Molecular Weight Dependence of Chain Packing and Semicrystalline Structure in Oriented Films of Regioregular Poly(3-hexylthiophene) Revealed by High-Resolution Transmission Electron Microscopy

Abstract

The semicrystalline structure of regioregular head-to-tail- (HT-) coupled poly(3-hexylthiophene-2,5-diyl) (Rr-P3HT) thin films grown by directional epitaxial solidification (DES) has been studied by high-resolution transmission electron microscopy (HR-TEM). A 10-fold increase of the weight-average molecular weight of the Rr-P3HT leads to several major structural changes: (i) an increase of the fluctuations of the lamellar thickness while preserving the average length of the polymer stems in the crystalline domains; (ii) an increase of the disorder in the crystalline packing of Rr-P3HT chains; (iii) an enhanced interconnectivity between crystalline lamellae via tie-crystallites and tie-chains; (iv) for M w ≥ 18.8 kDa equiv PS, preferential tilt angles of the polymer chains in the crystalline lamellae. Similar to stiff-chain polymers like poly(etheretherketone), the concomitant increase of the interchain distance (i.e., the a parameter in the unit cell) and the tilting of Rr-P3HT chains in the crystalline lamellae are attributed to chain folding exerting stress on the crystal packing because of the stiffness of comb-shaped Rr-P3HT chains.

Structure and Morphology Control in Thin Films of Conjugated Polymers for an Improved Charge Transport

“…Besides, molecular weight exerts effects on the crystallization morphology of conjugated polymers by controlling the connectivity between crystalline regions. As to the lamellar structure [71], the total lamellar thickness increased with the size of crystalline region nearly unchanged with the increasing molecular weight. The bending of chains near the ends gave rise to disorder region in lamella and tie chains connecting adjacent lamella were observed in thickened disorder region by HR-TEM for high molecular weight P3HT ( Figure 6I).…”

“…The folded chain model and fringed micelle model proposed for crystalline or semi-crystalline flexible polymers have been also applied in the examples of conjugated polymer polycrystalline films to explain crystal formation, tie chains and amorphous regions. As to the lamellar structure [71], the bending of chains near the ends gave rise to disorder region in lamella. With the increasing molecular weight, the total lamellar thickness increased but the size of crystalline region was nearly unchanged.…”

Structure and morphology control in thin films of regioregular poly(3‐hexylthiophene)

“…The study was performed on oriented epitaxied films of P3HT with different molecular weights M w . 57 Increasing M w from 7.3 to 69.6 kDa eq. PS results in several important structural changes.…”

Optical probes of chain packing structure and exciton dynamics in polythiophene films, composites, and nanostructures

“…24,[55][56][57][58] Figure 10 shows a schematic of the nanofiber structure along with a photoluminescence image from our work excited with 532-nm radiation. In crystalline poly(3-hexylthiophene) (P3HT) NFs, the polymer chains are aligned perpendicular to the NF growth axis, with thiophene rings oriented cofacially in adjacent stacks.…”