Nano Focus: Triblock polymers make square arrays with long-range order
- PDF / 547,882 Bytes
- 3 Pages / 585 x 783 pts Page_size
- 108 Downloads / 201 Views
trans
hν
their attachment to the CNT) both contribute to a large increase in ΔH. In the packed state, the researchers found that the position at which the azobenzenes were attached to the CNT (para, meta, or ortho with respect to the azobenzene nitrogen double bond) could be used to alter the value of ΔH. By also substituting hydroxyl groups for phenyl hydrogens, and thereby enabling the formation of hydrogen bonds between adjacent azobenzenes, the researchers found that they could simultaneously increase the
Three component triblock terpolymers extend the range of geometries which can be generated to include square arrays of dots, but thin films of triblock terpolymers suffer from the drawback that the patterns are usually broken up into small regions with only short-range order. Overcoming this problem, a team of researchers has now demonstrated that much longer range order can be imposed upon these triblock terpolymer structures through the selective application of surface topography and polymer brush coatings. As reported in the July 13 issue of
Nano Letters (DOI: 10.1021/nl201262f; p. 2849), C.A. Ross from the Massachusetts Institute of Technology, I. Manners from the University of Bristol, and their co-workers used a blend of polyisoprene-block-polystyrene-blockpolyferrocenylsilane (PI-b-PS-b-PFS) and 15% polystyrene homopolymer to create cylinders of PI and PFS arranged in alternating square arrays within a PS matrix. Coating sufficiently thin films (~32 nm) of this material onto silicon wafers covered with short polymer chains (a polymer brush) caused the cylinders to orient themselves perpen-
cis
Δ
excited state PES
relative stability of the cis and trans isomers and the energy barrier between them. In one example (Figure b), with a meta linkage and a total of six hydrogen bonds per azobenzene in the trans state, ΔH increased by 260% over gas-phase azobenzene and Ea increased by 20%, giving a half-life for the photoexcited state of the azobenzene-CNT fuel of greater than one year, a marked improvement over gas-phase azobenzene. Kolpak and Grossman said that their azobenzene-CNT fuel will be water soluble due to the hydroxyl groups on the solvent-accessible surfaces. Increased solubility can be achieved with substitution of additional polar groups at positions in contact with the solvent, which should not affect ΔH or Ea, so that formation of highly concentrated solutions is possible. The researchers estimated that the temperature of the heat released by their azobenzene-CNT fuel to be 620 K, but said that even higher temperatures could be achieved by lowering the heat capacity of the fuel. Kolpak and Grossman said, “In addition, our results suggest that one can generalize the concept of hybrid photoisomer-substrate nanostructures to other photoactive molecules, different substrates, and different linker chemistries, potentially leading to a wide range of novel degradation-resistant, highstability, and high-energy density solar thermal fuels.” Steven Trohalaki
b Azobenzene
a
Ea = 1.2eV hν
Gas-ph
Data Loading...
