Confined Crystallization within Cylindrical P3EHT Block Copolymer Microdomains

Abstract

Confinement of crystallites within block copolymer microdomains is a promising approach to study conjugated polymer crystallization due to interfacial chain tethering and defined geometries. The nanoscale organization of crystallites is often critical to determining the charge transport properties of conjugated polymers. Here, a poly(3-(2′-ethyl)hexylthiophene)block-poly(methyl acrylate) (P3EHT-b-PMA) system is leveraged to study the impact of confinement within cylindrical microdomains. The crystalline P3EHT permits accessible melting temperatures and robust formation of traditional microphaseseparated morphologies, while the rubbery PMA allows the local deformations required to permit P3EHT crystallization. Crystallites form with chains perpendicular to the diblock interface, causing domain expansion; TEM reveals that this is accommodated in the cylindrical geometry via local deformation. Complementary SAXS/WAXS of aligned diblocks shows preferential orientation of the alkyl chain stacks down domains. Furthermore, cylindrically confined P3EHT demonstrates a smaller window of thermal control over crystalline perfection via isothermal crystallization conditions than homopolymer P3EHT or block copolymer P3EHT in lamellar confinement. This work demonstrates that postcrystallization annealing is an alternative route to generating uniformly high quality crystallites in cylindrically confined P3EHT. These results are important for considering routes to optimizing and controlling crystallinity in nanoscale confined geometries.

Recent progress in block copolymer crystallization

“…The rubbery PMA provided soft confinement and molecular mobility to promote crystallization in the P3HT domains (as compared to PS‐ b ‐P3EHT materials) . They designed and confirmed melt microphase separation for lamellae‐forming ( f P3HT ≈ 0.46‐0.66) and cylinder‐forming ( f P3HT ≈ 0.10‐0.21) samples using TEM and SAXS. P3EHT crystallization was possible in the phase‐separated state for all samples, and break‐out crystallization did not occur with soft confinement.…”

“…Microphase separation in BCPs with conjugated blocks provides another methodology for patterning and templating the conductive domains. Several studies by Davidson and coworkers evaluated the crystallization of P3EHT in poly(methyl acrylate)‐ b ‐P3EHT (PMA‐ b ‐P3EHT) copolymers where the PMA block was above T g . The rubbery PMA provided soft confinement and molecular mobility to promote crystallization in the P3HT domains (as compared to PS‐ b ‐P3EHT materials) .…”

“…Far right : Cartoon of suggested relaxation and melting mechanism across the three regimes where the initial state depends on T c . (Reproduced from with permission from the American Chemical Society)…”

“…Copolymers with rod-like blocks have been the focus of recent work because the rod-like moiety often involves conjugated polymers, which can be used in photovoltaic, semiconductor, and other electronics technologies. [163,164] Much of the work has been focused on controlling the morphology of the conjugated block's domain, which can be accomplished by a variety of methods, including using varying solvent quality for solution assembly, [165][166][167][168][169][170][171][172] thermal history for bulk samples, [173][174][175][176][177][178][179][180] and varying compositions, [165][166][167][168][169][170][171][172][173][174][175][176][177][178][179][180] to improve the conductive qualities of the material. Most rod-like blocks studied here are thiophene-based and were combined with either a coil block that may or may not be semi-crystalline, [165][166][167][168][169][170][171][176][177]…”

“…There have been several studies concerning the morphological behavior of the most common BCP rod-like polymer, poly(3-hexylthiophene) (P3HT), because of its value as a conductor and light absorber. Because the alkyl chain on the 3-position of polythiophenes helps to organize the structure and provide increased solvent processability, work has focused on other polythiophene block copolymers such as poly(3-(2 0 -ethyl)hexylthiophene) (P3EHT) [172,[176][177][178] and other poly(alkylthiophenes), such as poly(3-butylthiophene) (P3BT) and poly(3-dodecylthiophene) (P3DDT). [175,179,180] With longer substituent alkyl chains, poly(alkylthiophenes) exhibit depressed melting temperatures relative to P3HT which allows them to robustly form microphaseseparated domains that confine the other block in block copolymer mixtures.…”