High-Temperature Rubbing: A Versatile Method to Align π-Conjugated Polymers without Alignment Substrate

Abstract

Mechanical rubbing of polymer films has been widely used in the liquid crystal display industry to prepare oriented alignment layers of polyimides. We show that this fast orientation method can be successfully applied to a large palette of different π-conjugated systems, i.e., p-and n-type semiconducting homopolymers and alternating copolymers. Transmission electron microscopy, grazing incidence X-ray diffraction and UV−vis absorption spectroscopy reveal that both, the temperature of the films during rubbing and the molecular weight of the polymer strongly influence the level of orientation. For polythiophenes and polyfluorenes, the dependence of the orientational order parameter on the rubbing temperature (T rub ) was determined. A strong increase of alignment with T rub is explained by the progressive alignment of higher molecular weight fractions at higher T rub . The disordering of alkyl side chains allows the hairy-rod shaped macromolecules to disentangle and align during rubbing. In addition, for certain conjugated polymers, the in-plane orientation, crystallinity, and polymorphism of the rubbed films can be substantially improved/modified by postdeposition thermal or solvent vapor annealing. This high level of orientation results in highly anisotropic optical and electronic properties (UV−vis absorption, fluorescence, charge transport).

Molecular Orientation of Conjugated Polymer Chains in Nanostructures and Thin Films: Review of Processes and Application to Optoelectronics

“…It is a well-known technique applied in the field of LCDs. 2D-XRD measurements revealed that rubbing P3HT layers generates molecular order whether the films are rubbed with velvet or nylon materials ( Figure 15) [33,[65][66][67][68]. In particular, depending on the temperature at which the rubbing process is performed, the amount of face-on and edge-on crystallites can be controlled to induce the adequate crystallite orientation depending on the application [66].…”

“…In particular, depending on the temperature at which the rubbing process is performed, the amount of face-on and edge-on crystallites can be controlled to induce the adequate crystallite orientation depending on the application [66]. For instance, the vertical hole mobility can be enhanced about twofold with the generation of face-on crystallites and mobility anisotropy can also be generated through rubbing [65][66][67]. Although most of the studies on rubbing focus on P3HT, this process is not limited to thiophene-based polymers.…”

Anisotropic thin films formation rate on PEDOT : PSS layer with high azimuthal anchoring

“…However, not only the chemical structure of organic materials but also molecular alignment within the thin film strongly influence device properties . Varying the molecular ordering, we can change the ionization potential, light absorption and luminescence, and increase carrier mobility and stability of the films . From this point, the main objective upon device fabrication is to achieve the definite control of molecular alignment in the thin films.…”

“…1,4,5 Varying the molecular ordering, we can change the ionization potential, 6 light absorption and luminescence, and increase carrier mobility and stability of the films. 5,[7][8][9] From this point, the main objective upon device fabrication is to achieve the definite control of molecular alignment in the thin films. There is a number of methods to align molecules: mechanical orientation, LB films deposition, self-organization of liquid crystal (LC) materials, deposition on specific alignment surfaces, 5,10 and so on.…”

Macroscopic alignment of poly(3‐hexylthiophene) for enhanced long‐range collection of photogenerated carriers

“…Large‐area alignment can be achieved using casting techniques such as directional solidification, solution shearing, and microchannel confinement, yielding anisotropic, and in some cases substantially improved, carrier mobilities . Mechanical post‐processing techniques such as stamping, rubbing, and straining have also been shown to induce polymer chains to adopt highly aligned configurations . Molecular design of polymers exhibiting liquid crystalline behavior has also been used to supplement the effectiveness of alignment techniques …”