Sadao Hibi

Molecular orientation behaviour of uniaxially stretched poly(vinyl chloride) film: 1. Birefringence: effect of plasticizers and draw ratio above and below Tg☆

Abstract In order to investigate the orientation behaviour of a non-crystalline chain polymer, plasticized and unplasticized poly(vinyl chloride) films (PVC) are chosen for investigation. The following two orientation distribution functions are postulated to calculate the second moment of the molecular orientation: (a) for the unplasticized film drawn below the glass transition temperature the distribution function derived from an affine deformation mechanism is applied; (b) for the all plasticized films or for the films drawn above the glass transition temperature a compound distribution function derived from the affine deformation mechanism and that of a rubber-like chain (Langevin model) proposed by Roe et al is applied. The parameters which are included in the distribution function, viz. the ratio of the affine part to the Langevin part, the ratio of polyene to carbonyl in a heat treated film and the segment number of the Langevin chain, can be evaluated experimentally. The intrinsic birefringences of normal and heat treated unplasticized and plasticized films are estimated theoretically on the basis of additivity of band polarizabilities. The calculated birefringences and orientation distribution functions show good agreement with measured values.

Effect of molecular weight on rolled high density polyethylene: 1. Structure, morphology and anisotropic mechanical behaviour

Abstract Rolled high density polyethylene with various molecular weights was used to study the effect of primary structure on the properties of the material. Keeping the molecular weight distribution at 2, and with similar processing conditions, it was found that the structure, morphology and anisotropic mechanical behaviour depended strongly on the molecular weight. The low molecular weight samples possessed high modulus and yield stress in the roll direction and showed brittle fracture in the direction perpendicular to the roll direction. The high molecular weight samples showed high modulus and yield stress in the direction perpendicular to the roll direction. The morphology of the internal surface showed a transitional change from fibrillar structure for low molecular weight samples to a smooth surface for high molecular weight samples. Wide angle X-ray diffraction revealed that, for samples with high molecular weight, the crystallographic axes were selectively correlated with the coordinate axes used in defining the samples. Small angle X-ray scattering showed four-point patterns for high molecular weight samples. It is suggested that samples with high molecular weight possess more entanglements among the tie chains connecting the lamella blocks. This results in higher orientation of the three crystallographic axes, a , b , and c , along the thickness, transverse and roll directions of the sample, respectively. Entanglement also reduces the anisotropic behaviour in the roll-transverse plane.

Molecular orientation behaviour of uniaxially stretched poly(vinyl chloride) film: 2. The fourth moment of molecular orientation measured by the polarized fluorescence method☆

Abstract In order to measure the fourth moment of molecular orientation of poly(vinyl chloride) film a new polarized fluorescent method is examined. The fundamental principle of the method is to employ the polyene and the carbonyl radicals formed in the molecular chain, at the working process or with heat treatment, instead of a fluorescent dyestuff. It is necessary, in this method, to estimate the ratio of polyene to carbonyl in the sample, and the value is found to be 1.5 for unheat treated PVC film or 1.0 for heat treated PVC film. The second moment 〈cos 2 ϑ〉 and the fourth moment 〈cos 4 ϑ〉 are evaluated from the fluorescence intensity pattern. The applicability of this method is verified by comparison with the second moment derived from the birefringence. Dependence of deformation mechanism on draw temperature and plasticizer content are also investigated on the basis of a structural model composed of the characterized molecular chain, the practicability of which was already established by observations of the change in birefringence with draw ratio.

Orientation behaviour of crystallites in cylindrical polyethylene rods under tension-torsion combined stress

Abstract A tension-torsion combined stress was loaded onto cylindrical rods of polyethylene to investigate the deformation mechanism of the crystalline phase under non-uniform stress. On applying the combined stress, a neck occurred at the centre of the sample and extended in the axial direction. After necking occurred, the twist deformation generated compressive stress in the radial direction, which suppressed the formation of micro-voids. Wide-angle X-ray diffraction revealed the following facts. The c axis, i.e. the crystal molecular chain axis, deviates from the axial direction owing to the combined loading of the tensile force and the shear force. The b axis, which is the long axis of the initial lamellae, orients selectively in the radial direction in the non-uniform stress state in which the shear stress increases in proportion to the distance from the centre of the cylindrical rod.

Effect of molecular weight on rolled high density polyethylene: 2. Fracture

Abstract In a previous paper, the effect of molecular weight on the structure, morphology and anisotropic behaviour of high desnity polyethylene (HDPE) has been studied. Entanglements among tie chains connecting the lamella blocks were believed to be the factor that initiated the intrinsic differences. A further study of the fracture behaviour was carried out. It was found that samples with low molecular weight showed brittle fracture with only a small amount of fracture energy being consumed. For high molecular weight samples, a large plastic deformation was associated with the crack extension. The energy consumed for crack extension was almost proportional to the molecular weight. Further stretching of the samples after rolling resulted in a different fracture mechanism and samples showing ductile fracture changed into brittle failure. The morphology of the fracture surface was also studied. It was found that two types of fracture surface existed even for samples with brittle fracture. The fractography was strongly related to the fracture energy. The fracture surface of the ductile failure type samples showed a fibrillar structure. It is suggested that peeling the plastically deformed part off the undeformed surface was the fracture mechanism for the high molecular weight samples. The fracture energy of HDPE without the effect of entanglement was obtained by extrapolating the fracture energy down to zero molecular weight to yield a value of ∼370 Jm−2.

Evaluation of the plastic zone at the crack tip of strain hardening low density polyethylene

Prediction of the plastic zone in front of the crack tip has been done using a modified Dugdales model for strain hardening low density polyethylene. By knowing the strain hardening index n, yield stress σY, crack length a, and average stress acting far away from the crack σ∞, the size of the plastic zone, rP, can be calculated. Simulation seems to be promising for the ratio of rP to the ligament length, b, falling in the range of 6–24%. If rPb is 24%, the plastic zone becomes so large that the HRR stress singularity together with the boundary condition used in this simulation may lose its significance and the resultant stress distribution deviates from the real situation. A detailed evaluation of the stress function inside the plastic zone is needed before further comment can be made.

Molecular orientation behaviours of uniaxially stretched poly (vinyl chloride) film: 3. Effect of molecular orientation on anisotropy of Young's modulus in uniaxially stretched poly (vinyl chloride) film

Abstract Using uniaxially stretched, unplasticized PVC film as an example of non-crystalline polymer film, an aggregation model is assumed consisting of a structural unit having an elastic compliance of the same value as that of an ideal sample, uniaxially stretched and with orientation of the structural unit that conforms to that of the molecular chain axis. By using orientation factors evaluated from polarized fluorescence measurements the anisotropy of Youngs modulus on the surface of the sample is investigated. The results of theoretical calculation explain the elastic anisotropy observed. The dependence of the anisotropy of Youngs modulus on draw ratio can be specified by the molecular orientation regardless of the temperature, above or below the glass temperature, at which the sample was stretched.

pH response of coloured copolypeptide hydrogel containing tryptophan treated with trifluoroacetic acid

Abstract The coloration phenomenon of l -tryptophan (Trp) treated with trifluoroacetic acid (TFA) was observed under varying pH, i.e., from yellow (above pH 5.5) to red (below pH 4.0). In order to elucidate the coloration mechanism, the coloured compounds were fractionated and investigated spectroscopically. It was found that Trp forms a yellow compound consisting of a tricyclic structure, and that the change in colour with the variation of pH is due to the dissociation of the nitrogen atom in an indole ring. Coloured copolypeptide hydrogel containing Trp residues was also prepared by a method similar to that for Trp. The colour change was also observed for the hydrogel. The pH dependence of such membrane properties as degree of swelling, solute permeability, and the mechanical property was investigated. These properties changed drastically in the pH region of the colour change. This can be explained by the dissociation of Trp residues.

Fracture energy analysis of single-edge-cracked isotropic ductile polyolefins

Abstract Evaluation of energies for crack initiation and propagation is performed on isotropic low density polyethylene (LDPE) and polypropylene (PP). Loading-unloading loops were used to calculate the energies involved during cracking. For PP, an uncracked sample shows necking upon stretching and exhibits high toughness. It was found that, when loaded, the cracked PP sample also showed necking from the crack tip. As a consequence, a crack propagated inside the fully plastic deformed domain. While uncracked LDPE undergoes uniform deformation with strain hardening upon loading, no necking phenomenon was observed at the crack tip of the cracked sample. As the crack propagated, it had to go through the ligament. We suggest that the ligament area is an important fracture parameter in evaluating the fracture energy. In both cases, energies were divided by the ligament area before being related to the crack length increment. Thus, the potential necessary to create unit crack propagation is represented as energy per unit ligament area. As our results show, the fracture resistance of PP is higher than that of LDPE and this is related to the difference in toughness of these two materials.

Finite element analysis of single-edgecracked rolled high-density polyethylene

Abstract Stress and strain distributions around the crack tip of rolled high-density polyethylene have been analysed using the two-dimensional finite element method. Mechanical parameters from two samples were used. The highly anisotropic sample showed distinct differences in Youngs moduli and yield stresses versus various off-axis angles while the sample of low anisotropy showed high moduli and yield stresses in both the roll and the transverse direction. Calculated results revealed a strong relation between the stress distributions at the crack front and the anisotropy of the materials. The sample with high anisotropy showed intense stress distributions coincident with the chain axis. In contrast, stress distributions of the sample with low mechanical anisotropy were less affected by the alignment of the chain axis. From experiment, kinky deformation was observed for some cases. Comparison of the experimental with the calculated results threw light on the mechanism of formation of a kink band during restretching of anisotropic polymers.