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SynopsisTensile properties of polyetheretherketone (PEEK) have been studied at 125, 25, and -100°C for thin films prepared with different therxnal histories. Initial morphology was controlled by rate of cooling from the melt. Amorphous films resulted from quenching the melt, while semicrystalline films were obtained by cooling the melt at Merent rates, or by crystallization of the rubbery amorphous state. The films were characterized using density, X-ray scattering, differential scanning calorimetry, and infrared spectroscopy. Scanning electron microscopy was used to examine fracture surfaces. Degree of crystahity and rate of cooling from the melt affected the tensile properties at all test temperatures. For films with nearly the same degree of crystallinity, those which were more slowly cooled from the melt fractured at the lowest strain. The amorphous films were most tough, drawing to 233% at -100°C and to over 500% at 125OC. Films crystallized from the rubbery amorphous state had streas-strain behavior intermediate between that of the amorphous and melt-crystallized films at all test temperatures. Density measurements on the drawn material indicate that void formation occurs simultaneously with the formation of fibrillar crystals. Necking resulted in density increases for amorphous films, and density decreases for the semicrystalline films.

A study was made on the stress relaxation behavior at 25°C of poly(methyl methacrylate) in uniaxial tension as a function of physical aging at both room temperature and 60°C. Test specimens were compression molded at 165°C, then quenched to room temperature and allowed to age for up to 30 days prior to testing. Stress relaxation curves measured after different aging times could be superposed to a single master curve for each aging temperature. Superposition was achieved by applying vertical and horizontal shifts. Hence, the shape of the response curves was not changed by aging. This is in accordance with observations made by Struik for tensile creep curves. Volume changes as a function of physical aging were also determined. Simple exponential relationships were observed between volume and both horizontal and vertical shifts. The horizontal shift implies a shift in the effective time scale caused by a change in free volume. The vertical shifts could be correlated with changes in Young's modulus caused by a change in density. For the range of aging studied, the response time scale varied over nearly two decades of log‐time. For the same conditions modules varied by 30 percent.

Considering the case where the relaxation time spectrum is preserved at finite deformations, a theoretical analysis of the tensile stress‐strain relation of elastomers at constant strain rates has been carried out. The finite strain effect is taken into account by replacing the Cauchy strain by a general strain function, ƒ(ϵ), in the Boltzmann superposition integral. The analysis shows that there are two cases where the time and strain effects are separable when: (1) the segment of the stress relaxation modulus which coincides with the experimental time of stretching can be represented by a single power law; and (2) the general strain function, ƒ(ϵ), is linearly proportional to the Cauchy strain. Separability of the time and strain effects, therefore, can be achieved by adjusting the stretching time (or strain) and temperature, if the relaxation time spectrum remains unchanged by the deformation. The tensile stress‐strain relations derived from the theoretical analysis were applied to analyze data on a crosslinked styrene butadiene rubber obtained in the temperature range −40 to 60°C. Γ(ϵ), which describes the strain dependence of tensile stress, Bϵ, the ratio of isochronal stresses at different strains, and ai, slope of a segment of the relaxation modulus Ei(t) on log t plot, were obtained directly from the experiment. Values of Γ(ϵ), Bϵ and ai obtained at −40°C are quite different from those obtained at −30°C or higher. Results obtained from our analysis are generally in agreement with those obtained by an empirical method for analyzing the experimental data.

A new transient network model for associative polymer networksThe viscoelastic behavior of entangled polymers is modeled by a three-dimensional transient network where the entangled points are considered to act as temporary crosslinks. Polymer chains are represented by beads and springs. The effects of entanglements on chain dynamics are introduced by assigning enhanced frictional coefficients to selected beads as well as extra elastic couplings between pairs of the entangled beads. The formation and disengagement of the entanglements can be envisioned to be in a dynamic equilibrium. The strength of elastic coupling is set to decrease with increasing distance between the entangled points. The resulting modified Rouse-Bueche-Zimm matrix is solved for the relaxation times from which the dynamic moduli, relaxation moduli, steady-state shear compliance, and zero-shear viscosity are computed. Results are in excellent agreement with experimental data on monodisperse polystyrene, poly(a-methylstyrene), poly(vinyl acetate) and polybutadiene.

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SynopsisThe stress-relaxation modulus of an unfilled ethylene vinyl acetate polymer at three different degrees of crosslinking was measured at 15 temperatures over a temperature span of about 160°C. At each temperature, the time response was measured for at least 3 decades of time. From these data it was possible to construct a master curve for each degree of crosslinking. The time-temperature shift factors, uT, were found to be related to temperature by the relation log uT = -A ( T -273), where the parameter A has a n average value of 0.234 for the three materials.

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