1999 рік

The structural changes occurring when amorphous cold-drawn poly(ethylene terephthalate) films are annealed at different temperatures (508C–2408C) for different annealing times (10–10 4 s) were investigated by means of X-ray diffraction and microhardness techniques. The X-ray results reveal the appearance of smectic order at 608C with a period of 10.7 Å. At 708C, a layer structure in the scale of 110 Å emerges. Finally, triclinic order is observed above 808C. The appearance of a layer structure prior to the development of triclinic crystals is associated with a density difference along the molecular direction produced by a molecular tilting mechanism. The microhardness behaviour of annealed cold-drawn PET films is correlated to the developing morphologies. At high annealing temperatures (.1008C), the plastic component of hardness is shown to vary with the occurring microstructural changes. Results indicate that the hardness of the amorphous intrafibrilar regions is higher than that of a fully amorphous material. The indentation anisotropy, DH, which is related to the elastic recovery of the material shows a conspicuous decrease at Ta t 708C, which is explained in terms of a relaxation of the fibrils in the chain direction.

Детальніше...  

We examined the mechanical characteristics of four major ampullate (MA) dragline silks during and after submersion in a range of solvents. The silks were reeled from four very different spiders: Araneus diadematus, Nephila edulis, Latrodectus mactans and Euprosthenops sp. They displayed significant differences in behaviour in the native state as well as during and after supercontraction in solvents such as water, urea solution and a set of alcohols. The different polarities of the solvents are thought to affect different regions of the silk’s molecular conformation. We hypothesise that the observed mechanical properties of dragline silks are those of a hard elastic polymer; and we explain the supercontraction of the silks as changes of orientation in the molecular chains.

Детальніше...  

A model combining both the thermodynamic and kinetic aspects of free-radical crosslinking copolymerization (FCC) is presented to predict the formation conditions and the properties of heterogeneous (porous) networks. The model involves thermodynamic equations describing the phase equilibria between the network and separated phases during FCC of vinyl/divinyl monomers and kinetic equations giving the concentration of reacting species and the polymer properties as a function of the monomer conversion. Calculation results are presented for styrene/m-divinylbenzene (S/m-DVB) copolymerization system in the presence of inert diluents. S/m-DVB copolymerization system at a high m-DVB concentration, or, at a low monomer concentration phase separates at the gel point and results in the formation of a microgel solution. The calculation results also show that the heterogeneity in S/m-DVB copolymer networks increases on increasing DVB or diluent concentration, or, on decreasing the solvating power of the diluent, in accord with the experimental data published previously. The model also predicts correctly the equilibrium volume swelling ratio of heterogeneous networks in solvents.

Детальніше...  

Surface segregation of components in semi- and full interpenetrating networks based on cross-linked polyurethane, poly(esteracrylate), poly(butyl methacrylate) and poly(vinyl acetate) has been studied at the interfaces with solids of high and low free surface energy. The method of attenuated total reflectance in IR spectra was used to estimate the composition of the system near the interface. It was found that the composition of the surface layers changes as compared with that of initial interpenetrating polymer networks (IPN) depending on the nature of the surface. It was also established that the surface segregation depends on the kinetic conditions of the reaction and on the method of curing. The latter effects are connected with microphase separation proceeding simultaneously with curing.

Детальніше...  

Uniaxial stress–strain measurements have been performed on two glassy polycarbonates and one polyimide in an atmosphere of CO 2, H 2O-vapour or acetone-vapour. The yield stress has been determined as a function of the concentration of sorbent molecules. In all observed cases the polymer is softened due to the sorption. The depression of the yield stress at a given concentration is dependent on the size of the sorbent molecules and of interchain volume sites within the polymer. For H 2O and CO 2 a unique relation is found between the softening and the volume dilatation accompanying the sorption. This dilatation has been associated with an internal isotropic stress via Hooke’s law. That enables us to explain the softening as a consequence of the well known dependence of the yield stress on the isotropic stress component.

Детальніше...  

The physical state of water, absorbed in silk fibroin (SF)/S-carboxymethyl kerateine (SCMK) blend films, was studied by differential scanning calorimetry (DSC) to investigate the change of internal structures of the blend films. The amount of non-freezing bound water in the blend film was decreased with the addition of SCMK because of the change in the secondary structure of SF from random coil form to bstructure form. This transition could be also monitored by thermal analysis using DSC. The maximum change of the secondary structure was observed at a blend ratio of 50/50 (wt/wt).

Детальніше...  

Granules of high density polyethylene grafted with 0.17 wt.% maleic anhydride (PEMA) were modified with an excess of 1,4-diaminobutane (DAB) by impregnation from near critical propane. After formation of amic acid groups, the excess of diaminobutane was extracted with a near critical propane–ethanol mixture (95/5 wt.%). Finally, the obtained PEMA–DAB was imidised quantitatively to the corresponding imide (PEMI) in the melt. The obtained PEMI showed no increased gel content with respect to the PEMA. The presence of primary amine groups was indirectly proven by selective extraction experiments. It appeared that PEMI samples had reacted with the anhydride groups of styrene-MA copolymer (SMA) during melt blending of SMA with PEMI, while the PEMA had not reacted. SMA/PEMI 80/20 blends consisted of a continuous SMA phase and PEMI droplets with a diameter of less than 1 mm. SMA/PEMA 80/20 blends showed a course morphology of PEMA strings in a continuous SMA phase. With this article we have shown that this new technique for the chemical modification of swollen HDPE particles in near critical propane has proven to be much better than the conventional modification in the melt, when it comes to avoiding crosslinking.

Детальніше...  

High char yield polybenzoxazines are obtained from acetylene-functional benzoxazine monomers. Polymerization of acetylene functional group, in addition to oxazine ring opening polymerization, contributes greatly to the exceptional thermal stability. A significant change in char yield is found for these polybenzoxazines by different polymerization environments. The polymerization behaviour of these compounds under air and nitrogen atmosphere has been studied by Fourier transform infra-red spectroscopy, ultra-violet–visible spectroscopy, differential scanning calorimetry and proton nuclear magnetic resonance spectroscopy.

Детальніше...  

The viscoelastic properties of microbial biodegradable copolymers containing 3-hydroxybutylate (3HB) and 3-hydroxyvalerate (3HV) were investigated as well as the reference homopolymer, poly(3-hydroxybutylate) (PHB), by plotting the first normal stress difference (N1) versus shear stress (t s) and the storage modulus (G 0 ) versus the loss modulus (G 00 ). At low shear rates, the N1s of the copolymers were higher than that of PHB, and at high frequencies, the G 0 s of the copolymers were lower than that of PHB. To examine the rheological properties of biodegradable copolymers, we used a modified Cole–Cole plot (N1 versus t s and G 0 versus G 00 ). From these plots, we found that the elastic behavior from steady shear experiment is closely related to that of the dynamic oscillatory experiments for the biodegradable copolymer.

Детальніше...  

Polysulfone gas separation hollow fiber membranes were manufactured using a dry/wet spinning process with forced convection in the dry gap. Hollow fibers were produced using two different bore coagulants: one pure water and one with reduced water activity. An optimized multi-component dope was used which proved to be a shear-thinning power-law fluid. For each bore system, membranes were spun at low and high dope extrusion rates (DERs) corresponding to shear rates of around 4000 s 21 and 10 000 s 21, respectively, at the outer spinneret wall. Plane polarized infrared spectroscopy was used to probe the membrane active layer. Pressure-normalized fluxes and selectivities were evaluated using pure carbon dioxide and methane. For the spinning conditions used here, the combination of reduced water activity in the bore and high DER produced highly selective membranes. Some selectivities reached about three times the recognized intrinsic value for the polymer.

Детальніше...