Takahiro Seki Talks About Material Nanotechnologies
Emerging Research Front Commentary, August 2010
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Article: Optical alignment and patterning of nanoscale microdomains in a block copolymer thin film
Authors: Morikawa, Y;Nagano, S;Watanabe, K;Kamata, K;Iyoda,
T;Seki, T |
Takahiro Seki talks with ScienceWatch.com and answers a few questions about this month's Emerging Research Front paper in the field of Materials Science.
Why do you think your paper is highly
cited?
An explosive number of investigations have recently been reported for thin films of block copolymers, which are expected to play important roles in material nanotechnologies. In light of technological applications, the development of alignment techniques of microphase separation (MPS) structures over large areas and patterning at desired scales are of particular importance.
The block copolymers with controlled molecular mass provide regular patterns of mesoscale size (typically 10 - 100 nm level) by self-assembly. Such mesoscopic sizes correspond to the size of polymer chains. The size features are fascinating since they are smaller than those typically obtained by photolithographic methods. So it bears the role of next-generation fabrications.
Without any guidance, the MPS structures are randomly directed. So, development of alignment methods of the MPS structure of block copolymer films over wide areas and patterning is of highly importance. Particularly, application of light irradiation technique is highly desirable in light of its practical versatility.
We (the present paper) and a group in Tokyo Institute of Technology (Yu H, et al., J. Am. Chem. Soc. 128: 11010, 2006) have first demonstrated the photoalignment for the MPS structures of block copolymer films. Besides, this work further demonstrates the additional function of surface relief formation of micrometer scales by simply irradiating interference patterned light.
We believe that the idea of this research project is fully new and attractive for a wide range of researchers engaged in block copolymers, polymer thin films, photochemistry, patterning, and lithographic technology, etc.
Does it describe a new discovery, methodology, or
synthesis of knowledge?
This paper describes a new method to align the MPS of block copolymers processed by light. To align the MPS structure, flow or shear effects, and magnetic and electric fields have been utilized, but this paper describes the first example of photoinduced alignment.
The interesting issue in this work is that the cylinder MPS structure of ca. 20 nm diameter is aligned normal or parallel to the supporting substrate of the film, depending on the film thickness undulations of the relief, and in the parallel regions, the orientation is governed by the polarization mode of the irradiated light.
Would you summarize the significance of your paper
in layman's terms?
Alignment technique of liquid crystals over large areas and patterning is the key to production of liquid crystal displays. In a similar way, once the alignment of MPS structure is established, significant applications of block copolymers to fabrication of optical and electrical elements, sensors, separation and catalyst technologies, etc., are expected. When a block copolymer showing the liquid crystalline nature is used, the photoalignment technique may be applied in the same way. This is the motivation of this work.
" Since the process requires no high-cost and high-energy procedures (the key process is the self-assembly of polymer material), we further expect that the block copolymer thin film technology can also contribute a great deal to environmental problems in industrial productions...."
Photoalignment of photochromic molecules (mostly azobenzene) in polymer films is now a well-known phenomenon. The principle of photoalignment is as follows: when rod-like photoexcitable molecules in polymer films are irradiated with linearly polarized light, the molecules are reoriented to a "non-excitable" direction, namely, orthogonal to the electric vector of the linearly polarized light.
In liquid crystalline systems, the molecules behave in collective fashions (cooperative manners), so the photoalignment becomes more and more effective.
However, there have been, up to now, no attempts to align the larger mesoscale structures of MPS of block copolymers driven by the photoinduced alignment of photochromic molecules. This was a highly challenging issue.
Interestingly, as the results, orientations of three types of hierarchical structures are controlled at once only by a simple light irradiation process followed by annealing. Orientation of photoresponsive azobenzene molecules (sub-nanoscale), mesoscopic MPS structure (mesoscopic scale 10 - 100 nm), and, finally, relief structure (micrometer scale) are controlled simultaneously in the film. We think that this is a particular example.
How did you become involved in this research, and
how would you describe the particular challenges, setbacks, and
successes that you've encountered along the way?
I have been involved in photoresponsive films since 1987 (AIST Research Center, Tsukuba at that time). The surface photoalignment of liquid crystals, which was pioneered by Prof. Kunihiro Ichimura (Group leader of AIST research center at Tsukuba, later Professor of Tokyo Institute of Technology), was the main research subject.
Also, the photoinduced mass transfer process leading to a surface relief formation since 2000 became another main project in our group after I moved to Tokyo Institute of Technology. The projects are still continuing after moving to Nagoya University. The present investigation is an extension of these systematic studies.
I had a chance to co-work with Prof. Tomokazu Iyoda (Tokyo Institute of Technology) who had newly developed a fascinating liquid crystalline azobenzene-containing block copolymers combined with poly(ethylene oxide). My background and the encounter of this excellent block copolymer inspired us to attempt the photoalignment and surface relief formation of this block copolymer film.
Of course, the attempt was not a simple journey; there were a lot of experimental variables to be adjusted. Our younger leader staff, Dr. Shusaku Nagano and PhD student, Yuichi Morikawa, made great efforts to optimize the conditions. This work is the embodiment of the ideals, efforts, and travails of all these members.
Where do you see your research leading in the
future?
Making use of the difference in chemical nature of the block components, one can selectively deposit functional substances and materials to one block domains. Therefore, the MPS patterns of block copolymer films may be utilized as the 2D scaffold or template for depositing other functional nanomaterials such as optically and electronically active polymers, metals, inorganic materials, biomolecules etc.
The work does not remain only in a limited field of block copolymer chemistry but will be expanded to the processing tool of many sorts of functional materials including organic, inorganic, metal, and biological systems. Therefore, establishing the on-demand photoalignment and patterning technique is of great significance.
Do you foresee any social or political
implications for your research?
It is of no doubt that, as exemplified by photolithography processes, the light-assisted method is promising to be involved in the processes to produce various types of elements. With regard to photoalignment of liquid crystals, SHARP Corporation very recently introduced a photoalignment process in the production of large-area liquid-crystal TV displays after 20 years of the first discovery of surface photoalignment by Prof. Ichimura.
With regard to the photoalignment and patterning of MPS structure of block
copolymer films, the projects are still at the starting point. We expect
that the knowledge obtained at this stage will find novel and wide
industrial applications in the future. Since the process requires no
high-cost and high-energy procedures (the key process is the self-assembly
of polymer material), we further expect that the block copolymer thin film
technology can also contribute a great deal to environmental problems in
industrial productions.
Takahiro Seki, Professor. Dr.
Department of Molecular Design and Engineering
Graduate School of
Engineering
Nagoya University
Nagoya, Japan
KEYWORDS: OPTICAL ALIGNMENT, PATTERNING, NANOSCALE MICRODOMAINS, BLOCK COPOLYMER THIN FILM, AZO POLYMER FILMS, LIQUID CRYSTAL, SURFACE, AZOBENZENE, STORAGE, MOIETIES, PEPTIDE, LIGHT, ORDER.