1992 рік

The intrinsic viscosity data reported in the literature for ethylene-butylene copolymers in several solvents are used to estimate the X parameter characterizing the polymer-solvent interaction. The dependences of this parameter on the copolymer composition display unexpectedly pronounced minima which indicate a repulsive interaction of ethylene and butylene units. In view of their chemical similarity, which makes the enthalpy contribution to the interaction low, the behaviour is assigned to entropy-of-mixing effects.

Детальніше...  

Simple considerations of the molecular-weight dependences of the intrinsic viscosity and melt viscosity of chain polymers are given.

Детальніше...  

The interaction parameters for polystyrene and tetramethyl bisphenol-A polycarbonate blends were evaluated by fitting experimental PVT behaviour and a lower critical solution temperature (LCST)-type phase boundary to the lattice fluid theory of Sanchez and Lacombe. The phase boundaries of these blends were obtained by using a differential scanning calorimetry (d.s.c.) method, and were compared with those obtained by light transmission and visual methods. The characteristic properties for these polymers were determined by the non-linear least-squares fitting of specific volume data as a function of temperature and pressure to the equation of state. The net bare interaction parameters from the Sanchez-Lacombe theory, AP*, calculated at several compositions yielded negative values (AP* = -0.17 _ 0.01 cal cm-3) and did not depend on composition. Flory-Huggins type interaction parameters reported in the literature for this system determined by a small-angle neutron scattering technique compare favourably with the results of this study except in terms of composition dependence.

Детальніше...  

Mixtures of hydrophilic and hydrophobic methacrylates were grafted onto amylopectin, using the ceric ion redox initiation system. The pairs hydroxyethyl methacrylate (HEMA)/methyl methacrylate (MMA), hydroxypropyl methacrylate (HPMA)/MMA, HEMA/ethyl methacrylate (EMA) and HPMA/EMA were grafted at different concentrations of the monomer feed. In all cases, a major formation of ungrafted acrylic polymer can be observed when the hydrophilic monomer used is HPMA. High yields were obtained with all the mixtures at certain concentrations of the monomer feed

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The aqueous polymerization of methyl methacrylate was carried out at 60°C using potassium persulphate and sodium metabisulphite as the redox pair initiation system. The viscosity average molecular weight for the prepared poly(methyl methacrylate) (PMMA) was 1.49 x 105. Adsorption of PMMA on the surface of Egyptian albite ore was carried out. The adsorption isotherm obtained was S-shaped and the monolayer surface coverage was 7 ? 10-Sgg - ~ adsorbed polymer. The stereoregularity of the prepared PMMA changed with the adsorption process. The sedimentation rate of the modified ore in the organic medium was also determined. The most stable state for the suspension of the ore was found at 4 x 10- 3 g g- 1 adsorbed polymer.

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This paper concerns mainly the effect of the shear rate on the reduced viscosity of sodium poly(styrene sulphonate) (Na PSS) in salt-free solution. A maximum in reduced viscosity is obtained for a critical concentration attributed to a preferential intermolecular distance. This maximum seems to have the same origin as that observed by light or neutron scattering. Then, in excess of external salt, the viscosity of Na PSS is compared with that of polystyrene in organic solvent. The main difference between these two polymers is the large increase in the stiffness of the chain in the presence of ionic charge, causing large non-Newtonian behaviour of the polyelectrolyte when the polymer concentration increases even in 0.1 M NaC1.

Детальніше...  

Nylon 6 films were prepared by gelation/crystallization from solutions. The solvents used were a formic acid-chloroform mixture, trifluoroethanol and benzyl alcohol. Maximum values for Young's modulus and the orientational degree of the b-axes (the crystal fibre axes ), were obtained for the drawn films prepared from formic acid-chloroform solution. These values, however, were lower than those of melt films. To study the reason for this, the deformation mechanism of gel films prepared from the formic acid-chloroform cosolvent was investigated and compared with the deformation mechanism of melt film using the orientation distribution function of crystallites. The distribution functions of both films exhibited similar profiles, indicating the preferential orientation of the b-axis with respect to the stretching direction by the rotation of crystallites around the c-axis, leading to taut tie molecules. This mechanism was found to be quite different from the molecular orientation of ultradrawn polyethylene gel films associated with significant crystal transformation from a folded to a fibrous type.

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A series of new poly (oxadiazole-imide)s that are soluble in polar solvents, and some of which are soluble in chloroform, have been prepared by incorporating the hexafluoroisopropylidene unit and other flexible linkages into the backbone. Films of these poly(oxadiazole-imide)s cast from solution are light-tan to yellow in colour and have ultimate tensile strengths ranging from 89.0 to 95.6 MPa. Initial weight loss occurs near 380°C in air by thermogravimetric analysis, while one polymer appears unchanged after isothermally ageing for 300 h at 300°C in air. The inherent viscosities ranged from 0.34 to 0.54 dl g- 1 in N,N-dimethylacetamide. The refractive indices of the poly(oxadiazole-imide)s are near 1.6 and the dielectric constants range from 2.9 to 3.2 at 1 MHz. Moisture absorptions of the poly(oxadiazole-imide)s are near 3% by weight. A new hexafluoroisopropylidene- and imide-containing dicarboxylic acid and its diacid chloride and six new ether- and imide-containing dicarboxylic acids and their diacid chlorides are also presented. These monomers were prepared from commercially available diamines and trimellitic anhydride without the use of an amide solvent.

Детальніше...  

In this paper a method of estimating diffusion rates in suitable polymer blend systems is described. Recent studies of blends of linear with branched polyethylenes have shown that in the melt the components mix under some conditions of temperature and composition, but segregate under others. In particular, for a large number of systems, blends with low linear content are demixed in the melt at the highest temperature at which crystallization is observed, but can be cooled into a metastable state in which the melt remixes before crystallization takes place. Such remixing is the subject of the present paper. The times of melt mixing were assessed from the thermograms and the distances over which the linear material diffused were assessed from electron micrographs of replicas. Combining this information, estimates were obtained for the diffusion rates of the various polymers (with molecular weights of between 2 x 10 6 and 2550) in the branched matrix. These diffusion rates vary with the inverse square of the molecular weight of the linear polymer, and are close to the self-diffusion rates expected from the Klein and Briscoe equation

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Crystalline polyhydroxyalkanoates (PHAs) such as poly-D(-)3-hydroxybutyrate (PHB) and its copolymers with poly-3-hydroxyvalerate (P(HB-HV)) are produced by a wide variety of bacteria and have uses in controlled drug delivery systems. The crystallization kinetics and morphology of P(HB-HV) polyesters both with and without the incorporation of a model drug, Methyl Red, have been investigated, as they are thought to influence drug release characteristics. The influence of copolymer composition and incorporation of Methyl Red on radial growth rates, G, of PHB and P (HB-HV) copolymer spherulites were investigated using polarized light video-microscopy. Growth curves could be obtained over a wide range of undercoolings for all the copolymers studied. At a given crystallization temperature, G decreased with increasing HV content and with increasing drug concentration in polymer spherulites. Spherulite morphology appeared to be a complex function of polymer molecular weight, copolymer composition, drug loading and crystallization temperature (T c). Release of Methyl Red from melt-crystallized matrices of these PHAs was a function of T? and copolymer composition. Drug release from isothermally crystallized copolymer films occurred progressively more rapidly with increasing HV content. This could be explained by the progressively poorer drug entrapment within copolymer matrices with increasing HV content. In PHB, Methyl Red was thought to be largely entrapped within spherulites (interlamellar regions) but for copolymers with increasing HV content, progressively greater amounts of drug were excluded at the spherulite surface and at interspherulitic boundaries.

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