Phase Separation in the Melt and Confined Crystallization as the Key to Well-Ordered Microphase Separated Donor-Acceptor Block Copolymers

Microphase-separated donor-acceptor block copolymers have been discussed as ideal systems for morphology control in organic photovoltaics. Typical micro-phases as known from coil-coil systems were not observed in such systems due to crystallization dominating over micro-phase separation. We show how this problem can be overcome by the synthesis of high molecular weight block copolymers leading to a high enough chi N parameter and microphase separation in the melt. A combination of copper-catalyzed azicle-allryne click reaction and nitroxide mediated radical polymerization (NMRP) was used for the synthesis of donor acceptor poly(3-hexylthiophene)-block-poly perylene bisimide acrylate (P3FIT-b-PPerAdr) block copolymers. With this synthetic strategy, high molecular weights are possible and no triblock copolymer byproducts are formed, as observed with former methods. Two different block copolymers with a high molecular weight P3HT block of 19.7 kg/mol and a PPerAcr content of 47 and 64 wt % were obtained. X-ray scattering measurements show that the diblock copolymers exhibit microphase separation in the melt state. Furthermore, upon cooling confined crystallization occurs inside the microphase separated domains without destroying the, microphase order. The observed microstructures fit well to the respective volume fractions and the crystalline packing within the individual blocks is analogous to those in the respective homopolymers. For the first time, typical lamellar or cylindrical phase separated structures as known for amorphous coil-coil systems are realized for a crystalline-liquid crystalline, donor-acceptor block copolymer. A similar block copolymer synthesized with an earlier method exhibits a Crystallization-induced microphase separation.