Physical Aging Of Poly Research Articles

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.

Physical aging and crystallization of amorphous poly(aryl ether ether ketone ketone)

Physical aging of poly(aryl ether ether ketone ketone) (PEEKK) was investigated. Heat flow responses were measured after annealing the amorphous samples that were obtained by quenching the melt into an ice-water bath at just below the glass transition temperature. 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 form was used to evaluate the relaxation activation energy of physical aging and the transport activation energy of isothermal crystallization. The activation energy of physical aging was similar in magnitude to that observed for the temperature dependence of crystallization under conditions of transportation control. Results obtained were interpreted as purely kinetic effects associated with the glass formation process. © 1998 John Wiley & Sons, Inc. J. Appl. Polym. Sci. 70: 907–912, 1998

Physical Aging in Poly(vinyl acetate). 2. Relative Rates of Volume and Enthalpy Relaxation

The isothermal volume contractions obtained on physically aging a sample of poly(vinyl acetate) at three temperatures below the glass transition were measured dilatometrically. These were compared with enthalpy aging data for the same polymer sample on the basis of characteristic relaxation times by fitting both types of relaxation to the Kolrausch−Williams−Watt (KWW) expression. It was found that the ratios of enthalpy and volume relaxation times depend on the temperature and that there is no simple correspondence between them. Further comparison of the isothermal aging using a time-dependent characteristic relaxation time indicated that conclusions on rates of aging must be considered within the context of the model chosen for data analysis. The possibility of predicting the rate of volume aging from enthalpy data was explored using an expression derived from the equation of state of Simha and Somcynsky, but the correlation between the actual behavior and that predicted was disappointing. An order param...

Stress relaxation and physical ageing in a blend of poly(styrene-co-acrylonitrile) and poly(methyl methacrylate)

The structural recovery during physical ageing of poly(styrene-co-acrylonitrile), poly(methyl methacrylate) and of a 50/50 blend of these polymers has been examined by determining the stress relaxation modulus, G(t), over a range of temperatures below their respective glass transitions. Time–ageing time superposition of the modulus curves is possible and this gives sets of shift factors which reflect the changing mechanical response of the polymers and their blend during isothermal ageing. The ageing rate of poly(styrene-co-acrylonitrile) decreases quite markedly in the region of the glass transition. The rates for poly(methyl methacrylate) and for the blend also decrease on approaching the glass transition, but with a lesser temperature dependence. By scaling this behaviour to the glass transition the rate of ageing of the blend is found not to be intermediate between that of the components but to be similar to that of poly(methyl methacrylate). This may be explained by proposing that the two blend components respond differently to the applied stress field.

The three successive stages of creep of PMMA between 55°c and 90°c

AbstractAfter cooling from 140°C, the physical aging of poly(methyl methacrylate), PMMA, has been studied at 55, 70, 84, and 90°C by means of temporary microcreep tests. The systematic analysis of recoveries reveals that at 90°C, creep follows successively three different stages. After stress application, creep starts by a logarithmic function of time, while recovery follows exactly the same kinetics. An Andrade creep begins after the logarithmic creep: It is in fact the beginning of the Kohlrausch‐Williams‐Watts creep with an exponent of 0.36. This creep is fully recoverable, but the duration of this recovery is ∼3 times the creep duration. The third stage of creep is superimposed on the Andrade creep and follows a power law in t0.8, but it is not recoverable in the usual duration of experiments. By comparison with the recoverable creep of PMMA studied by Plazek above Tg, which can be fitted by a KWW law with an exponent 0.8, this third creep is interpreted as the beginning of the rubbery deformation.

Physical ageing of poly(chlorotrifluoroethylene): A positron annihilation study

Isothermal annealing of poly(chlorotrifluoroethylene) (PCTFE) was carried out at a temperature of 70 °C (slightly above T g ) using the positron annihilation lifetime (PAL) technique. Both the o-Ps lifetime and intensity shows systematic variation with ageing time, and its intensity can be represented by two additive decay exponentials. The free volume which is estimated using the Nakanishi et al. equation developed from Tao's model, exhibits Doolittle type free volume relaxation. Physical ageing seems to yield close packing of polymer chains without crystallization. The relaxation time, and the nonexponentiality parameter, were estimated using a Kolrausch-Williams-Watt (KWW) function.

Physical aging of poly(ether sulfone) from enthalpy relaxation measurements

AbstractEnthalpy relaxation due to physical aging of hydroxyl‐terminated poly(ether sulfone)s (PES) with various molecular weights was studied by DSC at three aging temperatures between Tg − 45°C and Tg − 10°C. The experimental results revealed that both the average enthalpy relaxation time 〈τ〉 and limiting enthalpy relaxation ΔH∞ decrease slightly with increasing molecular weight for PESs aged at the same temperature intervals Tg − Ta (where Ta < Tg). This behavior is not quite consistent with the enthalpy relaxation process of polystyrene. Agrawal reported that the relaxation spectra for polystyrenes with different molecular weights could be superimposed when Tg − Ta was the same. The experimental results of the present work suggest that the motion of segments is affected by intermolecular hydrogen bonding through the OH groups of the PES chain ends.

Remarks on Single and Multiparameter Free Volume Calculations in Physical Aging of Poly(vinyl acetate) in the Tg-Region

Free volume plays an important role in the analysis of physical aging,both experimentally and theoretically. In this work, the Doolittle freevolume equation and the KAHR model are used to predict isothermal responsesfollowing temperature down- and up-jumps for poly(vinyl acetate), PVAc, inthe Tg-region. The constant B of the Doolittleequation is less than unity when the equation is combined with the Kovacsdifferential equation for free volume collapse for isothermal contractionpredictions. Using B values in the range 0.3 to 0.5 allows goodprediction of the isothermal contraction. It is shown that the equationyields aging rates approaching realistic values only for temperatures muchbelow Tg. Isothermal expansion could not be predicted. Volumerelaxation kinetics is analysed based on the Doolittle equation and comparedwith stress relaxation kinetics near Tg, where the slope ofthe inflexion region is calculated. The transient response of PVAc in thesame aging experiments is obtained using the KAHR model.

A model for nonlinear creep and physical aging in poly(vinyl chloride)

AbstractThe creep behavior of polymers depends on the physical age of the material at the time of stress application. Creep curves shift to longer times for more highly aged material and, in poly(vinyl chloride), (PVC), this can be modeled by an empirical equation in which the magnitude of an effective mean retardation time parameter is dependent upon, and increases with, the age of the polymer. Creep compliances for PVC also depend on the magnitude of the applied stress when this exceeds about 4 MPa. This nonlinear behavior is caused, at short creep times, by a reduction in the value for the retardation time parameter on application of the creep stress. Specimens appear therefore to be initially de‐aged by elevated stresses. Subsequently, this parameter increases with creep time implying that physical aging has been reactivated, but the rate of increase also depends on the stress level. These influences of elevated stresses can be described by an extension of the creep model, and parametric expressions have been derived which relate creep compliance values to time, stress, and the age of the polymer. It is shown how the parameters can be determined from a short series of creep experiments and thus how creep deformations can be calculated over wide ranges of time, stress, and age.

Physical aging in poly(ethylene naphthalene‐2,6‐dicarboxylate) in relation to sorbed water, enthalpic relaxation, and mechanical properties

AbstractAmorphous poly(ethylene naphthalene‐2,6‐dicarboxylate) (PEN) films (∼ 220 μm thick), stored in ambient atmosphere for different periods of time and after annealing at different temperatures below Tg = 123°C, for different times, have been investigated by infrared spectroscopy (FTIR), microhardness, and differential scanning calorimetry (DSC). FTIR spectroscopy and weight measurements reveal the presence of water which is easily removed by annealing of the films. Films again recover their initial weight and absorption bands after 1‐2 days storage in ambient atmosphere. Samples annealed at different temperatures Ta for different times ta show an increasing microhardness for relatively short times of ta. The microhardness passes through a maximum at an annealing time depending on Ta, and it decreases toward values somewhat larger than the initial ones. The changes observed in the microhardness and in the values of the excess enthalpy with storage time of the samples at room temperature depend on the physical aging as well as on the content of water of PEN films. © 1995 John Wiley & Sons, Inc.

Physical ageing of poly(methyl methacrylate): 2. Effects of non-linearity

Enthalpic ageing data for poly(methyl methacrylate) are reanalysed in terms of a modified version of the Cowie-Ferguson model which takes account of a time-dependent characteristic time, tc. In terms of describing the time evolution of enthalpy changes occurring upon ageing, no significant improvement in the fits to the data were found when compared with the original linear model.

The effect of temperature on creep and physical ageing of poly(vinyl chloride)

Tensile creep data have been obtained for poly(vinyl chloride) at a number of temperatures ranging from 21.5 to 58°C. The results were obtained from specimens of different age characterized by the elapsed time t e between quenching a sample from 85°C to the test temperature and of the creep test. A model for interpreting non-linear creep data has been developed to describe the creep behaviour of the material as a function of both age and temperature. The results show that the effect of physical ageing is to progressively reduce the level of the relaxed β compliance and to increase the mean retardation time of the glass-rubber α relaxation process. The effect of conducting creep tests on specimens of the same age at progressively higher temperatures is the opposite: the level of the relaxed β compliance increases whilst the mean retardation times of the α process decrease. Neither the ageing rate nor the shape of the distribution of retardation times are affected by temperature.

The kinetics of volume relaxation and related property changes during physical aging of poly(ether ether ketone) (PEEK)

Structural relaxation is accompanied by a simultaneous change in all structure-sensitive physical mechanical properties. Naturally, a fundamental correlation between structural relaxation and the variation of the properties of the glass would be of great interest to scientists and engineers. Changes in mechanical and dielectric properties during the course of structural relaxation are of particular interest, but one must remember that their measurements involve the use of external stress (or strain), which, regardless of its magnitude, will affect the ongoing structural relaxation. The application of even the smallest stress or strain could shorten the relaxation time by one or more orders of magnitude and hence caution must be exerted in seeking correlations between volume and enthalpy relaxation on the one hand, and viscoelastic (mechanical or dielectric) measurements on the other. The two approaches can be viewed as manifestations of the same structural relaxation although one must bear in mind that they differ fundamentally in terms of the cause and the effect, and while the theoretical background of correlation between structural and viscoelastic relaxation has not been explored, in this paper an empirical correlation between the two will be reported.

Relaxation behaviour in the transition region of glassy systems

Following the Adam-Gibbs procedure for describing the relaxation process of glass-forming liquids, a Vogel-Fulcher type equation has been obtained, determined by a new structural parameter called the isovolume fictive temperature, T x. The resulting expression for the relaxation time has been applied satisfactorily to the experimental data of dielectric relaxation during physical ageing of poly(vinyl acetate). For testing the reliability of the proposed model, comparison with other phenomenological approaches has been made.

Physical ageing of poly(methyl methacrylate) from enthalpy relaxation measurements

The enthalpic ageing behaviour of poly(methyl methacrylate) was studied by differential scanning calorimetry for several ageing temperatures, and ageing times up to 10 4 min. The data are interpreted in terms of both the phenomenological multiparameter model and the Cowie-Ferguson model. The phenomenological multiparameter model was found not to provide an adequate description of the enthalpic ageing process(es) occurring in poly(methyl methacrylate). Problems encountered in defining the equilibrium glassy state by extrapolating from liquid enthalpy data are also discussed.

Thermal properties and physical ageing of poly ( l-lactic acid)

Poly( l-lactic acid) (PLLA) was investigated by differential scanning calorimetry and dynamic mechanical and dielectric spectroscopies. In the dynamic mechanical and dielectric spectra no secondary relaxations were observed below the glass-to-rubber transition (70°C at 3 Hz). The thermal history strongly affects the physical properties of PLLA, inducing changes of crystalline: amorphous ratio as well as large physical ageing effects on the glassy amorphous phase. This latter phenomenon is clearly observed as a consequence of room-temperature storage and develops at a faster rate as the ageing temperature T a approaches T g. Ageing experiments carried out at the same undercooling ( ΔT = T g - T a) on PLLA samples of different molecular weight ( M ̄ v = 5300, 20 000, 691 000 ) show that a decrease in molecular weight increases the magnitude of the enthalpy relaxation at the glass transition.

Physical aging in poly(methyl methacrylate) / poly(styrene‐co‐acrylonitrile) blends. I. Stress relaxation

AbstractAn investigation was carried out on the effect of physical aging on viscoelastic properties of compatible polymer blends. Poly(methyl methacrylate) (PMMA) and poly(styrene‐co‐acrylonitrile) (SAN) were chosen for this study. Blends of six different ratios of PMMA and SAN were physically aged at different times and temperatures and then subjected to stress–relaxation tests at small strains. It was found that the shape of the stress‐relaxation curve does not change with a change in blend composition, aging temperature, or aging time. A single “master curve” is presented, to which all data may be fit using 0.408 for the exponential parameter of the Williams‐Watts equation. In creating the master curve, all data are shifted horizontally in time only. For a given set of experimental conditions, the relaxation time τ may be expressed in terms of blend composition, aging temperature, and aging time.

Physical aging in poly(methyl methacrylate)/poly(styrene-co-acrylonitrile) blends. 3. Simulation of enthalpy relaxation using the Moynihan model

The Moynihan parameters for each blend composition were obtained from the best fits of specific heat data in the glass transition region measured in the rate-heating approach experiments and optimized by using the Marquardt algorithm. The optimized parameters were then used in simulations of experiments following the isothermal approach, in which a period of isothermal relaxation was introduced between cooling and heating steps. Calculated values for the isothermal enthalpy relaxation agreed with previously reported experimental data

Physical aging in poly(methyl methacrylate)poly(styrene‐co‐acrylonitrile) blends. Part II: Enthalpy relaxation

AbstractAn investigation of the effect of physical aging on excess enthalpy of compatible polymer blends was carried out. Poly(methyl methacrylate) (PMMA) and poly(styrene‐co‐acrylonitrile) (SAN) were chosen for this study. Blends of different ratios of PMMA and SAN were physically aged at different times and temperatures below their glass transition (Tg) and then subjected to enthalpy relaxation measurement in a differential scanning calorimeter (DSC). An improved procedure was developed and, employed to analyze the data. The error associated with the calculation of the normalized deviation in enthalpy, known as the “Φ” function, was below 4%. The relaxation was observed to proceed faster at higher aging temperature. It was also found that at higher aging temperatures of Tg – 20 and Tg– 35°C, enthalpy relaxation in SAN‐rich blends proceeds faster than in PMMA rich blends, while at the low aging temperature of Tg– 50°C the rate of relaxation becomes independent of the composition.

Effects of crystallinity on ageing phenomena in poly(vinyl chloride)

The physical ageing of poly(vinyl chloride) (PVC) is affected by the degree of crystallinity which can be induced by annealing of the sample at temperatures above T g. Differential scanning calorimetry (d.s.c.) measurements showed that the enthalpy relaxation, induced by sub- T g annealing, of PVC samples with different crystallinity increases with increasing crystallization temperature ( T c). On the other hand, the enthalpy relaxation decreases with increasing crystallization time ( t c) at a given T c. These observations cannot be attributed only to the overall crystallinity percentage, but they are attributable mainly to the entire structure of the amorphous phase which depends upon above- T g annealing conditions ( T c, t c). The assumptions of different portions of the material crystallized at different T c and of decreasing extent of disturbed regions as t c increases seem to explain well the above findings. Solvent sorption experiments served to support our suggestions resulting from d.s.c. experiments.