Matthias Fischer†§, C. David Heinrich‡§, Mukundan Thelakkat‡*, Thomas Thurn-Albrecht†*
† Experimental Polymer Physics Group, Martin Luther University Halle-Wittenberg, Von-Danckelmann-Platz 3, 06120 Halle, Germany
‡ Applied Functional Polymers, Macromolecular Chemistry I, University of Bayreuth, Universitätsstr. 30, 95440 Bayreuth, Germany
*E-mail of corresponding authors: mukundan.thelakkat@uni-bayreuth.de, Thomas.thurn-albrecht@physik.uni-halle.de
§ Both authors contributed equally
Prepared for submission
Impact of Molecular Dynamics on Structure Formation of Donor-Acceptor Block Copolymers
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Abstract
Donor-acceptor block copolymers (BCPs) are attractive model materials for studying morphology-dependent processes in organic photovoltaics due to their intrinsic property to form an equilibrium nanostructure on the length scale of the excition diffusion length. As usually semiconductor polymers are crystalline or liquid crystalline and structure formation processes are complex in these BCPs. We here present a study of the interplay between phase separation, crystallization and glass transition of two poly(3-hexylthiophene)-b-polyperylene bisimide (P3HT-b-PPBI 1 and 2) diblock copolymers incorporation P3HT as a donor and a polystyrene with two different pendant perylene bisimides (PBI-N3 1 and 2) as acceptor blocks. These materials were synthesized by a modular approach, combining KCTP, controlled RAFT polymerization and click chemistry in order to obtain highly comparable polymers. We synthesized poly(3-hexylthiophene) (P3HT) with a high molecular weight (Mn,SEC = 18300 g mol-1), in a controlled manner, introduced a RAFT end group by click chemistry to form a macro initiator and subsequently polymerized propargyloxystyrene by sequential polymerization. In a post-polymerization step using click reaction, the polystyrene block was grafted with the PBI acceptor units. We obtained diblock copolymers with 70 wt% of the PBI block and 30 wt% P3HT. In order to study the effect of the backbone glass transition on side chain crystallization in the acceptor block, low molecular weight model compounds and homopolymers with both PBIs were also synthesized. The BCPs were characterized by temperature dependent small- and wide angle x-ray scattering (SAXS/WAXS) in combination with differential scanning microscopy (DSC) While microphase separation in the liquid state led to a cylindrical morphology in both cases, the crystallization of the functional side chains depend strongly on the backbone glass transition temperature as compared to the ordering temperature of the PBI unit.
Impact of Molecular Dynamics on Structure Formation of Donor-Acceptor Block Copolymers
59 Introduction
The combination of the controlled Kumada catalyst transfer polymerization (KTCP) of 3-hexylthiophene, controlled radical polymerization (CRP) of easily available monomers such as propargyloxystyrol and click chemistry of suitable semiconductor functionalities via copper-catalyzed azide-alkyne cycloaddition (CuAAc) opens a new versatile pathway towards defined donor-acceptor block copolymers (BCPs).1-3 P3HT is one of the most studied conjugated polymers that can be polymerized in a controlled manner by the KTCP. McCullough et al.4,5 and Yokozawa et al.6,7 independently reported the controlled synthesis of P3HT. This method allows the synthesis of polymers with controlled end groups aside from being able to adjust the appropriate molecular weight for the intended application.8,9 In the field of organic electronics and organic solar cells absolute molecular weights of P3HT in the range of around Mn,MALDI = 12000 gmol-1 exhibit optimum material properties in terms of a high charge carrier mobility in organic field transistors (OFET)10 as well as in space charge limited current (SCLC) devices.11 The first synthesis of acceptor polymers with pendant perylene bisimide side chains (PPBI) by a direct CRP of acrylate functionalized PBI monomer were reported in 2004 by Linder et al.12 Bringing P3HT and the PPBI polymers in one block copolymer gives access to fully functionalized donor-acceptor BCPs. The equilibrium micro-structure of nanoscale phase separated donor-acceptor BCPs with perpendicular alignment with respect to a substrate has been proposed to be a very interesting model system for organic photovoltaics (OPV).13 Such a structured material should in principle provide enough donor-acceptor interfaces, due to the small structure size, for charge separation as well as optimal pathways for charge carrier transport. Block copolymer microphase separation is well understood for amorphous blocks. The corresponding phase diagram in general depends on the incompatibility χN and the volume fraction.14 If crystalline or liquid crystalline blocks are involved, the self-assembly behavior becomes more complex since there is an additional competition between classical microphase separation and crystallization. Depending on the relative positions of the order-disorder transition temperature (TODT) and the crystallization temperature (Tc),
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structure formation can either be induced by microphase separation or by crystall-ization. If crystallization starts from a microphase separated structure, the existing nanostructure can either remain intact (confined crystallization) or be destroyed (breakout crystallization).15
While the microphase separation in numerous BCPs of conjugated poly(alkylthiophene)s with an electronically inactive second block were reported,16-20 reports on donor-acceptor polymers with well-ordered microphase separated structures are rare.21-25 In previous publications we explored microstructure formation for several donor-acceptor BCPs containing a crystallizable donor block and a liquid crystalline acceptor block. Lohwasser et al. reported the synthesis of donor-acceptor BCPs, P3HT-b-PPBIs, which showed well-ordered lamellar and cylindrical microstructures as expected for the respective volume fractions.3 Lohwasser et al.
pointed out the importance of the incompatibility factor χN for these systems.3 If χN, resp. the molecular weight, was high enough, microphase separation in the melt with subsequent confined crystallization during cooling was observed for a series of donor-acceptor BCPs consisting of P3HT and pendant PBIs attached to a polyacrylate back-bone (cf. Fig. 1 a). In a later study the additional importance of chain mobility became clear.24 In this case the acceptor block consisted of pendant phenyl-C61-butyric acid methyl ester (PCBM) attached to a polystyrene backbone. This combination led to a strong increase of the glass transition temperature (Tg) of the acceptor block, which limited the microphase separation and formation of long-range order (cf. Fig. 1 b).24,26
In our earlier report, for the synthesis of P3HT-b-PPBI, high molecular weight P3HT with alkyne end group was converted to a macro initiator for NMRP via CuAAc click chemistry. This initiator was used to directly synthesize the second block using PBI-acrylate monomers. The direct polymerization of this monomer is not trivial and the introduction of CuAAc in combination with a controlled radical polymerization gave easier access to defined PBI-pendant homopolymers.2 This concept was first attempted by Tao et al. in 2009.28 Lang et al. investigated this concept in detail by synthesizing poly(propargyloxystyrene) via nitroxide mediated radical polymerization (NMRP) and grafting this precursor polymer with different perylene bisimides with azide
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functionality.2 A second study was concerned with the influence of the spacer length between the perylene bisimide core and the polymeric backbone and the influence of hydrophilic swallow tails on the thermal properties of the grafted polymers.29 The hydrophilic PPBIs were amorphous materials and exhibited surprisingly high electron mobilities in SCLC devices.30 Therefore, it is very interesting to incorporate such hydrophilic pendant blocks into BCPs and to study their microphase separation.
In the current study, based on the above facts, we have synthesized two comparable donor-acceptor BCPs with differently substituted PBI acceptor units by a modular approach. The PBIs are selected in such a manner that one carries the conventional hydrophobic alkyl swallow tail substituents, whereas the second one has flexible oligoethylene glycol (OEG) substituents. In addition, comparable sets of pendant PBI homopolymers and PBI model compounds were synthesized to study the structure formation in these respective systems and to understand the structure formation in BCPs, which may be complex. We use a combination of temperature dependent small- and wide-angle X-ray scattering (SAXS/WAXS), atomic force micros-copy and transmission electron microsmicros-copy (AFM, TEM) to investigate the structure formation in BCPs with pendant PBIs attached to a polystyrene backbone. We expect that due to the lower Tg of the acceptor block microphase separation will again take place unhindered by molecular mobility.29 Furthermore, the effect of different solubilizing group attached to the PBI unit is investigated.
Fig. 1 Schematic illustration of the effect of the glass transition temperature on self-assembly and crystallization of a block copolymer with high Tg in one block. The gray color schematically indicates the temperature range with low molecular mobility of the acceptor block. (i) Above the order-disorder transition temperature (TODT), the BCP forms a disordered melt. (ii) While in case (a) the chain mobility is high in the relevant temperature range, so that a well ordered microphase separated state forms below TODT, microphase separation and the formation of long range order is limited by the high Tg in case (b). (iii) Below Tc crystallization takes place.
It is confined for large segregation strengths. (For simplicity, it is assumed here that the crystallization temperature is the same for both blocks).
Impact of Molecular Dynamics on Structure Formation of Donor-Acceptor Block Copolymers