1993 рік

The kinetics of the aqueous polymerizations of acrylamide and methacrylamide employing the cerium(IV) ammonium nitrate-sorbose redox pair under an inert atmosphere were studied. It was found that the rate of monomer disappearance is directly proportional to the concentration of sorbose and inversely proportional to the cerium(IV) ion concentration but depends on the square of the monomer concentration. The rate of cerium(IV) ion disappearance is directly proportional to the initial concentration of cerium(IV) ion and sorbose but independent of the monomer concentration. The effects of various additives, namely alcohols, inorganic salts and so on, on the rates of polymerization of acrylamide and methacrylamide were also studied. The viscometric average molecular weight My decreases on increasing the concentration of monomer and sorbose. A plausible mechanism based on experimental results is proposed

Детальніше...  

Dried gel and solution-cast films of atactic (at-) and isotactic (iso-)polystyrene/carbon disulfide (PS/CS2) systems were investigated by Fourier transform infra-red spectroscopy (FTi.r.) and the relation between gelation and crystallization was studied. Iso-PS gel films dried at gelation temperature were less turbid than homogeneous solution-cast films made at room temperature. No difference was found in the i.r. spectrum of dried gel and solution-cast films of at-PS/CS2. However, the i.r. spectrum of iso-PS dried gel film was different from that of solution-cast film. Comparison of these spectra indicated that spectral changes occurred at the bands near 1200, 560, 1085-1052 and 907-922cm-x. There were two bands at 909 and 922 cm-1 for solution-cast films, while for dried gel films there was only one which shifted from 912 to 921 cm-1 with increasing polymer concentration (10 to 40 g l -~) of the original gel. I.r. spectra of dried gel films of low concentration gels were like those of solution and quenched iso-PS samples. The intensity of the crystalline band at 983 cm- ~ decreased with decreasing iso-PS gel concentration. This means that gel containing no crystals should be formed at very low polymer concentration; this is consistent with our previous conclusion that gelation of the PS/CS2 system is independent of crystallization.

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The Lecyar model, derived by remodelling the McAllister model, is able to describe satisfactorily the viscosity-composition dependence for several blend systems exhibiting positive, negative and mixed deviations. In the present work, the model is applied to polycarbonate/acrylonitrile-butadiene-styrene blends and polyacetal (polyoxymethylene)/thermoplastic polyurethane blends, with the latter having an elastomer as one of the constituents. The model provides a good fit in both cases

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Complexation constants of divalent and trivalent metal ions with 2-acetamidoacrylic acid (4A) and its polymer (P4A) were determined by the modified Bjerrum method. P4A has carboxyl and acetamido (acetyl) groups. Although an additive effect of two functional groups was expected in complexing with metal ions, the complexation constants of P4A were essentially the same as those for poly(acrylic acid), while P4A has greater complexing ability than 4A. The interactions of P4A and 4A with Cu(II) and Mn(II) ions were studied using 13C n.m.r. The addition of Cu(II) ion to P4A and 4A solutions selectively broadens the a-carbon and carboxyi carbon signals, and the addition of Mn(II) ion broadens the acetyl and carboxyl carbonyl carbon signals. This finding suggests that Cu(II) and Mn(II) ions form five-membered and seven-membered chelate rings, respectively.

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Morphological alterations induced by plane-strain compression of semicrystalline poly(ethylene terephthalate) (PET) at 190°C were studied using small-angle and wide-angle X-ray diffraction techniques, polarized light microscopy and differential scanning calorimetry. Based on the observations, a scheme of texture development is outlined. An initially spherulitic morphology transforms into one that comprises stacks of fragmented crystalline lamellae with lamellar normals oriented towards the flow direction. After some initial deformation by interlamellar sliding in the amorphous material (100)[001] chain slip operated throughout the remainder of the deformation. Up to a compression ratio of 2.6 the (100)[001] chain slip mechanism orients lamellar normals towards the compression axis. Further deformation caused fragmentation of the thinned-out lamellae and subsequent reorientation of lamellar normals towards the flow direction. Pole figure analysis indicated cooperative activity of the (100)[001] chain slip and (100)[010] transverse slip during the later stages of texture development. The (100)[010] transverse slip mechanism controlled the orientation of the crystal structure in a plane orthogonal to the flow direction. There was no evidence of (010)[001] chain slip during texture development. At a compression ratio of 3.3 pole figure analysis revealed the existence of a dual unit cell orientation that renders an overall orthotropic symmetry to plane-strain compressed PET.

Детальніше...  

Methyl methacrylate (MMA)-methyl acrylate (MA) copolymerizations have been analysed over a wide range of conversions with the purpose of predicting the changes in copolymer composition and comonomer sequence distribution as a function of conversion. High resolution ~H n.m.r, spectra (200 MHz) of MMA-MA copolymers, prepared at two overall comonomer concentrations by free radical copolymerization in benzene solution at 50°C at low conversion, have been analysed in terms of comonomer composition, sequence distribution and stereoregularity. Reactivity ratios for the terminal model, statistical parameters, Pij, and co-isotacticity parameters, 0.11=0.23 and 0"=O'12=O'21=0.50, estimated at low conversion were used to describe the changes in copolymer composition and triad fraction intensities with conversion. The experimental sequence distribution intensities were in agreement with the estimated values. From the good agreement found between experimental and calculated values, it can be concluded that the terminal model, through reactivity ratios and Bernoullian statistics, provides a very precise description of copolymerization over the whole range of conversion and composition and overall concentration of monomers.

Детальніше...  

Advanced solid-state n.m.r, techniques using 1H spin diffusion are useful tools for the non-destructive investigation of polymer morphologies and domain sizes. Calibration measurements of the spin diffusion coefficients are reported in this communication, combining ~H spin diffusion with 13 C detection and electron microscopic measurements on a diblock copolymer of known morphology. The spin diffusion coefficients thus determined can now be used for quantitative determination of domain sizes in systems containing both rigid and mobile components.

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Chemical and physical tests were conducted to determine the effects of long-term exposure to nitrogen tetroxide on Kalrez 1045, a perfluorinated copolymer. The weight and Shore A hardness of samples from two different lots were determined after exposure at two different pressures, 7320 kPa (72.5 atm) and 101 kPa (1 atm), for times ranging between 1 and 80 days and for periods up to 21 days following their exposure. The largest changes occurred in the first 8 to 10 days of exposure. The weight increased between 15 and 30% and the Shore A hardness declined from 83 to 60. Both weight and hardness partially recovered to their original values after removal from liquid N204. However, samples exposed for more than 1 day never fully recovered. The chemical stability of the Kalrez as a function of time exposed to N204 was monitored with solid probe-mass spectrometry. The results indicated that there was no main chain scission for the first 30 days of exposure. Loss of the pendant groups from the main chain was observed within 16 h. Loss of the pendant group from the crosslink site monomer, and loss of the crosslink itself ocburred after 150 h of exposure to N20 4. The overall results indicated that Kalrez 1045 exhibited exceptional stability to N204, and seals manufactured from this material should be superior to the butyl rubber currently employed in oxidizer vessels of launch systems.

Детальніше...  

Ethylene-propylene block copolymer (B-PP) consists of two parts: propylene homopolymer and copolymer of ethylene and propylene. A method for determining the isotacticity, mmmm, of the propylene hornopolymer part in B-PP is proposed here, using only the 13C n.m.r, spectrum of B-PP. This is a conventional method to determine the isotacticity with high precision.

Детальніше...  

Analysis of the structure of nine polymers, in terms of the average size of free volume at Tso G for spin probe Tempo, reveals an empirical connection between this microstructural characteristic of matrices and the change in reorientational regime of the spin probe

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