmission values through the sample were normalized by the light transmission through o-xylene. At the highest blend solution concentration of o-xylene (ϕo = 0.990), the relative transmission was the same as the transmission through pure oxylene. At solution concentrations lower than ϕo = 0.960, the relative transmission was 0%. The relative transmissions are average values over seven measurements, and the error bars are the standard deviation. The error bars in composition are calculated through error propagation from the mass.

Figure 4. Spinodal (dashed line) curve for PS/P3HT blends in oxylene plotted as volume fraction of solvent (with respect to total solution volume) vs the mass fraction of P3HT (relative to the total polymer mass). The shaded region between the solid lines indicates the error in the calculated spinodal line, determined via error propagation from the interaction parameters. Time points indicate solvent volume fraction in the film during spin-coating at 1500 rpm, determined from a PS/P3HT drying curve in o-xylene (see Figure S2), and are spaced temporally by 2 s. The error bars are the error in the calculated volume from the film thickness measurements.

light through the sample to the transmitted light through oxylene in the cell. Digital photographs of all samples are shown in the Supporting Information (Figure S4). Solutions through which the background grid was visible were classified as transparent. These solutions had relative transmissions of 38% or higher. Starting with the highest concentration of solvent, the relative transmission (relative to transmission through pure o-xylene) at ϕo = 0.990 was ∼100%. As the o-xylene concentration decreased to ϕo = 0.983, the transmitted intensity decreased to 97%. Both the ϕo = 0.990 and ϕo = 0.983 solutions were transparent. A sharper decrease was seen between the ϕo = 0.983 and ϕo = 0.971 solutions (68% relative transmission) as well as the ϕo = 0.966 solution (38% relative transmission). Although ϕo = 0.971 and ϕo = 0.966 solutions were still optically transparent (Figure S4), they appeared darker in color than the bright orange ϕo = 0.983 solution. This sharp decrease in transmitted light intensity between ϕo = 0.983 and ϕo = 0.971 indicated the onset of phase separation in the polymer blend solution.66,67 Finally, at ϕo = 0.960 and ϕo = 0.950, the relative transmission was 0%. As expected from the transmission results, the ϕo = 0.960 and ϕo = 0.950 solutions were not optically transparent (Figure S4). The ϕo = 0.983 and ϕo = 0.971 phase separation threshold was in reasonable agreement with the theoretical phase diagram (see Figure 4), which predicted that phase separation should occur at ϕo = 0.978 for a 1:1 blend of PS:P3HT. From the experimentally determined onset of phase separation region, δP3HT ranged between 14.6 MPa 1/2 and 15.2 MPa 1/2 , corresponding to ϕo = 0.983 and ϕo = 0.971, respectively, which agreed with δP3HT determined via solvent vapor swelling (14.8 ± 0.2 MPa1/2). The results in Figures 4 and 5 illustrate the synergy between the experimental data and the calculated phase diagram,

volume, mi is the ratio of the molar volume of component i to o-xylene and χi−j is the Flory−Huggins interaction parameter between components i and j, in which o-xylene is written as “o” for simplicity. χo−j was determined through the application of eq 2 to our polymer solubility parameters for PS and P3HT (Table 2). The spinodal line was calculated by taking the determinant of the resulting 3 × 3 matrix of the derivatives of the partial molar Gibbs free energy expressions in eqs 4a−4c, as described elsewhere.64 The resulting spinodal line was converted from volume fraction of polymer to mass fraction of P3HT with respect to the total polymer mass using the literature values of the PS and P3HT densities (ρPS = 1.04−1.065 g/cm3, ρP3HT = 1.10 g/cm3 at 293 K).32,65 The polymer/polymer/solvent phase diagram (Figure 4) was asymmetric, as the degrees of polymerization for the two polymers differed by ∼35% (NPS = 163, NP3HT = 105), and the calculated o-xylene polymer interaction parameter for PS was much lower than that for P3HT (χo‑PS = 0.34, χo‑P3HT = 0.84 ± 0.06). To validate the spinodal line for the PS/P3HT/o-xylene ternary system, we measured the transmission of a HeNe laser (λ = 632.8 nm) through a 1:1 PS:P3HT blend in o-xylene in a 1 mm path length sample cell. We selected a HeNe laser, as the wavelength fell outside of the range over which P3HT absorbs strongly (400−600 nm). Furthermore, given that the solutions with the highest concentration of o-xylene had similar transmission intensity as pure o-xylene, P3HT did not absorb a significant amount of the incident light. Figure 5 shows the relative transmission of the polymer blend solutions, in which relative transmission was defined by the ratio of the transmitted 6538

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indicating the validity of the approach. Though we have used PS/P3HT blends in this work, the methods described herein are easily adapted to other systems of interest.

CONCLUSIONS We successfully employed a straightforward experimental setup to determine the solvent−polymer interaction parameters (χi−j) for PS and P3HT in THF and CHCl3 using the swelling behavior of polymer thin films in solvent vapor environments. From the solvent−polymer interaction parameters for P3HT in THF (χTHF−P3HT = 1.04 ± 0.04) and CHCl3 (χCHCl3−P3HT = 0.99 ± 0.01), we estimated the solubility parameter of P3HT (δP3HT = 14.8 ± 0.2 MPa1/2) using a much wider range of solvent compositions than reported previously in the literature. Using this value, combined with δPS and regular solution theory, the Flory−Huggins polymer−polymer interaction parameter between PS and P3HT (χPS−P3HT = 0.48 ± 0.06) was calculated at 23 °C. This polymer−polymer interaction parameter was employed to generate a polymer blend phase diagram as a function of solvent content. The resulting theoretical diagram was validated experimentally by measuring the relative transmission of PS/P3HT/o-xylene solutions made at solvent compositions near the spinodal line. These results indicated that onset of phase separation in a 1:1 PS:P3HT blend occurred between ϕo = 0.983 and ϕo = 0.971. In summary, we have demonstrated an approach to study solvent−polymer and polymer−polymer interaction parameters for PS and P3HT, which can be easily extended for use with other polymer/polymer combinations relevant to organic photovoltaics, composites, and other materials systems. ASSOCIATED CONTENT

S Supporting Information *

Uptake of solvent vapor by N2 gas stream in Figure S1; film thickness vs time during spin-casting measured by spectral reflectometry and constructive interference (stroboscopic illumination) in Figure S2; comparison of 17 kg/mol PS and 892 kg/mol PS swelling in CHCl3 in Figure S3; digital images of the 1:1 PS:P3HT blends in o-xylene from the transmission experiments in Figure S4. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] (E.M.F.); [email protected] (T.H.E.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge a research grant from the University of Delaware Research Foundation−Strategic Initiatives program for financial support. T.H.E. also thanks NSF CBET0930986 for partial support. J.A.E. and E.M.F. were partially supported by the U.S. Department of Energy, Office of Basic Energy Sciences, Division of Materials Sciences and Engineering, under Award DE-FG02-09ER46626. The authors also acknowledge Dr. Jim Elman (Filmetrics, Inc.) for assistance in fitting P3HT spectra with spectral reflectometry. Additionally, the authors thank Dr. M. Mackay and Dr. J. Swan for helpful discussions. 6539

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dx.doi.org/10.1021/ma400597j | Macromolecules 2013, 46, 6533−6540