Thermal Control of Confined Crystallization within P3EHT Block Copolymer Microdomains

The local, nanoscale organization of crystallites in conjugated polymers is often critical to determining the charge transport properties of the system. Block copolymer geometries, which offer controlled nanostructures with tethering of chains at interfaces, are an ideal platform to study the local organization of conjugated polymer crystallites. The model conjugated polymer poly(3-(2′-ethyl)hexylthiophene) (P3EHT) features a depressed melting temperature relative to the widely studied poly(3-hexylthiophene) (P3HT), which allows it to robustly form microphase-separated domains that confine the subsequent P3EHT crystallites. Importantly, P3EHT crystallization in confinement is coupled to a rubbery second block via interfacial tethering, mechanical properties, and chain stretching. Here, the impact of thermal processing on the diblock copolymer structure is examined to elucidate the key driving forces controlling the final coupled diblock copolymer and crystalline structures. Surprisingly, the diblock copolymer domain size is significantly impacted by the temperature at which the conjugated domain is crystallized. Decreasing amounts of domain extension are observed with increasing crystallization temperatures. This temperature-dependent domain structure appears to be correlated with the crystallization processes; these processes are inferred from precise changes in the lamellar structure across melting. By carefully tracking the changes in domain structure across melting, this work identifies three distinct regimes. We suggest a structural model of the conjugated block melting processes consisting of (I) excluded-chain relaxation, followed by (II) chain interdigitation during melt-recrystallization, and finally (III) complete melting that is independent of the initial crystallization conditions. These results suggest that P3EHT crystallization processes associated with temperature-dependent chain diffusion and nucleation are primarily responsible for the unexpected temperature-dependent crystallization behavior. They also emphasize that less perfect conjugated polymer crystals may actually be associated with a poorly interdigitated structure. Furthermore, this work demonstrates the utility of leveraging a diblock copolymer structure with a rubbery second block in order to precisely track changes in the crystallite structure.