Study on Self-Assembly Behaviors of Conjugated Rod-Coil Block Copolymers in Selective Solvents based on Dissipative Particle Dynamics

The development of rod-coil block copolymers in the past four decades is phenomenal to the field of nanoindustry. This type of block copolymers consists of two or more polymeric or oligomeric blocks which differ significantly from each other in physical properties, and are linked by chemical bonding. Rod-coils inherit the block copolymer characteristics of the highly-developed coil-coils; microphase separation occurs due to the incompatibility between the blocks and the spatial limitation from the chemical linkage. Moreover, the rigid block exhibits orientational ordering owing to the excluded volume effect and the mesogenic characters. The rod effect as well as molecular amphiphilic nature drives the block copolymers to self-assemble into a variety of intriguing morphologies with a number of potential nanoscaled applications.
Due to the prosperity of the optoelectronic industry in recent years, conjugated rod-coil block copolymers gain increasingly attention on account of the distinct optoelectronic properties. The applications of conjugated rod-coils can be seen in Lasers, PLED full color displays, energy savers and solar panels, etc. Theoretical works and experiments show that the optoelectronic properties of conjugated rod-coils are highly correlated to the molecular structures, supramolecular morphologies and detailed arrangements of rods. UV-visible and Photoluminescence (PL) spectroscopies are commonly adopted to examine the photophysical properties for conjugated rod-coils. Different types of rod-coil aggregation result in different spectroscopic results. Two major types of aggregates are usually used to account for the distinct spectral phenomena. If the absorption wavelength of the aggregates has a hypsochromic shift (blue-shift) compared to that of the dispersed phase, it is usually attributed to the formation of H-aggregates. On the other hand, if the absorption wavelength has a bathochromic shift (red-shift), it is attributed to the formation of J-aggregates. Both models originate from the well-studied dye molecules. Rods are parallel to each other in both types of aggregates and with a relatively smaller slippage extent for H-aggregates while the slippage extent is larger for J-aggregates.
The Dissipative Particle Dynamics was employed to study the self-assembly behavior of conjugated rod-coils in dilute selective solution, where the rods are solvophobic and the coils are solvophilic. The effects of the molecular structures (rod-coil, coil-rod-coil, rod-coil-rod, star, graft, and other 7 more structures), rod/coil length, and π-π strength on the final self-assembled results of the rod-coils are systematically investigated. It was found that the molecular structures of the rod-coils can be classified into two categories, the RC-type and the CRC-type structures, according to the molecular arrangement of rods in the equilibrium state. RC-type structures can be characterized by rods on both ends of the copolymer, or rod on the one end and coil on the other. CRC-type structures are characterized by coils at outer rims. RC-type rod-coils prefer to take the smectic-like patterns, which correspond to H-aggregates. CRC-type rod-coils tend to take the preferred-nematic patterns (nematic packing with preferred slippage extent), which correspond to J-aggregates. To justify the blue- and red-shift spectral phenomena caused by different types of aggregation, a routine based on quantum mechanics (VAMP) in the Material Studio was applied to estimate the absorption spectrum for several arrangements of conjugated rods, and the results were as what we have expected; blue-shift for the smectic–like and red-shift for the preferred-nematic. Nematic-like and isotropic patterns were also observed in the investigation, where the photophysical property of the former was unclear, and the latter should have no particular features due to the disordered arrangement of rods. In addition to the molecular structures, rod and coil length have certain degrees of effect on the orientational order as well as the positional order for the final organization of rods. On the other hand, π-π strength plays the crucial role in the above mentioned two properties. As to the self-assembled morphologies, many interesting supramolecular structures appear, such as spherical micelles, vesicles, cylinders, helices, honeycombs, and so forth. The classifications on the self-assembled morphologies are shown in the end of each chapter according to the molecular structures.
Results from our investigation are consistent with most theoretical predictions as well as experiment findings. This work can provide the insights from the molecular level and should be a valuable reference for future applications of conjugated rod-coils.