Physical aging is a ubiquitous phenomenon in glassy materials and originates from the fact that they are generally out-of-equilibrium. Due to the technological and fundamental implications, this phenomenon has been deeply investigated in the last decades especially in glassy polymers. Here we provide a critical review of the latest hot debated themes in the field of physical aging in polymers and polymer nanocomposites. We first summarize the fundamental aspects of physical aging, highlighting its relationship with the polymer segmental mobility. A review of the methods employed to monitor physical aging is also provided, in particular those probing the time dependent evolution of thermodynamic variables (or related to) and those probing the (quasi)instantaneous polymer segmental mobility. We subsequently focus our attention on the two following debated topics in the field of physical aging of polymers: (i) the fate of the dynamics and thermodynamics of glassy polymers below the glass transition temperature (Tg), i.e. the temperature below which physical aging occurs; (ii) the modification of physical aging induced by the presence of inorganic nanofillers in polymer nanocomposites. With respect to the former point particular attention is devoted to recent findings concerning possible deviations from the behavior normally observed above Tg of both dynamics and thermodynamics deep in the glassy state. Regarding the effect of the presence of nanofillers on the rate of physical aging, the role of the modification of the polymer segmental mobility and that of purely geometric factors are discussed with particular emphasis on the most recent advances in the topic. The modification of the rate of physical aging in other nanostructured systems, such as polymer thin films, is discussed with particular emphasis on the analogy in terms of a large amount of interface with polymer nanocomposites.

Physical aging in polymers and polymer nanocomposites: recent results and open questions

To describe the time-dependent creep behavior of viscoelastic material, fractional constitutive relation models which are represented by the fractional element networks are studied. Three sets of creep experimental data for polymer and rock are employed to demonstrate the effectiveness of these fractional derivative models. The corresponding constrained problem of nonlinear optimization is solved with an interior-point algorithm to obtain best fitting parameters of these fractional derivative models. The comparison results of measured values and calculated values versus time are displayed through graphics. The results demonstrate that the fractional PoyntingThomson model is optimal in simulating the creep behavior of viscoelastic materials. And it also shows that the interior-point method is effective in the inverse problem to estimate parameters of fractional viscoelastic models.

Creep constitutive models for viscoelastic materials based on fractional derivatives

We have measured the glassy-state structural relaxation of aqueous suspended polystyrene (PS) nanoparticles (the case of soft confinement) and the corresponding silica-capped PS nanoparticles (the ...

Structural relaxation of polymer nanospheres under soft and hard confinement: isobaric versus isochoric conditions.

Poly (phenylene sulfide) (PPS) is one kind of high‐performance polymer with high thermal stability that can be used widely in different industrial domains. However, according to an investigation of the literature, few reviews have comprehensively focused on the continuous development of PPS applications in the past decade. To meet this demand, this paper provides an overall investigation of PPS polymer and PPS‐based composites from synthesis and process to applications. Briefly, this paper introduces PPS materials according to the following topics. First, the molecular weight distribution and morphology of PPS, as well as their reinforced parts, are introduced. Afterward, the topic is focused on the synthesis, process, and blending of PPS. In the next part, this paper investigates the key points regarding PPS as a high‐performance polymer, focusing on the aspect of thermal behavior and mechanical properties. Finally, PPS composite applications are emphasized and overviewed from a wide range of aspects.

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Overall Investigation of Poly (Phenylene Sulfide) from Synthesis and Process to Applications—A Review

Morphology control of nanofillers in poly (phenylene sulfide): A novel method to realize the exfoliation of nanoclay by SiO2 via melt shear flow

This paper considers the experimental characterization of isothermal physical aging of PEEK and PPS films using a dynamic mechanical analyzer. Using the short-term test method established by Struik, momentary creep and stress relaxation curves were measured at several temperatures within 15–35°C below the glass transition temperature (Tg) at various aging times. Stress and strain levels were such that the materials remained in the linear viscoelastic regime. These curves were then shifted together to determine momentary master curves and shift rates using the PHYAGE program. In order to validate the obtained isothermal physical aging behavior, the results of creep and stress relaxation testing were compared and shown to be consistent with one another using appropriate interconversion of the viscoelastic material functions. Time–temperature superposition of the master curves was also performed. The temperature shift factors and aging shift rates for both PEEK and PPS were consistent for both creep and stress relaxation test results.

Isothermal physical aging characterization of Polyether-ether-ketone (PEEK) and Polyphenylene sulfide (PPS) films by creep and stress relaxation

Long-term creep of polyphenylene sulfide (PPS) subjected to complex thermal histories: The effects of nonisothermal physical aging

Modeling mechanical aging shift factors in glassy polymers during nonisothermal physical aging. I. Experiments and KAHR‐ate model prediction

Hybrid aluminum-graphite-epoxy fibre-metal laminates have been tested in open hole tension-tension fatigue at several stress levels. Crack initiation and growth in the outer aluminum layers was monitored by periodic digital imaging of the specimen surface. Image analysis was used to determine the crack length throughout the experiment. Crack growth results were consistent between specimens for each test configuration. A relationship between the crack growth results and stress level was developed using linear elastic fracture mechanics; this successfully characterized the data. The effect of shot peening and alternate hole finishing techniques were considered for fatigue behaviour improvement.

Characterization of fatigue crack initiation and growth in hybrid aluminium–graphite fibre composite laminates using image analysis

paper presents the structural relaxation behavior near the glass transition temperature (T g ) of poly (phenylene sulfide) (PPS). Since the test temperature is close to T g , the specimen is able to reach equilibrium after isothermal or nonisothermal temperature jumps in the experimental time scale; as such, the classic phenomenology of structural relaxation into equilibrium can be pursued by mechanical testing (physical aging) in a temperature range near T g . In our study, physical aging effects were observed by sequential creep test method using a dynamic mechanical analyzer (DMA). The physical aging of PPS near T g from mechanical testing clearly showed the similar behavior from the classic volumetric responses found by Kovacs in 1960s. The mechanical aging shift factors manifested all of the three “essential ingredients” in the kinetics of structural relaxation which constitute the most physical phenomenology of glassy-forming materials: intrinsic isotherms; asymmetry of approaching equilibrium; and memory effects. The findings in this study offer a valuable approach to investigate the relationship between the mechanical response and thermodynamic properties (volume, enthalpy, etc.) during structural relaxation of glassy materials.

Roger D. Bradshaw

Papers

Isothermal physical aging characterization of Polyether-ether-ketone (PEEK) and Polyphenylene sulfide (PPS) films by creep and stress relaxation

Long-term creep of polyphenylene sulfide (PPS) subjected to complex thermal histories: The effects of nonisothermal physical aging

Modeling mechanical aging shift factors in glassy polymers during nonisothermal physical aging. I. Experiments and KAHR‐ate model prediction

Characterization of fatigue crack initiation and growth in hybrid aluminium–graphite fibre composite laminates using image analysis

Structural Relaxation near the Glass Transition: Observing Kovacs Kinetic Phenomenology by Mechanical Measurements