The effect of noncrystallizable units on the crystallization and melting was studied for two octene copolymers of syndiotactic polypropylene, in an extension of previous investigations on a homopolymer sample with 3% meso diads. Using time and temperature dependent SAXS experiments and DSC, we determined the dependencies of the crystal thickness, the rate of crystallization, and the melting point on the chosen crystallization temperature. With an increase in the content of noncrystallizable units (octene units or meso diads; both show equal effects), we observed, as expected, a shift of the melting points to lower temperatures and similar shifts of the growth rate versus temperature curves, but surprisingly, no effect at all on the crystal thickness. The thicknesses of all three samples show a common temperature dependence, being inversely proportional to the supercooling below the equilibrium melting point of perfect syndiotactic polypropylene. The latter is located at 196 °C, as determined by an extrapolation based on measured melting points. Data demonstrate that the popular Hoffman-Weeks plot when applied to random copolymers does not yield the respective equilibrium melting points; it can only be used for perfect homopolymers. Crystal thicknesses and growth rates are, according to the observations, independent properties. The thicknesses are those of a specific native crystal form with high surface free energy. DSC experiments indicate that all crystals first form this native state and then become stabilized by relaxation processes that decrease the surface energy. These stabilization processes, which produce the difference between the temperatures of crystallization and melting, leave the crystallite thickness unchanged.
We have observed enhanced fluorescence and laser emission due to a photonic defect mode in a dye doped cholesteric polymer network. The defect is caused by a phase jump of the cholesteric helix at the interface of two stacked layers of a cholesteric polymer film. Fluorescence spectra show an additional resonant mode inside the photonic stop band. Pulsed excitation gives rise to laser emission of the defect mode, with an exceptionally low lasing threshold. The defect mode emission has a circular polarization whose sense of rotation is opposite to that of the cholesteric helix.
A crosslinked cholesteric network doped with a fluorescent dye exhibits lasing efficiencies that are more than one order of magnitude higher than those of a low molar mass cholesteric liquid crystal (CLC) with similar optical parameters. The emission from the samples has been found to be stable, whereby the lasing threshold is much lower than that of the CLC compound. Films of the crosslinked sample can easily be peeled off their glass substrates and can be microstructured or coated to change the polarization of the emitted laser light.
As shown by time-and temperature dependent SAXS experiments, crystals of s-PP do not change their thickness during isothermal crystallization and a subsequent heating to melting. This allows an accurate determination of the relations between crystallization temperature, crystallite thickness, rate of crystallization, and melting points. There are five main results obtained in a comprehensive SAXS and DSC investigation: (i) Crystals have greatly varying stabilities, in spite of their uniform thickness; the first crystals melt close to the crystallization temperature. (ii) Melting points are affected by the distance to neighboring crystals. (iii) Crystals perfect during heating and annealing. (iv) Recrystallization after melting, as observed for low enough heating rates, starts with crystal growth rates that are at least 2 orders of magnitude higher than for a primary crystallization, and then slows down progressively, being accompanied by an increase in crystal thickness. (v) The dynamic signals observed in a MDSC run are indicative of a smooth change between crystallization and melting at the growth front. Data evaluation yields the average time required for a melting of individual crystallites. A fourstate scheme allows a description of the behavior. It is based on the following assumptions: (i) Primary crystallization from the melt produces in a first step imperfect "native" crystals, which are subsequently stabilized by structural relaxation processes. These affect both the interior and the surface region. Crystallization proceeds under a small driving force, near to the equilibrium between melt and native crystals. (ii) The amount of structural relaxation is nonuniform thus producing crystals of different stability. (iii) With increasing crystallization temperature, i.e., decreasing growth rate, native crystals continuously approach the equilibrium state. (iv) The main part of fusion takes place near the equilibrium between the relaxed crystals and the disentangled melt.
We discuss the properties of photonic defect modes in cholesteric liquid crystals. Twist defects, isotropic defect layers, and combinations of both are considered. After deriving the reflection and transmission properties of the defects, we study the effect of a finite sample thickness on the defect mode's amplitude and on the required polarization of incident light to excite the defect mode.
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