End-on Chain Orientation of Poly(3-alkylthiophene)s on a Substrate by Microphase Separation of Lamellar Forming Amphiphilic Diblock Copolymer

We obtained the end-on orientation of poly(3-dodecylthiophene) (P3DDT) chains where the main chains are vertically oriented on a substrate by synthesizing poly(3-dodecylthiophene)-block-poly(3-(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)methyl thiophene) copolymer (P3DDT-b-P3TEGT) with two different weight fractions of P3DDT block (w_P3DDT = 0.48 and 0.65). Both block copolymers showed well-ordered lamellar microdomains in bulk, verified by small-angle X-ray scattering (SAXS). Because of the high incompatibility of the two blocks, P3DDT-b-P3TEGT thin films prepared by spin-coating on a substrate followed by thermally annealing showed parallel oriented lamellar microdomains to the substrate. Hydrophilic P3TEGT microdomains were located at the substrate/polymer interface, while hydrophobic P3DDT microdomains were located at the polymer/air interface. Thus, both P3DDT and P3TEGT backbone chains were oriented perpendicularly to the lamellar layer (namely, film thickness direction), and the end-on orientations of P3DDT and P3TEGT chains were obtained. The hole mobility was measured by fabricating a space-charge-limited current (SCLC) device. P3DDT-b-P3TEGT showed much enhanced mobility compared with the device made of neat P3DDT film with edge-on orientation, indicating that end-on orientation is very effective for improving the hole mobility along the vertical direction.