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Sep 4, 2018 - Modular Synthesis and Structure Analysis of P3HT‑b‑PPBI Donor−. Acceptor ... Experimental Polymer Physics Group, Martin Luther Uni...
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Modular Synthesis and Structure Analysis of P3HT‑b‑PPBI Donor− Acceptor Diblock Copolymers C. David Heinrich,† Matthias Fischer,§ Thomas Thurn-Albrecht,*,§ and Mukundan Thelakkat*,†,‡ Applied Functional Polymers, Macromolecular Chemistry I, and ‡Bavarian Polymer Institute, University of Bayreuth, Universitätsstr. 30, 95440 Bayreuth, Germany § Experimental Polymer Physics Group, Martin Luther University Halle-Wittenberg, Von-Danckelmann-Platz 3, 06120 Halle, Germany Macromolecules Downloaded from pubs.acs.org by KAOHSIUNG MEDICAL UNIV on 09/04/18. For personal use only.

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ABSTRACT: While donor−acceptor block copolymers (BCPs) are widely discussed to be optimal model materials for studying the morphology-dependent processes in organic photovoltaics, the observation of the desired microphaseseparated structures is rare. We present a novel modular approach for the synthesis of donor−acceptor block copolymers offering a fast and flexible way for chemical modifications leading to microphase separation accompanied by confined crystallization of the individual blocks. Two donor−acceptor BCPs of poly(3-hexylthiophene)-b-polyperylene bisimide (P3HT-b-PPBI 1 and 2) were synthesized incorporating P3HT as a donor and a polystyrene with two different pendant perylene bisimides (PBI-N3 1 and 2) as acceptor blocks to show the flexibility of the synthetic approach. The synthesis combines Kumada catalyst transfer polymerization (KCTP), controlled radical polymerization, and click chemistry to obtain highly comparable polymers via a modular approach. We synthesized poly(3-hexylthiophene) (P3HT) with a high molecular weight (Mn,SEC = 18300 g mol−1), in a controlled manner, introduced a chain transfer end group by click chemistry to form a macroinitiator, and subsequently polymerized propargyloxystyrene by sequential polymerization. In a postpolymerization click reaction, the polystyrene block was quantitatively grafted with two different PBI acceptor units. We obtained diblock copolymers with 70 wt % of the PBI block and 30 wt % P3HT. The BCPs were characterized in bulk by temperature-dependent small- and wide-angle X-ray scattering (SAXS/WAXS) in combination with differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). The same materials were also characterized in thin films by grazing incidence smallangle X-ray scattering (GISAXS) and atomic force microscopy (AFM). The attachment of the PBI units to the PS backbone was found to slow down the molecular dynamics, evidenced by an increase of the glass transition temperature (Tg). Despite the high Tg, the BCP morphology is dominated by microphase separation in the molten state. While the BCPs adopt a cylindrical morphology, the range of order is limited by the slow molecular dynamics. Subsequent crystallization of the individual blocks during cooling results in confined crystallization within the cylindrical, microphase-separated morphology. The results emphasize the importance of molecular dynamics for the formation of a well-ordered microphase-separated BCP morphology.

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of P3HT in the range of around Mn,MALDI = 12000 g mol−1 exhibit optimum material properties in terms of a high charge carrier mobility in organic field transistors (OFET)10 as well as in space charge limited current (SCLC) devices.11 The first synthesis of acceptor polymers with pendant perylene bisimide side chains (PPerAcr) by a direct CRP of acrylate-functionalized PBI monomer was reported in 2004 by Lindner et al.12 Bringing P3HT and the PPerAcr polymers in one block copolymer gives access to fully functionalized donor−acceptor BCPs.13 The equilibrium microstructure of nanoscale phaseseparated donor−acceptor BCPs with perpendicular alignment with respect to a substrate has been proposed to be an ideal

INTRODUCTION The combination of the controlled Kumada catalyst transfer polymerization (KTCP) of 3-hexylthiophene, controlled radical polymerization (CRP) of easily available monomers such as propargyloxystyrol, and click chemistry of suitable semiconductor functionalities via copper-catalyzed azide− alkyne cycloaddition (CuAAC) opens a new versatile pathway toward defined donor−acceptor block copolymers (BCPs).1,2 P3HT is one of the most studied conjugated polymers that can be polymerized in a controlled manner by the KTCP. McCullough et al.3,4 and Yokozawa et al.5,6 independently reported the controlled synthesis of P3HT. This method allows the synthesis of polymers with controlled end groups aside from being able to adjust the appropriate molecular weight for the intended application.7−9 In the field of organic electronics and organic solar cells absolute molecular weights © XXXX American Chemical Society

Received: June 21, 2018 Revised: August 13, 2018

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DOI: 10.1021/acs.macromol.8b01301 Macromolecules XXXX, XXX, XXX−XXX

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Macromolecules model system for organic photovoltaics (OPV).14 Because of the small structure sizes, such a material should provide enough donor−acceptor interfaces for charge separation as well as optimal pathways for charge carrier transport. Block copolymer microphase separation is well understood for amorphous blocks. The corresponding phase diagram in general depends on the incompatibility χN and the volume fraction.15 If crystalline or liquid crystalline blocks are involved, the self-assembly behavior becomes more complex since there is an additional competition between classical microphase separation and crystallization. Depending on the relative positions of the order−disorder transition temperature (TODT) and the crystallization temperature (Tc), structure formation can be induced either by microphase separation or by crystallization. If crystallization starts from a microphaseseparated structure, the existing nanostructure can either remain intact (confined crystallization) or be destroyed (breakout crystallization).16 While the microphase separation in numerous BCPs of conjugated poly(alkylthiophene)s with an electronically inactive second block were reported,17−21 reports on donor− acceptor polymers with well-ordered microphase-separated structures are rare.22−25 In previous publications we explored microstructure formation for several donor−acceptor BCPs containing a crystallizable donor block and a liquid crystalline acceptor block. Lohwasser et al. reported the synthesis of donor−acceptor BCPs, P3HT-b-PPerAcrs, which showed wellordered lamellar and cylindrical microstructures as expected for the respective volume fractions.2 Lohwasser et al. pointed out the importance of the incompatibility factor χN for these systems.2 If χN and the molecular weight were high enough, microphase separation in the melt with subsequent confined crystallization during cooling was observed for a series of donor−acceptor BCPs consisting of P3HT and pendant PBIs attached to a polyacrylate backbone (cf. Figure 1a). In a later study the additional importance of chain mobility became clear.24 In this case the acceptor block consisted of pendant phenyl-C61-butyric acid methyl ester (PCBM) attached to a polystyrene backbone. This combination led to a strong increase of the glass transition temperature (Tg) of the acceptor block, which limited the microphase separation and the formation of long-range order (cf. Figure 1b).24,26 In our earlier report, for the synthesis of P3HT-b-PerAcr, a high molecular weight P3HT with alkyne end group was converted to a macroinitiator for NMRP via CuAAC click chemistry. This initiator was then used to directly synthesize the second block using PBI-acrylate monomers. The synthesis and direct polymerization of the PBI-acrylate monomer are not trivial. On the other hand, we have already shown that different kinds of PBI pendants can be clicked to a polymer backbone (polyacrylate or polystyrene) using the CuAAC in combination with a controlled radical polymerization, giving easier access to well-defined PBI-pendant homopolymers.1,28 Tao et al. demonstrated this click concept for obtaining a donor− acceptor block copolymer by multistep postpolymerization reactions.29 Lang et al. compared this click concept with direct polymerization of PBI-functionalized acrylate monomers for obtaining acceptor polymers.1 In addition, the influence of the spacer length between the perylene bisimide core and the PS backbone and the influence of new hydrophilic swallow tails on the thermal properties of the grafted polymers were studied.28 The hydrophilic PPBIs were found to be amorphous materials, and they also exhibited surprisingly high electron mobilities in

Figure 1. Schematic illustration of the effect of the glass transition temperature on self-assembly of a block copolymer with high Tg in one block. The gray color schematically indicates the temperature range with low molecular mobility of the acceptor block. (i) Above the order−disorder transition temperature (TODT), the BCP forms a disordered melt. (ii) While in case (a) the chain mobility is high in the relevant temperature range, so that a well-ordered microphaseseparated state forms below TODT, microphase separation and the formation of long-range order are limited by the high Tg in case (b). (iii) Below Tc crystallization takes place. It is confined for large segregation strengths and for high Tg of one block. (For simplicity, it is assumed in (a) that the crystallization temperature is the same for both blocks.)

SCLC devices.30 Therefore, it is very interesting to incorporate such hydrophilic pendant blocks into BCPs and to study their microphase separation in comparison to hydrophobic analogues. In this study, on the basis of the results above, we have synthesized two comparable donor−acceptor BCPs with differently substituted PBI acceptor units by a modular approach. The PBIs are selected in such a manner that one carries the conventional hydrophobic alkyl swallow tail substituents, whereas the second one has flexible oligoethylene glycol (OEG) substituents. We use a combination of temperature-dependent small- and wide-angle X-ray scattering (SAXS/WAXS), grazing incidence small-angle X-ray scattering, atomic force microscopy, and transmission electron microscopy (GISAXS, AFM, and TEM) to investigate the structure formation in BCPs with pendant PBIs attached to a polystyrene backbone. We expect that due to the lower Tg of the acceptor block, as compared to the above-mentioned PCBM-containing polymer, microphase separation will again take place unhindered by molecular mobility.26,28 Furthermore, the effect of different solubilizing groups attached to the PBI unit is investigated. B

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to a published procedure.8 We have earlier shown that the distribution and nature of end groups of an alkyne-functionalized P3HT depend on the way of quenching the reaction·9 After quenching with methanol, it is reasonable to conclude that the formation of diethynylated species is negligibly low. The absolute molecular weight of P3HT-Alkyne was determined by MALDI-TOF to be 13300 g mol−1, which corresponds to a degree of polymerization N of 80 (Table 1 and Figure S1b). P3HT-Alkyne was then converted to the macroinitiator (P3HT-RAFT) for RAFT polymerization via CuAAC with an azide-functionalized RAFT agent. The successful functionalization with the RAFT end group can be detected by 1H NMR (Figure 2a). The signal of the alkyne proton at 3.50 ppm (2) cannot be detected anymore after the click reaction, and new signals that can be assigned to the RAFT end group (a−d) appear instead. We therefore assume that conversion from alkyne to RAFT end group was nearly quantitative. The third step toward the fully functionalized BCPs was the polymerization of the second block. The second block was aimed to have about 30 repeating units to synthesize donor− acceptor BCPs with ∼70 wt % non-P3HT block. The RAFT polymerization leading to the block copolymer P3HT-bPTMSPOS was optimized to achieve control of the N of the second block. At a [monomer]:[P3HT-RAFT] ratio of 440:1 the polymerization gave reproducible results. The propargyloxystyrene monomer was polymerized with P3HT-RAFT initiator until a conversion of 6.7% was reached. With the knowledge of the [monomer]:[P3HT-RAFT] ratio and the conversion, a N of 30 was expected. This value is in excellent agreement with the one that can be obtained by calculating the PTMSPOS content via NMR. The molar ratio of P3HT:PTMSPOS is 2.8:1, and the N of the second block should therefore be 29 under the assumption that every P3HT chain was bearing an alkyne end group that was also quantitatively converted to the RAFT group and started a new block. Also, the small fraction of coupled P3HT (see Figure S1a) is neglected in this calculation. This explain why the SEC curve of P3HT-b-PTMSPOS is not only shifted to higher molecular weights in comparison to the macroinitiator P3HT-RAFT but also shows a significant broadening of the distribution (Figure 2b). The block copolymer P3HT-bPTMSPOS had to be deprotected before the final grafting step. The trimethylsilyl group could be quantitatively cleaved at mild conditions with TBAF. The successful reaction could be monitored by 1H NMR (Figure S2a) and SEC (Figure 2b). This polymer was the precursor polymer (P3HT-b-PPOS) for the grafting with the acceptor PBI units.

RESULTS AND DISCUSSION Synthesis. Donor−acceptor BCPs were synthesized via a modular approach. A precursor polymer P3HT-b-PPOS was synthesized, and the second block was subsequently grafted with two different azide-functionalized PBI molecules via a CuAAC click reaction (see Scheme 1). The perylene bisimides Scheme 1. Synthesis Donor−Acceptors BCPs via CuAAC Click Chemistrya

a

The precursor polymer P3HT-b-PPOS is coupled with two different perylene bisimides (PBI-N3 1 and 2).

were chosen due to the very promising electronic/electrical properties of homopolymers with these side chains (PPBI).30 Likewise, it is possible to investigate the influence of the different acceptor units PBI-N3 1 and 2 on the grafting-to approach as well as on structure formation. Our objective was to obtain fully functionalized BCPs with a weight content of the acceptor block of around 70 wt % to obtain a diblock copolymer with cylindrical microphase separation. At first, an alkyne-functionalized P3HT polymer was synthesized via Kumada catalyst transfer polymerization (KCTP) and converted to a macroinitiator by attaching a chain transfer agent suitable for the RAFT polymerization of the TMS-protected POS. This protected precursor block copolymer P3HT-b-PTMSPOS was subsequently deprotected to get P3HT-b-PPOS and grafted with the two different acceptor units, PBI-N3 1 and PBI-N3 2. The synthesis of the precursor BCPs is shown schematically in a three-step synthesis in Scheme 2. Alkyne-functionalized P3HT (P3HTAlkyne) with high molecular weight was synthesized according

Scheme 2. Synthesis of the Precursor Block Copolymer P3HT-b-PPOSa

a

P3HT-Alkyne, synthesized by Kumada catalyst transfer polymerization (KCTP), is converted into the macro-RAFT-agent P3HT-RAFT via CuAAC click chemistry. The PPOS block can be sequentially polymerized from P3HT-RAFT via a RAFT polymerization of trimethylsily-protected 4-(propargyloxy)styrene. A deprotection of the alkyne group with TBAF results in the precursor polymer P3HT-b-PPOS. C

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Macromolecules Table 1. Overview of the Synthesized Polymers P3HT-Alkyne P3HT-RAFT P3HT-b-PTMSPOS P3HT-b-PPOS P3HT-b-PPBI 1 P3HT-b-PPBI 2

Mn,SEC (g mol−1)

Mp,SEC (g mol−1)

Đ

Mn,NMRa (g mol−1)

18300 18400 21900 22300 69500 77700

23000 23600 29200 28300 87300 101200

1.19 1.18 1.40 1.42 1.25 1.23

Mp,MALDI (g mol−1)

wt % (PPBI)b

13300 13300 20500 18000 48100 49700

72 73

a Calculated from and Mn,MALDI(P3HT-RAFT) = 13300 g mol−1 (∼ DP = 80) and ratio of P3HT:PPOS (80:29) from NMR. bCalculated from and Mn,MALDI(P3HT-RAFT) = 13300 g mol−1 (∼ DP = 80) and ratio of P3HT:PPBI (80:37) from NMR.

Figure 2. (a) Details of the 1H NMR spectral region of P3HT-Alkyne and the macroinitiator P3HT-RAFT. The signal of the alkyne proton (2) disappears after the click reaction with the RAFT agent, and new signals (a−d) can be assigned to the new end group. (b) SEC traces of P3HTRAFT and the BCPs P3HT-b-PTMSPOS and P3HT-b-PPOS.

Figure 3. (a) 1H NMR spectra of P3HT-b-PPOS and the donor−acceptor BCPs P3HT-b-PPBI 1 and P3HT-b-PPBI 2. The position of the alkyne proton in P3HT-b-PPOS (gray) and the signals of the protons of the CH2 groups adjacent to the triazole ring (blue and red) are highlighted. (b) SEC traces P3HT-b-PPOS and the two purified donor−acceptor BCPs P3HT-b-PPBI 1 and P3HT-b-PPBI 2 after CuAAC click reaction of P3HTb-PPOS with PBI-N3 1 and PBI-N3 2.

In the final step, two fractions of the same precursor block copolymer were grafted with two different PBI-N3 acceptors via CuAAC click reactions. These reactions were conducted at room temperature and resulted in a quantitative grafting of the second block. The complete conversion of the alkynes of the second block can be monitored via 1H NMR by the disappearance of the alkyne proton signal at 2.52 ppm (see Figure 3a). A new proton signal at 5.09 ppm is observed which is assigned to the CH2 on the acceptor side adjacent to the triazoles ring. The strongest indicator for quantitative grafting is the shift of signal of the methylene protons directly adjacent to the oxygen atom of the propargyloxystyrene. This O−CH2 proton signal appears at 4.62 ppm in the case of the precursor P3HT-b-PPOS and is shifted markedly to lower field (5.09

ppm) for both donor−acceptor BCPs (P3HT-b-PPBI 1 and 2). The SEC curves of the grafted material before final purification (Figure S2b) exhibit a clear low molecular weight peak even though the grafting of the second block was highly efficient. This peak appears at the exact position of the precursor polymer. Its presence stems from one of the aforementioned nonquantitative reactions during the previous steps e.g. that not all P3HT chains were functionalized with an alkyne end group. These side products could be efficiently separated from the final BCPs via a silica gel column. The strong interaction of the perylene bisimide containing polymers with the stationary phase was successfully utilized for this. Any residual P3HT polymers could be selectively washed from the column with toluene while residual PBI-N3 1 D

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Figure 4. Scattering intensity versus scattering vector q over full q range of P3HT-b-PPBI 1 (left) and P3HT-b-PPBI 2 (right) at different temperatures during cooling. Temperatures at which the BCPs are in the molten state are marked in red, and temperatures at which only the PPBI block shows molecular ordering are marked in orange. Data were joined together from measurements at three sample-to-detector distances (curves shifted for clarity).

ing PPBI homopolymer.28 Additionally, a second crystallization event takes place at 150 °C. At this temperature, the typical scattering pattern of P3HT with the (100) reflection at 3.7 nm−1 together with its higher order signals and the π−π stacking signal at ≈16.6 nm−1 appears.31,32 In contrast, only one crystallization event is observed in P3HT-b-PPBI 2. Here only P3HT crystallizes at 150 °C, and the PPBI block stays in an amorphous state. While the behavior of the PPBI block is in agreement with the corresponding homopolymer,28 the reason for its amorphous state is still topic of further investigation, and the results will be published separately. In the region of small scattering vectors (q < 1 nm−1) both BCPs show scattering signals corresponding to the BCP microstructure. The signals consist of a strong peak at around 0.2 nm−1 accompanied by higher order signals which are a clear indication for a microphase-separated structure. It is worth pointing out that the BCP nanostructure (SAXS peak shape and position) keeps unchanged over the whole temperature range studied from 270 to 20 °C (except for a minor shift due to thermal expansion). This observation carries two implications: (i) the order− disorder transition temperature is higher than 270 °C, and (ii) confined crystallization without breakout takes place within the morphology of the BCP nanostructures. To determine the BCP morphology, we performed an indepth analysis of the SAXS patterns of both samples at 20 °C after cooling from the molten state. Based on the chemical composition of 72 and 73 wt % PPBI for P3HT-b-PPBI 1 and 2, respectively, a morphology of hexagonally ordered P3HT cylinders in PPBI matrices would be expected according to the classical phase diagram of coil−coil BCPs assuming that P3HT and the PPBIs have a similar density. Figure 5 shows the SAXS data together with an empirical model function consisting of a power law background and three Gaussians with a common full width at half-maximum (fwhm) to describe the peaks up to the third order. Even though the higher order peaks are too close to be well separated, the peak positions fit to a ratio of 1:√3:2 consistent with the expected hexagonal lattice of cylindrical microdomains. From the position of the first-order reflections the distance between the lattice planes can be calculated to be 28 and 31 nm for P3HT-b-PPBI 1 and P3HT-b-PPBI 2, respectively. The fwhm of the SAXS peaks is a factor of 1.9 and 2.3 above the resolution limit of the instrument for P3HTb-PPBI 1 and P3HT-b-PPBI 2, respectively, indicating that the long-range order is limited within the samples. While usually the peak width is given by the resolution of the instrument for a microphase-separated sample, the existence of higher order

and 2 could be washed down with chloroform. The pure BCPs could finally be retrieved from the column by a 98:2 (v:v) mixture of chloroform: methanol with high yield. The SEC curves of the purified polymers (Figure 3b) are monomodal with no sign of residual P3HT or precursor block copolymer. The molar composition of the BCPs can be extracted from 1H NMR analysis of P3HT-b-PPBI 1 and 2 with a value of 2.16:1 for both polymers. This means that the second block constitutes 72 wt % (P3HT-b-PPBI 1) and 73 wt % (P3HT-b-PPBI 2) of the overall mass of the block copolymer. The N for the acceptor block was calculated as 37. The overall efficiency of the end group modification of P3HT and the initiation of the second block can be determined by comparing this value with the calculated DP of the precursor P3HT-b-PTMSPOS (DP = 29). The increased DP after purification therefore means that about 80% of the P3HT chains, formed during the initial P3HT polymerization, were functionalized with an alkyne, transformed to a macroinitiator, and subsequently started the polymerization of the styrene monomer. Altogether the described synthetic strategy gave access to highly comparable donor−acceptor BCPs with precise control over the composition. Especially the highly efficient quantitative grafting with different acceptor units and the lack of unreacted precursors or P3HT homopolymers ensures the structural purity and high comparability of the final BCPs. Additionally, the presence of monomodal distribution of the block copolymers without any visible high molecular weight shoulder is a clear indication against appreciable amounts of triblock in the final P3HT-b-PPBIs. Structure Formation in P3HT-b-PPBI 1 and P3HT-bPPBI 2. The diblock copolymers P3HT-b-PPBI 1 and 2 were characterized via temperature-dependent X-ray scattering measurements to understand the structure formation. Additionally, DSC, TEM, AFM, and GISAXS measurements were performed. Figure 4 shows temperature-dependent small- and wide-angle X-ray scattering measurements. The scattering curves can be divided into two regions: At larger scattering vectors (q > 1 nm−1) the Bragg reflections of the individual phases can be studied, while at small scattering vectors (q < 1 nm−1) the signals of the BCP nanostructure can be found. Two separate crystallization events take place in P3HT-b-PPBI 1 during cooling (we here subsume the formation of a crystalline and a liquid crystalline phase under the term crystallization). First, at 180 °C two scattering peaks form at 1.07 and 2.14 nm−1. These peak positions are in agreement with a lamellar liquid crystalline morphology as proposed for the correspondE

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Figure 5. Small-angle X-ray scattering intensity versus scattering vector q of P3HT-b-PPBI 1 (blue) and P3HT-b-PPBI 2 (red) at 20 °C with a model function consisting of a power law background intensity and three Gaussians to describe the peaks. Curves of P3HTb-PPBI 2 are shifted by a factor of 10 for clarity.

Figure 7. 2D GISAXS pattern of P3HT-b-PPBI 1 (left) and P3HT-bPPBI 2 (right) at room temperature after annealing in the molten state. Data are plotted on a logarithmic intensity scale.

SAXS signals is nevertheless a clear indication for an ordered microphase-separated structure. The cylindrical BCP morphology is further supported by TEM and AFM measurements as shown in Figure 6. In the

substrate. Accordingly, the observed scattering signal is centered around the reflected beam at qz ≈ 0.27 nm−1 and qy ≈ 0 nm−1 (not visible on the color scale of Figure 7).34 Starting with the scattering pattern obtained from P3HT-bPPBI 1, the following features can be identified. The two peaks at qy = ±0.19 nm−1, qz ≈ 0.36 nm−1 correspond to the fundamental reflection of the 2D-hexagonal lattice with the corresponding planes inclined with an angle of ±60° with respect to the qz-axis. The q value is in agreement with the value from the bulk measurements shown above. Directly below this signal, two peaks are visible in the range of qz between 0.25 and 0.29 nm−1. This signal is caused by the amplification of the scattering intensity showing up between the critical angles of the polymer and the substrate.34 The signal is modulated along the qz direction reflecting the finite film thickness.35 Finally, the signal at around the coordinate origin is caused by the primary beam, whose upper part is not blocked by the sample. The fact that the intensity here is apparently very low is an artifact due to the high intensity of the primary beam not masked by a beam stop. While the basic features in the scattering patterns from the two samples are similar, the additional nearly isotropic, ringlike intensity distribution in P3HT-b-PPBI 2 shows that the microdomains are much less well oriented. This orientational disorder is most likely also the reason for more distorted BCP fingerprint structure seen in the AFM measurements of this sample. In agreement with the temperature-dependent X-ray experiments, the DSC measurements also show melting and crystallization in both BCPs (Figure 8). One melting and one crystallization peak can be observed within the DSC traces of each sample. The peak temperatures of melting/ crystallization are 228 °C/146 °C and 222 °C/140 °C for P3HT-b-PPBI 1 and P3HT-b-PPBI 2, respectively. Taking differences in the thermal program between the X-ray and the DSC measurements into account (stepwise vs direct cooling), the crystallization temperatures are in good agreement with the crystallization of P3HT as observed in the temperaturedependent X-ray measurements. It is unclear why no separate crystallization/melting of the PPBI-block can be observed in the DSC traces of P3HT-b-PPBI 1. Possible explanations are either that the transition enthalpy of the liquid-crystalline ordering is too weak to be detected or that within the thermal program of the DSC the PPBI block crystallizes together with the P3HT.

Figure 6. TEM (top) and AFM height image (bottom) of P3HT-bPPBI 1 (left) and P3HT-b-PPBI 2 (right) after cooling from the molten state. For TEM, P3HT-b-PPBI 1 was annealed for 50 h in chloroform vapor at 40 °C stained with RuO4.

TEM images of both BCPs an arrangement of cylinders can be observed. The samples are stained with RuO4, which is known to selectively stain P3HT. In agreement with the chemical composition with P3HT as minority component of the BCP, the P3HT-cylinders appear in dark in the bright matrix of PPBI. While in the TEM images most of the cylinders are cross-cut (except of a few lying cylinders in P3HT-b-PPBI 1) the AFM images of both materials show the common orientation of flat lying cylinders caused by interaction with the surface.33 To investigate the thin film structure at not only the surface but also the internal structure of the films, GISAXS measurements were performed. Figure 7 shows the 2Dscattering pattern collected from the same melt-crystallized samples as studied with AFM. For both measurements, an angle of incidence of 0.18° was chosen, slightly above the critical angle of the polymer but below the critical angle of the F

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Experimental details, additional MALDI-TOF MS spectra, 1H NMR spectra, and SEC curves (PDF)

AUTHOR INFORMATION

Corresponding Authors

*(M.T.) E-mail: [email protected]. *(T.T.-A.) E-mail: [email protected]. ORCID

Thomas Thurn-Albrecht: 0000-0002-7618-0218 Mukundan Thelakkat: 0000-0001-8675-1398 Author Contributions

Figure 8. DSC traces of P3HT-b-PPBI 1 and P3HT-b-PPBI 2 measured with a heating/cooling rate of 10 K min−1. Curves of P3HT-b-PPBI 1 are shifted by 1 J g−1 °C−1 for clarity.

C.D.H. and M.F. contributed equally. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support from the Bavarian State Ministry of Education, Science and the Arts under the research network “Solar technologies go hybrid” (SolTech) and from the Ministry of Economy, Science and Digitalization of the State of Saxony-Anhalt under the network “Nanostructured Materials”. We thank Martin Hufnagel and Hubertus Burchhardt for the measurement of the MALDITOF MS spectra and Carmen Kunert for the TEM measurements.

The difference of around 80 °C between the melting and crystallization peak also shows that a considerable amount of undercooling is necessary to induce crystallization in the BCPs, which is in agreement with previous similar examples.24−26 Additionally, to the melting and crystallization peaks, both BCPs show a significant amount of cold crystallization between 60 and 170 °C, most likely due to the significant undercooling needed to induce crystallization. According to Lang et al., the glass transitions temperatures of the PPBI-blocks are with 166 and 142 °C for P3HT-b-PPBI 1 and P3HT-b-PPBI 2, respectively, in the temperature window of cold crystallization.28 Because both phenomena are superimposing each other, the glass transition of the acceptor block is not visible in the DSC measurements of the BCPs.

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CONCLUSION We synthesized two new donor−acceptor BCPs P3HT-b-PPBI 1 and P3HT-b-PPBI 2 by a modular approach. We could show that the side-chain grafting with different perylene bisimides is quantitative for the homopolymers as well as for the diblock copolymers. The critical step is therefore the end group functionalization of P3HT to obtain the macroinitiator and the sequential polymerization of the second block. About 80% of the P3HT were incorporated into the block copolymer, and we found a convenient way to separate the nonincorporated homopolymers from the diblock copolymers P3HT-b-PPBI 1 and P3HT-b-PPBI 2. We explored the interplay between microphase separation and molecular dynamics in two donor− acceptor BCPs consisting of P3HT-b-PPBI and confirmed the expectation that the decrease of the glass transition temperature as compared with a previously examined sample containing PCBM in the acceptor block facilitates the formation of a well-defined microphase-separated structure. Inside the existing block copolymer nanostructures, confined crystallization without alteration of the cylindrical morphology occurred. The results demonstrate that for efficient material design of phase separated donor−acceptor BCPs a number of different criteria have to be considered ranging from the optoelectronic function and thermodynamics of self-assembly up to the molecular mobility.

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DOI: 10.1021/acs.macromol.8b01301 Macromolecules XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.macromol.8b01301 Macromolecules XXXX, XXX, XXX−XXX