1992 рік

Aqueous solutions of a range of diblock copolymers with one stat-copoly (oxyethylene-oxypropylene) block and one poly(oxyethylene) block were investigated by a variety of techniques, including light scattering, photon correlation spectroscopy, surface tension and electron microscopy. Critical micelle concentrations and temperatures were determined and estimates made of micellar size and shape. The thermodynamic micellization functions were derived, and comparison made with those of a diblock-copoly (oxyethyleneoxypropylene). Thermally reversible gelation was noted, as were solubilization effects. The composition of the statistical block was of major importance in determining the hydrophobicity of a copolymer, and thereby its micellization and gelation properties.

Детальніше...  

A kinetic model which describes the network structure development during free-radical crosslinking copolymerization is proposed. The model was successfully applied to various batch copolymerization systems such as methyl methacrylate/ethylene glycol dimethacrylate, styrene/p-divinylbenzene, styrene/ ethylene glycol dimethacrylate, and acrylamide/N,N'-methylene-bis-acrylamide. The model calculations suggest that polymer networks synthesized by free-radical copolymerization are, in general, inhomogeneous at least on a microscopic scale. This model can be used to control the network structure, and a semi-batch policy to produce homogeneous polymer networks is proposed.

Детальніше...  

Several segmented polyurethanes comprising azo aromatic and hydrophilic groups were synthesized by reaction of m-xylylene diisocyanate with a mixture of m,m'-dihydroxyazobenzene, poly(ethylene glycol) (PEG, M, = 2000) and 1,2-propanediol (propylene glycol, PG). Their polymer films were made by solution casting, and pellets of a hydrophilic drug (FOY-305) were coated with these polymers. Both the films and the coated pellets were incubated anaerobically in a culture of human intestinal flora. It was found that the azo groups in the polymer were reduced to hydrazo groups during incubation, which induced degradation of the films and the coatings without decreasing the molecular weight of the polymer. The rate of drug release from the pellets depended on both the azo and PEG compositions. Based on these findings, a new degradation mechanism of the polyurethane films and coatings is proposed.

Детальніше...  

The adsorption process of end-hydroxylated poly (dimethyisiloxane) chains upon silica particles is compared with that of end-methylated chains. Four variables were used to analyse the adsorption process : the chain molecular weight M., the initial concentration of silica c~t, the amount of adsorbed polymer QI, and the relative number of fixed monomeric units Za, observed from n.m.r. Both polymer species were found to obey two simple equivalent laws of adsorption: Q~ 0c x/~. and zB oc 1/x/~ .. Several features characterize the adsorption process: equal specific amounts of hydroxylated chains are adsorbed for two silica concentrations, c~t = 0.17 or 0.29 (w/w); for a given chain length, the specific amount of adsorbed hydroxylated chains is higher than that of methylated ones ; a partial desorption of methylated chains is induced by washing mixtures with c~ = 0.17 (w/w); the process of adsorption of hydroxylated chains is about 10 times faster than that of methylated ones, at 343 K.

Детальніше...  

A concentrated emulsion has a very large volume fraction of dispersed phase (0.74-0.95 in this case) and the appearance of a gel. Three procedures based on concentrated emulsion polymerization are suggested for the preparation of polymer-supported quaternary onium salts. ( 1) Concentrated emulsions of vinylbenzyl chloride (VBC) in water are subjected to polymerization. The polymer resins thus obtained are composed of particles in the micrometre range. A large fraction of the pendant benzyl-chloride groups present in the poly(VBC) particles are converted to onium chloride by a quaternization reaction. (2) A small amount of VBC is added to a partially polymerized concentrated emulsion of styrene (containing a crosslinking agent) in water under vigorous stirring. The system is subsequently subjected to complete polymerization. The obtained polystyrene-poly(VBC) is found to consist of particles having a non-uniform poly (VBC) shell that covers a crosslinked polystyrene core. This polymer is then subjected to a quaternization reaction in order to generate a polymer substrate with bound quaternary onium chloride. (3) A concentrated emulsion of styrene in an aqueous solution of a quaternary onium chloride monomer is prepared. The onium chloride adsorbed on the surface of the dispersed phase polymerizes simultaneously with styrene when the concentrated emulsion is subjected to polymerization. The polymer-supported onium salts thus prepared were used as phase transfer catalysts in the alkylation of isopropylidene malonates. The catalysts containing larger pores had a higher activity than those with a more compact structure. The catalyst prepared by the second approach had a greater catalytic activity than that prepared by the first approach. The catalyst prepared by the third procedure had a low activity because of the small amount of supported onium salt it contains.

Детальніше...  

A model of particles formed in dispersion polymerization is suggested and the Gibbs energy of a single particle at infinite dilution is calculated. By minimizing the energy at fixed particle mass and composition, the particle dimensions are calculated and the degree of swelling of the particles is predicted. The estimated geometric radii are somewhat smaller than the experimental values of the hydrodynamic radii.

Детальніше...  

AM! calculations are reported for enines with aromatic substituents. It was found that the position of the unsaturated bonds in each molecule may account for the reactivity of each isomer via different polymerization pathways. The resonant structure of the charged form of one of the molecules allowed speculation about the observed inhibition of the polymerization reaction.

Детальніше...  

The electron spin resonance (e.s.r.) spectra of radicals observed during the copolymerization of methyl methacrylate (MMA) and ethylene glycol dimethacrylate (EGDMA) consist of 13 lines in the initial liquid phase, but change to 9 lines as the copolymerization proceeds. The experimental spectra have been simulated by use of a model based upon a Gaussian distribution of dihedral angles about a most stable conformation. The simulations suggest that the e.s.r, spectra are superpositions of spectra arising from three types of radicals: a liquid-phase radical giving rise to a 13-line spectrum and two solid-phase radicals giving rise to two slightly different 9-line spectra. The two types of 9-line spectra correspond to terminal radicals on EGDMA and MMA residues. The ratio of the two types of solid-phase radicals is correlated with the degree of disorder of the reacting system, which accounts for the observed dependence of both initiator efficiency and final radical concentration upon the weight fraction of EGDMA. When copolymerization proceeded to medium conversion, ultra-violet irradiation of the samples reinitiated the reaction and caused a temporary reappearance of the 13-1ine spectrum, indicating that some propagating radicals are trapped in the polymer matrix.

Детальніше...  

In order to investigate the contrasting propensities for crystallization between aryl sulphone and aryl ketone polymers, new aromatic monomers containing sulphone, ketone and ether linkages were synthesized and reacted to prepare new polyarylene ethers. The polymers obtained were characterized by solution viscosity, thermal gravimetric analysis and differential scanning calorimetry (d.s.c.). The effect of the presence of ketone and/or sulphone linkages in the polyarylene ethers during synthesis and on the final properties is discussed

Детальніше...  

A series of aryl bis-maleimides (BMIs) and bis-citraconimides (BCIs) were characterized by d.s.c, using both temperature scans and isothermal experiments. Impurities in the BMIs tend to increase the temperature at which thermal polymerization starts, while the converse is true for BCIs. This was attributed to the presence of an itaconimide impurity in the BCIs. In particular, the thermal polymerization kinetics of the compounds were investigated. Effects of structure and monomer purity on the thermal polymerization characteristics were identified

Детальніше...