Controlling Condensed Nanostructures Of Poly(3-alkylthiophene)-based Systems

Poly(3-alkylthiophenes)(P3ATs) have been recognized as a kind of very important conjugated polymers, with a higher carrier mobility, better chemical stability, and processability. Therefore, they can be widely used as an electron donor in organic semiconductor devices such as organic light emitting diodes, organic field effect transistors and organic solar cells. The morphologies of P3ATs nanostructures are closely related to the device performance. Tailoring and optimizing the nanostructures, which can enhance charge separation and transportation, and then lead to high charge carrier mobility, is of great importance for their device performances. Finding new ways to well control the nanostructures of P3ATs, which is significant for both fundamental research and practical applications.This thesis mainly focused on controlling nanostructures in the P3ATs systems. We first realized the structure transition from polythiophene-based nanofibers to spherical clusters in ultrafiltration, after that we controlled P3ATs crystallization behavior through controlling the formation and morphology of nucleus, in that way we prepared the star-like and linear P3ATs nanofibers, respectively. Finally, in order to further deepen the understanding of microphase separation of block copolymers, we investigate the morphology evolution in acetone-annealed polystyrene-poly(methyl methacrylate)(PS-b-PMMA) diblock copolymer thin films.1. The research of polymer chains passing through the nanopores under certain conditions is greatly motivated by many real applications, for example, ultrafiltration. So far, most of studies in this field have focused on the system of biopolymers, a voltage-driven technique is usually applied to detect single-molecule translocation such as DNA, RNA, and proteins. On the other hand, a hydrodynamic force is used to drive the non-biopolymer chain through the nanopores. Firstly, we reported the structure transition of P3BDDT nanofibers with a larger dimensional scale in ultrafiltration. Under a strong flow field, a fiber-to-cluster transition can be observed when the nanofiber solution concentration is above a critical value. While the nanofibers remain one-dimensional morphology after ultrafiltration under a weak flow field or when the solution concentration is lower than the critical concentration. Simple theoretical calculation based on the scaling theory explains our experimental observations well.2. Polymer crystallization involves two steps:the nucleus formation and the crystal growth, therefore the crystallization rate should include the rate of the nucleus formation and the crystal growth, and the total crystallization rate is determined by the two steps. As to P3AT systems, a rather simple approach is used to control the dissolution way of P3DDT in toluene:the way of stirring makes some lager nanograins survive in the solutions, and these nanograins can serve as nucleus to induce the isotropic growth of P3DDT, as a result, the star-like nnaofibers formed; while the way of heating makes P3DDT completely dissolve in the solutions, the dissolved P3DDT just self-organized into linear nanofibers through anisotropic π-π interactions. Because of the different nucleation and different crystallization rates, the star-like nanofibers formed with a faster kinetic process than the linear nanofibers.3. The strength of microphase separation of block copolymer is primarily determined by the enthalpy and entropy reasons and usually scaled by xN, the product of the Flory-Huggins parameter (x) between the blocks and the total degree of polymerization (N) of the copolymer. Based on this, we followed the time development of morphologies and structures of symmetric and asymmetric PS-b-PMMA diblock copolymer thin films after annealing in acetone vapor by AFM. When film thickness was controlled according to their equilibrium bulk lamellar period, all the investigated thin films of symmetric PS-b-PMMA with different molecular weights can change from disordered pattern to nanoscale depressions and at last to striped structures. In the case of asymmetric PS-b-PMMA systems, the morphology evolutions depend on the block ratios in the block copolymers. The formation mechanisms of different film morphologies have been discussed in detail.In conclusion, we take different methods to realize well controlling the nanostructures based on the P3ATs systems, which is promising for their real applications. Through the investigation on the time development of morphologies and structures of PS-b-PMMA diblock copolymer thin films after annealing in acetone vapor, we have deepened our understanding of microphase separation of block copolymers.