Direct Calorimetric Observation of the Rigid ... - ACS Publications

Letter Cite This: J. Phys. Chem. Lett. 2018, 9, 990−995

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Direct Calorimetric Observation of the Rigid Amorphous Fraction in a Semiconducting Polymer Jaime Martín,*,†,‡ Natalie Stingelin,§ and Daniele Cangialosi*,∥,⊥ †

POLYMAT, University of the Basque Country UPV/EHU, Avenida de Tolosa 72, 20018 Donostia-San Sebastián, Spain Centre for Plastic Electronics and Department of Materials, Imperial College London, Exhibition Road, London, SW7 2AZ, United Kingdom § School of Materials Science & Engineering and School of Chemical & Biomolecular Engineering, Georgia Institute of Technology, 311 Ferst Drive, Atlanta, Georgia 30332, United States ∥ Centro de Física de Materiales (CSIC-UPV/EHU), Paseo Manuel de Lardizabal 5, 20018 San Sebastián, Spain ⊥ Donostia International Physics Center (DIPC), Paseo Manuel de Lardizabal 4, 20018, San Sebastián, Spain ‡

S Supporting Information *

ABSTRACT: The performance of polymeric semiconductors is profoundly affected by the thermodynamic state of its crystalline and amorphous fractions and how they affect the optoelectronic properties. While intense research has been conducted on the crystalline features, fundamental understanding of the amorphous fraction(s) is still lacking. Here, we employ fast scanning calorimetry to provide insights on the glass transition of the archetypal conjugated polymer poly(3-hexylthiophene) (P3HT). According to the conceptual definition of the glass transition temperature (Tg), that is, the temperature marking the crossover from the melt in metastable equilibrium to the nonequilibrium glass, an enthalpy relaxation should be observed by calorimetry when the glass is aged below Tg. Thus, we are able to identify the enthalpy relaxations of mobile and rigid amorphous fractions (MAF and RAF, respectively) of P3HT and to determine their respective Tg. Our work moreover highlights that the RAF should be included in structural models when establishing structure/property interrelationships of polymer semiconductors.

A

circuitry, organic light-emitting diodes, and thermoelectric generators.10 The properties required for such applications can be profoundly affected by the degree of crystallinity and the thermodynamic state of the amorphous fractions. Intense research on the characterization of the glass transition of both the MAF6,11−18 and RAF;6,19 and on the effect of the thermodynamic state of the amorphous fractions, attained after aging in the glassy state,20 on charge transport21 has recently been pursued. The RAF’s calorimetric signature for poly(3-(2′-ethyl)hexylthiophene) was, for instance, identified when aging this specific semiconducting polymer above the Tg of the MAF. The presence of an endothermic overshoot in the specific heat was, thereby, attributed to the enthalpy relaxation of the glassy RAF.19 Similar results had previously been obtained for semicrystalline poly(phenyl oxide),22 poly(phenylene sulfide), and poly(ethylene terephthalate).23 In this Letter, we present a systematic study aiming at identifying the glass transition of both the MAF and the RAF. For this purpose, we recall the fundamental definition of the glass transition temperature (Tg), that is, the temperature at

class of materials of utmost fundamental and applied interest is that of semicrystalline polymers. These systems exhibit a complex structure composed of a crystalline fraction, as well as a mobile and rigid amorphous fractions (MAF and RAF, respectively).1 The latter is generally attributed to regions where chains are constrained at the boundary with the crystalline fraction.2 Its existence has been indirectly inferred by differential scanning calorimetry (DSC). For this purpose, two methods have so far been employed. One is based on the fact that the specific heat of various semicrystalline polymers between the glass transition (Tg) and the melting (Tm) temperatures cannot be described by a two-fraction model; rather the presence of a RAF must be taken into account.1,3−7 A second method relies on the fact that the sum of the crystalline fraction and the MAFobtained from the melting endotherm and the step in the specific heat at the glass transition, respectivelynever equals unity and, thereby, the remaining part of the material must be attributed to the RAF.6,8,9 Both methods provide evidence of the existence of the RAF, though identifying its precise Tg has so far been elusive. An important subclass of semicrystalline polymers is that of semiconducting conjugated polymers. The presence of delocalized π orbitals makes them attractive systems in applications ranging from solar cells to thin-film electronic © 2018 American Chemical Society

Received: November 23, 2017 Accepted: February 8, 2018 Published: February 8, 2018 990

DOI: 10.1021/acs.jpclett.7b03110 J. Phys. Chem. Lett. 2018, 9, 990−995

Letter

The Journal of Physical Chemistry Letters which the crossover from the equilibrium supercooled melt to the nonequilibrium glass takes place.24 Given this definition, a system residing in the glassy state will exhibit an evolution of its thermodynamic state toward lower free energiesa phenomenon known as physical aging.25 By contrast, no timedependent evolution of the thermodynamic state will be observed in the equilibrium supercooled melt. Here, we employ fast scanning calorimetry (FSC) to characterize the presence (or not) of physical aging effects, in terms of enthalpy relaxation, using 2-μm thick regioregular P3HT films. Such an approach has been successfully exploited to characterize the glass transition of thin polystyrene films.26,27 We investigate P3HT samples that were first quenched from the melt at 4000 K/s and subsequently aged for 30 min using a wide temperature range below Tm. Our results show that, after such an aging protocol, basically two distributions in the relaxed enthalpy are found: a low- and high-temperature distribution. These can be readily attributed to the glass transition of the MAF and RAF, respectively. We start our discussions with the heat flow rate scans for semicrystalline P3HT films obtained by cooling from the melt at 4000 K/s and then being aged for 30 min at the indicated temperatures (Figure 1, black curves). The corresponding reference curves, that is, those related to samples immediately heated after cooling without undergoing an aging step, are shown in the same figure (red curves). As can be observed, physical aging generally results in the appearance of an endothermic overshoot. The temperature range of this overshoot generally increases with increasing aging temperature. A detailed view of the effect of physical aging on the calorimetric response is provided in Figure 2b, where the heat flow rate of an aged sample in excess of that measured for an unaged reference is shown. From these we can identify the presence of three regimes (Figures 1 and 2). In each of these regimes, an increase followed by a decrease in the excess heat flow rate can be observed. The first regime is found for aging temperatures of −65 to 20 °C, exhibiting the largest excess specific heat at an aging temperature of about 5 °C. An intermediate range between 20 and 80 °C can be observed with a maximum in the excess heat flow rate at about 60 °C. Finally, above 80 °C, a steady increase of the excess heat flow rate is recorded. The endothermic overshoot in the high aging temperature regime is located in a range between 160 and 220 °C. This is the region where the crystalline moieties of P3HT are generally found to melt.28 Hence, we assign this observed overshoot (which is not observed in the reference samples) to the melting of crystals that have been generated during cold crystallization that occurs at these specific aging temperatures. In the low temperature regime, the endothermic overshoot is only observed when aging is carried out in a temperature range around the Tg of the MAF.11,12,17,29 We thus propose that this observed overshoot is associated with the enthalpy relaxation of the glassy MAF. To address the origin of the overshoot observed when aging is performed in the intermediate temperature regime (20−80 °C), we determined the reversing part of the specific heat by performing step-response experiments (see Supporting Information for details).30−32 Analysis of such experiments consists in Fourier transforming the heat flow rate and the instantaneous heating rate in each period, that is, a temperature step and an isotherm. The ratio of these two quantities delivers

Figure 1. Heat flow rate scans for P3HT samples aged for 30 min at temperatures indicated after initial quench at 4000 K/s (black), and the reference (red). The endothermic overshoots due to the relaxation of the MAF and the RAF are shadowed in red and blue, respectively. The area shadowed in gray color corresponds to the excess of heat flow rate originated from the melting of crystals that are generated during the aging step.

the complex heat capacity: C*p = Cp′ + iCp′′; and, therefore, the reversing specific heat: Cp,rev = Cp′2 + Cp″2 . The latter is shown in Figure 3a for the unaged reference and a sample aged at 65 °C for 60 min, where the endothermic overshoot is largest (see Figure 2b). As can be observed, beside a slight shift to higher temperatures of Cp,rev, which we attribute to the decrease in the MAF segmental dynamics, no effects of aging are observed. This result indicates that no reversible melting is observed in the time scale relevant to the step-response experiments (1 Hz). To provide further insight into the origin of the overshoot observed above the MAF Tg, we, moreover, determined the amount of the relaxed enthalpy after aging at 65 °C using different aging times. This was determined integrating the heat flow rate curves presented in the Supporting Information. Fitting the evolution of the enthalpy with time, shown in Figure 3b, using the Kohlrausch−Williams−Watts law:33ΔH = a − (a − b) exp(−t/τ)β, we can deduce a stretching exponent β = 0.34. This is typical of glass dynamics and can not be explained with with an Avrami-like crystallization kinetics [Avrami 991


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The Journal of Physical Chemistry Letters

Figure 2. (a) Schematic illustration of a three-fractions structure of semicrytalline P3HT. (b) Excess heat flow rate, that is, the difference between the heat flow rate of samples aged for 30 min at the indicated temperatures and that of the reference, which was not aged, as a function of temperature.

Figure 3. (a) Reversing specific heat as a function of temperature for a P3HT sample aged at the indicated conditions and the corresponding unaged reference sample. (b) Relaxed enthalpy at 65 °C as a function of aging time. The line is a fit through the KWW function with β = 0.34. (c) UV−vis absorption spectra of samples aged for the indicated aging times at 65 °C. The insets are enlargements of portions of the main panel. (d) Evolution of the UV−vis absorption at 594 and 579 nm (i.e., the 0−0 and 0−1 transitions) during aging at 65 and 110 °C, respectively.

spectral shape (0−0 transition absorption) nor the maximum absorption peak’s position, suggesting that, at these aging temperatures/times (65 °C for 60 min), no molecular ordering occurs. We note, however, that, when the aging is performed at 110 °C, a slight change in the 0−0 absorption intensity (I) is observed (see Supporting Information for the UV−vis spectra of samples annealed at 110 °C). While not drastic, this evolution can be readily followed when plotting the increase of 0−0 absorption intensity as 1 − (It=t’/It=0) against the aging time for isotherms of 65 °C (red circles) and 110 °C (blue circles) isotherms (see Figure 3d). Combining these observations, that is, the absence of any significant change in both the reversing specific heat and the UV−vis 0−0 absorption intensity upon aging at 65 °C, as well as the stretching exponent β = 0.34 deduced from the evolution of relaxed enthalpy with time, we conclude that the observed overshoot in the interim temperature regime is not dominated (if caused at all) by crystallization and remelting effects, but

exponents are generally found to be larger than one in homogeneously nucleated crystallization. Highly confined polymers have been shown to exhibit Avrami exponents smaller than the unity, but always larger than 0.5.34,35], supporting our view that the observed overshoot originates from one of the amorphous fractions. Finally, in order to shed further light on the nature of the overshoot and scrutinize whether this feature does not originate from melting/recrystallization/melting processes of imperfect crystals, UV−vis absorption spectra were recorded during aging for various periods of times at 65 °C (Figure 3c). We thereby exploited the fact that an increase of the degree of molecular order in P3HTe.g., due to aggregation/crystallization usually increases the 0−0 transition absorption feature at 579 nm because of enhanced interchain interactions. In some cases, a shift of the maximum absorption peak can also be found caused by an increase in conjugation length resulting from a backbone planarization. We do not find any change in the 992


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other studies using different techniques.12,14,15,18 A slight discrepancy can be expected; it has to be considered, for instance, that here the Tg is determined for much shorter time scales (4000 K/s as compared to 10 rad/s in ref 17). Furthermore, our metrics to define the Tg, based on the onset of aging effects, provides an upper bound of the Tg,26,27 whereas the Tg deduced in other studies rather represents an intermediate value. Finally, we would like to stress that the behavior of the excess of heat flow plotted in Figure 2 indicates that the relaxed enthalpy slowly decreases toward zero when increasing the aging temperature. This points toward a large heterogeneity of the MAF glass transitiona fact that can be expected to inevitably result in significant differences between the Tg obtained from either rheological experiments17 or the half step of the specific heat step,12,14,15 and the Tg onset used here. In order to determine the Tg of the RAF, we used a similar procedure. In this case, we extrapolated the high temperature flank of the RAF relaxed enthalpy below the onset of cold crystallization above 80 °C. Figure 4 illustrates that from such an extrapolation we can obtain an onset value of the Tg of the RAF of ∼180 °C. Given that the melting temperature of the crystalline P3HT moieties starts at temperatures around 200 °C (Figure 1), this implies that devitrification of the glassy RAF occurs before the melting is completed. Interestingly, the devitrification of the RAF and the cold crystallization seem to be interconnected in the case of P3HT, similarly to what has been previously proposed for poly(3-hydroxy butirate).41 Our results thus suggest that the formation of new P3HT crystals form solely when aging is conducted at temperatures above 80 °C, that is, when the RAF begins to become mobile. This implies that it is the segmental mobility of the RAF that is relevant for the further development of P3HT crystals via cold crystallization, while the impact of the MAF mobility for such cold crystallization processes is relatively minor. This may provide new insights into the design of efficient aging protocols for semiconducting polymer thin films for device applications. In summary, the present work aimed to identify the glass transition of the MAF and the RAF of the archetypal conjugated polymer, P3HT. To do so, we relied on the basic definition of glass transition, that is, the crossover from the melt in metastable equilibrium to the nonequilibrium glass. Such a definition implies that below Tg a system undergoes physical aging and, thereby, exhibits enthalpy relaxation. Our results show that, after performing aging experiments over a wide range of temperatures at fixed aging times, the relaxed enthalpy exhibits two main distributions. The low temperature distribution is identified as a manifestation of the glass transition of the MAF. At higher temperatures, enthalpy relaxation originating from aging of the RAF dominates. Finally, extrapolating the high temperature flanks of the distributions of relaxed enthalpies allows us to provide an estimation of the Tg of the MAF and the RAF. Our data clearly indicate that a RAF component is present in P3HT thin films and, thus, likely in other semiconducting polymers. This conclusion highlights the need to consider this often “forgotten” fraction in structural models to understand the optoelectronic properties of these interesting class of materials and, in general, to establish meaningful structure/processing/property interrelationships.

rather results from the enthalpy relaxation of the RAF, i.e., the amorphous regions constrained at the boundary with crystals, as illustrated in Figure 2a. We thus went on and performed a more quantitative analysis of the aging behavior of the MAF and the RAF. This requires the knowledge of the relaxed enthalpy during aging. This was calculated from the excess heat flow rates shown in Figure 2 employing the following equation: ΔH(Ta , ta) =

T ≫ Tg

∫T ≪T

(Cpa(T ) − Cpu(T )) dT

g

Cap(T)

(1)

Cup(T)

In this equation, and are the specific heats of an aged and the reference unaged samples, respectively. Their difference equals the excess specific heat, which can be calculated from the excess heat flow rate knowing the sample mass (see Supporting Information).27 Figure 4 shows the amount of relaxed enthalpy after aging at different temperatures for 30 min. In line with the data

Figure 4. Relaxed enthalpy as a function of aging temperature after aging for 30 min. The dashed lines represent a linear extrapolation of the high frequency flanks of the distributions of relaxed enthalpies.

presented in Figure 2, two distributions of the temperature dependence of the relaxed enthalpy can be deduced. Furthermore, the low temperature distribution, associated with the enthalpy relaxation toward equilibrium of the MAF, exhibits a shoulder at aging temperatures between −40 and −20 °C. This observation indicates that the glassy MAF exhibits two mechanisms of equilibrium recovery: (i) a standard mechanism at temperatures close to Tg of the MAF, responsible for the main peak in the relaxed enthalpy, and (ii) another, fast mechanism of recovery. Such a picture is consistent with previous observations in amorphous36,37 and semicrystalline polymers,9 as well as other glasses.38−40 Data of Figure 4 can be exploited to determine the onset of aging, which provides a definition of the Tg of both the MAF and the RAF. [Several studies use the convention Tg,on for the onset of devitrification on heating. In such a case, our definition of Tg would rather be an endset of the Tg. Here, we use the convention Tg,on to specify that this corresponds to the onset of nonequilibrium effects.] For this, we extrapolated the high temperature flank of the relaxed enthalpy to zero. It is thereby important to note that, for identifying the Tg of the MAF, one has to take into account that heat effects due to the aging of the RAF overlap with the amount of relaxed enthalpy above 20 °C. By doing so, we deduce a Tg of ∼50 °C for the MAF. This seems to deviate slightly from values reported, e.g., by Xie et al.,17 who found a value of 22 °C by oscillatory shear rheometry. Similar values to the latter have been reported in

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MATERIALS AND METHODS We used a P3HT of a weight-average molecular weight of 135 kg/mol, a dispersity of 1.6, and a regioregularity of 99.9%. This 993



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is a model system especially suitable for the purpose of our study, since it can be expected to form a semicrystalline fraction morphology with crystalline moieties being interconnected with mobile amorphous fractions. It was prepared by a Grignard metathesis polymerization at 55 °C with a 0.15 mol catalyst loading of Ni(dppp)Cl2, and purified by Soxhlet extraction with methanol followed by extraction with chloroform.42 Weightaverage molecular weight and dispersity were determined by size-exclusion chromatography (SEC) using an Agilent 1200 series GPC-SEC running chlorobenzene at 80 °C and fitted with two PL gel mixed-B columns in series, calibrated against polystyrene standards. We must note that because the rigidity of the P3HT backbone compared to that of PS,43,44 we expect that our molecular weight value is somewhat overestimated. This, however, does not change any interpretation as we are in a molecular weight regime well above the onset of entanglements. Regioregularity was determined by integration of the α-methylene region in the 1H NMR spectrum. FSC was carried out by means of a Mettler Toledo Flash DSC 1 with an intracooler, allowing temperature control between −90 and 450 °C, and nitrogen purge. P3HT films were deposited directly on the backside of the chip for FSC by spin coating (500 rpm, 60 s) from 10 mg/mL solutions in chloroform onto the Flash DSC chip sensors. The thermal protocol essentially consisted in recording heat flow rates of the sample aged at a given temperature and the corresponding reference, as shown in the Supporting Information. Moreover, a step response protocol, based on a step of 2 °C at 200 K/s and an isotherm of 0.1 s, was used to determine the reversing specific heat. Finally, isothermal aging at 65 °C was carried out at different aging times between 1 and 20 000 s in order to assess the time dependence of the enthalpy relaxation processes. These protocols are described in detail in the Supporting Information. For UV−vis spectroscopy, P3HT films were spin coated (500 rpm, 60 s) on microscopy glass slides and positioned in a microscope hot stage (Linkam Scientific Instruments, Ltd.). The system was then placed under the focus of a bright-field optical microscope (Zeiss Axio Scope with 10X objective and tungsten-halogen bulb). The light absorbed by the P3HT film was monitored with a fiber optic spectrometer (USB2000+, Ocean Optics), which was aligned to the system as a light collector. Subsequently, absorption spectra were recorded over the range of 400−700 nm every 1 s while the temperature was kept at 65 and 110 °C. To assess whether the P3HT molecularly orders during aging at either 65 and 110 °C agings, we thereby followed the time dependence of the absorption intensity of the 0−0 and 0−1 transitions (at 594 and 579 nm, respectively).

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Letter

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]; Phone: +34 943 018 959. *E-mail: [email protected]; Phone: +34 943 018 806. ORCID

Jaime Martín: 0000-0002-9669-7273 Natalie Stingelin: 0000-0002-1414-4545 Daniele Cangialosi: 0000-0002-5782-7725 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS J.M. acknowledges support from the Diputación de Guipúzcoa under the programme Fellow Gipuzkoa. Likewise, J.M. thanks MEC for the Ramón y Cajal contract and the Ikerbasque Foundation for the Ikerbasque Research Fellow program. Financial support from Fundación Iberdrola (Ayudas a la Investigación en Energı ́a y Medio Ambiente 2017) is also acknowledged. D.C. acknowledges the University of the Basque Country and Basque Country Government (Ref. No. IT-65413 (GV)), Depto. Educación, Universidades e investigación; and Spanish Government (Grant No. MAT2015-63704-P, (MINECO/FEDER, UE)) for their financial support. Authors thank John de Mello, James Bannock, and Walter Barnaby for the synthesis of the P3HT.

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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.jpclett.7b03110. Thermal protocols employed to determine the amount of relaxed enthalpy and the reversing specific heat; FSC experiments after annealing at 65 °C for different times; UV−vis experiments for annealing at 65 and 110 °C for different aging times (PDF) 994


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