1992 рік

The kinetics of the polymerization to form the rod-like poly( 1,4-phenylene-2,6-benzobisthiazole) in the nematic phase are discussed. The kinetics are not affected by a phase transformation from isotropic to nematic, provided that transformation occurs early in the polymerization. Suppressed diffusion rate parallel to the molecular axes appears to control the kinetics over much of the conversion, resulting in a constant rate of increase of the intrinsic viscosity with polymerization time. At high conversions, additional constraints due to restricted rotational diffusion may occur. The molecular-weight distribution is close to most-probable

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The results of F Ti.r. studies of blends of poly (N-vinyl pyrrolidone) (PVPr) and poly (methyl methacrylate) (PMMA) with poly(diglycidal ether of bisphenol-A) (epoxy) are discussed. The changes observed in the OH and carbonyl stretching frequency region of the blend compositions studied are discussed in terms of hydrogen bonding interaction between the OH groups of epoxy resin and carbonyl groups of PVPr and PMMA. Qualitative as well as quantitative data have been obtained about these interactions and are compared. The FTi.r. results indicate a high degree of mixing at the molecular level for both blend systems at all compositions studied. It is observed that in the PVPr/epoxy blend system the strength of the hydrogen bonding interaction between the OH groups of epoxy and C=O groups of PVPr is much stronger than the interaction between the self-associated OH groups in pure epoxy resin. A more uniform hydrogen bonding between the OH groups of epoxy and C=O groups of PMMA was observed and is also discussed

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The mechanical properties of mineral-filled polypropylene (PP) are determined not only by the size, shape and modulus of the filler particles, but also by microstructure. For example, poor impact strength is correlated with a high capacity for nucleation of crystallization. In the present study, optical microscopy has been used to measure the growth rate of spherulites in PP. In tandem with this, isothermal d.s.c. measurements have been made of the crystallization of the PP filled with talc, calcium carbonate and stearate-coated carbonate at different loading levels. Computer simulation of spherulite growth has been used to derive the number of nucleating sites per unit volume of polymer and, using surface area measurements, the number of sites per unit area of mineral surface was obtained. Values for talc were, as expected, considerably higher than those for carbonate (and especially coated carbonate) fillers. The presence of filler affects not only the nucleation and kinetics of the crystallization process but also the crystallinity and orientation indices and the proportion of//-phase crystallites present. The connection between these factors and impact strength is discussed. It is concluded that impact properties are determined by inter alia crack pinning and blocking by filler particles, stress concentrations at the edges of the filler particles, and the nucleating ability of the filler

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A systematic study using differential scanning calorimetry has been performed on the annealing behaviour of an aromatic polyetherimide (Ultem 5001). Although crystallization from the melt did not occur, crystallinity was easily induced in the presence of methylene chloride. With annealing, a sharp melt endotherm of the solvent-treated polyetherimide was observed. The melt temperature determined after annealing for 30 min increased progressively as the annealing temperature increased, leading to an extrapolation to an apparent equilibrium melt temperature of 369°C. However, the melt temperature, as well as crystallinity, increased with the increase of the residence time at each annealing temperature. X-ray diffraction data revealed two distinct crystalline phases; a low temperature (~) phase obtained by crystallization for 30 min at 248 and 258°C, and a high temperature (fl) phase obtained at 258°C for 3 h or exceeding 258°C. Values of heat of fusion per gram of crystallite were consistent with a range of 247-261 J g-1 for the samples of higher crystallinity

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With potassium persulphate as initiator and sodium dodecyl sulphate as emulsifier, the batch emulsion copolymerization rate behaviour of styrene (S) and methyl acrylate (MA) was investigated at SO”C, varying the monomer ratio and the monomer to water ratio. The composition drift occurring during the copolymerization is determined by the reactivity ratios and by the monomer partitioning between the various phases present in the emulsion system. Monomer partitioning studies show that the monomer ratio in the latex particles is equal to the monomer ratio in the droplets, although the total monomer swellability of the (co)polymer latex particles depends upon copolymer composition and monomer droplet composition. In the absence of monomer droplets the equilibrium monomer concentration in the aqueous phase is closer to its saturation value (i.e. water solubility) than the monomer concentration in the latex particles. As a consequence of the composition drift, the kinetic behaviour differs widely from the homopolymerizations depending upon the initial monomer ratio. The copolymerization rate strongly varies during copolymerization resulting in a conversion-time plot differing in shape from the sigmoidal shape usually observed in emulsion homopolymerization. The penultimate effect in S-MA copolymerization has been proved to be responsible for this, as can be seen clearly from the conversion dependence of the copolymerization rate during batch emulsion copolymerizations starting from an MA-rich monomer feed, leading to a strong composition drift.

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Blends of the aliphatic polyester poly(e-caprolactone) with two biphenyl-containing polyesters have been investigated. The aromatic polyesters differed in the number of phenyl groups incorporated in the repeating unit; in this way a semi-rigid liquid crystalline and a semicrystalline polyester were obtained. Blends of the aromatic polyesters with poly (e-caprolactone) were prepared by the dissolution/precipitation method and characterized by optical microscopy, d.s.c., small-angle laser light scattering and X-ray diffraction. A marked difference in crystallization behaviour and crystallinity was observed, although melting and crystallization temperatures were not influenced by changing the composition of the blends. The experimental results indicate the occurrence of miscibility phenomena in the isotropic state of blends of poly(e-caprolactone) with the most flexible biphenyl-containing polyester.

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Experimental results on the effects of specimen thickness and environmental temperatures on fatigue fracture behaviour of poly (ether ether ketone) (PEEK) are reported. Low cycle fatigue experiments are conducted on injection moulded single-edge notched specimens of 1.57, 2.70 and 5.42 mm in thickness at ambient temperatures, and on specimens 2.70 mm thick at environmental temperatures of 39, 50, 63, 75 and 100C. In all the thickness experiments and in the experiments with temperatures of 39 and 50'~C, the crack tip profile is initially round. At long crack lengths the crack tip profile changes to a triangular shape. When the test temperature is 63, 75 and 100°C, the crack tip remains round throughout the fracture process. The crack tip angle is primarily dependent upon the test temperature. Examinations of the fracture surfaces and transverse sections indicate that in the thickest specimen, relatively rough fracture surfaces are observed and a few discontinuities (crazes or cracks) underneath the main crack path. Thus, crack propagates in a 'brittle' manner. In all other experiments both 'brittle' and 'ductile' modes of fracture are observed. The point of transition from 'brittle' to 'ductile' fracture is dependent upon the specimen thickness and test temperature. Fatigue striations are seen throughout the fracture surfaces. Correlation of the striations and the number of cycles indicates a one-cycle crack growth mode. Hysteretic losses during fatigue crack growth are negligible until a few cycles prior to unstable fracture. Crack opening displacements are independent of the specimen thickness and increase with rise in temperature. When crack growth rates are correlated with the elastic energy release rate, they are independent of specimen thickness and increase with increase in temperature.

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The mechanical and thermal properties of the solution-cast films of chemically prepared polyaniline have been studied by means of dynamic mechanical analysis (d.m.a.), differential scanning calorimetry (d.s.c.) and thermogravimetric analysis (t.g.a.). The glass transition temperature (Tg) of the polyaniline films in emeraldine base form was determined and characterized for the first time to be in the range of ~ 105-220°C for the films containing ~ 16-0% of 1-methyl-2-pyrrolidinone (NMP) residual solvent, respectively, from a decrease in the storage modulus in d.m.a, and from d.s.c, studies. Depending on the content of NMP, the thin films of polyaniline have a storage modulus of ~ 200 MPa-2 GPa at temperatures below Tg. Other thermal transitions and reactions in the temperature range of ~ 25-500°C are also characterized by infra-red spectroscopy, X-ray diffraction and cyclic voltammetry and are discussed in relation to the mechanical properties of polyaniline.

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Cloud-point data to 160°C and 2000 bar are presented showing the effect of cosolvents on the phase behaviour of poly(ethylene-co-methyl acrylate) (EMA) (64 mo1%/36 mol%) with propane and chlorodifluoromethane (F22). Ethanol shifts the EMA-propane cloud-point curves to lower temperatures and pressures, but above _ 10 wt% ethanol, the copolymer becomes insoluble. Up to 40 wt% acetone monotonically shifts the EMA-propane cloud-point curves to lower temperatures and pressures. Acetone and ethanol both shift the cloud-point curves of EMA-F22 mixtures in the same monotonic manner for cosolvent concentrations of up to 40 wt%.

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A high-resolution 13C n.m.r, spectrum of soluble polyaniline in DMF-d7 solution was recorded. The assignment for the various resonance peaks in the spectrum was tentatively performed and the chain structure of polyaniline was analysed. It has been shown that the main chain of pristine state polyaniline is composed of alternating benzoid-quinoid and successive benzoid-quinoid sequences with the former being present in greater concentration. The sequence distribution is random. In addition to the benzoid-type and quinoid-type structures, there is a small amount of other structural units in the main chain.

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