Structure formation of liquid-crystalline isotropic AB block copolymers in nematic solvents

AB block copolymers with an isotropic and a nematic block are synthesized by living anionic copolymerization of isotropic and mesogenic methacrylates. The complete phase diagrams of the corresponding homopolymers with the nematic solvent have been established. In dilute solution (0.2 to 5% by weight) in a low-molar-mass nematic solvent, the copolymers exhibit two different types of structure formation. Polymers with high isotropic block fraction form irreversible networks as a results of phase separation followed by vitrification of the polymer-rich phase. Polymers with high nematic block fraction on the other hand form thermoreversible threadlike structures perfectly ordered parallel to the director of the nematic solvent. These structures cannot be explained by a macroscopic phase separation process but more likely by a formation of micellar-like aggregates.     

Quasielastic light scattering as a method for determining the distribution of relaxation times in a polymer system

Amongst other techniques, dynamic light scattering may be used to obtain molar mass distributions. The first step in this process consists in the Laplace inversion of the time correlation function that was measured by dynamic light scattering. This inversion gives a distribution of diffusion coefficients. In order to convert this distribution into the corresponding molar mass distribution, a relationship between diffusion coefficient and molar mass of monodisperse fractions has to be known. Such a relationship can be derived for linear and star-branched macromolecules from measurements of polydisperse systems, since the polydispersity of the distributions does not change with the molar mass. The problem is more involved with randomly branched materials, since in these cases the polydispersity increases strongly as the point of gelation is approached. A procedure is suggested for deriving the diffusion-molar mass dependence of monodisperse samples from polydisperse systems. After an outline of this background the method is applied to the three selected systems (i) radically polymerized linear PMMA, (ii) a star-branched microgel where monodisperse arms are attached to a microgel center and (iii) a randomly branched poly(dicyanate) sample based on bisphenol A. The results are compared with the combined column chromatography SEC/LALLS/VISC. Good agreement was found up to molar masses of about 10 millions g/mol, but systematic deviations occured in the high molar mass region. These differences result from the limitations of size permeation chromatography. Finally it is shown that the size distribution can be determind by this method, even for associates.     

Study on the Isomorphous Substitution of Silicon by Indium in the MFI Framework

In-ZSM-5 zeolites were prepared by hydrothermal synthesis and characterized by XRD, MAS NMR, IR, thermoanalysis, TPD of ammonia and catalytically tested. The data reveal the isomorphous substitution of silicon by indium in the framework, however, in comparison with other elements the substitution degree is low: Al > Ga > Fe > B > In. In-ZSM-5 shows a low catalytic activity.     

Synthesis by ATRP of triblock copolymers with densely grafted styrenic end blocks from a polyisobutylene macroinitiator

A macroinitiator was prepared from a triblock copolymer of polyisobutylene (PIB) with end blocks of poly(p‐methylstyrene) (P(p‐MeS)) by bromination to obtain initiating bromomethyl groups for atom transfer radical polymerization (ATRP). Controlled polymerization of styrene and p‐acetoxystyrene yields new triblock copolymer structures with densely grafted end blocks. Simultaneously, however, thermally initiated polymerizations can be observed by size exclusion chromatography (SEC) which were also controlled yielding low molecular weight polymers with narrow distributions. A tendency to crosslinking can be suppressed by selection of the polymerization conditions.     

Data evaluation in light scattering from solutions of branched polyethylene

The molecular dimensions of branched polyethylene published in the literature were re‐evaluated to obtain the radii of gyration R_G and other relevant topological parameters. By means of the exponent a_s in the relationships R_G = b · M_s^a the reliability of the data and reasons for deviations were discussed. Linear and branched polyethylenes are molecularly dissolved only in the good solvent, tetralin, whereas in 1‐chloronaphthalene and 1,2,4‐trichlorobenzene the exponent a₂ is lower than expected for good solvents. Microgels and aggregates affect the exponents. After correction of the data by a two component separation method in combination with model calculations and the analysis of scattering curves, a consistent picture is obtained. This procedure is demontrated with fractions of branched polyethylene in the theta solvent diphenylmethane, which are indicating incomplete molecular dissolution.     

Thermoreversible gelation of a polysaccharide with immunological activity: Rheology and dynamic light scattering

The β — 1.3/1.6 glucan schizophyllan is known to suppress tumor growth. Two different schizophyllan samples have been characterized by static and dynamic light scattering in dilute solution of 0.01M NaOH and dimethyl sulfoxide (DMSO). A thermoreversible, optically transparent gel of schizophyllan was obtained on addition of sorbitol to aqueous solution, and the gelation process was followed by low amplitude oscillatory shear and dynamic light scattering (DLS). Power law behavior of storage modulus G'(ω) and loss modulus G”(ω) with the exponent n=0.5 was found at the gel temperature. In the same temperature range intensity time correlation functions (TCF) were measured and the electric field TCFs were calculated, taking into account the heterodyne contributions. The occurrence of two motions gave evidence for inhomogeneities in the gel. The results of the slow motion from DLS agreed surprisingly well with those, estimated by oscillatory rheology. The viscoelastic properties and the influence of sorbitol and schizophyllan concentrations were investigated regarding the pharmaceutical application in cancer therapy.