P. van de Witte

Phase behavior of polylactides in solvent-nonsolvent mixtures

Isothermal phase diagrams for the semicrystalline poly-L-lactide (PLLA) and the amorphous poly-DL-lactide (PDLLA) in combination with several solvent-nonsolvent combinations (dioxane/water, dioxane/methanol, chloroform/methanol, and NMP/water) have been determined. The locations of the liquid-liquid miscibility gap, the solid-liquid miscibility gap and the vitrification boundary in the isothermal phase diagrams at 25°C were identified. The liquid-liquid miscibility gap for the systems with PLLA was located in the same composition range as the corresponding systems with PDLLA. For the systems containing PLLA solid-liquid demixing was thermodynamically preferred over liquid-liquid demixing. Attempts were made to correlate the experimental findings with predictions on the basis of the Flory-Huggins theory for ternary solutions using interaction parameters derived from independent experiments. Qualitative agreement was found between the theoretical predictions and the experimentally obtained liquid-liquid miscibility gap. No good agreement was found for the solid-liquid miscibility gap.

A morphological study of membranes obtained from the systems polylactide-dioxane-methanol, polylactide-dioxane-water and polylactide-N-methyl pyrrolidone-water

The influence of liquid-liquid demixing, solid-liquid demixing, and vitrification on the membrane morphologies obtained from several polylactide-solvent-nonsolvent systems has been investigated. The polymers investigated were the semicrystalline poly-L-lactide (PLLA) and the amorphous poly-DL-lactide (PDLLA). The solvent-nonsolvent systems used were dioxane-water, N-methyl pyrrolidone-water and dioxane-methanol. For each of these systems it was attempted to relate the membrane morphology to the ternary phase diagram at 25°C. It was demonstrated that for the amorphous poly-DL-lactide the intersection of a glass transition and a liquid-liquid miscibility gap in the phase diagram was a prerequisite for the formation of stable membrane structures. For the semicrystalline PLLA a wide variety of morphologies could be obtained ranging from cellular to spherulitical structures. For membrane-forming combinations that show delayed demixing, trends expected on the basis of phase diagrams were in reasonable agreement with the observed membrane morphologies. Only for the rapidly precipitating system PLLA-N-methyl pyrrolidone-water were structures due to liquid-liquid demixing obtained when structures due to solid-liquid demixing were expected. Probably, rapid precipitation conditions promote solid-liquid demixing over liquid-liquid demixing, because the activation energy necessary for liquid-liquid demixing is lower than that for crystallization.

Modification of the twist angle in chiral nematic polymer films by photoisomerization of the chiral dopant

A method for the production of polarization sensitive recordings in liquid crystalline polymers is presented. The system is based on local modification of the twist angle of chiral nematic polymer films. The twist angle of the polymer film is varied by modifying the chemical structure of the chiral dopant. Here a photoisomerizable menthone derivative is used that has a fivefold difference in helical twisting power between the E and the Z isomer. The twist angle of the film can be varied between −90° and 0° by introducing a nonphotosensitive chiral dopant with opposite twisting sense. Complex pictures with gray scales can be recorded in the films with black and white contrasts higher than 20.

Colourful Photo-Curable Coatings for Application in the Electro-Optical Industry

The formation of mirrors reflecting a single colour or reflecting the whole visible spectrum by application of a liquid crystalline coating followed by UV polymerisation (photo-curing) is described. Also, the formation of patterned coatings obtained by a sequence of UV exposure steps is discussed. Such coatings play an important role in the improvement of the performance of liquid crystal displays (LCDs).

Glass Forming Nematic Twin Molecules Synthesis and Polymerization Kinetics

Abstract In this paper we describe the synthesis of a nematic twin molecule with a lateral methyl substituent in the mesogenic group. This compound exhibits a broad nematic mesophase between -18°C and 135°C and can be supercooled into a liquid crystalline glass. The LC-phase was permanently fixed into a network by in-situ photopolymerization. The broad mesophase of the twin molecule allows the investigation of the photopolymerization behaviour by photo-DSC-measurements in a wide tem-perature range. The Tg of the polymer network was determined by TMA-measurements.

Colorful films for application in the electro-optical industry made by photopolymerization

The formation of mirrors reflecting a single colour or reflecting the whole visible spectrum by application of a liquid crystalline layer followed by UV polymerisation (photo-polymerisation) is described. Also, the formation of patterned films obtained by a sequence of UV exposure steps is discussed. Such films play an important role in the improvement of the performance of liquid crystal displays.