1999 рік

This study examines the relaxation process at different temperatures in partially cured samples of an epoxy resin and of the same resin modified with 11.1% carboxyl-terminated butadiene acrylonitrile copolymer (CTBN). A differential scanning calorimeter (DSC) was used, which enabled the relaxation enthalpy, the glass transition temperature (T g) and the relaxation peak temperature to be found. CTBN modifies the cure reaction and the relaxation process of the epoxy resin. The study of the relaxation process has a practical interest in addition to a theoretical one. It is known that, with these thermosets, a fully cured sample cannot be obtained at the first step. The introduction of an elastomeric modifier is a normal way of increasing the toughness of these materials.

Детальніше...  

The influence of solvent and temperature on the miscibility of poly(methyl methacrylate)/poly(vinyl acetate) (PMMA/PVAc) was investigated. Experiments using differential scanning calorimetry (DSC) and viscometry were performed. LCST curve was determined by measurements of transmitted light intensity against temperature, at several compositions of PMMA/PVAc, 15/85, 25/75, 50/50, 75/25 and 85/15. It was observed that the miscibility of the PMMA/PVAc blends depends on the solvent. The blends are miscible in chloroform at 308C and 508C, whereas in N,N-dimethyl formamide (DMF) at the same temperatures, the blends are immiscible. In toluene the miscibility depends on the temperature: the blends are miscible at 308C and immiscible at 508C. Results from viscometry match very well those of DSC. The film of PMMA/PVAc 50/50 cast from chloroform presents only one Tg. If cast from DMF, the blend presents two Tg values, close to those for pure polymers. The thermogram of the opalescent film, cast from DMF, is similar to the one obtained after a transparent film, cast from chloroform, was heated above the LCST curve.

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A positive-working photosensitive polyimide (PI) precursor based on fluorinated polyisoimide (FPII) and 2,3,4-tris( 1-oxo-2-diazonaphthoquinone-5-sulfonyloxy)benzophenone (DSSB) as a photosensitive compound has been developed. FPII was prepared by ring-opening polyaddition of dianhydrides, pyromellitic dianhydride and biphenyltetracarboxylic dianhydride, with diamine, 2,2’and followed by treatment with trifluoroacetic anhydride-triethylamine in N-methyl-2-pyrrolidinone. The FPII bis(trifluoromethyl)benzidine film showed a good solubility in a wide range of organic solvents. The dissolution behaviour of FPII containing 30 wt% of DSSB after exposure was studied and it was found that the difference of dissolution rate between the exposed and the unexposed parts was enough to get a positive pattern due to photochemical reaction of DSSB in the polymer film. The photosensitive fluorinated polyimide (FPI) precursor containing 30 wt% of DSSB showed a sensitivity of 250 mJ cm-1 and a contrast of 1.5 with 436 nm light, when it was developed with a mixture of 2.38% aqueous tetramethylanunonium hydroxide solution and 2-propanol at room temperature. The FF’I film cured up to 360°C had a low coefficient of thermal expansion of 10.8 ppm “Cm’ and a low dielectric constant of 2.89

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Rodlike crystals of polyoxymethylene (POM) were isothermally grown on the (001) face of NaCl or KCl from 0.1 wt% solutions (solvent: nitrobenzene, acetophenone, benzyl alcohol, m-cresol). Transmission electron microscopic observation in bright and dark-field imaging modes and selected-area electron diffraction analysis successfully evidenced that POM chain stems in the rodlike crystals are set parallel to the substrate surface and also perpendicular to the rod axis. That is to say, each rodlike crystal was well identified as an edge-on lamella. Further, it was confirmed that the width of bright striations in the dark-field images, which correspond to the crystalline-core thickness of the edge-on lamella, depended on the crystallization temperature (T c) and increased with increasing lamellar thickness. The crystalline-core thickness was, however, inevitably smaller than the corresponding lamellar thickness for any T c. It was accordingly concluded that the POM edge-on lamella has a surface layer (20%–25% of the lamellar thickness) containing folds on each side of the lamella, the layer is presumed to be composed of adjacent re-entrant folds which have some fluctuation in contour length and resultant fluctuation in conformation.

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A molecular dynamics (MD) study was performed to analyze the enthalpy of mixing between poly(vinyl chloride) (PVC) and various aliphatic polyesters. The enthalpy of mixing was calculated from the computed cohesive energy densities of: (1) pure PVC; (2) aliphatic polyesters with a molar ratio of CH2/COO groups from 2 to 14; and (3) mixtures of these components. In order to validate the calculation method, mixtures of homologue model compounds with low molar-masses were also examined. An immiscible n-octane/methyl acetate pair having a 50/50 volume ratio showed an enthalpy of mixing of 1 3.43 cal/cm 3 at 298 K, whereas the miscible 2,3-dichlorobutane/diethyl acetate pair of a 50/50 volume ratio showed the enthalpy of mixing of 2 3.20 cal/cm 3 at 298 K. The calculated results for model compound mixtures also showed good agreement with empirical observations from the literature. The cohesive energy density of pure aliphatic polyesters, as the CH2/COO value of polymer chains is increased, decreases gradually owing to the reduced polarity of aliphatic polyesters. The binary interaction parameter (B) of PVC and aliphatic polyester mixtures predicts a maximum miscibility around a CH2/COO ratio of 6, where the calculated B was equal to 2 3.85 cal/cm 3. These results are in semi-quantitative agreement with experimental values from the literature.

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A single-phase melt of the isotactic polypropylene (iPP)/hydrogenated poly(styrene-co-butadiene) (hSBR) blend, which has a virtual upper critical solution temperature (UCST)-type phase boundary below the melting point of iPP (T,), was prepared at 50/50 wt. ratio and quenched below UCST. By transmission electron microscopy and dynamic mechanical analysis, the blend was shown to be a phase-separated material in which hSBR-rich domains with uniform diameter of ca. 20 nm are dispersed in an iPP-rich matrix. The regular structure is believed to be formed by spinodal decomposition which is pinned at an early stage by crystallization in the iPP-rich region. The blend showed good strain recovery after large deformation, suggesting a potential for a new class of thermoplastic elastomer. Wide-angle X-ray diffraction (WAXD) studies showed that iPP crystallites in the blend are smaller than those in neat iPP and there exists an optimum size of ca. 10 nm (crystal size by the Scherrer equation) for this strain recovery. Such crystallites suffered from less plastic deformation and were hardly oriented under bulk deformation. That is, the crystallites that developed in the presence of polymer impurity (hSBR) seem to be different from those in neat iPP, and play the role of tie points to provide the elastomeric character of the matrix itself.

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The liquid–liquid (L–L) phase separation and its effect on crystallization in polypropylene (PP)/ethylene–propylene–rubber (EPR) blend obtained by melt extrusion were investigated by time-resolved light scattering (TR-LS), optical microscope and small-angle X-ray scattering (SAXS). The existence of the lower critical solution temperature (LCST) was found by the kinetic analysis of the L–L phase separation, that is, the apparent diffusion coefficient of spinodal decomposition (SD), obtained by the TR-LS, decreased with increasing temperature. The L–L phase-separated specimen at 190°C for various time periods was subjected to a temperature-drop to 130°C for the isothermal crystallization and then investigated effect of L–L phase separation on crystallization. Memory of L–L phase separation via SD remained even after crystallization and crystallization proceeded only in PP-rich phases. The crystallization rate decreased with increasing L–L phase-separated time at 190°C. The rapid crystallization for short L–L phase-separated time could be ascribed to the elevation of chain mobility of PP by relatively higher amounts of EPR in PP-rich phases. The amount of EPR between PP lamellae in PP-rich phases was quantified using the correlation function, which is given by the fourier transform of the SAXS intensity.

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In order to enhance the rate of crystallisation of polyhydroxybutyrate (PHB), and copolyhydroxybutyrate-co-valerate (CHB/HV), the effect of various seeding agents was investigated. Solid mixtures of PHB containing either saccharin or phthalimide, and a blend of CHB/HV with PHB were prepared. The effect of these materials as nucleating agents was compared with a commercial nucleating agent, boron nitride (BN). In assessing the relative seeding abilities, overall crystallisation rate, spherulite development and nucleation density were measured by DSC and optical microscope. FT-IR microscopy was used to determine the solubility of the nucleating agent in the molten polymer. The conclusions are drawn that saccharin and phthalimide are much less effective as nucleating agents than BN. They are comparatively soluble in molten PHB but are rejected from the crystalline PHB. There was little nucleation by saccharin and phthalimide, and whilst small crystallites of saccharin and phthalimide develop within the boundaries of the spherulite much of the material accumulates in the regions between the spherulites. PHB acted as a nucleating agent for CHB/HV although the activity was not found to be superior to BN as the nucleating agent.

Детальніше...  

The dynamic structure factor of moderately concentrated polymer solutions is often multimodal, roughly consisting of fast and slow modes. Wang’s formulation of concentration fluctuations predicts that the slow mode disappears when the polymer and solvent components have identical partial specific volumes, but the prediction is not supported by the formulation of Onuki (J. Non-crystalline Solids, 1994,172174, 1151) and that of Doi and Onuki as well (J. Phys. II (Paris), 1992, 2, 1631). The present paper shows that the primary origin of the disagreement is the difference in the basic equations of the two theories, in contrast to the prevailing notion that attributes it to the different constitutive equations assumed by the two theories for the partial stresses generated by concentration fluctuation.

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High density polyethylene batch thermolysis has been performed in dilute solutions to obtain waxy products with number-average molar masses in the range of 2000 to 6000 g mol ¹1. In relation to a classical bulk thermolysis (390°C, 6 h) which affords branched waxy polymers, dilution with phenylether permits controlled diffusion reactions modifying the product characteristics: low molar masses, narrow distribution, high crystallinity, rich in a-olefins and few long branching formation. A hydrogen donor solvent (tetralin), acting on the free radical degradation pathway, affords a lower fragmentation level of the main chains and short chains branching. In addition, a high level of tetralyl radicals grafting on the a-olefin is observed. Adjustment of the process conditions might give tailor-made waxy polymers to use just as they are or after chemical modifications of the olefinic functions.

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