Self-diffusion in block copolymer solutions

Self-diffusion of poly(sulphone-ethylene oxide) copolymer macromolecules in different solvents has been investigated by pulsed field gradient nuclear magnetic resonance. The experimental response indicates a wide distribution of self-diffusion coefficients P(D) for all samples studied. The distribution function P(D) is shown to be represented by a logarithmic normal function and parameters characterizing its width have been obtained. The width of P(D) is large at room temperature, indicating that the systems tend to gelation. The concentration dependence of the average self-diffusion coefficient D is described by the relation D ̄=φ{symbol}-α 2 (α=3.4 - 3.8) in the range of polymer concentrations φ{symbol}2=0.2 - 0.6. The effect of solvent thermodynamic quality on D ̄ is considered. Anomalous concentration dependences of D observed in a number of solutions are discussed. © 1987

NMR investigation of the molecular mobility and structure of polysulphone-polybutadiene and polysulphone-polyethylene oxide block copolymers

Molecular mobility in polyblock regular alternating block copolymers has been investigated by pulsed NMR in the temperature range 173-463°K. It was found that phase microsegregation in polysulphone-polybutadiene block copolymers is only very slightly dependent on their composition. This is due to the incompatibility of the oligobutadiene and oligosulphone components. The glass temperatures and molecular mobilities of flexible and rigid blocks in these copolymers coincide with corresponding parameters of the initial oligomers. However, at elevated temperatures there is a marked reduction in mobility of the oligobutadiene block compared with that of the initial oligomer. Owing to partial compatibility of the oligoethylene oxide and oligosulphone components in polysulphone-polyethylene oxide block copolymers the glass temperatures, phase micro-segregation and mobility of flexible and rigid blocks are largely dependent on the copolymer composition. © 1982

Novel transparent nanocomposite films based on chitosan and bacterial cellulose

New nanocomposite films based on different chitosan matrices (two chitosans with different DPs and one water soluble derivative) and bacterial cellulose were prepared by a fully green procedure by casting a water based suspension of chitosan and bacterial cellulose nanofibrils. The films were characterized by several techniques, namely SEM, AFM, X-ray diffraction, TGA, tensile assays and visible spectroscopy. They were highly transparent, flexible and displayed better mechanical properties than the corresponding unfilled chitosan films. These new renewable nanocomposite materials also presented reasonable thermal stability and low O(2) permeability.FCT - SFRH/BD/41388/ 2007FCT - SFRH/BPD/38515/200

Diffusion spin echo decay by pulsed magnetic field gradient NMR and some structure features in blockcopolymer solutions

Blockcopolymer solutions were investigated by pulsed field gradient NMR. Nonexponential diffusion decay of solvent molecules and the dependence of it's shape on time of observation uere found out. These features (unusual for solvent molecules) were explained on assumption of the solution heterogeneous structure. Consideration, performed, allow us to estimate the linear dimensions of the heterogeneities, which for concentrated systems occured to be near 10000 A. © 1985 Springer-Verlag

Marine Polysaccharides in Pharmaceutical Applications: An Overview

The enormous variety of polysaccharides that can be extracted from marine plants and animal organisms or produced by marine bacteria means that the field of marine polysaccharides is constantly evolving. Recent advances in biological techniques allow high levels of polysaccharides of interest to be produced in vitro. Biotechnology is a powerful tool to obtain polysaccharides from a variety of micro-organisms, by controlling the growth conditions in a bioreactor while tailoring the production of biologically active compounds. Following an overview of the current knowledge on marine polysaccharides, with special attention to potential pharmaceutical applications and to more recent progress on the discovering of new polysaccharides with biological appealing characteristics, this review will focus on possible strategies for chemical or physical modification aimed to tailor the final properties of interest