R. P. Kambour

Electron microscopy of crazes in polystyrene and rubber modified polystyrene: use of iodine-sulphur eutectic as a craze reinforcing impregnant

Abstract Iodine-sulphur eutectic has been impregnated into crazes in polystyrene and rubber-modified polystyrene to act as a reinforcing agent during microtomy. Craze filaments and holes are much larger than in sulphur-reinforced poly-(dimethyl phenylene oxide) crazes due to a physical aging process involving polymer flow. In polycarbonate crazes impregnated with sulphur similar effects are observed due to plasticization by the sulphur. Reinforcing impregnation and rubber-staining are complementary tools in studying crazing in rubber-modified polystyrene.

Compatibility and neutron scattering studies of mixtures of polystyrene with poly(2,6-dimethyl 1,4-phenylene oxide) and its brominated derivatives

Abstract Several narrowly disperse polystyrenes (PS) were mixed with three series of random copolymers of 2,6-dimethyl phenylene oxide (also known as xylenyl ether (XE)) and 3-bromo-2,6-dimethyl phenylene oxide (BrXE), each series having a fixed chain length. The critical BrXE comonomer concentration necessary to induce phase separation, xc′ was characterized for each blend series by a number of techniques. xc was extrapolated to a value of about 0.5 at infinite chain lengths of each polymer of the blend. This finding supports the negative heat of mixing of PS with PXE reported by other workers. Small-angle neutron scattering studies were carried out using small amounts of perdeuteropolystyrene in several matrices for which single phase behaviour had been shown to exist. The radius of gyration R2 and the second virial coefficient A2 were greatest in the pure PXE matrix reflecting the goodness of this material as a ‘solvent’ for PS. In matrices containing 50% PS 50% copolymer, A2 extrapolated to zero at xc ≥ 1 which was consistent with the phase behaviour of this series of blends.

Interaction of crazes with pre-existing shear bands in glassy polymers

Shear bands have been grown in bulk specimens of P3O(poly 2,6 diphenyl 1,4 phenylene oxide) and in thin films of two blends of polystyrene with poly(xylenyl ether). The subsequent interaction of crazes with these shear bands has been characterized by transmission electron microscopy. For the case of shear bands grown under the plane stress conditions of thin films, it is found that the bands act as preferential sites for craze nucleation. A fairly regularly-spaced array of short crazes grows within the shear bands and these crazes may thicken sufficiently to coalesce. When the crazes reach the end of the shear band they emerge and propagate into the unoriented polymer matrix. Within the shear band the craze growth direction does not lie normal to the tensile axis, but is rotated due to the molecular orientation of the shear band. The direction of craze growth is also affected under the plane strain conditions of bulk specimens. In this case the craze is diverted along the shear band before re-emerging into the matrix. Measurements of the craze fibril extension ratio, λ, within the shear band show an increase over typical values obtained outside the shear band in the same polymer. This high value of λ leads to an increased likelihood of craze break-down and crack nucleation within the shear band.

In situ polymerization of bisphenol-A-carbonate cyclic oligomers in miscible blends with a styrene-acrylonitrile copolymer: phase separation dynamics and the influence of phase dispersion on ductility

Abstract The in situ polymerization of bisphenol-A-carbonate cyclic oligomers (BPACY)/styrene-acrylonitrile copolymer (SAN) blends has been demonstrated to yield PC/SAN blends with morphologies unattainable via conventional melt blending. Extremely fine phase dispersion can be obtained by this method of blend preparation. Domain-coarsening kinetics have been shown to be quite sensitive to the volume fraction of the dispersed phase. The ‘pinning’ of domain coarsening, unique to polymer systems, can be attributed to the extreme barriers to diffusive coarsening mechanisms in these systems. Thus, phase coarsening is arrested when percolation ceases or domains no longer form local clusters. The dispersed phase size has been shown to have a dramatic effect on high-stress deformation in systems where a brittle phase is dispersed in a more compliant ductile matrix. The increased ductility of blends with finer phase dispersions has been rationalized based on a lower tendency for smaller brittle phases to craze and crack in addition to the influence of complex local stress fields in heterogeneous materials.

Heterogeneous nucleation of crazes below notches in glassy polymers

A slip-line field study of craze initiation at plastic zone tips below notches in polycarbonate has been carried out over a wide temperature range. Combination of the critical values of hydrostatic tension with Yees temperature data for bulk modulus leads to a calculated critical elastic dilation in the region beyond the zone tip that is nearly temperature-independent. Microscopic investigation, however, shows strong evidence that crazes in polycarbonate and other glassy resins initiate at micrometre-size foreign particles. Assuming that the foreign particle is a rigid adherent sphere, a stress analysis for the particle situated on the elastic side of the elastoplastic boundary has been effected; the particle is found to raise the hydrostatic tension over the value in the homogeneous field by 104%. In view of this analysis the polycarbonate experimental results suggest a temperature-independent, critical local elastic dilation of greater than 2% as a crazing criterion. Based on these results it may be suspected that all crazes — surface or internal — in glassy polymers, except at crack tips, are heterogeneously nucleated at impurity sites.

Single groove double cantilever beam fracture specimens versus Dugdale specimens for estimating shear lip propagation energies: BPA polycarbonate

Cracks in double cantilever beam specimens with single side grooves (S-G DCB) exhibit crack tip plastic zones that mimic closely the plastic zones that produce shear lips in mixed mode crack propagation in thick specimens of polycarbonate. In both the S-G DCB plastic zone and the shear lip the plastic strain is 30%. The S-G DCB strain energy release rate is a linear function of thickness, as a result of the fact that the cross-sectional area of the plastic zone increases linearly with the square of the net section thickness while the plastic strain in the zone is independent of thickness. In Dugdale specimens the thickness dependence of GC is different from and more complex than that in the S-G DCB specimens. For each geometry GC starts at zero thickness, but for Dugdale specimens rises eight times faster with the thickness at first. This greater thickness dependence of GC in the Dugdale specimens arises from a greater dependence of zone cross-sectional area on thickness and a greater zone plastic strain (50 to 60% mostly). Other advantages of S-G DCB specimens versus Dugdale specimens in assessing ductile fracture are discussed.

Structure and Properties of Alternating Block Polymers of Dimethylsiloxane and Bisphenol-A Carbonate

Evidence from several kinds of studies indicates that in many block polymers in the solid state a compositional fluctuation or so-called domain structure exists on the scale of 102 A ca.1–6 Many physical properties of these systems are well known to differfrom those of their random-copolymerized counterparts because of this structure. Block polymers of dimethylsiloxane and bisphenol-A carbonate have been synthesized by H. A. Vaughn7 which exhibit physical properties reminiscent of those of other block polymers composed of at least one rubbery block separating two glassy blocks.

Thickness dependence of Gc for shear lip propagation from a single crack propagation specimen: aluminium 6061-T6 alloy, cold-rolled copper and BPA polycarbonate

AbstractValues ofGc for ductile crack propagation in a series of double cantilever beam specimens, each with a single side groove of constant depth, increase linearly with net section thickness. If a single side groove with linearly varying depth is cut in a double cantilever beam specimen, the tapered net section thickness results in a plastic zone, the crosssection area of which increases linearly with crack length. The attendantGc increases in proportion to the plastic zone size in such a specimen. A single properly designed tapered section specimen appears to be capable of providing estimates of (a) the dependence of shear lipGc on shear lip width, (b) the natural shear lip width and shape, and (c) the shear lip plastic strain. Gc and plastic zone data from specimens of both kinds are reported for aluminium 6061-T6 alloy, cold-rolled copper and BPA polycarbonate. Results of uniaxial tensile tests and of centre-notch tensile tests are also reported for comparison purposes.Gcs, plastic zone sizes and plastic zone strains vary from material to material and appear to reflect in part the drawing and necking characteristics seen in uniaxial tensile tests.

Low temperature motions in, and mechanical properties of, glassy plasticized polymers

Abstract Ambient moduli and yield stresses of single-phase glassy plasticized thermoplastics reflect several characteristics of the diluent (D) and the resin in addition to T g reduction. Dynamic mechanical spectroscopy from -150°C to the blend T g shows broad new losses (I) starting at the NEAT D T g , low- T losses (II) from internal motion in the diluent, and retardations (III) of resin secondary relaxations (if any) (i.e., anti-plasticization). NMR shows that micro-Brownian rotation of D molecules causes I. However I dies out with Ds of higher T g (e.g., 0°). II arises from Ds with flexible rings. III occurs in BPA polycarbonate but not in polyphenylene oxide/polystyrene: this blend has little low- T loss, thus eliminating III as a possibility and making changes caused by D addition easier to understand. Finally, excess low- T g D existing as a second phase produces a very sharp loss peak (IV) at the D T g . The I–IV contrast indicates that I arises from dissolved isolated D molecules.

Gc versus crack velocity in single groove double cantilever beam specimens of polycarbonate

The dependence of Gc on crack velocity in single-groove double cantilever beams (SG DCB) is negligible over the range of crack velocities from 0.2 to 1100 in. min−1. The constancy of Gc appears to reflect a counterbalancing of the rise in yield stress by a decrease in crack opening displacement. The latter occurs through a decrease in the “gauge length” of material engulfed by yielding rather than by a decrease in the ultimate plastic strain in the crack tip plastic zone. The energy of shear lip formation at any crack velocity appears to be accurately estimated from Gcs for SG DCB specimens fractured at low velocity.