J. A. Sauer

Effects of Hydrostatic Pressure on the Mechanical Behavior of Polyethylene and Polypropylene

An apparatus has been designed and constructed that will allow determination of the mechanical behavior of polymer specimens subject to applied tensile and compressive loading, while the sample is simultaneously subject to a hydrostatic pressure environment. The moduli of high‐density polyethylene and polypropylene determined from compressive tests show a significant increase with pressure. Tensile nominal stress‐strain curves have been obtained at various pressures up to 100 000 psi. These show that the yield stress also increases significantly with increasing pressure for both materials. The nature of yielding and fracture is found to be quite different for the two polymers studied. Polyethylene tends to deform more by shear, and the necked region at high pressures reduces to a fine point before separation. In polypropylene, fracture occurs by plastic tearing across the cross section. An attempt is made to account for the experimental results by use of yield criteria that includes a hydrostatic pressure...

Morphology of Solution‐Grown Polypropylene Crystal Aggregates

Polypropylene aggregates, consisting of interwoven arrays of single‐crystal lamellae, have been obtained by crystallization of samples of isotactic polypropylene from dilute solutions held at constant temperatures between 85° and 115°C. The solvent used was α‐chloronaphthalene with polymer concentrations ranging from 0.4% to 0.01% by weight. The resulting crystalline aggregates were examined by means of phase contrast and electron microscopy. Similar crystal aggregates have been studied by Khoury and found to be incipient spherulites. These aggregates tend to occur in the form of open weave structures, with characteristic cusps. Lath‐like lamellar crystals, with well‐defined faces, are found at the edges of these structures. These edge lamellae produce a single‐crystal electron‐diffraction pattern corresponding to the monoclinic structural form of isotactic polypropylene. On the basis of the experimental findings, it is suggested that folding occurs in monoclinic polypropylene only along a given set of pa...

Effects of hydrostatic pressure on the mechanical behaviour of polytetrafluoroethylene and polycarbonate

Abstract The effects of hydrostatic pressure (up to 100,000 psi) on the stress-strain behaviour of a highly crystalline polymer (polytetrafluoroethylene) and an amorphous polymer (polycarbonate) have been determined. All measurements are made by an apparatus so designed that specimens can be subject to a tensile, or compressive, load while under any given constant hydrostatic pressure. For both polymers, the yield stress has been found to increase significantly with increase of pressure; the implications for various theories of yielding are discussed. The modulus also rises appreciably with increase of pressure; it is shown that part of this effect is due to the presence of finite strains. The ductility and fracture behaviour differ for the two polymers. Under increasing pressure, the polytetrafluoroethylene shows a more brittle type of failure and the ductility decreases; while for the polycarbonate, the test specimens become more ductile with increasing pressure.

Structure and Thermal Behavior of Pressure‐Crystallized Polypropylene

Samples of isotactic polypropylene have been crystallized at a pressure of 3.86 kbar and under a variety of crystallization conditions. These include isothermal crystallization at five different degrees of supercooling ranging from ΔT=70°C to ΔT=34°C and crystallization by slow cooling from 248°C. Each sample was studied by means of x‐ray diffraction at both room and elevated temperature, by differential scanning calorimetry, and by electron microscope observation of replicas of the fracture surface. The x‐ray diffraction results show that all the pressure‐crystallized samples have formed in the γ, or triclinic, modification. Depending upon crystallization conditions, this phase may be stable or metastable. For high values of ΔT, the triclinic phase shows a partial conversion to the α, or monoclinic, phase under elevated temperature annealing. For low values of ΔT, the γ phase does not convert to α even at elevated temperatures. The DSC measurements indicate that the melting temperatures of the pressure‐c...

Influence of plasticizers on poly(methyl methacrylate) ionomers

Abstract The influence of a polar plasticizer glycerol, and a nonpolar plasticizer, dioctylphthalate (DOP), on the microstructure and relaxation properties of poly(methyl methacrylate) ionomers has been investigated by dynamic mechanical thermal analysis. The test results indicate that glycerol strongly interacts with and weakens the ionic cluster ‘phase’, and also significantly increases the mobility of backbone hydrocarbon chains in the multiplet-containing matrix phase. In contrast, the nonpolar plasticizer DOP is more selective in that it appreciably reduces the glass transition temperature of the hydrocarbon-rich matrix phase, but has a much smaller effect than glycerol on the glass transition temperature of the ion-rich cluster ‘phase’. The present results are compared and contrasted with the effects of the same two plasticizers on polystyrene ionomers.

Effects of radiation and chain ends on fatigue behaviour of polystyrene

Abstract Fatigue lifetimes, under a given alternating stress amplitude, have been determined for a series of linear and branched polystyrenes. The branched polymers were obtained by a crosslinking reaction using γ-irradiation from a Co60 source. By control of irradiation time, a series of branched samples of progressively increasing weight average molecular weight ( M w), with little change in number average molecular weight ( M n, were obtained. From comparison of fatigue data for these irradiated and branched samples with fatigue data obtained on a series of linear polystyrenes of increasing molecular weight, it may by concluded that appreciable increases in fatigue endurance can be achieved by increase in M n and reduction in chain end density. For the irradiated samples, whether irradiated in air or in vacuum, fatigue lifetimes were comparable to or less than lifetimes to fracture for the unirradiated polymer, even though significant increases in M w had occurred. It is suggested that the improved fatigue performance with increase of M n is a consequence of increased craze stability resulting from the greater degree of chain entanglement and the smaller proportion of chain end defects.

Fatigue behavior of polystyrene and effect of mean stress

The behavior of polystyrene under cyclic loading has been investigated. Tests have been run under completely reversed axial stress and also at various superimposed mean stress values. In one series...

Mechanical Properties of Molecular Composites. I. Poly (p-phenylene terephthalamide) Anion Molecules Dispersed in Poly(4-vinylpyridine)

Molecular composites have been prepared by dispersing rigid-rod molecules of ionically-modified poly(p-phenylene terephthalamide) (PPTA anion) in a polar poly(4- vinylpyridine) (PVP) matrix. For concentrations up to 5 wt % of the rigid-rod reinforce- ment, the resulting composites are transparent and possess a single glass transition temperature that increases with concentration of the PPTA anion. The mechanical properties of the molecular composites are found to increase with concentration and to attain maximum values at about 5 wt % of the PPTA anion. The enhancement in properties, and the miscibility induced between the two component polymers, is attrib- uted to the development of specific interactions between the ionic groups of the PPTA anion and the polar units of the PVP matrix. When such interactions are not present, as in composites reinforced with non-ionic PPTA, the samples are opaque and their properties are significantly reduced compared to those of the PPTA anion/PVP composites.

Craze and fatigue resistance of glassy polymers

Abstract The fracture characteristics of several glassy polymers subject to monotonic tensile loading and to alternating tension-compression loading are described and compared. In both cases, a reflected light method is used to detect craze initiation at an early stage. Various methods have been investigated to enhance the craze and fatigue resistance of PS. It is shown that increases in mean fatigue life of 2 to 3 decades are possible by increase of molecular weight and reduction in number of chain ends but that no increases occur if molecular weight is raised by cross-linking and radiation. The use of surface treatments and surface coatings as additional methods for enhancing fatigue performance is explored. It is found that chemical treatments can be even more effective, in some cases, than mechanical polishing and that surface coatings of flexible compatible oligomers, as well as surface layers of viscous hydrogen-bonding liquids, can enhance fatigue lifetimes by inhibiting or delaying crazing. Severa...

Influence of diluents on mechanical relaxation behavior of nylon 6 and polyimide

Abstract The mechanical relaxation behavior of nylon 6 and of a polyimide have been investigated as a function of water content. In addition, tests have been carried out on nylon 6 samples of both low and high monomer content. Both diluents, water and monomer, act as a plasticizer in nylon 6. They lower the modulus values in the temperature range above 300°K and shift the αa transition to lower temperatures. Water is the more effective plasticizer, the average shift for 1% added diluent being about 20°K as compared to about 2°K for added monomer. Both diluents act like antiplasticizers at low temperatures, raising the modulus and decreasing the intensity of the γ relaxation. As either water or monomer is added, a β relaxation rises in the vicinity of 250°K (103 Hz). It is concluded that the β-relaxation process may arise from reorientational motions of CH2 segments containing nonbonded amide groups as well as from combined motions of such units and associated water molecules. In polyimide water has almost...