1994 рік

Real-time dielectric relaxation and rheological measurements are reported during the course of cure for three thermoplastic-modified thermoset resin blends. Two thermoset resin systems were used, one containing only a difunctional epoxy and the other containing a mixture of tri- and difunctional epoxies. The latter mixture was used to react two reactively terminated polysulfone thermoplastics with slightly different molecular weights. Positron annihilation measurements are reported on one of the thermoplastic-thermoset resin systems as a function of the thermoplastic composition, and indicate that changes in composition lead to variations in the free-volume content. Mechanical properties are presented for all three systems. These data indicate that the phase structure has a significant effect on mechanical performance, but the most dominant effect results from change in the thermoset matrix material.

Детальніше...  

The formation of ether bonds by reaction of benzylic bromides with phenyl silyl ethers has been investigated as an alternative to the classical Williamson ether synthesis. The reaction has been applied to the synthesis of high-molecular-weight polyethers that are not readily obtained by other methods. An AB-type monomer, such as 3-bromomethyl-trimethylsilyloxybenzene, reacts in the presence of a stoichiometric amount of carbonate to afford the corresponding polyether with molecular weight as high as 300 000 depending on the reaction conditions. While the polyetherification reaction of 3-bromomethyi phenol under classical conditions leads to polymers rich in C-alkylated and branched units, conditions may be found to reduce or prevent C-alkylation and branching with 3-bromomethyl-trimethylsilyloxybenzene. The reaction is believed to proceed via activation of the O-Si bond through carbonate addition to afford a species containing a pentacoordinate silicon. The role of carbonate and the stoichiometry and mechanism of the reaction have been investigated with the help of a model etherification reaction

Детальніше...  

Refractive indices along the three principal directions of uniaxial planar deformed poly(ethylene terephthalate) (PET) films have been used to calculate the second moments of the orientation distribution of the phenyl ring's normal with respect to the three principal directions of the sample. Under our stretching conditions, the phenyl rings show a strong tendency to orient their plane parallel to the plane of the films. The orientation of the chain axis can also be evaluated from the same data. A good correlation is observed between this indirect measurement and direct, but more tedious, techniques such as infra-red dichroism.

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Thermoplastic polyurethane elastomers (TPUs) from diol-terminated poly(tetramethylene glycol) (PTMG), 1,4-butanediol (BD) and 4,4'-diphenylmethane diisocyanate (MDI) were modified by copolymerizing with diamine-terminated poly(7-methyl-L-glutamate) oligomer (7-PMG). In TPUs modified by the 7-PMG segment, the hard segment showed a decreased crystallinity and a melting point depression, as some of the hard segments were replaced by the 7-PMG segment. This behaviour is probably due to the strong interaction and phase mixing between ~,-PMG and hard segments. The melting behaviour of the PTMG segment suggested a weaker interaction between 7-PMG and PTMG segments compared with that between hard segments and PTMG segments. When a small amount of hard segments was replaced by 7-PMG segments, the modulus increased and the elongation at break decreased.

Детальніше...  

The effect of spinning rheology on gas-separation hollow-fibre membranes has been investigated. For polysulfone fibres, permeability decreased with increasing dope concentration. The lowest fibre selectivities occurred with the medium spinning-dope concentration, suggesting that both surface pore area and active layer thickness were high. Both permeability and selectivity increased with increasing dope extrusion rate, possibly due to enhanced orientation of the membrane skin. Permeability increased and selectivity decreased with increasing jet stretch. Polyacrylonitrile membranes, because of the low polymer permeability, proved insensitive to spinning conditions, showing no separation capability due to the dominating effect of surface pores.

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Poly(4-vinyl pyridine (P4VP) core-polystyrene (PS) shell type polymer microspheres were synthesized by crosslinking the P4VP spherical microdomains of poly(styrene-b-4-vinyl pyridine) (P(S-b-4VP)) diblock copolymers with 1,4-dibromobutane (DBB). The packing structures of well characterized microspheres, both on one layer and also in the bulk were investigated by transmission electron microscopy (TEM). For the AB05 microspheres (diameter of P4VP core = 40.7 nm, aggregation number = 1300), an ordered packing of the microspheres on to a hexagonal lattice was observed for one layer, while in the bulk of the film, AB05 ordered on to a face-centred cubic (f.c.c.) lattice

Детальніше...  

The effect of dehydration on the molecular structures and physical properties of fibroin gels was investigated. The physical properties that are discussed include strength, water content and thermal stability. It was found that the molecular structure of fibroin was not changed by the change in water content of the gel, while the physical properties of the gel, however, were changed significantly. A quantitative analysis of the molecular structures was carried out by separating the components of the i.r. spectra. The spectral parameters of the amide I, II, and III absorption bands, corresponding to the silk I, 8, and random coil conformations, respectively, have been derived. In addition, the fibroin gels were characterized by their water contents, strength and thermal stability.

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N.m.r. analysis of the hydration and temperature behaviour of resolved carboxyl carbon resonances in the acid and neutralized ~t,~o-dicarboxylatopolybutadiene telechelics reveal a complex interaction between water and chain end groups participating in the formation and subsequent stability of crosslinks. A multiplicity of environments are detected above the glass transition of the host polymer matrix arising from chemical and physical inequivalences. Reasonable interpretations are advanced to account for the observed carboxyl peaks. Analysis of the temperature dependence of 13C linewidths offers further insight into the manner in which end groups influence overall motion in the polymer. Extraction of meaningful activation energies from 13C linewidth data, however, must take correlation frequency distributions into account. The anomalous behaviour of water and its influence on structure and motion is explored.

Детальніше...  

To modify the surface properties of aramid fibre, graft polymerization of acrylamide (AAm) and glycidyl methacrylate (GMA) was performed onto the surface of Kevlar 49. Following plasma treatment and subsequent exposure to air to introduce peroxides onto the fibre surface, the polymer peroxides were decomposed in the monomer solution containing riboflavin by ultra-violet (u.v.) irradiation to effect graft polymerization of the monomers onto the fibre. The monomer solution was prepared from water and dioxane for AAm and GMA, respectively. After removal of homopolymers, the grafted fibre was subjected to surface analysis with attenuated total reflection Fourier-transform infra-red spectroscopy and X-ray photoelectron spectroscopy (XPS). It was found that grafted PAAm and PGMA chains were present in the surface region of the fibre. Graft polymerization was greatly affected by u.v. irradiation time, monomer concentration and plasma treatment time. The reaction of propylamine with the PGMA-grafted surface was accompanied by the appearance of a new nitrogen peak in the XPS spectrum, suggesting the presence of epoxy groups on the surface of PGMA-grafted fibre

Детальніше...  

Electropolymerization of pyrrole in aqueous solutions of sodium nitrate with additions of different steric stabilizers (e.g. polyvinylpyrrolidone, poly(vinyl alcohol), poly(ethylene oxide)) was conducted using potentiostatic, galvanostatic and potentiodynamic techniques. Different substrates (Pt, glassy carbon and stainless steel) were used. The electropolymerization processes were investigated and the resultant polymers were characterized in terms of morphology, adhesion and electroactivity. Thus, under galvanostatic conditions, the growth potential of the polymers was affected little by the presence of stabilizers. For potentiostatic and potentiodynamic methods, however, the presence of stabilizers reduced the rate of growth of the polymer. Furthermore, stabilizers were found to affect the morphology of the plate side and solution side of films. Adhesion to the substrate increased with increasing concentration of stabilizer until a maximum was attained, beyond which further additions of stabilizer began to reduce the adhesion. On some substrates, such as stainless steel, the presence of stabilizer improved the electroactivity of the deposited polymer.

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