Unraveling the Phase Behavior of All-Conjugated Donor-Acceptor Block Copolymers through Molecular Engineering

Despite recent advances in all-conjugated poly(3-alkylthiophene) (P3AT)-based block copolymers (BCPs) with identical backbones, investigations into the phase behavior of conjugated donor-acceptor (D-A) BCPs containing two disparate backbones are somewhat limited. Herein, we report tailoring the phase behavior of a series of new all-conjugated D-A BCPs, i.e., poly(3-alkylthiophene)-block-poly(isoindigo-alt-bithiophene) (P3AT-b-PII2T), by tuning their intrinsic factors (alkyl chains of P3ATs and block weight ratio of P3AT to PII2T) in conjunction with post thermal annealing. P3ATs include poly(3-hexylthiophene) (P3HT), poly(3-octylthiophene) (P3OT), and poly(3-decylthiophene) (P3DT), where the alkyl chain lengths gradually increase and approach that of PII2T. When the block weight ratio of P3AT to PII2T was approximately 1:1, the larger alkyl chain difference between the P3AT and PII2T blocks and the increased thermal annealing temperature favored microphase segregation (i.e., producing respective P3AT and PII2T crystals) over the crystallization of P3AT and PII2T as one block. Moreover, when the block weight ratios of P3OT to PII2T in P3OT-b-PII2T were changed to approximately 4:1 and 1:4, the major component dominated the crystallization of the entire BCPs, with depressed crystallization of the other minor component. The rule that similar alkyl chain length and a comparable block weight ratio between P3AT and PII2T favor their crystallization as one block can be readily extended to another conjugated D-A BCP, that is, poly(3-alkylthiophene)-block-poly(naphthalene diimide-alt-bithiophene) (P3AT-b-PNDI2T). This study highlights the phase behaviors in P3AT-b-PII2T and P3AT-b-PNDI2T BCPs tuned by synergistic molecular engineering and post thermal treatment, which could strengthen the fundamental understanding of conjugated D-A BCPs for their application in organic optoelectronic devices.