Gradient multilayered films & confined crystallization of polymer nanolayers by forced assembly coextrusion

Chapter 1. A review of recent progress of micro- and nanolayer coextrusion for polymeric systems with good layer uniformity is described. Coextrusion through a series of layer multiplying die elements has enabled the production of film containing tens to thousands of layers with individual layer thicknesses from the micro- to the nanoscale. Improvements in layer uniformity are discussed through optimization of layer multiplying die design, selection of viscosity matched polymer systems, and incorporation of coextrusion surface layer capabilities. Coextrusion of layered polymer films with individual layer thicknesses in the nanoscale has resulted in the production of novel systems with improved properties. Nanolayered polymer films were utilized to develop an all-plastic laser, to fabricate gradient refractive index lenses, and to investigate gas barrier enhancement of crystalline polymer nanolayers confined to induce a high aspect ratio, in-plane, single-crystal-like lamellar structure.;Chapter 2. Forced assembly polymer coextrusion utilizes layer multiplication to produce films with tens or thousands of microns to nanometer thick layers. The development of novel uneven split layer multiplying dies has produced gradient multilayer films with at least a 10X difference between the thickest and thinnest layers. Coextrusion through a series of equal and uneven split multiplier dies allows for flexibility in the unique design of layer thickness distributions by: (1) altering the multiplier offset and (2) changing the sequence of a series of uneven split multiplying dies with different splitting ratios. This new technology has created highly reflective, multilayered photonic films with gradient layer thickness distributions exhibiting, as examples, a 600 nm wide reflection band and dual optical reflection bands within a single film. Also, gradient multilayers exhibit unique mechanical behavior. A layer thickness dependent craze to shear banding deformation mechanism was observed. In addition, gradient controlled buckling was observed across a single film due to foaming-induced layer delamination.;Chapter 3. Polycaprolactone multilayered films were produced exhibiting highly oriented confined crystallization as a result of reducing individual layer thicknesses from the micron to the nanoscale. WAXS/SAXS measurements of crystalline PCL layers exhibited a thickness dependent confinement mechanism from unoriented spherulites (micron layers) to high-aspect ratio, single or stacked lamellae (nanolayers) oriented parallel to the layer boundary. Highly oriented, single lamellae PCL layer films exhibited more than a two order of magnitude reduction in oxygen gas barrier as a result of the increased diffusion path length tortuosity around the high aspect ratio lamellae crystals with large estimated lateral dimensions up to 5 mum. The effect of the confining layer on PCL properties was examined by coextrusion of PCL against a series of amorphous materials, PS, PMMA, and PC with increasing chemical compatibility in micro- and nanolayered films. Increasing PCL and confining material interaction from a non-interacting PS to more interactive PMMA glassy confining substrate resulted in a decrease of confined crystalline layer orientation and oxygen permeability in thicker layers while broadening the temperature transition window from edge-on to in-plane lamellae in melt recrystallization experiments. An extreme case of nanolayering PCL against a highly interacting miscible confining layer, PC, resulted in no layer thickness effect on PCL crystalline phase orientation in coextruded multilayered films. Confinement against another crystalline polymer, such as polyamide or polypropylene, resulted in affinity for PCL to crystallize as edge-on lamella at ambient temperatures. A subsequent shifting of to higher melt recrystallization temperatures was necessary to induce the edge-on to flat-on orientation transition as a result of the crystalline confining layer substrate.