ABSTRACT
Driven by molecular affinity and balance in the crystallization kinetics, the ability to co-crystallize dissimilar yet self-crystallizable blocks of a block copolymer (BCP) into a uniform domain may strongly affect its phase diagram. In this study, we synthesize a new series of crystalline and monodisperse all-π-conjugated poly(2,5-dihexyloxy-p-phenylene)-b-poly(3-(2-ethylhexyl)thiophene) (PPP-P3EHT) BCPs and investigate this multi-crystallization effect. Despite vastly different side-chain and main-chain structures, PPP and P3EHT blocks are able to co-crystallize into a single uniform domain comprising PPP and P3EHT main-chains with mutually interdigitated side-chains spaced in-between. With increasing P3EHT fraction, PPP-P3EHTs undergo sequential phase transitions and form hierarchical superstructures including predominately PPP nanofibrils, co-crystalline nanofibrils, a bilayer co-crystalline/pure P3EHT lamellar structure, a microphase-separated bilayer PPP-P3EHT lamellar structure, and finally P3EHT nanofibrils. In particular, the presence of the new co-crystalline lamellar structure is the manifestation of the interaction balance between self-crystallization and co-crystallization of the dissimilar polymers on the resulting nanostructure of the BCP. The current study demonstrates the co-crystallization nature of all-conjugated BCPs with different main-chain moieties and may provide new guidelines for the organization of π-conjugated BCPs for future optoelectronic applications.
ABSTRACT
Solution processing of π-conjugated polymers constitutes a major low-cost manufacturing method for the fabrication of many new organic optoelectronic devices. The solution self-assembly kinetics of π-conjugated rod-coil block copolymers of symmetric poly(3-hexyl thiophene)-b-poly(2-vinyl pyridine) (P3HT-P2VP) during drying and the phase transformations of the subsequently dried samples were studied by using a combination of TEM, SAXS, WAXS and DSC measurements. During solution drying in chlorobenzene, a good solvent for the copolymer, P3HT-P2VP first formed nanoseed aggregates followed by the directional growth of nanofibrils driven by the formation of prevailing form II P3HT crystals within its nanofibril core confined by the surrounding domain of P2VP blocks. This result was in sharp contrast when a similar molecular weight P3HT homopolymer was solution self-assembled in chlorobenzene, nearly free from confinement, in which case the resulting nanofibrils consisted of a mixture of majority form I and form II crystals. Solvent-cast films of P3HT-P2VP nanofibrils with form II crystals were heat-/cold-treated and showed solid-state phase transformations from form II crystals to form I crystals, both within nanofibrils with annealing, indicating the metastability of the form II crystals with temperature. A disordered state followed with increasing temperatures which, when cooled, induced the formation of a thermodynamically stable lamellar phase with only form I P3HT crystals. Correspondingly, the study provides new strategies for controlling polymorphs and nanostructures of π-conjugated block copolymers for future applications using solution processing and subsequent heat treatment.
