Nanoporous poly(3-hexylthiophene) thin films based on “click” prepared degradable diblock copolymers

Abstract

A facile approach to control the nanostructured organization of conjugated polymer platforms is proposed, based on the synthesis and characterization of copolymers containing a cleavable group inside the chain. This is illustrated by copolymerization of electroconjugated units representing regioregular poly(3-hexylthiophene) obtained by Grignard metathesis polymerization with a terminal alkynyl group and a sacrificial unit of poly(ethylene oxide) with a functionalized end azide group. The diblock copolymers were successfully synthesized via a click coupling reaction between blocks, where an acid degradable acetal group was incorporated. The strategy for formation of thin porous poly(3-hexylthiophene) films includes acid treatment of diblock copolymers, accompanied by the degradation of the acetal linker and easy removal of the poly(ethylene oxide) block. The study displays an appropriate strategy to convert nanoporous poly(3-hexylthiophene) thin films into suitable high surface area matrices, which, after successful filling with electron acceptor material, could create novel nanostructures for organic photovoltaics.

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Introduction

Rod–coil block copolymers containing regioregular poly(3-hexylthiophene) (P3HT) as a “rod-like segment” have attracted considerable attention in recent years as promising materials for applications in optoelectronic devices.^1,2 Such materials combine both the optical and electronic properties of conjugated polymers with the remarkable self-assembly properties of block copolymers. P3HT is one of the most reported conjugated polymers due to its good solubility and chemical stability related to excellent electronic properties based on well-defined, and highly organized three-dimensional polycrystalline structure.^3–6 The synthesis of highly regioregular (rr) P3HT obtained by Grignard metathesis (GRIM) process provides 95–100% head-to-tail couplings, narrow molar mass dispersities and good end-group fidelity as reported in the literature.^7–9

A number of block copolymers containing a “rod-like” P3HT segment and associated to a “coil-like segment” such as polystyrene,^10–12 polymethylacrylate,^13–15 polyacrylic acid,¹⁶ poly(ε-caprolactone),¹⁷ poly(2-(dimethylamino)ethyl methacrylate),^18,19 poly(4-vinylpyridine)²⁰ and polyethylene glycol^21,22 have been prepared, and tested in organic electronic devices as active layers or as compatibilizers. The presence of the second block not only seems to have an effect on the crystallinity of the materials but also leads to new morphological behaviors. To date, small difference in the lamellar film microstructure occurred upon changing the second block nature except for the poly(2-vinylpyridine),²³ stereocomplexed polylactide,²⁴ and inclusion of regiorandom P3HT.²⁵

Another strategy also consists on the preparation of donor–acceptor block copolymers containing a P3HT segment as a donor unit and a second block bearing pendent attached acceptor moieties such as fullerene^26–28 or perylene.^29,30 Recent investigations show that although an improved phase separation, the associated electronic characteristics do not exceed the ones of classical P3HT:PCBM bulk heterojunction organic photovoltaic devices.³¹

In order to optimize the nanoscale morphology between the two semiconducting materials, i.e. the donor–acceptor interface, a maximization of the charge separation as well as the diffusion process and the generated current collection has also been obtained by a “sacrificial structure-directing” technique. Such a strategy involves the preparation of block copolymers composed by a conjugated electron donor block (i.e. P3HT) and a removable segment such as poly(D,L-lactide),³² poly(L-lactide),^33,34 or polystyrene.^35,36 After deposition, the removable segment is sacrificed and replaced by an electronically active electron acceptor material in the patterned ordered scaffolds. Usually, the zone where every created exciton must meet an interface prior to recombination i.e. donor–acceptor interface in the device occurs every 10 nm, and also the donor and acceptor phases must form continuous pathways to the electrodes allowing for efficient charge transport and collection. Above mentioned 10 nm length scale patterning is difficult to be achieved via conventional lithographic techniques or by phase separation of homopolymer blends. In that case conjugated rod–coil block copolymer self-assembly was used as a tool to obtain control over nanoscale morphology.^1,37,38

To the best of our knowledge there is no data in the literature commenting the preparation and the interest of diblock copolymers based on rr P3HT and polyethylene oxide (PEO) as obtained by “click” chemistry process and bearing a “simple” cleavable group. Herein we report a facile approach to control the nanostructured organization of conjugated polymer platforms by the development of such copolymers. Diblock copolymers were successfully synthesized via an alkyne–azide click coupling reaction including the use of a degradable acetal group between rr P3HTs modified with terminal alkynyl group and PEOs functionalized end azide group. The goal of the study is the formation of thin film of nanoscale templates which after selective etching of cleaved PEO unit to be converted into porous high surface area matrices. Further successful filling with electron acceptor compound could create novel nanostructured architectures for organic photovoltaics.

Results and discussion

Synthesis and characterisations of P3HT-b-PEO block copolymers

Poly(3-hexylthiophene)-block-poly(ethylene oxide) (P3HT-b-PEO) block copolymers bearing a central cleavable acetal bond (Scheme 1) were synthesized via copper mediated “click” coupling reaction³⁹ between preliminary modified P3HT and PEO units. Briefly, P3HT blocks were synthesized by using a Grignard metathesis (GRIM) polymerization, followed by three-step modification reactions leading to the as-obtained alkyne functionalized poly(3-hexylthiophene) macroreagent (P3HT-C≡CH) (Scheme 2a). Functionalized PEO segments were obtained by a two-step modification of commercially available poly(ethylene oxide) macroreagents consisting of both acetalization and azide end-group functionalization yielding (PEO-N₃) (Scheme 2b).

Scheme 1 Poly(3-hexylthiophene) and poly(ethylene oxide) “click” coupling reaction.

Scheme 2 Modification reaction for preparation of (a) P3HT-alkyne and (b) PEO-azide macroreagents.

The GRIM method^3,40–42 was used for the preparation of the well-defined head-to-tail P3HT with high regioregularity content (>95%), controlled molecular weight, low molar mass dispersity (Đ_M) and H/Br as terminal groups. A three-step modification was then needed to convert the as-obtained H/Br P3HT to Br/alkyne P3HT. In a first step, Br/aldehyde P3HTs were obtained quantitatively from H/Br P3HTs via a Vilsmeier–Haack reaction. Aldehyde end-groups were easily detected by the appearance of a strong band showing up at 1649 cm⁻¹ in the FTIR spectrum (see ESI, Fig. S1†). In a second time, those Br/aldehyde groups were converted to Br/methylol (–CH₂OH) groups via a reduction reaction. Sodium borohydride was used as selective and mild reduction reagent,⁴³ and the successful functionalization was proven by the appearance in the ¹H NMR spectrum of a clear singlet at 4.76 ppm corresponding to the new generated –CH₂OH end-capping function (Fig. 1a). In order to fully confirm the end-group fidelity, P3HT-OH has been characterized by MALDI-ToF mass spectrometry (ESI, Fig. S2†). As expected, one main distribution is present which well corresponds to the poly(3-hexylthiophene) end-capped with the expected moieties (Br/CH₂OH).

Fig. 1 ¹H NMR spectrum of (a) P3HT-OH macroinitiator and (b) P3HT-C≡CH macroreagent.

The third step deals with the conversion of methylol end-groups in alkyne ones via a Steglich esterification method with 4-pentynoic acid.⁴⁴ FTIR analysis (ESI, Fig. S3†) confirms the modification step with a band of small intensity at 3314 cm⁻¹ typical of a carbon–carbon triple bond vibration and supplemented by a clear carbonyl (C=O) group showing up at 1740 cm⁻¹. The structure of the expected P3HT-C≡CH was also confirmed by ¹H NMR spectroscopy by comparing the integration of the signals coming from the –CH₂OC(O)– protons (δ = 5.26 ppm) and from the alkyne C≡CH proton (δ = 1.98 ppm) (Fig. 1b). The MALDI mass spectrum recorded after esterification of P3HT-OH also confirms the perfect functionalization and the generation of the expected alkynyl end-functionalized P3HT-C≡CH macroreagent. As compared to the P3HT-CH₂OH, the MALDI-ToF mass distribution is shifted by 80 dalton to a higher molar mass distribution which corresponds to the addition of a pentynoyl moiety (ESI, Fig. S4†).

It was established that method with additional functionalization of P3HT end groups was preferred to direct quench of the polymerization with ethynylmagnesium bromide.⁴⁵ In a latter case it was discovered that storage of the alkynyl P3HT in air under ambient conditions results to significant crosslinking and poor solubility of the product in organic solvents.⁴⁶

Functionalization of PEO monomethyl ether (PEO-OH) with cleavable acetal and clickable azide terminal group was achieved in two steps. Firstly, commercially available PEO-OH (with ¹H NMR spectrum shown on Fig. 2a) with a molecular weight of 2000 (or 5000) g mol⁻¹ was reacted with 2-chloroethyl vinyl ether in an electrophilic addition reaction leading to the acetal end-functionalization of the polyether. This reaction has been performed in presence of pyridinium p-toluenesulfonate used as catalyst.⁴⁷ While FTIR analysis confirms the presence of the chloride atom by the appearance of a small vibrational band at 667 cm⁻¹ (ESI, Fig. S5†), the quantitative end-functionalization of the PEO-OH precursor into PEO-Cl is confirmed by ¹H-NMR spectroscopy (Fig. 2b) when comparing the intensity of protons assigned to both chain-ends such as methoxy (Ha, δ = 3.37 ppm), methine (Hc, δ = 4.82 ppm), and methyl protons (Hd, δ = 1.27 ppm).

Fig. 2 ¹H NMR spectrum of (a) PEO-monomethyl ether, (b) PEO-acetal, and (c) PEO-azide.

In a second step, the conversion of the chloride to an azide end-group was achieved in DMF at 120 °C with sodium azide in a time optimised reaction.⁴⁸ After 3 h, the completion of the reaction was attested by FTIR analysis by the appearance of the azido function at 2110 cm⁻¹ (ESI, Fig. S6†) while the precipitated PEO-azide (PEO-N₃) had the required ¹H-NMR analysis (Fig. 2c) to show a multiplet at 3.43–3.35 ppm for N₃CH₂– protons. The molecular properties of such modified macroreagents and corresponding diblock copolymers are shown in Table 1.

Table 1 Molecular characteristics of homopolymer precursors and corresponding diblock copolymers

(Co-)polymer	Mn NMR^a, (g mol^−1)	Mn theo^b, (g mol^−1)	Mn SEC^c, (g mol^−1)	Mw/Mn^c
a Number-average molecular weight of (co-)polymer as determined by ^1H NMR spectroscopy in CDCl3.b Theoretical molecular weight.c Number-average molecular weight of (co-)polymer and dispersity index as determined by SEC in THF (+2 wt% NEt3) vs. polystyrene standards.d Number-average molecular weight of the trade product.
P3HT-C≡CH	7450	8000	8450	1.18
PEO2000-N3	—	2000^d	3000	1.09
PEO5000-N3	—	5000^d	8350	1.08
P3HT-b-PEO2000	9550	10 000	10 950	1.33
P3HT-b-PEO5000	13 800	13 000	12 700	1.43

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Copper mediated “click” coupling reaction between alkyne modified P3HT and acetal containing azide modified PEO was applied to afford on the preparation of poly(3-hexylthiophene)-block-poly(ethylene oxide) block copolymers. The advantages of the azide/alkyne click reaction are well known and consist primarily on its high efficiency, its high tolerance to functional groups and on the possibility to use a broad range of solvents.^39,49,50 Concerning that, in the current investigation we explored the procedure for “click” coupling reaction providing polymers of controlled molecular weight and dispersity values, similar to those described in literature.^22,44

Practically, “click” reactions between PEO-N₃ and P3HT-C≡CH were realized in the presence of a CuBr/PMDETA catalytic complex in THF at 55 °C for 24 h. An excess of PEO-N₃ was used to slightly boost the kinetics of the reaction while a 55 °C temperature is absolutely necessary for the good dissolution of PEO-N₃ macroreagent during copolymerization reaction. The molecular weight of P3HT block was kept unchanged at 8450 g mol⁻¹ whereas the molecular weight of the PEO was varied at 2000 and 5000 g mol⁻¹. In such way two block copolymers with different length of PEO block were obtained, P3HT-b-PEO2000 and P3HT-b-PEO5000, respectively. The molecular characteristics of the resulted P3HT-b-PEO block copolymers are summarized in Table 1 and their ¹H NMR spectra are presented on Fig. 3a and b.

Fig. 3 ¹H NMR spectrum of (a) P3HT-b-PEO2000 and (b) P3HT-b-PEO5000 copolymer.

¹H NMR spectra display the complete disappearance of the P3HT-C≡CH ethynyl group signal initially present at 1.98 ppm for alkyne end group of P3HT-C≡CH macroreagent, and the appearance of a new singlet at 7.46 ppm attributed to the methine proton of the as-obtained triazole ring in P3HT-b-PEO block copolymers. Despite its poor resolution, the presence of the –CHCH₃ signal of the acetal function showing up at 4.72 ppm confirms well the smooth reaction conditions adopted for the “click” reaction. Characteristic proton signals were integrated for both blocks and compared to the one of the acetal and triazole function. Values are reported on Fig. 3a and b for protons a, c, e, g, i and j and allowed to calculate the molecular weight of both copolymer segments. Additionally, the full disappearance of the vibrational azide band initially present at 2110 cm⁻¹ in the FTIR spectrum also confirms the successful synthesis of P3HT-b-PEO copolymers (ESI, Fig. S7†).

Fig. 4 SEC traces of (a) P3HT and PEO macroreagents, and their block copolymers and (b) the degradation products by cleaving of acetal moiety in P3HT-b-PEO2000 block copolymer.

Finally, unambiguous evidence for the successful “click” coupling reaction between the two P3HT and PEO blocks was confirmed from the data evaluated by SEC analyses shown on Fig. 4a which clearly shows the shift to lower elution volumes of the diblock copolymers compared to the trace initially recorded for the macroreagents. It seems however important to note that the presence of some remaining traces of unfunctionalized P3HT macroreagent cannot be totally excluded since chromatograms are partially overlapped.

To attest on the degradation of the as-prepared P3HT-b-PEO block copolymers, the hydrolytic treatment of the inner acetal group has been studied on the P3HT-b-PEO2000. Practically this was achieved by the use of a trifluoroacetic acid (TFA) (9 vol%) in THF/H₂O (14/1, v/v) solution for 24 h at room temperature and obtained products were analysed by SEC (Fig. 4b).⁴⁷

After treatment, the solvent was evaporated, the mixture dissolved in chloroform, neutralised and extracted with solution of NaHCO₃(aq.). The obtained degraded product (black trace) was redissolved in THF and precipitated in methanol. The purple solid product obtained after filtration was analyzed by SEC and derived mass characteristics match closely to the one of the P3HT alkyne macroreagent (bottom red trace). The product dissolved in methanol after evaporation was also subjected to a SEC investigation and shows mass characteristics of the PEO2000 macroreagent (bottom blue trace). Obtained data confirm well the successful acid degradation of the inner acetal group of block copolymers without degradation of any segments. Additional evidence for cleavage of acetal bond was obtained ¹H NMR spectrum of separated products after degradation of P3HT-b-PEO5000 block copolymer (ESI, Fig. S8 and S9†).

Thermal analyses

Thermal behaviour of synthesised block copolymers was analysed by DSC. Thermal properties (thermograms) of the prepared two block copolymers P3HT-b-PEO2000 and P3HT-b-PEO5000 as well as P3HT alkyne and PEO azides macroreagents are shown on Fig. 5.

Fig. 5 DSC thermograms of P3HT and PEO macroreagents, and P3HT-b-PEO block copolymers recorded under nitrogen at 10 °C min⁻¹.

In the present work was found that the nanofibrillar well-ordered P3HT block allows and do not obstruct the crystallization formation of PEO blocks. DSC thermogram of P3HT-b-PEO2000 diblock copolymer reveals two melting temperatures (T_m) of P3HT and PEO2000 block which are situated at 208 °C and 53 °C, respectively. They were very identical to T_m values obtained from P3HT macroreagent at 210 °C and PEO2000 macroreagent at 53 °C. The melting enthalpy (ΔH_m) value of P3HT and PEO2000 block is 19 J g⁻¹ and 56 J g⁻¹, respectively, whereas the ΔH_m of P3HT and PEO2000 macroreagent is 14 J g⁻¹ and 181 J g⁻¹, respectively.

Similar behaviour exposes also thermogram of P3HT-b-PEO5000 diblock copolymer where the presence of two melting temperatures typical for every segment of the copolymer is available. T_m for P3HT segment in both homopolymer and copolymer are situated at 203 °C and 210 °C, respectively, whereas T_m for PEO5000 segment in both homopolymer and copolymer are located at 59 °C and 55 °C, respectively. The enthalpy value of P3HT and PEO5000 block is 8 J g⁻¹ and 28 J g⁻¹, respectively, compared with the ΔH_m of P3HT and PEO5000 macroreagent values at 14 J g⁻¹ and 196 J g⁻¹, respectively.

X-ray diffraction (XRD) analyses

XRD profiles of P3HT and PEO homopolymers, and P3HT-b-PEO and degraded P3HT-b-PEO block copolymers, were measured on a cast films from chloroform solution in 2θ interval from 3 to 50°. Targeted signals are pointed out by arrows (Fig. 6).

Fig. 6 XRD profile of PEO and P3HT homopolymers, and P3HT-b-PEO and degraded P3HT-b-PEO block copolymers.

PEO homopolymers show diffraction profile with intensive signal at 2θ = 19.1° for (1 2 0) reflection type, highly intensive signal at 2θ = 23.1° for (1 3 2) reflection type, and weak signals for 2θ = 26.1° and 2θ = 26.8°, respectively. Obtained XRD data for PEO homopolymers is in good agreement with the data published in literature.^51–53

Typical diffraction peaks of P3HT macroreagent are highly intensive signal at 2θ = 5.4° for (1 0 0) reflection type, low intensive signal at 2θ = 10.8° for (2 0 0) reflection type and non intensive signal at 2θ = 16.3° for (3 0 0) reflection type. Broad XRD reflection, typical for (0 1 0) reflection type of the π–π stacking distance in P3HT, is located at about 2θ = 23.5°. Above data characterising P3HT unit was already revealed by many authors.^6,7,54,55

Crystalline diffraction peaks of the synthesized P3HT-b-PEO block copolymers with degradable acetal bond are a superposition of XRD patterns of the two P3HT and PEO homopolymers. XRD profile is situated at similar 2θ positions as in P3HT block with intensive signal at 2θ = 5.4° for (1 0 0) reflection type, low intensive signals at 2θ = 10.8° and 2θ = 16.3° for (2 0 0), and (3 0 0) reflection type, respectively, and also appearance of less intensive signals at 2θ = 19.2° and 2θ = 23.2° for (1 2 0) and (1 3 2) reflection type, respectively, which are characteristic of the presence of PEO block.

XRD crystalline profile of the degraded P3HT-b-PEO block copolymer resembles the one of P3HT macroreagent with intensive signal at 2θ = 5.4° for (1 0 0) reflection type, low intensive signals at 2θ = 10.8° for (2 0 0), and non intensive signal at 2θ = 16.3° for (3 0 0) reflection type. It has to be noted that diffraction peaks for PEO block at 2θ = 19.2° for (1 2 0) reflection type and at 2θ = 23.2° for (1 3 2) reflection type are not appeared in XRD crystalline profile of the degraded P3HT-b-PEO block copolymer which attests for selective film etching process of PEO domains and formation on the surface only of P3HT domains with nanofibrillar structure.

Optical properties

All polymer films drop-casted from solution with concentration of 10 mg ml⁻¹ in chloroform showed three maxima over the range of their absorbance spectra (Fig. 7) which is typical for the organized structure of regioregular P3HT.^7,36

Fig. 7 Optical properties of the films of pristine P3HT, P3HT-b-PEO block copolymers and corresponding degraded materials.

In parallel to two main absorptions at 510 nm and 544 nm, the films also showed a shoulder at around 605 nm which is related to a vibronic absorption and corresponds to a high degree of ordering in P3HT homopolymer, as well as in the pristine and the degraded P3HT-b-PEO block polymers.

The presence of PEO block has a minor impact on absorbance spectrum of the block copolymers and leads to a slight decrease in the absorption as compared to the P3HT homopolymer. Chemical etching/hydrolysis of P3HT-b-PEO block copolymer films with TFA vapours for 4 h and followed by successful washing with methanol/triethylamine solution (100/1, v/v) for additional 4 h implies no change in the absorbance spectrum of P3HT building block suggesting no degradation of the electroconjugated block.

That observation confirms the concept that the sacrificial PEO block is selectively cleaved and remained nanofibrillar matrix of P3HT block stays intact after TFA treatment.³⁵

XPS analyses

The surface composition of P3HT-b-PEO and degraded P3HT-b-PEO block copolymer films was analyzed by X-ray photoelectron spectroscopy (XPS) and obtained C1s spectra are shown on Fig. 8a and b.

Fig. 8 C1s XPS spectrum of (a) P3HT-b-PEO, and (b) degraded P3HT-b-PEO block copolymer films.

The spectrum of P3HT-b-PEO copolymer film represents two deconvoluted peaks corresponding mainly for peak of (–CH–CH–) groups at binding energy 285.1 eV and small peak of 15% intensity for (–C–O–C–) groups at binding energy 286.8 eV (Fig. 8a). The surface composition of degraded P3HT-b-PEO copolymer film can be fit into three deconvoluted peaks.

They can be assigned as a main peak for (–CH–CH–) groups at binding energy 285.2 eV, and two very small peaks of 6% intensity for (–C–O–C–) groups at binding energy 286.6 eV and of 4% intensity for (–C=O) groups at binding energy 288.0 eV (Fig. 8b). It can be assumed that during degradation treatment with TFA acid, the peak corresponding possibly for PEO content on the film surface decreased more than two times and also weak formation of carbonyl groups on the surface was detected.

Morphological structure of the thin films

Thin films of the copolymers P3HT-b-PEO2000 and P3HT-b-PEO5000 were obtained by spin-coating onto ITO glass substrates and both AFM phase images revealed a microscopic morphology made of fibrillar objects with a long-range parallel arrangement (Fig. 9c and d). These objects are often referred to as nanofibrils or nanoribbons, and are typical of regioregular poly(3-hexylthiophene) crystallization, as shown in Fig. 9a for the neat P3HT macroreagent. In P3HT-b-PEO2000 thin films, the average width of the fibrils is 16 ± 2 nm, consistent with an assembly made of parallel π-stacked P3HT macromolecules (perpendicular to the fibrils long axes) and PEO2000 in a compact configuration on the edges of P3HT lamellae, as for other rod–coil copolymers comprising a P3HT rod block.^17,24 The fibrillar arrangement of the structure in P3HT-b-PEO films suggests that the crystallization of the P3HT blocks (in light) dominates and it is barely disturbed by domains of the PEO blocks (in dark) which adopting likely coiled conformation of their segments. On Fig. 9b is shown the morphology of pure PEO2000 macroreagent thin film which represents a matrix of untextured morphology. Longer length of PEO block in P3HT-b-PEG5000 copolymer also insignificantly influences the self-assembly behavior in morphological structure of the thin film compared with shorter length of PEO block in P3HT-b-PEO2000 copolymer.

Fig. 9 Tapping mode AFM phase images of thin films of: (a) P3HT-C≡CH macroreagent; (b) PEO5000-N₃ macroreagent; (c) P3HT-b-PEO2000 block copolymer; (d) P3HT-b-PEO5000 block copolymer; (e) degraded P3HT-b-PEO2000 block copolymer; (f) degraded P3HT-b-PEO5000 block copolymer. The white scale bars in the bottom left corners are 200 nm-long.

Further were studied the thin films of P3HT-b-PEO2000 and P3HT-b-PEO5000 block copolymers prepared in the same conditions but exposed to TFA vapors, accompanied by degradation of acetal linker and easily removal of PEO block by methanol (Fig. 9e and f). In that case fibrillar structure morphology is still available but partially disturbed compared to the morphology of unhydrolysed films. Increase of the roughness and formation of porosity in the thin film surface of the films in both hydrolysed block copolymers were observed and it was investigated on the tapping mode AFM height images of P3HT-b-PEO2000 and degraded P3HT-b-PEO2000 block copolymers (Fig. 10a and b).

Fig. 10 Tapping mode AFM height images of thin films of: (a) P3HT-b-PEO2000 block copolymer, and (b) degraded P3HT-b-PEO2000 block copolymer.

Height distance of the degraded P3HT-b-PEO2000 block copolymer film increased to 20.0 nm compared with one of nondegraded P3HT-b-PEO2000 block copolymer film of 8.0 nm. Similarly the root-mean-square (rms) roughness values were 2.2 ± 0.2 nm for non etched and 4.9 ± 0.4 nm for etched film of P3HT-b-PEO2000, and 2.0 ± 0.1 nm for non etched and 5.2 ± 0.5 nm for etched film of P3HT-b-PEO5000, respectively.

That can be attributed to some partial lateral collapse of the obtained P3HT domains, as has been reported in other nanostructured systems in the literature.^33,35,36 The partial domain collapse was assumed to come from the surface tension force during evaporation of the liquids from the film and was attempted to be reduced by utilizing supercritical CO₂ as solvent in the final treatment of nanoporous film surface.³⁴

The nanoporous film formation of degraded P3HT-b-PEO5000 block copolymer was also confirmed by obtained scanning electron microscopy (SEM) surface image (Fig. 11). The pores with average size similar to that of P3HT fibrils width were observed on whole polymer surface.

Fig. 11 SEM surface image of nanoporous thin film of degraded P3HT-b-PEO copolymer.

Above structural analysis reveals that selective film etching process of PEO domains by acidic cleavage of acetal bonds left on the surface only P3HT domains with nanofibrillar ordered structure. That was accompanied by increase of the surface roughness and such formed polythiophene layer presents nanoporous structured film suitable to serve as high surface area matrix for organic solar cells.

Experimental

Synthesis of P3HT-b-PEO2000 block copolymer containing cleavable acetal bond

A flask maintained under nitrogen atmosphere was charged by alkynyl end-functionalized P3HT (50.0 mg, 0.006 mmol, 1 eq.), PEO2000 azide (24.0 mg, 0.012 mmol, 2 eq.), and CuBr (17.2 mg, 0.12 mmol, 20 eq.). After three freeze–pump–thaws degassing cycles, anhydrous THF (5 ml) was added via a syringe and the solution was purged with nitrogen and stirred vigorously for 20 min. PMDETA (30.0 μl, 0.144 mmol, 24 eq.) was added via a syringe and the solution was degassed by nitrogen for additional 20 min. The reaction mixture was heated up to 55 °C and stirred for 24 h. After addition of 40 ml THF, the solution was passed through a neutral Al₂O₃ column to remove copper salts. The purified solution was evaporated on a rotary evaporator, dissolved in small amount of THF and precipitated in cold methanol to selectively remove the excess of pristine PEO. The filtered product was washed with acetone and hexane to afford a purple solid (49.5 mg, yield 80%); ¹H-NMR (CDCl₃, δ ppm): 7.46 (s, 1H, –N₃HC₂– triazole ring), 6.98 (s, 45H, Ar-H P3HT), 5.22 (s, 2H, Ar-OCH₂– P3HT), 4.72 (q, J = 5.4 Hz, 1H, H₃C–CH(OR)(OR′) PEO), 4.48 (t, J = 5.4 Hz, 2H, –CH₂–N₃HC₂– PEO), 3.92 (t, J = 5.4 Hz, 2H, –CH₂–CH₂–N₃HC₂– PEO), 3.80–3.50 (m, 180H, –(CH₂CH₂O)_m– PEO), 3.38 (s, 3H, CH₃O– PEO), 3.05 (t, J = 6.8 Hz, 2H, –CH₂–N₃HC₂– P3HT), 2.95 (t, J = 6.8 Hz, 2H, –CH₂–CH₂–N₃HC₂– P3HT), 2.80 (t, J = 6.8 Hz, 90H, Ar-CH₂– P3HT), 1.78–1.64 (m, 90H, Ar-CH₂–CH₂– P3HT), 1.50–1.15 (m, 270H, Ar-CH₂–CH₂–(CH₂)₃–CH₃ P3HT), 0.91 (t, J = 6.8 Hz, 135H, Ar-(CH₂)₅–CH₃ P3HT); M_n (SEC) = 10 950 g mol⁻¹, Đ_M = 1.33.

Synthesis of P3HT-b-PEO5000 block copolymer containing cleavable acetal bond

The procedure for the “click” coupling reaction between the alkynyl end-functionalized P3HT and PEO5000 azide macroreagent is the same as described for the preparation of P3HT-b-PEO2000 block copolymer but instead of using methanol as precipitation solvent, the recovery of the final diblock was done by using isopropanol. Yield 51%, ¹H-NMR (CDCl₃, δ ppm): 7.46 (s, 1H, –N₃HC₂– triazole ring), 6.98 (s, 45H, Ar-H P3HT), 5.22 (s, 2H, Ar-OCH₂– P3HT), 4.72 (q, J = 5.4 Hz, 1H, H₃C–CH(OR)(OR′) PEO), 4.48 (t, J = 5.4 Hz, 2H, –CH₂–N₃HC₂– PEO), 3.92 (t, J = 5.4 Hz, 2H, –CH₂–CH₂–N₃HC₂– PEO), 3.80–3.50 (m, 450H, –(CH₂CH₂O)_m– PEO), 3.38 (s, 3H, CH₃O– PEO), 3.05 (t, J = 6.8 Hz, 2H, –CH₂–N₃HC₂– P3HT), 2.95 (t, J = 6.8 Hz, 2H, –CH₂–CH₂–N₃HC₂– P3HT), 2.80 (t, J = 6.8 Hz, 90H, Ar-CH₂– P3HT), 1.78–1.64 (m, 90H, Ar-CH₂–CH₂– P3HT), 1.50–1.15 (m, 270H, Ar-CH₂–CH₂–(CH₂)₃–CH₃ P3HT), 0.91 (t, J = 6.8 Hz, 135H, Ar-(CH₂)₅–CH₃ P3HT); M_n (SEC) = 12 700 g mol⁻¹, Đ_M = 1.43.

Preparation of drop-casted and spin-coated polymer films

Block copolymer solutions prepared for a concentration of 10 mg ml⁻¹ in chloroform were drop-casted on a glass substrate (20 × 25 mm) suitable for XRD measurement, on a quartz substrate (40 × 10 mm) appropriate for UV-VIS analysis, on a Si/SiO₂ substrate (10 × 10 mm) for XPS analysis or alternatively, spin-coated (60 s, 2000 min⁻¹) on a ITO glass substrate (15 × 15 mm) followed by solvent annealing in CHCl₃ overnight to allow AFM spectroscopy. After evaporation of the solvent, all films were dried under vacuum at room temperature. For sake of comparison films were also prepared from homopolymers alone by using the same procedure.

Acidic degradation of spin-coated polymer films

The thin films were exposed to trifluoroacetic acid (TFA) vapors in a closed chamber for 4 h. After that they were simply dipped into a methanol/triethylamine (100/1, v/v) solution for 4 h to selectively remove the PEO cleaved chains. Then the films were dried under vacuum and used for characterizations.

Conclusions

In summary, a facile approach to control the nanostructured organization of conjugated polymer platforms is proposed, based on the synthesis and characterization of copolymers containing cleavable group inside the chain. It is illustrated by block copolymerization of electroconjugated unit representing regioregular poly(3-hexylthiophene) (rr-P3HT) obtained by Grignard metathesis (GRIM) polymerization with terminal alkynyl group and sacrificial unit of poly(ethylene oxide) (PEO) with functionalized end azide group. The block copolymers were successfully synthesized via a click coupling reaction including the use of acid degradable acetal group between blocks. The strategy for formation of thin porous P3HT films includes treatment of P3HT-b-PEO block copolymers in TFA vapors, accompanied by the degradation of acetal linker inside the polymer chain and easily removal of PEO block by methanol. AFM images of the block copolymer films represented a smooth surface with nanofibrillar structured arrangement where average width of fibrils is 16 ± 2 nm and length is few hundred nanometers. However, removal of PEO blocks was followed by increase of the surface roughness due to pore formation and some partially collapse of the domains. The study displays an appropriate strategy to convert nanoporous P3HT thin films into suitable high surface area matrices which after successful filling with electron acceptor material could create novel nanostructures for organic photovoltaics.

Acknowledgements

This work was supported by project POLINNOVA in the FP7-REGPOT-2012-2013-1 of European Commission and project DFNI-E02/5 of Bulgarian National Science Fund. O. C. & P. G. thank the financial support from the Wallonia Region, the European Commission (FSE, FEDER), the National Fund for Scientific Research (F.R.S.-FNRS), and the Belgian Federal Science Policy Office (PAI 6/27). The authors deeply thank Dr Mathieu Surin and Dr David Moerman for discussions on the AFM surface analysis.

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Footnote

† Electronic supplementary information (ESI) available: Materials, apparatus, synthesis and modification of P3HT, and PEO macroinitiators, FTIR, NMR and MALDI-ToF analyzes. See DOI: 10.1039/c6ra00952b

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