Influence of Composition of Amphiphilic Double-Crystalline P3HT-b

Jul 25, 2016 - On addition of MeOH as a nonsolvent for the P3HT block, these block copolymers are able to form stable micellar aggregates if the PEG f...
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Influence of Composition of Amphiphilic Double-Crystalline P3HT‑b‑PEG Block Copolymers on Structure Formation in Aqueous Solution Paul M. Reichstein,† Sebastian Gödrich,‡ Georg Papastavrou,‡ and Mukundan Thelakkat*,† †

Applied Functional Polymers, Department of Macromolecular Chemistry I, and ‡Physical Chemistry II, University of Bayreuth, Universitaetsstr. 30, 95440 Bayreuth, Germany S Supporting Information *

ABSTRACT: A series of poly(3-hexylthiophene)-block-poly(ethylene glycol) (P3HT-b-PEG) with constant P3HT block length block and variable PEG block lengths are presented. Alkyne-functionalized P3HT with high absolute molecular weight of 11.4 kg/mol is combined with azide-functionalized PEGs via copper-catalyzed alkyne−azide cycloaddition (CuAAC). The resulting P3HT-b-PEG block copolymers have PEG weight fractions between 15 and 64 wt %. In bulk materials the crystallinity in the conjugated block is similar to pure P3HT, while the crystallinity of PEG is influenced with decreasing PEG block length. On addition of MeOH as a nonsolvent for the P3HT block, these block copolymers are able to form stable micellar aggregates if the PEG fraction is >31 wt %. In AFM, DLS, and cryo-TEM, P3HT-b-PEG micelles are found to have a spherical or cylindrical shape with diameters of around 25 nm and lengths between 40 nm and some hundred nanometers. It is found that short PEG blocks lead to bigger block copolymer micelles. Thus, a correlation of composition on solution structure and its consequences on the crystallization of both blocks is given.

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INTRODUCTION In the past decade poly(3-hexylthiophene) (P3HT) has become one of most studied conjugated polymers.1 The most important reason for this was the development of a controlled synthesis of regioregular P3HT which is referred as Kumada catalyst transfer polymerization (KCTP).2,3 P3HT was also used in the synthesis of different block copolymers, which are still of high interest because of their unique properties, especially due to possible self-assembled structures at thermodynamic equilibrium.4 The controlled end-functionalization of P3HT led the way toward the synthesis of well-defined block copolymers with P3HT blocks.5 Although P3HT has a decreased flexibility in the backbone due to conjugation, its block copolymers have been shown to form typical cylindrical and lamellar morphologies of coil−coil block copolymers.6 Among the block copolymers involving P3HT, there are a number of examples where the second block is polar such as P3HT-b-poly(acrylic acid), 7,8 P3HT-b-poly(2-vinylpyridine),9−12 P3HT-b-poly(4-vinylpyridine),13−15 P3HT-b-poly(ethylene glycol),16−27 and others. One of the possibilities in such amphiphilic block copoylmers is that the interaction of the polar blocks can be utilized to coordinate inorganic or metal nanoparticles. The infiltration and incorporation of nanoparticles into block copolymer domains are often not controlled, but for example in P3HT-b-P2VP it was shown that a very well-defined distribution of CdSe nanoparticles could be achieved by varying the regioregularity of P3HT blocks.12 Still the main issues that hinder the use of these © XXXX American Chemical Society

hybrid materials in electronic devices are the insufficient amount of inorganic materials causing weak percolation of charges, less preferential incorporation for high amounts of inorganics, and the low molecular weight of P3HT used in many of the reported systems. It has been shown that the hole mobility in P3HT increases with the molecular weight and has a maximum for a Mn(MALDI) P3HT ≈ 12 000 g/mol (corresponding to 20 000 g/mol in SEC).28 Especially, P3HT-b-PEGs are promising candidates for the study of self-assembly behavior and the application in the field of organic−inorganic hybrid materials due to the ability of PEG to coordinate inorganic semiconductor and metallic nanoparticles.29 PEG blocks have recently shown to facilitate very high loading with Au nanoparticles in self-assembled block copolymer nanodomains.30 The methods for synthesis of P3HT-b-PEGs in the literature range from Suzuki coupling of Br-functionalized P3HT and boronic ester-functionalized PEG,16−19 anionic polymerization of ethylene oxide followed by KCTP through a suitable end group,20 to the coupling of prior synthesized blocks of P3HT and PEG via Steglich esterification21 or copper-catalyzed alkyne−azide cycloaddition (CuAAC) reactions.22−27 P3HT-b-PEG via CuAAC first demonstrated by Javier et al.22 was in the meantime used in several other publications.23−27 In most of these, the P3HT had Received: June 17, 2016 Revised: July 6, 2016

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Scheme 1. Synthesis of P3HT-b-PEG Block Copolymers via CuAAC Reaction of P3HT-Alkyne and PEG-N3s to Yield Four Different Block Copolymers with Different PEG Block Lengthsa

a

Subscripts on PEG denote the degree of polymerization.

Scheme 2. (a) Synthesis of P3HT-Alkyne by KCTP and End-Capping with Ethynylmagnesium Chloride and (b) Synthesis of PEG-N3s with Different Lengths by Tosylation of PEG-OHs and Reaction with NaN3

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RESULTS AND DISCUSSION Synthesis of P3HT-b-PEGs. We use the coupling of the prior synthesized P3HT-Alkyne and PEG-N3 blocks by a CuAAC reaction for the synthesis of amphiphilic P3HT-b-PEG block copolymers (Scheme 1). To allow for well-defined block copolymers via a polymer−polymer click reaction, we are capable of controlling the molecular weight, polydispersity, and end-group functionality of the individual blocks. Thus, we can create well-defined block copolymers with systematic variation of composition by utilizing the same P3HT block and different PEGs as second block. This allows us a comprehensive study of structure formation in amphiphilic block copolymers with comparable lengths of the donor block and increasing PEG length. Starting from 2,5-dibromo-3-hexylthiophene, the active monomer, (2-bromo-3-hexylthiophene-5-yl)magnesium chloride for the KCTP is formed by a metathesis reaction.33 The active monomer is polymerized by using 1,3-bis(diphenylphosphino)propane nickel(II) chloride (Ni(dppp)Cl2) as catalyst. In the reaction a monomer:catalyst ratio of 1:80 is chosen to yield P3HT with a high molecular weight (for optimum charge charrier transport). By the reaction with ethynylmagnesium chloride at the end of the polymerization, the P3HT chains are end-capped with alkyne groups on one end (Scheme 2a).5 This P3HT-Alkyne has a Mn of 11.4 kg/mol in MALDI-ToF equivalent to a Mn of 17.7 kg/mol measured by SEC.34 This high Mn of P3HT favors efficient hole transport properties.28 Furthermore, a high molecular weight of P3HT can also promote the phase separation in block copolymers and enhance the self-assembly of the resulting P3HT-b-PEGs.4 1H NMR spectroscopy proves the expected molecular structure and the synthesized P3HT-Alkyne has a regioregularity of >96% (Figure S1). The successful insertion of the alkyne group

a rather low molecular weight and the PDI of the synthesized P3HT-b-PEG was quite high. Nevertheless, CuAAC reactions offer a good strategy for the tailored synthesis of block copolymers because they allow the modular variation of the individual block length and therefore the final composition31 in addition to the synthetic advantages of click chemistry such as high yield and fewer side reactions.32 For the preparation of donor−acceptor hybrid systems, first one has to understand the aggregation properties and phase morphologies of P3HT-b-PEO systems without any inorganic or metal particles. For this, a systematic variation of composition of the block copolymer maintaining the advantages of high Mn P3HT is required. Further, structure formation in solution for the different compositions needs to be elucidated to work out the synthetic routes toward hybrid systems for different applications. In this paper, we address these questions of synthesis and properties of well-defined P3HT-b-PEGs by incorporating four different lengths of PEG to a high molecular weight P3HT. The synthesis is done using KCTP of 2,5-dibromo-3-hexylthiophene followed by a polymer−polymer click reaction of P3HT-Alkyne and different PEG-N3s. We demonstrate that the systematic variation of PEG length influences the structure formation in solution considerably, and colloidal aggregates with different shape and size can be prepared. Thus, these aggregates in polar solvents have a crystalline P3HT core with a corona of PEG available for interaction with inorganic materials. Thus, these findings report the micellar structures of P3HT containing amphiphilic block copolymers in water for the first time. This opens up new venues for biosensor/bioimaging involving aqueous conjugated polymer micelles, and this also allows possible strategies to combine P3HT-b-PEG micelles and inorganic materials into hybrid nanocomposites with self-assembled structures. B

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bad because P3HT-b-PEG seems to form small aggregates, which are removed with the MeOH solution. Therefore, Soxhlet extraction with MeOH was applied to remove the excess PEG-N3 to minimize the losses. Afterward, the block copolymer could be retrieved by Soxhlet extraction with dichloromethane (DCM) followed by freeze-drying. For example, the SEC curve of the purified product for the P3HT-b-PEG454 synthesis (Figure 1a) shows a new signal at higher molecular weight compared to both the two precursor polymers. The measured Mp(SEC) of the new peak is 56.3 kg/ mol, which is very close to the sum of the Mps of the precursor polymers P3HT-Alkyne (Mp = 21.0 kg/mol) and PEG454-N3 (Mp = 33.8 kg/mol). The same is valid for other block copolymers as well. This proves the successful synthesis of the P3HT-b-PEG block copolymers via polymer−polymer click reaction. Additionally, in comparison to a SEC of the crude reaction product (Figure S6), the peak of the excess PEG-N3 as observed in the RI detector completely disappears, which demonstrates the effective purification by Soxhlet extraction with MeOH. However, in the SEC curve of the purified product, a second peak/shoulder is visible which is located at the Mp of the P3HT-Alkyne precursor. This signal is probably due to residual P3HT homopolymers. This can arise from either incomplete end-group functionalization of P3HT or incomplete conversion of the polymer−polymer click reaction. Several methods such as Soxhlet extraction, reprecipitation, and column chromatography, even though decreases the residual amount of P3HT homopolymer in the BCP, could not lead to a complete and effective removal of P3HT homopolymer. The amount of the residual P3HT could be estimated to be around 12 wt % by fitting and comparing the areas under the SEC curve of P3HT-b-PEG454 (Figure S7). The SEC curves of the four final block copolymers (Figure 1b) show a shift of Mp for the P3HT-b-PEGs according to the PEG block length to higher molecular weights. In all cases, the measured Mp for the block copolymer is in good agreement with the sum of the Mptheos of the two precursor polymers (Table 1). Indications of unreacted P3HT can be observed in all these SEC curves, but only for P3HT-b-PEG454 the amount could be estimated because the P3HT homopolymer peak and the block copolymer peak are separated enough in the SEC curve. The click reaction of the two precursor blocks was also monitored by comparing FT-IR spectra of the precursors and the resulting BCPs. As typical example, the data for P3HT-bPEG227 and the two precursor polymers are shown (Figure 2). The FT-IR spectrum of P3HT-Alkyne shows the characteristic vibrational band of the alkyne group at 3310 cm−1. This band disappears in the block copolymer P3HT-b-PEG227 as does the vibrational band of the azide group at 2100 cm−1 observed in PEG227-N3. The total absence of the alkyne specific vibrational bands indicates that all P3HT-Alkyne was clicked with PEG-N3, and the small residual P3HT homopolymer in the product can be attributedto P3HT with Br/Br end groups, which observed in MALDI-ToF analysis. In conclusion, a series of four P3HT-b-PEG block copolymers were successfully synthesized. The molecular weight of the PEG block was systematically varied from 2 to 20 kg/mol to have block copolymers with increasing weight fractions of PEG varying between 15 and 64 wt %. Thus, for example P3HT-b-PEG227 has almost equal weight fractions of P3HT (53 wt %) and PEG (47 wt %), whereas P3HT-b-PEG454 has a majority weight fraction of PEG (64 wt %) and a P3HT content of 36 wt %. It is to be noted that the values of Mn

at the end of the polymer can be shown by MALDI-ToF mass spectroscopy (Figure S2). In the mass spectrum one main series of peaks is visible, where the individual peaks are separated by 166.3 g/mol, which is the mass of the P3HT repeating unit. Here the end groups correspond to Br/alkyne. Additionally, there are two minor peak series which have either H/alkyne or Br/Br end groups,33 but Br-P3HT-Alkyne is according to MALDI-ToF analysis the major product. It is to be noted that P3HT with Br/Br end groups cannot take part in the click reaction. For the preparation of PEG-N3 blocks, four different poly(ethylene glycol) monomethyl ethers (PEG-OHs) are treated in a postpolymerization reaction to obtain monotosylated PEGs (PEG-Tos) which are then reacted with NaN3 (Scheme 2b). The facile conversion of PEG-OH to PEG-Tos and PEG-N3 in high yields is documented by 1H NMR (Figure S3) and FT-IR spectroscopy (Figures S4 and S5). On the basis of these polymer building blocks, four P3HT-bPEGs were obtained by using a CuAAC reaction with gradual increase in PEG length. The success of block copolymer synthesis was first verified by using SEC analysis (Figure 1).

Figure 1. (a) SEC curves of P3HT-Alkyne, PEG454-N3, and the purified P3HT-b-PEG454 product measured with THF containing 0.25 wt % tetrabutylammonium bromide (TBAB) as eluent. (b) SEC curves of the four block copolymers P3HT-b-PEG45, P3HT-b-PEG113, P3HTb-PEG227, and P3HT-b-PEG454 and the precursor P3HT-Alkyne measured with THF containing 0.25 wt % TBAB as eluent.

It is to be noted that the removal of unreacted PEG-N3 from the block copolymers is much easier than the removal of residual P3HT precursor polymer. Therefore, the synthesis was carried out using a 2-fold excess of PEG-N3 for all cases in order to convert P3HT-Alkyne to a maximum extent in the polymer−polymer click reaction. Higher ratios of PEG-N3 have no more beneficial effect on the conversion. The excess PEGN3 could be removed by precipitation in MeOH, which is a good solvent for PEG. P3HT is not soluble in MeOH. However, by precipitating in MeOH, we found that the yield is C

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Table 1. Overview on the Synthesized Polymers and Their Molecular Characterization by SEC Using THF + 0.25 wt % TBAB as Eluent and MALDI-ToF Mass Spectroscopy polymer P3HT-Alkyne PEG454-N3 PEG227-N3 PEG113-N3 PEG45-N3 P3HT-b-PEG454 P3HT-b-PEG227 P3HT-b-PEG113 P3HT-b-PEG45

Mn(MALDI) [kg/mol]

Mn(SEC) [kg/mol]

Mp(SEC) [kg/mol]

11.4 20.4 10.0 5.3 1.8

17.7 31.8 15.8 7.4 2.7 32.0 25.9 22.8 20.0

21.0 33.8 16.7 8.0 2.8 56.3 39.4 30.3 23.8

Mptheo(SEC) [kg/mol]

PDI (SEC)

repeating units P3HT/PEG

f(P3HT) [w/w]

68/− −/454 −/227 −/113 −/45 68/454 68/227 68/113 68/45

1.00

54.8a 37.7a 29.0a 23.8a

1.15 1.06 1.06 1.04 1.05 1.44b 1.47b 1.32b 1.22b

0.36 0.53 0.69 0.85

a Calculated for 100% click yield as sum of Mps of the individual precursor blocks. bThe broad PDI values originate from the residual P3HT present in the samples.

Figure 2. Comparison of FT-IR spectra of P3HT-Alkyne, PEG227-N3, and P3HT-b-PEG227 as a typical example for the block copolymer synthesis. The characteristic band of the alkyne end group at υ = 3310 cm−1 in P3HT-Alkyne and the characteristic azide band at υ = 2100 cm−1 in PEG227-N3 completely disappear in the product P3HT-bPEG227.

Figure 3. Second heating and cooling curves from differential scanning calorimetry for P3HT-Alkyne, P3HT-b-PEG454, P3HT-b-PEG227, P3HT-b-PEG113, and P3HT-b-PEG45 measured with a heating and cooling rate of 10 K/min.

(SEC) and PDI (SEC) in Table 1 are calculated for complete elution curves, where still some amount of residual P3HT is present and which broadens the molecular weight distribution. Thermal and Optical Characterization of P3HT-bPEGs. The synthesized block copolymers were characterized by thermal gravimetric analysis (TGA) and differential scanning calorimetry (DSC). In TGA measurements, all block copolymers show two-step degradation (Figure S8). In a first step, the PEG block decomposes starting at about 200 °C as can be inferred from the TGA of pure PEG454-N3. In a second step at temperatures above 350 °C, the P3HT block starts thermal degradation. Compared to the pure P3HT-Alkyne and the pure PEG-N3, the TGA curves of the P3HT-b-PEG block copolymers show the characteristics of both precursor polymers. As expected from the weight fractions of the two blocks, the shape of the TGA curves is dominated by PEG degradation for P3HT-b-PEG454 and changes with decreasing PEG block length to a more and more P3HT-like decomposition. The TGA curves with two-step decomposition for the block copolymers give additional evidence for the successful synthesis of P3HT-b-PEG. Since both PEG and P3HT are semicrystalline polymers, the DSC analysis can give an insight regarding structure formation and phase separation of the bulk block copolymer material (Figure 3). For comparison the DSC curve of P3HT-Alkyne is also given. The DSC curves of the individual PEG-N3s are shown in Figure S9. All the four block copolymers exhibit the melting and crystallization peaks corresponding to the individual blocks. This is a clear indication that the two segments do not mix and they are phase separated. To

understand the influence of one block on the crystallinity as well as kinetics of crystallization of the other block, the melt enthalpies (ΔHm) and crystallization temperatures (Tc) are compared with those of the homopolymers. The DSC curves of P3HT-Alkyne show a melting point Tm at 237 °C and a crystallization temperature Tc of 172 °C. These thermal transition signatures of P3HT are observed in all of the four block copolymers. For P3HT-b-PEG454 and P3HT-bPEG227 there is a small increase of Tc (180 °C and 178 °C), whereas for the block copolymers with shorter PEG blocks the increase of Tc is even higher (189 °C for P3HT-b-PEG113 and 196 °C for P3HT-b-PEG45). This indicates that on cooling from melt, the crystallization of P3HT in block copolymers is favored by attaching a second block, and there is no kinetic hindrance whatsoever. Obviously, the absolute ΔHm values for the P3HT block decrease with increasing PEG content. Therefore, to investigate the crystallinity of P3HT in the block copolymers, the melting enthalpies ΔHm of the P3HT melting peaks (Table 2) are compared to the expected ΔHm(theo) values. These values are calculated by taking into account the weight fraction of P3HT and ΔHm of an unfunctionalized P3HT. A P3HT with comparable molecular weight (Mn = 10.7 kg/mol from MALDI-ToF) as used in this study exhibits a ΔHm of 22.9 J/g, which is 69% of ΔH∞ m (33 J/ g) for crystalline P3HT reported in the literature.36 The measured values of ΔHm for the four block copolymers are almost similar to those calculated ΔHm(theo) based on the corresponding wt % of the components, and there is no systematic trend in correlation to PEG block length. So overall the melting enthalpy of the P3HT block in the P3HT-b-PEG D

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Table 2. Thermal Properties of the Synthesized P3HT-b-PEG Block Copolymers and the Corresponding Precursors P3HTAlkyne and PEG-N3 Homopolymersa PEG block polymer

Tm [°C]

ΔHm [J/g]

P3HT block ΔHm(theo)b [J/g]

Tm [°C]

ΔHm [J/g]

237.3 217.3

22.9 16.5

102.8

215.8

8.7

8.3

77.8

220.7

10.3

12.2

51.5

222

13.6

15.9

22.8

222

17.9

19.5

c

P3HT-Br P3HT-Alkyne PEG454-N3 P3HT-b-PEG454 PEG227-N3 P3HT-b-PEG227 PEG113-N3 P3HT-b-PEG113 PEG45-N3 P3HT-b-PEG45

64.2 63.3 62.7 57.5 60.2 49.8 54 31.9

161.3 80.4 166.7 47.6 168.8 25.2 153.1 9.2

ΔHm(theo)b [J/g]

All the ΔHm values are taken from second heating cycle. bExpected ΔHm value for the corresponding wt % in each sample assuming the same crystallinity as in pristine blocks. cComparable nonfunctionalized P3HT as used in this study Mn(MALDI) = 10.7 kg/mol.

a

block copolymers is hardly affected by the incorporation of the PEG block at one chain end. Additionally, the DSC curves of the P3HT-b-PEG block copolymers show the characteristic melting and crystallization peaks of the PEG blocks at lower temperatures. Compared to the Tm and Tc of the individual precursor PEG-N3 polymers (Figure S9), a clear influence of PEG block length on thermal properties of PEG blocks in block copolymers can be observed. For example, P3HT-b-PEG454 with the highest weight fraction of PEG, the melting temperature of the PEG block (Tm = 64 °C) is the same as for the pure PEG-N3 polymer. With decreasing PEG block lengths, the Tm of PEG blocks in block copolymers are shifted to lower temperatures. In P3HT-bPEG227 Tm(PEG) is 5 °C lower, in P3HT-b-PEG113 Tm(PEG) is 11 °C lower, and in P3HT-b-PEG45 Tm(PEG) is even 18 °C lower than for the corresponding PEG homopolymer. This is a clear indication that the crystallization of PEG in block copolymers is kinetically hindered. Furthermore, the melting enthalpies of the PEG blocks in P3HT-b-PEGs, in contrast to aforementioned enthalpies of P3HT blocks, are strongly influenced by the composition. The comparison of ΔHm(theo) for the PEG blocks, calculated in a similar way as for the P3HT blocks, shows a considerable decrease in crystallinity with decrease of the PEG block length. The difference (ΔHm dif) in P3HT-b-PEG454 is 22% whereas in P3HT-b-PEG227 the difference increases to 39%, and in P3HT-b-PEG113 to 51%, and in P3HT-b-PEG45 ΔHm is even 60% smaller than that expected for the crystallization of the same length of PEG block, taking into account the decrease in PEG content. This clearly shows that the PEG crystallization in an already crystallized P3HT matrix is hindered considerably. This is also in accordance with the observed lowering of Tc of PEG block for P3HT-b-PEG113 and PEG-b-PEG45. In conclusion, the DSC analysis shows that for all P3HT-b-PEGs the crystallinity of PEG is reduced considerably, whereas the P3HT crystallization is not influenced to that extent. Especially for the block copolymers with shorter overall molecular weight P3HT-b-PEG113 and P3HT-b-PEG45 (i.e., smaller molecular weight of the PEG blocks) the decrease of Tm and crystallinity in the PEG blocks is drastic. Besides the thermal properties, absorption studies can deliver information on aggregation due to crystallization of P3HT block in P3HT-b-PEG block copolymers. For this, absorption spectra in solution and thin films were measured and compared (Figure 4). The UV−vis spectra in chloroform solution (Figure

Figure 4. (a) UV−vis spectra of P3HT-Alkyne and all P3HT-b-PEG block copolymers in chloroform solution with a concentration of 0.04 mg/mL. (b) UV−vis spectra of thin films on glass obtained by spincoating P3HT-Alkyne and the four P3HT-b-PEGs from 1 wt % chloroform solution. The spectra are normalized to the absorption peak at 550 nm (0−1 transition).

4a) show an absorption band for both the P3HT-Alkyne and the P3HT-b-PEG block copolymers ranging from 300 to 550 nm with a maximum at about 450 nm. Since the measurements were done in solution with a constant concentration (0.04 mg/ mL), the absolute intensities of the absorption are lower for P3HT-b-PEGs than for pure P3HT-Alkyne, and the intensities correspond to the weight fraction of P3HT in the individual block copolymers. The measurements indicate that the block copolymers are fully dissolved in chloroform because all samples show the typical absorption for amorphous P3HT caused by the π−π* excitation in the chromophores of the coiled P3HT main chain. As expected, PEG does not contribute to the absorption spectra between 300 and 700 nm, and the length of the PEG block coupled to P3HT only influences the concentration of the P3HT chromophores in the material and thereby the absolute intensity of the absorption. E

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Macromolecules Second, the UV−vis spectra of as-cast thin films obtained by spin-coating from 1 wt % solutions in chloroform were measured to investigate the aggregation behavior of P3HT-bPEGs in solid state. As known for P3HT, there is a red-shift of the absorption in comparison to the solution spectra. All block copolymers show an absorption in the wavelength range 350− 680 nm which has a maximum at 520 nm and two distinct shoulders at 550 and 605 nm which are assigned to 0−2, 0−1, and 0−0 transitions. These additional vibronic bands of the absorption curve are also seen in the pure P3HT-Alkyne film spectrum and arise from weakly bound H-J aggregates in the P3HT crystallites.37−39 In detail, there is no systematic change of the intensity of the two bands in correlation with the PEG content in the four P3HT-b-PEGs. Thus, the film spectra show that P3HT crystallizes in all the P3HT-b-PEGs in a similar way as homopolymers, which supports the crystallization behavior of P3HT seen in DSC. Preparation of Block Copolymer Micelles from P3HTb-PEGs. One main motivation for the synthesis of P3HT-bPEGs is to study the nature of aggregation in solutions which is possible due to the amphiphilic character of such block copolymers. While P3HT is a nonpolar polymer which is soluble e.g. in THF, chloroform, and dichlorobenzene, PEG is soluble in many nonpolar and polar solvents including THF, dichloromethane, EtOH, MeOH, ethyl acetate, and water. Therefore, we assume that we can induce the formation of selfassembled microstructures when we change the solvent from a good solvent for both blocks to a selective solvent for PEG. As shown in the literature, P3HT-b-P4VP, P3HT-b-P2VP, and P3HT-b-PEG are able to form stable aggregates in selective solvents.11,13,24 To initially prove the concept of microstructure formation in a simple experiment, MeOH is added to solutions of P3HTAlkyne and P3HT-b-PEG454 in THF. While the P3HT-Alkyne fully precipitates after addition of MeOH, P3HT-b-PEG454 forms a transparent solution without visible precipitation. Further, the color of the P3HT-b-PEG454 solution turns to violet after MeOH addition (Figure S10), and the resulting solution shows no precipitation of the material even after weeks. For further investigation of the structure formation of P3HTb-PEG block copolymers in solution under controlled conditions, we built an experimental setup (Figure 6a) with a syringe pump, which can dose MeOH (or other solvents) with a defined addition rate through a syringe into a flask with the block copolymer dissolved in THF (1 mg/mL). The overall added amount of MeOH is chosen to have a volume ratio of MeOH:THF of 9:1 at the end. The block copolymer solution is stirred strongly to get a homogeneous mixing and to avoid local oversaturation. After the addition of MeOH, a violet colored clear solution is formed, which has a concentration of ∼0.1 mg/ mL. To investigate the nature of the formed aggregates, the solution of P3HT-b-PEG454 in MeOH:THF (9:1 v/v) as a typical example is measured by transmission electron microscopy (TEM). A sample for TEM is prepared simply by dropping the solution onto a TEM grid. The TEM images (Figure 5a) show small spherical and cylindrical structures with a size of about 20 nm diameter and 80 nm length which are visible without any additional staining of the sample. We assume that the structures are block copolymer micelles with P3HT in the core surrounded by a PEG corona. The surrounding PEG chains solubilize the micelles in MeOH:THF

Figure 5. (a) TEM image of the dried colloidal aggregates formed by P3HT-b-PEG454 directly after preparation in the THF:MeOH solution on the TEM grid. (b) Cryo-TEM image of the same P3HT-b-PEG454 sample after dialysis against water.

where pure P3HT is not soluble, and thus a precipitation is hindered. To further analyze these micelles, dynamic light scattering (DLS) measurements were conducted on the P3HTb-PEG454 solutions in MeOH:THF. The hydrodynamic radius Rh which can be measured is ∼30 nm, which is in the range of the particles sizes observed by TEM. It is to be noted that the structures observed in TEM (Figure 5a) are not direct solution structures, but after drying on the TEM grid and therefore there may be agglomeration of individual micelles and other surface effects that change the shape of the structures due to different local concentrations during solvent evaporation. To get a real insight into the solution structures of P3HT-b-PEG micelles, we figured out a way to transfer the block copolymer micelles into aqueous solutions. The MeOH:THF solution directly after preparation of the P3HT-b-PEG micelles is exchanged by deionized water through dialysis. Aqueous solutions have the big advantage that the samples can be vitrified and investigated by cryo-TEM, which provides an image of the as-formed BCP micelles in solution. DLS measurements before and after dialysis prove F

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Macromolecules that the structures are not changed in size during the process (Figure S11). For example, the Rh values for P3HT-b-PEG454 are almost the same, and also the calculated size distribution is comparable. Thus, we conclude that we do not change the shape and size of the micelles by dialysis. The final aqueous solutions of P3HT-b-PEG454 micelles are stable over months and have a concentration of about 0.1 mg/mL. The cryo-TEM images, for example, of P3HT-b-PEG454 micelles (Figure 5b) show small specific objects, which have a compact round shape or a little longer wormlike appearance. We assume that the contrast in the unstained P3HT-b-PEG454 samples in cryo-TEM arises from the crystalline P3HT-core which has a much higher electron density than the solubilizing and dissolved PEG corona. The distance between the individual objects is much bigger than in the TEM image of the dried micelles (Figure 5a). Thus, with the dialysis method, we are able to obtain block copolymer micelles from P3HT-b-PEG block copolymers and can image them in their original aqueous solution state.

were interested whether the size, shape, and stability of micelles are influenced by the composition of the P3HT-b-PEG block copolymer. Therefore, the above-described preparation of micelles was used for all of the four P3HT-b-PEG block copolymers, the MeOH addition rate being 0.2−0.33 mL/min. The first finding from these experiments is that on addition of MeOH P3HT-b-PEG227 and P3HT-b-PEG113 form stable micelles similar to P3HT-b-PEG454. All the resulting solutions are without any precipitation and have a violet color indicating aggregated P3HT species. In contrast, P3HT-b-PEG45 does not form stable micelles in aqueous solution, and similar to pure P3HT-Alkyne the material precipitates completely. It seems that a weight fraction of 15 wt % PEG is not enough to stabilize the aggregates that form by crystallization of P3HT segments. So we concentrate in the following discussion on the three block copolymers P3HT-b-PEG454, P3HT-b-PEG227 and P3HT-b-PEG113, which form stable colloidal solutions during MeOH addition. To compare the micelles from the different P3HT-b-PEGs, the MeOH:THF solutions of the three longer block copolymers were dialyzed against water and analyzed by DLS. The measured autocorrelation functions of the three solutions (Figure 6b) show a decay that includes only one mode which is an indication for the uniformity of the scattering particles in the solutions. While P3HT-b-PEG454 and P3HT-b-PEG227 micelles show a comparable decay, the decay of the autocorrelation function of P3HT-b-PEG113 is slower, which indicates bigger micelles sizes for the shortest PEG chain. As a next question, we addressed the state of the P3HT in the core of the P3HT-b-PEG micelles. For this, we measured UV−vis spectra of the micellar solutions in water (Figure 7).

Figure 6. (a) Preparation method for solution structures of P3HT-bPEG block copolymers. To a P3HT-b-PEG in THF solution (0.5−1.0 mg/mL), MeOH is added in a controlled way by a syringe pump up to a volume ratio of THF:MeOH of 1:9. By dialysis, the solvent of the colloidal aggregates is changed to water. (b) Results from DLS measurements at 90° for the different P3HT-b-PEG micelles in water: Intensity−time autocorrelation function as a function of the delay time.

Figure 7. UV−vis spectra of the P3HT-b-PEG454, P3HT-b-PEG227, and P3HT-b-PEG113 micelles in water. All spectra were normalized to the 0−1 transition at 550 nm.

The absorption maximum of all the three block copolymers is shifted to higher wavelength compared to the chloroform solutions. The absorption curves have shoulders at 550 and 610 nm which have already been seen in the film spectra (Figure 4) and which are a clear indication that P3HT in the micelles is in a crystalline state. From UV−vis data we conclude that P3HT in the P3HT-b-PEG micelles is in a crystalline state and the degree of crystallization in micelles is comparable for P3HT-bPEG454 and P3HT-b-PEG227, whereas the 0−2 vibration shoulder is less intensive for the block copolymer P3HT-bPEG113. Furthermore, the aqueous solutions were vitrified and investigated by cryo-TEM (Figure 8a−c). For P3HT-bPEG454 with the longest PEG chain, we observe mostly spherical micelles with an average diameter of 26 ± 8 nm

It is to be noted that a direct preparation of micelles in aqueous solution by adding water instead of MeOH to the THF solution of P3HT-b-PEG454 is not possible. In this case, the block copolymer does not form stable micelles, but the material precipitates from a water:THF mixture. Therefore, it is necessary to do the micelle preparation by addition of MeOH and afterward dialysis against water. Influence of Composition on Colloidal Structures of P3HT-b-PEG in Aqueous Solutions. After the development of a method for micelle preparation and characterization, we G

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the surrounding PEG corona. However, the AFM measurements confirm that the micellar structures observed by cryoTEM after vitrification are also present after spin-coating from aqueous solutions. The micelles formed by P3HT-b-PEG454 have a spherical or oval shape with an average diameter of 35 ± 9 nm (Figure 8d). On AFM overview images (scan size 3 μm, Figure S12) the micelles seem to cluster on the substrates, and the arrangement is roughly hexagonal. In AFM, the micelles formed by P3HT-bPEG227 (Figure 8e) have a similar shape compared to P3HT-bPEG454. But with 54 ± 8 nm the average size of P3HT-bPEG227 micelles is a bit bigger than the size of P3HT-b-PEG454. Also for P3HT-b-PEG113, imaging of single micelles was done by AFM (Figure 8f). Here, we observe a comparable shape as seen in cryo-TEM, which means that the micelle shape is illdefined and the micelles seem to have branches and consist of different compartments. The diameter of the micelles is bigger than in cryo-TEM images, which is again resulting from the AFM imaging of both the PEG corona and the P3HT core, while in cryo-TEM only the crystalline P3HT core is visible. All together the structures found by AFM affirm the structures observed in cryo-TEM and the change in micelles sizes is in good agreement with the DLS measurements. Based on the above detailed analysis of size and shape of the micelles and taking into account the fact that P3HT is in crystalline phase, a schematic model for the micellar structures for different PEG lengths is shown in Figure 8g.

Figure 8. Cryo-TEM (a−c) and AFM height images (d−f) of P3HTb-PEG454, P3HT-b-PEG227, and P3HT-b-PEG113 micelles. (g) Schematic model shows a possible structure of the formed aggregates. Height scale of AFM images is −10 to 15 nm (d + e) and −20 to 25 nm (f).

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CONCLUSION In conclusion, we could successfully synthesize a series of welldefined P3HT-b-PEG block copolymers via a polymer− polymer click reaction between P3HT-Alkyne and different PEG-N3s. Keeping the Mn of P3HT constant, we systematically changed the composition of the resulting P3HT-b-PEG by using four different PEG block lengths. The thermal and optical properties of these block copolymers were systematically investigated, and we found that for all P3HT-b-PEGs phase separation in bulk can be confirmed. Second, we could confirm the drastic influence of crystallinity of PEG blocks compared to negligible effects on the crystallinity of P3HT blocks on cooling the block copolymers from melt. Further, we investigated the ability of P3HT-b-PEGs to form self-assembled block copolymer micelles when the solvent is changed to a selective solvent for PEG. For P3HT-b-PEG454, P3HT-b-PEG227, and P3HT-b-PEG113, we could prepare stable micelles in THF/ MeOH. These micelles were transferred to stable aqueous solutions, which were imaged using cryo-TEM and AFM. These micelles consist of a crystalline P3HT core which is surrounded by a solubilizing PEG shell, and the shape and size of the micelles are influenced by the length of the PEG block. These findings help to understand the structure formation of amphiphilic block copolymers with one conjugated block in solution. This systematic study of micellar structures of amphiphilic semiconductor block copolymers allows further investigation in different fields such as self-assembled hybrid materials, infiltration of biomembranes, and their optical imaging.

(Figure 8a). In contrast, the micelles formed by P3HT-bPEG227 have a more wormlike or cylindrical shape, but also smaller spherical micelles are present in the sample (Figure 8b). The diameter of the cylindrical micelles is 21 ± 5 nm, and their length ranges from 40 up to 300 nm. The reason for this change in structural size is the difference in the weight fraction of PEG and the corresponding change in volume of the PEG chains that are coupled to the P3HT chains. This trend continues also for P3HT-b-PEG113 (Figure 8c), where the cryoTEM images show long micellar aggregates that have additionally branched shapes and the overall size is much bigger compared to the both P3HT-b-PEGs with longer PEG blocks. The diameter of the individual parts of the aggregated micelles is 24 ± 8 nm, which is quite similar to the diameters of the other P3HT-b-PEG micelles. Therefore, we assume that in cryo-TEM the crystalline cores of the P3HT-b-PEG micelles have a comparable structure due to stabilization by PEG, and only the length and the tendency to grow together are influenced by the PEG block length. However, it is to be noted that the measured structural sizes may not exactly reproduce the core size alone, and it can be influenced by the surroundings, which interact with the PEG shell. Additionally, we conducted complementary atomic force microscopy (AFM) measurements to confirm the micellar structures seen in electron microscopy. The advantage of AFM is that the whole micelle can be imaged and no staining is necessary. The aqueous solutions of P3HT-b-PEG micelles were spin-coated on freshly cleaved mica substrates and then imaged by PeakForce Tapping AFM in air. On the resulting images (Figure 8d−f) the block copolymer micelles can clearly be seen as individual particles on the flat mica substrate. It is to be noted that during the spin-coating process the micelles are dried and probably the PEG also crystallizes when getting dried. Thus, the AFM images show the complete micelle, and it is not possible to distinguish between the crystalline P3HT core and

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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.6b01305. H

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Detailed synthesis route, 1H NMR spectra, MALDI-ToF spectra, data from TGA, DSC, FT-IR, SEC, DLS, and additional AFM images (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail [email protected] (M.T.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We thank Dr. Markus Drechsler (BIMF, University of Bayreuth) for cryo-TEM measurements and Martin Hufnagel (University of Bayreuth) for MALDI-ToF measurements. We kindly acknowledge financial support from DFG (SFB 840-TP B7 and TP C4). P.M.R. thanks the Elite Network Bavaria Macromolecular Science programme at the University of Bayreuth and the Max Weber-Programm for financial support during his studies.

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