1999 рік

Co-continuous blends of polyethylene with polypropylene or polystyrene are shown to exhibit high tensile moduli. The experimental results exceed predictions of the moduli using existing models for co-continuous morphologies, and approach the level of the parallel model. These high moduli are found to be isotropic to a good degree, in contrast to fibrillar polymer blends which can have such high moduli only in one direction.

Детальніше...  

Poly(ethylene oxide) (PEO) with three different salts, ZnCl 2, ZnBr 2 and ZnI 2 has been characterized at various temperatures and compositions by optical microscopy and differential scanning calorimetry. A molecular thermodynamic model based on both an extended Flory–Huggins theory and a Debye–Hückel theory modified by Guggenheim is developed to interpret the liquidus curves in the resulting phase diagram. The proposed model considers not only short-range specific interactions between the salt ions and the polymer or solvating base groups (oxygen in PEO) of the polymer leading to loose complex formation but also long-range electrostatic forces between salt ions. Quantitative description according to the proposed model is in good agreement with experimentally observed transition temperature of a given system including an eutectic point.

Детальніше...  

Polyimides with enhanced solubility have been synthesized from various aromatic tetracarboxylic dianhydrides and sterically hindered diamines. Intrinsic viscosities in 1-methyl-2-pyrrolidinone (NMP) ranged from 0.28 to 1.05 dL/g. Most of the polyimides were soluble in common solvents such as N,N-dimethylacetamide, NMP, chloroform and tetrahydrofuran. Polyimides derived from thianthrene-2,3,7,8tetracarboxylic dianhydride (TDAN) and diamino mesitylene (DAM) or diethyltoluene diamine (DETDA) were insoluble in all solvents indicating that polyimide solubility decreased as anhydride rigidity increased. Glass transition temperatures ranged from 252 to 398°C and above with the polymers showing little or no weight loss by TGA up to 400°C in both air and nitrogen. The glass transition temperatures of the polyimides increased 15 to 98°C (compared to unhindered polyimide analogs) when one or more methyl group was placed ortho to the imide nitrogen, hindering backbone rotation, chain packing and flexibility. Tough, transparent films of the soluble polyimides were cast from solution.

Детальніше...  

The morphology of blends of polystyrene and polyethylene prepared by single screw extrusion and static mixing is shown to consist initially of sheets, which subsequently break up into droplets or threads. The rapid decrease of phase dimensions during blending can quantitatively be described by deformation of sheets in shear flow and not by deformation of droplets into threads. The final phase dimensions are found to be largely determined by the sheet thickness at the onset of break-up of the sheets. The type of morphology obtained after breakup can change during further processing without changing the phase dimensions significantly. At high values of the capillary number threads remain stable, whereas at low values droplets are formed. The final phase sizes appear to be almost independent of the capillary number.

Детальніше...  

The effect of acrylonitrile (AN) content on the adhesion of poly(styrene-co-acrylonitrile) (SAN) to polycarbonate (PC) was examined by measuring the delamination toughness of PC/SAN microlayers in the T-peel test. Experiments were carried out on microlayers with thin SAN layers, to ensure that delamination would occur predominantly by an interfacial mechanism, and with PC layers that were thick enough to prevent the delamination crack from tearing through the PC. Layer thicknesses of approximately 4.5 mm/0.5 mm (PC/SAN) gave satisfactory results. A broad maximum in the interfacial toughness occurred at about 20% AN. When microlayers with thicker SAN layers were tested, only SAN compositions with 15–25% AN crazed. This increased the delamination toughness and greatly exaggerated the maximum at 20% AN. Crazing only occurred if the interfacial toughness of PC-SAN exceeded the SAN crazing condition. When the crazing condition, which increased linearly with AN content, was overlayed with the interfacial toughness, the intersections correctly predicted the AN content for transitions in failure mode.

Детальніше...  

The partial pyrolysis of poly(dimethyl siloxane) is reported. Pyrolysis is completed in a two step process, inert pyrolysis in a nitrogen atmosphere followed by oxidative pyrolysis in air. Treatment temperatures were from 300–400°C for the inert processing and 260–300°C for oxidative treatment. This thermal processing required no solvents or bulky catalysts, whose transport into the matrix is diffusion controlled and thus limit product uniformity. Therefore, high sample uniformity was achieved. Hybrid materials with reduced carbon and hydrogen content and an increased oxygen/silicon ratio resulted. These composites were shown to be stronger than composites reported in the literature with similar elemental compositions, but prepared in such a way as to achieve only limited interphase interaction. At high oxidation temperatures, microporosity developed in the materials.

Детальніше...  

Stable co-continuous morphologies are found over a wide composition range in blends of styrene/(ethylene–butylene) based block copolymers (SEBS) and polypropylene (PP) when they are processed below the block copolymer’s order–disorder transition (ODT), i.e. when the block copolymers are microphase-separated. Blending at higher temperatures, i.e. when the block copolymer shows a single-phase melt, also leads to a reasonably wide composition range of co-continuity but when annealing takes place at these temperatures the blends show an increase in their phase domains and the composition range decreases significantly. Annealing of the co-continuous PP/SEBS blends when the block copolymers are microphase-separated hardly influences the phase sizes and composition range. Blending the same block copolymers with polymethyl methacrylate or polyoxymethylene, leading to blends with much higher interfacial tensions, results in a much smaller composition range of co-continuous morphologies than was found in the PP/SEBS blends, whatever the processing temperature be. It is demonstrated that breakup and retraction can be severely limited or even stopped at lower blending temperatures, therefore fulfilling the condition for stability of co-continuous morphologies. The (non-)breakup or (non-)retraction behaviour of elongated structures strongly depends on a complex combination of parameters, including phase size, yield stress and interfacial tension. Therefore the formation of stable co-continuous morphologies also strongly depends on these parameters.

Детальніше...  

Miscibility in a series of blends, derived from poly(benzimidazole) (PBI) and a range of poly(vinyl acetate-stat-vinyl alcohol) [PV(Ac/ OH)] copolymers with vinyl alcohol contents between 20 and 73 mol%, has been studied by differential scanning calorimetry, dynamic mechanical thermal analysis, Fourier transform infra-red spectroscopy (FTi.r.), scanning electron microscopy and optical haze measurements. The results show that as the vinyl alcohol content of the copolymer component increases, the level of dispersion of the two components of the blend improves. While no one-phase blends, miscible at the molecular level, were obtained, the levels of dispersion in blends with PV(Ac/OH) compositions of 56.0 and 73.0 mol% vinyl alcohol were good enough to allow the formation of optically clear films. An enhanced storage modulus (E⬘) for blend 5, which was larger than that for PBI alone, was also obtained. Evidence for significant hydrogen bonding between the blend components was found from FTi.r. measurements, but none of the blends exhibited a single glass transition temperature. Consequently it is estimated that the phase-domain sizes for the well-dispersed blends lie in the range: 20 nm < domain size < 500 nm, which are larger than one would expect from single-phase miscible blends.

Детальніше...  

Hydrogels were prepared by free radical polymerization in aqueous solution of N-isopropylacrylamide (NIPA) and of NIPA with sodium acrylate (SA) and sodium methacrylate (SMA) as comonomers. For each copolymerization three different compositions were used and for all systems three concentrations of a tetrafunctional crosslinker were employed. Swelling ratios, r ˆ (mass hydrogel/mass dry polymer), were measured on heating from 20°C and then on cooling back to 20°C, and the swelling/deswelling was shown to be thermoreversible. Lower critical swelling temperatures (Tc) were exhibited in copolymers with low concentration of sodium salt. The type of comonomer did not affect the swelling–deswelling behaviour in hydrogels with low concentration of comonomer, but there were strong differences at higher concentrations. These variations were attributed to small differences in the degree of hydrolysis of the SA and SMA units within the hydrogel.

Детальніше...  

Poly(butyl methacrylate) particles were prepared using an emulsion polymerization procedure with b-cyclodextrin (b-CD) in place of the surfactant. The rate of polymerization was lower than in the equivalent sulphate surfactant stabilized emulsion polymerizations. Particle size and molecular weight distributions were unimodal and did not show any dependance on b-CD concentration. However, the average molecular weight was, in all b-CD containing polymerizations, significantly higher than in polymerizations in the absence of both surfactant and b-CD. b-CD had little effect on the behaviour of surfactant-containing systems. A tentative explanation, involving the enhancement of colloidal stability by poly(butyl methacrylate) included by b-CD amphiphilic polymers, is proposed.

Детальніше...