Physical Aging Of Poly Research Articles

Physical aging of a biodegradable alicyclic polymer: poly (pentamethylene trans-1,4-cyclohexanedicarboxylate)

Physical aging of poly (pentamethylene trans-1,4-cyclohexanedicarboxylate) (PPeCE), a biodegradable alicyclic polyester, was investigated using Fast Scanning Calorimetry (FSC), a recent calorimetric technique allowing to accelerate physical aging and study the associated relaxation processes at different aging temperatures in an experimentally-reasonable time scale. Different mechanisms were highlighted by varying the aging temperature on a temperature range of more than 60 °C. At aging temperatures well below the glass transition temperature, several relaxation mechanisms were evidenced, probably related to secondary relaxation processes (β relaxations). When the aging temperature approaches the glass transition temperature, the primary relaxation process (α relaxation) becomes predominant.

Crazing Mechanism and Physical Aging of Poly(lactide) Toughened with Poly(ethylene oxide)-block-poly(butylene oxide) Diblock Copolymers

Sustainable polymers are important alternatives to plastics and elastomers derived from petroleum resources. Poly(lactide) (PLA), a commercially available sustainable plastic, is a well-known succe...

Physical ageing of Poly(Lactic acid): Factors and consequences for practice

Injection molded specimens were prepared from poly(lactic acid) (PLA) and their properties were determined as a function of time to study physical ageing. The mechanical testing of specimens showed that properties change rapidly with time. The stiffness of the specimens increases considerably, while their deformability decreases drastically from 250% after injection molding to a few percent after less than a day of ageing. Thermal analysis showed that both relaxation enthalpy and the change in the glass transition temperature (Tg) of the polymer increased with time. Tg decreased with increasing ageing time that could not be explained by the generally accepted approach of decreasing free volume. The analysis of literature data showed that the decrease or increase of Tg depends on the temperature of ageing and on the rate of cooling. Tg decreases at relatively low ageing temperatures, while it increases when ageing temperatures are closer to the Tg of the polymer. Besides the decrease of free volume, the development of internal stresses also plays a role in the determination of the glass transition temperature. Internal stresses result in a decrease of Tg, while decreased free volume leads to an increase. Internal stresses determine deformation and failure mechanism as well; large stresses lead to crazing/cracking and finally to brittle failure, which may hinder the application of PLA in many areas.

Simultaneous Thermal Cross-Linking and Decomposition of Side Groups to Mitigate Physical Aging in Poly(oxyindole biphenylylene) Gas Separation Membranes

Physical aging in amorphous polymers causes a decrease in specific volume and thus in the gas transport properties of their membranes. In this work, the effect of simultaneous thermal decomposition of a thermolabile tert-butyl carbonate group, BOC, and cross-linking by a propargyl group (−CH2–C≡CH) on the gas selectivity–permeability properties of the resulting membranes is studied to learn how membranes with mitigated variations in the gas permeability coefficients with aging time may be produced. The model copolymer is a poly(oxyindole biphenylylene) that bears BOC and propargyl groups, [(PN-BOC)x-(PN-Pr)y]n. Systematic studies on the structure/processing/property relationship assessed by TGA, DSC, and permeation measurement using pure gases reveal that a single thermal treatment for 1 h at 240 °C on a neat copolymer membrane, 12–20 μm thickness, is enough to produce chemically robust membranes (insoluble in NMP and DMSO) and that are physically more resistant to aging since the permeability reduction r...

Contribution of the rigid amorphous fraction to physical ageing of semi-crystalline PLLA

The physical ageing of poly(l-lactic acid) (PLLA) samples crystallized in a wide temperature range, is compared, through the analysis of the enthalpy relaxation, with the behavior of a completely amorphous sample, in the same undercooling conditions. A preliminary determination of the percentage of the different phases in the semi-crystalline samples (crystalline, mobile amorphous and rigid amorphous fractions) has been performed. The enthalpy loss (Δha), normalized to the mobile amorphous content, has allowed to differentiate PLLA samples crystallized at different temperatures, because the samples crystallized at lower temperatures, characterized by a slightly constrained mobile amorphous fraction, exhibit a smaller enthalpy loss. In addition, the normalized Δha data relative to the semi-crystalline PLLA samples have been found to deviate from the Δha values of the amorphous sample at high undercooling. This trend has been ascribed to the contribution of the rigid amorphous fraction to the structural relaxation process. The experimental data have been explained by supposing that structural relaxation in amorphous and semi-crystalline PLLA occurs at low undercooling via cooperative segmental motions, whereas, at high undercooling, through small-scale local motions, which take place also in the rigid amorphous fraction.

Physical Aging in Poly(L‐lactide) and its Multi‐Wall Carbon Nanotube Nanocomposites

AbstractSummary: In this work the effects of the structure evolution of neat PLLA and its multi‐wall carbon nanotube nanocomposites at an aging temperature of 45 °C (Tg‐12 °C) have been investigated. The analysis of enthalpy relaxation rate (βH) of composites by means of differential scanning calorimetry (DSC) reveals the existence of a critical concentration of 1.25–2.5 wt. % in which aging rate reaches a mi–imum value of 1.351 J/g. Mechanical tests showed that neat polymer is much more sensitive to the aging process than the reinforced one. As shown by transmission electron microscopy (TEM) all the samples present randomly dispersed nanotubes, yielding an agglomerated structure at MWCNT contents exceeding 1.25–2.5 wt.%. The obtained results are explained from the viewpoint of polymer/filler interfacial effects that perturb the structural relaxation of glassy polymer due to the drastic reduction on the molecular mobility of polymer chains in the vicinity of carbon nanotube surfaces. In this way, nanocomposite approach represents an efficient strategy to successfully reduce the physical aging process of polymers.

Thermal degradation and physical aging of poly(lactic acid) and its blends with starch

AbstractPoly(lactic acid) (PLA) and its blends with starch and methylenediphenyl diisocyanate (MDI) were extruded in a twin‐screw extruder and compress‐molded in a dog‐bone shaped tensile bars to form test specimens. The thermalgravimetric profile and thermal endurance of these samples were characterized. The effect of physical aging on mechanical and thermal properties of these samples was evaluated. For the aging study, samples were stored at 25°C and relative humidity fluctuating between 90 and 30%, from 2 to 180 days, with or without a polyethylene bag as a moisture barrier. Physical aging of pure PLA samples stored in a controlled environment from 2 to 360 days was also evaluated. The presence of MDI in the PLA/starch composite did not affect the thermal decomposition profile. The PLA showed the highest Arrhenius activation energy and strongest thermal endurance of all samples, followed by PLA/starch/MDI and PLA/starch. Exposure of the samples to storage conditions with fluctuating relative humidity (RH) significantly affects thermal‐mechanical performance of PLA and its composites. But, the samples stored in plastic bags can minimize such effects. Mechanical properties of PLA and PLA/starch‐based composites sealed in plastic bags had no significant change during 30‐day storage in fluctuating humidity conditions (30–90% RH). POLYM. ENG. SCI., 2008. © 2008 Society of Plastics Engineers

Effects of microcrystallinity and morphology on physical aging of poly(ethylene terephthalate)

AbstractPhysical aging characteristics of poly(ethylene terephthalate) have been evaluated in relationship to volume fraction levels of crystallinity up to 25%. Changes in the enthalpies of relaxation, monitored at aging temperatures from 55 to 65°C, are found to give good fits with the Cowie‐Ferguson model. Overall equilibrium enthalpy of relaxation values decrease linearly with increased crystallinity. They increase with decreased aging temperature, providing extrapolated lower temperature results that are validated in terms of specific heat relationships. Activation energies for enthalpic relaxations are found to increase from 337 to 361 kJ/mole as crystallinity increases up to 25%. Overall relaxation endotherms are further resolved into contributions from interspherulitic and intraspherulitic amorphous regions. Interspherulitic, equilibrium enthalpies of relaxation decrease with increased levels of crystallinity, while intraspherulic values show corresponding increases. Characteristic relaxation times of the intraspherulic regions increase greatly, as levels of crystallinity increase; however, interspherulitic relaxation times decrease very slightly. Dynamic differential scanning calorimetry results show two glass transitions in the case of a 25% crystalline sample and a single transition for noncrystallized material. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007

Effect of molecular weight and temperature on physical aging of thin glassy poly(2,6‐dimethyl‐1,4‐phenylene oxide) films

AbstractThe effects of polymer molecular weight and temperature on the physical aging of poly(2,6‐dimethyl‐1,4‐phenylene oxide) (PPO) is examined. Gas permeability and refractive index were monitored during the aging process for PPO film samples at three aging temperatures below the glass transition temperature. Comparisons between the two samples of PPO that differ widely in molecular weight reveal an insignificant difference, which support the notion that above a critical molecular weight range there is little influence on aging rate. Increased temperature, over the limited range of 35–55 °C, results in higher aging rates for films made from both PPO materials. The rate of aging decreases strongly with increasing film thickness over the range examined, ∼0.4–25 μm. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1390–1398, 2007

Physical Aging and Refractive Index of Poly(methyl methacrylate) Glass

Physical aging of poly(methyl methacrylate) (PMMA) glass was studied by refractive index measurement. We measured change of refractive index of PMMA glass with annealing at temperatures below the glass transition temperature (Tg) by optical prism coupling, and determined volume relaxation by physical aging using the Lorentz-Lorenz equation. In this paper, the effect of aging temperature on volume relaxation of PMMA glass was investigated. Maximum volume relaxation for PMMA glass was determined at 18 °C below the Tg.

Influence of vinylidenefluoride on the physical ageing of poly(chlorotrifluoroethylene)

The influence of vinylidenefluoride (3% composition) on the physical ageing of poly(chlorotrifluoroethylene) at 70 °C has been investigated using the positron annihilation lifetime technique. Both the ortho-positronium ( o-Ps) pick-off lifetime and its intensity show a systematic variation with ageing time. The o-Ps intensity exhibits non-exponential character which can be fitted with two additive exponential decay curves and the free volume is found to exhibit the Doolittle type of free volume relaxation. The relaxation times were evaluated from the structural relaxation function and the non-exponentiality parameter was estimated using the Kohlrausch–Williams–Watts (KWW) function, which indicates the deviation from exponential relaxation. Based on the relaxation times, the activation energies calculated seems to label the different kinetic units/irregularity in the chain backbone of PCTFE structure participating in the relaxation process. Physical ageing seems to yield close packing of polymer chains.

Physical aging of poly(diethylene glycol‐p,p′bibenzoate)

AbstractThe structure of a quenched sample of a liquid crystalline polyester, poly(diethylene glycol‐p,p′‐bibenzoate) (PDEB), and its physical aging was studied by microhardness methods (MH), wide‐angle X‐ray scattering (WAXS), and positron annihilation lifetime spectroscopy (PALS). It was established that the smectic layers in the liquid crystalline domain are oriented parallel to the sample plane. Physical aging leads to an increase of either the Vickers microhardness, directly related to the elastic modulus of the material, or the total microhardness, which includes all the components of the overall deformation. These mechanical changes are due to a decrease in the sizes of nanopores and the consequential increase of the density of the disordered phase. © 2002 Wiley Periodicals, Inc. J Appl Polym Sci 83: 2363–2368, 2002

Physical Aging of Poly(vinylpyrrolidone) under Different Humidity Conditions

The physical aging of poly(vinylpyrrolidone) (PVP K25) was studied under different storage conditions by positron lifetime spectroscopy and scanning electron microscopy. The transition from the glassy state (at RT and 55% RH) to the completely plasticized wet rubbery state (at RT and 75% RH) is not continuous in PVP K25. It was found that not only the actual water content of the material but also its storage history determines the size distribution of free volume holes in it. At 65% RH a slow anomalous structure formation was observed. By the means of ab initio calculations on simplified model molecules, it was determined that polymer chains and water molecules are able to form a hydrogen bound “network” under certain humidity conditions.

Role of a small amount of comonomer on the physical aging of poly(methyl methacrylate) copolymer investigated by dynamic viscoelasticity

The physical aging of a poly(methyl methacrylate) (PMMA) copolymer having a small amount of ethyl methacrylate was investigated below its glass transition temperature ( T g) using dynamic viscoelastic measurements. The results were compared with those from homo-PMMAs to investigate how the comonomer in the copolymer affects the physical aging of the glass. Annealing temperatures were classified into two categories by the glass transition temperature of a poly(ethyl methacrylate) ( T gPEMA; ca. 65°C). The relaxation rate of the copolymer was faster than those of the homo-PMMAs when the samples were aged below T gPEMA, but the rates were equal when aged at above T gPEMA. The experimental results are discussed in terms of the segmental motion.

Physical aging of poly(vinyl acetate). A thermally stimulated depolarization current investigation

The influence of physical aging on the dielectric α-relaxation of poly(vinyl acetate) (PVAc) is investigated by means of the thermally stimulated depolarization currents (TSDCs) technique. The obtained TSDC data are analyzed in terms of the Kohlrausch–Williams–Watts (KWW) equation for an isothermal decay of a stored charge. Assuming that at temperatures less than the glass transition temperature the shape of the KWW function depends neither on the temperature nor on the thermal history, the generalized Bucci and Fieschi equation allows us to obtain the dependence of the KWW time. The results obtained are interpreted in the Adam and Gibbs theory framework.

Physical aging in poly(ethylene oxide)/ atactic-poly(methyl methacrylate) blends

Physical aging in poly(ethylene oxide)(PEO)/ atactic-poly(methyl methacrylate) ( a-PMMA) blends with a low content of PEO is discussed. Behaviour in the T g-region is analysed. Different compositions are prepared by mixing in the melt, and were compared at the same temperature distance to mid-point enthalpic T g. Macrolevel properties (enthalpy and external volume) are discussed in terms of the Tool–Narayanaswamy–Moynihan (TNM) model. In particular, the non-linearity x parameter (determined using the peak shift method), the apparent activation enthalpy Δ h * , and the combination θ·(1– x), where θ=Δ h */ R· T g 2, are used to discuss overall segmental co-operativity and relaxation in the blends. Based on aging in viscoelastic properties (aging rate), structural mobility is discussed. From obtained results, an increase of co-operativity, and decrease of both, relaxation rate and change of structural mobility, with the increasing content of PEO in the blend, is concluded for temperatures between the dilatometric and enthalpic T g. Agreement with data from studies at the microlevel by other authors, where an onsetting with time phase separation in PEO/PMMA has been postulated, is found.

Effect of physical aging on the microstructure and the crystallization of amorphous poly(aryl ether ether ketone ketone) (PEEKK)

Physical aging of poly(aryl ether ether ketone ketone) (PEEKK) has been investigated. Heat flow responses were measured after annealing the amorphous samples obtained by quenching the melt into an ice-water bath close to, but below, the glass transition temperature. The extent of aging is related to the supercooling from the glass transition temperature and to the aging time. The activation energy of the aging process, which was estimated by a Williams-Watt expression, is similar in magnitude to that obtained for the cold crystallization for the aged samples. The quenched glass is a metastable glass. The conformation of molecular chains rearranges with physical aging which results in the formation of a denser packing in the amorphous phase. The dense amorphous phase may form an initial nucleus for crystallization. Isothermal cold crystallization of the aged samples was carried out. The Avrami equation was used to determine the kinetic parameters, and the Avrami constant n is about 2. An Arrhenius expression was used to evaluate the activation energy of relaxation upon physical aging and the activation energy of transportation upon isothermal crystallization. The activation energy of relaxation is similar in magnitude to that of crystallization for aged samples. Results obtained are interpreted as kinetic effects associated with the glass formation process.

Effect of a Thermotropic Liquid Crystalline Polymer on the Physical Ageing of PET

In this work the effect of the presence of a thermotropic liquid crystalline polymer (TLCP) on the physical ageing of poly(ethylene terephthalate) (PET) was investigated. Physical ageing of PET and its blends with a TLCP was carried out by annealing the samples at 60[ddot]C for several ageing times. Structural relaxation has been revealed by DSC through the analysis of position and magnitude of the endothermic peak superposed on the glass transition. It was found that the peak increases and shifts towards higher temperatures as the annealing time increases for all materials analysed. With respect to the PET matrix, blends submitted to physical ageing show an increase in the ageing rate by a percentage that increase with the amount of TLCP. Moreover, increasing the fraction of liquid crystalline component, a continuos decrease of the cold crystallisation temperature has been also observed.

Physical aging in poly(methyl methacrylate) glass: densification via density fluctuation

The physical aging in poly(methyl methacrylate) during annealing just below T g was investigated by light scattering and oscillating-DSC in which a sinusoidal temperature rise was imposed over the conventional linear temperature rise. Light scattering intensity decreased monotonously with increasing scattering angle. The intensity was found to increase rapidly and then to decrease gradually with annealing time t a. The correlation length and the mean-square density fluctuations by Debye–Bueche plot increased at an early stage and then decreased with t a. The width of the glass transition, obtained by the Fourier transform treatment of the oscillated DSC heat flow, increased with t a at an early stage and then decreased. The endothermic peak, given by the non-reversing part of the oscillating DSC heat flow, appeared even at an early stage of the aging. These results suggest that: (1) as-quenched PMMA glass is partially densified to yield an inhomogeneity in density with a correlation length about 200 nm; (2) by the annealing, the density fluctuations grow at first by a decrease in density at the low density regions and an expansion of the low density regions to increase the correlation length; and (3) the densification then proceeds at the low density regions to reduce the correlation length and to get higher density as a whole, by approaching a rather homogeneous glassy state.

Influence of water on the physical aging of poly(ethylene terephtalate)

AbstractPhysical aging in polyethylene terephtalate (PET) has been studied on samples well characterized in terms of aging history and their content of water. This allows us to better understand the role played by water in the complex phenomenology of the physical aging in a hygroscopic material as PET. No appreciable changes in the thermal behavior in the sample aged in water at room temperature for one month were noted, while substantial changes were observed in samples aged in more severe conditions like 50°C under vacuum and 50°C in water for one month. The results are consistent with the formation of small regions in which segments of chains are closely packed.