1. Crystallization in Microphase-Separated Block Copolymers
We have been studying the crystallization behavior of chain molecules confined in a variety of microdomain and vesicle geometries using block copolymer blends as nanoscaled templates. The effect of spatial confinement on the nucleation mechanism and the correlation between microdomain/vesicle morphology and crystallization kinetics are of particular interests. Crystallization-induced confinement disruption and the novel morphology thus generated are also being explored. We have also been investigating the co-crystallization behavior of the crystalline blocks with dissimilar lengths in the blends of two crystalline-amorphous block copolymers. In contrast to the kinetically trapped solid solution formed in the corresponding homopolymer blends, the co-crystallization behavior in the binary block copolymer blends may be a thermodynamically driven process.
2. New Packing Symmetries of the Classical Microdomains of Block Copolymers in the Bulk State
Molecular self-assembly in diblock copolymer and its blend with the corresponding homopolymer can generate a series of long-range ordered microdomains, including 1-D stacked lamellae, perforated lamellae, gyroid, hexagonally packed cylinder, and BCC-packed sphere. Recently, we have discovered two new morphologies in diblock copolymer systems, namely, FCC-packed spheres in a diblock copolymer/homopolymer blend and Square-packed cylinders in a supramolecular comb-coil diblock copolymer. Systematic studies are being conducted to reveal the driving forces and mechanisms associated with the formation of these structures.
3. Molecular Aggregation of Electroluminescent Conjugated Polymers in Solution State
Since the first successful fabrication of a polymer light emitting diode (PLED) using poly(phenylene vinylene) (PPV) as the active material, there has been a widespread research interest in conjugated electroluminescent (EL) polymers. The dilute solutions of EL polymers exhibit a unique feature in the presence of some sort of segmental aggregates with mysterious structure and origin. The presence of these aggregates in even a trace amount can have drastic impact on the photophysics of the polymer. We have been studying the aggregation behavior of PPV- and PFO-based EL polymers in the solution state using SANS and SAXS. The effects of chemical architecture, solvent quality, concentration, long-term aging and temperatures on the aggregation behavior are being investigated. Our present results suggest that the aggregates in the dilute solution of MEH-PPV are nanoscale nematic domains formed by the association of the hairy-rod segments with relatively large aspect ratio. This kind of nematic domain is distinguished from the macroscopic nematic phase formed in the classical lyotropic liquid crystalline polymer solutions in the sense that they are nanoscale microphase-separated domains formed at the solution concentration far below the threshold lyotropic concentration prescribed by the mean-field theory.
4. Higher-Order Self-Assembly of Biomacromolecular Complexes
We have been studying the self-assembly behavior of the electrostatic complexes of polyanionic DNA with cationic lipids/surfactants, dendrimers, and block copolymers. These complexes are usually of interests to the biomedical community in that they may serve as effective non-viral gene delivery systems for gene therapy. Our interests in these complex systems are however oriented toward the materials science aspect. We attempt to use these complexes as the templates for constructing spatially ordered opto-electronic nanowires because the complexations may result in 1-D to 3-D ordering of the DNA chains. This project is working closley with the researchers of the Materials Research Laboratory at ITRI, where we have been centering on a series of fundamental studies including the basic self-assembled structures of the complexes, metallization of DNA and computer simulation of the self-assembly behavior.