Periodic and quasiperiodic polymeric structures at the sub-micron and nano scale offer many interesting scientific opportunities for fundamental studies of material behavior at these length scales. This proposal describes a unified approach for the experimental investigation and theoretical modeling of periodic polymer materials which can simultaneously act as hypersonic phononic crystals and visible wavelength photonic crystals. Structures will be fabricated using block polymer self-assembly and interference and electron beam lithography. We plan to explore novel photonic bandgap structures fabricated via double inversion techniques as well as through direct fabrication with unique organic-inorganic hybrid monomers. Brillouin light scattering is an ideally suited method for the direct experimental measurement of phonon dispersion relations while transmission and reflection measurements of incident light will be employed to assess the optical dispersion relations. FEM modeling provides the ability to model elastic wave propagation in a wide range of bicomponent periodic structures and numerical techniques are well established for modeling light wave propagation. The combination of these tools and approaches constitutes a complete methodology for fabrication and characterization, measurement and modeling of this exciting new class of materials.
NON-TECHNICAL SUMMARY Dual band gap materials for sound and light (?Deaf and Blind Materials?) are a step towards creation of material systems with unusual properties. Success in the present endeavor will provide a pathway forward for construction of multicomponent, hierarchically structured materials designed to provide a set of key properties. This work will help us understand the basic nature of the propagation of light and sound through nanostructured polymeric materials. This work promises to enhance the foundations for experimental studies of phononic, photonic and importantly dual band gap photonic-phononic crystals and open new pathways towards achieving new material properties (e.g.tailored thermal conductivity, significantly enhanced acousto-optical coupling) that can have important technological applications since the properties of the periodically structured material are no longer just due to the inherent material properties, but can be dominated by the role of wave interference within the structure to give novel and indeed revolutionary properties (e.g. localization of sound and light to specific places in the material) that are simply unattainable otherwise. Our efforts will develop both experimental techniques and skilled people to use them at the cutting edge of what is really the emerging new field of ?periodic materials.? Moreover, working with sound and light waves is a tremendous advantage for inspiring young minds to the wonders of science. This is because light waves and sound waves are ubiquitous ? we are essentially immersed in them every day and are continually receiving and sending such waves. The non-intuitive interactions of these waves with periodic structures elicits genuine awe. We plan to provide block copolymer films on substrates that can be readily manipulated by ?kitchen chemistry? using various stimuli such as vinegar and salt solutions. Motivated by our interests to introduce students to the interesting ways that waves interact with periodically structured materials at the micro- and nano- scale to create new properties as well as to highlite/motivate the study of certain topics in freshman year math, including Fourier series, we have just completed a monograph, ?Periodic Materials and Interference Lithography: photonics, phononics and mechanics,? to be published in summer, 2008 by Wiley-VCH.
Structures that are periodic abound in modern life - a 2 dimensional example is the pattern of tiles on bathroom floors. The key is the unit cell (or single tile) that is simply repeated over and over again creating a periodic pattern. Waves, like sound waves and light waves are also periodic and when the wavelength of the wave and the spacing of the unit cell are about the same size, something special happens: the wave constructively or destructively interferes with itself, resulting in the wave either being reflected or transmitted by the structure. For waves that have wavelengths greater than or smaller than the unit cell size, the waves are transmitted, while waves right near the structural period are reflected. The important work we did in this NSF grant on "Periodic Polymer Materials: Deaf and Blind" was to fabricate various novel two and three dimensional polymeric structures that have dimensions that are approximately the same as visible light (resulting in so called structural color due to reflection of visible wavelengths) or sound waves that are in the hypersonic regime - waves that are very, very small (about a million times times smaller!) relative to the sound waves we listen to (which are pretty big - wavelengths are about a foot or so in size !). By making such periodic polymers we can reflect sound and light such that someone sitting inside our material would neither hear the sound nor see the light (hence the name of the proposal: Deaf and Blind) ! The scientific terms for such structures are phononic crytals and photonic crystals. Phonons are sound waves and photons are light waves. Designing and then making these structures and then sending the waves at them and recording the reflectivity allows testing of the theory and of our fabrication skills. We made our structures two ways. One was by using interference lithography with a polymer photoresist. The photoresist takes on the intensity pattern of the light as it interferes with itself inside the material. So we are using light to create structures (photonic crystals) to interact with light ! The second way was to use self assembly - that is we just put our polymers (but special ones called block copolymers) in a solvent, poured some of the solution onto a glass slide and let the molecules spontaneously arrange themselves (self assemble) as the solvent evaporated. Because we choose diblock A/B molecules with two equal lenght blocks, the structure formed was an alternating set of A and B layers. This periodic structure then acted like a mirror for wavelengths of incident light that were on the same size as the repeat period of the layers. Now we also were able to change the period by exposing the polymer to solvents or by changing the temperature or by adding some salt to a water solution or by mechanically squeezing the layers. This ability to tune (tailor the period) meant we had a material that could sense the presence of various kinds of chemicals and temperature and applied force! By choosing the A and B component polymer blocks and a stimulus (like salt content) we could manipulate the reflected color completely across the visible spectrum (from deep blue to orange-red). We also wanted to selectively reflect sound waves. In air or other fluids like water, there is only one kind of sound wave - acoustic waves. But in solids, there are three kinds of sound waves (called elastic waves). One is longitudinal just like in fluids and two are transverse waves. The wavelengths are really small - nanometers, so we needed to make very small structures. We also wanted to be able to tune our phononic structures just like we managed to do for photonic structures. In this case we made rubbery nanostructures and then mechanically stretched them to see changes in the phonons that could propagate inside our materials. In summary, in this project my research group was able to design and fabricate and then test and compare with theoretical predictions, the fascinating behavior of waves encountering periodic structures.
