Showing papers on "Glass transition published in 1999"
TL;DR: This paper showed that the fragility of a liquid can be determined from purely thermodynamic data (as opposed to measurements of kinetics) near and below the melting point, and that for most liquids the fragilities estimated this way are consistent with those obtained by previous methods and by a new method (ref.
Abstract: If crystallization can be avoided when a liquid is cooled, it will typically form a glass. Near the glass transition temperature the viscosity increases continuously but rapidly with cooling. As the glass forms, the molecular relaxation time increases with an Arrhenius-like (simple activated) form in some liquids, but shows highly non-Arrhenius behaviour in others. The former are said to be ‘strong’ liquids, and the latter ‘fragile’1,2. Here we show that the fragility of a liquid can be determined from purely thermodynamic data (as opposed to measurements of kinetics) near and below the melting point. We find that for most liquids the fragilities estimated this way are consistent with those obtained by previous methods and by a new method (ref. 3 and K.I., C.A.A. and C.T.M., unpublished data) at temperatures near the glass transition. But water is an exception. The thermodynamic method indicates that near its melting point it is the most fragile of all liquids studied, whereas the kinetic approach indicates that near the glass transition it is the least fragile. We propose that this discrepancy can be explained by a fragile-to-strong transition in supercooled water near 228 K, corresponding to a change in the liquid's structure at this point.
666 citations
TL;DR: In this article, the importance of T-g in relation to drying process and dried foods is discussed, which results in an increased rate of physicochemical changes in dried products, such as sticking, collapse, caking, agglomeration, crystallization, loss of volatiles, browning and oxidation.
571 citations
TL;DR: In this article, the effect of crystalline phase, uniaxial drawing and temperature on the real (e′) and imaginary (e″) parts of the relative complex permittivity of poly (vinylidene fluoride) (PVDF) was studied in the frequency range between 102 and 106 Hz.
Abstract: The effect of crystalline phase, uniaxial drawing and temperature on the real (e′) and imaginary (e″) parts of the relative complex permittivity of poly (vinylidene fluoride) (PVDF) was studied in the frequency range between 102 and 106 Hz. Samples containing predominantly α and β phases, or a mixture of these, were obtained by crystallization from a DMF solution at different temperatures. α phase samples were also obtained from melt crystallization and from commercial films supplied by Bemberg Folien. Different molecular orientations were obtained by uniaxial drawing of α and β phase samples. The results showed that the crystalline phase exerts strong influence on the values of e′ and e″, indicating that the αa relaxation process, associated with the glass transition of PVDF, is not exclsively related to the amorphous region of the polymer. An interphase region, which maintains the conformational characteristics of the crystalline regions, should influence the process decisively. The molecular orientation increased the values of e′ for both PVDF phases and modified its dependence with temperature over the whole frequency range studied. The influence of the crystallization and molecular orientation conditions on the dc electric conductivity (σdc) were also verified. The value of σdc was slightly higher for samples crystallized from solution at the lowest temperature and decreased with draw ratio.
465 citations
TL;DR: In this article, the complex dielectric permittivity e ν above and below the glass transition temperature, T g, was investigated in the frequency range 10 −3 ÂHz 9 Â Hz and a quantitative line-shape analysis was carried out including both processes by applying the Williams-Watts (WW) approach.
382 citations
TL;DR: In this article, high modulus and thermally stable clay/polyimide (pyromellitic dianhydride−4,4‘-oxydianiline, PMDA−ODA) nanocomposites were synthesized from reactive organoclay and poly(amic acid).
Abstract: High modulus and thermally stable clay/polyimide (pyromellitic dianhydride−4,4‘-oxydianiline, PMDA−ODA) nanocomposites were synthesized from reactive organoclay and poly(amic acid). The reactive organoclay was formed by using p-phenylenediamine as a swelling agent for silicate layers of montmorillonite. The swelling process was first carried out through ion exchange between the onium of one amine end group of p-phenylenediamine and the sodium ion in montmorillonite. The other amine end group of p-phenylenediamine reacted with the dianhydride end group of poly(amic acid), producing an irreversible swelling. This irreversible swelling resulted in a nanostructured material containing intercalated nanometer silicate layers dispersed in PMDA−ODA as confirmed by both X-ray and transmission electron microscopy. The glass transition temperature and the thermal decomposition temperatures of this type of clay/PMDA−ODA are higher than that of pure PMDA−ODA. A 2.5-fold increase in the modulus of clay/PMDA−ODA film wa...
340 citations
TL;DR: In this article, the authors use computer simulations to investigate a melt of short, unentangled polymer chains over a range of temperatures for which the mode-coupling theory remains valid.
Abstract: Many liquids cooled to low temperatures form glasses (amorphous solids) instead of crystals. As the glass transition is approached, molecules become localized and relaxation times increase by many orders of magnitude1. Many features of this ‘slowing down’ are reasonably well described2 by the mode-coupling theory of supercooled liquids3. The ideal form of this theory predicts a dynamical critical temperature T c at which the molecules become permanently trapped in the ‘cage’ formed by their neighbours, and vitrification occurs. Although there is no sharp transition, because molecules do eventually escape their cage, its signature can still be observed in real and simulated liquids. Unlike conventional critical phenomena (such as the behaviour at the liquid–gas critical point), the mode-coupling transition is not accompanied by a diverging static correlation length. But simulation4,5,6,7,8,9,10 and experiment11,12 show that liquids are dynamically heterogeneous, suggesting the possibility of a relevant ‘dynamical’ length scale characterizing the glass transition. Here we use computer simulations to investigate a melt of short, unentangled polymer chains over a range of temperatures for which the mode-coupling theory remains valid. We find that although density fluctuations remain short-ranged, spatial correlations between monomer displacements become long-ranged as T c is approached on cooling. In this way, we identify a growing dynamical correlation length, and a corresponding order parameter, associated with the glass transition. This finding suggests a possible connection between well established concepts in critical phenomena and the dynamics of glass-forming liquids.
339 citations
TL;DR: In this paper, the authors characterized the molecular mobility of selected amorphous systems (i.e., indomethacin, sorbitol, sucrose, and trehalose) below Tg using a combined experimental and theoretical approach.
Abstract: Increased interest in molecular time scales below the glass transition temperature, Tg, has arisen from the desire to identify the conditions (e.g., temperature) where the molecular processes which lead to unwanted changes in amorphous systems (e.g., chemical reactivity, crystallization, structural collapse) are improbable. The purpose of this study was to characterize the molecular mobility of selected amorphous systems (i.e., indomethacin, sorbitol, sucrose, and trehalose) below Tg using a combined experimental and theoretical approach. Of particular interest was the temperature where the time scales for molecular motion (i.e., relaxation time) exceed expected lifetimes or storage times. As a first approximation of this temperature, the temperature where the thermodynamic properties of the crystal and the equilibrium supercooled liquid converge (i.e., the Kauzmann temperature, TK) was determined. TK values derived from heat capacity and enthalpy of fusion data ranged from 40 to 190 K below the calorimet...
321 citations
TL;DR: In this article, the authors investigate microscopic transport in supercooled liquids around the glass transition regime, and demonstrate that two distinct processes contribute to long-range transport in the super cooled liquid state: single-atom hopping and collective motion, the latter being the dominant process.
Abstract: The mechanisms of atomic transport in supercooled liquids and the nature of the glass transition are long-standing problems1,2,3,4. Collective atomic motion is thought to play an important role4,5,6 in both phenomena. A metallic supercooled liquid represents an ideal system for studying intrinsic collective motions because of its structural similarity to the “dense random packing of spheres” model7, which is conceptually simple. Unlike polymeric and network glasses, metallic supercooled liquids have only recently become experimentally accessible, following the discovery of bulk metallic glasses8,9,10,11,12. Here we report a 9Be nuclear magnetic resonance study of Zr-based bulk metallic glasses8,9 in which we investigate microscopic transport in supercooled liquids around the glass transition regime. Combining our results with diffusion measurements, we demonstrate that two distinct processes contribute to long-range transport in the supercooled liquid state: single-atom hopping and collective motion, the latter being the dominant process. The effect of the glass transition is clearly visible in the observed diffusion behaviour of the Be atoms.
304 citations
TL;DR: In this article, the X-ray diffraction and infrared spectroscopy (IR) analysis confirm that polystyrene macromolecules can be inserted between lamella layers and whose layer separation is consequently higher than in the polymer-free clay.
Abstract: Polystyrene-Na+-montmorillonite(PS-Na+-MMT) nanocomposites are prepared by a simple emulsion polymerization. The X-ray diffraction(XRD) and infrared spectroscopy (IR) analysis confirm that polystyrene(PS) macromolecules can be inserted between lamella layers and whose layer separation is consequently higher than in the polymer-free clay. The enhanced thermal properties of composites are measured by differential scanning calorimetry(DSC) and thermogravimetric analysis(TGA) thermogram and indicate that the glass transition and the decomposition onset temperature of obtained nanocomposites are found to be moved to the higher temperature region. The increased Young's modulus of the obtained nanocomposites is ascribed to the intercalation of PS in clay galleries as well as the fine dispersion of clay particles into the polymer matrix.
301 citations
TL;DR: In this paper, the synthesis and characterization of poly(lauryl methacrylate)-b-poly[oligo(oxyethylene) methacarylate]-based block copolymer electrolytes (BCEs) are reported.
Abstract: For nearly 20 years, poly(ethylene oxide)-based materials have been researched for use as electrolytes in solid-state rechargeab le lithium batteries. Technical obstacles to commercialization derive from the inability to satisfy simultaneously the electrical and mechanical performance requirements: high ionic conductivity along with resistance to flow. Herein, the synthesis and characterization of a series of poly(lauryl methacrylate)- b-poly[oligo(oxyethylene) methacrylate]-based block copolymer electrolytes (BCEs) are reported. With both blocks in the rubbery state (i.e., having glass transition temperatures well below room temperatu re) these materials exhibit improved conductivities over those of glassy-rubbery block copolymer systems. Dynamic rheological testing verifies that these materials are dimensionally stable, whereas cyclic voltammetry shows them to be electrochemically stable over a wide potential window, i.e., up to 5 V at 55 8C. A solid-state rechargeable lithium battery was constructed by laminating lithium metal, BCE, and a composite cathode composed of particles of LiAl0.25Mn0.75O2 (monoclinic), carbon black, and graphite in a BCE binder. Cycle testing showed the Li/BCE/LiAl0.25Mn0.75O2 battery to have a high reversible capacity and good capacity
298 citations
TL;DR: Several features of the MCT nonergodicity transition in this system agree qualitatively with experimental observations on the colloidal gel transition, suggesting that the gel transition is caused by a low temperature extension of the glass transition.
Abstract: Colloidal gel and glass transitions are investigated using the idealized mode coupling theory (MCT) for model systems characterized by short-range attractive interactions. Results are presented for adhesive hard sphere and hard core attractive Yukawa systems. According to MCT, the former system shows a critical glass transition concentration that increases significantly with introduction of a weak attraction. For the latter attractive Yukawa system, MCT predicts low temperature nonergodic states that extend to the critical and subcritical region. Several features of the MCT nonergodicity transition in this system agree qualitatively with experimental observations on the colloidal gel transition, suggesting that the gel transition is caused by a low temperature extension of the glass transition. The range of the attraction is shown to govern the way the glass transition line traverses the phase diagram relative to the critical point, analogous to findings for the fluid-solid freezing transition.
TL;DR: In this paper, the authors explore the temperature dependence of the viscoelastic response of polycarbonate in the vicinity of the glass transition and find that the results are consistent with a transition from Vogel-F... to an Arrhenius-type of temperature dependence.
Abstract: In a recent paper DiMarzio and Yang [J. Res. Natl. Inst. Stand. Technol. 102, 135 (1997)] predicted that transport properties such as viscosity and diffusion coefficient do not follow the typical Williams, Landel, and Ferry (WLF) [J. Am. Chem. Soc. 77, 3701 (1955)] or Vogel–Fulcher-type of temperature dependence as the glass transition is approached. Rather, a transition to an Arrhenius-type of temperature dependence is predicted. Here we describe long term aging experiments that explore the temperature dependence of the viscoelastic response of polycarbonate in the vicinity of the glass transition. Aging the material for long times below the nominal glass transition temperature, assures that equilibrium is attained and we can directly test the DiMarzio–Yang prediction. In tests in which glassy samples of polycarbonate were aged into equilibrium at temperatures up to 17 °C below the conventionally measured glass transition temperature, we find that the results are consistent with a transition from Vogel–F...
TL;DR: Although the theoretical percentage of CPM on aging remained fairly constant over the processing temperature ranges in this study, the HC levels remaining in the extruded films during storage were a function of time and temperature.
Abstract: Hydroxypropylcellulose (HPC) films containing drugs or hydrophilic or hydrophobic plasticizers were prepared by a hot melt extrusion process. Polyethylene glycol 8000 (PEG 8000) 2%, triethyl citrate (TEC) 2%, acetyltributyl citrate (ATBC) 2%, and polyethylene glycol 400 (PEG 400) 1% were the plasticizing agents studied. In addition, either hydrocortisone (HC) 1% or chlorpheniramine maleate (CPM) 1% was incorporated into the films as a model drug. The physical-mechanical properties of the films that were investigated included tensile strength (TS), percentage elongation (%E), and Young's modulus (YM). Differential scanning calorimetry (DSC) was utilized to determine glass transition temperatures (Tg' s). These parameters were studied as a function of time and temperature. The glass transition temperatures initially decreased with the inclusion of the drugs and plasticizers. However, after 6 months aging, films containing PEG 400 and HC showed a marked increase in Tg. The films containing PEG 400 showed phy...
TL;DR: In this paper, a model based on the Halpin-Kardos equation, with the assumption of an immobilized phase around the whiskers, is developed to account for significant decrease in the modulus drop, on passing above the glass transition temperature.
Abstract: New nanocomposites are processed with a plasticized poly(vinyl chloride) matrix reinforced by cellulose whiskers whose characteristics are a high aspect ratio and a large interface area. Dynamic mechanical analysis performed on samples rein- forced with a filler fraction of up to 12.4 vol % gives the viscoelastic properties of the composite above and below its glass transition temperature. Different theoretical predictions are proposed to describe this behavior, but none of them is found wholly satisfactory for describing the reinforcing effect of these fillers. A model based on the Halpin-Kardos equation, with the assumption of an immobilized phase around the whiskers, is developed to account for significant decrease in the modulus drop, on passing above the glass transition temperature. The small discrepancy between this model and the experimental modulus measured in the rubber plateau is discussed as a possible effect of a percolating whisker network whose crosslinks are assured by chains adsorbed onto the whisker surface. Swelling experiments support this hypothesis. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 1797-1808, 1999
TL;DR: In this paper, the authors investigated the thermal and viscoelastic property enhancement on crosslinked epoxy using two types of nanoreinforcement, namely, organoion exchange clay and polymerizable polyhedral oligomeric silsesquioxane (POSS) macromers.
Abstract: The properties of nanostructured plastics are determined by complex relationships between the type and size of the nanoreinforcement, the interface and chemical interaction between the nanoreinforcement and the polymeric chain, along with macroscopic processing and microstructural effects. In this article, we investigated the thermal and viscoelastic property enhancement on crosslinked epoxy using two types of nanoreinforcement, namely, organoion exchange clay and polymerizable polyhedral oligomeric silsesquioxane (POSS) macromers. Glass transitions of these nanocomposites were studied using differential scanning calorimetry (DSC). Small-strain stress relaxation under uniaxial deformation was examined to provide insights into the time-dependent viscoelastic behavior of these nanocomposites. Since the size of the POSS macromer is comparable to the distance between molecular junctions, as we increase the amount of POSS macromers, the glass transition temperature Tg as observed by DSC, increases. However, for an epoxy network reinforced with clay, we did not observe any effect on the Tg due to the presence of clay reinforcements. In small-strain stress relaxation experiments, both types of reinforcement provided some enhancement in creep resistance, namely, the characteristic relaxation time, as determined using a stretched exponential relaxation function increased with the addition of reinforcements. However, due to different reinforcement mechanisms, enhancement in the instantaneous modulus was observed for clay-reinforced epoxies, while the instantaneous modulus was not effected in POSS–epoxy nanocomposites. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 1993–2001, 1999
TL;DR: In this article, the authors showed that Ag-centered local structures apparently phase separate from the host network, and one observes bimodal glass transition temperatures! Tg", which provides an attractive starting point to model electrical transport and light-induced effects in chalcogenide glasses.
Abstract: Ag as an additive in chalcogenide glasses (CGs), and particularly thin films of such glasses, has attracted widespread interest in glass science [1‐3]. The interest stems inpartfromlight-inducedeffectsrelevanttoopticalrecording and information processing [3]. It also stems from the drastically increased electrical conductivity [4] of Ag-CGs, some of which are solid electrolytes [4]. The structure of glasses containing group IB (Cu, Ag) additives in CGs, as for the celebrated cases of group IV (Si, Ge) and group V (P, As) additives in CGs, has been generally modeled [5], and diffraction results analyzed [6‐8], in the spirit of homogeneous random networks. In Ag-CGs, Ag-centered local structures apparently phase separate from the host network, and one observes bimodal glass transition temperatures ! Tg" . These new structure results provide an attractive starting point to model electrical transport and light-induced effects in Ag-CGs. Glass forming compositions [9], and particularly the composition variation [10] of Tg in network glasses, contain vital clues on the connectivity of the backbone. Ideas on constraint counting [11] have provided the tools to decode these clues and to gain insights into the structure of glasses. Binary GexSe12x glasses are rather well studied [12], and it thus appeared attractive to closely examine the chemical role of Ag as an additive in such glasses. Ternary ! GexSe12x" 12yAgy glasses form in two distinct compositional regions [7,13,14], a Se-rich region ! 0 , x , 1 " , la
TL;DR: Compared on a fractional weight basis, it could be concluded that the plasticizing effect of glycerol and sorbitol on wheat gluten proteins is less important than the plasticized effect of water.
Abstract: The glass transition temperature of wheat gluten, plasticized with water, glycerol, or sorbitol, has been studied using dynamical mechanical thermal analysis. For the three plasticizers studied, the general behavior of the glass transition temperature broadly followed the Couchman-Karasz relation using a wheat gluten DeltaC(p)() of 0.4 J g(-)(1) K(-)(1). Compared on such a fractional weight basis, it could be concluded that the plasticizing effect of glycerol and sorbitol on wheat gluten proteins is less important than the plasticizing effect of water. A continuous curve was obtained with the three plasticizers when the evolution of the glass transition temperature was presented on a fractional molecular basis. This was related to the similar chemical structure of these three components containing hydroxyl groups.
TL;DR: In this article, the effects of 2,2′-disubstituted groups of both the dianhydride and diamine constituents on the solubility and thermal and optical properties as well as the relaxation behavior of these polyimides were identified.
TL;DR: The migration rate of the plasticizers in the polymer was related to their physicochemical characteristics, and the temperature at which a thermoplastic resin of plasticized CGM could be formed was closely connected to the T(g) of the blend.
Abstract: Homogeneous blends of corn gluten meal (CGM) and "polar" plasticizers (water, glycerol) or "amphiphilic" plasticizers [octanoic and palmitic acids, dibutyl tartrate and phthalate, and diacetyl tartaric acid ester of mono-diglycerides (DATEM)] were obtained by a hot-mixing procedure. The glass transition temperature (T(g)) of the blends was measured by modulated differential scanning calorimetry and dynamic mechanical thermal analysis, as a function of plasticizer type and content (0-30%, dwb). The plasticizing efficiency (i.e., decrease of T(g)) at equal molar content was found to be proportional to the molecular weight and inversely proportional to the percent of hydrophilic groups of the plasticizer. The migration rate of the plasticizers in the polymer was related to their physicochemical characteristics. It was assumed that polar substances interacted with readily accessible polar amino acids, whereas amphiphilic ones interacted with nonpolar zones, which are buried and accessible with difficulty. The temperature at which a thermoplastic resin of plasticized CGM could be formed was closely connected to the T(g) of the blend.
TL;DR: In this article, the authors compared the cure behavior and properties of phthalonitrile polymers derived from three different monomers, namely, 4,4′-bis(3,4-dicyanophenoxy)biphenyl, 2,2-bis[4,4]-hexafluoropropane and 2.2]-propane, and showed that the char yields are high and the total heat release upon exposure to 50 kW/m2 flux for each polymer is low, compared to other thermosets.
Abstract: This article compares the cure behavior and properties of phthalonitrile polymers derived from three different monomers, namely, 4,4′-bis(3,4-dicyanophenoxy)biphenyl, 2,2-bis[4-(3,4-dicyanophenoxy)phenyl]hexafluoropropane and 2,2-bis[4-(3,4-dicyanophenoxy)phenyl]propane. Rheometric measurements with monomer melt in the presence of an aromatic diamine curing agent reveal that the rate of the cure reaction differs for the three monomers. The rate is dependent on the concentration of the curing agent. The glass transition temperature advances with increasing extent of cure and disappears upon postcure at temperatures in excess of 350°C. Based on thermogravimetric analysis, the thermal stability of all three polymers are comparable, whereas the fluorine-containing resin shows the best oxidative stability at elevated temperatures. Microscale calorimetric studies on all three polymers reveal that the char yields are high and the total heat release upon exposure to 50 kW/m2 flux for each polymer is low, compared to other thermosets. Flexural strength ranges between 80–120 MPa. The water uptake under ambient conditions is less than 3% by weight after submersion in water for seven months. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 2105–2111, 1999
TL;DR: In this paper, the fragility parameter, m, in the general classification scheme of glass-forming liquids has been estimated from data provided in the literature for viscosity measurements close to and below the calorimetric glass transition temperature for 21 metallic alloys consisting of ternary, quaternary and quinary alloys.
Abstract: The fragility parameter, m, in the general classification scheme of glass-forming liquids has been estimated from data provided in the literature for viscosity measurements close to and below the calorimetric glass transition temperature for 21 metallic alloys consisting of ternary, quaternary and quinary alloys. These alloys are found to display an intermediate fragility strength with 32 m 66 and an average of m 50. Of this group of alloys, the bulk metallic glass formers (those with critical cooling rates less than 100 K s-1) are stronger liquids with m lying between 30 and 40. There appears to be a general positive correlation between exceptionally good glass-forming ability and stronger dynamical behaviour of supercooled metallic liquids.
TL;DR: In this paper, a novel phosphorus-containing epoxy resins (1-3% phosphorus content) were synthesized by the reaction of 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO) and the diglycidyl ether of bisphenol A and then cured with 4,4′-diaminodiphenyl sulfone or phenol novolac.
Abstract: Novel phosphorus-containing epoxy resins (1–3% phosphorus content) were synthesized by the reaction of 9,10-dihydro-9-oxa-10-phosphaphenanthrene 10-oxide (DOPO) and the diglycidyl ether of bisphenol A and then cured with 4,4′-diaminodiphenyl sulfone or phenol novolac. Differential scanning calorimetry, high performance liquid chromatography, and epoxide equivalent weight titration were used to trace the reaction between the DOPO and the epoxy. The thermal stability and flame retardancy were checked by thermal gravimetric analysis, the limiting oxygen index, and the UL-94 vertical test. The glass transitions were measured by dynamic mechanical analysis. The relation between these properties (thermal stability, flame retardancy, and glass transition) and the DOPO contents (phosphorus content) were discussed. © 1999 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 37: 3903–3909, 1999
TL;DR: In this article, two poly(p-phenylenevinylene) (PPV) based polymers functionalized with an electron-deficient oxadiazole segment as the side chain by mimicking the chemical structure of 2-(4-biphenylyl)-5-( 4-tert-butylphenyl)-1,3,4-oxadiazoles (PBD) have been successfully synthesized through the Gilch route and Wittig method.
Abstract: Two poly(p-phenylenevinylene) (PPV) based polymers functionalized with an electron-deficient oxadiazole segment as the side chain by mimicking the chemical structure of 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) have been successfully synthesized through the Gilch route and Wittig method. The obtained polymer II, which is a copolymer, is completely soluble in conventional organic solvents. However, polymer I, which is a homopolymer, is not soluble in any common organic solvents tried. The structure and purity of II have been characterized by FT-IR, 1H NMR, 13C NMR, gel permeation chromatography (GPC), thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), UV−vis and photoluminescence (PL) spectroscopy, and electrochemical analysis. The TGA results indicate that II has very high thermal stability, while DSC investigation demonstrates that the glass transition temperature (Tg) of II is higher than 200 °C, which might be a merit for the long-life operation of light-emi...
TL;DR: In this paper, an organosoluble polyimide/silica hybrid materials were prepared using the sol-gel process using pyromellitic anhydride (PMDA) and 4,4′-diamino-3,3′-dimethyldiphenylmethane (MMDA).
Abstract: Organosoluble polyimide/silica hybrid materials were prepared using the sol–gel process. The organosoluble polyimide was based on pyromellitic anhydride (PMDA) and 4,4′-diamino-3,3′-dimethyldiphenylmethane (MMDA). The silica particle size in the hybrid is increased from 100–200 nm for the hybrid containing 5 wt % silica to 1–2 µm for the hybrid containing 20 wt % silica. The strength and the toughness of the hybrids are improved simultaneously when the silica content is below 10 wt %. As the silica content is increased, the glass transition temperature (Tg) of the hybrids is increased slightly. The thermal stability of the hybrids is improved obviously and their coefficients of thermal expansion are reduced. The hybrids are soluble in strong polar aprotic organic solvents when the silica content is below 5 wt %. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 73: 2977–2984, 1999
TL;DR: Preliminary in vitro biocompatibility studies revealed that the glasses of higher sodium oxide content were associated with a cytotoxic response and the measurement of media pH indicated that this cytotoxicity effect was due to ion exchange reactions at the glass surface.
Abstract: The rate of in vivo degradation and level of bioactivity of bioactive glasses are composition dependent [1]. By altering bioactive glass composition, the rate of resorption can be controlled. The network connectivity of a glass can be used to predict various physical properties of the glass including its solubility and, hence, its bioactivity [2]. Glass solubility increases as network connectivity is reduced. Glasses in the soda-lime phosphosilicate system were studied. The initial choice of composition was based on phosphate content and low network connectivity. A systematic substitution of calcium oxide for sodium oxide on a molar basis was made in order to examine the influence of sodium oxide content on the glass properties while keeping the network connectivity constant. The glass transition temperature and the peak crystallization temperature were seen to decrease linearly with increasing sodium oxide content. Thermal expansion coefficient and glass density were also seen to be related to sodium oxide content. Preliminary in vitro biocompatibility studies revealed that the glasses of higher sodium oxide content were associated with a cytotoxic response. The measurement of media pH indicated that this cytotoxic effect was due to ion exchange reactions at the glass surface.
TL;DR: In this article, a series of 12 aromatic diamines, 4,4′-diamino-2,2′-disubstituted biphenyls, has been designed and synthesized.
TL;DR: In this article, Fourier transform infrared spectroscopy (FTIR) was used to characterize the interaction of water and nylon 6,6 films and the results showed that the sorbed water acted as an effective plasticizer in depressing the glass transition temperatures of the polyamide.
Abstract: The sorption and transport of water in nylon 6,6 films as functions of the relative humidity (RH) and temperature were studied. Moisture-sorption isotherms determined gravimetrically at 25, 35, and 45°C were described accurately by the GAB equation. Water-vapor transmission rates were enhanced above ≈ 60–70% RH, primarily due to the transition of the polymer from glassy to rubbery states. The glass transition temperatures (Tg's) of nylon 6,6 were measured at various moisture contents using differential scanning calorimetry. The results showed that the sorbed water acted as an effective plasticizer in depressing the Tg of the polyamide. Fourier transform infrared spectroscopy (FTIR) was utilized to characterize the interaction of water and the nylon. Evidence from FTIR suggested that the interaction of water with nylon 6,6 took place at the amide groups. Based on the frequency shift of the peak maxima, moisture sorption appeared to reduce the average hydrogen-bond strength of the NH groups. However, an increase was seen for the CO groups. © 1999 John Wiley & Sons, Inc. J Appl Polym Sci 71: 197–206, 1999
TL;DR: The high temperature behavior of a sol-gel derived silicon oxycarbide glass containing 12 at.% carbon has been characterized by means of creep and in-situ ultrasonic echography measurements as discussed by the authors.
Abstract: The high temperature behavior of a sol-gel derived silicon oxycarbide glass containing 12 at.% carbon has been characterized by means of creep and in-situ ultrasonic echography measurements. Temperature induced changes include structural relaxation and densification from 1000 to 1200°C, and crystallization to form a fine and homogeneous β-SiC/glass-matrix nanocomposite with 2.5 nm large crystals above 1200°C. Young's modulus measurements clearly reveal a consolidation of the material upon annealing below 1200°C. Crystallization is almost complete after few hours at 1300°C and results in a significant increase in Young's modulus. The viscosity of the oxycarbide glass is much higher than that of fused silica, with two orders of magnitude difference at 1200°C, and the glass transition temperature ranges from 1320 to 1370°C.
TL;DR: In this article, changes in microstructure and mechanical properties are investigated as a function of epoxy-amine stoichiometry, and the epoxy system exhibits a two-phase structure consisting of a hard microgel phase and a dispersed phase of soft, unreacted and/or partially reacted material.
Abstract: Changes in microstructure and mechanical properties are investigated as a function of epoxy–amine stoichiometry The epoxy–amine system studied exhibits a two-phase structure consisting of a hard microgel phase and a dispersed phase of soft, unreacted and/or partially reacted material The size distribution of the microgel regions tends to increase with increasing amine content Concurrently, the connectivity of the softer phase increases dramatically This two-phase structure is inherently fractal, exhibiting a single glass transition temperature, Tg The Tg and elevated-temperature properties of the epoxy are directly correlated with crosslink density and the percentage of microgel phase observed in microstructure studies The fracture toughness at room temperature increases with increasing amine content, most likely due to the increased presence of the soft phase, which absorbs more energy during crack growth Changes in modulus values at 30°C with stoichiometry are explained by considering the effective aspect ratio of the polymer structure in the determination of sample rigidity Relationships between microgel sizes and the sizes of interphase regions that form in composite and adhesive systems are also discussed in terms of interphase properties © 1999 John Wiley & Sons, Inc J Appl Polym Sci 71: 699–712, 1999