Poly(3-alkylthiophene)-block-poly(3-alkylselenophene)s: Conjugated Diblock Co-polymers with Atypical Self-Assembly Behavior

Abstract

Understanding self-assembly behavior and resulting morphologies in block co-polymer films is an essential aspect of chemistry and materials science. Although the self-assembly of amorphous coil−coil block co-polymers is relatively well understood, that of semicrystalline block co-polymers where each block has distinct crystallization properties remains unclear.Here, we report a detailed study to elucidate the rich selfassembly behavior of conjugated thiophene−selenophene (P3AT-b-P3AS) block co-polymers. Using a combination of microscopy and synchrotron-based X-ray techniques, we show that three different film morphologies, denoted as lamellae, cocrystallized fibers, and patchy fibers, arise from the self-assembly of these block co-polymers over a relatively narrow range of overall degrees of polymerization (30 < N < 90). Crystallization-driven phase separation occurs at a very low N (<35), and lamellar films are formed. Conversely, at medium N (50−60) and high N (>80), the thiophene and selenophene blocks cocrystallize into nanofibers, where medium N leads to much more mixing than high N. The overall tendency for phase separation in these systems follows rather different trends than phase separation in amorphous polymers in that we observe the greatest degree of phase separation at the lowest N. Finally, we demonstrate how each morphology influences transport properties in organic thin-film transistors comprised of these conjugated polymers.

Unravelling the Correlation between Microphase Separation and Cocrystallization in Thiophene-Selenophene Block Copolymers for Organic Field-Effect Transistors

“…At this point, it is interesting to deduce the effects of the P3HT/P3BS block ratio and thermal annealing temperature on the microphase-separated or cocrystalline structures of these P3HT- b -P3BS BCPs. It is reported that polymers possess similar structures, potential energies, and crystallization kinetics may form cocrystals, , and only several examples of polymer cocrystals have ever been reported to date. ,,− For the three P3HT- b -P3BS BCPs casting from toluene solution, they exhibit microphase-separated structures composed of sole P3HT crystalline domains or individual P3HT and P3BS crystalline domains depending on the block ratio of P3HT/P3BS (Figure d). After 200 °C-annealing these BCP samples, P3BS form II changed to form I of higher crystallinity and larger d 100 -spacing in HT55BS45 and HT42BS58.…”

Programmable Assembly of π‐Conjugated Polymers

“…[64][65][66][67] Interestingly, high-curvature morphologies can be induced by synthesizing block copolymers comprising P3AT and P3AS components. Work from our group has shown the synthesis of patchy nanofibers formed from the chain-folding of poly(3-dodecylthiophene) (P3DDT) blocks around poly(3dodecylselenophene) (P3DDS), where these high-curvature morphologies are accessed with increasing degree of polymerization (N) [68,69] (Figure 2a). Notably, decreasing phase segregation was found to increase charge mobility, with films exhibiting the cocrystallization having moderate hole mobilities as high as 1.2 × 10 −2 cm 2 V −1 s −1 .…”

Synthesis and Self-Assembly of Conjugated Block Copolymers

“…However, when N is increased, thiophene and selenophene blocks co-crystallize into fibers (N = 50-60) and patchy fiber (N > 80). Field-effect transistors testing showed that co-crystalline fibers have the best device performance [114].…”

“…This probably is because the miscibility of P3HT and PFTBT has become better, resulting in weaker phase separation [14] . [114]. Copyright American Chemical Society).…”