Physical aging of poly(ether sulfone)-modified epoxy resin

The physical aging process of 4,4′-diaminodiphenylsulfone (DDS) cured diglycidyl ether bisphenol-A (DGEBA) blended with poly(ether sulfone) (PES) was studied by differential scanning calorimetry (DSC) at four aging temperatures between T_g-50°C and T_g-10°C. At aging temperatures between T_g-50 and T_g-30°C, the experimental results of epoxy resin blended with 20 wt% of PES showed two enthalpy relaxation processes. One relaxation process was due to the physical aging of PES, the other relaxation process was due to the physical aging of epoxy resin. The distribution of enthalpy relaxation process due to physical aging of epoxy resin in the blend was broader and the characteristic relaxation time shorter than those of pure epoxy resin at the above aging temperatures (between T_g-50 and T_g-30°C). At an aging temperature between T_g-30 and T_g-10°C, only one enthalpy relaxation process was found for the epoxy resin blended with PES, the relaxation process was similar to that of pure epoxy resin. The enthalpy relaxation process due to the physical aging of PES in the epoxy matrix was similar to that of pure PES at aging temperatures between T_g-50 and T_g-10°C. © 1997 John Wiley & Sons, Inc.     

Physical Aging of a Tetrafunctional/phenol Novolac Epoxy Mixture Cured with Diamine. DSC and DMA measurements

The physical aging of a system containing tetraglycidyl-4-4′-diaminodiphenylmethane (TGDDM), with a multifunctional novolac glycidyl ether resin hardened by 4,4′-diaminodiphenylsulphone (DDS) has been investigated by differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA). Samples fully cured were aged at temperatures between 200 and 250°C, during periods of time from 1 to a maximum of 336 h. Furthermore, the dynamic mechanical relaxation behaviour annealed at temperature of 220°C, was studied, aging during 24 and 168 h. The effect of the enthalpy relaxation during DSC heating scan is shown by the presence of an endothermic peak whose position and intensity depends on the aging conditions, both temperature and time. DSC studies suggest that enthalpy relaxation increases gradually with aging time to a limiting value for each temperature where structural equilibrium is reached. DMA results show that the effect of aging is to cause chain stiffening and a decrease in the height of the peak value of the loss factor. This revised version was published online in August 2006 with corrections to the Cover Date.     

Physical aging studies in epoxy resins. I. Kinetics of the enthalpy relaxation process in a fully cured epoxy resin

The physical aging of an epoxy resin based on diglycidyl ether of bisphenol-A cured by a hardener derived from phthalic anhydride has been studied by differential scanning calorimetry. The isothermal curing of the epoxy resin was carried out in one step at 130°C for 8 h, obtaining a fully cured resin whose glass transition was at 98.9°C. Samples were aged at temperatures between 50 and 100°C for periods of time from 15 min to a maximum of 1680 h. The extent of physical aging has been measured by the area of the endothermic peak which appears below and within the glass transition region. The enthalpy relaxation was found to increase gradually with aging time to a limiting value where structural equilibrium is reached. However, this structural equilibrium was reached experimentally only at an aging temperature of T_g-10°C. The kinetics of enthalpy relaxation was analysed in terms of the effective relaxation time τ_eff. The rate of relaxation of the system given by 1/τ_eff decreases as the system approaches equilibrium, as the enthalpy relaxation tends to its limiting value. Single phenomenological approaches were applied to enthalpy relaxation data. Assuming a separate dependence of temperature and structure on τ, three characteristic parameters of the enthalpic relaxation process were obtained (In A = ?333, E_H = 1020 kJ/mol, C = 2.1 g/J). Comparisons with experimental data show some discrepancies at aging temperatures of 50 and 60°C, where sub-T_g peaks appears. These discrepancies probably arise from the fact that the model assumes a single relaxation time. A better fit to aging data was obtained when a Williams-Watts function was applied. The values of the nonexponential parameter β were slightly dependent on temperature, and the characteristic time was found to decrease with temperature. © 1994 John Wiley & Sons, Inc.     

Epoxy resins (DGEBA): The curing and physical aging process

Differential scanning calorimetry (DSC) and infrared spectroscopy (IR) were used to monitor the degree of cure of partially cured epoxy resin (Epon 828/MDA) samples. The extent of cure, as determined by residual heat of reaction, concurred with that determined by monitoring the infrared radiation absorbance of the epoxide group near 916 cm^?l. The fictive temperature T_{f, g} was found to increase with the degree of cure, increasing rapidly during cure until reaching a value near the cure temperature T_c of 130°C (approximately 80% cure) where the material vitrified. The greatly reduced reaction rate during the final 20% of cure was not only a consequence of vitrification but, as revealed by infrared spectroscopy, the result of the depletion in the number of reactive epoxide groups. The endothermic peak areas and peak temperatures evident during the DSC scans were used as a measure of the extent of “physical aging” which took place during the cure of this resin, and after, fully cured samples were aged 37°C below their ultimate glass temperature for various periods of time. The rate of physical aging slowed as the temperature increment (T_t,g ? T_c) increased. Although an endothermic peak was evident after only 1 h of cure (T_{f, g} = 138.3°C), such a peak did not appear until fully cured samples were aged for 16 h or more. Enthalpy data revealed that for partially cured material, the fictive temperature T_{f, a}, reflecting physical aging, increased with curing time. In contrast, the T_{f, a}, for fully cured samples decreased with sub-T_g aging time. The characteristic jump in the heat capacity ΔC_p which occurred at the T_{f, g} decreased as curing progressed. This decrease appears to be dependent upon the rotational and vibrational degrees of freedom of the glass. Finally, a graphical method of determining the fictive temperature T_{f, a}, of partially and fully cured epoxy material from measured endothermic peak areas was developed.     

Enthalpy relaxation in an epoxy-cycloaliphatic amine resin

Enthalpy relaxation in a system containing the diglycidyl ether of bisphenol A (DGEBA) resin and a diamine, 1,3-bisaminomethylcyclohexane (1,3-BAC) as curing agent, has been investigated by differential scanning calorimetry (DSC). Samples fully cured were annealed at temperature T_g–15 °C for periods of time from 1 h to a maximum of 168 h. The enthalpy relaxation is analyzed by the peak shift method, in which the sample is heated at 10 °C/min following cooling at various rates through the glass transition region. The key parameters of structural relaxation determined were the non-linearity parameter x=0.47 ± 0.02, the apparent activation energy Δh*=1264 ± 48 kJ/mol or Δh*/R=152 ± 6 kK and the non-exponentiality parameter β ≈ 0.3. The results, obtained by the same method, were compared with those for other systems based on fully cured DGEBA. The correlations among these parameters with the peak shift model should be considered with caution. However, the results show that a correlation between crosslink lengths and the value of Δh* can be considered. The relaxation process for DGEBA/1,3-BAC proves to be highly cooperative. Received: 28 June 2000 Accepted: 6 September 2000     

Influence of physical aging on the performance of corona‐charged amorphous polymer electrets

The influence of physical aging on the electret properties before corona charging of three amorphous polymers, polyetherimide (PEI), poly(phenylene ether) (PPE), and polystyrene (PS), as well as with blends of PPE and PS, was investigated. The degree of aging was monitored by determining the enthalpy relaxation Δh using differential scanning calorimetry (DSC). The electret performance was evaluated by isothermal potential decay (ITPD) at elevated temperatures and by thermal stimulated discharge (TSD) measurements. It was demonstrated that physical aging below the glass transition temperature substantially improves the electret performance of amorphous polymers by reducing the free volume and thus hindering charge motion. As an example, the performance of nonaged PEI was improved by physical aging at 200 °C for 4 days from 18 to 95% retained charge after 24 h at 120 °C. A similar beneficial influence of physical aging on the charge storage capability was achieved using blends of PPE with PS. © 2010 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 48: 990–997, 2010     

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.     

Changes of mechanical properties in cold-crystallized syndiotactic polypropylene during aging

Many semicrystalline polymers undergo a process of aging when they are stored at temperatures higher than their glass-transition temperature (T _g). Syndiotactic polypropylene was quenched from the melt to −40 °C, crystallized from the glassy state at 20 or 40 °C and stored at the respective temperature for different aging times up to 7200 h. A significant increase in the tensile modulus and stress at yield and a decrease in strain at yield were observed for both aging temperatures. Differential scanning calorimetry (DSC) scans of aged material showed an endothermic annealing peak 15–30 °C above the previous aging temperature, the maximum temperature and enthalpic content of which increased with aging time. The position and the shape of the melting peak were not affected by aging. Scans of the storage modulus obtained from dynamic mechanical analyser measurements indicated a softening process starting at about 20 °C above the aging temperature and correlating with the annealing peak detected by DSC. Density measurements and wide-angle X-ray scattering investigations revealed that neither the crystallinity increased significantly nor did the crystal structure change. So the observed property changes induced by aging are attributed to microstructural changes within the amorphous phase. Furthermore, it could be shown by annealing experiments carried out at 60 °C, that aging above T _g is, analogous to aging below T _g (physical aging), a thermoreversible process. Received: 18 September 2000 Accepted: 2 January 2001     

Physical aging of a polyetherimide: Creep and DSC measurements

Creep and differential scanning calorimetry (DSC) measurements have been used to study the physical aging behavior of a polyetherimide. Isothermal aging temperatures ranged from 160°C to T_g with aging times ranging from 10 min to 8 days. The only measurable effect of physical aging on the short-time creep curves is a shift of the creep compliance to longer times. Andrade plots of the compliance versus the cube root of time are linear at short times with the slope β decreasing with increasing aging time to a constant value once equilibrium is reached. Log β³ is related directly to the degree to which the creep curves shift to longer times with physical aging, and is used in this work as a measure of physical aging. A reduced curve of log β³ versus log aging time is obtained for the aging temperatures investigated by appropriate vertical and horizontal shifts. The enthalpy change during aging increases linearly with the logarithm of the aging time, t_a, leveling off at equilibrium at values which increase with decreasing aging temperature. Hence, both nonequilibrium and equilibrium temperature shift factors can be calculated from the DSC data. Good agreement is observed between the equilibrium temperature shift factors obtained from the creep and DSC data. The temperature dependence of the nonequilibrium temperature shift factors is found to be an order of magnitude smaller than that of the equilibrium shift factors. The time scales to reach equilibrium for enthalpy and for mechanical measurements are found to be the same within experimental error. © 1995 John Wiley & Sons, Inc.     

Enthalpy relaxation of an epoxy–anhydride resin by temperature‐modulated differential scanning calorimetry

The enthalpy relaxation of an epoxy–anhydride resin was studied by physical aging and frequency‐dependence experiments with alternating differential scanning calorimetry (ADSC), which is a temperature‐modulated differential scanning calorimetry technique. The samples were aged at 80 °C, about 26 K below the glass‐transition temperature, for periods up to 3800 h and then scanned under the following modulation conditions: underlying heating rate of 1 K min⁻¹, amplitude of 0.5 K, and period of 1 min. The enthalpy loss was calculated by the total heat‐flow signal, and its variation with the log (aging time) gives a relaxation rate (per decade), this value being in good agreement with that calculated by conventional DSC. The enthalpy loss was also analyzed in terms of the nonreversing heat flow, revealing that this property is not suitable for calculating enthalpy loss. The effect of aging on the modulus of the complex heat capacity, |Cp*|, is shown by a sharper variation on the low side of the glass transition and an increase in the inflexional slope of |Cp*|. Likewise, the phase angle also becomes sharper in the low‐temperature side of the relaxation. The area under the corrected out‐phase heat capacity remains fairly constant with aging. The dependence of the dynamic glass transition, measured at the midpoint of the variation of |Cp*|, on ln(frequency) allows one to determine an apparent activation energy, Δh*, which gives information about the temperature dependence of the relaxation times in equilibrium over a range close to the glass transition. The values of Δh*, determined from ADSC experiments in a range of frequencies between 4.2 and 33 mHz and at an amplitude of 0.5 K, and an underlying heating rate of 1 K min⁻¹, were analyzed and compared with that obtained by conventional DSC from the dependence of the fictive temperature on the cooling rate. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 2272–2284, 2000     

Physical aging of an amorphous polyimide: Enthalpy relaxation and mechanical property changes

The physical aging behavior of an isotropic amorphous polyimide possessing a glass transition temperature of approximately 239°C was investigated for aging temperatures ranging from 174 to 224°C. Enthalpy recovery was evaluated as a function of aging time following sub‐T_g annealing in order to assess enthalpy relaxation rates, and time‐aging time superposition was employed in order to quantify mechanical aging rates from creep compliance measurements. With the exception of aging rates obtained for aging temperatures close to T_g, the enthalpy relaxation rates exhibited a significant decline with decreasing aging temperature while the creep compliance aging rates remained relatively unchanged with respect to aging temperature. Evidence suggests distinctly different relaxation time responses for enthalpy relaxation and mechanical creep changes during aging. The frequency dependence of dynamic mechanical response was probed as a function of time during isothermal aging, and failure of time‐aging time superposition was evident from the resulting data. Compared to the creep compliance testing, the dynamic mechanical analysis probed the shorter time portion of the relaxation response which involved the additional contribution of a secondary relaxation, thus leading to failure of superposition. Room temperature stress‐strain behavior was also monitored after aging at 204°C, with the result that no discernible embrittlement due to physical aging was detected despite aging‐induced increases in yield stress and modulus. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 1931–1946, 1999     

Enthalpy relaxation in a partially cured epoxy resin

The enthalpy relaxation of a partially cured (70%) epoxy resin, derived from diglycidyl ether of bisphenol-A cured by methyl-tetrahydrophthalic anhydride with accelerator, has been investigated. The key parameters of the structural relaxation (the apparent activation energy Δh*, the nonlinearity parameter x, and the nonexponentiality parameter β) are compared with those of the fully cured epoxy resin. The aging rates, characterized by the dependences of the enthalpy loss and peak temperature on log(annealing time), are greater in the partially cured epoxy than they are in the fully cured resin at an equivalent aging temperature (T_a = T_g − 20°C). There is a significant reduction in Δh*, from 1100 kJ mol⁻¹ for the fully cured system to 615 kJ mol⁻¹, as the degree of cure is reduced. The parameter x determined by the peak-shift method appears essentially independent of the degree of cure (x = 0.41 ± 0.03 for the partially cured resin compared with 0.42 ± 0.03 obtained previously for the fully cured resin), and does not follow the usually observed correlation of increasing x as Δh* decreases. This invariability of the parameter x seems to indicate that it is determined essentially by the local chemical structure of the backbone chain, and rather little by the supramolecular structure. On the other hand, the estimated nonexponentiality parameter β lies between 0.3 and 0.456, which is significantly lower than in the fully cured epoxy (β ≅ 0.5), indicative of a broadening of the distribution of relaxation times as the degree of cross-linking is reduced. Like the parameter x, this also does not follow the usual correlation with Δh*. These results are discussed in the framework of strong and fragile behavior of glass-forming systems, but it is difficult to reconcile these results in any simple way with the concept of strength and fragility. © 1996 John Wiley & Sons, Inc.     

Preparation and Properties of Novel Cyclophosphazenes Containing Cyanato Groups

Abstract

Novel cyclotriphosphazenes containing cyanato group (PZCN) derivatives were synthesized by a substitution reaction of 4-hydroxyphenoxycyclotriphosphazenes and cyanogen bromide (BrCN) in the presence of triethylamine (TEA). The PZCNs were characterized by FT-IR, liquid chromatography–mass spectrometry (LC-MS), ¹H NMR, ¹³C NMR, and ³¹P NMR spectroscopy. Curing reactions of the PZCNs were evaluated by FT-IR spectroscopy, thermogravimetry/differential thermal analysis (TG/DTA), and differential scanning, calorimetry (DSC). The PZCNs exhibited an exothermic peak due to curing within the temperature range of 140–300°C by DSC. The PZCNs were completely cured at 220°C. The cured PZCNs exhibited high thermal stability up to 350°C, a high char-forming capability, and electrical properties, such as dielectric constants (Dks) between 2.68 and 2.87, and dissipation factors (Dfs) between 0.008 and 0.013 at 1 MHz.

Supplemental materials are available for this article. Go to the publisher's online edition of Phosphorus, Sulfur, and Silicon and the Related Elements to view the free supplemental file.     

Rate of physical aging of polycarbonate at a constant tensile strain

A study was made of the physical aging of an annealed polycarbonate film at a constant extension of 2.6% at 5 temperatures from 30 to 110°C. During stress relaxation at the constant extension, the storage modulus in tension, E′, was determined by imposing a sinusoidal strain of small amplitude at frequencies up to 25 Hz during an aging period, commonly of 5 h. Plots of log E′ against log f, where f is the frequency, gave parallel straight lines, each at a different aging (elapsed) time t_e. These lines were superposed by shifts along the abscissa. The obtained shift factors log a varied linearly with log t_e, the slope being the aging rate μ, a quantity introduced by Struik. The results show that μ is about 1.37 at 30°C and that it increases progressively with the temperature and becomes approximately 2.13 at 110°C. Another measure of the aging rate is the rate of increase of E′ with t_e, expressed as the percent increase per decade of the aging time. This quantity also increases progressively with the temperature from about 2.6% at 30°C to nearly 3.5% per decade of time at 110°C.     

Thermal relaxation in poled non-linear optical sidechain polymers: Influence of annealing

The influence of different annealing treatments has been investigated on the stability of the Electro-Optic coefficient in a poled DANS (4-dimethylamino-4′-nitrostilbene) sidechain polymer, which was poled near its T_g-onset of 140°C. The annealing treatments and the EO relaxation experiments were performed at 120°C. The EO stability can be significantly increased both by an annealing treatment after poling and by annealing before poling. This latter effect is concluded from the reduction of the EO relaxation rates in successive relaxation cycles performed in the same polymer sample - completely depoled and repoled in between the cycles. The effect from the different annealing treatments on the time-dependence of the EO relaxation process is different and is analyzed in terms of a Debye-like model with a single, time-dependent Debye relaxation time r(t)=r_i+C. t³. The time-dependence of r and the observed effects from annealing are attributed to the parallel relaxation process of physical ageing.     

Effect of previous thermal history on physical aging of amorphous poly (ethylene terephthalate)

Physical aging of amorphous PET, at aging temperatures (T_a) of 40 and 60°C and different aging times (t_a), has been studied by DSC using two kinds of samples with different thermal history: Liquid-Nitrogen-Quenched samples (LNQ) and DSC cooled samples at a controlled cooling rate of 60 K/min (CC). At T_a = 40°C, a sub-T_g peak appears in LNQ samples but is not clearly observed in the CC samples. At T_g = 60°C, a superposed peak to T_g is observed in both kinds of samples. This different behaviour can be explained considering the distribution of relaxation times in the polymer.     

Preparation and properties of novel benzoxazine and polybenzoxazine with maleimide groups

A benzoxazine compound with a maleimide group, 3‐phenyl‐3,4‐dihydro‐2H‐6‐(N‐maleimido)‐1,3‐benzoxazine (HPM‐Ba), was prepared from N‐(4‐hydroxyphenyl)maleimide, formaldehyde, and aniline. The chemical structure of HBM‐Ba was identified by FT‐IR, ¹H‐NMR, and elemental analysis. HPM‐Ba showed a melting point of 52–55 °C and good solubility in common organic solvents. HPM‐Ba showed a two‐stage process of thermal polymerization. The first stage arose from the polymerization of maleimide groups, and the second one was the ring‐opening reaction of benzoxazine groups. Fusible polymaleimides with a Tg of around 100 °C could be obtained by thermally polymerizing HPM‐Ba at 130 °C. Further polymerizing the polymaleimides at 240 °C resulted in a completely cured resin showing a Tg at 204 °C. Good thermal stability and self‐extinguishing behavior was observed with the cured polybenzoxazine resins. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5954–5963, 2004     

Thiourethane‐based thiol‐ene high Tg networks: Preparation,thermal, mechanical,and physical properties

Thiourethane‐based thiol‐ene (TUTE) films were prepared from diisocyanates, tetrafunctional thiols and trienes. The incorporation of thiourethane linkages into the thiol‐ene networks results in TUTE films with high glass transition temperatures. Increases of T_g were achieved by aging at room temperature and annealing the UV cured films at 85 °C. The aged/annealed film with thiol prepared from isophorone diisocyanate and cured with a 10,080‐mJ/cm² radiant exposure had the highest DMA‐based glass transition temperature (108 °C) and a tan δ peak with a full width at half maximum (FWHM) of 22 °C, indicating a very uniform matrix structure. All of the initially prepared TUTE films exhibited good physical and mechanical properties based on pencil hardness, pendulum hardness, impact, and bending tests. © 2007 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 45: 5103–5111, 2007     

Effects of physical aging,crystallinity, and orientation on the enzymatic degradation of poly(lactic acid)

The effects of physical aging, degree of crystallinity, and orientation of poly(lactic acid) (PLA) were studied using differential scanning calorimetry (DSC) and wide angle X-ray scattering (WAXS). The samples of PLA with 96% [L] and 4% [D] contents were prepared by injection molding. The physical aging of PLA strongly depended on time and temperature. The change of rate of physical aging was very fast initially and slowed down as time increased. The enzymatic degradation of PLA was carried out with proteinase K at 37°C at a pH value of 8.6 in a Tris/HCl buffer solution. The enzymatic degradation rate was found to decrease as a function of physical aging (i.e., excess enthalpy relaxation). The rate of enzymatic degradation of PLA decreased with the increase in crystallinity. A threshold was observed when the heat of fusion was less than 20 J/g. The weight loss of PLA with a low level of crystallinity had no apparent change during any period of testing time. The rates of enzymatic degradation of stretched and injection-molded specimens were comparable. © 1996 John Wiley & Sons, Inc.     

Molecular motion during physical aging in polystyrene: Investigation using probe reorientation

A photobleaching method has been used to observe the reorientation of tetracene and rubrene in polystyrene during physical aging. Rotation times change more than an order of magnitude during isothermal aging after a temperature quench from above T_g. Down‐ and up‐jumps of the temperature show the expected asymmetry due to the nonlinearity of the aging process. The rotation times of tetracene and rubrene require the same amount of time to reach equilibrium after a temperature change (10³ − 10⁵ s in the range 93–99 °C). These equilibration times are the same order of magnitude as equilibration times for volume and enthalpy relaxation, but have a somewhat weaker temperature dependence. Very near equilibrium, the rates of aging are different for the two probes, with rubrene approaching equilibrium more rapidly at very long times. This may be understood if the aging process is spatially heterogeneous, that is, if aging occurs more rapidly in some small regions of the sample than in others. © 2000 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 38: 68–79, 2000