Erasure below glass‐transition temperature of effect of isothermal physical aging in fully cured epoxy/amine thermosetting system

The erasure below the glass‐transition temperature (T_g) of the effect of isothermal physical aging (at aging temperature T_a) in a fully cured epoxy/amine thermosetting system is investigated using the torsional braid analysis (TBA) dynamic mechanical analysis technique and the differential scanning calorimetry (DSC) technique. From the TBA temperature scans, the intensity of the localized perturbation of the moduli in the vicinity of the T_a (90°C), due to isothermal physical aging, is decreased by heating to below the T_g (T_g∞ = 177°C), indicating that the physical aging effect can be eliminated by heating to below the T_g. The isothermal aging effect in the vicinity of the T_a is almost completely eliminated by heating to 50°C above the T_a (i.e., 140°C); however, a competing aging effect occurs above T_a at higher temperatures during the heating. Erasure below T_g of the isothermal physical aging effect is inferred from DSC experiments from the diminished relaxation enthalpy in the vicinity of the T_g, which is measured from the difference in areas between the aged (T_a = 150°C) and deaged thermograms. A comparison of the TBA and DSC results is made. Implications on the heterogeneous nature of the amorphous glassy state of polymers are discussed. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 81: 396–404, 2001     

Conversion—temperature—property relationships in thermosetting systems: Property hysteresis due to microcracking of an epoxy/amine thermoset—glass fiber composite

A single specimen of an epoxy/amine thermoset—glass fiber composite was examined, using a freely oscillating torsion pendulum operating at ∼ 1 Hz, for different conversions (as measured by T_g) from T_g0 = 0°C to T_g∞ = 184°C during cooling and heating temperature scans. T_g was increased for successive pairs of scans by heating to higher and higher temperatures. The data were used in two ways: (i) vs. temperature for a fixed conversion to obtain transitions, modulus, and mechanical loss data, and (ii) by crossplotting to obtain isothermal values of the mechanical parameters vs. conversion (T_g). Hysteresis between cooling and subsequent heating data was observed in temperature scans of essentially ungelled material (T_g < 70°C) and was attributed to spontaneous microcracking. Hysteresis was analyzed in terms of the following three parameters: T_crack, the temperature corresponding to the onset of microcracking on cooling; T_heal, the temperature at which the specimen heals on subsequent heating; and the difference between isothermal cooling and heating data vs. conversion. Results were incorporated into a more general conversion—temperature—property diagram which serves as a framework for relating transitions (relaxations) to macroscopic behavior. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 39–53, 1997     

Physical aging deep in the glassy state of a fully cured polyimide

Physical aging of a fully cured polyimide/glass fiber specimen has been investigated deep in the glassy state using a freely oscillating torsion pendulum (TBA). A single specimen, the physical aging effects of which could be erased by heating to above T_g = 304°C (0.8 Hz), could be used for all experiments. Data were obtained during isothermal aging at different aging temperatures, T_α, (from 10°C to T_g) and during subsequent temperature scans (T_α to 5 to 315°C). The aging rate depended upon the value of T_α relative to both T_g and the β-relaxation temperature, T_β = 139°C (1.3 Hz). Changes in thermomechanical behavior due to aging were localized about T_α. This suggests that only an intermediate portion of the relaxation spectrum participates in, and is affected by, isothermal physical aging. It follows, and is observed, that the intensity of the β-relaxation mechanical loss peak is perturbed most significantly by aging at T_α near T_β. The effect of isothermal aging deep in the glassy state could be essentially eliminated by heating to below T_g. © 1992 John Wiley & Sons, Inc.     

Isothermal physical aging of poly(methyl methacrylate): Localization of perturbations in thermomechanical properties

Effects of isothermal physical aging on poly(methyl methacrylate) (PMMA) were investigated for isothermal aging temperatures (T_a) from T_g?13 to T_g?128°C using a freely oscillating torsion pendulum (TBA). A single PMMA-glass fiber specimen, the effects of the thermal history of which could be erased by heating above T_g (=116°C, 0.7 Hz), was used for all experiments; this facilitated comparison of the unaged and aged specimen. The modulus was observed to increase linearly with the logarithm of isothermal aging time. Thermomechanical properties of the aged versus unaged specimen showed perturbations (e.g., increased modulus) principally in the vicinity of T_a. This suggests that different intermediate portions of the relaxation spectrum are specifically involved in the process of aging for different values of T_a.     

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.     

Annealing studies on a fully cured epoxy resin: Effect of thermal prehistory,and time and temperature of physical annealing

The dynamic mechanical behavior at about 1 Hz of a fully cured epoxy resin (maximum glass transition temperature, T_g∞, ca. 170°C) ahs been studied during and after isothermal annealing in terms of the influence of thermal prehistory, time of annealing, and temperature of annealing (T_a). Annealing temperatures ranged from T_g ? 15 to T_g ? 130°C. The rate of isothermal annealing was observed to decrease by a decade for each decade increase of annelaing time when the material was far from equilibrium. Annealing at high temperatures did not measurably affect the mateiral properties during cooling (for T ? T_a); similarly the effect of annealing at low temperatures was not measurale during heating (for T ? T_a).     

Relaxation behavior and activation energy of relaxation for polyimide and polyimide–graphene nanocomposite

The relaxation behavior of polyimide and its nanocomposite containing 10 wt % of graphene was studied by using the dynamic mechanical spectrometer. Dynamic mechanical analysis of polyimide and its composite was performed as a function of temperature and frequency in the temperature range of 25–480 °C and frequency range between 0.05 and 100 Hz. The effect of increasing frequency of testing from 0.05 to 100 Hz is a significant shift from the glass transition temperature, T_g, to higher temperature from 360 °C at 0.05 Hz to 420 °C at 100 Hz. The tan δ peak height for both α and β transitions decreased with increasing test frequency from 0.24 at 0.05 Hz to 0.08 at 100 Hz, due to increasing restriction to chain motion. At any given testing frequency, the T_g for the composite was shown to be higher than that for the matrix by about 5–10 °C. The Arrhenius equation was used to calculate the activation energy for both α and β transitions. The activation for α and β transitions for the composite and polyimide matrix were determined to be 688 and 537 kJ/mol and 313 and 309 kJ/mol, respectively, indicating that a significant increase in the energy barrier to chain relaxation occurred as a result of reinforcement of polyimide with low weight fraction of graphene. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43684.     

Dynamic Mechanical Properties of Carbon Black Filled Ethylene Vinyl Acetate Rubber

Abstract

The dynamic mechanical properties of ethylene vinyl acetate (EVA) rubber filled with different loadings of carbon black and at different degrees of crosslinking were studied over a wide range of temperatures (-150° to +200°C). The loss tangent (tan δ) versus temperature plots indicated presence of different transitions. The α-transition (or the glass-rubber transition) corresponding to the maximum in tan δ value, occurred at ?17°C, which is the principal glass-transition temperature (abbreviated as T _g) of EVA rubber. The γ-transition occurred in the temperature region of ?125° to ?135°C, while the β-transition appeared as a shoulder in the temperature region of ?65° to ?75°C. Besides, there was also a high tempeature transition around +62°C which is known as liquid to liquid transition (T _1.1). Incorporation of carbon black filler did not cause any shift of T _g, while the tan δ peak values at T _g decreased sequentially with increase in filler loading. The γ- and β-relaxations were found to be insensitive to filler loading. The T _1.1 transition, however, was found to be suppressed by incorporation of carbon black filler particularly at high loading. Extent of crosslinking did not influence the T _g But, the T _1.1 transition, which was prominent with the lightly crosslinked system was found to be suppressed at high level of crosslinking. Strain dependent dynamic mechanical properties under isothermal conditions showed that the secondary structure breakdown of carbon black filler under the effect of strain amplitude is influenced by the degree of crosslinking of EVA rubber.     

Evolution of properties with increasing cure of a thermosetting epoxy/aromatic amine system: Physical ageing

Isothermal physical ageing below the glass‐transition temperature (T_g) of a high‐T_g thermosetting difunctional epoxy/tetrafunctional aromatic amine system was investigated at different ageing temperatures (T_a) and chemical conversions (monitored by the T_g) using the torsional braid analysis freely oscillating torsion pendulum technique. In the absence of chemical reaction during an isothermal ageing process, the rate of isothermal physical ageing passes through a minimum with increasing conversion. The minimum is related to the minimum in mechanical loss between the secondary relaxation in the glassy state (T_β) and the glass‐transition relaxation (T_g) (the temperatures of both of which increase with increasing conversion). If isothermal ageing rates for all conversions (beyond gelation) would have been measured directly from temperatures below T_β to above T_g, it is concluded that two maxima in isothermal ageing rate would have been observed corresponding to the two relaxation processes. There exists a superposition in isothermal ageing rate versus T_g ? T_a [by shifting horizontally (and vertically)], which implies that the ageing rate is independent of the details of the changing chemical structure attributed to cure. Controlling mechanisms during physical ageing are segmental mobility associated with the T_g region and more localized motion associated with the glassy‐state relaxation T_β. © 2003 Wiley Periodicals, Inc. J Appl Polym Sci 90: 2665–2675, 2003     

Physical aging in the glassy state of a thermosetting system vs. extent of cure

The physical aging behavior of a high-T_g amine/epoxy thermosetting system has been investigated vs. change of chemical structure induced by cure and vs. aging temperature (T_a) using the Torsional Braid Analysis (TBA) technique. The chemical structure was changed systematically from monomer to highly crosslinked polymer by curing in the equilibrilium state (T > T_g). The aging temperatures ranged from just below the glass transition temperature to deep in the glassy state (T_a > T_g). In the absence of chemical reaction, the physical aging rate at a given temperature, T_a, passes through a minimum with increasing chemical conversion (i.e., change of chemical structure). Analysis of this behavior is simplified by using T_g as an index of measurement of extent of cure. There is a superposition principle for normalizing the physical aging behavior of the thermosetting glasses, which involves a shift of T_g–T_a and a shift of C(T_a) (a function of aging temperature), regardless of chemical structure. Analysis reveals that: (1) this behavior is the consequence of the T_g and T_β transitions, (2) the segmental mobility (1/τ) is a function of the deviation from equilibrium (as measured by T_g–T_g and the aging time), (3) the segmental mobility, which is involved in the physical aging process in the glassy state, is insensitive to the extreme changes of chemical structure (from monomer, to sol/gel polymer, and to highly crosslinked polymer), and (4) physical aging deep in the glassy state affects both segmental mobility and cohesive energy density. © 1993 John Wiley & Sons, Inc.     

Isothermal Cure of an Epoxy System Containing Tetraglycidyl-4,4′-diaminodiphenylmethane (TGDDM), a Multifunctional Novolac Glycidyl Ether and 4,4′-Diaminodiphenylsulphone (DDS): Vitrification and Gelation

Times to gelation and vitrification have been determined at different isothermal curing temperatures between 200 and 240°C for an epoxy/amine system containing both tetraglycidyl-4,4′-diaminodiphenylmethane (TGDDM) and a multifunctional Novolac glycidyl ether with 4,4′-diaminodiphenylsulphone (DDS). The mixture was rich in epoxy, with an amine/epoxide ratio of 0·64. Gelation occurred around 44% conversion. Vitrification was determined from data curves of glass transition temperature, T_g, versus curing time obtained from differential scanning calorimetry experiments. The minimum and maximum values T_g determined for this epoxy system were T_g0=12°C and T_gmax=242°C. Values of activation energy for the cure reaction were obtained from T_g versus time shift factors, a_T, and gel time measurements. These values were, respectively, 76·2kJmol^-1 and 61·0kJmol^-1. The isothermal time–temperature–transformation (TTT) diagram for this system has been established. Vitrification and gelation curves cross at a cure temperature of 102°C, which corresponds to glass transition temperature of the gel. © of SCI.     

Dynamic Mechanical Analysis of Amorphous-Phase Organization in Acrylic Polymers

Factors affecting polymer network organization were studied in highly crosslinked acrylics of the type used in dental adhesive resins. The variables tested were comonomer content and processing conditions. BisEMA (2,2,-bis[4-(2-methacryloyloxyethoxy)-phenyl]-propane) and BisEMA + TEGDMA (triethyleneglycol dimethacrylate) were cured with and without 25% comonomer. Comonomers had characteristics that are expected to influence intrachain organization in amorphous phases: TEGDMA, crosslinking; methyl methacrylate (MMA), monomer conversion; isobornyl methacrylate (IBM), low cure shrinkage; tetrahydrofurfuryl methacrylate (THFM), antiplasticization. Dynamic mechanical analysis temperature scans were run at 0.1 Hz 2h or 24h after ambient cure, or 24h postcure after heating at 75° or 125°C. After 24h, tan δ maxima occurred in ranges centering on approximately -30°, 75° and 150°C (T_g). Heating at 125°C nearly eliminated all peaks except T_g, reduced tan δ peaks and increased T_g by 0–14°. T_g increased in the order: TEGDMA>125°C>IBM>MMA>75°C>2h>24h>THFM. The ability to crosslink, and postcure heating at 125°C, were the more important factors found to increase intrachain organization in amine-promoted, unfilled BisEMA resins of the type used in dental sealants, luting cements and bulk-filling resin composite materials.     

Short term isothermal aging of epoxy resin and epoxy-carbon fiber composites

Short term isothermal aging of a neat epoxy resin and one ply epoxy-carbon fiber composite has been performed. The glass transition temperature, T_g of the neat epoxy resin aged at 204°C increased with aging time. The weight loss of the neat epoxy resin due to aging increased with aging temperature and aging time. The weight loss of the epoxy–carbon fiber composite during aging was slightly less than that of the neat resin. SEM microscopy showed the presence of voids and pores on the surface of the composite due to loss of low molecular weight volatiles. The amount and the size of the voids formed during aging increased with aging temperature and time.     

Anomalous behavior of thermosetting systems after cure vs. chemical conversion: A normalized conversion–temperature–property diagram

Isothermal properties of thermosetting materials after cure, such as density and modulus, pass through maximum and minimum values with increasing chemical conversion. In this report observed decreases in modulus and density at isothermal temperatures below the glass-transition temperature, T_g, are termed “anomalous.” Four diepoxide (diglycidyl ether of bisphenol A) and tetrafunctional diamine (trimethylene glycol di-p-aminobenzoate) high T_g thermosetting systems with different ratios of amine to epoxy were investigated for the purpose of analyzing the evolution of the isothermal properties with increasing conversion. The density, T_g, and modulus of the materials with increasing conversion were measured by a combination of dilatometric, differential scanning calorimetry, and torsional braid analysis techniques. The results are presented in the form of conversion–temperature–property (T_gTP) diagrams with modulus and density as the properties. T_g is used as a direct measure of conversion based on the one-to-one relationship between T_g and conversion. The property-conversion behavior of the systems with different ratios of amine to epoxy show similar behavior if T_g is used as the measure of conversion and the data are normalized with respect to T_g at a conversion corresponding to the lower limit of the conversion range at which a maximum in the isothermal modulus occurs. The conversion corresponding to molecular gelation, _gelT_g, correlates with the lower limit of the conversion range at which the maximum in isothermal modulus occurs; _gelT_g also marks a change in the behavior of the sub-T_g mechanical relaxations vs. conversion. The conversion corresponding to the maximum in isothermal modulus vs. conversion correlates with the conversion corresponding to the maximum in isothermal density vs. conversion. © 1995 John Wiley & Sons, Inc.     

Observations of physical aging in a polycarbonate and acrylonitrile–butadiene–styrene blend

The effects of physical aging of a 75 : 25 PC/ABS blend have been studied using differential scanning calorimetry (DSC) and Fourier transform infrared spectroscopy (FTIR). From DSC, two distinct peak endotherms at about 90°C and 110°C, which are associated with the glass transition of ABS (T_g,ABS) and PC (T_g,PC) components, respectively, were observed. When progressive aging was monitored at 80°C for over 1000 h, the changes in enthalpic relaxation, glass and fictive temperatures for the blend followed similar trends to those already seen in the literature for PC aged between 125 and 130°C. The rate of enthalpy relaxation was also comparable. The plot of peak endotherm against logarithmic aging time for the PC blend constituent, however, behaved quite differently from the linear relationship known for highly aged PC. The ABS peak component also appeared to be insensitive to aging. Both observations were confirmed to be statistically significant using analysis of variance methods. Using temperature modulated‐DSC, there is evidence that aging increases the blend miscibility as the T_g,PC shifts toward the stationary T_g,ABS during aging. Parallel FTIR investigations found oxidation of butadiene during aging to be even at this relatively low temperature, forming hydroxyl and carbonyl degradation products. The presence of ABS in the blend also appeared to have prevented the shifting from the trans‐cis to trans‐trans arrangement of the carbonate linkage, which is a well‐known phenomenon during elevated temperature aging of PC alone. Moreover, the carbonate linkage appears to have been at the lower energy, trans‐trans, arrangement prior to the aging process. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008     

Straining phenomena of POY revealed by thermal retraction and other techniques

Partially oriented polyesters yarns (POY) were strained at different strain rates (0.03–12.00 min^?1) and temperatures above and below T_g (3–92°C). Thermal retraction, density, DSC, and WAXS techniques show that strain-induced crystallization takes place by straining at temperatures above as well as below T_g. Above T_g, depending upon the strain rate, two regimes are observed: Below the strain rate of 1.5 min^?1, the flow regime; the degree of crystallinity is reduced as the strain rate increases. Above the strain rate of 1.5 min^?1, the strain-induced crystallization regime; the degree of crystallinity increases as the strain rate increases. Thermal retraction, stress–relaxation, and sonic modulus techniques indicate that, upon cold straining, instead of the original T_g at 65–69°C, two glass transitions occur: an upper T_g (u) and a lower T_g (l). For POY strained at 3°C and at a strain rate of 10 min^?1, the values are 78°C and 37°C, respectively. The higher the strain rate and the lower the straining temperature, the large the difference between T_g (u) and T_g (l).     

Thermal transition behavior of polybutadiene containing polyurethanes

The thermal transition behavior of a series of hydroxy terminated polybutadiene (HTPBD) containing segmented polyurethanes has been studied by differential scanning calorimetry (DSC) and thermal mechanical analysis (TMA). Four transition regions are observed; the soft segment T_g, at ?74°C, two hard segment transitions T₁, at 40°C and T₂ at 103°C and a softening region by TMA at 180°C, presumed to arise from the dissociation of allophonate bonding, The low T_g, only 7°C higher than the T_g of free HTPBD, indicates nearly complete phase segregation despite the amorphous nature of the hard segment structure. The dependence of T₁, on hard segment length and thermal cycling suggests that it represents domains consisting primarily of shorter hard segments units. Factors contributing to the rather low mechanical properties of HTPBD polyurethanes are also discussed.     

Crosslinking-property relationships in PMR polyimide composites. Part I

This study examines for the first time how matrix crosslinking affects the composite physical and mechanical properties of a graphite fiber reinforced PMR polyimide composite during long-term isothermal aging. Unidirectional composite specimens of Celion 6000/PMR-P1 were isothermally exposed at 288°C in air for various time periods up to 5000 h. The matrix crosslink densities were estimated from the kinetic theory of rubber elasticity and shifts in the glass transition temperatures (T_gs). The T_g, coefficient of thermal expansion, density, weight loss, moisture absorption, and elevated temperature flexural and interlaminar shear properties were also determined. Several linear relationships were found between the matrix crosslink density and composite physical and mechanical properties. The T_g, initial weight loss and density, and elevated temperature interlaminar shear strength increase with an increase in crosslink density. Conversely, the initial moisture absorption and coefficient of thermal expansion decrease with increasing crosslink density. As expected, the elevated temperature flexural strength and modulus show no direct correlations with crosslink density. Further, after achieving the highest matrix crosslink density, several of the composite properties begin to decrease rapidly. These findings suggest that time-temperature dependent nature of attaining the maximum matrix crosslinking is closely linked to the onset of the composite property degradation. Though much more work is needed, a fundamental understanding of the relationships between matrix crosslinking and composite physical and mechanical property can provide a scientific basis for the prediction of the extent of composite service life not only for PMR polyimides but also for other thermosetting matrix resins, such as epoxies and bismaleimides.     

Automated torsion pendulum analysis of the formation and properties of a polyphthalocyanine

The cure and transitions of a C₁₀ diamide phthalocyanine resin have been investigated by the torsional braid method. Two relaxations (T′ > T_g and T > T_g) occurring above the glass transition temperature (T_g) are shown to correspond to two relaxations which develop prior to vitrification on isothermal cure. They bear directly on processibility of the material. Also investigated was the coupling between a low temperature transition (～ ? 146°C/2.1 Hz) inherent to the material and a low temperature water-induced transition (～ ? 76°C/1.9 Hz).     

Curing kinetics and thermal properties of vinyl ester resins

The curing kinetics of dimethacrylate-based vinyl ester resins were studied by scanning and isothermal DSC, gel time studies, and by DMTA. The rate of polymerization was raised by increased methyl ethyl ketone peroxide (MEKP) concentration but the cocatalyst, cobalt octoate, retarded the reaction rate, except at very low concentrations. By contrast, the gel time was reduced for all increases in either peroxide or cobalt concentration. This contradictory behavior was explained by a kinetic scheme in which the cobalt species play a dual role of catalyzing the formation of radicals from MEKP and of destroying the primary and polymeric radicals. The scanning DSC curves exhibited multiple peaks as observed by other workers, but in the present work, these peaks were attributed to the individual influence of temperature on each of fundamental reaction steps in the free radical polymerization. Physical aging appeared to occur during the isothermal polymerization of samples cured below the “fully cured” glass transition temperature (T_g). For these undercured materials, the difference between the DSC T_g and the isothermal curing temperature was approximately 11°C. Dynamic mechanical analysis of a partially cured sample exhibited anomalous behavior caused by the reinitiation of cure of the sample during the DMTA experiment. For partially cured resins, the DSC T_g increased monotonically with the degree of cure, and this dependence was fitted to an equation related to the Couchman and DiBenedetto equations. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 769–781, 1997