Effect of isothermal aging on post-imidization and glass transition temperature of LaRC-IA polyimide resin

The isothermal aging of partilly imidized NASA Langley Research Center, LaRC-IA polymide resin containing 70 wt% N-methyl pyrrolidone, NMP was performed in a vacuum oven at 65, 95, 135, 165 and 200°C for 0.5, 1.0, 1.5 and 2 h. The weight loss and chemical changes that occurred during aging was determined gravimetrically and by FTIR spectroscopy, respectively. The imide absorption peak at 1778 cm⁻¹, increased in intensity as the aging temperature was increased from 65 to 200°C. The expulsion of NMP (70 wt%) was completed after ∼2.5 h of aging at 135°C. Additional weight loss ≤4 wt%, after the expulsion of NMP, was attributed to post-imidization. The imide carbonyl peak absorption at 1721 and 1778 cm⁻¹, respectively, were broadened after aging at 200°C. The broadening of the imide absorption peaks was marked by the disappearance of the amide peak near 1660 cm⁻¹ and is attributable to post-imidization of the partially imidized polyamic-acid. Dissolution of the polyimide aged at T ≥ 165°C in dimethyl formamide, DMF, was unsuccessful even after long times of stirring (∼12 h) at elevated temperature (T ∼ 85°C). The DSC thermogram for the LaRC-IA resin showed a series of broad endothermic peaks between 150–180°C and narrow endothermic peaks at 210°C. The low temperature endotherm disappeared after aging at T ≥ 135°C for t ≥ 1 h. The high temperature endotherm decreased with increased aging temperature and time. The glass transition temperature of the polyimide increased with increased aging temperature and time.     

Rate of imidization of polymerizable reaction mixtures: PMR-15

Imidization of PMR-15 was investigated using Fourier transform infrared spectroscopy (FTIR) as a function of time and temperature. Imidization was performed at 65 ≤ T ≤ 300°C for 3 ≤ t ≤ 150 min. FTIR spectroscopy showed that imidization (measured by the changes in the imide carbonyl absorption at 1778 cm⁻¹) increased with temperature and time. Imidization was found to be nearly completed in 2.5 h at 300°C. Imidization of PMR-15 occurred in three stages: (i) the initial imidization region characterized by gradual reaction followed by (ii) a very rapid reaction region that spans about 0.5 h and (iii) a final imidization region characterized by a gradual reaction and spans about 2 h. An Avrami-type kinetic analysis was used to obtain the reaction order of 1.5 and 1.7 and the rate constant for imidization of 1.3 × 10⁻³ and 1.5 × 10⁻³ min^−3/2; at 135 and 165°C, respectively. Comparison with other kinetic models shows agreement at low conversions (p ≤ 15%). At high conversions of p > 20%, a second-order kinetic model seems to fit the data reasonably well in agreement with the observed order. © 1997 John Wiley & Sons, Inc. J Appl Polym Sci 66: 2529–2538, 1997     

Modeling the imidization kinetics of AFR700B polyimide

AFR700B imidization was monitored using Fourier transform infrared spectroscopy (FTIR) by following the increase in the carbon-nitrogen bond of imide groups displayed by the 1360 cm⁻¹ band over several isotherms. Imidization occurred until ∼300°C (572°F). This was confirmed by the presence of water up to 300°C (572°F) in thermogravimetric analysis-mass spectroscopy (TGA-MS) data. First, reaction kinetics were described by first-order kinetics with a fast and slow region of reaction. However, this model did not accurately describe the data in the transition from the fast to the slow reaction region. Imidization is kinetically controlled at short times. However, when the monomers rapidly react to form a high molecular weight polymer gel, imidization is controlled by diffusion. Therefore, a second-order kinetic/diffusion model was used to describe the imidiation kinetics. This resulted in a substantial improvement in the fit of the data. The kinetic/diffusion model can also be used to describe imidization in polyimides other than AFR700B.     

Synthesis and properties of 6FDA‐BisAAF‐PPD copolyimides for microelectronic applications

In this work, fluorine‐containing copolyimides were synthesized from 6FDA dianhydride and different ratios of BisAAF and PPD diamines. Properties, such as composition, viscosity, dielectric constant, glass‐transition temperature, thermal decomposition temperature, tensile characteristics, and transmittance, were investigated by using elemental analysis, viscometry, Fourier transform infrared spectrometry, differential scanning calorimetry, a thermogravimetric analyzer, a tensile tester, and UV–visible spectrophotometry. After curing at 300°C for 1 h, imidization was observed, as indicated the appearance of an absorption peak of the carbonyl of the imide at 1780 cm^?1 (C?O asymmetry stretching). The inherent viscosity increased with an increasing PPD mole fraction, from 0.40 dL/g of pure 6FDA‐BisAAF to 0.84 dL/g of pure 6FDA‐PPD. The dielectric constant decreased with increasing fluorine content. The glass‐transition temperature increased with an increasing PPD mole fraction; the values increased from 317°C with pure 6FDA‐BisAAF polyimide to 364°C with pure 6FDA‐PPD polyimide. The 5% weight loss temperature (T_d) of the copolyimides was around 530°C in air and 540°C in a nitrogen atmosphere. The tensile modulus and tensile strength gradually increased with an increasing PPD molar fraction. The transmittance of 6FDA‐BisAAF‐PPD copolyimides was greater than 90% at wavelengths above 500 nm. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 98: 2064–2069, 2005     

In situ cyclization reactions during the preparation of high‐performance methacrylic acid/acrylonitrile/acrylamide ternary copolymer foam

A new high‐performance methacrylic acid/acrylonitrile/acrylamide ternary copolymer foam was prepared by radical bulk copolymerization, and the reaction mechanism of in situ cyclization taking place during copolymer foaming as well as the heat treatment was examined too. Then, the crucial mechanism was validated via optical microscopy, infrared absorption spectroscopy (Fourier transform infrared), differential thermal analysis (differential scanning calorimetry), and thermogravimetric analysis. The results showed a weak exothermic peak at 149.17°C and a strong endothermic peak at 270.85°C in differential scanning calorimetry curves of the methacrylic acid/acrylonitrile/acrylamide copolymer after the foaming and heat treatment at 160°C for 2 h and at 200°C for 2 h. The peak temperature of the differential thermogravimetry curve was 175.87°C, whereas the weight‐loss rate was less at 276.58°C in the thermogravimetry curves. In the case of the Fourier transform infrared curves, the ? OH absorption peak at 930–970 and 1480 cm^?1 weakened, and the C? N absorption peak of the imide increased. The >C?O absorption peak at 1700 cm^?1 occurred as an excursion phenomenon toward the low‐frequency field; at the same time, the second absorption peak increased. Furthermore, the ? C?N absorption peak at 2240 cm^?1 weakened, and a new ? C?N? absorption band appeared. All these data revealed that in situ cyclizations had taken place in the copolymer molecule chains, so some rigid ring structures appeared in the copolymer molecule chains, such as six‐membered imide rings and ladder polymer structures. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci 2007     

Characterization of the imidization of the aromatic polyimide LARC-160

The polyimide resin LARC-160 was prepared from diethyl-3,3′,4,4′-benzophenone tetracarboxylate (BTDE), ethyl-5-norbornene-2,3-dicarboxylate (NE), and Jeffamine AP-22. The imidization reactions of NE and BTDE were studied by HPLC, ¹³C-NMR, and IR. NE imidizes slowly at 12°C; BTDE imidizes when the resin is heated above 100°C. Both imidization reactions proceed directly to the imide. Neither amic acid is present in significant quantities at any stage of the imidization reactions. The monomer mixture has been stored at 12°C for periods up to 14 months. The effects of resin aging at this temperature on the chemical composition of the resin monomer mixture and the imidized polymer formed on curing were investigated. Aging the resin monomer mixture has the effect of partially advancing the imidization reaction. Aging also results in the formation of slightly higher-molecular-weight polyimide chains after curing of the resin at 140 and 180°C. Bisnadimide (BNI) is observed as a major reaction product, regardless of resin age.     

Electrochemical absorption of hydrogen in transition metal oxides—II. Sorption of hydrogen in titanium monoxide TiO

The absorption of hydrogen in titanium “monoxide” was studied electrochemcially by potential-step and ac techniques. The absorption equilibrium obeys the Crandall—Faughnan—Armand isotherm and the chemical diffusion coefficient is a linear function of concentration. The heat of absorption is 80 kJ (g atom H)⁻¹ and the true diffusion coefficient D₀ is 4 × 10⁻¹³ cm²s⁻¹ at 20°C. The absorption occurs in oxygen vacancies.     

Infrared study of the effect of heat on wool

The infrared spectra of wool heated in vacuum and in air to different temperatures ranging from 120 to 250°C were investigated. It was found that certain absorption bands disappear when wool is heated in vacuum to 180°C and in air to 120°C for 2 hr. Also, the results showed that the intensities of the C=O stretching band at 1660 cm⁻¹, N–H stretching band at 3325 cm⁻¹ and the C–H stretching band at 2940 cm⁻¹ decrease when wool is heated in vacuum to 180°C and in air to 120°C. The spectra of the samples heated in vacuum to 250°C and in air to 225°C exhibited strong absorption bands belonging to the carboxyl and sulfonate groups.     

Kinetics study of isothermal nicotine release from poly(acrylic acid) hydrogel

The isothermal kinetic of the release of nicotine from a poly(acrylic acid) (PAA) hydrogel was investigated at temperature range from 26°C to 45°C. Specific shape parameters of the kinetic curves, the period of linearity and saturation time were determined. The change in the specific shape parameters of the kinetic curves with temperature and the kinetic parameters of release of nicotine E_a and ln A were determined. By applying the “model fitting” method it was established that the kinetic model of release of nicotine from the PAA hydrogel was [1 − (1 − α)^1/3] = k_Mt. The limiting stage of the kinetics release of nicotine was found to be the contracting volume of the interaction interface. The distribution function of the activation energy was determined and the most probable values of activation energies of 25.5 kJ mol⁻¹ and 35 kJ mol⁻¹ were obtained. Energetically heterogeneity of the interaction interface was explained by the existence of the two different modes of bonding the nicotine molecules onto the hydrogel network by hydrogen bond and electrostatic forces. © 2010 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

The turning point on plots of log ϕ and log t of Mo's equation

BACKGROUND: Mo's equation based on the Avrami equation and Ozawa equation has been successfully used in non‐isothermal crystallization kinetics by many researchers. However, in recent years we have found that plots of log ?–log t of Mo's equation are not straight lines, but there appears a turning point at ? = 7 °C min^?1 or so. The aim of this article is to analyze in detail the reason for the occurrence of this turning point by studying the non‐isothermal crystallization of poly(vinylidene fluoride) (PVDF) using differential scanning calorimetry. RESULTS: A turning point at about 7 °C min^?1 appeared on plots of log ?–log t. The cooling rate where the turning point occurred showed little change with increasing relative crystallinity, but the temperature decreased. It is noted that this turning‐point temperature is lower than transition temperature of regime I → II reported for PVDF. CONCLUSION: We deemed that the occurrence of turning points on plots of log ?–log t of Mo's equation can be ascribed to the different crystallization regimes based on the Hoffman nucleation theory, and presumed that the turning point corresponded to transition of regime I → II. Finally, we considered that Mo's equation should be analyzed using two beelines, instead of one beeline. Copyright © 2009 Society of Chemical Industry     

A novel polymer electrolyte based on polydioxolane polyurethane with Na+ single‐ionic conductivity

A series of novel polyurethane ionomers with polydioxolane (PDXL) as soft segment was prepared and characterized by Fourier transform infrared spectroscopy, differential scanning calorimetry, and dynamic mechanical analysis. The ionomers obtained were Na⁺ single‐ionic conductors. Their ionic conductivity and water absorption were tested. At medium temperature (> 75°C), the conductivity of ∼ 10⁻⁵ s cm⁻¹ was reached. The temperature dependence of conductivity could not be well expressed by both Arrhenius and VTF equations. When ionization level was fixed, the conductivity increased as the Mn of PDXL decreased. We also discussed the effect of ionization level on water absorption. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1711–1719, 1999     

Chemical copolymerization of aniline with o‐chloroaniline: thermal stability by spectral studies

Poly(aniline‐co‐o‐chloroaniline) salts were synthesized by chemical copolymerization of aniline with o‐chloroaniline using three different acids. The polymer salt samples were heat treated at four different temperatures (150, 200, 275 and 375 °C) and the thermal stability of the polymer salts were studied by conductivity, electron paramagnetic resonance (EPR), infrared (IR) and electronic absorption spectral measurements. The conductivity of the copolymers could be controlled in a broad range from 10 S cm⁻¹ for homopolymer of aniline to 10⁻⁴ S cm⁻¹ for those of o‐chloroaniline. No structural changes took place up to 200 °C and this was confirmed from EPR, IR and electronic absorption spectra. No definite correlation exists between conductivity and spin concentration. © 2000 Society of Chemical Industry     

Studies on the Non‐isothermal Crystallization Behavior of Aluminum Nanopowder‐Filled Poly(3,3‐bis‐azidomethyl Oxetane)

Non‐isothermal crystallization of aluminum nanopowder‐filled poly(3,3‐bis‐azidomethyl oxetane) (PBAMO) from melt at various constant cooling rates from 1 to 10 °C min⁻¹ was studied. Ozawa's approach was used to analyze non‐isothermal crystallization kinetics. It was found that Ozawa's approach was effective at medium constant cooling rate. The crystallization activation energy derived from Kissinger's equation was 80.8 kJ mol⁻¹. The experimental results showed that the aluminum nanopowder acted as a nucleating agent to raise the peak crystallization temperature and to accelerate the crystallization rate at higher constant cooling rates. Uniform crystals which showed one melting peak could be obtained at a constant cooling rate of 1 °C min⁻¹.     

Thermal and electrical properties of additive-free rapidly hot-pressed SiC ceramics

The thermal and electrical properties of newly developed additive free SiC ceramics processed at a temperature as low as 1850 °C (RHP0) and SiC ceramics with 0.79 vol.% Y₂O₃-Sc₂O₃ additives (RHP79) were investigated and compared with those of the chemically vapor-deposited SiC (CVD-SiC) reference material. The additive free RHP0 showed a very high thermal conductivity, as high as 164 Wm⁻¹ K⁻¹, and a low electrical resistivity of 1.2 × 10⁻¹ Ω cm at room temperature (RT), which are the highest thermal conductivity and the lowest electrical resistivity yet seen in sintered SiC ceramics processed at ≤1900 °C. The thermal conductivity and electrical resistivity values of RHP79 were 117 Wm⁻¹ K⁻¹ and 9.5 × 10⁻² Ω cm, respectively. The thermal and electrical conductivities of CVD-SiC parallel to the direction of growth were ∼324 Wm⁻¹ K⁻¹ and ∼5 × 10⁻⁴Ω⁻¹ cm⁻¹ at RT, respectively.     

A “chain fold band” in the infrared spectra of nylon 6

An absorption band found at 974 cm⁻¹ in the infrared spectra of “amorphous” nylon 6 films has been assigned to CH₂ wagging or twisting vibrations in molecular chain folds. Annealing amorphous films in air at increasing temperatures up to 150°C has been shown to induce a steady increase in the intensity of the absorption band at 974 cm⁻¹. Annealing at higher temperatures resulted in a sharp decrease in intensity up to an annealing temperature of 210°C. This sharp decrease coincided with a sharp increase in the long period determined by low-angle x-ray diffraction studies. Drawing amorphous films has been shown to induce a decrease in the intensity of the band at 974 cm⁻¹. Polarized infrared investigations of amorphous drawn films revealed that the band at 974 cm⁻¹ was strongly perpendicular in character. Treatment of amorphous films in water induced no frequency change in the band at 974 cm⁻¹.     

Kinetic study of the transformation of sodalite to nepheline

The kinetics and mechanism of the development of nepheline (NaAlSiO₄) through thermal transformation of sodalite (Na₄Al₃Si₃O₁₂Cl) was studied by means of differential thermal analysis at different heating rates (10°C min⁻¹ to 50°C min⁻¹) to control the evolution of the crystallization fraction. The conversion of sodalite (cubic crystal) to pure nepheline (hexagonal crystal) took place in the 800°C–900°C interval. The activation energy for nepheline crystallization from sodalite was determined by isothermal and non-isothermal methods. The ratio t_0.75/t_0.25, together with the Avrami exponent (n) and the numerical factor of the dimensionality of crystal growth (m) parameters indicated that dominant crystallization mechanism in nepheline development is bulk crystallization mechanism controlled by interface reaction.     

Thermal stabilization by polyols of β-xylanase from Bacillus amyloliquefaciens

Purified endo-β-1,4-xylanase of Bacillus amyloliquefaciens MIR 32 retained 100% of its activity after 4 days of incubation at 50°C. Sorbitol (400 mg cm⁻³) produced a 63-fold increase in the half-life of the enzyme at 65°C, which was only 29 min at this temperature in the absence of the polyol. This thermal stabilizing activity increased exponentially in respect to sorbitol concentration in the range 250–400 mg cm⁻³ and was dependent on the pH, showing a maximum at pH values between 5·25 and 8·0. The circular dichroism (CD) thermal scanning profile (50°C h⁻¹) at 224 nm showed that changes in the secondary structure of xylanase started at 65°C, while in the presence of sorbitol (400 mg cm⁻³) these modifications started at 80°C. This study indicated that sorbitol might be a valuable stabilizer for the use of β-xylanase from B. amyloliquefaciens at high temperatures. © 1998 SCI     

Water sorption in amorphous poly(ethylene terephthalate)

The water sorption characteristics of poly(ethylene terephthalate) (PET) amorphous samples of 250 μm thickness have been studied at various temperatures in a saturated atmosphere. Concerning diffusivity, one can distinguish the following two domains characterized by distinct values of the activation energy: E_D ≈ 36 kJ mol⁻¹ at T > 100°C, and E_D ≈ 42 kJ mol⁻¹ at T < 60°C, with a relatively wide (60–100°C) intermediary domain linked to the glass transition of the polymer. The crystallization of this latter occurs in the time scale of diffusion above 80°C but doesn't change the Fickian character of sorption curves. The equilibrium concentration m_∞ is an increasing function of temperature, but the solubility coefficient S decreases sharply with this latter, with the apparent enthalpy of dissolution ΔH_s being of the order of −28 kJ mol⁻¹ at T < 80°C and −45 kJ mol⁻¹ at T > 80°C. Density measurements in the wet and dry states suggest that water is almost entirely dissolved in the amorphous matrix at T < 80°C but forms partially a separated phase at T > 80°C. Microvoiding can be attributed to crystallization-induced demixing. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 1131–1137, 1999     

In-Situ Infrared Study of the Synthesis of Polyaniline Under Acid and Neutral pH

In-situ infrared study of polyaniline (PANI) synthesis showed that the reaction initiated at pH = 1.5 produced a granule PANI microstructure via para-linked dimers of 4-aminodiphenylamine, exhibiting γ(C–H) at 802 cm⁻¹; the reaction initiated at pH = 5.0 and 7.0 produce fiberous, and planar microstructures via ortho-linked dimers of 1,2-aminodiphenylamine and phenazine, exhibiting γ(C–H) at 738 and ν(C=N) at 1446 cm⁻¹. The doped PANI that was produced at pH less than 5.0 showed a feature-less IR background absorption above 1600 cm⁻¹. This absorption could correspond to π-electron delocalization as an indicative of polyaniline conductivity.

     

Humid aging of polyetherimide. I. Water sorption characteristics

The sorption and desorption kinetics of water into polyetherimide (ULTEM 1000) were studied at various temperatures ranging from 20 to 100°C. The water equilibrium concentration increases slightly with temperature from 1.39% (by weight) at 20°C to 1.50% at 100°C. The solubility coefficient, S, calculated from these data, and the water vapor pressure decrease with temperature. The calculated heat of dissolution H_s is close to −43 kJ mol⁻¹, which explains the low effect of temperature on the equilibrium concentration. The diffusion coefficient, D, varies from about 1.10⁻¹² m² · s⁻¹ at 20°C to about 16.10⁻¹² m² · s⁻¹ at 100°C. The apparent activation energy of diffusion, E_D, and the heat of dissolution, H_s, of water in the polymer have opposite values (respectively, +43 and −42 kJ · mol⁻¹). From this observation and a comparison of these data with water diffusion characteristics in other glassy polar polymers, it is hypothesized that the transport rate of water is kinetically controlled by the dissociation of water–polymer complexes. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 1439–1444, 2000