Impact of π-conjugated gradient sequence copolymers on polymer blend morphology

Abstract

A gradient sequence copolymer containing 3-hexylthiophene (3HT) and 3-(6-bromohexyl)thiophene (3BrHT) with a linear change in comonomer composition was synthesized via a controlled, chain-growth semi-batch method. For comparison, random and block copolymers with the same molecular weight and comonomer ratio (1 : 1) were prepared. All three copolymers exhibited similar molecular weight (M_n ∼ 32 kDa), low dispersity (Đ < 1.2) and high regioregularity (>99%), suggesting that any differences among the three copolymers can be attributed to the different copolymer sequences. The optical and thermal properties, as well as the thin film morphologies, of the gradient copolymer were compared to the random and block copolymers and the physical blend of the homopolymers. The physical blends showed extensive micron-scale phase separation by AFM and TEM. Adding the gradient copolymer to the blend resulted in a dramatic reduction in the domain size. Moreover, the domain size decreased as the amount of the gradient copolymer increased, suggesting that the copolymer is compatibilizing the polymer blend. By comparison, the random and block copolymers were less effective compatibilizing agents, which indicates that the gradient sequence copolymer is well suited to tailor the morphology of immiscible polymer blends.