1992 рік

Surfaces of ultra thin films of isotactic polystyrene (iPS) drawn from the melt were investigated by scanning force microscopy (SFM). SFM micrographs of iPS surfaces exhibit the morphology of uniaxial orientation resulting from the preparation technique as well as structures pseudomorphic to regularly arranged shish crystals. Their diameters were -60% larger compared to earlier results obtained from dark-field transmission electron microscopy investigations. Furthermore, different defects of the polymer surface morphology were observed. It is supposed that macromolecules of a lower orientational order cover the shish crystals (hair dressing model)

Детальніше...  

In this communication we demonstrate the feasibility of determining the orientational distribution functions of various morphological components in ultrahigh molecular weight polyethylene fibres by two-dimensional rotor-synchronized 13C cross-polarization/magic angle spinning nuclear magnetic resonance (n.m.r.) techniques. For comparative purposes we have also included a drawn polyethylene fibre of normal molecular weight. The results showed that these fibres contain two crystalline components, designated C~ and C2, and an amorphous (non-crystalline) component, A. Order parameters and orientational distribution functions of these components can be obtained through analysis of the two-dimensional ' 3C n.m.r, spectra.

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A computer-controlled bending system has been constructed for characterizing the bending properties of commercial membranes composed of polymer-bitumen blends supported by a non-woven polyester type base. The system allows the bending elasticity and yield point to be obtained for a membrane, and also permits the stress relaxation after a distortion to be characterized. These properties are related to the polymer present in the bitumen.

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Butyl acrylate (BA) - styrene (ST) copolymerization was carried out in bulk without any free-radical initiator at 110, 150 and 170°C. The Kelen-Tudos (K-T) transformation of comonomers-copolymer composition data shows that the copolymerization can be sufficiently described by a terminal copolymerization model. Reactivity ratios for BA (r,) and ST (r2) were estimated by the K-T method as well as by the error-in-variables method. Together with previous reports on BA-ST copolymerization at 50 and 60°C, these values of rl and r 2 served to find the Arrhenius relationships: In r 1 = 1.3510- 1034.1/T, and In r2 =0.05919-131.6/T, where T is absolute temperature. Thus, although r, can be strongly affected by temperature, r 2 is practically temperature insensitive. This can be explained by the activation energy differences between homo- and cross-propagations (8.6 and 1.1 kJ mol- 1, respectively). Effect of conversion on copolymer and comonomers composition is also discussed.

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This communication describes the application and extension of a method for calculating the helical parameters with which to describe molecular conformations. The method, which was originally developed by Shimanouchi and others, is applied to a polyimide of 3,3’,4,4’-benzophenonetetracarboxylic dianhydride (BTDA) and 2,2-dimethyl-1,3-(Caminophenoxy)propane (DMDA) which has eight torsional bonds in the chemical repeat unit. Discrete low energy states for these torsions were determined by Ramanchandran energy maps of sequential dihedral pairs or single bond torsional energy diagrams. The total number of possible low energy conformations for these states is 1152 including conformationally related isoenantiomorphs. The method conveniently generates the conformations for subsequent crystal structure packing and refinement. Consideration of these together with the X-ray data of Cheng and co-workers reduces the number to about 15 with about a 2/l conformation and a c axis ofapproximately49.2 A. Of these, about half appear to be good candidates for crystal packing.

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The electric field orientation of x-carrageenan in aqueous solution was investigated by the reversing-pulse electric birefringence method at 633 nm and 33°C. The observed field orientation was attributed to the fast-induced polarization of the counterions (K + ions) in the ionized polymer domain. Since the charge density parameter ~ of non-helical and elongated x-carrageenan is 0.66, the fast-induced ionic polarization is probably due to the counterions loosely bound by Debye-Hiickel type electrostatic forces, but not due to the condensed counterions.

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Rheology, viscoelastic measurements and morphological aspects of blends of a commercial liquid crystalline polymer (LCP) and a polyarylate of bisphenol A (PAr) are presented. Viscoelastic results reveal incompatibility between the components and in the case of 60 LCP/40 PAr blends the physical properties are different, depending on the mixing method. This is attributed to the relationship between rheology, morphology and properties. The influence of the composition and the draw ratio on the storage tensile modulus is also studied.

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The flow properties of polyelectrolyte solutions have been studied using narrow molecular weight polystyrene sulphonate (PSS) samples of different molecular weight. In salt-free solutions the viscosity behaviour was characteristically dependent on the molecular weight of the polyelectrolyte. For low molecular weight samples the simple classical behaviour was observed, the reduced viscosity of the solution increasing sharply on dilution. The shear dependence of viscosity was low in general, but increased on dilution in parallel with the increase of reduced viscosity. For high molecular weights the reduced viscosity reached a maximum on dilution, followed by a rapid decrease on further dilution of the solutions. The viscosity was strongly shear-dependent and approached infinity at low shearing stresses. At low concentrations the reduced viscosity of HPSS solutions was markedly higher than that of NaPSS solutions, the difference vanishing at higher concentrations. These findings indicate that the viscosity behaviour of salt-free polyelectrolyte solutions is essentially determined by strong electrostatic interactions between highly expanded polyion chains, with the possible formation of ordered quasicrystalline structures in dilute solutions.

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Numerical finite element analyses have shown how the sequence of initiation between the two main mechanisms of toughening observed in rubber-toughened epoxy polymers is strongly dependent upon the properties of the rubbery particle and, in particular, the value of Poisson’s ratio. A high Poisson’s ratio of the dispersed second phase favours the initiation of internal cavitation before localized shear yielding of the epoxy matrix.

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Lamellar crystals of poly(ethylene oxide) grown in toluene solutions were investigated by atomic force microscopy (AFM). Typical lamellae had a square habit with a thickness of 12.5 nm and an edge length in the range of0.01-0.2 mm depending on the crystallization time. On some lamellae large screw dislocations were observed which showed a growth face at the diagonal. When the crystals were exposed to air, the lamellae gradually disintegrated. AFM images of disintegrated regions showed that polymer chains were contracted in bundles which formed tree-like patterns. Diagonal features observed on both dislocation terraces and disintegrated lamellae support the existence of fold domains separated by the diagonals and the chain folding mechanism proposed by Kovacs.

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