Variation Of Tg Research Articles

Electrical conductivity of lithium borosilicate glasses modified with TiO2

The variation of electrical conductivity with addition of TiO2 in lithium borosilicate system has been studied. The addition of TiO2 to the optimum conducting composition in lithium borosilicate system further enhances the conductivity. A maximum conductivity is obtained at 10 mol% TiO2. Also the TiO2 addition results in increase in density and decrease in molar volume. The variation in glass transition temperature Tg and CTE with addition of TiO2 is explained on the basis of changes in glass structure.

Crystallization kinetics and phase transformation in superionic lithium metaphosphate(Li2O–P2O5) glass system

The ionic conductivity of mol%50Li2O–50P2O5 melt quenched glass shows an anomalous increase after its glass transition temperature(Tg) around590 K. On further increasing the temperature gradually, the conductivity decreases owing to the devitrificationof Li2O–P2O5 glass. The evolution of devitrified crystallites was evidenced by XRD patterns. To understand thedevitrification process, isothermal and non-isothermal DSC studies have been carried out on mol%50Li2O–50P2O5 glass.Tg aswell as Tc values are found to increase monotonically with increasing heating rates. Variation ofTg as a function of heating rates has been investigated to evaluate the lower limiting temperature ofTg and the activation energy for structural relaxation. Results of the DSC studies indicate(i) single-stage bulk crystallization of the glass, with DSC traces exhibiting a single transition, (ii) an order parameter (Avrami constant) of2.8 ± 0.1,suggesting internal (bulk) crystallization of the glass, (iii) an activation energy for crystallization equalto 121.7 kJ mol−1 and (iv) the activation energy for structural relaxation,Eg, tobe 558.8 kJ mol−1. The crystallization mechanism is closely associated with the JMA model and theexperimental dataset have been fitted to a non-isothermal Avrami expression and theobtained parameters confirm the experimental results.

Vortex-glass phase transition and superconductivity in an underdoped(Ba,K)Fe2As2single crystal

Measurements of magnetotransport and current-voltage (I-V) characteristics up to 9 T were used to investigate the vortex phase diagram of an under-doped Measurements of magnetotransport and current-voltage (I-V) characteristics up to 9 T were used to investigate the vortex phase diagram of an under-doped (Ba,K)Fe2As2 single crystal with Tc=26.2 K. It is found that the anisotropy ratio of the upper critical field Hc2 decreases from 4 to 2.8 with decreasing temperature from Tc to 24.8 K. Consistent with the vortex-glass theory, the I-V curves measured at H=9 T can be well scaled with the vortex-glass transition temperature of Tg=20.7 K and critical exponents z=4.1 and v=1. Analyses in different magnetic fields produced almost identical critical exponent values, with some variation in Tg, corroborating the existence of the vortex-glass transition in this under-doped (Ba,K)Fe2As2 single crystal up to 9 T. A vortex phase diagram is presented, based on the evolution of Tg and Hc2 with magnetic field.

Miscibility and interactions between 17β-estradiol and Eudragit® RS in solid dispersion

Miscibility of 17β-estradiol and Eudragit® RS in solid dispersions was determined by modulated temperature differential scanning calorimetry (MTDSC). A reduction of the 17β-estradiol melting point in Eudragit® RS solid dispersions was observed by MTDSC using heating program I in which the maximum temperature in the first heating run was lower than the 17β-estradiol melting point. The melting point depression of 17β-estradiol in solid dispersions as a function of composition could be explained by the Nishi–Wang equation indicating an interaction between 17β-estradiol and Eudragit® RS in the system. A variation of glass transition temperature (Tg) of 17β-estradiol in Eudragit® RS solid dispersion was observed by MTDSC using heating program II in which the maximum temperature in the first heating run reached the 17β-estradiol melting point. In the second heating run of heating program II, 17β-estradiol was in an amorphous form blended with Eudragit® RS. The variation in Tg of amorphous 17β-estradiol blended with Eudragit® RS could be explained by the Kwei equation, a modified version of the Gordon–Taylor equation. The parameter estimates from the Kwei equation were consistent with an interaction between 17β-estradiol and Eudragit® RS, which was due to inter-associated hydrogen bonding as deduced from the FTIR spectra of the blends.

Influence of DGEBA crosslinking on Li+ ion conduction in poly(ethyleneimine) gels

We characterize the DC conductivity (σ0) of solution electrolytes prepared by adding LiCF3SO3 (LiTf) salt to a (50/50 w/w) solution of branched poly(ethyleneimine) (PEI) in N,N-dimethylformamide (DMF). The value of σ0 increases with decreasing LiTf concentration over the range of compositions studied due to the formation of contact ion pairs at higher LiTf concentrations, with the highest value of σ0 exceeding 10−3Scm−1 at 20°C. Rubber-like gel electrolytes are prepared by epoxide–amine crosslinking of selected solutions by addition of diglycidyl ether of bisphenol A (DGEBA). Holding the [N]:[Li] mole ratio fixed, increasing the crosslink density dramatically decreases σ0 at all temperatures studied. The decrease in σ0 cannot be attributed to an increase in the glass transition temperature, as little variation in Tg is noted amongst the samples due to their high solvent content. Rather, we propose that the decrease in conductivity is due to loss of fast segmental motions associated with chain ends, which become tethered to the network upon crosslinking.

Glass transition temperature of allyl methacrylate‐n‐butyl acrylate gradient copolymers in dependence on chemical composition and molecular weight

AbstractGlass transition temperatures (Tgs) of P(AMA‐co‐BA) copolymers and the corresponding homopolymers, where AMA is allyl methacrylate and BA isn‐butyl acrylate, obtained by means of atom transfer radical polymerization were measured using differential scanning calorimetry. Because of the (pseudoliving) nature of this polymerization technique an increase in molecular weight (MW) is produced as the reaction progresses, which gives rise to an increase inTgs. This increment can be adequately described by the Fox–Flory's equation in both homopolymers. However, in the spontaneous gradient copolymers of P(AMA‐co‐BA), the expected increase inTgwith the augment of the monomer conversion is compensated by the enrichment of BA as the polymerization reaction progresses. These opposite effects with respect to theTgvalues almost balance each other, and therefore no significant influence on the MW or on conversion is found. This fact establishes thatTgs can be used to describe the profile of these gradient copolymers, and can be theoretically determined because of its dependence on the molar fraction in the copolymer. From this dependence on chemical composition along with the experimental behavior, a prediction of theTgvariation with the MW was performed. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 1845–1855, 2007

Morphological effects on glass transition behavior in selected immiscible blends of amorphous and semicrystalline polymers

The effects uncompatibilized immiscible polymer blend compositions on the Tg of the amorphous polymer were studied in the systems polystyrene/polypropylene (PS/PP), polystyrene/high density polyethylene (PS/PE) and polycarbonate/high density polyethylene (PC/PE). In the two similar systems of PS/PP and PS/PE, the Tg of PS increased with decreasing PS percentage in the blends. This variation in glass transition is attributed to the polymer domain interactions resulting from the different morphologies of various blend compositions. Experiments were conducted to study these effects by preparing blends with various polymers that varied the relationship between the Tg of the amorphous polymer and the crystallization behavior of the semicrystalline polymer. Results show that the variation in amorphous component Tg with composition depends strongly on the physical state of the semicrystalline domains. Whereas the Tg of PS in PS/PE blends changed with composition, the Tg of PC in the PC/PE blend did not change with composition.

Adjusting drug release by using miscible polymer blends as effective drug carries

In the present study PVP/HPMC and PVP/Chitosan polymer blends were prepared by using the solvent evaporation technique. From DSC studies were revealed that both blends are completed miscible in the entire composition range since only one glass transition temperature was detected. Miscibility can be attributed to the strong interactions evolved between the carbonyl group of PVP, which acts as strong proton acceptor, with hydroxyl and amino-groups of HPMC and Chitosan, which are proton donors. Thus hydrogen bonds are easily formed, as was verified by FTIR, producing miscible blends. However, the extent of interactions depends from polymer composition and mainly from the ratio and the kind of reactive groups. In PVP/HPMC blends a negative variation of Tg is recorded while in PVP/Chitosan the variation has a sigma form. The miscibility of these systems creates matrixes with completely different physical properties in order to use as effective drug carriers. PVP/HPMC blends can be used as pulsatile chronotherapeutics systems adjusting exactly the time of the drug release while PVP/Chitosan blends can be used to control the release profile of a poorly water soluble drug. In these blends HPMC and Chitosan respectively are the control factors for the corresponding applications.

Acoustic relaxation and ionic conductivity of oxyethylene aliphatic polyionenes

Polyionenes composed of oxyethylene and aliphatic hydrocarbon units were studied by differential scanning calorimetry, wide-and small-angle X-ray scattering, and dielectric spectroscopy. These polymers are characterized by the variation of Tg with the concentration of ionic centers. Polyionenes with short ethylene oxide segments are amorphous; however, as the average number of monomer units in the oxyethylene segments increases to 20, a crystalline structure typical of poly(ethylene oxide) is formed. Oxyethylene-aliphatic polyionenes are microphase-separated systems. Polyionenes from this series are characterized by a high ionic conductivity, which increases with an increase in the concentration of ionic sites—the conductivity at room temperature is 10−5–10−4 Ω−1 cm−1. By means of acoustic spectroscopy, it was found that the isotherms of the ultrasound absorption (frequency domain) and ultrasound speed had two dispersion regions. The mechanism of the dispersions was associated with the softening of the quasi-lattice produced by cationic sites and with the motion of chain segments connecting these sites, The speed of sound in polyionene is abnormally high (1800–2100 m/s) for polymers, a result which is due to a high level of intermolecular interactions.

Free Volume, Glass Transition and Degree of Branching in Ethylene/α‐Olefin Copolymers: Positron Lifetime, Differential Scanning Calorimetry, Wide‐Angle X‐Ray Scattering, and Density Studies

AbstractSummary: Positron annihilation lifetime spectroscopy, differential scanning calorimetry, wide‐angle X‐ray scattering, and density measurements were used to systematically study the variation of the glass transition temperature Tg and the mean size vh of holes (local free volumes) in n‐alkyl branched polyethylenes. The samples were commercial ethylene‐rich copolymers with 1‐propene, 1‐butene, and 1‐octene comonomers. From the total specific volume V and the crystallinity Xc the specific volume of the amorphous phase Va was estimated and used to calculate the specific hole‐free volume Vf. It was found that Tg and Xc decrease and V, Va, Vf, and vh increase with increasing weight fraction of comonomers. Both the real crystalline and amorphous specific volumes decrease with increasing crystallinity Xc. For not too high contents of comonomers, Tg decreases and vh increases linearly with the number and with the length of n‐alkyl branches. This behavior was attributed to an increased segmental mobility caused by branching. Both Tg and vh follow linear master curves as a function of the degree of branching if this is defined as the fractional number of carbon atoms in the side chains compared with the total number of carbon atoms. A method for estimating Tg from vh measured at room temperature is shown. The number density of holes Nh shows a slight variation from 0.6 to 0.8 (±0.1) nm−3 with increasing branching. magnified image

Synthesis and characterization ofN‐acryloylcarbazole/methyl methacrylate copolymers

AbstractCopolymers ofN‐acryloylcarbazole (A) and methyl methacrylate (M) were synthesized in different in‐feed ratios. The composition of the copolymer was determined by the help of1H NMR spectrum. The comonomer reactivity ratios determined by Kelen‐Tudos (KT) and nonlinear error‐in‐variables methods wererA= 1.12 ± 0.16,rM= 0.94 ± 0.14, andrA= 1.05,rM= 0.90, respectively. Complete spectral assignments of the1H and13C1H NMR spectra of the copolymers were done by the help of distortionless enhancement by polarization transfer (DEPT) and two‐dimensional NMR techniques, such as heteronuclear single quantum coherence (HSQC), total correlation spectroscopy (TOCSY), and heteronuclear multiple bond correlation (HMBC). The methine, α‐methyl, and carbonyl carbon resonances were found to be sequence sensitive. The signals obtained were broad because of the restricted rotation of bulky carbazole group and the quadrupolar effect of nitrogen present in carbazole moiety. Glass transition temperatures (Tg) were determined by differential scanning calorimetry and were found to be characteristic of copolymer composition. As theN‐acryloylcarbazole content increases, theTgincreases from 378.3 K for poly(methyl methacrylate) to 430.4 K for poly(N‐acryloylcarbazole). Variation inTgwith the copolymer composition were found to be in good agreement with theoretical values obtained from Johnston and Barton equations. © 2006 Wiley Periodicals, Inc. J Appl Polym Sci 100: 2667–2676, 2006

Water absorption mechanism and some anomalous effects on the mechanical and viscoelastic behavior of an epoxy system

AbstractDuring the past decades there has been a great accumulation of important data on the diffusion of water molecules in polymeric solids and its effect on the mechanical and viscoelastic behavior of polymers. It has become apparent that in many cases diffusion in polymers as well as its effect exhibits features that cannot be expected from classical theories and that such departures are related to the molecular structure characteristics of polymers. In the present investigation, the mechanical and viscoelastic behavior of an epoxy resin system is studied as a function of absorbed water, temperature, and time of immersion. Water sorption was achieved by immersing the material in distilled water at constant temperature of 60°C and 80°C for 2, 5, 8, 13, 32, 74, 128, 266, 512, 1024, and 1536 h. Subsequently the specimens were tested in static and dynamic three‐point bending tests to study their mechanical and viscoelastic behavior. The variation of Tg, tan δ, bending modulus, and strength was measured as a function of exposure time and respective percentage of water uptake for both temperatures. Some anomalies in their behavior due to water absorption were observed, and a model for the description of the experimentally observed mechanical behavior due to hygrothermal aging is proposed. The results show that the model predictions are in good agreement with experimental findings. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci 99: 1328–1339, 2006

Prognostic value of an increase in the serum thyroglobulin level at the time of the first ablative radioiodine treatment in patients with differentiated thyroid cancer

The aim of this study was to assess the prognostic factors for treatment efficacy, and in particular the increase in serum thyroglobulin (Tg) level at the time of the first ablative radioiodine treatment, in patients with differentiated thyroid carcinoma (DTC). A retrospective chart review was performed on 407 patients treated for DTC by total thyroidectomy and (131)I ablation between 1995 and 2002, and examined 5-12 months later with diagnostic (131)I whole-body scan and serum Tg measurement after thyroid hormone treatment withdrawal. At the time of the ablative radioiodine treatment, serum Tg level was determined just before (131)I administration (TgD0) and 5 days later (TgD5); Tg variation was expressed as the ratio TgD5/TgD0. At the first post-ablation follow-up examination, unsuccessful ablation was defined by a Tg level > or =2 ng/ml and/or abnormal (131)I uptake. Ablation was unsuccessful in 51 patients. Univariate analysis showed high TgD0 level, low TgD5/TgD0 ratio extrathyroidal invasion, (131)I uptake in the neck (excluding the thyroid bed) during the ablative treatment and distant metastases to be significantly associated with unsuccessful ablation. On logistic multivariate analysis, TgD0 level <5 ng/ml and TgD5/TgD0 ratio > or =20 were independently associated with successful ablation. A receiver operating characteristic curve analysis determined that a TgD5/TgD0 ratio greater than 20 had a 97% positive predictive value for successful ablation. When both TgD0 and TgD5/TgD0 ratio were considered, that is, TgD0 <5 ng/ml or TgD0 > or =5 ng/ml but TgD5/TgD0 ratio >20, ablation was unsuccessful in only 12/301 patients. Our data show that the TgD5/TgD0 ratio may be used as a new prognostic indicator of (131)I treatment efficacy in patients with DTC.

Hyperbranched Copolymers versus Linear Copolymers: A Comparative Study of Thermal Properties

Copolymerization of two AB2-type monomers that incorporate spacer segments of similar lengths but different flexibility permitted, for the first time, the preparation of a range of hyperbranched copolymers, wherein the backbone rigidity was varied while maintaining similar branching densities. The copolymers were prepared via a recently developed melt transetherification methodology to yield moderately high molecular weight polymers, with molecular weights ranging from 20 000 to 50 000. 1H NMR spectroscopic studies revealed that the composition of the copolymers varied linearly with monomer composition, confirming the formation of truly random copolymers. Analogous linear copolymers based on suitably designed AB-type monomers, containing the same two spacers, were also prepared for comparison. Thermal analysis of these copolymers using DSC indicated that the Tg's of both linear copolymers and hyperbranched copolymers varied with composition in a manner that was in complete accordance with the Fox equation, although all the linear copolymers exhibited significantly higher Tg values than their hyperbranched counterparts. It is interesting that, despite their very different topology and the presence of large number of chain ends, hyperbranched copolymers exhibit a similar Tg variation as their linear analogues. The generality of this observation in the broader context of hyperbranched copolymers, such as those possessing different branching densities and terminal functionalities, remains to be tested.

The Glass Transition Temperature of Polymer Melts

We develop an analytic theory to estimate the glass transition temperature T(g) of polymer melts as a function of the relative rigidities of the chain backbone and side groups, the monomer structure, pressure, and polymer mass. Our computations are based on an extension of the semiempirical Lindemann criterion of melting to locate T(g) and on the use of the advanced mean field lattice cluster theory (LCT) for treating the thermodynamics of systems containing structured monomer, semiflexible polymer chains. The Lindemann criterion is translated into a condition for T(g) by expressing this relation in terms of the specific volume, and this free volume condition is used to calculate T(g) from our thermodynamic theory. The mass dependence of T(g) is compared to that of other characteristic temperatures of glass-formation. These additional characteristic temperatures are determined from the temperature variation of the LCT configurational entropy, in conjunction with the Adam-Gibbs model for long wavelength structural relaxation. Our theory explains generally observed trends in the variation of T(g) with polymer microstructure, and we find that T(g) can be tuned either upward or downward by increasing the length of the side chains, depending on the relative rigidities of the side groups and the chain backbone. The elucidation of the molecular origins of T(g) in polymer liquids should be useful in designing and processing new synthetic materials and for understanding the dynamics and controlling the preservation of biological substances.

EPR investigation and thermal study of yA2O-(1-y)[0.25(WO3)2-0.75(P2O5)] (A=Li, Na) glasses

Glasses belonging to the series yA2O-(1-y)[0.25(WO3)2-0.75P2O5] (y=0.10-0.60) are elaborated. DTA experiments reveal that the glass-network of the glasses are broken with increasing of modifier content. EPR spectra show the presence of two signals due to W5+ and Mo5+, as impuritie. The intensity of these EPR centres decreases when the concentration of A2O increases. The variation of Tg upon the modifier content is ascribed to structural changes occurring in the matrix of these glasses.

Composition, preparation procedure and physical properties of linear homogeneous polyurethanes for membranes

AbstractLinear polyurethanes (PUs) were prepared in solution in two steps. In the first step, toluene‐diisocyanate (TDI) 80:20 (T) was reacted with a short‐chain poly(oxypropylene) diol (V) to obtain prepolymers characterized by various ratios of R = [T]°/[V]°. In the second step, the prepolymers were reacted with the extender 1,4‐cyclohexanedimethanol (E) with various extension ratios RE = (R − 1)[V]°/[E]°. The PU properties were analyzed by various techniques. In particular, the molecular weights determined by gel permeation chromatography (GPC), the thermal properties such as glass transition temperature Tg and specific heat variation ΔCp measured by differential scanning calorimetry (DSC), the chemical composition of the PUs and the various types of hydrogen bonds present in the polymers evidenced by Fourier‐transform infrared (FTIR) spectroscopy, the monophasic structure evidenced by small‐angle X‐ray scattering (SAXS), and the existence of only one Tg confirmed that these PUs were linear, amorphous and monophasic. Positron annihilation lifetime spectroscopy (PALS) analysis showed that the mean volume of the nanoholes increased with increasing temperature, but was not dependent on the composition, as expected for a monophasic system. A stoichiometric semi‐empirical model was proposed that relates the PU blocks' micro‐composition to the R and RE macro‐parameters, chosen for the synthesis. The polymer assumes various expressions of the general formula X[(TV)k(TE)n]mX for different values of the R and RE ratios. The micro‐parameters k and m have a direct connection with the experimental mean molecular weights of the prepolymer and the polymer, respectively: n depends only on R. The model could foresee the density of hydrogen bonds and distinguished the bonds connected to either V or E, which could be shown by FTIR analysis. This paper shows that, when using stereo‐irregular diols and blends of 2,4‐ and 2,6‐TDI, non‐stereoregular PUs are obtained. If low‐molecular‐weight diols are used and R &lt; 3.3, it is quite improbable that the PU blocks separate into macrophases and therefore monophasic amorphous PUs are obtained. Monophasic PUs can be useful for applications such as in the field of membrane gas and vapour separation. Copyright © 2005 Society of Chemical Industry

Risk of non-medullary thyroid cancer influenced by polymorphic variation in the thyroglobulin gene.

Benign thyroid disorders are strong risk factors for non-medullary thyroid cancer (NMTC). Germline variation in Tg (thyroglobulin) and TSHR (thyroid stimulating hormone receptor) confers an increased risk of benign thyroid disorders. To explore the hypothesis that polymorphic variation in these genes affects the risk of NMTC we compared the frequency of TgQ2511R, TSHR-P52T and TSHR-D727E genotypes in two series of NMTC cases and controls (group 1, Canadian 102 cases and 102 controls; group 2, British 202 cases and 298 controls). No significant association was seen with TSHR-P52T and TSHR-D727E genotypes and risk of NMTC. However, the frequency of the R-allele of TgQ2511R was over represented in NMTC cases in both study populations. The odds ratios associated with hetero- and homozygosity for the R-allele were 1.6 (95% confidence interval, 1.1-2.5) and 2.0 (95% confidence interval, 1.2-3.3), respectively. Although the risk of NMTC associated with the TgQ2511R R-allele is modest, its high prevalence in the general population suggests it may make a significant contribution to the incidence of NMTC.

Monomer reactivity ratios and glass‐transition temperatures of copolymers based on dimethyl amino ethyl methacrylate and two structural hydroxy‐functional acrylate isomers

AbstractTwo chemical structural isomers, 2‐hydroxyethyl methacrylate and ethyl α‐hydroxymethyl acrylate, were copolymerized with dimethyl amino ethyl methacrylate in bulk at 50 °C with 1.0 × 10−2 mol/L 2,2′‐azobisisobutyronitrile as an initiator. Experiments showed differences in the values of the monomer reactivity ratios due to electronic or steric contributions to the cross‐propagation reaction. The homopolymer glass‐transition temperature (Tg), along with the intramolecular and intermolecular structure and Johnston's equation, enabled a description of the experimental variations of the copolymer Tg. © 2003 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 41: 2659–2666, 2003

Cooling rate dependence of the glass transition temperature of polymer melts: Molecular dynamics study

A coarse-grained bead spring model of short polymer chains is studied by constant pressure molecular dynamics (MD) simulations. Due to two competing length scales for the length of effective bonds and the energetically preferred distance between nonbonded beads, one observes a glass transition when dense melts are cooled down (as shown in previous work, at a pressure p=1 the mode coupling critical temperature is at Tc≈0.45 and the Vogel–Fulcher temperature is T0≈0.33, in Lennard-Jones units). The present work extends these studies, estimating a cooling-rate-dependent glass transition temperature Tg(Γ) by cooling the model system from T=0.6 down to T=0.3, applying cooling rates from Γ≈10−3 to Γ≈10−6 (in MD time units), and attempting to identify Tg(Γ) from a kink in the volume versus temperature or potential energy versus temperature curves. It is found that Tg(Γ) lies in the range 0.43⩽Tg(Γ)⩽0.47, for the cooling rates quoted, and the variation of Tg(Γ) for Γ is compatible with the expected logarithmic variations. We will show why a detailed distinction between competing theories on these cooling rate effects would need an excessive amount of computer time. To estimate also the melting transition temperature Tm of this model, the sytem was prepared in a crystalline configuration as an initial state and heated up. The onset of diffusion, accompanied by an isotropization of the pressure tensor was observed for Tm≈0.77. This implies that the model is suitable for studying deeply supercooled melts.