Search

The microstructure and m of a material composed of an oriented lamella stack are described. The material used for these studies was an S-B-S block copolymer exhibiting a lamellar morphology. By processing this material, using a novel injection moulding procedure, large specimens can be produced in which the lamellar microphases are uniformly oriented. This provides a unique opportunity for studying the anisotropy of the mechanical properties of a well characterized lamella stack, in which the two phases are molecularly bonded together. The experimental data are compared with predictions derived from composite mechanics. Excellent agreement between theory and experiment is found and this provides information concerning Poisson's ratio for the rubbery phase.

Torsion pendulum measurements of the longitudinal shear modulus in 'single-crystal' samples of an S-B-S block copolymer are reported. These samples consisted of a hexagonal array of polystyrene cylinders embedded in a matrix of polybutadiene, with the cylinder axes uniaxially aligned. Sample morphology and associated anisotropic physical properties resemble those of a miniature fibre-reinforced system. The shear modulus calculated from the torsional stiffness by conventional means, assuming St Venant's principle, is found to depend upon the sample length/width ratio, the results for short specimens being double those for very long specimens. It appears that St Venant's principle expressed in its usual form is not applicable to materials possessing a large mechanical anisotropy. An alternative interpretation of experimental results is given by invoking the shear-lag theory of fibre reinforcement.

A lamellar composite is approximated by a stack of very long longitudinal strips, of rectangular cross section, of elastic material bonded to rigid strips on their upper and lower faces and extended in a direction perpendicular to these faces. The lateral faces of the strips are assumed stress-free. The stresses and displacements in the cross section are calculated using two-dimensional stress analysis giving solutions in Fourier series. The effective modulus of the material is calculated for a range of thickness/width ratios and compared wth literature values. The analysis has relevance for the deformation of lamellar composites in polymeric and other materials.

For pt.I see ibid., vol.8, p.34 (1975). The two-dimensional problem in infinitesimal elasticity of a series of rectangular blocks of two different materials interleaved in a stack of infinite extent, loaded along its axis is solved by two complementary techniques. These are (i) a Fourier series solution of the Airy stress function and (ii) a finite element calculation. Tables and graphs of representative calculations are given. The shape of the distorted layers is of barrel and pincushion type and the positions of the maximum stresses differ from the predictions of more elementary calculations. The analysis is applied to the prediction of elastic moduli and failure mechanisms in real lamellar polymers modelled by long strips, such as oriented linear polyethylene, oriented polychloroprene, and cis-polyisoprene and 'elastic-hard' fibres.

A powerful new experimental method of determining the dynamic mechanical behaviour of viscoelastic materials is described. The method, Fourier transform mechanical spectroscopy (FTMS), consists of superposing and deconvoluting discrete Fourier transforms obtained from a single stress relaxation experiment using a novel sampling scheme. The theory of the sampling scheme and the superposition of the transforms is described elsewhere. In this paper the authors present the results of using the new method on Nylon over the frequency range 5*10-4 to 20 Hz and temperatures from 25 to 110 degrees C. There is good agreement between the values of storage and loss moduli, obtained by means of the transform method, and those obtained by conventional means. An activation energy of 143 kcal mol-1 is obtained for the relaxation in this temperature range. The new method has considerable potential for the rapid determination of the mechanical properties of novel polymers and composites.

An ultrasonic critical-angle method and an ultrasonic resonance method have been successfully used to monitor the cross-linking reaction in amine-cured epoxide resins in both unfilled and filled states by measuring the ultrasonic velocity. Glass beads and glass fibre have been used as fillers to produce isotropic and unidirectional glass-fibre composites. The values of modulus measured on the isotropic composite were in agreement with an exact theory developed by Kerner. Theoretical results are also presented over a range of fibre loading showing how the modulus changes for the uncured and cured glass-fibre composites, with the fibre in a variety of geometrical arrays.

A method is described of improving the high frequency components of the spectrum derived by the fast Fourier transform. It is particularly applicable to the analysis of time series which decay with time such as in stress relaxation. The method involves the addition of components at the same frequency derived from fast Fourier transforms taken over a succession of increasing time intervals. A numerical example is given.

Spectral analysis of Johnson noise has been used to determine the ionic conductivity of the super-ionic polycrystalline material rubidium silver iodide. The paper demonstrates the direct accessibility of conductivity data, eliminating the need for an often complex model of the electrode-electrolyte interface.

The mechanical properties of a novel fibre-reinforced material formed from the segregated microphases of a three-block copolymer are described. Such a material was prepared by extruding a commercial polystyrene-polybutadiene-polystyrene block copolymer, followed by an annealing process. Low-angle x ray diffraction and electron microscopy revealed that the dispersed polystyrene phase was in the form of cylinders which form a hexagonal lattice with the cylinder axes parallel to the original extrusion direction. As expected, the resulting mechanical anisotropy was found to be very pronounced. In particular, the dependence of Young's modulus on the angle between the stress direction and the original extrusion direction was compared with the predictions of some current theories of fibre reinforcement based on hexagonal symmetry. Complete agreement with these theories can only be obtained if the Poisson's ratio of the polybutadiene matrix is taken as 037 compared with the bulk value of 050. The morphological consequences of this result are also discussed. Torsional modulus measurements were also conducted but led to unexpectedly large values compared with those derived from the Young's modulus data alone. This is undoubtedly associated with the extreme anisotropy exhibited by these samples and requires further detailed examination.

The extraction of a stainless steel wire embedded in an epoxy resin has been studied as a function of the embedded length. The models developed to elucidate the dependence of the debonding stress and the pull-out stress on the embedded length are applied successfully to the experimental results. The nonlinear variation of the debonding stress with the embedded length is caused by the distribution of shear stress along the wire. The variation of pull-out stress with embedded length is caused by the decrease of the frictional stress due to the Poisson contraction of the wire under tensile stress. The effect of wire contraction on the friction process was confirmed by using different initial wire tensions. The residual compressive stress at the interface due to resin shrinkage and the coefficient of friction were determined from the analysis of the pull-out process.