Thermodynamic Features of Perfectly Crystalline Poly(3

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Thermodynamic Features of Perfectly Crystalline Poly(3hexylthiophene) Revealed through Studies of Imperfect Crystals Mina Alizadehaghdam,†,‡ Barbara Heck,§ Silvia Siegenführ,§ Farhang Abbasi,*,†,‡ and Günter Reiter*,§ †

Institute of Polymeric Materials and ‡Faculty of Polymer Engineering, Sahand University of Technology, Tabriz, Iran Physikalisches Institut, Albert-Ludwigs-Universität Freiburg, Hermann-Herder-Straße 3, 79104 Freiburg, Germany

§

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S Supporting Information *

ABSTRACT: A reliable value for the enthalpy of fusion of a perfect poly(3-hexylthiophene) (P3HT) crystal (ΔH°m) is still in doubt. In the published works, ΔH°m ranging from 33 to 50 J/g, obtained from measuring the dependence of the heat of fusion on crystallinity and crystal thickness, based on the Mandelkern relation, is in clear contradiction to ΔH°m = 99 J/g, resulting from measuring melting point depression of P3HT crystals in the polymer−diluent mixtures, based on the Flory relation. In this work, we satisfied a requisite of the Flory equation, ignored in the literature, by presenting a new conception as the “dry melting temperature”. We confirmed that the correct value for the melting temperature of the undiluted polymer to use in the equation is the “dry melting temperature” of the P3HT crystals prepared in the presence of the diluent and not the melting temperature of those prepared in the absence of the diluent. We also employed UV−vis spectroscopy along with a Franck−Condon analysis to determine crystallinity of the P3HT samples and revealed that the ΔH°m obtained by the Mandelkern approach (76 ± 5 J/g) could support that concluded from the Flory approach (74 ± 4 J/g).

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INTRODUCTION Poly(3-alkylthiophene)s (P3ATs) have become a model system for the research on conjugated polymers in terms of both fundamental and application-oriented studies. 1 In particular, regioregular poly(3-hexylthiophene) (rr-P3HT) has been widely studied because it provides excellent electrical properties with great solubility and processability due to the hexyl substituents.2−5 Unfortunately, important thermodynamic characteristics of rr-P3HT, such as enthalpy of fusion for a perfect P3HT crystal (ΔH°m), are still not accurately defined. Several groups of researchers have worked on clarifying the heat of fusion of a perfect P3HT crystal. Malik and Nandi6 measured the melting points of semicrystalline P3HT samples in diluent (acetophenone) mixtures as a function of acetophenone concentration. On the basis of Flory equation, ° = 99 J/g for P3HT. Balko et al.7 employed they obtained ΔHm wide-angle X-ray scattering (WAXS) to estimate the crystallinity of their P3HT samples. Then, the enthalpy of fusion was measured by differential scanning calorimetry (DSC) and divided by the crystallinity to give an average value ° = 33 J/g for a perfect P3HT crystal. At the same time, of ΔHm Koch and co-workers8 published results on low molecular weight homologues of P3HT (3HTn, n = 10−36). They extrapolated the enthalpy of fusion of crystallized samples of 3HTn to an infinite molar mass of P3HT to reach ΔHm ° = 39 J/ g for P3HT. Snyder et al.9 included finite crystal size effects in their calculations for crystals of P3HT of different thicknesses and molecular weights. Solid state nuclear magnetic resonance (NMR) and DSC were employed to deduce the crystallinity © XXXX American Chemical Society

and the enthalpy of fusion of their samples. Extrapolating the heat of fusion divided by the crystallinity to the infinite lamellar thickness led to ΔHm ° = 49 J/g for a perfect crystal. Lee and Dadmun10 determined the enthalpy of fusion as a function of the density for their P3HT samples. Knowing the density of a perfect P3HT crystal from the unit cell structure, extrapolation of the heat of fusion to this density yielded ΔH°m in a range from 37 to 50 J/g. The approaches used in the literature for obtaining values of ΔH°m for P3HT can be categorized into two classes: (i) measuring the melting temperature in polymer−diluent mixtures as a function of concentration 6 based on the Flory relation and (ii) investigating the dependence of the heat of fusion on crystallinity7,10,11 and crystal thickness8,9,12 based on the Mandelkern relation. Applying the first method led to ΔH°m = 99 J/g, while the second approach yielded far lower values of ΔHm ° in the range from 33 to 50 J/g for 100% crystalline defect-free P3HT. This discrepancy is the issue we address in this work. Working with the same polymer crystals is a central requisite of the Flory equation often neglected in the literature when applying this equation. We dried the P3HT crystals after preparing them in the presence of the diluent. The similar “dry melting temperatures” confirmed that the crystals prepared in different P3HT−diluent mixtures were the same. Commonly Received: November 2, 2018 Revised: February 8, 2019

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DOI: 10.1021/acs.macromol.8b02350 Macromolecules XXXX, XXX, XXX−XXX

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temperatures” of P3HT crystallites. To identify a baseline and to subtract it from the DSC curves, a heating run from 25 to 260 °C at a rate of 20 °C/min was performed with an empty sealed DSC pan. Small-Angle X-ray Scattering (SAXS). To provide samples for SAXS measurements, P3HT was crystallized in the DSC apparatus similar to the samples prepared for DSC measurements. Pans were opened, and the crystalline samples of P3HT were taken out. To accomplish random orientations, these samples were divided into small pieces with a sharp blade and then transferred to a sample holder for SAXS measurement. The overall sample mass was about 14 mg. The SAXS experiments were performed at room temperature with the aid of a Kratky camera attached to a conventional Cu Kα X-ray source having a wavelength λ = 0.1542 nm. The scattering vector is S = 2 sin θ/λ, with Bragg angle θ. Scattering curves were registered using a position-sensitive metal wire detector. The obtained slitsmeared data were deconvoluted using an algorithm developed by Strobl.15 During the measurements, the sample chamber was under vacuum. UV−Vis Absorption Spectroscopy. P3HT was dissolved in chloroform at a concentration of 20 mg/mL in an oil bath at 60 °C for 1 h. Films of a thickness of ca. 200 nm (measured by AFM) were made by spin-coating this solution onto glass substrates, cleaned previously in an ultrasonic bath at 50 °C for 15 min using ethanol. The spinning speed and time were 1000 rpm and 60 s, respectively. First, to erase memory effects, films were brought to 260 °C in a hot stage purged with nitrogen (Linkam Scientific Instruments, UK), and the temperature was controlled to a precision of about 0.1 °C. The sample temperature was subsequently taken to a chosen Tc at the maximum possible cooling rate of the Linkam hotstage (100 °C/min) where it remained there for 5 h to crystallize the polymer. Afterward, the temperature was lowered to the room temperature as fast as possible (at the rate of 100 °C/min). The value of Tc ranged from 150 to 200 °C (Table 2). In addition, two samples were prepared by cooling them very slowly (1 °C/min) and very rapidly (100 °C/min) from the molten state at 260 °C, labeled “slowly cooled” and “quenched” in Table 2. To identify contributions from ordered and disordered regions within the sample, UV−vis absorption spectra for the P3HT crystallized films were recorded over areas with a diameter of ca. 40 μm using a Zeiss Axio 100 microscope connected to a spectrometer (Ocean Optics USB 2000) via an optical fiber of 400 μm diameter. A halogen lamp was used as the light source, and the spectra were collected in the transmission mode using a 10× objective lens. Atomic Force Microscopy (AFM). AFM measurements on the samples prepared for UV−vis absorption spectroscopy were performed with a JPKBioMAT setup consisting of a NanoWizard AFM, the BioMAT base, and a Zeiss Axio Imager optical microscope. AFM images were measured in the tapping mode at ambient conditions. AFM cantilevers were made of silicon (type: PPP-NCLW) purchased from Nanosensors. The Si tips used had a resonance frequency of about 180 kHz and a constant force of about 30 N/m.

in the literature, the melting temperature of the undiluted polymer in the equation is replaced with a value obtained from heating of the polymer crystals prepared in the absence of the diluent.6,13,14 However, we revealed that for satisfying the requirement of Flory equation, it is necessary to replace the melting temperature of the undiluted polymer in the equation with the “dry melting temperature” of the crystals prepared in the presence of the diluent. In another approach, we employed UV−vis spectroscopy along with a Franck−Condon analysis in parallel to SAXS measurements to determine crystallinity and crystal size of P3HT samples and reached at the conclusion that approach (ii) can yield the same ΔH°m values as deduced from approach (i).

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EXPERIMENTAL DETAILS

Materials. Poly(3-hexylthiophene) (P3HT) was obtained from Merck Chemicals (Germany). The number-average molecular weight, M̅ n = 19 kg/mol, and dispersity, Đ = 1.67, were measured by gel permeation chromatography (GPC). The regioregularity of P3HT (= 93%) was determined using proton nuclear magnetic resonance (1H NMR). 3-Hexylthiophene was purchased from Sigma-Aldrich Co. The materials were used as received. Differential Scanning Calorimetry (DSC). P3HT semicrystalline samples were prepared using a differential scanning calorimeter (DSC), PerkinElmer Model DSC4, operated under a nitrogen atmosphere. Seven milligrams of the polymer was introduced into a DSC pan. The pan was sealed and heated to 260 °C (well above the nominal melting temperature of P3HT around 240 °C), with a heating rate of 20 °C/min. The sample was kept at 260 °C for 1 min to ensure complete melting of the polymer and to erase memory effects. After the sample temperature reached Tc at the maximum possible cooling rate accessible with our DSC apparatus (200 °C/ min), it remained at Tc for 5 h to crystallize the polymer. Afterward, the temperature was lowered to room temperature as fast as possible (at a rate of 200 °C/min). The temperature was then raised at a rate of 20 °C/min to record the peak of the melting curve identified as the melting temperature (Tm) and the area under this peak as the enthalpy of fusion (ΔHm). The value of Tc ranged from 150 to 200 °C (Table 2). In addition, two samples were prepared by cooling them very slowly (1 °C/min) and very rapidly (200 °C/min) from the molten state at 260 °C, labeled “slowly cooled” and “quenched” in Table 2. P3HT was also crystallized in its mixture with 3-hexylthiophene (3HT) as a diluent. First, the desired mass of 3HT ranging from 0 to 1.5 mg was introduced into a DSC pan using a suitable micropipet. Then, 4 mg of the polymer was added. The pan including the mixture was sealed and heated to 260 °C, at a heating rate of 20 °C/min, and kept there for 1 min to ensure complete melting of the polymer, resulting in a homogeneous P3HT-3HT solution. The sample temperature was then lowered to room temperature at a cooling rate of 2 °C/min to crystallize the polymer in the presence of the diluent. The crystallization peak, in the presence of the maximum amount of 3HT (1.5 mg), appeared at around 152 °C. For the lower diluent compositions, the crystallization peaks were at higher temperatures. Afterward, the temperature was raised at a rate of 20 °C/min to record the “wet melting temperatures” of P3HT crystallites. The “dry melting temperatures” of these crystallites were obtained in another similar experiment. After the P3HT−3HT mixture was cooled to room temperature, the sealed pan was opened and heated to 100 °C for 30 min to remove 3-HT by evaporation, far below the melting temperature of P3HT crystals. This temperature was also below the lowest crystallization peak of our samples (152 °C), supporting the assumption of negligible reorganization of our P3HT crystals during the diluent evaporation process. Weighing the DSC pan before and after heating confirmed the complete evaporation of the diluent. The dry P3HT crystalline film was subsequently transferred to a new DSC pan. This pan was sealed and heated at a rate of 20 °C/min to record the “dry melting

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RESULTS AND DISCUSSION Based on the Flory equation, the equilibrium heat of fusion can be derived from the melting point depression caused by adding a diluent.13,14,16 The addition of a diluent having volume fraction vd to a crystallizable polymer reduces its melting point from that of the undiluted polymer, Tm ° , to that of the polymer−diluent system, T°m′, according to eq 1. ij 1 1 yzz jj jj T ° ′ − T ° zzz m{ k m

νd =

VpR VdΔHm°

−

VpB νd ΔHm° Tm° ′

(1)

where R is the gas constant and ΔH°m is the heat of fusion per repeating unit of the crystallizable polymer. Vp and Vd are the partial molar volumes of the polymer repeating unit and the diluent, respectively, and they were considered to be the same. B

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Table 1. Melting Temperatures of P3HT Crystals Prepared in the Absence or Presence of Different Amounts of 3HT, Crystallized by Cooling the Samples from the Melt or Solution State (at 260 °C) to 25 °C at a Cooling Rate of (2 °C/min); Fifth and Sixth Columns Present the Modified Melting Points Obtained by a Linear Fitting of the Measured “Wet Melting Temperatures” in the Third Column When the Melting Point for 0% 3HT Is Considered To Be 241.3 and 236.8 °C, Respectively P3HT (mg)− 3HT (mg)

3HT vol fraction

melting temp measured in the presence of 3HT (wet melting temp) (°C)

4−0.0 4−0.5 4−0.8 4−1.0 4−1.2 4−1.5

0.00 0.13 0.19 0.23 0.26 0.31

218.3 207.7 200.8 194.6 186.3

melting temp measured after 3HT fitting values of the “wet melting fitting values of the “wet melting elimination from the crystallized structure temp” with 241.3 °C as the mp temp” with 236.8 °C as the mp of P3HT (dry melting temp) (°C) of the undiluted P3HT (°C) of the undiluted P3HT (°C) 236.8

241.422 218.417 207.260 200.672 194.661 186.569

241.3 241.5 241.3 241.3 241.2

B is a constant related to the temperature dependence of Flory’s interaction parameter (χ): χ = BVd/RT. Measuring the melting temperature of a semicrystalline polymer in polymer− diluent mixtures of different compositions and then plotting ° ′ should give a straight the left-hand side of eq 1 versus νd/Tm line whose intercept suggests the heat of fusion of the perfect crystalline polymer. Upon use of eq 1, one significant difficulty is that polymer crystallization in polymer−diluent mixtures, with different contents of diluent, should produce polymer crystals having the same lamellar thickness and perfection.14 This implies that the presence of the diluent or varying its content in the polymer−diluent mixture does not affect the crystallization process. However, this assumption cannot always be fulfilled as the presence of the diluent often affects the resulting crystal size (increase the lamellar thickness) caused, e.g., by a decrease of the crystallization rate.17 In addition, the Flory equation is based on the equilibrium state, which means that both T°m and Tm° ′ should be interpreted as the equilibrium melting temperatures, defined as melting temperature of a perfect crystal formed from a polymer of infinite molecular weight.14 However, in the case of polymers, this equilibrium is not expected to be reached. To get the experimental melting temperatures closer to the equilibrium value, usually slow cooling and heating rates are employed to crystallize and melt polymers.6 This can be an appropriate choice when the difference between experimentally achieved melting temperature of a semicrystalline polymer and the corresponding equilibrium value obtained by extrapolative methods is not large (0.05). Thus, a change in 1/l within the observed range does not lead to a statistically significant change in the ΔHm/xc values; the uncertainties are too large. Furthermore, analyzing the data by Snyder et al.9 carefully shown in Figure 5b reveals that within the large error bars we may also obtain a value of ΔH°m = 64 J/g. On the other hand, on the basis of our extensive DSC measurements, we could reduce the uncertainty in determining the melting point to better than 1 °C. Assuming error bars of ±1 °C for each measured data point shown in Figure 1, one-way ANOVA led

Figure 4. (a) Tapping mode AFM phase image and (b) absorbance spectrum taken on a P3HT film crystallized by cooling slowly at 1 °C/ min from the melt at 260 °C to room temperature (slowly cooled sample in Table 2) (circles). The solid curve shows the modified Franck−Condon fit to the spectrum consisting of three peaks (dashed curves): A0−0, A0−1, and A0−2 at 2.07, 2.24, and at 2.42 eV, respectively. See Table 2 for quantitative results of the other fitting parameters.

similarly crystallized P3HT film. Applying the Franck−Condon principle, the spectrum can be interpreted as the superposition of three distinct bands representing the low-energy absorbance vibronic peaks A0−0 (at 2.07 eV), A0−1 (at 2.24 eV), and A0−2 (at 2.42 eV) separated by about 175 meV, attributed to πstacked crystalline chains forming weakly interacting Haggregates, in accordance with the lamella structure seen in the AFM image. The remaining high-energy contributions to the spectrum arose from intrachain states of coiled (amorphous) chains.21−26 The fraction of the spectrum attributed to absorption from aggregates was determined using a modified Franck−Condon fit (eq 3) to the spectrum, put forward by Spano and co-workers:26 F

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depression by adding a diluent, are more easily fulfilled than measuring precise values of enthalpy of fusion and crystallinity, required for the application of eq 2. Thus, in general, the value of ΔH°m obtained through Flory relation (eq 1) is more reliable than the value of ΔH°m obtained through Mandelkern relation (eq 2). As we mentioned earlier, the Flory equation is based on the equilibrium state which means that both Tm ° and Tm °′ should be interpreted as the equilibrium melting temperatures. In our future work we will fulfill this requirement by applying the equation for perfect P3HT crystals and investigate its effect on the resulting ΔHm °.

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CONCLUSIONS By satisfying the requirement of the Flory equation, based on “working with the same polymer crystals”, we could present a correct application of this equation for poly(3-hexylthiophene) and obtained a value of 74 ± 4 J/g for the heat of fusion of 100% crystalline defect-free P3HT. In a complementary approach, we employed a Franck−Condon analysis of UV− vis spectra for the determination of the crystallinity of partially crystalline P3HT films. Extrapolating the enthalpy of fusion normalized by crystallinity to crystals of infinite lamellar thickness yielded ΔH m° = 76 ± 5 J/g for poly(3hexylthiophene), being in agreement with ΔHm° resulted from measuring melting point depression of P3HT crystals in the P3HT−diluent mixtures.

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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.8b02350. Figure S1: DSC heating curves, in the “dry” and “wet state”, for P3HT crystals already prepared in the presence of 13 vol % 3HT by cooling from the solution state (at 260 °C) to 25 °C at a cooling rate of (2 °C/ min); Figure S2: SAXS pattern of P3HT crystallized by cooling the sample slowly at 1 °C/min from the melt at 260 °C to room temperature (PDF)

Figure 5. (a) Enthalpy of fusion divided by crystallinity (ΔHm/xc) plotted versus the inverse crystal thickness (1/l) for the variously crystallized P3HT samples when L1 (squares) or L2 (circles) is considered as the thickness of crystalline layer (see Table 2). The value of the crystal thickness (l) used in eq 2 was calculated by l = L1or2/0.38, with 0.38 being the length of one repeating unit of P3HT in nm. The dashed lines represent linear fits to the experimental points of squares and circles. The error bars are shown by the dotted lines. (b) Enthalpy of fusion divided by crystallinity (ΔHm/xc) plotted versus the inverse crystal thickness (1/l) for data points extracted from the published work by Snyder and co-workers (squares with their dotted error bars);9 circles represent the squares moved within the error bars. The dashed lines represent linear fits to the squares and circles. The dotted lines continue the dashed lines to show the intercepts.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected].

to a P value of about zero (P < 10−8) and 0.03 (P < 0.05) in Figures 1 and 2a, respectively. In addition, determination of the melting temperature through the position of the maximum of the peak is largely independent of the baseline used in the analysis of the DSC curve. However, the determination of the enthalpy of fusion depends strongly on the correct identification of the baseline. As another advantage in comparison to eq 2, eq 1 does not require the knowledge of xc. This can be especially important when different methods for measuring the crystallinity, such as wide/small-angle X-ray scattering (WAXS/SAXS),7,12 nuclear magnetic resonance (NMR),9,11 density measurements,10 and UV absorption,24 lead to different values of xc for the same sample.27 Thus, the correctness of values of ΔH°m obtained by employing eq 2 (for example, ΔH°m = 49 J/g or ΔH°m = 76 J/g shown in Figure 5) has to be considered with some precaution. In summary, we believe that the requirements of eq 1, i.e., deriving the value for ΔH°m of P3HT from the melting point

ORCID

Farhang Abbasi: 0000-0001-9770-4255 Notes

The authors declare no competing financial interest.

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REFERENCES

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