摘要:

Averagerubber fatigue resistance has previously been related to such factors as elastomer type, cure system, cure temperature, and stress history. This paper extends this treatment to a full statistical analysis of rubber fatigue data. Analyses of laboratory fatigue data are used to predict service life. Particular emphasis is given to the prediction of early tire splice failures, and to adaptations of statistical fatigue analysis for the particular service conditions of the rubber industry.

ISSN:0021-8979    

DOI:10.1063/1.329825

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

Many physical properties of polymer glasses change spontaneously during isothermal aging by a process commonly modeled as collapse of free volume. The model has not been verified rigorously because free volume cannot be unambiguously measured. In the present investigation we tentatively identify the free‐volume fraction with the fraction of empty sites in the equation of state of Simha and Somcynsky. With this theory, volume recovery measurements can be analyzed to yield directly the time‐dependent, free‐volume fraction. Using this approach, recent volume measurements on poly(methyl methacrylate) are analyzed. The resulting free‐volume fractions are then used in the Doolittle equation to predict the shift in stress relaxation curves at 23 °C. These predicted shift factors agree with the experimental measurements of Cizmeciogluetal. In addition, it is shown that previous assumptions concerning temperature dependence of free volume are inconsistent with the theory.

ISSN:0021-8979    

DOI:10.1063/1.329826

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

Structural relaxation in a temperature range from 32 to 62° K glass transition has been investigated, for the first time, for a nearly monodisperse polystyrene using a differential scanning calorimeter and a thermal mechanical analyzer. Low temperature anneals in the vicinity of 320 K stabilize the glassy structure and lead to volume contraction. Upon heating, the annealed samples show an excess endothermic peak and exhibit a gradual expansion associated with structural recovery above the annealing temperature. Significantly, the samples recover the initial heat content and volume without reheating through the glass transition temperature. This sub‐Tgstructural relaxation behavior is in many respects distinct from that commonly observed near the glass transition. Kinetics of relaxation processes are also discussed.

ISSN:0021-8979    

DOI:10.1063/1.329827

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

The &agr;‐ phase chain of poly(vinylidene fluoride) has a component of dipole moment parallel to the chain axis which can be reversed by propagation of a solitary wave along the chain. Calculations of the energy required to create such a wave in a perfect chain yield larger values than are implied by experiment. This suggests that either head‐to‐tail defects or regions of stress at the lamellar boundary might play an important role in causing the observed dielectric behavior.

ISSN:0021-8979    

DOI:10.1063/1.329828

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

Water‐soaked, epoxy resin‐graphite fiber composites show a waterline in their broadline proton NMR spectrum which indicates a state of intermediate mobility between the solid and free water liquid states. The line is still present at −42 °C, but shows a reversible decrease in amplitude with decreasing temperature. The line is isotropic upon rotation of the fiber axis with respect to the external magnetic field.

ISSN:0021-8979    

DOI:10.1063/1.329829

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

Single crystals of poly(vinylidene fluoride) PVF2have been grown from dilute solution and sedimented to form oriented mats. The product was &agr;‐PVF2or a mixture of &agr;‐ and &ggr;‐PVF2and different morphologies were observed for the two phases. Annealed mats exhibited complex melting behavior, due in part to melting and recrystallization. Plots of melting temperature against reciprocal lamellar spacing indicate equilibrium melting points of 443.5 and 457.3 K for rapidly crystallized &agr;‐ and &ggr;‐PVF2after 1‐h anneal, and fold surface free energies of 15.6 and 27.2 mJm−2for &agr;‐PVF2from different solvents, 24.8 mJm−2for &ggr;‐PVF2. Long annealing times or high annealing temperatures tend to increase the melting point without changing the lamellar thickness probably due to rejection of head‐to‐head defects.

ISSN:0021-8979    

DOI:10.1063/1.329830

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

Thermally stimulated depolarization currents in low‐density polyethylene containing Irganox 1076 are investigated. The thermal sampling technique has been used to study the elementary processes constituting &bgr; and &ggr; relaxations. Irganox 1076 is found to increase the intensity of &bgr; and &ggr; peaks and not to change the activation parameters. The participation of the antioxidant in the molecular relaxation processes is discussed.

ISSN:0021-8979    

DOI:10.1063/1.329831

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

Selected mechanical properties of symmetrically (±45°) reinforced 16‐lamina graphite/epoxy composites have been found to be affected by sub‐Tgannealing. Postcured specimens of Thornel 300 graphite/Narmco 5208 epoxy were sub‐Tgannealed at 140 °C for ca. 10, 102, 103, 104, and 105min, with a prior quenching from aboveTg. The ultimate tensile strength strain‐to‐break, and static toughness of the composite materials were found to decrease as a function of sub‐Tgannealing time. No weight loss was observed during the sub‐Tgannealing. The time‐dependent change in properties can be explained on the basis of physical aging which is related to free volume decrease in the non‐equilibrium glassy state of network epoxies. The results imply possible changes in composite properties with service time.

ISSN:0021-8979    

DOI:10.1063/1.329832

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

Transition from unoriented &ggr; to oriented &bgr; phases is followed during stretching poly(vinylidene fluoride) at 150 °C, using both x‐ray and small angle light scattering techniques. It is observed that the &ggr;→&bgr; transition occurs simultaneously with the morphologic transition from spherulitic to fibrillar state. The threshold draw ratio is &lgr;?2, corresponding to a chain orientation functionfH?0.5; the conversion into &bgr; phase is complete at draw ratios &lgr;?4.

ISSN:0021-8979    

DOI:10.1063/1.328520

出版商:AIP    

年代:1981

数据来源: AIP

摘要:

An important aspect of the thermodynamic properties of chain molecules in solids is the amount of entropy that is associated with rotations about bonds. Of particular interest is the high‐pressure hexagonal phase of polyethylene. This system is modeled for the purpose of this calculation as an assembly of chains confined to small cylinders. This paper describes an integral equation approach to the problem that may be viewed as a continuum limit of the transfer matrix technique. Two cases are solved: (1) the freely jointed chain confined to a cylinder small enough to prevent the chain from folding back on itself, and (2) the fixed bond angle chain confined to the same type of cylinder. The result of calculation (2) is discussed in light of a model for the high‐pressure hexagonal phase of polyethylene.

ISSN:0021-8979    

DOI:10.1063/1.328521

出版商:AIP    

年代:1981

数据来源: AIP