Poly(3-hexylthiophene)-block-poly(tetrabutylammonium-4

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Poly(3-hexylthiophene)-block-poly(tetrabutylammonium-4styrenesulfonate) Block Copolymer Micelles for the Synthesis of Polymer Semiconductor Nanocomposites Paul M. Reichstein, Johannes Brendel, Markus Drechsler, and Mukundan Thelakkat ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.9b00108 • Publication Date (Web): 14 Mar 2019 Downloaded from http://pubs.acs.org on March 20, 2019

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ACS Applied Nano Materials

Poly(3-hexylthiophene)-blockpoly(tetrabutylammonium-4-styrenesulfonate) Block Copolymer Micelles for the Synthesis of Polymer Semiconductor Nanocomposites Paul M. Reichstein, † Johannes C. Brendel, †,§ Markus Drechsler, ‡ Mukundan Thelakkat*,†,‡ †

Applied Functional Polymers, Macromolecular Chemistry I and ‡ Bavarian Polymer Institute

(BPI), University of Bayreuth, Universitätsstr. 30, Bayreuth 95440, Germany

KEYWORDS: Nanocomposites, P3HT, PSS, TiO2, block copolymer, self-assembly

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ABSTRACT: We present the synthesis of a new amphiphilic Poly(3-hexylthiophene)-blockpoly(tetrabutylammonium-4-styrenesulfonate)

P3HT-b-PSS-/TBA+

block

copolymer

via

polymer-polymer click reaction. The P3HT-alkyne block has a molecular weight of 17.6 kg/mol (SEC) / 11.3 kg/mol (MALDI-ToF) and the block copolymer has a high purity due to the complete removal of both precursor blocks by a column chromatographic approach. The fluorinated styrene sulfonate polymer P(SS-CH2-CF3)-N3 is synthesized by Reversible Addition Fragmentation Transfer (RAFT) polymerization and a specific RAFT agent is used to avoid labile ester linkages. SEC with THF as solvent overestimates the molecular weight (MW) of P(SS-CH2-CF3)-N3, but 1H-NMR spectroscopy allows for the exact calculation of the molecular weight. We show that the precursor block copolymer P3HT-b-P(SS-CH2-CF3) forms wormlike micelles by self-assembly with a crystalline P3HT core (diameter < 30 nm) in polar solvents. In these BCP micelles the P(SS-CH2-CF3) precursor in the corona can be deprotected and P3HT-bPSS-/TBA+ micelles are formed. Additionally, the compartmentalized in-situ synthesis of highly crystalline TiO2 (anatase) nanocrystals (< 10 nm) stabilized within the PSS corona of the micelles is demonstrated. The resulting P3HT-TiO2 nanocomposites with well-defined nanoscale morphology are water-processible and they are investigated in detail by cryo-TEM and UV-vis spectroscopy.

The organization of materials on the nanoscale offers access to unprecedented properties and has been in the focus of research for several decades now. In particular, block copolymers are one main class of materials used in this context due to their ability to form well-defined selforganized nanostructures.1,2 The synthesis of well-defined block copolymers allows on the one hand the control of microdomains with sizes down to a few nanometers3,4 and on the other hand


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the formation of complex self-assembled structures in solution.5 In order to realize a morphology-controlled semiconductor nanocomposite, which has potential applications in hybrid solar cells, one recent approach utilizes the direct self-assembly of inorganic semiconductors and organic hole transport materials on the nanoscale.6 For this concept, a hydrophilic block such as Polyethylene oxide (PEO),7 Poly(2-vinylpyridine) (P2VP),8 Poly(3-hexylthiophene)-graft-PEO (P3HT-g-PEO)9 or Poly(styrene sulfonate) (PSS)10 is necessary to interact and coordinate an inorganic electron transport material such as TiO2 or CdSe or their precursors. Usually a welldistributed nanocomposite is achieved, which is not attractive for phase separated applications such as catalysis and photovoltaics. Therefore, in this paper we concentrate on the well-defined synthesis of a hole conductor block copolymer, which allows the formation of a donor-acceptor hybrid nanocomposite having compartmentalized nano-domains of TiO2 in a matrix of P3HT. Additionally, block copolymers with a semiconducting P3HT block such as P3HT-b-PEO11 can be synthesized that show amphiphilic behavior and can be directly used to get hybrid nanocomposites. However, the formation of crystalline inorganic materials remains a crucial issue to maintain the excellent charge transport properties of these semiconductors. In particular, inexpensive metal oxides such as TiO2 usually require high temperatures (> 400 °C) to form the beneficial crystal structures. These high temperatures are not compatible with most semiconducting polymers, because their thermal stability usually does not exceed 300 °C. A potential solution is the formation of highly crystalline TiO2 during the reaction of the precursors using a catalyst. For this purpose, the introduction of coordinating groups such as sulfonates into the polymer is suitable, because they can catalyze the formation of TiO2 crystals at low temperatures (< 50°C).12,13 Block copolymers with PSS blocks have been synthesized and investigated in different fields of soft matter research. Among them are studies on Polystyrene-b-


3


PSS,14,15

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PEO-b-PSS,16 PSS-b-Poly(N-isobutyl acrylamide) (PNIPAAm)17 and PSS-b-

Poly(methylbutylene) (PSS-b-PMB).18-22 Up to now, only three recent publications have introduced block copolymers based on P3HT and PSS. Jiang and coworkers23 synthesized a vinyl end-capped P3HT and used it as macromonomer in a free radical polymerization of sodium styrenesulfonate to obtain P3HT-b-PSS-/Na+. By using impedance spectroscopy, they found that the copolymer had a better conductivity and explained this finding with a self-doping of the material where the PSS chains dope the P3HT. They also could show that mixing with CdSe nanoparticles in solution, results in a uniformly distributed nanocomposite without percolation pathways for electrons even though photoluminescencequenching in the composite films indicated charge transfer. Furthermore, a molecular weight Mn = 6.6 kg/mol (SEC) of P3HT block will hinder a high charge carrier mobility which shows maximal values24 for molecular weights of 18.5 kg/mol (SEC) or 12.4 kg/mol (MALDI-ToF) respectively, and the structure formation of the amphiphilic copolymer was not investigated in this study. A similar synthetic approach was used by Yao et al. for the synthesis of P3HT-b-(PSstat-PSS-/H+).25 They oxidized vinyl-capped P3HT to yield terminal hydroxyl functionalities and attached a RAFT agent by esterification there. The RAFT polymerization of styrene resulted in a P3HT-b-PS block copolymer and by uncontrolled partly sulfonation in a polymer analogous reaction the final block copolymer was formed. They obtained an Mn of about 10.0 kg/mol (SEC) for the P3HT precursor which corresponds to an absolute molecular weight of about 6.3 kg/mol (MALDI-ToF). Finally, 90 % of the PS units are sulfonated to yield a water-soluble block copolymer with a high ionic conductivity of 1 S/m. The synthesized P3HT-b-(PS-stat-PSS-/H+) was moreover used for the dispersion of single-walled carbon nanotubes to get easily processible highly conducting inks.


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The first approach for a controlled synthesis of P3HT-b-PSS-/H+ block copolymers was reported by Erothu et al.26 using click chemistry to combine an azide-functionalized PSS with P3HT-alkyne. A series of three Poly(neopentyl styrenesulfonate)s PNSS with molecular weights between 2.6 and 6.2 kg/mol was synthesized using a RAFT agent bearing an azide group suitable for CuAAC and attached to a P3HT-alkyne with Mn = 12.6 kg/mol (SEC). By thermal treatment of these precursor polymers at 150 °C the neopentyl group could be remove to yield one block of poly(styrene sulfonic acid). GIWAXS measurements of thin films showed that microphase separated structures are only present in P3HT-b-PSS-/H+ and not in the precursor polymer. Both precursor and P3HT-b-PSS-/H+ were finally used as electron blocking layers in P3HT:PCBM blend solar cells. In this manuscript, we are not aiming at the preparation of free-standing TiO2 as such in anatase form or a mesoporous TiO2 film. Our objective is the incorporation of anatase TiO2 in an in-situ method at room temperature without sacrificing the semiconductor block copolymer (structure directing template), which itself is one of the active materials. Thus we demonstrate a non-sacrificial approach towards semiconductor hybrid materials. This is more elegant since we can use our polymer semiconductor nanocomposites directly in thin films, e.g. for hybrid solar cells, where the P3HT acts as hole-conductor and the TiO2 nanocrystals acts as electron conductor, without any additional steps of calcination, backfilling etc. We chose the specific synthesis route to ensure the combination of P3HT (with a sufficient molecular weight for good charge transport properties) and highly crystalline TiO2 leading to the required nanoscale morphology in a hybrid material for charge transport paths. To realize this, we prepared a welldefined block copolymer combining the semiconductor P3HT and PSS-, which has previously been demonstrated to effectively form anatase TiO2 at low temperatures. For obtaining the PSS-


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block we used a new and elegant route involving trifluoroethanol as protecting group for the sulfonic acid. The precursor block copolymer P3HT-b-P(SS-CH2-CF3) was prepared using the copper catalyzed azide-alkyne cycloaddition (CuAAC) of heterotelechelic P3HT and the protected precursor poly(styrene trifluorethyl sulfonate)

P(SS-CH2-CF3)-N3 which was

synthesized by RAFT polymerization. To enable the successful click reaction between the polymers a novel chain transfer agent (CTA) had to be developed which excludes labile ester linkages. After self-assembly and deprotection of the P(SS-CH2-CF3) block we introduced TiO2 by the controlled in-situ hydrolysis of the common precursor titanium(IV) isopropoxide. This results in polymer semiconductor nanocomposites, wherein TiO2 nanocrystals are exclusively formed in the PSS corona of the wormlike P3HT-b-PSS micelles. All polymers were characterized with SEC and NMR, while the click reaction was followed by FT-IR spectroscopy and the micelles as well as the nanocomposites were investigated by transmission electron microscopy (TEM) and cryo-TEM. To the best of our knowledge, we present for the first the combination of TiO2 and P3HT-b-PSS block copolymers in a self-assembled hybrid material.

RESULTS AND DISCUSSION Synthesis of block copolymer P3HT-b-P(SS-CH2-CF3) For the synthesis of P3HT-b- PSS-(CH2-CF3) we use a polymer-polymer click approach via CuAAC reaction since both blocks and thus the composition of the block copolymer can be tailored individually in this way. For a defined block copolymer, it is important to synthesize both blocks in a controlled way and with an exclusively one end group suitable for the desired coupling reaction.27 Herein we use Kumada catalyst transfer polymerization (KCTP) for the


6

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synthesis of P3HT-alkyne 3 and RAFT polymerization for the synthesis of PSS-(CH2-CF3)-N3 2 as precursor for PSS (Scheme 1).

S

N3

O S O O CF3

S S C12H25 AIBN 2-Butanone 80°C

N3

O S O O CF3 2

C6H13 S

C6H13 C12H25

S

1

Br

S

S m

n

Br

CuBr/PMDETA

S

Br

S 3

S

S m

S

4

C12H25 C4H9

C4H9 N+ + OHC4H9 C4H9

O S O CF3 O

m

Br

S

n

C12H25

S

O S O O CF3

4

P3HT-b-P(SS-CH2-CF3)

C6H13 MeOH

S

S

N N N

n

P(SS-CH2-CF3)-N3

N N N

n

THF C6H13

5 P3HT-b-PSS-/TBA+

S

S

N N N

m

C12H25

S

C H C H O S O 4 9 N+ 4 9 O C4H9 C4H9

Scheme 1. Overview on the synthesis of the block copolymer P3HT-b-P(SS-CH2-CF3) 4 via CuAAC reaction of P3HT-alkyne 3 and PSS-(CH2-CF3)-N3 2. The precursor polymers are synthesized in a controlled way by KCTP and RAFT polymerization and the polymer analogous reaction with tetrabutylammonium hydroxide yields the final P3HT-b-PSS-/TBA+ 5. As shown in previous publications of our group P3HT-alkyne can be synthesized with molecular weights of about 18 kg/mol (SEC) corresponding to an absolute molecular weight of 11 kg/mol (MALDI-ToF), high regioregularity > 96 % and with defined alkyne end groups, which can be utilized for polymer-polymer coupling reactions.28,29 Following this procedure, we synthesized a P3HT-alkyne with a molecular weight of 11.3 kg/mol measured by MALDI-ToF (Fig. S1A). A detailed analysis of the MALDI-ToF spectrum (Fig. S1B-C) revealed that the main series of peaks originates from P3HT with one bromine and one alkyne end group. A second series carries an alkyne group and a hydrogen instead of bromine. Two minor series can be


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identified as Br-P3HT-Br and H-P3HT-Br, which are removed via chromatography at a later stage. The presence of the alkyne end group can also been shown in the 1H-NMR (Fig. S2). To ensure a similar solubility of the two precursor polymers we use sulfonic acid protected by 2,2,2trifluoroethanol as a precursor which was already used for block copolymer synthesis in our group.30 The synthesis of the precursor monomer (Scheme 2A) is done in two steps and the final product 2,2,2-trifluoroethyl-4-vinylbenzenesulfonate (SS-CH2-CF3) 1 is purified by column chromatography. The overall yield is 68 % and the 1H-NMR spectrum (Fig. S3) shows the purity of the monomer SS-(CH2-CF3) which is a white crystalline material at RT.

+ Cl

A O Na

O S

Cl

CF3

+ TEA

1. 0°C, DMF

+

2. RT, 3h

1. 0°C, 2h, DCM 2. RT, 48 h

O S O Cl

O

OH

O S O O CF3 1

Br

N3

N3

NaN3

NBS AIBN

DMF

CCl4 / 80°C

B

Br

CS2 C12H25SH

N3 S

S

C12H25

S

K3PO4 Acetone

Scheme 2. Synthesis of the precursor monomer SS-CH2-CF3 1 in two steps with 4-vinylbenzene1-sulfonyl chloride

as

intermediate

(A).

Synthesis

of

the

RAFT

agent

3-azido-1-

phenylpropyl dodecyl trithiocarbonate with adjacent azide group for CuAAC coupling and without ester linkage in the molecule (B). For the RAFT polymerization of SS-CH2-CF3, we use trithiocarbonate as a special chain transfer agent (CTA) in order to avoid side reactions involving the labile ester linkage during the deprotection of the P(SS-CH2-CF3) block. This CTA also carries an azide group for the CuAAC


8

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coupling of the both precursor blocks. The successful synthesis of the CTA 3-azido-1phenylpropyl dodecyl trithiocarbonate (Scheme 2B) can be shown by 1H-NMR spectroscopy (Fig. S4) and the presence of the azide group in the CTA is clearly proven by FT-IRspectroscopy (Fig. S5). The RAFT polymerization of SS-(CH2-CF3) 1 was done with AIBN as initiator and 2-butanone as solvent. The reaction proceeds over 7h and the conversion of the reaction is monitored by 1H-NMR-spectroscopy. In general, the reaction time in RAFT polymerization lies in a broad range depending on the reaction conditions, especially the choice of the solvent, the temperature, the used initiator and the used CTA.31 The reaction time of seven hours used here can be further lowered by optimizations, which was not the main focus of this work. The signals of the three vinyl-protons at δ = 6.74 ppm, δ = 5.97 ppm and δ = 5.53 ppm decrease, while simultaneously a broad peak of the P(SS-CH2-CF3) backbone in the area of δ = 1.25 ppm to δ = 2.51 ppm is formed (Fig. S6). Also the sharp signal at δ = 4.37 ppm of the methylene protons in the trifluoroethanol protecting group is converted to a broad peak indicating polymer formation. An end group analysis using these methylene protons and the methyl group protons of the CTA at δ = 0.83 ppm gives a molecular weight of 12.4 kg/mol. During the polymerization, SEC samples are taken to analyze the molecular weight at different conversions. The SEC analysis is done with THF + 0.25 wt.% tetrabutylammonium bromide (TBAB) as eluent and the molecular weights are calculated using a PS calibration. The molecular weight distributions of the SEC samples (Fig. S7A) show the growing of the P(SSCH2-CF3) during the RAFT polymerization. A first-order kinetic plot of ln(M0/Mt) against time t (Fig. 1A) shows some retardation of the polymerization within the first hours. After that a pseudo-first-order kinetic is observed for the RAFT polymerization up to 4 h followed by a decreasing polymerization rate, which is related to


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the reduced concentration of propagating radicals. If the molecular weight is plotted against the conversion (Fig. 1B), a linear correlation can be seen up to 75% conversion, which shows the possibility to control the molecular weight in this range. Additionally, the PDI decreases during the polymerization to 1.12 for the final polymer which is a typical behavior for controlled radical polymerizations.

Figure 1. The first-order kinetic plot of the RAFT polymerization of 2,2,2-trifluoroethyl-4vinylbenzenesulfonate (SS-CH2-CF3) 1 shows a retardation in the beginning, a short pseudo-first order kinetic region and a decrease of polymerization rate after 4 h (A). The molecular weight linearly increases with the conversion and the PDI decreases down to 1.12 for the final polymer (B).


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The presence of the azide group at one end of the P(SS-CH2-CF3) is important for the following polymer click coupling to P3HT-alkyne. Here FT-IR spectroscopy (Fig. 2) proves the presence of the azide group in the polymer. The low intensity of the azide band vibrational signal is caused by the low concentration of -N3 end groups relative to the mass of the whole polymer, since each chain can have only one single -N3 group. After the successful synthesis of the two precursor polymers with monofunctional end groups alkyne and azide, a CuAAC coupling is done (Scheme 1). For purification, a column chromatographic method is applied. The block copolymer P3HT-b-P(SS-CH2CF3) sticks to the silica when eluted with pure CHCl3 as mobile phase. Subsequently all unreacted P3HT can be removed, since it is soluble in CHCl3. The block copolymer P3HT-b-P(SS-CH2-CF3) 5a and the unreacted P(SS-CH2-CF3)-N3 2 can be washed down with THF + 2 vol.% MeOH. Precipitation in ethyl acetate removes the excess of P(SSCH2-CF3)-N3. In the FT-IR spectrum of the purified block copolymer the characteristic bands of the alkyne and azide end group at 3054 cm-1 and 2100 cm-1 respectively disappear in comparison to the two precursor polymers (Fig. 2). This is a clear indication that the precursor blocks were successfully coupled in a CuAAC reaction under formation of a triazole ring as covalent linker.


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Figure 2. FT-IR-spectra of the block copolymer P3HT-b-P(SS-CH2-CF3) (red) and the two precursor polymers P3HT-alkyne (blue) and P(SS-CH2-CF3)-azide (black). The characteristic bands for both end groups, alkyne at 3050 cm-1 and azide at 2100 cm-1 vanish due to the successful coupling by CuAAC reaction. The block copolymer was analyzed further by using SEC analysis with THF+TBAB as eluent and PS calibration (Fig. 3). In a direct comparison of the precursor polymers and P3HT-b-P(SSCH2-CF3), a clear shift of the molecular weight distribution towards higher molecular weights can be observed (Fig. 3A). The block copolymer has a relative molecular weight of Mn = 102.6 kg/mol and a very narrow distribution with PDI = 1.15. Taking into account the amount of monomer, CTA and the conversion for the RAFT polymerization, a theoretical molecular weight for P(SS-CH2-CF3)-N3 was calculated as 12.6 kg/mol which is considerably smaller compared to the measured MW in SEC (THF+TBAB eluent and PS calibration) of Mn = 84.9 kg/mol. It is known32 that fully or partly fluorinated materials interact with SEC columns when THF is used


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as an eluent. To investigate if this is the case for our polymer we measured the polymer by SEC using hexafluoroisopropanol (HFIP) as eluent and a PMMA calibration (Fig. 3B). This measurement reveals a smaller molecular weight of Mn = 8.1 kg/mol and a narrow PDI of 1.14. Thus, the molecular weight of P(SS-CH2-CF3) measured by HFIP as eluent is in better agreement with the theoretically calculated MW of 12.6 kg/mol. A direct comparison between the two elugrams (Fig. S7) shows that the elution time of the polymer is completely different depending on the eluent used. There seems to be a repulsive interaction with the column material in THFSEC, which causes a fast elution and thus an overestimation of the molecular weight. Nevertheless the kinetic investigations described earlier by SEC with THF+TBAB as eluent (Fig. 1) are valid if one takes into account that the molecular weights of all P(SS-CH2-CF3) containing polymers are overestimated.


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Figure 3. SEC curves of the block copolymer P3HT-b-P(SS-CH2-CF3) (red) and the two precursor polymers P3HT-alkyne (blue) and P(SS-CH2-CF3)-azide (A) show the successful synthesis of the block copolymer. A comparison of SEC with different solvents, HFIP (PMMA calibration) and THF+TBAB (PS calibration) shows the overestimation of molecular weight of P(SS-CH2-CF3) when using THF+TBAB as solvent and a PS calibration (B). In the same way the molecular weight of the block copolymer is also overestimated by SEC due to the interaction of the P(SS-CH2-CF3)-block with the column as discussed above. Interestingly the peak molecular weights (Mp) of all three polymers are in a good agreement relative to each other, since the sum of the Mps of the two precursor polymers P3HT-alkyne Mp = 21.0 kg/mol and P(SS-CH2-CF3)-N3 Mp = 93.3 kg/mol is almost exactly the Mp of the


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block copolymer Mp = 115.1 kg/mol. Furthermore, in the SEC curve the successful purification of the product can be seen. In the SEC trace of the purified block copolymer (Fig. 3A), there are no peaks or shoulders at the position where the precursor polymers are found. Additionally, the block copolymer is investigated by 1H-NMR-spectroscopy. From the 1HNMR spectrum of P3HT-b-P(SS-CH2-CF3) (Fig. 4), information on its composition can be extracted.

Figure 4. 1H-NMR spectrum of the block copolymer P3HT-b-P(SS-CH2-CF3) in d8-THF with assignment of all visible peaks. By using the polymer protons signals of both blocks the molecular weight of P(SS-CH2-CF3) in the block copolymer can be calculated, because the absolute MW of P3HT was measured absolutely by MALDI-ToF.


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In the 1H-NMR spectrum all expected signals of both polymer blocks are present. An additional peak of the triazole proton appears at a chemical shift of δ = 10.85 ppm. Under the assumption that the block copolymer is completely purified, which was shown by SEC, and from the molecular weight of the P3HT block obtained from MALDI-ToF ( Mn = 11.3 kg/mol), the molecular weight of the P(SS-CH2-CF3) block in the block copolymer can be calculated. The signals b and c correspond to the four protons in the repeating unit of P(SS-CH2-CF3) and one proton from P3HT respectively, whereas signal d corresponds to two protons in P(SS-CH2-CF3) only. From the ratio of the integrals a molecular weight for the P(SS-CH2-CF3) block of MW = 15.9 kg/mol (NMR) can be calculated. This is a little higher than the expected theoretical MW and the molecular weight of Mn = 12.4 kg/mol (NMR) calculated for P(SS-CH2-CF3)-N3 by NMR end group analysis. In conclusion, we could show the successful synthesis and complete purification of a P3HT-bP(SS-CH2-CF3) block copolymer, were all unreacted chains of both precursor polymers could be removed. The molecular and thermal characterization of all three polymers can be found in Table 1. Table 1. Molecular characteristics and thermal properties of all used polymers. Polymer

Mn

Mw

MP

PDI

Tm

ΔHm

Tc

Tg

kg/mol kg/mol kg/mol °C J/g °C °C 17.6a 20.0a 21.0a 1.14a P3HT-alkyne 3 224 19.6 187 11.3b 11.4b 84.9a 92.3a 93.3a 1.09a 8.1c c c P(SS-CH2-CF3)-N3 2 9.2 9.0 1.14c 80 15.9d 12.4e P3HT-b-P(SS-CH2-CF3) 5a 102.6a 117.4a 115.1a 1.14a 208 3.5 164 79 a a a P3HT-b-P(SS-CH2-CF3) 5b 83.5 91.6 87.9 1.10 a determined by SEC (THF + 0.25 wt% TBAB / PS calibration), b determined by MALDI ToF MS, c determined by SEC (HFIP / PMMA calibration), d calculated by NMR using MALDI-ToF MS of P3HT e calculated by NMR end group analysis


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Thermal characterization of P3HT-b-P(SS-CH2-CF3) 5a The thermal properties of the two precursor polymers and the block copolymer were investigated by TGA and DSC. The DSC curve of P3HT-Alkyne (Fig. 5) shows the melting of the polymer at a temperature of Tm = 224 °C and crystallization at Tc = 187 °C and a glass transition is found at Tg = -19 °C. According to the DSC and TGA measurements (Fig. S7), P(SS-CH2-CF3)-N3 is an amorphous material as expected for a polystyrene derivative. A glass transition is measured at Tg = 82°C and from the TGA curve a decomposition temperature T5% = 337°C is obtained. In the DSC curve of the block copolymer P3HT-b-P(SS-CH2-CF3) (Fig. 5) both characteristic thermal transitions of the blocks can be found. The glass transition of the P(SS-CH2-CF3) is negligibly shifted to Tg = 90 °C. In contrast, the melting peak of the P3HT block is shifted to a lower temperature with Tm =208 °C and the crystallization temperature is also lower at Tc = 163 °C.


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Figure 5. DSC curves of second heating and cooling of P3HT-alkyne (blue), P(SS-CH2-CF3)-N3 (black) and the block copolymer P3HT-b-P(SS-CH2-CF3) (red) measured with 10 K/min. The individual thermal characteristics of both blocks are maintained in the block copolymer which once more proves the successful coupling of P3HT-alkyne and P(SS-CH2-CF3)-N3 by the CuAAC click reaction and indicates a phase separation of the blocks. We emphasize that the complete SEC and thermal characterization of the block copolymer P3HT-b-P(SS-CH2-CF3) has to be done before removing the trifluoroethanol protecting group. Once the final block copolymer, P3HT-b-PSS-/TBA+ is formed after a post polymerization reaction, it is difficult to carry out SEC analysis due to the presence of an ionic block in the system.


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Synthesis of P3HT-b-PSS-/TBA+ in a micellar approach The block copolymer is treated with a solution of tetrabutylammonium hydroxide (TBAH) for the deprotection of the P(SS-CH2-CF3) block. For this, the block copolymer P3HT-b-P(SS-CH2CF3) 5b is dissolved in THF and DMSO is added dropwise under continuous stirring, up to a ratio of 1:1 (v:v). This leads to the formation of micelles by self-assembly which is clearly indicated by a colour change from orange to purple. Afterwards THF is removed under reduced pressure to yield a DMSO solution containing P3HT-b-P(SS-CH2-CF3) micelles. This observation is in agreement with the micelle formation of other amphiphilic block copolymers with one P3HT block and in non-solvents for P3HT.11 In these micelles, P3HT forms a crystalline core and the hydrophilic block forms a surrounding corona, leading to the formation of wormlike colloidal structures, which are stable in solution over a long time. In this micellar form it is possible to deprotect the dissolved P(SS-CH2-CF3) block in the corona of the micelles. TBAH in MeOH is added to the P3HT-b-P(SS-CH2-CF3) micelles in DMSO. The complete removal of the trifluoroethanol protection group is controlled by 1H-NMR measurements. After 20 h of reaction, the polymer micelles are precipitated in ethyl acetate and purified by several cycles of centrifugation and re-dispersion in ethyl acetate. The micelles of the resulting P3HT-b-PSS-/TBA+ block copolymer are investigated by TEM in dried state (Fig. S9). Here the micelles have a wormlike shape and a length between 100 nm and 500 nm. Some of the micelles also show branching, but since the micelles are measured in a dried state this may also be an effect of aggregation during drying. A much better insight into the solution structures of P3HT-b-PSS-/TBA+ micelles is possible by cryo-TEM measurements, which give a direct image of the micelles in solution. In order to vitrify and investigate the


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P3HT-b-PSS-/TBA+ micelles by cryo-TEM, a micellar solution is transferred to water by dialysis. The cryo-TEM images of the aqueous solution (Fig. 6A) confirm the wormlike shape of the of P3HT-b-PSS-/TBA+ micelles. The contrast arises from the crystalline P3HT in the core of the micelles and no staining is necessary to image the micelles core.


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Figure 6. cryo-TEM images of the block copolymer P3HT-b-PSS-/TBA+ micelles without staining and a schematic representation of the P3HT-b-PSS-/TBA+ micelle (A). cryo-TEM images of the P3HT-b-PSS + TiO2 nanocomposites without staining and a schematic representation of the nanocomposite (Fig. 6B). In a more detailed view on the nanocomposites the single TiO2 crystals can be seen directly attached to the individual micelles (Fig. 6C). In cryo-TEM high resolution images of the nanocomposites even the lattice spacing of TiO2 can be seen (D). The inset shows a rotational integrated SAED pattern from the nanocrystals and the reflexes of anatase TiO2 can be labelled.


21


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In contrast to the TEM measurements of the dried micelles, no branched micelles were observed in cryo-TEM, but some smaller spherical structures can be seen (Fig. S10). The process of micelle formation by dropwise addition of a non-solvent to the THF solution of the block copolymer is less defined than other approaches using crystallization driven self-assembly (CDSA)33,34 which may explain the large variation in the micelles length ranging from 50 nm to 600 nm. The resulting block copolymer in the micelles now carries a hydrophilic, ionic PSS/TBA+ block as corona which can be utilized for the in-situ synthesis of TiO2 nanocrystals from a molecular precursor of Ti.

Compartmentalized in-situ synthesis of TiO2 nanocrystals in P3HT-b-PSS-/TBA+ micelles PSS is known to act as a catalyst for the hydrolysis of Ti-precursors12,13 under formation of TiO2 nanocrystals. We applied this methodology on our P3HT-b-PSS micelles to synthesize TiO2 selectively inside of the PSS corona of the micelles, using titanium(IV) isopropoxide as Tiprecursor. After 2h of reaction at room temperature, the reaction mixture was purified by dialysis against water. The aqueous P3HT-b-PSS micelle dispersion is stable after the in-situ synthesis of TiO2 and no precipitation occurred, while it remains in dispersion. Once dried, the P3HT-b-PSS nanocomposites cannot be re-dispersed again. This is probably caused by the strong interaction between the corona polymer chains and the TiO2 nanocrystals of different micelles in the dried state, which leads to a quasi-crosslinking of the micelles. The successful synthesis of TiO2 was investigated by TEM. In TEM images of the dried nanocomposites (Fig. S11) the wormlike micelles are visible as well as additional smaller material with high contrast which is TiO2. Although it seems that the TiO2 is distributed more around the micelles, the whole samples are covered with the smaller structures. Standard TEM measurements cannot uncover the native structure of the formed soluble nanocomposites. For a clear analysis of the nanocomposite’s


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composition cryo-TEM measurements were performed (Fig. 6B and Fig. 6C), since they show the actual structure of the nanocomposites in solution. The TiO2 nanocrystals here appear as separated particles which can be clearly differentiated from the BCP micelles since they have a much stronger contrast in the cryo-TEM images (Fig. 6B). The most important information derived by the cryo-TEM images is the location of the TiO2 nanocrystals. They are exclusively found in the P3HT-b-PSS-/TBA+ micelles, which are visible as lighter (grey) wormlike structures in the cryo-TEM images (Fig. 6C). Obviously, the PSS- corona of the wormlike micelles on the one side catalyzes the hydrolysis of titanium(IV) isopropoxide and one the other side coordinates to the formed nanocrystals to bind them to the micelles. The cryo-TEM images also provide further analysis of the TiO2 nanocrystals. In high-resolution images (Fig. 6D) the distance between individual layers in the atomic lattice of the single nanocrystals is visible and found to be 0.355 nm. In contrast to powder X-ray diffractometric measurement which cannot show the presence of TiO2 in samples of the synthesized nanocomposites (due to small amount or small crystallites of TiO2) selected area electron diffraction (SAED) of the material in cryo-TEM is able to show the diffractogram (Fig. 6D). Here the rotational integrated reflexes can be indexed and show that the TiO2 nanocrystals have a highly pure anatase modification with a high crystallinity. The optical properties of the P3HT-b-PSS solutions and the P3HT-b-PSS-/TBA+ + TiO2 nanocomposites were investigated with UV-vis spectroscopy (Fig. S12). The P3HT-b-PSS/TBA+ micelles in water show the typical absorption spectra of crystalline P3HT with a maximum at 510 nm and two distinct vibronic bands at 550 nm and 600 nm. After in-situ synthesis of TiO2 nanocrystals inside the PSS- corona, the spectrum stays unaffected for wavelengths higher than 400 nm, but in the UV region of wavelengths below 350 nm a very intense absorption peak is added. This absorption is caused by the TiO2 anatase nanocrystals and


23


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shows the increased absorption of the synthesized nanocomposites. Our micellar route to incorporate TiO2 in a single domain of a semiconductor block copolymer is very successful. However, it is not clear at this stage, if we require well-defined polymer micelles of uniform size for the preparation of microphase separated nanocomposite film (in which the micellar structure is destroyed at the end). If necessary, there are efficient methods of vortexing and sonication for rapid exchange of polymer chains in micelles leading to kinetically controlled uniform micelles which can be used.35,36 In the samples shown in cryo-TEM, the mass content of TiO2 is low. The next challenge is to incorporate higher amounts of the inorganic material to guarantee efficient charge percolation through the particles in the solid state without losing the advantages of microphase separated percolation pathways, their thin film optimization and applications. CONCLUSIONS Herein we could demonstrate the successful synthesis of an ionic amphiphilic semiconducting block copolymer P3HT-b-PSS-/TBA+ with an absolute P3HT molecular weight of 11.3 kg/mol and a PSS- block by a polymer-polymer click reaction. Due to the very different solubility of the two blocks, we were able to remove both precursor polymers and yield the block copolymer P3HT-b-P(SS-CH2-CF3) with a high purity. The addition of a non-solvent for P3HT induces the formation of block copolymer micelles with a crystalline P3HT core and P(SS-CH2-CF3) as a solvating corona. Due to these self-assembly properties the P(SS-CH2-CF3) block in the micelles can be deprotected and the resulting nanostructured P3HT-b-PSS-/TBA+ micelles can be transferred to water or ethanol, wherein they form stable colloidal solutions. The investigation of these solutions with cryo-TEM reveals that the micelles have a wormlike shape. Furthermore the PSS- corona is utilized to perform a compartmentalized in-situ hydrolysis of a titanium precursor at room temperature which leads to the formation of highly crystalline TiO2 anatase nanocrystals


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inside the PSS- corona. The P3HT-b-PSS-/TBA+ + TiO2 nanocomposites form stable colloidal solutions in for example water or ethanol, which can be concentrated and used to prepare hybrid polymer semiconductor nanocomposite films by e.g. solution-casting.

METHODS Chemicals and Reagents. Ethylmagnesium chloride solution, tert-Butylmagnesium bromide solution, sodium azide (> 99 %), anhydrous sodium sulfate, ammonium chloride, copper(I)iodide, copper(I)bromide, methanol, chloroform, diethyl ether, dimethylformamide (dry), tetrahydrofuran (extra dry), triethylamine, N,N,N′,N′,N′′-pentamethyldiethylenetriamine (PMDETA), thionyl chloride (≥ 99 %), triethylamine (≥ 99.5 %), N-bromosuccinimide (99 %), sodium 4-vinylbenzenesulfonate (≥ 90 %) sodium thiolsulfate (99 %), potassium phosphate (≥ 99 %), carbon disulfide (≥ 99.9 %), 1-dodecanthiol (≥ 98 %) and dichlorobenzene were purchased from Sigma-Aldrich. PMDETA was distilled under reduced pressure. Sodium sulfate (≥ 99 %) was purchased from Carl Roth. 2,2,2-trifluoroethanol (99.8 %) was purchased from ABCR. Azobisisobutyronitrile (98 %) was purchased from Fluka and recrystallized from methanol. Carbon tetrachloride (99 %) was purchased from Riedel-de Haën. 2,5-Dibromo-3hexylthiophene and 1,3-bis(diphenylphosphino)-propanenickel(II) chloride Ni(dppp)Cl2 were synthesized according to reported procedure.11 All solvents were bought from commercial sources and were used as received or after distillation.

Instrumentation. 1H-NMR spectra were recorded in deuterated chloroform CDCl3, deuterated d6-DMSO or deuterated d8-THF on a Bruker Avance 250 spectrometer at 300 MHz at room temperature. Chemical shifts are noted in ppm and coupling constants in Hz. All spectra were


25


Page 26 of 37

calibrated according to the residual solvent peak. Size exclusion chromatography (SEC) was performed

utilizing

a

Waters

510

HPLC

pump

and

THF

with

0.25

wt%

tetrabutylammonium bromide (TBAB) as eluent at a flow rate of 0.5 mL/min. A volume of 100 μL of polymer solution (1-2 mg/mL) was injected into a column setup comprising a guard column (PSS, 5 × 0.8 cm, SDV gel, particle size 5 μm, pore size 100 Å respectively) and two separation columns (Varian, 30 × 0.8 cm, mixed C gel, particle size 5 μm). Molecular weight distributions were monitored with a WATERS 486 tunable UV detector at 254 nm and a Waters 410 differential RI detector. Polystyrene was used for calibration and 1,2-dichlorobenzene as an internal reference. HFIP-SEC was measured using HFIP containing 0.5% w/w potassium trifluoroacetate as eluent with a flow rate of 0.5 mL/min using an Agilent 1200 isocratic pump. The injection volume was 20 µL and three PSS-PFG separation columns (particle size = 7 µm) with porosity range from 100 to 300 Å were used (PSS, Mainz, Germany). Molecular weight distributions were recorded by using a Gynkotek refractive index detector at 23 °C. Poly(methyl methacrylate) was used for calibration and toluene as an internal reference. Matrix assisted laser desorption ionizations spectroscopy with time of flight detection mass spectroscopy measurements (MALDI-ToF MS) were performed on a Bruker Reflex III. For P3HT polymers 1,8-dihydroxy-9,10-dihydroanthracen-9-one (dithranol) was used as matrix. The ratio of the mixtures was 1000 : 1 (Matrix : Polymer). Fourier transform infrared (FT-IR) spectra were measured on a Perkin Elmer Spectrum 100 FTIR spectrometer with an attenuated total reflection (ATR) unit. UV-vis spectroscopy was done with a Jasco V-670 Spectrophotometer using quartz cuvettes (1 cm path length). Thermogravimetric analysis (TGA) of polymers was conducted using a Mettler Toledo TGA/SDTA 851e instrument under nitrogen atmosphere (20 mL/min) at a heating rate of 10 K/min and in the temperature range of 30-700°C. Differential scanning


26

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calorimetry measurements were done on a Mettler Toledo DSC 2 STARe System with a heating and cooling rate of 10 K/min und N2 atmosphere. For cryogenic transmission electron microscopy (cryo-TEM) studies the specimens covered with a thin film of aqueous solution of P3HT-b-PSS micelles were instantly vitrified by rapid immersion into liquid ethane, cooled to approximately 90 K by liquid nitrogen in a temperature controlled freezing unit (Zeiss Cryobox). After freezing the specimens, the specimen was inserted into a cryo-transfer holder and transferred to a Zeiss EM922 Omega EF-TEM instrument and measured.

Synthesis of P3HT-alkyne. 7.86 g 2,5-Dibromo-3-hexylthiophene ( 24.1 mmol, 1 eq.) were placed in a secured 500 mL-Schlenk flask and 48 mL LiCl solution in THF (0.5 M) were added under argon. 17.9 mL of t-BuMgCl solution in THF (1.30 mol/L, 23.14 mmol, 0.96 eq.) were added to the flask and the mixture was stirred overnight. After 16 h the reaction mixture was diluted

by

175 mL

of

dry

THF.

159.3 mg

1,3-bis(diphenylphosphino) propane

nickel(II) chloride (Ni(dppp)Cl2, 0.289 mmol, 0.0125 eq.) were dissolved separately in a 10 mL pear-shaped Schlenk-flask and mixed with 2 mL of dry THF. The catalyst suspension was completely injected to the reaction flask with the monomer solution. After 25 min the reaction was cooled in an ice bath, and 12.3 mL of ethinylmagnesiumchloride in THF (Ethinyl-MgCl, 0.473 mol/L, 5.818 mmol, 20 eq.) were injected to the reaction mixture and the mixture was stirred for 15 min. The resulting polymer was purified by precipitation in MeOH, two times in hexane and dried under vacuum. 1H-NMR (300 MHz, CDCl3): δ (ppm) = 7.03-6.93 (s, 1H, Har), 6.89 (s, Har alkyne end group), 6.83 (s, Har –Br end group), 3.53 (s, 1H, alkyne-H), 2.67-2.93 (m, 2H, α-CH2), 2.66-2.73 (m, α-CH2 alkyne end group), 2.52-2.60 (m, α-CH2 Br end group), 1.771.63 (m, 2H, β-CH2), 1.51-1.21 (m, 6H, CH2), 0.99-080 (m, 3H, CH3), yield = 2.7502 g, SEC:


27


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Mn = 17.6 kg/mol, Mw = 20.0 kg/mol, PDI = 1.14 (THF+TBAB/PS calibration), MALDI-ToF MS: Mn = 11.3 kg/mol, Mw = 11.4 kg/mol,.

Synthesis of 4-vinylbenzene-1-sulfonyl chloride. A 100 mL flask was charged with 17.6 mL thionyl chloride (242.83 mmol, 5 eq.) and 10.0 g sodium 4-vinylbenzenesulfonate (48.5 mmol, 1 eq.) were slowly added within 5 min to under argon at 0 °C. 22.0 mL dry DMF was added dropwise at 0 °C under vigorous stirring. Afterwards, the reaction mixture was stirred for 10 min at 0 °C and then allowed to warm to RT and was stirred again for 3 h. The reaction was stopped by pouring the reaction mixture slowly onto ice in a beaker. The aqueous suspension was allowed to warm to RT and was thereafter extracted three times with diethyl ether. The organic phase was washed twice with water and 2 wt% HCl solution, dried over NaSO4 and filtered. The residual solvent was removed under reduced pressure and the obtained oil is stored in the freezer. 1H-NMR

(300 MHz, DMSO-d6): δ (ppm) = 14.49 (s, 1H), 7.56 (d, J = 9.9 Hz, 1H), 7.43 (d, J =

8.2 Hz, 1H), 6.72 (dd, J = 17.7, 10.9 Hz, 1H), 5.85 (dd, J = 17.7, 0.9 Hz, 1H), 5.27 (dd, J = 10.9, 0.9 Hz, 1H), yield = 90 % (8.88 g, 43.82 mmol).

Synthesis of 2,2,2-trifluoroethyl-4-vinylbenzenesulfonate. In a 100 mL flask 3.51 mL 2,2,2trifluoroethanol (48.09 mmol, 1.1 eq.) and 6.70 mL triethylamine (48.09 mmol, 1.1 eq.) were added to 43.7 mL DCM at 0 °C. After 5 min of stirring 8.86 g 4-vinylbenzene-1-sulfonyl chloride (43.72 mmol, 1.0 eq.) were added dropwise within 5 min at 0 °C, which resulted in a turbid reaction mixture. The solution was stirred for 68 h and then washed twice with water and the aqueous phase was extracted with DCM. The organic phase was dried over Na2SO4, filtered and the solvent was removed under reduced pressure. The obtained brownish crystals were


28

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stored in the freezer overnight and purified via column chromatography (silica gel, pentane/DCM 2:1) to yield white crystals. 1H-NMR (300 MHz, CDCl3): δ (ppm) = 7.88 (dt, J = 8.4, 1.8 Hz, 1H), 7.59 (dt, J = 8.4, 1.8 Hz, 1H), 7.26 (s, 0.35H) 6.77 (dd, J = 17.6, 10.9 Hz, 1H), 5.94 (d, J = 17.6 Hz, 1H), 5.51 (d, J = 10.9 Hz, 1H), 5.30 (s, 1H), 4.37 (q, J = 7.9 Hz, 1H), 1.56 (s, 1H), 1.25 (s, 1H), yield = 76 % (8.83 g, 33.17 mmol).

Synthesis of 3-azidopropylbenzene. A 50 mL flask was charged with 7.63 mL 1-Brom-3phenylporpane (50.2 mmol, 1 eq.) and 8.16 g sodium azide (126 mmol, 2.5 eq.), the educts were dissolved in 25 mL N,N-dimethylformamid and heated to 80 °C for 12 h. After cooling to RT the reaction mixture is poured into 200 mL of diethyl ether and washed with saturated NaCl solution and with water twice. The organic phase is dried over Na2SO4 and the solvent is removed under reduced pressure to obtain the product. 1H-NMR (300 MHz, CDCl3): δ (ppm) = 7.35 – 7.16 (m, 5H, Har), 3.30 (t, J = 6.7 Hz, 2H, CH2-N3) 2.72 (t, J = 7.8 Hz, 2H, α-CH2), 1.99 – 1.86 (m, 2H, βCH2), yield = 99 % (8.10 g, 50.2 mmol).

Synthesis of 3-azido-1-bromopropylbenzene. Under argon, 4.01 g (3-azidopropyl)benzene (24.89 mmol, 1.0 eq.), 40.9 mg azobisisobutyronitrile (2.49 mmol, 0.01 eq.) and 4.88 g Nbromosuccinimide (27.42 mmol, 1.10 eq.) were dissolved in 54 mL carbon tetrachloride in a 100 mL flask. The reaction mixture was stirred for 3 h at 80 °C under reflux. The colour of the reaction mixture changed slowly from yellowish to orange and then suddenly to dark brown. After cooling to RT the reaction mixture was diluted with 100 mL chloroform and washed twice with saturated NaS2O3 solution and water and the aqueous phase was extracted with chloroform. The combined organic phases were diluted with 200 mL hexane, dried over Na2SO4, filtered and


29


Page 30 of 37

the solvent was removed under reduced pressure. The obtained product was purified via column chromatography (silica gel, hexane/ethyl acetate 19:1). 1H-NMR (300 MHz, CDCl3): δ (ppm) = 7.46 – 7.27 (m, 5H), 5.08 (dd, J = 9.1, 5.8 Hz, 1H), 3.60 – 3.35 (m, 2H), 2.61 – 2.40 (m, 1H), 2.39 – 2.19 (m, 1H), yield = 45 % (2.69 g, 11.21 mmol).

Synthesis of 3-azido-1-phenylpropyl dodecyl trithiocarbonate. In a 50 mL flask, 1.5 mL 1dodecanethiol (6.262 mmol, 1.00 eq.) were added under argon to a suspension of 1.328 g potassium phosphate (6.256 mmol, 0.99 eq.) in 13 mL acetone and stirred for 1 h at RT. Afterwards, 1.3 mL carbon disulfide (21.57 mmol, 3.43 eq.) were added dropwise and the mixture stirred for 1 h at RT. Then 1.51. g (3-azido-1-bromopropyl)benzene (6.289 mmol, 1.00 eq.) was added dropwise and the mixture stirred for 72 h at RT. The reaction mixture was filtered and the precipitate washed three times with acetone. The solvent was removed under reduced pressure and the obtained product purified via column chromatography (silica gel, hexane/ethyl acetate 19:1). 1H-NMR (300 MHz, CDCl3): δ (ppm) = 7.40 – 7.27 (m, 5H), 5.33 (dd, J = 9.2, 6.2 Hz, 1H), 3.39 – 3.13 (m, 4H), 2.52 – 2.35 (m, 1H), 2.31 – 2.14 (m, 1H), 1.67 (q, J = 9.0 Hz, 2H), 1.44 – 1.20 (m, 18H), 0.88 (t, J = 6.7 Hz, 1H), fraction 1: yield = 27 % (638.3 mg, 1.458 mmol), purity: 96 %; fraction 2: yield = 61 % (1470.9 mg, 3.360 mmol), purity: 84 %.

Synthesis of P(SS-CH2-CF3)-N3. 2-butanone was degassed with argon for 25 min prior to the reaction. 1.122 g 2,2,2-trifluoroethyl 4-vinyl-benzenesulfonate (4.214 mmol, 54.89 eq.), 37 mg 3-azido-1-phenylpropyl dodecyl carbonotrithioate (0.077 mmol, 1.00 eq.), 2.5 mg AIBN (0.015 mmol, 0.20 eq.) and 1.3 mL 2-butanone were added to a 10 mL flask. The reaction mixture was degassed with argon for 5 min and then stirred for 7 h at 80 °C. The reaction mixture was


30

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stopped by cooling the flask in liquid nitrogen and was precipitated in 120 mL methanol. The precipitate was filtered and dried in vacuum. 1H-NMR (300 MHz, DMSO-d6): δ (ppm) = 7.96 – 6.44 (m, 4H, Har), 4.83 (s, 2H, CH2-CF3), 2.41 – 1.27 (m, 3H, backbone), yield = 79 % conversion (562.9 mg), SEC: Mn = 84.9 kg/mol, Mw = 92.3 kg/mol, PDI = 1.09 (THF+TBAB/PS calibration), Mn = 8.1 kg/mol, Mw = 9.2 kg/mol, PDI = 1.14 (HFIP/PMMA calibration).

Synthesis of P3HT-b-P(SS-CH2-CF3). A 50 mL round bottom flask with a screw cap was charged with 50 mg P3HT-alkyne (0.005 mmol, 1 eq.) and 80 mg P(SS-CH2-CF3)-N3 (0.010 mmol, 2 eq.). The polymers were dissolved in 25 mL dry THF, the reaction mixture was degassed with argon for 30 min and heated to 60 °C. 0.15 mL of a 0.085 M CuBr/PMDETA solution in (o-)dichlorobenzene were injected to start the reaction. After 4 d the reaction was cooled to RT and the solvent was evaporated under reduced pressure. The product was dissolved in a few mL of CHCl3 and purified by column chromatography (silica gel, 1. chloroform, 2. THF + 2 vol.% MeOH). The product sticks to the column and P3HT can be eluted with chloroform. After an eluent change to THF + 2 vol.% MeOH the block copolymer and remaining P(SS-CH2CF3)-N3 can be eluted, the solvent is removed and the material is dried in vacuum. A concentrated solution of the product is precipitated into 150 mL of ethyl acetate to remove the excess P(SS-CH2-CF3)-N3 and the resulting suspension is filtered and freeze-dried from benzene. 1H-NMR

(300 MHz, THF-d8): δ (ppm) = 10.88 (s, 0.023 H, triazol-H), 7.90-7.57 (m, 2H, PSS-

Har), 7.08 (m, 1H, P3HT-Har), 6.98-6.62 (m, 2H, PSS-Har), 4.82-4.56 (m, 2H, P(SS-CH2-CF3)), 2.92-2.79 (m, 2H, P3HT α-CH2), 1.54 - 1.24 (m, 11H, PSS-backbone + P3HT-CH2), 0.98-0.83 (m, 3H, CH3), yield = 45 mg, SEC: Mn = 102.6 kg/mol,

Mw = 117.4 kg/mol,

PDI = 1.14

(THF+TBAB/PS calibration).


31


Page 32 of 37

Synthesis of P3HT-b-PSS-/TBA+. In a 10 mL round bottom flask 0.060 g P3HT-b-P(SS-CH2CF3) are dissolved in 0.5 mL THF and 0.5 mL DMSO are added dropwise under continuous stirring. THF is removed from the resulting violet suspension under reduced pressure. To the micellar solution in DMSO 0.6 mL of a 1.55 M solution of tetrabutylammonium hydroxide in MeOH is added and the reaction mixture is stirred for 20 h. For purification, the polymer was precipitated in ethyl acetate and centrifuged. After washing with ethyl acetate and centrifugation once more the material was re-dispersed in water.

Synthesis of TiO2 nanocrystals in P3HT-b-PSS-/TBA+. The synthesis of the TiO2 nanocrystals was adapted from literature.37 In a small vial 10 mL ethanol are added to 0.010 g P3HT-b-PSS-/TBA+ dispersed in water. 0.038 mL of titanium(VI) isopropoxide are dissolved in 1 mL of ethanol and added dropwise over 10 min to the solution of P3HT-b-PSS-/TBA+ micelles. The reaction was stirred for 2h at room temperature and subsequently dialyzed against water to remove the ethanol and any side products yielding the final polymer semiconductor nanocomposite dispersion.

ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge on the ACS Publications website at DOI: Additional 1H-NMR spectra, MALDI-ToF spectra, FT-IR spectra, SEC cuves, TEM images, cryo-TEM images and UV-vis measurements.


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ORCID Mukundan Thelakkat: 0000-0001-8675-1398 Johannes Brendel: 0000-0002-1206-1375 Markus Drechsler: 0000-0001-7192-7821 Present Addresses §

Institute of Organic Chemistry and Macromolecular Chemistry (IOMC), Friedrich Schiller

University Jena, 07743 Jena, Germany Author Contributions The manuscript was written by P.M.R. P.M.R. and J.C.B. conducted synthesis and characterized the materials. M.D. performed all electron microscopy measurements. M.T. supervised the project and corrected the manuscript. All authors have given approval to the final version of the manuscript. Funding Sources Bayerisches Staatsministerium für Bildung und Kultus, Wissenschaft und Kunst within project “Solar technologies go hybrid” (SolTech) and Deutsche Forschungsgemeinschaft (DFG) under SFB 840 - TP 7. Notes


33


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The authors declare no competing financial interest. ACKNOWLEDGMENT We acknowledge funding from the Bavarian State Ministry of Education, Science and the Arts under

the

program

“Solar

technologies

go

hybrid”

(SolTech)

and

Deutsche

Forschungsgemeinschaft (DFG) under SFB 840 - TP 7. P.M.R. thanks the program “Elite Network Bavaria - Macromolecular Science” at the University of Bayreuth for support during his studies. We thank Dominic Rosenbach and Tobias Klein for help in synthesis during lab courses.


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Poly(3-hexylthiophene)-block-poly(tetrabutylammonium-4

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