Modular Synthesis and Structure Analysis of P3HT-b-PPBI Donor-Acceptor Diblock Copolymers

While donor-acceptor block copolymers (BCPs) are widely discussed to be optimal model materials for studying the morphology-dependent processes in organic photovoltaics, the observation of the desired microphase-separated structures is rare. We present a novel modular approach for the synthesis of donor-acceptor block copolymers offering a fast and flexible way for chemical modifications leading to microphase separation accompanied by confined crystallization of the individual blocks. Two donor-acceptor BCPs of poly(3-hexylthiophene)-b-polyperylene bisimide (P3HT-b-PPBI 1 and 2) were synthesized incorporating P3HT as a donor and a polystyrene with two different pendant perylene bisimides (PBI-N-3 1 and 2) as acceptor blocks to show the flexibility of the synthetic approach. The synthesis combines Kumada catalyst transfer polymerization (KCTP), controlled radical polymerization, and click chemistry to obtain highly comparable polymers via a modular approach. We synthesized poly(3-hexylthiophene) (P3HT) with a high molecular weight (M-n,(SEC) = 18300 g mol(-1)), in a controlled manner, introduced a chain transfer end group by click chemistry to form a macroinitiator, and subsequently polymerized propargyloxystyrene by sequential polymerization. In a postpolymerization click reaction, the polystyrene block was quantitatively grafted with two different PBI acceptor units. We obtained diblock copolymers with 70 wt % of the PBI block and 30 wt % P3HT. The BCPs were characterized in bulk by temperature-dependent small- and wide-angle X-ray scattering (SAXS/WAXS) in combination with differential scanning calorimetry (DSC) and transmission electron microscopy (TEM). The same materials were also characterized in thin films by grazing incidence smallangle X-ray scattering (GISAXS) and atomic force microscopy (AFM). The attachment of the PBI units to the PS backbone was found to slow down the molecular dynamics, evidenced by an increase of the glass transition temperature (T-g). Despite the high T-g, the BCP morphology is dominated by microphase separation in the molten state. While the BCPs adopt a cylindrical morphology, the range of order is limited by the slow molecular dynamics. Subsequent crystallization of the individual blocks during cooling results in confined crystallization within the cylindrical, microphase-separated morphology. The results emphasize the importance of molecular dynamics for the formation of a well-ordered microphase-separated BCP morphology.