Heat capacities and thermodynamic properties of one manganese-based MOFs

A metal-organic framework [Mn(4,4′-bipy)(1,3-BDC)] _n (MnMOF, 1,3-BDC = 1,3-benzene dicarboxylate, 4,4′-bipy = 4,4′-bipyridine) has been synthesized hydrothermally and characterized by single crystal XRD and FT-IR spectrum. The low-temperature molar heat capacities of MnMOF were measured by temperature-modulated differential scanning calorimetry for the first time. The thermodynamic parameters such as entropy and enthalpy relative to reference temperature 298.15 K were derived based on the above molar heat capacity data. Moreover, the thermal stability and the decomposition mechanism of MnMOF were investigated by thermogravimetry analysis-mass spectrometer. A two-stage mass loss was observed in air flow. MS curves indicated that the gas products of oxidative degradation were H₂O, CO₂, NO, and NO₂.     

Thermal analysis on influence of compatibilizing agents

Studies have been made on differential scanning calorimetry (DSC), thermogravimetric analysis (TG) and dynamic mechanical analysis (DMA) of binary blends of isobutylene-isoprene (IIR) copolymer and polychloroprene (CR) elastomers. Blends of IIR and CR are incompatible and showed separateT _g peaks in DSC curves similar to Tanδ peaks. However, addition of chlorinated polyethylene (CM) elastomer, as compatibilizer, imparts miscibility between IIR and CR which could be judged both through DSC as well as by dynamic loss measurements (Loss modulusE″ and Tanδ). The storage modulus (E′) showed variation of stiffness due to structural changes associated with the addition of compatibilizer. TG plots for these blends showed improvement of thermal stability both by addition of a suitable compatibilizer as well as due to formation of crosslinked structures associated with the vulcanization of the blends by standard curative package.     

Liquid crystalline and isotropic epoxy thermosets: Mechanism and kinetics of non-isothermal degradation

Thermal non-oxidative degradability of two epoxy thermosets was studied. Investigations were carried out on a non-commercial liquid crystalline structure and its isotropic homologue in order to provide further insight into the mechanism and kinetics of thermal degradation of the proposed systems. The studies were done by thermogravimetric analysis (TGA) and dynamic mechanical analysis (DMA). For the first time the degradation of a liquid crystalline epoxy was studied using an advanced isoconversional kinetic method. The results were used to predict the thermal stability of both types of epoxy networks. GC-MS analysis was applied on evolved gas during degradation to elucidate the degradation mechanism in accordance with the kinetic results. The liquid crystalline structure has a different mechanism of decomposition in comparison with its isotropic homologue. In spite of a higher T_g value, it shows a similar thermal stability but a lower release of degradation compounds.     

The investigation of methyl phenyl silicone resin/epoxy resin using epoxy-polysiloxane as compatibilizer

Styrene–butadiene rubber was subjected to long-term thermal aging treatment at 80 °C with aging period up to 180 days. The degradation kinetics of the aged sample was analyzed by thermogravimetric analysis. Multiple heating rate experiments were carried out in nonisothermal conditions and three isoconversional model-free methods (Friedman; Kissinger–Akahira–Sunose; Li and Tang methods) were employed. The results showed that the temperature for 5 % mass loss increased, whereas the maximum mass loss temperature decreased after aging. Activation energies (E _a) derived from the three methods were found to be dependent on conversion degree (α). E _a increased with increasing α in the whole range of conversion for samples aged for 0, 60, and 120 days, while the aged samples displayed higher E _a values. However, samples aged for 180 days showed declining E _a versus α. The changes on the degradation kinetics were associated with the modification on the chemical structure after thermal aging.     

Synthesis and thermal degradation behaviors of hyperbranched polyphosphate

A novel epoxy-terminated hyperbranched polyphosphate (E-HBPP) was synthesized by employing an A₂ + B₃ polycondensation and characterized by FTIR, ¹H NMR and GPC. E-HBPP was used as a reactive-type flame retardant for diglycidyl ether of bisphenol-A/m-phenylene diamine (DGEBA/mPDA) system. A series of flame retardant resins were prepared and their flame retardancy was monitored by the limiting oxygen index (LOI). The results showed that the LOI value of the cured samples and the degree of expansion of the formed char after burning increased along with the E-HBPP content. Their thermal degradation behaviors were investigated by thermogravimetric analysis and in situ FTIR and showed that the phosphate group of E-HBPP first degraded to form poly(phosphoric acid)s at around 300 °C, which had a major contribution to form the compact char to protect the sample from further degradation. The dynamic mechanical thermal properties were studied by dynamic mechanical thermal analysis (DMTA) and the results showed a good miscibility between E-HBPP and DGEBA. The mechanical properties of the cured films were also investigated. Less than 20% E-HBPP addition improved both the tensile strength and elongation at break.     

Effect of ZnO nanoparticles on kinetics of thermal degradation and final properties of ethylene–propylene–diene rubber systems

The morphology, thermal degradation behavior in addition to static and dynamic mechanical properties of various ethylene?Cpropylene?Cdiene (EPDM) rubber compounds containing nano-zinc oxide (NZnO) were investigated compared to those of EPDM with ordinary-sized ZnO (OSZnO). The field-emission scanning electron microscopy studies showed that unlike the conventional system, the formation of large size ZnO agglomerates was discouraged for NZnO filled systems. Thermogravimetric analysis (TG) revealed that the thermal degradation of EPDM system was delayed upon the inclusion of NZnO instead of OSZnO in the compound. The kinetic analysis of TG data based on Friedman and Kissinger methods showed that the nanocomposite samples exhibited higher activation energy (E _a ) and lower order of reaction (n) over the conventional system, suggesting the enhancement of thermal stability upon decreasing ZnO particle size. The results obtained from dynamic mechanical analysis and static mechanical characterizations in terms of hardness, resilience, and abrasion tests interestingly indicated that NZnO not merely could act as a thermal insulator, but also could perform as a nano-filler to improve the final performance of EPDM elastomers.     

Determination of the activation energies for α and β transitions of a system containing a diglycidyl ether of bisphenol a (DGEBA) and 1,3-bisaminomethylcyclohexane (1,3-BAC)

Using dynamic mechanical analysis (DMA) we have studied the variation with the frequency of the dynamic mechanical properties (storages modulus,E'; loss modulus,E'' and loss tangent or tan σ) for a system containg a diglycidyl ether of bisphenol A (DGEBA) and 1,3-bisaminomethylcyclohexane (1,3-BAC). These properties were measured both in the glass transition and β transition regions. An increase in frequency caused a shift of tan σ peak positions in both regions toward higher temperature. Finally, we report the activation energies of a DGEBA/1,3-BAC expoxy system for α and β transitions.     

Thermal-oxidative aging of DGEBA/EPN/LMPA epoxy system: Chemical structure and thermal-mechanical properties

The evolvement of chemical structure and thermal-mechanical properties of diglycidyl ether of bisphenol-A and novolac epoxy resin blends cured with low molecular polyamide (DGEBA/EPN/LMPA system) during thermal-oxidative aging were investigated by Attenuated Total Reflectance Fourier Transform Infrared spectrometry (ATR-FTIR) and Dynamic Mechanical Thermal Analysis (DMTA). The results revealed that the chemical reactions during thermal-oxidative aging contained oxidation and chain scission. Some possible chemical reaction processes were given. There was a new compound formed during aging processes and the change of its glass transition temperature (T_g) with aging time followed an exponential law. In addition, the changes of dynamic mechanical behavior of this epoxy system aged at four different temperatures (110 °C, 130 °C, 150 °C, 170 °C) were compared. An empirical formula was obtained through kinetic analysis and this formula can be used to predict the oxidative degree of the surface at different aging temperature.     

Reversal and activation of physical aging by applied deformations in simple compression and extension

The differential (incremental) storage modulus E′ was measured intermittently at 1 Hz during the stress relaxation of cylindrical specimens of polycarbonate subjected to finite static strains in both simple compression and extension. (In measuring E′, the amplitude of the applied sinusoidal strain was 0.2%.) Application of each static strain gave a value of E′/E′₀ less than unity, where E′₀ is the storage modulus at 1 Hz of the undeformed specimen. This behavior results from an increase in the mobility of short molecular segments; it signifies a partial erasure of the state of physical aging, a change also termed de-aging or rejuvenation. After a static strain had been applied, E′ increased continuously, a reflection of physical aging that results from a progressive decrease in segmental mobility. Plots of E′/E′₀ at an aging time of 100 s against the absolute value of the static strain show that simple extension de-ages a specimen somewhat more than does a strain of the same magnitude in compression. This difference results from the increase or decrease in volume (or free volume) effected by strains in extension or compression, respectively. The dominant cause of de-aging, however, is the deviatoric (shear) component of the strain tensor. The increase or decrease in volume produced by an applied strain modifies only slightly the segmental mobility.     

Vibrational and electronic spectral studies of substituted 1,3-dihydro-1,3-diphenyl-2-thioxo-2H,5H-pyrimidine-4,6-diones

The vibrational spectra of 1,3-diphenyl-; 1,3-di (4′-chlorophenyl)-; 1,3-di(3′-methoxyphenyl)-1,3-dihydro-2-thioxo-2H,5H-pyrimidine-4,6-diones in solid state (KBr pellet) and in solution (CHCl₃ and CCl₄) have been studied and assignments made. Tautomeric and hydrogen bonding behaviour are discussed. Electronic spectra in various solvents at different pH values are recorded. The effect of substituents, change of solvent on the π—π* and n—π* transitions of all the compounds is explained. The bathochromic and hypsochromic shifts observed when the neutral form changes to the cationic or anionic form depending on the pH of solution, are also discussed.     

An analysis of the thermal aging behaviour in high-performance energetic composites through the glass transition temperature

Differential scanning calorimetry (DSC) was used to analyze the thermal aging behaviour in energetic composite materials where a hydroxyl-terminated polybutadiene (HTPB)/isophorone diisocyanate elastomer is the polymeric matrix. Different parameters from the analysis of the glass transition, such as the glass transition temperature (T_g), were used in order to monitor this isothermal aging at 65 °C during a total time of 3000 h, finding an increasing and broadening T_g. In addition, the accelerated aging behaviour of these materials was also studied by a classical method, based on the change of mechanical properties such as those of Young's modulus or strain at break. The correlation between both methodologies was examined, demonstrating that an analytical technique such as DSC allows the evaluation of the actual state of composite solid propellants with a small sample and a straightforward measurement.     

Accelerated degradation behaviour of poly(?-caprolactone) via melt blending with poly(aspartic acid-co-lactide) (PAL)

Poly(?-caprolactone) (PCL) has many favourable attributes for tissue engineering scaffold applications. A major drawback, however, is its slow degradation rate, typically greater than 3 years. In this study PCL was melt blended with a small percentage of poly(aspartic acid-co-lactide) (PAL) and the degradation behaviour was evaluated in phosphate buffer solution (PBS) at 37 °C. The addition of PAL was found to significantly enhance the degradation profile of PCL. Subsequent degradation behaviour was investigated in terms of the polymer's mechanical properties, molecular weight (M_w), mass changes and thermal characteristics. The results indicate that the addition of PAL accelerates the degradation of PCL, with 20% mass loss recorded after just 7 months in vitro for samples containing 8 wt% PAL. The corresponding pure PCL samples exhibited no mass loss over the same time period. In vitro assessment of PCL and PCL/PAL composites in tissue culture medium in the absence of cells revealed stable pH readings with time. SEM studies of cell/biomaterial interactions demonstrated biocompatibility of C₃H₁₀T_1/2 cells with PCL and PCL/PAL composites at all concentrations of PAL additive.     

Thermo-oxidative degradation kinetics and mechanism of the system epoxy nanocomposite reinforced with nano-Al2O3

We have synthesized epoxy nanocomposites with various percents of nanoalumina by using ultrasonic dispersion treatment. Scanning calorimetry studies revealed that the composition having 1% nanoalumina results in the highest value of cross-link density as evidenced by the glass transition temperature (T _g). Thermal degradation of the systems consisting of diglycidyl ether bisphenol A (DGEBA)/1,3-Poropane diamine and with 1% and without nanoalumina were studied by thermogravimetry analysis to determine the reaction mechanism in air. The obtained results indicated that a relatively low concentration of nanoalumina led to an impressive improvement of thermal stability of epoxy resin. The Coats?CRedfern, Van Krevelen, Horowitz?CMetzger, and Criado methods were utilized to find the solid state thermal degradation mechanism. Analysis of our experimental results suggests that the reaction mechanism is depending on the applied thermal history. For the nanocomposite, the mechanism was recognized to be one-dimensional diffusion (D₁) reaction at low heating rates and it changes to be a random nucleation process with one nucleus on the individual particle (F₁) at high heating speeds. The results also indicated that the degradation mechanism of organic phase is influenced by the presence of inorganic nanofiller.     

Dynamic mechanical and thermal characterization of high-performance polybenzoxazines

Two polybenzoxazines are cured in an autoclave from the polyfunctional benzoxazine monomers, 8,8′-bis(3,4-dihydro-3-phenyl-2H-1,3-benzoxazine) and 6,6′-bis(2,3-dihydro-3-phenyl-4H-1,3-benzoxazinyl) ketone. The density and tensile properties of these polybenzoxazines are measured at room temperature. Dynamic mechanical tests are performed to determine the T_g, crosslink density, and the activation enthalpy of the glass-transition process for these two polybenzoxazines. The effect of postcure temperature on the T_g of the polymers is investigated and discussed in terms of crosslink density. Fourier transform infrared (FTIR) spectroscopy is also applied for the molecular characterization of the curing systems. Thermal properties of these polybenzoxazines are studied in terms of isothermal aging and decomposition temperature via thermogravimetric analysis. These two polybenzoxazines show mechanical and thermal properties similar to or better than bismaleimides and some polyimides. They also show very high char yield after being carbonized in a nitrogen atmosphere. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 3257–3268, 1999     

Thermal characterizations of multifunctional epoxy/anhydride systems used for pultruded composite ropes

The viscoelastic characterization and thermal stability property of some multifunctional epoxy/anhydride systems cured at different schedules were investigated by dynamic mechanical thermal analysis (DMTA) in single cantilever mode at fixed frequency, and by non-isothermal thermogravimetric (TG) analysis, respectively. According to the DMTA results, three obviously different glass transition temperatures (T _g), were observed, among which TGDDM/MHHPA system exhibits the largest T _g. While from the TG curves, the results of the mass loss and thermal stability showed that, after cured for a prolonged duration, the TGDDM/MHHPA system possessed the most excellent performance in heat resistance.     

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

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

Enhanced thermal stability of polypyrrole hexagonal microplates fabricated by organic crystal surface-induced polymerization

The stability of polypyrrole hexagonal microplates (PHMs) fabricated by organic crystal surface-induced polymerization (OCSP) in the presence of 4-sulfobenzoic acid monopotassium salt (SBAK) crystals was examined during thermal aging at 150 °C for 10 h under air and nitrogen atmospheres. Thermal stability of PHMs and conventional polypyrroles (CPPys) was evaluated in terms of the resistivity (R_t) after aging for t h, normalized to the initial resistivity (R₀) before aging, R_t/R₀. Although the PHMs maintained R₁₀/R₀ values of 21.9 and 3.0 under air and nitrogen, respectively, the CPPys exhibited much higher R₁₀/R₀ values, of 853.8 and 14.6, respectively. A possible explanation for the enhanced thermal stability of the PHMs is the higher thermal stability and the antioxidant effect of SBAK dopant molecules. Thermo-oxidative degradation was accelerated due to direct chemical attack on the cationic pyrrole rings of atmospheric water and oxygen, leading to a steep increase in surface resistivity. The development of carbonyl defects on PPy chains during thermal aging was monitored using Fourier transform-infrared (FT-IR) spectroscopy and X-ray photoelectron spectroscopy (XPS). Ultraviolet-visible (UV-vis) spectroscopy revealed that the PHMs essentially retained the bipolaron structures, even after thermal aging for 10 h in air, whereas the CPPys showed almost no bipolaron structures.     

Concept of hard clusters in the interpretation of thermal and mechanical properties of polyurethane and polyurethane acrylate networks

Polyurethane (PU) and polyurethane acrylate (PUA) networks based on hydroxyl-terminated polycaprolactone (PCL), 1,3-bis-2,2′(2-isocyanatopropyl)benzene (m-TMXDI), trimethylolpropane (TMP) for PU or hydroxyethyl methacrylate (HEMA) for PUA were synthesized. Glass transition temperature, T_g, dynamic mechanical relaxation, α, and equilibrium tensile modulus, E′, were measured to compare the two kinds of networks. To explain thermal and mechanical properties of networks, the concept of hard clusters has been introduced. PU networks exhibit a single-phase structure with modulus and T_g dependent on the concentration of elastically active network chains (EANC) per unit volume calculated by considering hard crosslink clusters. The rigidity of the clusters comes from small diisocyanate and trimethylolpropane units connected by urethane bonds. They are embedded in a continuous soft phase of macrodiol urethane. Physical equivalence between several kinds of network models has been demonstrated for full conversion of isocyanate-alcohol reaction. PUA networks exhibit thermodynamically one-phase structures that become a two-phase structure for high molar mass of macrodiol when the molar fraction of isocyanate groups increases. For those networks, the calculated modulus considering clusters based on polyacrylate chains seems to be a good way to approach the experimental value of the equilibrium modulus. For the same molar ratio of OH to NCO groups the range of dynamic moduli is larger for PUA than for PU. This difference can be explained by a different concentration of crosslinks in the networks. © 1996 John Wiley & Sons, Inc.     

Synthesis of <Emphasis Type="Italic">N,N′</Emphasis>-bis-[3-alkoxy-4-(hydroxy,alkoxy, acyloxy)-phenylmethylene- and -Phenylmethyl]-1,3-phenylenediamines

Starting with the vanillin series aldehydes, by reaction with 1,3-phenylenediamine in absolute methanol E,E-N,N′-bis-[3-alkoxy-4-(hydroxy-, alkoxy-, acyloxy)phenylme-thylene]-1,3-phenyl-enediamines (Shiff bases) are synthesized from reduction with Na[BH(OAc)₃] in benzene were prepared respective N,N′-bis-[3-alkoxy-4-(hydroxy-, alkoxy-, acyloxy)phenylmethyl]-1,3-phenyl-enediamines.