Sonocrystallization of conjugated polymers with ultrasound fields

Abstract

Ultrasound acoustic waves are demonstrated to assemble poly-3-hexylthiophene (P3HT) chains into nanofibers after they are fully dissolved in what are commonly considered to be 'good' solvents. In the absence of ultrasound, the polymer remains fully dissolved and does not self-assemble for weeks. UV-vis spectroscopy, ultra-small angle X-ray scattering (USAXS) and small angle neutron scattering (SANS) are used to characterize the induced assembly process and to quantify the fraction of polymer that forms nanofibers. It is determined that the solvent type, insonation time, and aging periods are all important factors affecting the structure and final concentration of fibers. The effect of changing polymer regio-regularity, alkyl chain length, and side chain to thiophene ratio are also explored. High intensity focused ultrasound (HIFU) fields of variable intensity are utilized to reveal the physical mechanisms leading to nanofiber formation, which is strongly correlated to cavitation events in the solvent. This in situ HIFU cell, which is designed for simultaneous scattering analysis, is also used to probe for structural changes occurring over multiple length scales using USAXS and SANS. The proposed acoustic assembly mechanism suggests that, even when dispersed in 'good' solvents such as bromobenzene, dichlorobenzene and chloroform, P3HT chains are still not in a thermodynamically stable state. Instead, they are stabilized by local energy barriers that slow down and effectively prevent crystallization. Ultrasound fields are found to provide enough mechanical energy to overcome these barriers, triggering the formation of small crystalline nuclei that subsequently seed the growth of larger nanofibers.

Origin of the Anisotropy and Structure of Ellipsoidal Poly(fluorene) Nanoparticles

“…Ultrasonication was demonstrated to enable the sonocrystallization of dissolved conjugated polymers which subsequently solidified as nanofibers. 152 In a fully dissolved state, the polymer chains were considered to be thermodynamically instable, but located in a local energetic minimum. However, the mechanical energy provided by the ultrasound-induced cavitation events was sufficient to overcome the energetic barrier, resulting in the formation of crystalline nuclei and the growth of larger nanofibers.…”

“…However, the mechanical energy provided by the ultrasound-induced cavitation events was sufficient to overcome the energetic barrier, resulting in the formation of crystalline nuclei and the growth of larger nanofibers. 152 As the intrinsic order in anisotropic particles is considered to be a decisive factor in overcoming the tendency to surface minimization, insight into the internal particle structure and its correlation with the external shape is of major interest to understand and control particle anisotropy.…”

Functionalized Cellulose Nanocrystal-Mediated Conjugated Polymer Aggregation

“…The self-assembly of π-conjugated semiconducting polymers is a topic of substantial interest in the field of organic electronics. , This self-assembly results in the formation of semicrystalline “aggregates” through chain folding and π–π stacking, facilitating charge-carrier transport along both the conjugated backbone and the π–π stacking direction. , Further, it has been shown that the presence of these polymer aggregates in solution can lead to films with enhanced optoelectronic properties, as determined by improved charge-carrier mobilities of thin films comprising the active layer in organic field-effect transistor devices , or improved power conversion efficiencies from bulk heterojunctions used in solar cells. , Methods to induce aggregation in polythiophenes, in particular, poly­(3-hexylthiophene) or P3HT, have included sonication, , UV irradiation, aging over time, , and poor solvent addition. , However, many of these strategies require significant amounts of energy input or involve the addition of hazardous organic solvents.…”

“…3,4 Further, it has been shown that the presence of these polymer aggregates in solution can lead to films with enhanced optoelectronic properties, as determined by improved charge-carrier mobilities of thin films comprising the active layer in organic field-effect transistor devices 2,5 or improved power conversion efficiencies from bulk heterojunctions used in solar cells. 6,7 Methods to induce aggregation in polythiophenes, in particular, poly(3-hexylth-iophene) or P3HT, have included sonication, 8,9 UV irradiation, 10 aging over time, 11,12 and poor solvent addition. 13,14 However, many of these strategies require significant amounts of energy input or involve the addition of hazardous organic solvents.…”

In−situ monitoring Poly(3-hexylthiophene) nanowire formation and shape evolution in solution via small angle neutron scattering

“…Several processing conditions have been investigated in order to optimize this process and some have been shown to successfully promote nanowire formation such as the above mentioned favorable solvent selection Han (2015); Chang (2004), solution doping Kim (2010), use of ultrasound Xi (2018), use of an appropriate film deposition technique Diao (2014); Katz (2004), and post-treatments such as thermal ; Cui (2003) and solvent annealing Fu (2015); Dickey (2006); Hu (2014). A long-term (> 2 hours) ageing of the P3HT solution to self-assemble nanowires is one of the simplest methods of P3HT nanowire formation.…”