J.A. Sauer

Effects of sorbed water on properties of low and high molecular weight PMMA: 1. Deformation and fracture behaviour

Abstract The influence of sorbed water on deformation and fracture behaviour of both low and high molecular weight poly(methylmethacrylate) has been investigated. In the low molecular weight polymer, addition of water produces a rise in internal friction in the −100°C region but it has no apparent effect on the β-relaxation process. The tensile strength falls gradually with increasing concentration of water and more rapidly at high concentrations. Ductility initially increases with increasing water content but it reduces at high concentration. In the high molecular weight polymer, in contrast to the low molecular weight material, a yield maximum is observed for both dry and air-equilibrated samples of low moisture content and some samples show both necking and cold drawing prior to fracture. Observed deformation modes include shear bands, crazes, and diamond-shape surface cavities. However, water saturated samples fail in a brittle manner. From the observed deformation behaviour, as well as from observation of fracture surface morphology, it is suggested that sorbed water acts as a mild plasticizer for PMMA up to a concentration of about 1.1 % and, at higher water concentration, water clustering occurs.

Dynamic mechanical properties of sulphonated polystyrene ionomers

Abstract Measurements have been made of the dynamic mechanical properties of a series of partially sulphonated polystyrene ionomers. Results obtained indicate that two glass transition temperatures are present, with the lower temperature transition arising from a phase containing only ionic multiplets and the higher temperature transition arising from a phase containing ionic clusters. The loss peak height associated with the multiplet containing phase decreases with increasing ionic content while that associated with the cluster phase rises. The cluster phase appears to become the dominant phase at ion contents of about 6 mol%. The T g of PS increased linearly with ionic content at the rate of about 3°C per mol%. For each ion content, the average molecular weight between ionic crosslinks was determined from values of the rubbery plateau modulus and compared with calculated values, assuming all ion pairs participate in forming crosslinks. It was also observed that the rubbery plateau region extended over a considerably wider temperature range when samples were prepared with a divalent metal ion, such as Ca, instead of a monovalent ion.

EFFECT OF SAMPLE HISTORY ON IONIC AGGREGATE STRUCTURES OF SULPHONATED POLYSTYRENE IONOMERS

The effects of thermal treatment and of solvent (dimethyl formamide (DMF)) history on the ionic aggregate structures of lightly sulphonated polystyrene ionomers were studied by dynamic mechanical measurements. It was shown that thermal treatment of the ionomers with 2.5 mol% ion content enhanced cluster formation. It was also found that DMF worked as a ‘dual’ plasticizer that not only destroyed ionic aggregates but also decreased the Tg of matrix materials. Thermal treatment of cast samples from DMF recovered ionic aggregate structures that were comparable to those seen in compression-moulded samples.

Effects of environment and surface coatings on the fatigue properties of polystyrene

Abstract The influence of surface condition, environmental media and surface coatings on the fatigue lifetime of polystyrene specimens has been explored. It is shown that surface flaws such as machining marks, are much more detrimental to fatigue lifetime than to static strength. The effect of alcohols on fatigue lifetime is primarily one of plasticization rather than of molecular size and mobility. Hence n-butanol is a more aggressive environment for polystyrene than is methanol. For a wide variety of organic media, a fairly good correlation was found between fatigue lifetime and solubility parameter. Highly polar media, like glycerol and water, are shown to be favourable media, rather than aggressive ones, in that they increase average fatigue lifetimes of polystyrene specimens by about one decade. It is suggested that any media that inhibit or delay crazing, either by increasing surface energy or by blunting flaws and reducing stress concentration, should also be beneficial to fatigue performance. A surface coating that performs this latter function is a 600 molecular weight polystyrene oligomer. It is shown that application of this compatible, viscous coating to polystyrene specimens increases the average fatigue life, for both polished and unpolished specimens by a decade or more.

Effects of sorbed water on properties of low and high molecular weight PMMA: 11. Fatigue performance

Abstract The influence of sorbed water on average fatigue life and on fracture surface morphology for unnotched samples of low and high molecular weight poly(methyl methacrylate) has been investigated. For air-equilibrated samples, the effects of test frequency on fatigue performance, and on associated thermal effects, have been determined. Average fatigue lifetimes are about two decades higher for the high molecular weight polymer. Sorbed water, at concentrations from 0 to 1%, produces a significant drop in fatigue resistance. At higher water contents, fatigue life tends to become independent of water content. It is suggested that the transition in behaviour near 1% is associated with onset of water clustering.

Molecular composites of poly(p-phenylene terephthalamide) anion and poly(ethylene oxide): Mechanical properties

Molecular composites were prepared by the solution blending of poly(ethylene oxide) (PEO) of three different molecular weights with low concentrations of poly(p-phenylene terephthalamide) (PPTA) anion serving as a rigid-rod reinforcement. For all of the composites, the tensile strength, stiffness, and fracture energy were enhanced relative to values for the three PEO homopolymers. Composites made with PEO of high molecular weight (∼106 or higher) exhibited good extensibility and improved resistance to stress relaxation and attained maximum values of strength and toughness at a PPTA-anion content of about 6 wt %. The enhancement in mechanical properties is primarily attributed to good miscibility between the components arising from the presence of intermolecular, ion–dipole interactions between the ionic units of the PPTA anion and the dipole units of the PEO. Some consideration is also given to possible effects of a nucleating agent and an annealing treatment, both of which lead to additional increases in the tensile strength of the composites.

The effect of water on the tensile yield of polystyrene

Abstract When PS is tested in tension but immersed in water the yield stress is unchanged and the crazing stress is increased, as compared with tests conducted in air. The crazing pattern is otherwise unchanged. If the PS is first equilibrated with water and then tested under water, both crazing stress and yield stress are reduced. A very large number of small crazes are generated which result in a 23% increase in the fracture strain.

Molecular composites of poly( p-phenylene terephthalamide) anion and poly(propylene oxide): mechanical properties

Abstract Molecular composites have been prepared by mixing rigid rod molecules of PPTA anion (K-salt) and poly(propylene oxide) in a common solvent and compression molding the precipitated material. As the PPTA anion content of the composites is increased, an upward shift occurs in both the glass transition temperature and the decomposition temperature. The tensile strength, stiffness and resistance to stress relaxation of the composites are significantly increased, even at relatively low concentrations of the rigid rod reinforcement. The enhancement in mechanical properties results from a good dispersion of the rigid PPTA anion molecules and from the presence of ion-dipole interactions between the ionic groups of the PPTA anion and the dipolar units of the poly(propylene oxide) matrix. Additional increases in strength and rigidity may be achieved by changing the counterion from monovalent K + to divalent Ca 2+ .

Deformation modes of poly(styrene-co-sodium methacrylate) ionomers

Abstract The influence of ion content and ionomer morphology on deformation modes of poly(styrene-co-sodium methacrylate) ionomers has been examined. Dynamic mechanical thermal analysis indicates a morphology characteristic to amorphous ionomers: two separate loss peaks are observed, one associated with the glass transition of the multiplet-containing matrix phase and the higher temperature loss peak with the glass transition of an ion-rich cluster ‘phase’. As ion content rises, the intensity of the matrix loss peak diminishes, while the intensity of the cluster loss peak rises. At a critical ion content of about 6 mol%, the cluster phase begins to dominate. Such ‘two-phase’ morphology of the ionomers has an appreciable effect on deformation modes. At low ion contents, where the matrix phase is the major one, only crazing is observed and the craze morphology is generally similar to that of polystyrene. At higher ion contents where the cluster phase becomes dominant, some shear deformation is present and the crazes tend to be shorter and less well defined. The effect of thermal treatment on crazing behaviour of the ionomers is also studied; two competing factors, i.e. simple physical aging and enhancement of the ion-rich cluster phase, determine the overall deformation modes. In general, thermal treatment enhances cluster formation, which in turn suppresses crazing and shifts the deformation mode towards shear deformation. Results of this study are compared with those reported on other types of ionomers, such as partially sulfonated polystyrene ionomers and ionomers based on poly(methyl methacrylate).

Molecular composites of poly(p-phenylene terephthalamide) anion and poly(ethylene oxide) : Thermal behavior and morphology

Molecular composites, in which a small concentration of ionically modified poly(p-phenylene terephthalamide) (PPTA) is dispersed in a poly(ethylene oxide) matrix, have been prepared. With the content of PPTA anion increasing to about 5 wt %, the glass-transition temperature rises and the melting temperature decreases. From the equilibrium-melting-temperature depression data that were obtained from Hoffman–Weeks plots, the Flory–Huggins interaction parameter was determined to be negative (−1.10). These indications of enhanced miscibility between the components are attributed to intermolecular ion–dipole interactions. The presence of rigid PPTA-anion reinforcement alters the morphology; for example, the spherulite size is reduced, and the degree of crystallinity is lowered. Possible models of how the reinforcement is incorporated into the composite are presented.