Effect of Molecular Weight on Competitive Self-Assembly of Poly(3-dodecylthiophene)-block-poly(methyl methacrylate) Copolymers

The fabrication of poly(3-alkylthiophene) (P3AT) nanopatterns with 10$\sim$20 nm scale using block copolymer self-assembly is one of key issues to achieve highly efficient organic optoelectronic devices. However, most P3HT-containing rod-coil block copolymers show only fibril structures due to their strong rod/rod interaction. P3DDT containing block copolymer shows well defined nanostructure, however, when P3DDT block chain is much longer than coil block, it also shows fibril structure. We suggest a simple but effective strategy to induce block copolymer microphase separation: increasing $\chi$N$_{total}$ with larger N$_{total}$ instead of reducing rod/rod interaction. We investigated, via small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM), the microphase separation of P3DDT-b-PMMA at high weight fractions of P3DDT. A high molecular P3DDT-b-PMMA (w$_{P3DDT}$ = 0.76) formed cylindrical morphology, which is quite different from fibril morphology of a lower molecular weight P3DDT-b-PMMA having the same w$_{P3DDT}$. Furthermore, the crystallinity of the P3DDT block chains confined in self-assembled microdomains showed higher than that of P3DDT homopolymer, which is verified by differential scanning calorimetry (DSC).