1992 рік

Two different apparent activation energies of the direct current electrical conductivity of polyhexandiolmaleate resins, with and without styrene, were determined in the range between 290 K and 350 K. The temperature at which the activation energy change was detected, was assigned to the upper liquid-liquid transition, T~p, where the intramolecular short range local ordering based on hydrogen bonding disintegrated. No effect by the degree of the radiation crosslinking on the Tip transition proved its intramolecular nature. Increasing the radiation crosslinking dose resulted in an increase in the apparent activation energies up to the vitrification dose which was directly dependent on the maleate content.

Детальніше...  

The structure, crystallization and morphology of poly (aryl ether ketone ketone)s (PEKKs) prepared from diphenyl ether (DPE), terephthalic acid (T) and isophthalic acid (I) and having different T/I ratios have been investigated. As prepared, these copolymers can be thought of as consisting of 'phthalate diads' containing -DPE-T DPE-T- (TT) and/or-DPE T-DPE-I- (TI). Melt-crystallized PEKKs (all T/I ratios) form a structure similar to that observed in other poly(aryl ether ketone)s (form 1 ; a = 0.769 nm, b = 0.606 nm and fibre axis c = 1.016 nm). However, depending on composition, both TT and TI crystals were observed. All PEKK materials grow in the form of spherulites having negative birefringence. The incorporation of isophthaloyl moieties is observed to increase the chain flexibility and decrease the rate of crystallization. The equilibrium melting temperatures of various PEKKs were estimated using the Hoffman Weeks approach, and showed a linear correlation with the meta isomer content. In contrast to other poly(aryl ether ketone)s, a form 2 crystalline modification (a = 0.393 nm, b = 0.575 nm and c = 1.016 nm) can be induced either by exposure to solvents or by cold crystallization. The two crystalline modifications differ from each other in the placement of the chains and, consequently, the interchain interactions.

Детальніше...  

This paper is aimed at working out the part played by particle/particle interfaces in the viscoelastic behaviour of latex films. More precisely, a thorough analysis of the effect of neutral or ionized carboxyl functional groups, chemically incorporated in the latex particles and at the surface of the particles, is provided. The tensile micromechanical properties of styrene-butadiene copolymer latex films, with various carboxyl group contents and various neutralizing agents, are studied using dynamic mechanical analysis. Then, the structure of the latex films is investigated, using transmission electron microscopy (TEM) performed on stained ultramicrotomed films. The features of the interfacial crosslinking which result from ionic or hydrogen bonding interactions at the particle/particle interfaces are shown up, in close correlation with structural information gained from TEM investigations. The opportunity offered by the combination of the techniques used in this work, for a comprehensive prediction of the cohesive strength of the latex films is emphasized.

Детальніше...  

A new electron microscopy technique was used to observe two-phase morphology in polymer blends. It consisted of preparing a fiat specimen surface by microtoming, staining the unsaturated domains with Os04, and scanning electron microscope observation of the composition images using backscattered electrons. The new technique was applied to the melt blends of polypropylene (PP)/polybutadiene and PP/ethylene-propylene-diene rubber systems. Compared with conventional scanning electron microscope observation using secondary electron imaging for fractured surfaces, the new technique yielded a much clearer phase contrast.

Детальніше...  

The growth rate of isotactic polystyrene (iPS) crystals has been investigated for crystallization in concentrated dimethyl phthalate solutions and in the melt over a wide range of supercooling from 20 to 170 K. For most of the supercoolings studied, the growth faces are rough; nucleation is not present. However, the growth rate G for each concentration shows approximately the same dependence on supercooling AT as growth controlled by secondary nucleation: log(Gift) depends linearly on K/TAT, where fl is a transport factor and K is a constant. The value of K increases with decreasing concentration. Possible mechanisms for crystal growth are discussed, taking account of the deposition of one stem on growth faces; the side surface free energy increases with decreasing concentration

Детальніше...  

The solution properties of poly(methylphenyl)silane are studied in tetrahydrofuran at room temperature. Combined measurements by gel permeation chromatography (g.p.c.) and light scattering indicate that the absolute molecular-weight values of poly(methylphenyl)silane do not differ from the g.p.c, values obtained using a classical g.p.c, calibration with polystyrene standards. Some calculations are performed on the molecular-weight values using the theory of Yamakawa and Fujii. These results allow one to conclude that the behaviour in solution of poly(methylphenyl)silane can be described using a model based on rigid-rod-like elements involving about 40 Si atoms.

Детальніше...  

The structure of the PCL spherulite in poly(~-caprolactone)/poly(styrene-co-acrylonitrile) (PCL/SAN) blends was investigated by optical microscopy and small angle light scattering. The spherulite structure with a Maltese cross has been observed in pure PCL. Similar PCL/SAN blends exhibited not only spherulites with a Maltese cross, but also distinct extinction rings. The Hv light scattering pattern especially caused diffraction rings in PCL/SAN blends but not in pure PCL. The spherical symmetry of spherulite PCL becomes more incomplete and the twist of the lamella becomes more irregular with increasing SAN content. It is found that the spherulite structure of PCL/SAN blends is dependent on the crystallization temperature and the concentration of SAN in PCL/SAN blends.

Детальніше...  

The equilibrium triple point and pressure and temperature phase diagram of polyethylene were obtained by in situ optical microscopic and X-ray observations of the melting temperature of hexagonal and orthorhombic isolated extended chain single crystals at high pressure. The melting temperatures of extended chain crystals still showed a AT dependence at high pressure, which has been neglected in previous studies. The triple point pressure and temperature were 0.5 x 108 1.5 x 108 Pa and 20-30°C higher than those previously obtained by the d.t.a, method. These results suggest that lamellar thickness dependence is important even in extended chain crystals to obtain equilibrium thermodynamic physical quantities of polymers.

Детальніше...  

The fluorescence in dilute solution has been measured for five polyesters with terephthalate as the rigid aromatic unit and diols derived from cyclohexane as the flexible spacer. The spacers are !,2-cis-cyclohexanediol, 1,2-trans-cyclohexanediol, a 1:2 mixture of 1,3-cis- and 1,3-trans-cyclohexanediols, a 1:2 mixture of 1,4-cis- and 1,4-trans-cyclohexanediols, and a 1:3 mixture of 1,4-cis- and 1,4-trans-cyclohexanedimethanol. The two polymers with the largest ratio of the intensities of excimer and monomer emission are those that contain either the 1:2 mixture of 1,3-cis- and 1,3-trans-cyclohexanediol or the 1:3 mixture of 1,4-cis- and 1,4-trans-cyclohexanedimethanol. A conformational analysis concludes that the spacers most conducive to excimer formation are the 1,3-cis-cyclohexanediol and 1,4-cis-cyclohexanedimethanol. This result from calculations is compatible with experimental results

Детальніше...  

Blends of poly(N-vinyl-2-pyrrolidone) (PVP) with a copolyamide (CoPA) randomly composed of 1:1:1 (wt) nylon 6, nylon 66 and nylon 610 structural units were prepared by casting from a common solvent. They were found to be miscible and show a single, composition-dependent glass transition temperature (Tg). The addition of PVP to CoPA significantly lowers the crystallinity owing to an increasing T~ of the system. The observed miscibility is proposed to be the result of specific interactions between the proton acceptor groups of PVP and the amide groups of CoPA.

Детальніше...