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

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 Mw ≥ 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.