Phase Behavior of Mixed Polymer Brushes Grown from Ultrathin Coatings.

Experimental validation of the predicted melt phase behavior of A/B mixed brush on planar substrate is presented using poly(methyl methacrylate) (A)/ polystyrene (B) (PMMA/PS) with equal number of A/B chains as an example. Well-defined mixed A/B brushes are synthesized using a single component inimer coating to achieve high grafting density (0.9 chains/nm2), uniformity of grafting sites, and predictable chain length. The inimer coating is a copolymer of nitroxide-mediated polymerization (NMP) inimer, atom transfer radical polymerization (ATRP) inimer, styrene, and glycidyl methacrylate (GMA). Cross-linking of the film provides the required stability to probe the melt morphology. Our studies show that even with equal grafting density of the A and B the morphology can be modulated by varying the length of B chains while keeping that of A fixed. We show the transition of self-assembled structures from disorder to cylinder to ripple phase at sub-30 nm length scale on a planar surface by thermal annealing of mixed brushes. These results are supported by a phase diagram established through Monte Carlo simulation using a coarse-grained particle-based model.

Photoisomerization Dynamics in a Densely Packed Optically Transformable Azobenzene Monolayer.

Molecular monolayers that can be reconfigured through the use of external stimuli promise to enable the creation of interfaces with precisely selected dynamically adjustable physical and electronic properties with potential impact ranging from electronics to energy storage. Azobenzene-containing molecular monolayers have multiple stable molecular conformations but face a challenging nanoscale problem associated with understanding the basic mechanisms of reconfiguration. Time-resolved X-ray reflectivity studies show that the reconfiguration of a densely packed rhenium-azobenzene monolayer occurs in a period of many seconds. The degree of reconfiguration from trans to cis forms depends on the integrated UV fluence and has kinetics that are consistent with a mechanism in which the transformation occurs through the nucleation and growth of nanoscale two-dimensional regions of the cis isomer.

Optically Reconfigurable Monolayer of Azobenzene Donor Molecules on Oxide Surfaces.

The structural configuration of molecules assembled at organic-inorganic interfaces within electronic materials strongly influences the functional electronic and vibrational properties relevant to applications ranging from energy storage to photovoltaics. Controlling and characterizing the structural state of an interface and its evolution under external stimuli is crucial both for the fundamental understanding of the factors influenced by molecular structure and for the development of methods for material synthesis. It has been challenging to create complete molecular monolayers that exhibit external reversible control of the structure and electronic configuration. We report a monolayer/inorganic interface consisting of an organic monolayer assembled on an oxide surface, exhibiting structural and electronic reconfiguration under ultraviolet illumination. The molecular monolayer is linked to the surface through a carboxylate link, with the backbone bearing an azobenzene functional group and the head group consisting of a rhenium-bipyridine group. Optical spectroscopy, X-ray photoelectron spectroscopy, atomic force microscopy, and X-ray reflectivity show that closely packed monolayers are formed from these molecules via the Langmuir-Blodgett technique. Reversible photoisomerization is observed in solution and in monolayers assembled on Si and quartz substrates. The reconfiguration of these monolayers provides additional means to control excitation and charge transfer processes that are important in applications in catalysis, molecular electronics, and solar energy conversion.