Stephen F. Hahn

Catalytic Hydrogenation of Polystyrene

The use of ultra wide pore silica as a catalyst for the hydrogenation of polystyrene to give poly(cyclohexylethylene) (PCHE) has facilitated the development of a dramatically improved process with much milder conditions. The large pores allow rapid diffusion into and out of the catalyst, so that the interior surfaces are also available for reaction. The mild conditions give rise to high-quality PCHE, whose structure is shown in the Figure.

Extrusion of triblock and pentablock copolymers: Evolution of bulk and surface morphology

We report studies on the extrusion of nearly monodisperse triblock and pentablock copolymers composed of poly(cyclohexylethylene) (C) and poly(ethylene) (E). Lamellae forming CEC triblock and CECEC pentablock copolymers were extruded through a slit die using a capillary rheometer at various processing speeds and temperatures below the order-disorder transition temperatures. Extrudate microstructure was characterized by small angle x-ray scattering and the surface characteristics were investigated by optical microscopy and profilometery. At low extrusion rates, the triblock and pentablock copolymers display different bulk orientation behavior, leading to relatively smooth surfaces in both cases. Above a characteristic wall shear stress, associated with a slope change in the flow curve, both polymers exhibit perpendicular lamellae alignment but dramatically different surface properties. The CEC material contains relatively low amplitude (∼1–3 μm) random roughness while the CECEC surface is highly textured, ...

Birefringent properties of cyclic block copolymers and low-retardation-film development

— Cyclic block copolymers (CBCs) are a new class of optical polymers made by fully hydrogenating block copolymers of styrene and conjugated diene. This class of materials has excellent optical transparency, photostability, and good thermal resistance. By changing the copolymer composition and the resulting block-copolymer morphology, a unique set of birefringence properties can be achieved. The focus of this work was to study various sources of birefringence in block copolymers using a series of model CBC materials. One particularly interesting finding relates to the development of an ultra-low-phase-retardation CBC film. Unlike the conventional approach of using an additive or blend, a CBC film prepared by melt extrusion can readily achieve near-zero retardation in both the film plane and thickness direction. This nearly isotropic CBC film is useful as a polarizer protection film in flat-panel displays. When used as the inner protective layer of a polarizer, CBC film helps to reduce the color shift of IPS-LCDs at oblique angles and offer a wider viewing angle.

Estimation of the Molecular Weight between Crosslinks of Crosslinked Semicrystalline Polyolefins

The molecular weight between crosslinks (MXL) of crosslinked semicrystalline polyolefins, polyethylene in particular, has been estimated from uniaxial tensile tests in the melt state with the Sentmanat Extensional Rheometer (SER). Applying the Mooney–Rivlin equation to these stress‐strain data resulted in the determination of an apparent molecular weight between crosslinks. This fast and convenient technique correlates well with the conventional approach where MXL is obtained via the plateau modulus from shear viscosity.