Dynamic mechanical studies of thermal aging in poly(vinyl chloride)

The effects of thermal aging on the dynamic mechanical properties of rigid poly(vinyl chloride) (PVC) and chlorinated poly(vinyl chloride) (CPVC) are examined in the glass transition (T_g) and β transition (T_β) regions. PVC, when quenched through T_g and then annealed at 40°C, exhibits a sub-T_g peak in the loss modulus function. The position of the peak moves to higher temperatures with increasing annealing time. The effect is analogous to sub-T_g endothermic peaks previously observed by differential scanning calorimetry (DSC). A sub-T_β peak in the tan δ or the loss modulus functions can be produced by quenching through T_β. The effect of heating rate on the sub-T_β peak is explored.     

Physical aging in graphite/epoxy composites

Matrix-dominated mechanical behavior of a graphite/epoxy composite has been found to be affected by sub-T_g annealing. Postcured (±45°)_4S specimens of Thornel 300 graphite/Narmco 5208 epoxy were quenched from above T_g and given a sub-T_g annealing at 140°C for times up to 10⁵ min. The ultimate tensile strength, strain-to-break, and toughness of the composite material were found to decrease as functions of sub-T_g annealing time. No weight loss was observed during the sub-T_g annealing. The time-dependent change in mechanical behavior is explained on the basis of free-volume changes that are related to the physical aging of the nonequilibrium glassy network-epoxy. The results imply possible changes in composite properties with service time.     

Physical aging of linear and network epoxy resins

Network and linear epoxy resins principally based on the diglycidyl ether of bisphenol-A and its oligomers have been prepared and studied. Both diamine and anhydride crosslinking agents were utilized. In addition, some rubber modified epoxies and a carbon fiber reinforced composite was investigated. All of these materials display time-dependent changes in many of their properties when they are stored (following quenching) at temperatures below their glass transition temperature (sub-T_g annealing). For example, the degree of stress relaxation for a given time period is observed to decrease in a linear fashion with the logarithm of time during sub-T_g annealing. Young's modulus and yield stress were also found to increase ire physical aging. Solvent sorption experiments initiated after different sub-T_g annealing times have demonstrated that the rate of solvent uptake can be indirectly related to the free volume of the epoxy resins. The effect of water on the physical aging of these epoxy resins was not found to be a significant variable. Residual thermal stresses were also found to have little effect on the physical aging process, although this variable was not studied in detail. Finally, the physical aging process also affected the sub-T_g properties of uniaxial carbon fiber reinforced epoxy material and the effects were as expected. The importance of the recovery or physical aging phenomenon, which affects the durability of epoxy glasses, is considered in view of the widespread applications for these resins as structural materials.     

Low-pressure CO2 sorption in poly(vinyl benzoate) conditioned to high-pressure CO2

Sorption of CO₂ in poly(vinyl benzoate) was gravimetrically measured at pressures up to 1 atm. Sorption isotherms were determined above and below the glass transition temperature T_g from 5 to 85°C. The isotherms were analyzed by the dual-mode sorption model assuming that the plasticizing effect of sorbed CO₂ is negligible at this pressure range. The solubilities and Henry's law dissolution parameters were compared with those obtained by the high-pressure sorption and permeation measurements. Henry's law dissolution parameters were in good agreement with one another. However, the solubilities first determined here were smaller than those determined by the high-pressure sorption experiment at the same temperature. It was clear that the Langmuir capacity of the present specimen was smaller in spite of similar high-pressure CO₂ exposure. Relaxation of the polymer was expected to be one of the reasons. This expectation was confirmed from the observation and analysis of sorption isotherms after two kinds of treatments. After annealing above T_g, the Langmuir capacity was shown to be decreased to 1/2 or even to 1/3 from the sorption isotherms below 45°C. This means that the conditioning to the high-pressure CO₂ surely has a large effect on the nature of glassy polymer. Just after high-pressure CO₂ exposure at 25°C, increased solubility was observed. Furthermore, the slow decrease of solubility, that is, the decrease of conditioning effect, was also followed from the continual measurements at 25°C. This result reflects not only the characteristic of sorption capacity after high-pressure CO₂ exposure, but also the relaxation of polymer in glassy state.     

Effect of physical annealing on the dynamic mechanical properties of a high-Tg amine-cured epoxy system

Physical annealing of a fully cured amine/epoxy system has been investigated using the freely oscillating TBA torsion pendulum technique. The material densifies spontaneously during annealing in an attempt to reach equilibrium, thereby changing material behavior. The dynamic mechanical behavior of a film specimen (T_g = 174°C, 0.3 Hz) and of a glass braid composite specimen (T_g = 182°C, 0.9 Hz) was monitored during isothermal annealing at sub-T_g temperatures (ranging to 230°C below T_g); after annealing, the behavior was measured vs. temperature and compared with that of the unannealed state. Isothermally, the storage modulus (G′) of the film specimen and the relative rigidity (1/P²) of the composite specimen increased almost linearly with log time, whereas the logarithmic decrement (Δ) decreased with time. The isothermal rates of annealing were determined from the rates of changes in G′ and in 1/P² for the film and composite specimens, respectively. In a wide temperature range between T_g and the secondary transition temperature, T_sec (≈ ?30°C, 2.3 Hz by TBA), the isothermal rates of annealing at the same annealing time appeared to be the same. Thermomechanical spectra of the isothermally annealed material revealed a maximum deviation in thermomechanical behavior from the unannealed material in the vicinity of the annealing temperature. The effects of physical aging were the same for the film and composite specimens. Effects of sequential annealing at two isothermal temperatures on the thermomechanical behavior were also investigated; when the second temperature was higher than the first, the effect of only the high-temperature annealing was evident, whereas the effect of annealing at both temperatures was revealed when the second temperature was lower than the first. Results suggest that physical annealing at different temperatures involves different length scales of chain segment relaxation and that the effects of isothermal aging can be eliminated by heating to below T_g.     

Quantitative study of the annealing of poly(vinyl chloride) near the glass transition

Poly(vinyl chloride) displays a normal DSC of DTA curve for the glass transition when quenched from above its T_g. However if cooled slowly or annealed near the glass transition temperature, a peak appears on the DSC or DTA curve at the T_g. In this paper quantitative studies of the time and temperature effects on the production of this endothermal peak during the annealing of PVC homopolymer and an acetate copolymer are presented. The phenomenon conforms to the Williams, Landell, and Ferry equation for the relaxation of polymer chains, the rate of the peak formation becoming negligible at more than 50°C below T_g. The energy difference between the quenched and annealed forms is small. For a PVC homopolymer annealed 2 hr at 68°C, which is T_g ?10°C, the difference is 0.25 cal/g. For a 13% acetate copolymer of PVC similarly annealed, the difference is 0.36 cal/g. The measured rates of the process give a calculated activation energy of 13–14 kcal/mole for PVC homopolymer and copolymer. This appearance of a peak on the T_g curve for a polymer when annealed near the glass temperature appears to be a general phenomenon.     

Sorption of propane in poly(ethyl methacrylate) near Tg

The sorption of propane in poly(ethyl methacrylate) was investigated above and below the polymer T_g and the dual-mode sorption model parameters were evaluated. The Langmuir capacity decreased as the temperature was raised to T_g and the apparent Henry's law constant exhibited a discontinuity at T_g. The enthalpy of microvoid filling ΔH_H is more exothermic than the enthalpy of dissolution at T < T_g by about 8 kJ/mol.     

Effect of annealing at temperature around glass transition temperature on molecular orientation and shrinkage of ethylene–vinyl alcohol copolymers

The effect of annealing on the molecular arrangement of ethylene–vinyl alcohol copolymers (EVA) was studied by optical methods. Drawn samples were annealed at temperatures near the glass transition temperature T_g in two cases; the first case allowed thermal shrinkage and the second case did not. For annealing below T_g, the hydroxyl groups were rearranged only to the stable position. Thermal shrinkage occurred by annealing at temperatures between T_g and T_g + 20–30 K. In the case of annealing at temperatures above T_g + 30 K, thermal shrinkage and crystallization occurred. The crystallization by annealing made the drawn sample difficult to shrink.     

Sorption kinetics and equilibria in annealed glassy polyblends

The sorption kinetics and equilibria of n-hexane in glassy polyblends of polystyrene and poly(2,6 dimethyl-1,4 phenylene oxide) were studied as a function of annealing conditions. Cast film samples were annealed 20°C above their respective glass transition temperatures for two hours and twenty-four hours. The rate of relaxation-controlled (Case II) sorption of n-hexane in these films was reduced markedly consequent to annealing. The effect of annealing on the sorption kinetics and the independently determined film densities was more pronounced for the poly(phenylene oxide)-rich samples. Although sorption rates were reduced by as much as a factor of 100, the sorption equilibrium was insignificantly affected by annealing. Super Case II transport was observed for the slow absorbing annealed samples whereas the more rapid sorption in the unannealed samples followed ideal Case II kinetics. The more pronounced effects of annealing for the poly(phenylene oxide)-rich samples on sorption rates and film densities were explained by considering the increasing difference between the film T_g and the drying temperature used in the original film preparation for the poly(phenylene oxide)-rich samples. These results suggest that glassy polymers, cast and dried well below their glass transition temperatures, will be subject to large long-term reductions in absorption rates and specific volume. Moreover, residual, excess free volume significantly affects relaxation-controlled absorption of vapors in partially annealed glassy polymers.     

Polymer crystallization induced by sorption of CO2 gas

Previous work has shown that sorption of CO₂ at relatively high pressures by glassy polymers reduces their glass transition temperatures and may convert the glass into a rubber under certain conditions. It is shown here that this plasticization by a gas can induce crystallization just as sorption of vapors or liquids is known to do. This point is extensively explored for miscible blends of poly(vinylidene fluoride) and poly(methyl methacrylate) and to a lesser extent for poly(ethylene terephthalate). In some cases, this secondary crystallization process results in small crystals whose melting endotherms are just above the glass transition and are very similar to peaks resulting from heat capacity overshoots, or enthalpic relaxation, caused by sub-T_g annealing; however, by appropriate techniques peaks arising from these two separate mechanisms can be distinguished. For oriented materials, evidence is shown which demonstrates that the additional crystals formed on CO₂ sorption have the same preferential orientation as the original material.     

Crystallization Mechanism of CVD Si3N4–SiCN Composite Ceramics Annealed in N2 Atmosphere and Their Excellent EMW Absorption Properties

To tailor a new electromagnetic wave (EMW) absorbing material with lower reflection coefficient (RC) and larger operating frequency band, the CVD Si₃N₄–SiCN composite ceramics were prepared from SiCl₄–NH₃–C₃H₆–H₂–Ar system and then annealed at the temperatures of 1400–1700°C in N₂ atmosphere. Effect of the annealing temperatures on the microstructure, phase composition, permittivity, and microwave‐absorbing properties of the ceramic were investigated. Results showed that the CVD Si₃N₄–SiCN ceramics gradually crystallized into nanosized SiC grains, Si₃N₄ grains and graphite (T ≤ 1600°C), and then the grains grew up at T = 1700°C. The permittivity, dielectric loss, and electrical conductivity of as‐annealed CVD Si₃N₄–SiCN ceramics (T ≤ 1600°C) increased firstly due to the formation of conductivity and polarity network and the increase in nanograin boundary, and then decreased at 1700°C because of the growth of nanograins and the disappearance of nanograin boundary. The minimal RC and effective absorption bandwidth of the as‐annealed CVD Si₃N₄–SiCN ceramic at 1600°C was ?41.67 dB at the thickness of 2.55 mm and 3.95 GHz at the thickness of 3.05 mm, respectively, demonstrating that the totally crystallized CVD Si₃N₄–SiCN ceramic (T = 1600°C) had the superior microwave‐absorbing ability.     

Permeation and sorption of CO2 through amine-contained polyurethane and poly(urea–urethane) membranes

As an absorbent of CO₂, tetraethylenepentamine (TEPA) and/or N-methyldiethanolamine (MDEA) was introduced into the hard segment as chain extenders of the series urethane polymer (PU), urea–urethane polymer (PUU), and segmented urethane/urea–urethane copolymer (SPU) based on 4,4′-diphenylmethane diisocyanate (MDI) and either poly(ethylene glycol (PEG)-400 or -600. The obtained polymers thus contained a nearly equal weight composition of both soft and hard segments and were prepared as polymer membranes for permeation and sorption of CO₂. The properties of the polymer membranes were characterized using a Fourier transform IR spectrophotometer, thermal gravimetric analysis, and rheometric measurement. The permeation and sorption isotherms as a function of temperature and pressure as well as the activation energy of permeability and diffusivity and the enthalpy change of the solution were measured. The test temperatures were carried out above and below the T_gs of soft-segment domains or the T_gh of hard-segment domains. The steady-state permeability (P) and diffusion coefficients (D) obtained ranged from 1.01 to 12.9 barrer and 1.04 to 4.04 cm²/s, respectively, and the solubility coefficients (S), from 0.529 to 3.43 cm³(STP)/cm³ polymer-atm at 10 atm and 35°C. The TEPA-containing polymer membranes showed a smaller P and D but a larger S than did MDEA-containing ones. The membranes comprising PEG-600 exhibit the values of P, D, and S at about 11.5-, 2.5-, and 4.5-fold the PEG-400 ones, respectively. For SPU membranes, above T_gs or T_gs, the pressure dependencies of P followed the modified free-volume model. On the other hand, below T_gs, they exhibited a minimum permeability at a certain pressure caused by the plasticization action of sorbed CO₂. The sorption isotherms of CO₂ indicated that the membranes comprising PEG-400 can be described by a dual-mode sorption model below T_gs. Also, the SPU polymer membranes obied the Henry's law above T_gs as well as T_gh. The characteristic constants of the sorption model were also determined and are discussed. © 1996 John Wiley & Sons, Inc.     

Oxygen transport in crosslinked,silicon-containing copolymers of methyl methacrylate

Copolymers of methyl methacrylate and 1,3-bis(methacryloxy methyl)-1,1,3,3-tetramethyl disiloxane were prepared by chemically induced copolymerization/crosslinking at 60°C and 49 mm Hg. Crosslinked, glassy copolymers were obtained with copolymer mole fraction of the silicon-containing monomer varying from 0.09 to 0.55. Oxygen transport studies were performed with thin films as prepared and after sub-T_g annealing. The results of this study indicated that an enhancement of both the steady state oxygen permeation rate and the oxygen diffusion coefficient was achieved through copolymerization. The oxygen diffusion coefficients through the copolymers were found, within experimental error, to be independent of silicon content and ranged from 0.80 × 10^?7 to 1.90 × 10^?7 cm²/s vs. oxygen diffusion coefficient for pure poly(methyl methacrylate) of 1.5 × 10^?8 cm²/s. Sub-T_g annealing effected a reduction of approximately equal magnitude in both the oxygen diffusion coefficient and the steady state oxygen flux for the copolymers. In addition, the normalized oxygen flux data were predicted with Fick's law, assuming constant boundry conditions and diffusion coefficient. These results were explained in terms of the free volume theory and the combined effects of increased crosslinking density, chain mobility, and oxygen solubility with increased copolymer silicon content.     

The crystallization of a copoly(chlorotrifluorethylene-vinylidene fluoride) from the amorphous rubbery state

A random copoly(chlorotrifluorethylene-vinylidene fluoride) in the ratio of 3 : 1 was annealed at the temperature range of T_g < T < T_m. The copolymer slowly crystallizes, attaining a rather low ultimate degree of crystallinity, depending on the annealing temperature, in the form of randomly distributed ribbonlike lamellae. The crystallites' melting temperatures are much lower than those of the corresponding homopolymers, increasing with annealing temperature and time. The crystallization kinetics, analyzed using the Avrami equation, indicates the formation of small, low-ordered crystallites. The crystallization process results in a dramatic increase in the elastic modulus at T > T_g. Annealing of stretched samples results in oriented crystallization at much higher rates than in the unstretched material, without markedly affecting the ultimate degree of crystallinity. The oriented crystallites, distributed in an isotropic amorphous matrix, exhibit lower thermal stability than the corresponding unoriented crystals. Their melting temperatures increase with annealing time; however, they decrease with the extent of stretching, suggesting a strong kinetic effect on the crystallites' degree of order.     

Miscibility and esterification in the poly(styrene-co-maleic anhydride)/phenoxy blends

The “miscibility” and esterification in poly(styrene-co-maleic anhydride) (PSTMA)/phenoxy blends were investigated by DSC and FTIR. The blends prepared by casting exhibited a single composition-dependent but broad T_g during the first scanning. The broadness of the T_g transition range is due to the presence of microphases in the blends, which acquired some stability because of the hydrogen-bonding interactions with the continuous phase. However, the blends displayed two distinct T_gs during the second scanning, which can be attributed to phenoxy-rich and PSTMA-rich phases dispersed one in another at a scale larger than the initial one. To investigate the effect of esterification, the samples subjected previously to two scannings have been additionally heat-treated several times between 30 and 220°C and annealed each time at 220°C for increasing periods of time. During the additional scannings, the two T_gs identified during the second scanning increased with increasing annealing time but remained distinct. The fact that the fraction soluble in tetrahydrofuran decreased with increasing annealing time indicates that crosslinking due to esterification has occurred in both phases. The two phases generated after the first scanning were stabilized by the esterification reaction at the interfaces. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:913–919, 1998     

Physical aging of heavy metal fluoride glasses—sub-Tg enthalpy relaxation in a ZrF4-BaF2-LaF3-AlF3 glass

The enthalpy changes during structural relaxation or physical aging of a 58ZrF₄-33BaF₂-5LaF₃-4AlF₃ (ZBLA) glass during annealing well below the glass-transition temperature have been studied using differential-scanning calorimetry at several sub-T_g temperatures. Substantial relaxation within the course of several hours was detected at temperatures as low as sixty degrees below T_g (585K). The relaxation process is extremely nonlinear and self-retarding. The time dependence of the enthalpy during the initial stages of annealing was modeled approximately using the Narayanaswamy-Tool approach. The structural-relaxation parameters obtained from this fit were used to predict rates of physical aging for rapidly cooled ZBLA glass at temperatures close to ambient.     

Synthesis and thermally stimulated current study of molecular relaxations of the grafted polymethylphenylsiloxane segment in thiodiphenol-modified epoxy resin

A series of grafted polymethylphenylsiloxane (PMPS) segments in thiodiphenyl-containing epoxy resin (ESTP) was prepared. The structure was evaluated by IR, ¹H-NMR, and ¹³C-NMR measurements. DSC measurements on the grafted ESTP epoxy resins showed a decreasing trend for T_g with increasing content of PMPS siloxane. The TSC measurements further confirmed this trend. This result suggests that the matrix of ESTP copolymer was less rigid with grafting of PMPS oligomer, due to the Si—O—C linkage at the opened glycidyl ether chain located approximately between the BPA and 4,4′-thiodiphenoxyl chains in the epoxy backbone. A new and broad sub-T_g transition appeared at −60°C in the TSC spectrum was observed for the cooperative motion of this siloxane moiety. All copolymers showed sub-T_g relaxations of γ- and δ-modes, observed at −100 and −130°C, respectively. These two relaxations may be attributed to the motions of BPA and the terminal groups in the epoxy matrix. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 69: 1523–1530, 1998     

Low pressure CO2 sorption in poly(vinyl cyclohexane-carboxylate)

A gravimetric method was applied to determine the solubility of CO₂ in poly(vinyl cyclohexanecarboxylate) below 1 atm. The temperatures were varied from 5 to 85°C, above and below the glass transition temperature. The sorption isotherms were concave to the pressure axis below T_g and they were tentatively analyzed by the dual-mode sorption model. Above T_g, the isotherms were linear and described by the Henry's law. Below 75°C, the diffusion coefficients of CO₂ were also obtained from the half-time of the sorption process.     

Studies on the thermal properties of poly(phenylene sulfide amide)

Thermal properties of poly(phenylene sulfide amide) (PPSA) prepared using sodium sulfide, sulfur, and thiourea as sulfur sources which reacted with dichlorobenzamide (DCBA) and alkali in polar organic solvent at the atmospheric pressure, were studied. The glass transition temperature (T_g), melting point temperature (T_m), and melting enthalpy (ΔH_m) of the related polymers were obtained by use of differential scanning calorimetry analysis. The results are: T_g = 103.4–104.5°C, T_m = 291.5–304.7°C, and ΔH_m = 104.4–115.4 J/g. Thermal properties such as thermal decomposition temperature and decomposition kinetics were investigated by thermogravimetric analysis under nitrogen. The initial and maximum rate temperatures of degradation were found to be 401.5–411.7°C and 437–477°C, respectively. The parameters of thermal decomposition kinetics of PPSAs were worked out to be: activation energy of degradation was 135 to 148 kJ/mol and the 60-s half-life temperature was 360 to 371°C. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 1227–1230, 1997     

Properties of syndiotactic-rich poly(vinyl alcohol) thin film in water. V. Effect of initial temperature of heating on elongation

The effect of the initial temperature of heating on the elongation of syndiotactic-rich poly(vinyl alcohol) thin films was investigated in water under a load. The elongation ratios E_i after 4 h at fixed temperatures increased roughly with an increase in the initial temperature T_i and a decrease in the annealing temperature. E_i after 4 h was the smallest at T_i = 45°C for the films annealed at temperatures below 100°C. E_i was 6.8 at T_i = 60°C for the unannealed film and 1.12 at T_i = 70°C for the film annealed at 200°C. The elongation at break decreased and the temperature at break increased with an increase in annealing temperature, but those at the annealing temperature of 100°C were the smallest. The films annealed at 200°C did not break even at 98°C (boiling temperature) in water and the elongation ratio was 1.42–1.97 in the initial temperature range of 10–70°C. From these results, the relation between the elongation in water and the state of polymer chains in film was discussed.