The physical properties of bisphenol-a-based epoxy resins during and after curing. II. Creep behavior above and below the glass transition temperature

The creep behavior of a series of fully cured epoxy resins with different crosslink densities was determined from the glassy compliance level to the equilibrium compliance J_e at temperatures above T_g and at the glassy level below T_g during spontaneous densification at four aging temperatures, 4,4-diamino diphenyl sulfone DDS was used to crosslink the epoxy resins. The shear creep compliance curves J(t) obtained with materials at equilibrium densities near and above T_g were compared at their respective T_gs. T_gs from 101 to 205°C were observed for the epoxies which were based on the diglycidyl ether of bisphenol A. Creep rates were found to be the same at short times, and equilibrium compliances J_e were close to the predictions of the kinetic theory of rubberlike elasticity. Time scale shift factors determined during physical aging were reduced to T_g. At compliances below 2 × 10^?10 cm²/dyn, Andrade creep, where J(t) is a linear function of the cube root of creep time, was observed. The time to reach an equilibrium volume at T_g was found to be longer for the epoxy resin with lower crosslink densities. The increase of density during curing is illustrated for the epoxy resin with the highest crosslink density.     

The evolution of the viscoelastic retardation spectrum during the development of an epoxy resin network

The evolution of the viscoelastic behavior of an epoxy resin at various stages of curing has been followed with the changes in the retardation spectrum. The creep J(t) and recoverable creep compliance J_r(t) curves of the neat epoxy resin Epon l00lF (Shell) were determined at temperatures between 30 and 77°C. The viscosity decreased over 8 orders of magnitude as the temperature was increased. Specimens with eight stages of network development were prepared by reacting all of the epoxy resin's oxirane rings with amine hydrogens from varying ratios of a monofunctional amine (methyl aniline) and a tetrafunctional amine 4,4′-diamino diphenyl sulfone (DDS). Preparations in which 25, 35, and 40% DDS were used did not result in a molecular network, so they were viscoelastic liquids. With 45% DDS, the product had a nascent network and was judged to be just beyond the point of incipient gelation. The remaining preparations from 0.50, 0.60, 0.70, and 1.0 DDS yielded tighter less compliant molecular networks. The creep and recoverable compliance curves were measured over a range of temperatures above the glass transition temperature, T_g. They were reduced to T_g, and retardation spectra L(ln τ) were calculated.     

Dynamic mechanical relaxation effects in some poly(arylene sulfones)

Poly-4,4′-oxydiphenylenesulfonyl and poly-4,4′-methylenediphenylenesulfonyl were synthesized by an electrophilic substitution polymerization of the arylene monosulfonyl chloride monomers. The glass-transition temperatures T_g of these polymers were determined by calorimetric and dynamic mechanical measurements, and the number-average molecular weights were determined by vapor-pressure osmometry. Both polymers were found to have the same T_g at equivalent molecular weight; the limiting value at high molecular weight is 238°C. Both polymers have two dynamic mechanical relaxation peaks at temperatures far below T_g. One is in the neighborhood of 0°C, and the other is at ?110°C. Plausible origins for these relaxations, and the absence of any near 0°C in poly(4,4′-isopropylidenediphenylene-co-4,4′-sulfonyldiphenylene dioxide), are discussed.     

Wholly aromatic thermotropic liquid crystalline polyesters of 3,3′-bis(phenyl)-4,4′-biphenol with 4,4′-benzophenone dicarboxylic acid

A series of wholly aromatic, thermotropic polyesters, derived from 3,3′-bis(phenyl)-4,4′-biphenol (DPBP), nonlinear 4,4′-benzophenone dicarboxylic acid (4,4′-BDA), and various linear comonomers, were prepared by the melt polycondensation reaction and characterized for their thermotropic properties by a variety of experimental techniques. The homopolymer of DPBP with 4,4′-BDA had a fusion temperature (T_f) at 265°C, exhibited a nematic phase, and had a liquid crystalline range of 105°C. All of the copolyesters of DPBP with 4,4′-BDA and either 30 mol % 4-hydroxybenzoic acid (HBA), 6-hydroxy-2-naphthoic acid (HNA), or 50 mol % terephthalic acid (TA), 2,6-naphthalenedicarboxylic acid (2,6-NDA) had low T_f values in the range of 220–285°C, exhibited a nematic phase, and had accessible isotropization transitions (T_i) in the range of 270–420°C, respectively. Their accessible T_i values would enable one to observe a biphase structure. Each of the copolymers with HBA or HNA had a much broader range of liquid crystalline phase. In contrast, each of the copolymers with TA or 2,6-NDA had a relatively narrow range of liquid crystalline phase. Each of these polyesters had a glassy, nematic morphology that was confirmed with the DSC, PLM, WAXD, and SEM studies. As expected, they had higher glass transition temperatures (T_g) in the range of 161–217°C than those of other liquid crystalline polyesters, and excellent thermal stabilities (T_d) in the range of 494–517°C, respectively. Despite their noncrystallinity, they were not soluble in common organic solvents with the exception that the homopolymer and its copolymer with TA had limited solubility in CHCl₃. However, they were soluble in the usual mixture of p-chlorophenol/1,1,2,2-tetrachloroethane (60/40 by weight) with the exception of the copolymer with 2,6-NDA. © 1997 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 35: 769–785, 1997     

Effects of high-pressure CO2 on the glass transition temperature and mechanical properties of polystyrene

Hydrostatic pressure usually increases the glass transition temperature T_g of a polymer glass by decreasing its free volume; if the pressurizing environment is soluble in the polymer, however, one might expect an initial decrease in T_g with pressure as the polymer is plasticized by the environment. Just such a minimum in the T_g of polystyrene (PS) is observed as the pressure of CO₂ gas is increased over the range 0.1–105 MPa from both ultrasonic (1 MHz) measurements of Young's modulus E and static measurements of the creep compliance J. A time-temperature-pressure superposition law is obeyed by PS which allows a master curve for the compliance to be constructed and shift factors to be determined. A master curve for E is then obtained by using the Boltzmann superposition principle. The compliance J reaches a maximum, and E and T_g reach minima, at a CO₂ pressure of ca. 20 MPa at both 34 and 45°C, which are above the critical temperature (31°C) of CO₂. At the minimum, T_g is 41 at 45°C and 36 at 34°C, the larger depression at 34°C evidently corresponding to the higher solubility of CO₂ at the lower temperature. The plasticization effect due to CO₂ can be isolated by subtracting the effect of hydrostatic pressure alone from the experimental data. The results leave no doubt that at high pressures CO₂ gas is a severe plasticizer for polystyrene.     

Amorphous copolyesters based on bibenzoic acids and neopentyl glycol

High T_g amorphous copolyester thermoplastics were synthesized by incorporating 4,4′‐bibenzoate (4,4′BB) and 3,4′‐bibenzoate moieties into the polyester backbone via melt polycondensation. The high levels of crystallinity typically associated with 4,4′BB containing polyesters were suppressed through copolymerization of ethylene glycol, 1,4‐cyclohexane dimethanol, and neopentyl glycol (NPG) diols. NPG was shown to be highly effective in suppressing crystallization and was used to produce amorphous compositions with T_g’s as high as 129 °C. Diol ratios were determined by ¹H NMR spectroscopy and molecular weights were assessed with inherent viscosity (η_inh). Thermogravimetric analysis showed single‐step weight losses in the range of 395 – 419 °C. Differential scanning calorimetry was used to determine melting points and glass transition temperatures over a wide range of copolyester compositions and identified amorphous compositions. Dynamic mechanical analysis confirmed T_g’s and was used to study β‐relaxations below the T_g. Rheological analysis revealed the effect of NPG structures on shear thinning and thermal stability. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019 , 57, 579–587     

Time-temperature-transformation analysis of an acrylic-amino resin system

The curing behaviour of a thermosetting acrylic-amino-formaldehyde resin matrix has been investigated by rheological, thermogravimetric, and thermomechanical analysis. The time-temperature-transformation (TTT) diagram of the solventless resin matrix has been constructed. The gelation curve could be fitted with a power-function of T _gel=118.14·t _gel ^-01238. With the help of this function the conditions for gelation can be calculated, and storability of the matrix can be predicted. The onset temperature of thermooxydative decomposition is 220°C, and the maximum loss of mass belonging to this temperature was found to be 16.3%, corresponding to the ultimate conversion of curing. From iso-T_cure diagrams determined by isothermal thermogravimetric analysis, the iso-curing time (iso-t _cure) and iso-mass loss curves of the TTT diagram have been constructed. The iso-T_cure diagrams determined by TMA measurements, served the construction of the iso-T_cure and iso-glass transition temperature (iso-T _g) diagrams. The T _g of the fully cured system (T _g) was found to be 30°C. The iso-T_g line of 30°C represents the ultimate conversion of the cured system, up to which no degradation takes place. This curve runs below the ultimate conversion determined by thermogravimetric measurements, meaning that above 160°C decomposition takes place simultaneously with curing reactions as supported by the increasing tangent of the straight lines fitted to the last section of thermogravimetric iso-T _cure diagrams. The T _g of the matrix before the cure (T _g0) is less than 0°C. This means that the system is in its rubbery physical state during the curing process.     

Wholly aromatic thermotropic liquid crystalline polyesters of 4,4′-biphenol,substituted biphenols,and 1,1′-binaphthyl-4,4′-diol with 3,4′-benzophenone dicarboxylic acid

Wholly aromatic, thermotropic homopolyesters, derived from 4,4′-biphenol, substituted biphenols, or 1,1′-binaphthyl-4,4′-diol and 3,4′-benzophenone dicarboxylic acid, and two copolyesters, each of which contained 30 mol % of 6-hydroxy-2-naphthoic acid, were prepared by acidolysis polycondensation reactions and characterized for their liquid crystalline properties. The solubility behavior of these polymers has also been investigated. The two homopolymers of phenyl-substituted biphenols with 3,4′-benzophenone dicarboxylic acid were soluble in many common organic solvents. All of the homopolymers had lower T_m/T_f values than those with terephthalic acid, which was attributed to the incorporation of the asymmetric 3,4′-benzophenone dicarboxylate units in a head-to-head and head-to-tail fashion along the polyester chain. Two copolymers had lower T_m values than those of the respective homopolymers, as expected. They formed nematic phases which persisted up to 400°C, except those of phenyl-substituted biphenols with 3,4′-benzophenone dicarboxylic acid. Each of these two polymers also exhibited an accessible T_i transition, and had a broad range of LC phase. They had glass transition temperatures, T_g, in the range of 139-209°C and high thermal stabilities in the temperature range of 465-511°C. © 1995 John Wiley & Sons, Inc.     

Fully aromatic thermotropic liquid crystalline polyesters of 3-phenyl-4,4′-biphenol with 4,4′-benzophenone dicarboxylic acid

A series of fully aromatic, thermotropic polyesters, derived from 3-phenyl-4,4′-biphenol (MPBP), nonlinear 4,4′-benzophenone dicarboxylic acid (4,4′-BDA), and various other comonomers was prepared by the melt polycondensation method and characterized for their thermotropic liquid crystalline behavior by a variety of experimental techniques. The homopolymer of MPBP with 4,4′-BDA had a fusion temperature (T_f) at 240°C, exhibited a nematic liquid crystalline phase, and had a narrow liquid crystalline range of 60°C. All of the copolyesters of MPBP with 4,4′-BDA and either 30 mol % 4-hydroxybenzoic acid (HBA), 6-hydroxy-2-naphthoic acid (HNA) or 50 mol % terephthalic acid (TA), 2,6-naphthale-nedicarboxylic acid (2,6-NDA) and low T_f values in the range of 210–230°C, exhibited a nematic phase, and had accessible isotropization transitions (T_i) in the range of 320–420°C, respectively. As expected, each of them had a broader range of liquid crystalline phase than the homopolymer. They had a “frozen” nematic, glassy order as determined with the wide-angle X-ray diffraction (WAXD) studies. The morphology of each of the “as-made” polyesters had a fibrous structure as determined with the scanning electron microscopy (SEM), which arises because of the liquid crystalline domains. Moreover, they had higher glass transition temperatures (T_g) in the range of 167–190°C than those of other liquid crystalline polyesters, and excellent thermal stabilities (T_d) in the range of 500–533°C, respectively. © 1995 John Wiley & Sons, Inc.     

Aromatic polyamides. III. Copoly(4,4′-oxanilideterephthalamide—4,4′-phenyleneterephthalamide): Synthesis,wet-spinning,fiber thermal characterization,and stability in sulfuric acid

Copoly(4,4′-oxanilideterephthalamide—4,4′-phenyleneterephthalamide) (A-202/PPD) was synthesized by reaction of 4,4′-diaminooxanilide, p-phenylenediamine, and terephthaloyl chloride in organic solvents. Copolymer inherent viscosities in H₂SO₄ as high as 10.3 were obtained. Isotropic copolymer solutions (4%—5% concentration) of A-202/40%–80% PPD were spun to fibers with tenacity/elongation/modulus at 1% extension in the 13–14 gpd/1.5%–2%/700–1000 gpd range. Oxamide and amide stabilities in 98–100% H₂SO₄ and 20% oleum were compared. Poly(4,4′-oxanilideterephthalamide) (A-202), A-202/PPD copolymers, and poly(4,4′-phenyleneterephthalamide) (PPT) were unstable in 20% oleum, but all proved relatively stable in 100% H₂SO₄. However, the oxamide linkage proved less stable than the amide linkage in 98% H₂SO₄. A-202 and A-202/PPD copolymers formed stable anisotropic spinning solutions in 1% oleum at 10–20% concentrations. Dynamic mechanical analyses (Vibron) showed no glass transition temperature (T_g) below 200°C. Dilatometric measurement of A-202/50% PPD revealed a T_g at 257°C. Differential thermal analyses of A-202/40–80% PPD exhibited endotherms at 470–480°C. Thermogravimetric analyses showed no significant weight loss below 400°C.     

Relaxations in thermosets. XIX. Dielectric effects during curing of diglycidyl ether of bisphenol-A with a catalyst and the properties of the thermoset

Changes in the dielectric permittivity ε′ and loss epsiv;″ during the curing of DGEBA catalyzed by 10 mole % dimethylbenzylamine have been studied from sol to gel to glass formation regions at different temperatures from 323 to 390 K. The ε′ monotonically decreases with time of cure, and ε″ initially decreases by several orders of magnitude and then increases to reach a peak value before finally decreasing to a low value characteristic of the glassy state. The features shift to shorter times and the peak vanishes as the curing temperature is increased. The decrease of ε″ at the initial stage of cure has been analyzed in terms of dc conductivity σ₀, which follows a power law, σ₀ ∝? (t_g–t)^x, as well as a new singularity equation, σ₀ ∝? exp[–B/(t₀–t)] where t_g, x, B, and t₀ are empirical constants that vary with the curing temperature; t_g is close to the time for gelation; and t₀ ≥ time for vitrification. The dielectric properties of the thermoset formed after different periods of cure have been studied from 77 to 325 K. Similar studies of the thermosets formed at different temperatures have been made. Increase in the curing period decreases the heights of both the γ-and α-relaxation peaks and increases their separation, while a β-relaxation peak emerges. Isothermal curing at high temperatures decreases the height of the γ peak to a vanishingly small value and increases that of the β peak from a vanishingly small value. In both the uncured and fully cured states, there is only one sub-T_g relaxation process named γ for the uncured and β for the cured state. These results are discussed in terms of our general physical concepts of local mode motions in an amorphous matrix. © 1993 John Wiley & Sons, Inc.     

Twinkling fractal theory of the glass transition

In this paper we propose a solution to an unsolved problem in solid state physics, namely, the nature and structure of the glass transition in amorphous materials. The development of dynamic percolating fractal structures near T_g is the main element of the Twinkling Fractal Theory (TFT) presented herein and the percolating fractal twinkles with a frequency spectrum F(ω) ∼ ω^df–1 exp −|ΔE|/kT as solid and liquid clusters interchange with frequency ω. The Orbach vibrational density of states for a fractal is g(ω) ∼ ω^df–1, where d_f = 4/3 and the temperature dependent activation energy behaves as ΔE ∼ (T² − T). The key concept of the TFT derives from the Boltzmann population of excited states in the anharmonic intermolecular potential between atoms, coupled with percolating solid fractal structures near T_g. The twinkling fractal spectrum F(ω) at T_g predicts the correct dynamic heterogeneity behavior via the spatio-temporal thermal fluctuation autocorrelation relaxation function C(t). This function behaves as C(t) ∼ t^−1/3 (short times), C(t) ∼ t^−4/3 (long times) and C(t) ∼ t⁻² (ω < ω_c), which were found to be in excellent agreement with published nanoscale AFM dielectric force fluctuation experiments on a glassy polymer near T_g. Using the Morse potential, the TFT predicts that T_g = 2D_o/9k, where D_o is the interatomic bonding energy ∼ 2–5 kcal/mol and is comparable to the heat of fusion ΔH_f. Because anharmonicity controls both the thermal expansion coefficient α_L and T_g, the TFT uniquely predicts that α_L×T_g ≈ 0.03, which is found to be universal for a broad range of glassy materials from Pyrex to polymers to glycerol. Below T_g, the glassy structure attains a frustrated nonequilibrium state by getting constrained on the fractal structure and the thermal expansion in the glass is reduced by the percolation threshold p_c as α_g ≈ p_cα_L. The change in heat capacity ΔC_p = C_pL–C_pg at T_g was found to be related to the change in dimensionality from D_f to 3 in the Debye approximation as the ratio C_pL/C_pg = 3/D_f, where D_f is the fractal dimension of the glass. For polymers, the TFT describes the molecular weight dependence of T_g, the role of crosslinks on T_g, the Flory-Fox rule of mixtures and the WLF relation for the time-temperature shift factor a_T, which are traditionally viewed in terms of Free-Volume theory. The TFT offers new insight into the behavior of nano-confined glassy materials and the dynamics of physical aging. It also predicts the relation between the melting point T_m and T_g as T_m/T_g = 1/[1−p_c] ≈ 2. The TFT is universal to all glass forming liquids. © 2008 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 46: 2765–2778, 2008     

Diglycidyl ether of bisphenol-A with 4,4′-methylenedianiline: A pulsed NMR study of the curing process

The curing process, during which the monomeric diglycidyl ether of bisphenol-A is cured with 4,4′-methylenedianiline, has been studied by pulsed NMR. Values of the proton relaxation times T₁, T_1ρ, and T₂ have been measured as a function of time as the resin system cures at constant temperature. The relaxation times are interpreted in terms of the decrease in general molecular motion which results from the cure. Plots of correlation frequency versus time for the constant-temperature cure were constructed for three temperatures. It is shown that these three plots can be represented by a reduced curve. With certain simplifying assumptions, the shape of this reduced curve. With certain simplifying assumptions, the shape of this reduced curve can be accounted for in terms of the chemical rate constant and an exponent relating molecular weight to viscosity. The activation energy for the cure is estimated to be 11.7 kcal/mole.     

Time-temperature transformation (TTT) cure diagram of an unsaturated polyester resin

The curing of an unsaturated polyester resin was studied by differential scanning calorimetry (DSC), thermal mechanical analysis (TMA), and Fourier-transform infrared spectroscopy (FTIR). The results are presented in the form of a time-temperature-transformation (TTT) diagram. The kinetic analysis was performed by means of the dynamic Ozawa method. This analysis was used to determine the curing times (t) at various conversions (α) and temperatures (T) (isoconversional lines ln t = A + E/RT). The equivalence of the Ozawa method and the isothermal isoconversional adjustment ln t = A + E/RT were demonstrated. The relationship between the glassy transition temperature (T_g) and the conversion α was determined by DSC. It was established that this relationship is one-to-one and independent of mass, initiation system, and curing temperature (T_c). The T_g-α relationship was adjusted using the DiBenedetto equations and heat capacity data. Using the T_g-α relationship and the isoconversional lines, the vitrification curve was determined and it was observed that the vitrification times obtained are consistent with those obtained experimentally when T_c = T_g. Gelation was determined by TMA, the material being considered gelled when it reached sufficient mechanical stability for the TMA measuring probe to become embedded in it. At that moment the conversion reached was determined by DSC. It was seen that the material always gels at constant conversion, regardless of the curing temperature. The gelation line (gel times) were traced from the corresponding isoconversional line. © 1997 John Wiley & Sons, Inc.     

Synthesis and comparative study on polyetherimides from isomeric 4,4′-isopropylidenediphenoxy bis(phthalic anhydride)s

Two isomers of commercial 4,4′-(4,4′-isopropylidenediphenoxy) bis(phthalic anhydride) (4,4′-BPADA), that is, 3,4′-(4,4′-isopropylidenediphenoxy) bis(phthalic anhydride) (3,4′-BPADA) and 3,3′-(4,4′-isopropylidenediphenoxy) bis(phthalic anhydride) (3,3′-BPADA), were synthesized through aromatic nucleophilic substitution from nitrophthalonitrile and bisphenol A. 3,4′-BPADA was first synthesized from two intermediates, that is, 3-(4-[4-hydroxyphenylisopropylidene] phenoxy) phthalonitrile (3-BPADN) and 3,4′-(4,4′-isopropylidenediphenoxy) bis(phthalonitrile) (3,4′-BPATN). The corresponding three series of polyetherimides (PEIs) were prepared with two representative aromatic diamines (4,4′-oxydianiline and m-phenylenediamine (m-PDA)) via two-step procedure and chemical imidization. Isomeric polyimides showed T_gs from 206 to 256°C in nitrogen and T_d5%s from 488 to 511°C in argon, good mechanical properties (tensile moduli of 2.3–3.3 GPa, tensile strengths of 70–96 MPa, and elongations at break of 3.2%–5.1%), and good solubility. With the introduction of 3-substituted phthalimide unit, PEIs displayed higher T_g values, lower strengths and elongations, better solubility and larger d-spacings. The rheological properties of thermoplastic polyimide resins based on the BPADA isomers were investigated, which showed that polyetherimide PEI-3b derived from 3,3′-BPADA and m-PDA had the lowest melt viscosity among the isomers, indicating that the melt processibility had been greatly improved.     

Positron annihilation lifetime studies of changes in free volume on cross-linking cis-polyisoprene,high-vinyl polybutadiene,and their miscible blends

Measurements of average free volume hole sizes, 〈v_f〉, and the fractional free volumes, f_ps, in vulcanized cis-polyisoprene (CPI), high-vinyl polybutadiene (HVBD), and their 50 : 50 blend were made via determination of orthopositronium annihilation lifetimes. The results are compared to corresponding data on the uncured materials. On crosslinking, 〈v_f〉 decreases in the rubbery state but remains essentially unchanged in the glass. This is consistent with the expectation that the crosslinks greatly restrict the thermal expansion of the chains above the glass transition temperature (T_g) but have less influence on the packing density in the glass. Scaling relationships between 〈v_f〉, f_ps, the thermal expansion coefficient α_f = df_ps/dt, and T_g are examined. We find that 〈v_f〉_g, the hole volume at T_g, and f_ps,g, the fractional free volume at T_g, each increase significantly with increasing T_g. This behavior is consistent with previous observations reported in the literature and has been interpreted as a manifestation of the kinetic character of the glass transition. High-T_g polymers need a larger free volume to pass into the liquid state. The change in expansion coefficient on passing from the glass to the liquid, Δα_f = α_f,l − α_f,g, increases slowly with T_g, as predicted by free volume theory. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2754–2770, 1999     

Synthesis and properties of poly(ether imide)s based on the bis(ether anhydride)s from hydroquinone and its derivatives

Four bis(ether anhydride)s, 4,4′‐(1,4‐phenylenedioxy)diphthalic anhydride (IV), 4,4′‐(2,5‐tolylenedioxy)‐diphthalic anhydride (Me‐IV), 4,4′‐(2‐chloro‐1,4‐phenylenedioxy)diphthalic anhydride (Cl‐IV), and 4,4′‐(2,5‐biphenylenedioxy)diphthalic anhydride (Ph‐IV), were prepared in three steps starting from the nucleophilic nitrodisplacement reaction of 4‐nitrophthalonitrile with the potassium phenoxides of hydroquinone and various substituted hydroquinones such as methylhydroquinone, chlorohydroquinone, and phenylhydroquinone in N,N‐dimethylformamide, followed by alkaline hydrolysis and dehydration. Four series of poly(ether imide)s were prepared from bis(ether anhydride)s with various aromatic diamines by a classical two‐step procedure. The inherent viscosities of the intermediate poly(amic acid)s were in the range of 0.40–2.63 dL/g. Except for those derived from p‐phenylenediamine and benzidine, almost all the poly(amic acid)s could be solution‐cast and thermally converted into transparent, flexible, and tough polyimide films. Introduction of the chloro or phenyl substituent leads to a decreased crystallinity and an increased solubility of the polymers. The glass transition temperatures (T_g) of these polyimides were recorded in the range of 204–263°C. In general, the methyl‐ and chloro‐substituted polyimides exhibited relatively higher T_gs, whereas the phenyl‐substituted ones exhibited slightly lower T_gs compared to the corresponding nonsubstituted ones. Thermogravimetric analysis (TG) showed that 10% weight loss temperatures of all the polymers were above 500°C either in nitrogen or in air. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 665–675, 1999     

Cobalt(II) Tri‐tert‐butoxysilanethiolates with Bidentate Spacer Ligands

A new series of Co^II tri‐tert‐butoxysilanethiolate complexes with bidentate N,N′‐ligands (L) such as pyrazine, quinoxaline and 4,4′‐bipy was obtained: for pyrazine and quinoxaline the complexes are binuclear {[Co{SSi(tBuO)₃}₂]₂(μ‐L)} with metal atoms linked by an adequate heterocyclic base L. The use of 4,4′‐bipy resulted in a coordination polymer [Co{μ‐SSi(tBuO)₃}{SSi(tBuO)₃}(μ‐4,4′‐bipy)]_n and two polymorphic forms of {[Co{SSi(tBuO)₃}₂]₂(μ‐4,4′‐bipy)}. Pyridyl rings in one polymorph form a torsion angle of 0.57°, whereas a rotation about the linking C–C bond of 4,4′‐bipy in second polymorph is significant and results in a torsion angle of 72.4°. Complexes were analysed and characterised using elemental analysis, solid state IR and UV/Vis spectroscopy, and single‐crystal X‐ray analysis.     

Synthesis of copolymers of poly(ether sulfone ether ketone ketone) and poly(ether ketone diphenyl ketone ether ketone ketone) by electrophilic Friedel–Crafts solution polycondensation

A new monomer, 4,4′‐bis(4‐phenoxybenzoyl)diphenyl(BPOBDP), was synthesized via a two‐step synthetic procedure. A series of novel poly(ether sulfone ether ketone ketone)/poly(ether ketone diphenyl ketone ether ketone ketone) copolymers were prepared by electrophilic Friedel–Crafts solution copolycondensation of isophthaloyl chloride (IPC) with a mixture of 4,4′‐diphenoxydiphenylsulfone (DPODPS) and 4,4′‐bis(4‐phenoxybenzoyl)diphenyl (BPOBDP), in the presence of anhydrous aluminum chloride and N‐methylpyrrolidone (NMP) in 1,2‐dichloroethane (DCE). The copolymers with 10–50 mol% DPODPS are semicrystalline and have remarkably increased T_gs over commercially available PEEK and PEKK. The copolymers with 40–50 mol% DPODPS had not only high T_gs of 170–172°C, but also moderate T_ms of 326–333°C, which are extremely suitable for melt processing. These copolymers have tensile strengths of 96.5–108.1 MPa, Young's moduli of 1.98–3.05 GPa, and elongations at break of 13–26% and exhibit excellent thermal stability and good resistance to acidity, alkali, and common organic solvents. Copyright © 2009 John Wiley & Sons, Ltd.     

Thermal characterization of carbonate rocks,Kozani area,North-western Macedonia,Greece

The curing of a thermoreactive alkyd-melamine-formaldehyde resin system was investigated by rheologycal, TG and TMA-analysis, in order to construct the time-temperature-transformation diagram. The points of the gelation curve were determined by measuring the increase in viscosity during isothermal curing at different temperatures. A power-function could be fitted to the gelation curve, which is suitable to estimate gelation at any curing conditions, as well as to establish storage conditions. The reaction in the resin matrix was followed by monitoring the loss of mass during isothermal curing at different temperatures. The final section of the resulted iso-curing temperature (iso-T _cure) diagrams could be fitted with logarithmic functions, which may be used for estimating the conditions needed to a given, desirable mass loss, i.e. conversion. The steepness of the curves increases with temperature suggesting the forthcoming of degradation during cure with increasing temperature. From these data the iso-mass loss curves of the TTT-diagram were constructed. For determining the iso-Tg curves of the TTT-diagram isothermal curing was carried out in a drying oven at different temperatures, followed by TMA measurements. The iso-Tcure diagrams served to determine T _{g ￥}, and to construct the iso-T _g curves of the TTT diagram. Vitrification curve is far beyond conditions of storage, curing and degradation, meaning that the resin matrix is in rubbery physical state before, during and after the cure. Curing conditions resulting degradation can also be estimated from the TTT-diagram. This revised version was published online in August 2006 with corrections to the Cover Date.