1994 рік

Dielectric measurements on three dodecyloxy-substituted poly(p-phenylene terephthalate)s are presented and compared with dynamic mechanical measurements reported earlier. The results are interpreted on the basis of a molecular laminate model for the structural arrangement, i.e. main-chain layers separated by partially ordered side chains. Most of the observed relaxations are assumed to originate from side-chain motions. The low-temperature y relaxation around - 160°C is ascribed to local motions of the CH, units in the side chains. The b relaxation around - 35°C involves a cooperative motion of the amorphous parts of the side chains, while the B* relaxation just above room temperature is supposed to occur mainly in the crystalline parts of the side chains. Although some main-chain motions may already be involved in the relaxations at lower temperatures, they fully account for the a relaxation observed around 120°C

Детальніше...  

The reaction between 'living' t-butyl acrylic anions and benzaldehyde (BZA) is shown to be highly selective. Advantage is taken of this fact in order to prepare well defined hydroxyl-terminated poly(t-butyl acrylate) (tBuA) samples. When a dialdehyde such as terephthalaldehyde (TAP) is used in place of BZA, both aldehyde functions of the electrophilic reagent participate in the deactivating reaction of the acrylic anions. Multiblock copolymers consisting of polyvinylic and poly(t-butyl acrylate) sequences can therefore be obtained by stoichiometric coupling reactions between the 'living' triblock copolymer and terephthalaldehyde.

Детальніше...  

The effect of crosslink density in interpenetrating polymer networks (IPNs) with controllable hydrogen bonding composed of poly(butyl acrylate) (PBA) and polystyrene modified with hydroxyl-containing units [PS(OH)] on miscibility and morphology is studied. The results from d.s.c, and transmission electron microscopy (TEM) show that the crosslink density of PBA displays a two-fold effect on miscibility of the IPNs. For the IPNs made from immiscible polymer pairs with low hydrogen bonding density, crosslinking improves the degree of mixing, while for the IPNs composed of miscible polymer pairs due to hydrogen bonding, crosslinking causes microheterogeneity. An apparent discrepancy between the results of dynamic mechanical analysis and d.s.c, or TEM is observed and explained.

Детальніше...  

High density polyethylene (HDPE) was drawn at 80°C with draw ratios from 1 to 3.5. The resistance to slow crack growth was measured on notched tensile specimens under plane strain conditions and over a range of temperatures. The notches were made parallel or perpendicular to the direction of orientation. The failure times for crack growth perpendicular to the orientation direction varied exponentially with the draw ratio, and for crack growth parallel to the orientation direction the time of failure decreased to zero for a draw ratio of about 1.5. The activation energy was 110 kJ tool- 1 and was independent of the draw ratio.

Детальніше...  

The effect of exposing conducting polypyrroles to elevated temperatures on the electroactivity of the polymer has been considered. In all cases the redox properties were improved with mild heating but were degraded at higher temperatures. Thermal gravimetric analysis and dry state conductivity measurements have been used to obtain an understanding of the results observed.

Детальніше...  

The structural development during radical polymerization of a mixture of methyl methacrylate (MMA) and poly(ethylene-co-vinyl acetate) (EVA) was investigated by light scattering and optical microscopy. In the early stage of polymerization, the mixture of MMA/EVA was a single phase. As the polymerization of MMA proceeded, phase separation took place via spinodal decomposition (SD). This was supported by the characteristic change in light-scattering profile with reaction time. Analysis by the linearized and scaling theories showed that the reaction-induced phase separation behaviour was different from the familiar thermally induced SD during isothermal annealing after temperature jump. The scaling analysis showed that the final phase-separated structure may be formed by the interruption of a co-continuous percolation structure. Moreover, to investigate the effect of graft copolymer on SD, a poly(vinyl acetate) (PVAc) macromonomer with a methacrylate end-group was added to the MMA/EVA mixture to yield PMMA-PVAc graft copolymer during the polymerization. The incorporation of PVAc macromonomer increased the dominant wavenumber of the final frozen phase-separated structure (smaller periodic distance). The reasons seem to be that the concentration fluctuation growth is suppressed until the system reaches a deep quench depth at which the fluctuation should have a large wavenumber, and that the coarsening is retarded at the late stage by the graft copolymer formed in situ.

Детальніше...  

The fl relaxations of novolac phenol-formaldehyde (NPF) resin were investigated by whole and partial thermally stimulated depolarization current (t.s.d.c.) measurements in the temperature region from 137 to 270 K. The whole measurements showed two peaks with maximum current at 158 and 187 K, respectively. The peaks obtained by partial polarization were characterized by the activation energy (E,) and relaxation time at the temperature of the peak maximum. The relaxation map and the compensation diagram were drawn. The mean E a for both relaxations is 0.51 eV. The initial part of f12 obeys the compensation law with T~ = 308 K, 26 K below T r The appearance of extremely low convergence of the Arrhenius lines and thus irrational compensation points due to the narrow width of the activation energy (AE,) were discussed. The relaxations are supposed to be caused by the motions of the phenyl rings around the methylene links.

Детальніше...  

Differential scanning calorimetry (d.s.c.) confirms the greater stability of deuterated polystyrene compared to normal polystyrene with respect to oxidation in air. However, the same technique shows that the stability of deuterated cis-polyacetylene with respect to oxidation in air is no better than that of normal cis-polyacetylene. Our d.s.c, results also suggest that there may be a weak isotope effect for the hydrogen migration reaction under nitrogen but not for the isomerization of deuterated cis-polyacetylene to the trans isomer.

Детальніше...  

Polyimide oligomers (prepolymers) and resins of the PMR-15 type have been prepared from 5-norbornene- 2,3-dicarboxylic half acid ester (NE), 3,3',4,4'-benzophenonetetracarboxylic diester (BTDE) and a series of diamines including, and related to, 3,3'-diaminobenzophenone, viz. 3,3'-diaminobenzhydrol, 3-[(m,m'- diamino)benzhydryloxylpropyne (MDA-OMA) and 3,3'-diaminodiphenyl-~t,~t'-difluoromethane (FMDA). The PMR synthetic pathway (monomers~prepolymer--*resin) has been assessed by dynamic mechanical thermal analysis. The T~ values for resins incorporating BTDE are significantly lowered compared to that for PMR-15 itself. The fluorine-containing PMR-15 type of resin (from FMDA) is of higher thermal stability than PMR-15 itself.

Детальніше...  

New high-temperature adhesives based on polyisoimide (PII) have been developed. PIIs were prepared by the ring-opening polyaddition of 4,4'-hexafluoroisopropylidenebis(phthalic anhydride) and 4,4'-[hexafluoroisopropylidenebis( p-phenyleneoxy)]dianiline or 4,4'-[isopropylidenebis(p-phenyleneoxy)]dianiline, followed by treatment with trifluoroacetic anhydride-triethylamine in N-methyl-2-pyrrolidone. Compared to those of polyimides (PIs), the glass transition temperatures (T~) of PIIs were lower and the elasticities were greatly reduced above their Tg. Furthermore, PIIs showed stronger adhesion to copper foils. The interfacial surface morphologies of PII films after adhesion testing were investigated by scanning electron microscopy and energy-dispersive X-ray analysis. The results suggested that the good adhesion bond strength was produced by favourable anchoring between PII and copper foil due to a good flow of PII.

Детальніше...