Confined, Templated, and Break-Through Crystallization Modes in Poly(3-dodecylthiophene)-block-poly(ethyl methacrylate) Block Copolymers

Abstract

The crystallization modes of conjugated-amorphous block copolymers (BCPs) are determined by an interplay between segregation strength (chi N), softness of the amorphous block (i.e., rubbery or glassy), and the driving force for crystallization. Herein, we examine crystallization modes in poly(3-dodecylthiophene)-block-poly(ethyl methacrylate) (P3DDT-b-PEMA) using in situ X-ray scattering and ex situ electron microscopy. The segregation strength was approximately constant among all materials, and the PEMA block was rubbery under all conditions studied. The principal variables for our studies were the driving force for crystallization, which was tuned by varying the regioregularity (RR) of P3DDT in the range of 65 to 96%, and the volume fraction of P3DDT in the BCP. In the melt state, all BCPs assembled into well-ordered lamellar (LAM) or hexagonally packed cylindrical (HPC) phases. However, when cooled below the crystallization temperature, the crystallization mode depended on the symmetry of the BCP lattice and RR of P3DDT. In LAM phases, P3DDT crystallization was templated by the self-assembled structure when RR was high (76, 81 and 94%) and confined when RR was low (65 and 70%). In cylindrical phases, P3DDT crystal growth broke-through the rubbery PEMA domains when RR was high (76, 81 and 94%) but was templated when RR was low (65 and 70%). This morphology-dependent behavior reflects geometric frustration due to mismatch in the natural dimensions of P3DDT crystals and the initial BCP lattice. These studies underline the importance of geometric effects and RR control for balancing self-assembly and crystallization in conjugated-coil BCPs.