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Highlighting work from the research group of Dr J. Peng

and Dr F. Qiu in the Department of Macromolecular Science,

Fudan University.

Controlling the morphology and crystallization of a

thiophene-based all-conjugated diblock copolymer

by solvent blending

Trees, mimicking the polythiophene blocks, are connected

together to represent the polythiophene-based diblock

copolymer. This article describes a facile strategy to tailor

the morphology and crystallization of a polythiophene-based

diblock copolymer by solvent blending.

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See Juan Peng, Feng Qiu et al.,Soft Matter, 2017, 13, 5261.

This journal is©The Royal Society of Chemistry 2017 Soft Matter, 2017, 13, 5261--5268 | 5261

Cite this: SoftMatter, 2017,

13, 5261

Controlling the morphology and crystallizationof a thiophene-based all-conjugated diblockcopolymer by solvent blending†

Huina Cui, Xiubao Yang, Juan Peng* and Feng Qiu*

We report the crystallization and microphase separation behavior of an all-conjugated poly(3-hexyl-

thiophene)-b-poly[3-(6-hydroxy)hexylthiophene] (P3HT-b-P3HHT) block copolymer in mixed solvents

and demonstrate how the conformations of P3HT and P3HHT chains influence the photophysical

properties of the copolymer. It is shown that the balance among p–p stacking of P3HT, P3HHT and

microphase separation of the copolymer can be dynamically shifted by controlling the rod–rod inter-

actions of the copolymer via changing the block ratio and solvent blending. A series of nanostructures

such as well-ordered nanofibers, spheres and lamellar structures are formed and their formation

mechanisms and kinetics are discussed in detail. The variations in P3HT-b-P3HHT conformations are

concomitant with a hybrid photophysical property depending on the competition between intrachain and

interchain excitonic coupling, resulting in the transformation between J- and H-aggregation. Overall, this

work demonstrates how the P3HT-b-P3HHT conformations crystallize and phase-separate in the solution

and solid state, and the correlation between their structures and photophysical properties, which improves

our understanding of all-conjugated rod–rod block copolymer systems.

Introduction

Among conjugated polymers, poly(3-alkylthiophenes) (P3ATs)have attracted considerable attention over the past few yearsbecause of their promising applications in organic electronicssuch as photovoltaic cells,1–3 organic field-effect transistors,4,5

and light emitting diodes.6 They have a typical hair-rod configu-ration, consisting of a rigid thiophene backbone with pendantalkyl side chains that facilitate solubilization. P3ATs can pre-ferentially crystallize into nanofibers through anisotropicp–p interactions between planar rigid backbones and weakvan der Waals interactions between alkyl side chains.7,8 Sincethe optoelectronic properties of P3ATs are closely related totheir molecular packing and nanoscale morphology, control-ling their structures using various methods is important for theconsequent device performance.

Designing block copolymers (BCPs) containing P3ATs hasbeen considered as a promising strategy to control the nano-structure which can self-organize into well-defined microphase-separated structures on the nanometer scale. Indeed, thesynthesis, self-assembly and properties of P3AT-based rod-coil

BCPs have been extensively studied, including poly(3-hexyl-thiophene)-b-polystyrene (P3HT-b-PS),9 poly(3-hexylthiophene)-b-poly-(methyl methacrylate) (P3HT-b-PMMA),10,11 poly(3-hexyl-thiophene)-b-poly(vinylpyridine) (P3HT-b-PVP),12 and poly(3-alkylthiophene)-b-poly(ethylene oxide) (P3AT-b-PEO),13,14 etc.Compared to the extensively studied rod-coil BCPs, the researchon P3AT-based rod–rod BCPs is relatively not much reported.Without the insulating coil blocks which may decline the totalelectronic properties, such all-conjugated rod–rod BCPs havebecome a focus of interest.7,15–34 For example, Tu et al. reportedthe first amphiphilic all-conjugated diblock copolymer poly[9,9-bis(2-ethylhexyl)fluorene]-b-poly[3-(6-diethylphosphonatohexyl)-thiophene] (PF2/6-b-P3PHT), which showed vesicle formationin mixtures of non-selective and selective solvents.33 Hollingeret al. reported poly(3-hexylselenophene)-b-poly(3-hexylthiophene)(P3HS-b-P3HT) and discovered that the microphase separationcould be controlled by the heterocycle in the polymer chain.34

Our group reported that a series of diblock copolythiophenesmicrophase-separate into two crystal domains when the sidechain length of the two blocks is different more than twocarbon atoms.17–19 Yet, there are still some disadvantages inrod–rod BCPs. Usually, these BCPs are composed of two or morehydrophobic conjugated blocks, in which the reduced coil-likeability and strong rod–rod interactions between conjugatedblocks often lead to the formation of vesicles or lamellae inthe solution or the solid state, respectively, largely independent

State Key Laboratory of Molecular Engineering of Polymers, Department of

Macromolecular Science, Fudan University, Shanghai 200433, China.

E-mail: juanpeng@fudan.edu.cn, fengqiu@fudan.edu.cn

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c7sm01126a

Received 6th June 2017,Accepted 27th June 2017

DOI: 10.1039/c7sm01126a

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of their specific chemical structure and composition.15,29 Theseproperties may prevent the access to more complex or well-defined nanostructures. Overall, if the rigidity and the rod–rodinteractions are controlled, a variety of ordered nanostructuresmay be found in rod–rod BCPs.

Side chain functionalization has been proven to be a goodand convenient synthetic way to control the rod–rod interactions ofpolythiophenes. For example, the Segalman group introduced ashort branch into the hexyl side chains of P3HT, which causedpoly(3-(20-ethyl)hexylthiophene) (P3EHT) to have a lower meltingtemperature, a lower liquid crystalline transition temperature andreduced rod–rod interactions.35 The Seferos group introduced a2-ethylhexyl side chain into selenophene–thiophene BCPs and foundthat disordered fibrillar or ordered lamellar structures were formedthrough side chain engineering on different blocks.30 The side chainfunctional groups may also be ionic36 or polar groups,33,37 which canendow the polythiophenes with hydrophilic properties and expandmorphology diversity in polar solvents or water. Our group recentlyreported a class of rod–rod BCPs, poly(3-hexylthiophene)-b-poly-(3-(6-diethylphosphonatohexyl)thiophene) (P3HT-b-P3PHT) with sidephosphonate group substitution.38 Such BCPs have an amphiphilicnature and reduced rod–rod interactions, which can form well-controlled nanostructures in solutions and greatly improve theperformance of the photovoltaic device when used as additives. Todate, side chain substitution of P3AT-based rod–rod BCPs remainsless explored and needs extensive investigation.

In our previous report, we synthesized an all-conjugated rod–rod poly(3-hexylthiophene)-b-poly[3-(6-hydroxy)hexylthiophene](P3HT-b-P3HHT) with hydroxyl groups as side substitutiongroups.39 The thermal crosslinking of hydroxyl groups greatlyimproved the film crystallinity and ductility that may findapplications in flexible electronic devices. The introduction ofside hydroxyl groups may also be used to control the rod–rodinteractions of the copolymer to form ordered nanostructures,which are intriguing and remain unexplored yet. Consideringthe different polarity between P3HT and P3HHT, one mightnaturally think that the nanostructures of such BCPs in thesolution are related to the solvent selectivity. Motivated by thesethoughts, we investigated the self-assembling behavior ofP3HT-b-P3HHT in different quality solvents and tried to controlits assemblies, crystallization and dynamics. It showed that theP3HT-b-P3HHT morphology was quite dependent on the sol-vent selectivity for the two blocks. By blending solvents(i.e., methanol/pyridine or chloroform/pyridine) to control therod–rod interactions of the copolymer, the balance between p–pstacking of P3HT and P3HHT could be dynamically shifted,yielding diverse nanostructures such as well-ordered nanofibers,spheres and lamellar structures. Formation mechanisms andkinetics of different nanostructures were discussed. The correla-tion between solution structures and the photophysical proper-ties has been studied in detail.

Experimental section

MaterialsP3HT-b-P3HHT BCPs were synthesized using the modified

Grignard metathesis polymerization (GRIM) starting from

2-bromo-5-iodo-3-hexylthiophene. The synthesis details havebeen described in ref. 39. The resulting P3HT-b-P3HHT BCPswith varying block ratios and molecular weights are summar-ized in Table 1. The solvents pyridine, methanol, and chloro-form were purchased from Sinopharm Chemical Reagent Co.,Ltd (SRC) and used as received.

Sample preparation

P3HT-b-P3HHT was dissolved in pyridine (5 mg mL�1) byheating at 80 1C for 1 h to reach sufficient dissolution. Then,a certain pyridine, methanol or chloroform were added to it todilute the solution to the desired mixed solvent volume ratioswith the final concentration fixed at 0.8 mg mL�1. The solu-tions were cooled to room temperature (B30 1C) and stirred at250 rpm for 6 h. Thin films were prepared by spin-coating orsolvent-casting the copolymer solutions onto the silicon wafers.Prior to spin-coating, the wafers were cleaned using Piranhasolutions (H2SO4/H2O2, 3 : 1 in volume) and then thoroughlyrinsed with deionized water and dried.

Characterization

Transmission electron microscopy (TEM) images were obtainedon a Tecnai G2 20, FEI electron microscope operated at 200 kV.For the TEM analysis, the samples were prepared by drop-castingthe copolymer solution on copper grids, followed by evaporationof the solvent under ambient conditions. Atomic force micro-scopy (AFM) measurements were carried out on a Bruker Multi-mode Nanoscope IV with tapping mode. UV-vis spectroscopywas performed using Perkin-Elmer Lambda 750 equipment.Photoluminescence (PL) spectra were recorded on a QM 40spectrophotometer (Photo Technology International, Inc.).Grazing-incidence X-ray diffraction (GIXRD) experiments wereperformed at the BL14B1 beamline of Shanghai SynchrotronRadiation Facility (SSRF) with a fixed wavelength of 1.24 Å. Theincidence angle of the X-ray beam line was 0.151 and the exposuretime was 30 seconds.

Results and discussion

For a given A-b-B BCP system, a solvent that is good for one blockcan be classified as neutral or selective, according to whether it isa good or nonsolvent for the other block. The relative affinity ofsolvents for each block is governed by the polymer–solventinteraction parameter,40 wP–S (P = polymer and S = solvent),which can be estimated by wP–S = VS (dS � dP)2/RT + 0.34, whereVS is the molar volume of the solvent, R is the gas constant,

Table 1 Summary of compositions and molecular weights of P3HT-b-P3HHT BCPs

Polymers n/ma Mn PDI

P3HT-b-P3HHT (1 : 3) 1 : 3 9900 1.33P3HT-b-P3HHT (1 : 1) 1 : 1 14 800 1.35P3HT-b-P3HHT (3 : 1) 3 : 1 10 900 1.23

a n, m: the molar amount of the first and the second block in thesynthesized BCPs, respectively.

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T is the Kelvin temperature, and dS and dP are the solubilityparameters of the solvent and polymer, respectively. Using theFlory–Huggins criterion, the complete solvent-polymer misci-bility can be realized when wP–S o 0.5. In the present work,the solubility parameter of P3HT is dP3HT = 19.4 (J cm�3)1/2,41

while that of P3HHT cannot be referenced. By calculation,wP3HT–pyridine = 0.5157, wP3HT–methanol = 2.0828, wP3HT–CHCl3 =0.3452. According to the calculated values and with the helpof the dissolution experiment, pyridine is selective for P3HHTwhile chloroform is selective for P3HT. Methanol is a poorsolvent for both blocks but poorer for P3HT.

Our previous studies have shown that varying the solventselectivity is an efficient way to control the coil–coil copolymerphase behavior.42,43 In the following, we will show that varyingthe solvent selectivity is also effective in rod–rod P3HT-b-P3HHT systems to tune their phase behavior and yield diversenanostructures. First, P3HT-b-P3HHT BCPs with different blockratios form different nanostructures in pure pyridine. Then wefocus on solvent blending (methanol/pyridine mixtures andchloroform/pyridine mixtures) to finely tune the solvent selec-tivity to better control the rod–rod interactions and assemblybehavior. Methanol is a poor solvent for both blocks whichstrengthen the rod–rod interactions while chloroform weakensthe interactions. In addition, we show that the copolymer chainsdisplay a hybrid photophysical property with the transformationbetween J- and H-aggregation tuned by solvent blending. Theformation mechanisms of different solution structures havebeen discussed in detail.

Nanostructures formed from a pure pyridine solvent

It is known that P3HT exists in two different conformations inthe solution: a random coil conformation and a planarizedconformation which are sensitive to the quality of the solventused. With a hydroxyl group in the hexyl side chains of P3HT,P3HHT has similar coil and planarized conformations. In thefollowing, three P3HT-b-P3HHT BCPs with different blockratios were dissolved in pure pyridine to investigate their self-assembly behavior. As shown in Fig. 1a and b, P3HT-b-P3HHT(1 : 3) showed a dominant amorphous surface with some nano-fibers in very weak contrast. The soluble major P3HHT blockresulted in the BCP dissolving very well in pyridine and forminga featureless structure. As the length of the P3HT blockincreased, the fibrillar morphology with an average width ofB13.8 nm was observed in the P3HT-b-P3HHT (1 : 1) system(Fig. 1c and d). Interestingly, the fibers have highly orderedpacking, which could have advantages in charge transport(Fig. 1d). Further increasing the length of P3HT in the P3HT-b-P3HHT (3 : 1) system led to the formation of more nanofibers(Fig. 1e and f).

To fully elucidate the effect of the block ratio on the P3HT-b-P3HHT crystalline structures, UV-vis measurement was per-formed. In pure pyridine, the P3HT-b-P3HHT BCPs with variousblock ratios showed a single absorption peak at lmax = 450 nmdue to the intrachain p–p* transition (Fig. S1, ESI†). No vibronicpeaks were observed in P3HT-b-P3HHT (1 : 3), indicating thatthe polythiophene chains dissolved well in pyridine and

exhibited the coil conformation. While for (1 : 1) and (3 : 1)P3HT-b-P3HHT, the vibronic peak at 600 nm was observed,indicating the presence of the polythiophene crystalline structure.However, it is hard to distinguish which block crystallized insolutions since both P3HT and P3HHT have the same vibronicpeak position. To help identify the crystalline structure in thesolution, the UV-vis spectra of the P3HHT homopolymer inpyridine were compared which did not show the vibronic peak.It indicated that the P3HHT block was well-solvated while theP3HT block crystallized in (1 : 1) and (3 : 1) P3HT-b-P3HHT inpyridine. These spectroscopic results indicate that the block ratioinfluenced the rod–rod interactions. Since pyridine is a selectivesolvent for P3HHT, rod–rod interactions became stronger with theincreased P3HT content and the p–p stacking of P3HT graduallydominated in the P3HT-b-P3HHT (1 : 1) and (3 : 1) systems, result-ing in the formation of much more nanofibers.

After solvent-casting the copolymer solution on the siliconwafers, crystalline structures of the P3HT-b-P3HHT thin filmswere analyzed using GIXRD. For comparison, P3HT and P3HHThomopolymers were also studied. P3HT and P3HHT homo-polymers showed the family of (h00) diffraction patternsalong the out-of-plane (qz) direction, which are characteristic

Fig. 1 (a, c and e) TEM and (b, d and f) AFM images of P3HT-b-P3HHT ofvarious block ratios in pyridine. The block ratio of P3HT to P3HHT is (a and b)1 : 3, (c and d) 1 : 1, and (e and f) 3 : 1.

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of edge-on orientation with the direction of p–p stacking andthe layers of hexyl side chains parallel and perpendicular to thesubstrate, respectively (Fig. S2a and b, ESI†). From 1D GIXRD,the strong (100) diffractions of P3HT and P3HHT homopolymerswere at a scattering vector (qz) of 3.90 nm�1 and 3.65 nm�1,corresponding to a d100-spacing of 16.1 Å and 17.2 Å, respectively(Fig. S2c, ESI†). The p–p stacking distance between P3HT andP3HHT was 3.8 Å, coming from the scattering vector qx,y of16.54 nm�1 (Fig. S2d, ESI†). It showed that the introduction ofhydroxyl groups enlarged the d100-spacing between conjugatedbackbones while had little influence on the p–p stackingdirection. Compared to P3HT and P3HHT homopolymers,P3HT-b-P3HHT copolymer films also exhibited the arc patterns(h00) along the qz direction, indicating the edge-on orientation(Fig. 2a–c). From 1D GIXRD, since the (100) diffraction peaksof P3HT and P3HHT blocks in the copolymers were tooapproximate to separate, their corresponding higher order(300) diffraction peaks were further analyzed (Fig. 2d). In theP3HT-b-P3HHT (1 : 3) film, there was a single (300) diffractionpeak (qz = 10.78 nm�1) associated with P3HHT, indicating thatthe major P3HHT block crystallized in the solid state. While(1 : 1) and (3 : 1) P3HT-b-P3HHT films showed two pronounced(300) diffraction peaks associated with P3HT (qz = 11.79 nm�1)and P3HHT (qz = 10.76 nm�1) blocks, implying that both P3HTand P3HHT blocks crystallized. Comparing the P3HT-b-P3HHTcrystalline structures in the solution and solid state, it is easy tounderstand that the solvent evaporation process indeed furtherpromoted the crystallization of polythiophene chains. For (1 : 1)and (3 : 1) P3HT-b-P3HHT, the assembly of nanofibers wasdriven by the p–p interactions in the P3HT block in the solution

followed by the crystallization of the P3HHT block during thesolvent evaporation process. A schematic that displayed the(1 : 3) and (1 : 1) P3HT-b-P3HHT chains in the pyridine solutionand solid state is shown in Fig. 3.

Nanostructures formed from methanol/pyridine mixed solvents

In the following, we demonstrate how the assembly behaviorof the copolymer was influenced by the addition of a poormethanol solvent. P3HT-b-P3HHT (1 : 3) was chosen as anexample. Fig. 4 shows the structure evolution of the copolymer inmethanol/pyridine mixed solvents with the addition of methanol.After adding methanol as low as 20% into pyridine solution, theinitial featureless morphology (Fig. 1a and b) disappeared andsome nanofibers appeared (Fig. 4a). When the volume ratio ofmethanol/pyridine was 40 : 60, the nanofibers disappeared andsome ordered spherical micelles with an average diameter ofabout 8–10 nm could be faintly seen (Fig. 4b). Since methanol ispoorer to P3HT than P3HHT, P3HHT was more swollen and

Fig. 2 2D GIXRD patterns of (a) P3HT-b-P3HHT (1 : 3), (b) P3HT-b-P3HHT(1 : 1) and (c) P3HT-b-P3HHT (3 : 1) thin films cast from pyridine solutions.(d) 1D GIXRD profiles reduced along the out-of-plane direction fromdifferent P3HT-b-P3HHT films. The inset shows the corresponding (300)diffraction peak.

Fig. 3 Schematic illustration of P3HT-b-P3HHT (1 : 3) and (1 : 1) chains inthe pyridine solution and solid state.

Fig. 4 TEM images of P3HT-b-P3HHT (1 : 3) in the methanol/pyridinesolutions with different solvent ratios. The volume ratio of methanol/pyridine is (a) 20 : 80; (b) 40 : 60, and (c) 70 : 30. The inset of (b) is amagnified AFM image.

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became bigger in volume than P3HT with the addition ofmethanol. The copolymer transformed into spherical micelleswith P3HT as the core surrounded by the P3HHT corona tominimize the interfacial energy.44 Further increasing the ratioto 70 : 30, these spherical micelles aggregated to form muchlarger aggregates (Fig. 4c). It should be noted that with theaddition of methanol, the mixed solvents became poorer andpoorer to both P3HT and P3HHT and the copolymer wouldprecipitate finally.

The variations in P3HT-b-P3HHT conformations were con-comitant with a series of solvatochromic changes in the photo-physical properties reflected by UV-vis and PL spectra (Fig. 5).The UV-vis spectra also helped the analysis of the copolymercrystalline structures during solvent blending. Overall, the addi-tion of methanol led to a gradual decrease and a red-shift of themain absorption peak and the appearance of three vibronicpeaks at 520, 550 and 600 nm, respectively. During this process,copolymer chains changed from flexible coils to rigid rods andp–p stacked with each other to form crystalline structures. Thedistinct isosbestic point at 485 nm confirmed the presenceof two conformational forms, coil and aggregate form. Whenanalyzing the spectra in detail, at a methanol/pyridine ratio of25 : 75, vibronic peaks appeared in the copolymer while almostno vibronic peaks were observed in the P3HHT homopolymer(Fig. S3, ESI†), indicating the crystallization of the P3HT blockat this ratio. Further increasing the methanol/pyridine ratioto 40 : 60, both the P3HHT homopolymer and the copolymerexhibited obvious vibronic peaks at 520, 550 and 600 nm,

suggesting that both P3HT and P3HHT blocks crystallized inthe solution. Overall, the addition of methanol strengthenedthe rod–rod interactions, therefore, P3HT-b-P3HHT (1 : 3) changedfrom coil conformation in pure pyridine to the P3HT crystallinestructure at a methanol/pyridine ratio of 25 : 75, and to both theP3HT and P3HHT crystalline structures at a methanol/pyridineratio of 40 : 60.

The corresponding PL spectra further supported the for-mation of aggregates and confirmed their aggregate types. Inpyridine solution, the spectra exhibit an emission maximum at595 nm with a resolved vibronic shoulder at 635 nm, corres-ponding to the 0–0 and 0–1 singlet transitions, respectively(Fig. 5b). With the addition of methanol, the emission graduallyquenched with the decreased intensity of both 0–0 and 0–1emission due to the aggregation of polythiophene chains in thesolution. The 0–0/0–1 intensity ratio (I0–0/I0–1) in the PL wasmeasured to identify the type and strength of the dominantexcitonic coupling in the copolymer solution. According to Spanoand coworkers,45 the dominant intrachain excitonic coupling leadsto photophysical behavior of J-aggregates (i.e., the 0–0 transition isstronger than the 0–1 sideband). Whereas the dominant interchainexcitonic coupling results in H-aggregates (i.e., the 0–0 transition isweaker than the 0–1 sideband). Herein, I0–0/I0–1 gradually decreasedfrom 1.07 to 0.01 as the volume ratio of methanol/pyridine changedfrom 0 : 100 to 40 : 60, during which the 0–1 emission began toexceed 0–0 emission when the methanol/pyridine ratio was 15 : 85.This indicated that the dominant excitonic coupling type changedfrom the intrachain (weak J-type) to interchain (H-type) couplingwith the increased methanol content.

The corresponding P3HT-b-P3HHT (1 : 3) thin films castfrom different methanol/pyridine ratios were investigated usingGIXRD. The 1D GIXRD profiles showed that the scatteringvector (qz) of the (100) diffraction peak shifted from 3.70 nm�1

to 3.73 nm�1 with the increase of the methanol ratio from 20%to 40%, corresponding to a d100-spacing of 16.98 Å and 16.84 Å,respectively (Fig. 6). The smaller d100-spacing indicated that the

Fig. 5 (a) UV-vis and (b) PL spectra of P3HT-b-P3HHT (1 : 3) in themethanol/pyridine solutions with different solvent ratios.

Fig. 6 1D GIXRD profiles reduced along the out-of-plane direction fromthe P3HT-b-P3HHT (1 : 3) thin films cast from the methanol/pyridine solu-tions with different solvent ratios. The inset shows the corresponding (300)diffraction peak.

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copolymer packed more tightly in the case of the methanol/pyridine (40 : 60) system. The (300) diffraction peaks at 10.73 nm�1

were associated with P3HHT (inset in Fig. 6). Comparing theP3HT-b-P3HHT crystalline structures in the solution and solidstate, the solvent evaporation process promoted the crystallizationof P3HHT. Due to the overlap of diffraction peaks of P3HT andP3HHT in this case, the (300) diffraction peak of P3HT was notobvious in the solid state.

Nanostructures formed from chloroform/pyridine mixedsolvents

We then discussed the morphological transformation processof P3HT-b-P3HHT (1 : 3) in chloroform/pyridine mixed solventswith the addition of chloroform selective for the P3HT block(Fig. 7). When the ratio of chloroform/pyridine was 30 : 70, theinitial featureless morphology (Fig. 1a) changed into microphase-separated lamellar structures (Fig. 7a and b). The average lamellarperiod was B10.7 nm measured from the AFM image with theinset showing the schematic microphase-separated lamellar struc-tures in the solid state (Fig. 7b). Morphology transition frommicrophase separation to the p–p stacking of the copolymer waswell controlled by changing the solvent quality. Further increas-ing the ratio to 60 : 40 (Fig. 7c) and 70 : 30 (Fig. 7d), the lamellarstructure disappeared and isolated nanofibrils aggregated witheach other to form bundles.

Parallel investigation of the photophysical properties ofP3HT-b-P3HHT (1 : 3) in chloroform/pyridine mixtures was alsoconducted. With the addition of chloroform, the main absorp-tion peak at 450 nm gradually red-shifted to 480 nm with theappearance of two vibronic peaks at 550 nm and 600 nmattributed to the presence of crystalline aggregates in solutions.

(Fig. 8a). Since chloroform is a selective solvent for the P3HTsegment, the main absorption peak remained and the vibronicpeaks became weaker compared to the peaks with the methanoladdition. When analyzing the UV spectra in detail, the absence ofvibronic peaks in the copolymer when the chloroform/pyridineratio was less than 55 : 45 indicated the coil conformation of bothP3HT and P3HHT blocks. Upon further adding the chloroform,vibronic peaks at 550 and 600 nm appeared in the copolymerand P3HHT homopolymer (Fig. S4b, ESI†) systems instead ofthe P3HT homopolymer (Fig. S4a, ESI†) system, suggesting thedomination of P3HHT crystallization.

From the corresponding PL spectra, the addition of chloro-form first increased the emission until the ratio of chloroform/pyridine of 40 : 60 because a small amount of chloroform pro-moted the copolymer dissolution in the solution and weakenedthe rod–rod interactions. Then further adding the chloroforminduced the quenching of emission due to the aggregation ofpolythiophene chains in the solution (Fig. 8b). By quantitativeanalysis, I0–0/I0–1 gradually increased from 1.07 to 1.17 and thendecreased to 0.26 as the volume ratio of chloroform/pyridinechanged from 0 : 100 to 75 : 25. The 0–1 emission began toexceed 0–0 emission when the ratio of chloroform/pyridine was55 : 45. Similar to the addition of methanol, this indicated thatthe weak J-like type gradually gave way to the H-like type withthe increased content of chloroform.

Similarly, the GIXRD results of P3HT-b-P3HHT (1 : 3)thin films cast from various chloroform/pyridine ratios wereanalysed (Fig. 9). As the chloroform ratio increased, the (100)

Fig. 7 (a, c and d) TEM and (b) AFM phase image of P3HT-b-P3HHT (1 : 3) inthe chloroform/pyridine solutions with different solvent ratios. The volumeratio of chloroform/pyridine is (a and b) 30 : 70, (c) 60 : 40, and (d) 70 : 30.The inset in (b) shows the schematic microphase separation.

Fig. 8 (a) UV-vis and (b) PL spectra of P3HT-b-P3HHT (1 : 3) in thechloroform/pyridine solutions with different solvent ratios.

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diffraction peak shifted from 3.69 nm�1 to 3.74 nm�1, corres-ponding to the decrease of d100-spacing from 17.03 Å to 16.80 Å.The appearance of P3HHT (300) diffraction peaks indicatedthat the P3HHT block crystallized in the solid state cast fromchloroform/pyridine 30 : 70 and 60 : 40 solutions.

Conclusions

In summary, we have investigated the crystallization, micro-phase separation, and photophysical properties of P3HT-b-P3HHT BCPs in mixed solvents. During solvent blending, therod–rod interactions of the copolymer were dynamically con-trolled which were strengthened by the addition of methanolwhile weakened by the chloroform addition. The characteristicsof P3HT and P3HHT were gradually changed between rigid orflexible, resulting in a series of nanostructures such as well-ordered nanofibers, spheres and lamellae. The conformationsof the P3HT and P3HHT chains in the solutions influencedtheir photophysical properties greatly. The interplay betweenthe intrachain, J-favoring and the interchain, H-favoring exci-tonic coupling was shifted and resolved using absorption andPL spectroscopy. Overall, the significance of this work is todemonstrate how the rod–rod interactions in P3HT-b-P3HHTBCPs were controlled by changing the block ratio and thesolvent selectivity, and the correlation between their structuresand photophysical properties, which can improve our under-standing of all-conjugated rod–rod systems.

Acknowledgements

This work was financially supported by the National NaturalScience Foundation of China (Grant No. 21674024, 21274029and 21320102005) and Ministry of Science and Technology ofChina (2016YFA0203301). We gratefully acknowledge the sup-port from Shanghai Synchrotron Radiation Facility of China forusing the BL14B1 and BL16B1 beamlines.

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Fig. 9 1D GIXRD profiles reduced along the out-of-plane direction forP3HT-b-P3HHT (1 : 3) films cast from the chloroform/pyridine solutionswith different solvent ratios. The inset shows the corresponding (300)diffraction peak.

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