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.     

Effect of shade and thermo-mechanical viscosity stimulation methods on the rheological properties of nanohybrid resin composite

The aim of the present study is to measure the rheological properties of nanohybrid resin composite of three shades in pre-polymerized phase using different thermomechanical stimulations. Nanohybrid composite (Kerr Herculite XRV Ultra) in enamel, dentin, and incisal shades was included. Rheological measurements were made with a rotational rheometer in dynamic oscillation mode using three methods: (a) Strain Sweep test explored a range of deformation γ₀ from 0.025 to 3% with a frequency ω = 1 Hz (temperature set at 25 and 65?°C), (b) Frequency Sweep test explored frequencies between 1 and 100 rad/s applying a deformation γ₀ = 0.5% (temperature set at 25; 45; 65?°C), and (c) Ramp Temperature test explored a heating phase from 25 to 75?°C then a cooling phase back to 25?°C applying a γ₀ = 0.5% and a ω = 10 rad/s. Data were analyzed using a three-way ANOVA and Tukey’s test (α = 0.05). Viscosity measurement (p < 0.05) and shade of the composites (p < 0.05) significantly affected the results. Viscosity turned out to be subordinate to strain amplitude, frequency, temperature, and axial force applied during each test. Enamel shade was the most viscous whereas dentin shade was 8% less viscous (p < 0.05). The incisal shade was significantly less viscous (70%) than enamel (p < 0.05). Pre-heating decreased viscosity of incisal shade (30%) above 50?°C but this value was 90 and 98%, respectively, for strain and frequency sweep test. Preheating had a side effect as in the cooling phase, viscosity increased from 66 to 450% exceeding the value recorded at the beginning of the test. Preheating was not effective to reduce viscosity, and may reveal some side effects. The composite tested might not be pre-heated above 45?°C.     

Electrochemical behavior of multifunctional graphene–polyimide nanocomposite film in two different electrolyte solutions

The effect of solvent on specific capacitance, bulk resistance, and charge/discharge capacity of graphene/polyimide composite films is studied by electrochemical methods. Composite films are synthesized by in situ condensation polymerization of poly (amic acid) in the presence of 50 wt % partly exfoliated graphene sheets followed by thermal curing at 250°C. Raman spectrum of the exfoliated graphene sheets show an increase in the ratio of I_D to I_G peak intensities from 0.167 to 0.222, suggesting increased defects in graphene basal planes. Electrochemical measurements carried out by using 0.4M potassium hexafluorophosphate (KPF₆) dissolved in propylene carbonate and N‐methylpyrrolidone at 25°C show that the composite system exhibits both pseudocapacitance and supercapacitance behaviors, with an average capacitance of 40 and 36.5 F g^?1, respectively. Bulk resistance of the composite obtained by using KPF₆–propylene carbonate electrolyte solution is 300% lower than that obtained in KPF₆–N‐methylpyrrolidone solution, with a fairly stable specific capacity of 85 μAhr g^?1, with 80% retention observed after 30 charge–discharge cycles. Fourier transform infrared spectroscopy measurements show shifts in the cyclic imide carbonyl peak from 1778 to 1774 cm^?1, which suggests that some form of interaction exists between the graphene and polyimide. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42673.     

Electrical properties of flexible graphene reinforced polyimide composites

The electrical properties of polyimide and the composite at different volumes fractions were studied in the frequency range 200–20 kHz and in the temperature range 30–200 °C. Increasing the volume fraction of graphene up to 10%, resulted in an extremely large increase in the dielectric constant, which indicates the composites remarkable ability to store electric potential energy under the effect of alternating electric field. An increase in dielectric constant was also observed with increasing temperature and decreasing frequency. The outstanding dielectric properties of polyimide graphene nanocomposites are attributed to the large volume fraction of interfaces in the bulk of the material. The measured increase in dielectric constant with increasing temperature was attributed to the segmental mobility of the polymer chains. The AC conductivity for polyimide and the composites was calculated from the loss factor and a remarkably high conductivity was obtained for the composites due to the formation of conducting paths in the matrix by the graphene sheets. Also this study showed that the thermal conductivity of the composites increased sharply with increasing graphene concentration. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45372.     

Isothermal physical aging of a fully cured epoxy—amine thermosetting system

The rate and effects of isothermal physical aging of a fully cured epoxy—amine/glass fiber composite specimen were studied for a wide range of isothermal aging temperatures (−180 to 200°C) using a freely oscillating torsion pendulum technique: torsional braid analysis (TBA). As assigned from the maxima in the mechanical loss vs. temperature, the glass transition temperature, T_g, was 182°C (0.9 Hz), and the principal glassy-state secondary transition temperature, T_β, was ≈ −30°C (1.9 Hz). Plots of the increase in the isothermal modulus and of the decrease in the isothermal mechanical loss were linear vs. log aging time; their slopes provided aging rates. It was found that the isothermal aging rate varies with isothermal aging temperature (T_a) and that there are two maxima in the aging rate vs. T_a. A correlation presumably exists between the two maxima in the aging rate and the two transitions. This is not surprising since mechanical loss maxima (i.e., transitions) and aging rate maxima both correspond to specific, localized, and restricted submolecular motions. Effects after isothermal physical aging were investigated vs. temperature in terms of change of modulus of the specimen. The effect of isothermal aging existed primarily in a narrow temperature region localized about T_a. The majority of the isothermal aging effect can be eliminated by heating to temperatures above T_a, but below T_g. Theoretical and practical implications of this observation are discussed. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 64: 55–67, 1997     

Green facile fabrication of polyimide by microwave-assisted hydrothermal method and its decomposition dynamics

A microwave-assisted hydrothermal technique was proposed in this work to fabricate polyimide using ethanol as sole solvent, and the decomposition dynamics of the resulting polyimide was also investigated. The preparation process involved that the ammonium carboxylic ester intermediates could be able to cyclize de-alchoholation, thus generating imide ring structures. The obtained polyimide exhibited excellent thermal properties. Specifically, the glass transition temperature of the polyimide was around 267.8 °C, and it begun to decompose gradually from about 550 °C. A novel method was employed to choose the dynamics model according to the linear correlation between g(α) and 1/β. As a result, the three-dimensional diffusion (Jander) model was much more suitable for the thermal decomposition process of the resulting polyimide, and the thermal decomposition mechanism was explained by using the model. The apparent activation energy (E _a) values calculated by Flynn–Wall–Ozawa and Kissinger methods were similar showing the same variation tendency. Furthermore, a series of fitting polynomials for E _a and pre-exponential factor (A) values were achieved to reflect the trend more accurately. The dynamics results revealed that the decomposition process was complex and could be regarded as multistep reactions. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48484.     

Accelerating the graphitization process of polyimide by addition of graphene

In this article, a composite film from polyimide and thermally exfoliated graphene was prepared through in situ polymerization, and the weight percent of thermally exfoliated graphene with respect to the polyimide monomers was 5 wt %. The film was carbonized at 1000°C for 1 h and then heat‐treated at various temperatures up to 2100°C. For comparison, the corresponding pure polyimide film was also prepared and heat‐treated at the same condition. It was found that the addition of graphene could effectively prevent the polyimide film from shrinking in the direction parallel to the film surface during the heat treatment. Furthermore, the results of density, X‐ray diffraction, shrinkage in the direction perpendicular to the film surface and scanning electron microscopy revealed that the graphitization process of the polyimide could be accelerated by addition of graphene obviously. In view of the above phenomena, a reasonable explanation was presented. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41274.     

Phase transformation and microstructure of MgO–Al2O3–SiO2 system glass–ceramics under different heat treatment conditions

Abstract

Effects of heat treatment conditions on phase transformation, microstructure and thermal expansion coefficient (TEC) in MgO–Al₂O₃–SiO₂ system glass–ceramics were investigated by means of differential thermal analysis, X-ray diffraction and scanning electron microscopy. The magnesium aluminium titanate (MAT) precipitated firstly at 850°C and β-quartz solutions (β-QSS) formed at 950°C. Further increasing temperature to 1000°C, MAT disappeared and β-QSS became master phase, following little amount of α-cordierite, MgTi₂O₅, rutile and sapphirine. When glass was treated at 1050°C, β-QSS content decreased and α-cordierite became master phase. As temperature reached higher than 1100°C, β-QSS and sapphirine disappeared, and α-cordierite became master phase accompany with rutile and MgTi₂O₅ as secondary phase. The microstructure transformed gradually from particle shape crystallites to slat shape network with the increase in heat treatment temperature. By controlling heat treatment condition, an ideal glass–ceramics with proper TEC for matching sealing to 4J29 alloy has been obtained.     

Thermal and physical properties of allyl PPO and its composite

The thermal behavior of allyl PPO and its cured resin were investigated. In the allyl PPO curing process, the specific temperatures were T_gel = 173.6°C, T_cure = 225.4°C, and T_treat = 237.7°C, and the activation energy (E_a) was 122 kJ/mol. The average number of PPO molecular units between two crosslinking points was about 20. In the degradation process of cured allyl PPO resin, the temperature at which mass loss equaled 1% was much higher than 300°C. The E_a for degradation was calculated as 125 kJ/mol, with a degradation fraction (α) in the range of 0.15–0.65, or 117 kJ/mol with an α of 0.10–0.90. The most probable mechanism function of decomposition of the cured allyl PPO resin was f(α) = 2(1 ? α)^3/2 or g(α) = (1 ? α)^?1/2 ? 1. The thermocompressed laminate of the allyl PPO blending with an additive resin (made from BDM and DP) exhibited the desired properties. ©2006 Wiley Periodicals, Inc. J Appl Polym Sci 102: 4111–4115, 2006     

The effect of isothermal aging on transverse crack development in carbon fiber reinforced cross-ply laminates

An investigation into the effect of isothermal aging on the development of transverse cracks in cross-ply laminates of two high temperature composite systems was performed. The composite materials investigated were BASF X5260/640–800 and DuPont Avimid K/IM6. Changes in the glass transition temperature, composite weight loss, crack density, and mode I intralaminar fracture toughness were monitored during isothermal aging in air at 177°C for up to 2232 h. The two laminate configurations used in this study include two variations of the generic cross-ply configuration [0₂/90_n]_s, in which n equals 1 and 2. The results of this investigation show that a layer of degraded material forms at the surface of the X5260/640–800 bismaleimide laminates and that the thickness of the degraded layer increases with aging time. After 744 h of aging, transverse cracks form in the surface plies and an increasing crack density evolves as aging time is increased; however, transverse cracks do not form in the inner 90° ply groups with aging during the time period investigated. The Avimid K/IM6 thermoplastic polyimide laminates, which show evidence of cracking prior to aging, do not exhibit any significant change in crack density with aging. The results of the aging experiments also show that the bismaleimide system exhibits a weight loss of 1.5% and an increase in glass transition temperature from 250°C to 300°C after 2232 h of aging at 177°C, while the thermoplastic polyimide system shows a weight loss of only 0.05% and an increase in glass transition temperature from 280 to 285°C after 2232 h. Changes in the resistance to crack formation are also seen in these materials during aging. The mode I intralaminar fracture toughness, a measure of resistance to transverse crack formation, shows a 50% decrease after aging for 2232 h for the bismaleimide system, while the behavior exhibited by the thermoplastic polyimide shows little evidence of a reduction.     

Effect of atomic oxygen on corrosion and friction and wear behavior of polyimide composites

Thermosetting polyimide (TPI) composite with better wear resistance and lower friction was obtained with various functional fillers after a high–low temperature cycle test. Nevertheless, taking into consideration of harsh space condition, the friction and wear behavior of TPI composite were also explored after atomic oxygen (AO) irradiation. It reveals that TPI composite behaves excellent properties against AO mainly attributed to g-C₃N₄ with more negative electronegativity N than carbon atom. Moreover, the mechanism of friction and wear demonstrated that degradation caused by AO led to TPI composite surface with special “carpet-like” morphology and this resulted into lubrication failure, especially when temperature was increased from 200 °C to 350 °C. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48441.     

Preparation and characterization of high‐temperature resistance polyimide foams

A new high‐temperature resistance polyimide foam was synthesized from 2,3,3′,4′‐biphenyltetracarboxylic dianhydride (α‐BPDA) and p‐phenylenediamine (p‐PDA). The structures and foaming process of polyimide precursor powders were characterized by wide‐angle X‐ray diffractometer (WXRD) and the self‐made visualization device, respectively. The imidization degree, thermal mechanical properties and thermal stability of the polyimide foams with different post‐treatment temperatures were also measured by fourier transform infrared spectrometer spectrum (FTIR), dynamic thermal mechanical anaylsis (DMTA), and thermogravimetric analysis (TGA). Results showed that the inflation onset temperatures of polyimide precursor powders ranged from 122 to 135°C with varying the heating rate. And the increase in the imidization degree, glass transition temperatures (T_g) and temperatures for 5 wt% mass loss of high‐temperature resistance polyimide foams can be achieved with increasing post‐treatment temperature. It was quite surprising to find that T_g of high‐temperature resistance polyimide foam post‐treated at 420°C was up to above 450°C, and the char yield at 800°C was more than 60%. POLYM. ENG. SCI., 2010. © 2010 Society of Plastics Engineers     

3D-printed strontium-doped BG-CaSiO3-HA composite scaffolds promote critical bone defect repair by improving mechanical strength and increasing osteogenic activity

The release of silicon and calcium elements contained in silicon-based materials promotes the formation of bone. For bioactive glass prepared by the sol-gel method, water-soluble binders are usually added when preparing 3D printed scaffolds. However, the obtained scaffolds are prone to collapse when exposed to water and have low strength. At the same time, the binder needs to be removed for clinical applications, so the 3D printed scaffolds need to be sintered. Under high temperature, bioactive glass scaffolds will be transformed into composite scaffolds composed of bioglass, CaSiO₃ and hydroxyapatite, while different sintering temperatures will form different crystal types of CaSiO₃. In this study, SrBG-βCS-HA and BG-βCS-HA were obtained at a heating rate of 5 °C/min to 1100 °C and at the same rate to room temperature. SrBG-αCS-HA and BG-αCS-HA were obtained at a heating rate of 2 °C/min to 1200 °C and at the same rate to room temperature. In vitro and in vivo experiments verified that the presence of strontium in the obtained scaffolds after sintering further enhanced the osteogenic properties of the scaffolds. SrBG-αCaSiO₃-HA and SrBG-βCaSiO₃-HA were found to have relatively better osteogenic properties. The results show that SrBG-CaSiO₃-HA 3D printing scaffolds have excellent clinical application potential.     

Synthesis of photo-crosslinked hybrid fluoropolymer and its application as releasing coating for silicone pressure-sensitive adhesives

In this study, a novel photo-crosslinked polymer composite was made from NCO-terminated fluoropolymers by the incorporation of functionalized graphene through the in situ polymerization method. This polymer composite can be photoinduced to form a network under UV radiation without photoinitiators, displaying an excellent transmittance (T% > 90% at 550 nm), a high thermostability with a glass transition temperature (T _g < −15 °C), and a 10% weight loss temperature (410 °C). With UV radiation, the hybrid coating exhibited a controlled releasing force between 100 and 500 g/25 mm for silicone PSAs, which was an important breakthrough in the heavy-releasing industry. Their aging releasing performance and residual adhesion force were satisfactory. More importantly, antistatic agents are not needed any more before the coating of this novel releasing agent. In addition, the coating possessed good hydrophobicity with a contact angle of 110° and great resistance to solvents. These metrics indicate that incorporating functionalized graphene as curable points into composites can greatly improve the performance of fluoropolymer properties. The obtained polymers have potential applications in the releasing and protective industry. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 48322.     

Mechanical and dielectric relaxations in polytrifluoroethylene

Molecular motions in polytrifluoroethylene are studied by dynamic mechanical and dielectric measurements. Two absorption peaks (α_a, β) have been observed in the frequency range from 3 Hz to 300 kHz and in the temperature range from ?150° to 130°C. The molecular mechanisms for these absorptions are discussed, and the results are compared with ultrasonic, heat capacity and n.m.r. measurements. It is concluded that the α_a absorption located at 45°C (10 Hz) is associated with the large-scale motion of the main chains and the β absorption at ?35°C (10 Hz) is attributed to the limited motion of a few segments of the frozen main chains. In contrast to polychlorotrifluoroethylene the α_c absorption associated with crystalline regions has not been observed in polytrifluoroethylene. At high temperatures a steep rise in dielectric absorption is observed, which has been ascribed to a d.c. ionic conduction process.     

Microwave dielectric properties of SnO‐SnF2‐P2O5 glass and its composite with alumina for ULTCC applications

Ultralow‐temperature sinterable alumina‐45SnF₂:25SnO:30P₂O₅ glass (Al₂O₃‐SSP glass) composite has been developed for microelectronic applications. The 45SnF₂:25SnO:30P₂O₅ glass prepared by melt quenching from 450°C has a low T_g of about 93°C. The SSP glass has ε_r and tanδ of 20 and 0.007, respectively, at 1 MHz. In the microwave frequency range, it has ε_r=16 and Q_u × f=990 GHz with τ_f=?290 ppm/°C at 6.2 GHz with coefficient of thermal expansion (CTE) value of 17.8 ppm/°C. A 30 wt.% Al₂O₃ ‐ 70 wt.% SSP composite was prepared by sintering at different temperatures from 150°C to 400°C. The crystalline phases and dielectric properties vary with sintering temperature. The alumina‐SSP composite sintered at 200°C has ε_r=5.41 with a tanδ of 0.01 (1 MHz) and at microwave frequencies it has ε_r=5.20 at 11 GHz with Q_u × f=5500 GHz with temperature coefficient of resonant frequency (τ_f)=?18 ppm/°C. The CTE and room‐temperature thermal conductivity of the composite sintered at 200°C are 8.7 ppm/°C and 0.47 W/m/K, respectively. The new composite has a low sintering temperature and is a possible candidate for ultralow‐temperature cofired ceramics applications.     

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).     

Dynamic viscoelasticity of PUF under small strain and prediction of dynamic viscoelasticity in a wide frequency range

The dynamic viscoelasticity of polyurethane fiber (PUF) under small strain was studied with dynamic mechanical analysis and differential scanning calorimetry, and its viscoelasticity was predicted in a wide frequency range. The results revealed that the loss factor (tan δ) was about 0.05 and the phase angle (δ) was 2.86°, very close to the δ of a perfect elastomer in the range from −100°C to −80°C. The master curves superimposed by time-temperature superposition (TTS), the frequency range of the storage modulus master curve spanned 17 orders of magnitude from 10⁻⁷ Hz to 10¹⁰ Hz, and the loss modulus master curve spanned 13 orders of magnitude from 10⁻³ Hz to 10¹⁰ Hz. The apparent activation energy of the soft segment relaxation decreased with increasing temperature in the range from −70 to −26°C. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47070.     

Mechanical loss,creep, diffusion and ionic conductivity of ZrO2-8 mol%Y2O3 polycrystals

Polycrystalline ZrO₂-8 mol%Y₂O₃ was investigated by combining several experimental techniques on identical materials sintered out of the same high purity powder. The mechanical loss spectrum (damping and elastic modulus) was measured in a large frequency and temperature range (10⁻²Hz–1.5kHz; −150 to 1400°C). Damping due to point defect relaxation at low temperature and to viscoelastic relaxation at high temperature was revealed. The creep resistance was investigated with four-point bending tests (stress and temperature ranges: 20–75 MPa, 1100–1290°C), indicating Nabarro-Herring creep as the main rate-controlling mechanism. Both viscoelastic deformation and creep seem to be controlled by cation diffusion. Measurements of the ⁹⁶Zr tracer diffusivity by secondary ion mass spectrometry at 1125–1460°C yielded an activation enthalpy of 460 kJ/mol. Close values were obtained for creep (440 kJ/mol) and viscoelastic relaxation (530 kJ/mol). Finally, the ionic DC-conductivity of these electrolytes was measured with high accuracy in the range 300–1250°C.     

Preparation and dielectric properties of (Ba0.5Sr0.4Ca0.1)TiO3/polystyrene composites

Polymer composites with polystyrene (PS) as matrix and (Ba_0.5Sr_0.4Ca_0.1)TiO₃ (BSCT) as fillers are prepared by solution casting method. It is found that the dielectric constant of the prepared BSCT/PS composites increases with increasing filler content over the frequency range from 100 Hz to 500 MHz. And the dielectric properties of the composites show a good temperature independency over the range of ?30°C to 80°C. For the composite with 50 vol % filler content, the dielectric constant and dielectric loss are comparable with the literature values reported for other PS composites used for microwave substrate. Several theoretical models are used to compare with the experimental data of the dielectric constant of the composites. The microstructure and thermal properties of the composites were also studied. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41398.