Fundamental and applied research enabled by polymer nanolayer coextrusion technology
Polypropylene (PP) and polystyrene (PS) nanolayer films were prepared. The crystal structure of extremely thin PP layers confined between PS layers was studied. Changes in structure were observed as the PP layer thickness decreased to the nanoscale. A dispersion of isotactic polypropylene (PP) particles was produced by interfacial-driven breakup of PP nanolayers. Particle size analysis indicated that breakup of PP microlayers produced a bimodal particle size distribution. A population of submicron particles formed due to the Rayleigh instability, and a second population of large particles formed by relaxation. Breakup of 12 nm layers resulted in primarily submicron particles, which crystallized into smectic form by homogeneous nucleation at 40 °C. The fraction of PP as submicron particles dropped dramatically as the layer thickness increased to 40 nm. Fractionated crystallization gave rise to four crystallization exotherms at higher temperatures, which represented fractionated crystallization of the large micron-sized particles in the PP a-form. The effect of a particulate nucleating agent on crystallization of polypropylene (PP) in particles was found to be vastly different the effect of a sorbitol nucleating agent. A new class of hierarchically structured polymer optical materials was demonstrated that possess an internal refractive index gradient. The structure of the polymer material is analogous to the layered structure found in biological optical materials. Any practical refractive index distribution can be achieved with this flexible technology within the refractive index range of available coextrudable optical materials. An important application demonstrated for these materials is the construction of a bio-inspired GRIN lens. Lenses with gradients in both the radial and axial directions were fabricated. Materials with gradients with On ~ 0.17 have been made and larger index ranges are possible. Experimentally, these lenses were shown to have the parabolic radial and nearly linear axial refractive index distribution expected from the design, similar to a section of the spherical parabolic distribution found in the lens of an octopus. Comparing to a conventional spherical lens, GRIN lens exhibited improved on-axis spherical aberrations. In the end, a carefully designed lens was shown to have an on-axis focal ability exceeding any off-the-shelf spherical lenses.