Effect Of Physical Aging Research Articles

Aspects of physical aging, mechanical rejuvenation, and thermal annealing in a new copolyester

Long-term effects of physical aging and mechanical rejuvenation are monitored through dynamic mechanical properties of an amorphous glassy polymer. These phenomena are investigated through dynamic mechanical testing that evaluates in situ the evolution of the storage modulus with time during annealing and physical aging. Comparisons are made on samples with different thermal histories and mechanical treatment (rejuvenation). The polymer characterized in this contribution is a new commercially available copolyester under the tradename of Tritan™. The results are discussed in context to different aging rates obtained from the various thermal and mechanical treatments; both measurements of aging rates are compared. POLYM. ENG. SCI., 55:1941–1950, 2015. © 2014 Society of Plastics Engineers

Effect of physical aging on Johari-Goldstein relaxation in La-based bulk metallic glass.

The influence of physical aging on the β relaxation in La60Ni15Al25 bulk metallic glass has been investigated by mechanical spectroscopy. The amplitude of the β relaxation (ΔG″) decreases while its relaxation time (τ(β)) increases during aging. We find that, as in organic glasses, the changes of ln (τ(β)) and ln (ΔG(max) ) are linearly correlated with ln (τ(β)) = b - a ln (G(max)″). This behavior is discussed in term of the asymmetric double-well potential (ADWP) model, with U and Δ the energies characterizing the ADWP. It is suggested that during aging the ratio U/Δ remains approximately constant, with a value close to the coefficient describing the linear correlation between ln (τ(β)) and ln (G(max)″)(U/Δ ~ a). Moreover, the evolution versus aging time of ΔG(max) can be described by a simple stretched exponential equation giving values of τ(aging) consistent with tan(δ) measurements during aging. The very similar behavior of the β relaxation during aging in metallic glasses and organic material strongly suggests a common nature for this relaxation.

The Aging of America's Reservoirs: In‐Reservoir and Downstream Physical Changes and Habitat Implications

AbstractReservoirs are important for various purposes including flood control, water supply, power generation, and recreation. The aging of America's reservoirs and progressive loss of water storage capacity resulting from ongoing sedimentation, coupled with increasing societal needs, will cause the social, economic, environmental, and political importance of reservoirs to continually increase. The short‐ and medium‐term (<50 years) environmental consequences of reservoir construction and operation are well known and include an altered flow regime, lost connectivity (longitudinal, floodplain), an altered sediment regime, substrate compositional change, and downstream channel degradation. In general, reservoir‐related changes have had adverse consequences for the natural ecosystem. Longer term (>50 years) environmental changes as reservoirs enter “old” age are less understood. Additional research is needed to help guide the future management of aging reservoir systems and support the difficult decisions that will have to be made. Important research directions include assessment of climate change effects on aging and determination of ecosystem response to ongoing aging and various management actions that may be taken with the intent of minimizing or reversing the physical effects of aging.

Aging behavior of thermoplastic elastomers in the laser sintering process

Abstract

Adhesion properties of soy protein crosslinked with organic calcium silicate hydrate hybrids

ABSTRACTThe objective of this work was to investigate if inorganic calcium silicate hydrate (CSH) hybrids would improve soy protein wet adhesion properties. 3‐aminopropyltriethoxysilane (APTES) was used as a crosslinking agent to make covalent linkage between organic soy protein and inorganic CSH phases. Soy protein–calcium silicate hydrate (MSP‐CSH) composites with different mole ratio of APTES were prepared and the effect of crosslinking reaction on physicochemical properties such as thermal, rheological, FTIR spectroscopic, and morphological and adhesion properties were studied with physical aging effect. Covalent linkage was observed between CSH and soy protein using the FTIR technique. With aging effect, the denaturation temperature (Td) and enthalpies (ΔHd) of each fraction of soy protein increased in DSC thermograms, representing higher thermal stability, and the viscoelasticity of the composites also increased. The roughly coated surface of the MSP‐CSH composite was observed in SEM images. All these changes further confirmed the interaction between CSH and soy protein molecules. Dry and wet adhesion strength of the MSP‐CSH composites was higher than the control MSP alone. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014, 131, 40693.

TSDC and DSC Study of Effects of Physical Aging on Polybutylene Terephthalate (PBT)

The impact of the physical aging process on the electrical and thermal properties of semicrystalline polybutylene terephthalate (PBT) was investigated by means of thermally stimulated depolarization currents (TSDC) and differential scanning calorimetry (DSC). The TSDC technique was used to study the relaxation modes of PBT in the temperature range −50° to + 120°C. The obtained spectra revealed two peaks located at temperature maxima of 45° and 93°C. The peak appearing at 45°C corresponds to the dielectric manifestation of the glass transition phenomenon (α-relaxation). The aim of this work is to study the effect of physical aging on this relaxation. The recording of TSDC peaks of aged PBT at different temperatures for different times revealed a reduction in their intensities and their shift towards higher temperatures when the aging becomes significant. This result can be explained by the diminution of molecular chain mobility, which is directly related to the area under the peak representing the polarization of the sample. This result was confirmed by DSC measurements, which revealed the growth and the shift of the peak, which is superimposed on the jump of the heat capacity, located around 38°C and characteristic of the glass transition, towards higher temperatures where aging becomes significant.

Effect of bulk aging on surface diffusion of glasses

The effect of physical aging on surface diffusion has been determined for two organic glasses, Indomethacin and Nifedipine. The two systems exhibit similar aging kinetics typical of organic glasses. Surface diffusivity remains unchanged despite significant bulk aging that nearly equilibrates the systems and increases the bulk relaxation time by orders of magnitude. The finding is relevant for understanding the stability of amorphous materials and the formation of low-energy glasses by vapor deposition.

Glass transitions and physical aging of cassava starch – Corn oil blends

Glass transition temperatures and physical aging of amorphous cassava starch and their blends with corn oil were assessed by differential scanning calorimetry (DSC). Two enthalpic relaxation endotherms, well separated in temperature values, were exhibited by neat amorphous cassava starch with 10.6% moisture content, evidencing two amorphous regions within the starch with different degrees of mobility. The phase segregation of these two amorphous regions was favored by added corn oil at low moisture contents during storage. The presence of amylose–lipid complexes in this matrix, may also affect the molecular dynamics of these two amorphous regions at low moisture contents. Increasing moisture content, leads to a homogeneous amorphous phase, with an aging process characterized by a single enthalpic relaxation peak. In all cases, after deleting the thermal history of the samples only one glass transition temperature was detected (during DSC second heating runs) indicating that a single homogeneous amorphous phase was attained after erasing the effects of physical aging. Trends of the enthalpic relaxation parameters were also different at the two moisture contents considered in this work.

Effects of Thermal Treatment and Physical Aging on the Gas Transport Properties in Matrimid®

Carbon dioxide and methane transport in a commercial polyimide, Matrimid 5218<sup>®<sup/>, has been characterized in order to evaluate the effect of membrane thermal treatment and physical aging on its potentialities for CO<sub>2<sub/>/CH<sub>4<sub/> separation. In particular, CO<sub>2<sub/> and CH<sub>4<sub/> permeabilities and diffusion coefficients were measured at three different temperatures (35, 45 and 55°C) in films pretreated at 50, 100, 150 and 200°C, respectively. The performances of each sample were examined for a period of more than 3 000 hours. Permeability and diffusivity values for both penetrants showed a marked decrease with increasing the pretreatment temperature up to 150°C and remained, then, substantially unchanged for specimens pretreated at the highest temperature. Interestingly, the samples characterized by the higher flux after film formation also showed a faster aging phenomenon, leading to a 25 % decrease of CO<sub>2<sub/> permeability in the period inspected. Conversely, the samples pretreated at temperatures of 150°C, or higher, displayed very stable transport properties for the entire duration of the monitoring campaign. At the end of the aging period considered, the differences among samples were definitely reduced, suggesting that the initial behaviors are related to different polymer structures induced by pretreatment, which slowly evolve in time towards more similar and more stable configurations. Such aging rearrangements affect both CO<sub>2<sub/> and CH<sub>4<sub/> permeability in a similar way, so that no significant changes were observed for selectivity, which showed only a slight increment by increasing the temperature of the thermal treatment or the duration of the aging period.

Enthalpy relaxation phenomena of epoxy adhesive in operational configuration: Thermal, mechanical and dielectric analyses

Thermal cycling in space environment can cause physical aging of polymers used in structural adhesive bonded joint. Later, they can initiate failure. A methodology to follow physical aging effects on their thermal, mechanical and dielectric properties is applied to a commercial epoxy adhesive. The analytic description, using Tool, Narayanaswamy and Moynihan model gives a good description of the enthalpy relaxation. It is completed by a phenomenological analysis of the evolution of the adhesive thermal transitions, mechanical properties and molecular mobility. Tested samples with bonded assembly are representative of in service configurations. The influence of physical aging on the adhesive and the associated bonded assemblies is analyzed.

Effect of a small amount of poly(3‐hydroxybutyrate) on the crystallization behavior of poly(l‐lactic acid) in their immiscible and miscible blends during physical aging

The miscibility and effect of physical aging on the crystallization behavior of poly(l‐lactic acid) (PLLA)/poly(3‐hydroxybutyrate) (PHB) blends with a small amount of PHB (≤10 wt%) have been investigated using differential scanning calorimetry and Fourier transform infrared spectroscopy. It is found that the miscibility of PLLA/PHB blends with a very small percentage of PHB can be modulated by varying the molecular weight of the PHB. That is, a PLLA/PHB blend with low‐molecular‐weight PHB is miscible, whereas that with high‐molecular‐weight PHB is immiscible. It is found that physical aging at temperatures far below the glass transition temperature can promote the cold crystallization kinetics of PLLA in PLLA/PHB blends with high‐molecular‐weight PHB rather than in those with low‐molecular‐weight PHB. These findings suggest that the effect of physical aging on the crystallization behavior of the main component in a crystalline/crystalline blend with a small percentage of the second component is strongly dependent on the miscibility of the blend system. Enhanced chain mobility of PLLA in the interface region of PLLA matrix and PHB micro‐domains is proposed to explain the physical aging‐enhanced crystallization rate in immiscible PLLA/PHB blends with high‐molecular‐weight PHB. © 2013 Society of Chemical Industry

Modeling of structural mechanics, moisture diffusion, and heat conduction coupled with physical aging in thin plastic plates

This paper offers an approach to model the coupled physics of structural mechanics, moisture diffusion, and heat conduction together with physical aging. The modeling is performed on a glassy polymer blend and hence incorporates the material viscoelastic behavior. Determination of the coupling coefficients, which link the governing equations of the mechanisms to each other, is particularly challenging. In this work, an estimation procedure has been used to determine the coupling coefficients, based on the experimental data reported in our recent papers. The effects of physical aging on mechanical stress–strain are also included using the shift factors, which were specified experimentally. The effect of physical aging on moisture diffusion is also modeled using a time-varying boundary condition. Experimental verification of the model shows that the developed model is capable of predicting the deflection of plastic parts subjected to hygrothermal conditions; i.e., conditions where moisture diffusion and physical aging phenomena are influential.

Influence of storage temperature and moisture on the performance of microsphere/hydrogel composites

The current study involved investigation of the effect of storage temperature and moisture on the performance of poly(lactide-co-glycolide) (PLGA) microsphere/poly(vinyl-alcohol) (PVA) hydrogel composites. Physical aging occurred in composites stored at 25°C due to structural relaxation. The glass transition temperature (Tg) and enthalpy of relaxation of the composites increased leading to a slower cumulative % release. The Tg of composites incubated at 40°C, 75% RH decreased significantly due to the plasticization effect of absorbed water, whereas no change was observed in the Tg of microspheres alone; indicating that the hydrogel component enhanced water absorption. PLGA degradation occurred leading to significantly faster dexamethasone release following incubation at 40°C, 75% RH for 1 month. No significant change was observed in the in vitro release profiles of composites after 6 months storage at 25°C, 60% RH, however, release was accelerated following 12 months storage. Accordingly, exposure of the composites to ambient temperature/moisture during storage, shipping or handling may cause physical aging, plasticization, and degradation and hence, their performance may be affected. The extent to which the performance of the composite is affected by storage temperature and moisture is a net effect of physical aging and moisture induced plasticization/hydrolytic degradation.

Creep and fatigue behavior of a novel 2-component paste-like formulation of acrylic bone cements

The fatigue and creep performance of two novel acrylic bone cement formulations (one bone cement without antibiotics, one with antibiotics) was compared to the performance of clinically used bone cements (Osteopal V, Palacos R, Simplex P, SmartSet GHV, Palacos R+G and CMW1 with Gentamicin). The preparation of the novel bone cement formulations involves the mixing of two paste-like substances in a static mixer integrated into the cartridge which is used to apply the bone cement. The fatigue performance of the two novel bone cement formulations is comparable to the performance of the reference bone cements. The creep compliance of the bone cements is significantly influenced by the effects of physical ageing. The model parameters of Struik's creep law are used to compare the creep behavior of different bone cements. The novel 2-component paste-like bone cement formulations are in the group of bone cements which exhibit a higher creep resistance.

Modeling the glass transition of amorphous networks for shape-memory behavior

In this paper, a thermomechanical constitutive model was developed for the time-dependent behaviors of the glass transition of amorphous networks. The model used multiple discrete relaxation processes to describe the distribution of relaxation times for stress relaxation, structural relaxation, and stress-activated viscous flow. A non-equilibrium thermodynamic framework based on the fictive temperature was introduced to demonstrate the thermodynamic consistency of the constitutive theory. Experimental and theoretical methods were developed to determine the parameters describing the distribution of stress and structural relaxation times and the dependence of the relaxation times on temperature, structure, and driving stress. The model was applied to study the effects of deformation temperatures and physical aging on the shape-memory behavior of amorphous networks. The model was able to reproduce important features of the partially constrained recovery response observed in experiments. Specifically, the model demonstrated a strain-recovery overshoot for cases programmed below Tg and subjected to a constant mechanical load. This phenomenon was not observed for materials programmed above Tg. Physical aging, in which the material was annealed for an extended period of time below Tg, shifted the activation of strain recovery to higher temperatures and increased significantly the initial recovery rate. For fixed-strain recovery, the model showed a larger overshoot in the stress response for cases programmed below Tg, which was consistent with previous experimental observations. Altogether, this work demonstrates how an understanding of the time-dependent behaviors of the glass transition can be used to tailor the temperature and deformation history of the shape-memory programming process to achieve more complex shape recovery pathways, faster recovery responses, and larger activation stresses.

Physical ageing in Se–Te–Sb glasses

Bulk Se60−xTe40Sbx glasses in the composition range 0≤x≤14 were prepared by the melt quenching method. Differential Scanning Calorimetric (DSC) and thermal crystallization studies were performed to understand the thermodynamic property like glass transition and structural transformations. These glasses exhibit sharp endothermic peak at the glass transition (Tg). Disappearance of the endothermic peak at Tg in the rejuvenated samples clearly indicates the ageing effect in these glasses. Addition of Sb to Se–Te increases the connectivity of the structural network which is evidenced from the increase in Tg. A distinct change in the slope of the Tg at x=6, indicates a major change in the way the network is connected. The glass forming ability and the thermal stability also exhibit a maximum at x=6. Tg increases with the ageing time and the corresponding fictive temperature (Tf) calculated from the specific heat curves shows a decreasing trend. The molecular movements along the polymeric Se chains might cause the structural relaxation and the physical ageing. The physical ageing effect has been understood on the basis of the Bond Free Solid Angle (BFSA) model proposed by Kastner. Thermally crystallized samples show the formation of rhombohedral Sb2Te3, rhombohedral Sb2Se3 and hexagonal Te phases.

Stress relaxation of a polycarbonate blend after hygrothermal aging

This work has studied the competing effects of physical aging and moisture absorption on the relaxation behaviour of a polycarbonate/acrylonitrile-butadiene-styrene (PC/ABS) polymer blend after being subjected to both thermal and hygrothermal aging. Physical aging was simulated by thermal aging the blend at temperatures up to 80 ∘C which is below the glass transition temperatures of ABS and PC in the blend, respectively. Nine different combinations of relative humidity and temperature were investigated for hygrothermal aging. In both cases, two sets of aging times were applied: “short” term (≤16 hour aging) and “long” term (@168 hour aging). The short term tests were used to generate momentary master curves by applying time/aging-time and time/moisture superposition principles. Using the Kohlrausch–Williams–Watts (KWW) expression, the model parameters determined from curve fitting master curves were used to predict longer term stress relaxation behaviour. The predictions, verified against experimental results, showed reasonable agreement for all hygothermal exposure conditions except for full immersion. Moreover, comparisons of KWW parameters for both short and long term tests suggested that physical aging processes dominate absorbed moisture effects in terms of influencing viscoelastic behaviour, especially when the aging temperature approaches 80 ∘C.

Molecular modeling of physical aging in epoxy polymers

AbstractEpoxy resins are often exposed to prolonged periods of sub‐Tg temperatures which cause physical aging to occur. Because physical aging can compromise the performance of epoxies and their composites and because experimental techniques cannot provide all of the necessary physical insight that is needed to fully understand physical aging, efficient computational approaches to predict the effects of physical aging on thermomechanical properties are needed. In the current study, a new method is developed to efficiently establish molecular models of epoxy resins that represent the corresponding molecular structure at specific aging times. Although this approach does not simulate the physical aging process directly, it is useful in establishing molecular models that resemble physically aged states of epoxies. Such models are useful for predicting the thermomechanical properties of aged epoxy resins to facilitate the design of durable engineering structures. For demonstration purposes, the developed method is applied to an EPON 862/diethylene toluene diamine epoxy system for three different crosslink densities. © 2012 Wiley Periodicals, Inc. J. Appl. Polym. Sci., 2013

The effect of physical aging on the embrittlement of steam-sterilized polycarbonate

Polycarbonate is known to suffer from dramatic reductions in ductility upon exposure to hot, humid environments, such as during steam sterilization. Two phenomena have been proposed to be the main causes of this embrittlement: hydrolysis and microcavity formation. The present study focuses on a third phenomenon, whose contribution to the embrittlement has until now been considered insignificant: (physical) aging. By studying the influence of steam sterilization on the tensile deformation behavior of polycarbonate, it is shown that aging actually is one of the dominant factors in the embrittlement.

Effect of physical aging on the shape-memory behavior of amorphous networks

This paper presents an experimental and modeling study of the effects of physical aging on the shape-memory performance of (meth)acrylate-based networks composed of tert-butyl acrylate (tBA) crosslinked by various concentrations of poly(ethylene glycol dimethacrylate) (PEGDMA). The experiments measured the unconstrained recovery response of samples stored at 20 °C (Tg − 36 °C) for zero to 180 days and evaluated the effects of storage on the strain fixity, activation temperature, and initial recovery rate. A thermoviscoelastic model recently developed for amorphous networks near the Tg was applied to study the influence of structural and viscoelastic relaxation and the aging time and temperature on the recovery response. Results showed that the activation temperature and the initial recovery rate increased with the aging time, producing a sharper initial recovery response. The thermoviscoelastic model predicted that the magnitude of these effects depended on the aging temperature. There was an optimum aging temperature that maximized the initial recovery rate. These results suggest that physical aging can be manipulated to accelerate the recovery performance of shape-memory polymer devices.