Homogeneous and Segmented Nanofibers with a Conjugated Poly[3-(2′-ethylhexyl)thiophene] Core via Living Crystallization-Driven Self-Assembly

The ambient temperature living crystallization-driven self-assembly (CDSA) seeded growth method for polymeric amphiphiles with a crystallizable core-forming block has emerged as a promising approach to morphologically pure samples of uniform, length-controlled one-dimensional nanofibers as well as more complex architectures such as segmented assemblies. Access to nanofibers with poly(3-hexylthiophene) (P3HT) as the core-forming block is highly desirable as a result of their optoelectronic properties. However, application of the living CDSA method to P3HT diblock copolymers has proven to be a challenge due to rapid P3HT crystallization. This causes uncontrolled homogeneous nucleation and defect formation, resulting in a loss of length control and increased nanofiber length dispersity. Herein, we have explored the replacement of the P3HT core-forming block with a more soluble poly[3-(2′-ethylhexyl)thiophene] (P3EHT) core and demonstrate improved control over nanofiber formation. Specifically, we have studied two P3EHT-containing diblock copolymers, P3EHT23-b-PEG113 [PEG = poly(ethylene glycol)] and P3EHT19-b-P2VP138 [P2VP = poly(2-vinylpyridine)]. Seeded growth of P3EHT19-b-P2VP138 nanofibers at 22 °C produced low dispersity nanofibers with length control up to ∼1 μm, while P3EHT23-b-PEG113 only provided length control up to ∼300 nm under similar conditions. Self-nucleation was postulated to impair efficient seeded growth of P3EHT23-b-PEG113 and solution-state variable temperature UV–vis spectroscopy was used to locate temperatures where self-nucleation was suppressed. These studies revealed that homogeneous nucleation is suppressed for at least 24 h at or above 30 °C for P3EHT23-b-PEG113 in a 1:1 n-butanol:methanol solution. In the case of P3EHT23-b-PEG113, this allowed us to efficiently perform seeded growth under conditions (≥30 °C) that resulted in the formation of near-uniform nanofibers with controlled lengths of up to ∼1 μm. Self-nucleation suppression was also successfully utilized during thermal self-seeding experiments, wherein P3EHT23-b-PEG113 nanofibers with controlled lengths up to 2.8 μm and low length dispersities were formed. Seeded growth of P3EHT19-b-P2VP138 from the termini of P3EHT23-b-PEG113 seeds at 22 °C provided access to well-defined B-A-B triblock comicelles with a segmented coronal structure.