Glass Transition Process Research Articles

Molecular dynamics investigation of the glass transition at high-pressure in the phase separation liquid

Molecular dynamics simulation is performed to study the phase separation and the glass transition of a binary liquid mixture when the external pressure is increased from 0 to 2.75 GPa. The structure and dynamic characteristics in the glass transition process are calculated. We find that external pressure will promote the phase separation, and make the glass transition temperature increase. The external pressure will make the transition temperature become higher, at which the β relaxation emerge, the time of β relaxation becomes longer, and the diffusibility of the system becomes lower. We first found that in the phase separation liquid microscopic heterogeneity exists during the glass transition.

Effect of Sb and Sn additives on the activation energies of glass transition and crystallization in binary Se85Te15 alloy

Calorimetric measurements have been performed in glassy Se75Te15M10 (M=Sb, Sn) alloys to study the effect of Sb and Sn additives on the kinetics of glass transition and crystallisation in glassy Se85Te15 alloy. Activation energy of glass transition (E g) and the activation energy of crystallisation (E c) have been determined using different non-isothermal methods. An excellent agreement is observed in the values of E c and E g for all the non-isothermal methods used in the present study. The composition dependence of the activation energies of glass transition and crystallisation processes shows that the value of E g is higher in case of Sn additive while the value of E c is higher in case of Sb additive. The results are explained in terms of Pauling's concept of average heat of atomisation and mean atomic weights of ternary alloys, respectively.

Influence of molten status on nanoquasicrystalline-forming Zr-based metallic glasses

Quasicrystals, formed during the primary crystallization stage of annealing Zr70−XCu25Pt5AlX metallic glasses, decrease in size from 100 to 5 nm as the Al content is increased from 8 to 10 at. %. The glass transition and crystallization processes during annealing at fixed heating rate were found to be influenced by the thermal history of the melt. Pair distribution function analyses of neutron scattering data revealed that the broad peak from the nearest atomic pairs separates into multiple peaks with lower melt quenching temperatures (TL). These results suggest that the short-range order is enhanced with decreased TL. Such ordered regions remain in the glass upon quenching and contribute to nanoquasicrystal formation upon heating.

Effect of thermal crosslink conditions on dynamic mechanical behaviors of flexible epoxy

The dynamic mechanical behavior of a new kind of flexible epoxy FE-1, which was crosslinked under four different thermal crosslink conditions, was studied. Dynamic mechanical measurement was carried out from 10℃ to 120℃, and loss factor, tanδ and the storage modulus as functions of temperature were presented under five different frequencies of 0.1 Hz, 1 Hz, 5 Hz, 50 Hz and 100 Hz. The experimental results show that temperature has dramatic effects on the dynamic mechanical behavior of flexible epoxy. Compared with other common available epoxy, the flexible epoxy has higher loss factor over broad frequency and common temperature range. Activation energy corresponding to glass transition process of FE-1 was calculated from the temperature corresponding to tan δ(subscript max) values, obtained at different measurement frequencies. The maximum value of loss factor is 0.75 and the T(subscript g) varies from 6℃ to 50℃, indicating the flexible epoxy can be used as damping polymer materials at common temperature or frequency range.

Conformational and microstructural characteristics of poly(L-lactide) during glass transition and physical aging

The glass transition and physical aging processes of poly(L-lactide) (PLLA) were studied by variable-temperature Fourier transform infrared (FTIR) spectroscopy and (13)C solid-state NMR spectroscopy. The glass transition temperature (T(g)) of PLLA can be well determined from the temperature-dependent FTIR intensity. Nearby T(g), a distinct change in the slope of spectral intensity versus temperature plot is detected. FTIR results suggest that the energy-favorable gauche-trans (gt) conformers rearrange into the less energy-favorable gauche-gauche (gg) counterparts with heating over the glass transition region, which becomes more distinct at temperature above T(g). Besides, the 1267 cm(-1) band, which shows different trends of variation from the other bands upon heating, was assigned to be more sensitive to the nu(as)(C-O-C)+delta(CH) vibration mode of the less energy-favorable gg conformers in PLLA. By comparing the FTIR spectra of the aged and deaged PLLA, it was demonstrated that the rearrangement from the high- to low-energy conformers, i.e., gg to gt, occurs with physical aging. (13)C spin-lattice relaxation measurements indicate that the relaxation rate distribution broadens with aging, which agrees with the previous suggestion that the locally ordered domains are formed during physical aging. Because of the larger variation in the conformational state and microstructure, the FTIR intensities vary much more abruptly for the aged sample with heating to nearby T(g).

Kinetics of the glass transition in Se 72Te 23Sb 5 chalcogenide glass: Variation of the activation energy

The heating rate dependence of the glass transition temperature was investigated by differential scanning calorimetry (DSC) technique for the Se 72Te 23Sb 5 glass. Different definitions of the glass transition temperature were considered. The activation energy of the glass transition was determined using different kinetic models as well as isoconversional methods. Analysis of experimental data based on Moynihan and Kissinger equations as well as using different isoconversional methods shows that the activation energy of glass transition process is not constant but varies with the degree of transformation from the glassy to the supercooled phase. The applicability and limitations of the theoretical models to describe the kinetics of the glass transition are discussed.

Observation of two α-relaxation peaks in a nanoclay-filled epoxy compound

For over 10 years, the effect of nanosilicate layers on glass transition temperature of epoxy resins has been studied by various research groups. Unfortunately, there appears to be no consensus in the literature regarding the role of nanoclay addition on a-relaxation peak (Tg-peak) in montmorillonite (MMT)-filled epoxy compounds [1–8]. Several studies reported an increase in the overall glass transition temperature (Tg) of MMT-filled epoxies compared to the neat epoxy [1–3], while other studies claimed suppression of the Tg in these nanocomposites [4–6]. There are even some results indicating that the addition of nanoclay has no effect on a-relaxation peak of epoxy resins [7, 8]. Table 1 contains a description of the materials investigated in these studies. The following sections contain discussions of glass transition behavior in several types of nanoclay-filled epoxies. Messersmith and Giannelis [1] reported broadening of the tan d peak in the glass transition region and a slight increase in the Tg of mica-filled epoxy compound with the interlayer distances larger than 100 A. The glass transition temperature was characterized using a dynamic mechanical analyzer (DMA). The increase in Tg was attributed to the strong interactions between silicate platelets and epoxy network that restrict molecular motion of polymer segments near the silicate layer surface. Regarding the observations of Tg depression, several hypotheses have been proposed. Chen et al. [4] attributed this phenomenon to the presence of interphase region between the silicate layers where the surface modifiers of clay act as plasticizer in epoxy resin. They predicted the effective size of interphase region in the order of 50 A [4]. It was reported that the glass transition temperature decreased as the volume fraction of the interphase region inside the galleries increased by increasing the interlayer distance between silicate platelets [4]. However, Jackson and McKenna [9] found that confining organic liquids (1,2-diphenylbenzene and benzyl alcohol) in both surface treated and untreated nano-size pores resulted in similar Tg suppression. Becker et al. [5] believed that the presence of unreacted epoxy monomers leads to Tg depression in clay nanocomposites. Specifically, it was claimed that the drop in the Tg can be ascribed to the lower crosslink density of epoxy network between silicate layers, i.e. in the galleries [5]. The purpose of this letter is to report an observation on the effect of nanoclay fillers on the a-relaxation in an epoxy resin. This observation, which is a small portion of a more detailed study on MMT-filled epoxies, provides useful insight on glass transition processes in nanoclay-filled epoxy composites.

Composition dependence of the activation energy of the glass transition in some chalcogenide glasses

The activation energy (E t) of the glass transition process has been obtained for glassy Se80− x Te20M x (M = Ag, Cd, In, Sb) and Se100− x M x (M = In, Te, Sb) systems using differential scanning calorimetry. The composition dependence of E t is explained in terms of fragility index for the systems. A reversal in the trend of the E t is explained using topological models.

Glass transition in As10Se90 chalcogenide glass: A kinetic study

Abstract The kinetics of the glass transition in the As10Se90 chalcogenide glass was studied using differential scanning calorimetry (DSC) technique The activation energy of the glass transition was determined from the heating rate dependence of the glass transition temperature. Different kinetic models as well as isoconversional methods were used. Isoconversional analysis of the experimental data shows that the activation energy of glass transition process is not constant but varies with the degree of transformation (and hence with temperature) from the glassy to the supercooled phase. The variability of the activation energy with temperature was correlated with the dynamic fragility parameter in comparison with the previously published data of the glassy selenium.

New Paradigm of Glass Structure and Physicochemical Essence of Glass Transition

The new conception of the polymeric-polymorphoid structure of glass and glassforming liquid of individual chemical substances (ICS) is presented. The analysis of numerous data of X-ray diffractometry, DSC, Raman spectroscopy etc. shows that such glassforming ICS as SiO2, GeO2. H2O, Se, GeS2, GeSe2, As2S3, AsSe, BeCl2 and other are the copolymers of the structural fragments (polymorphoids) having no translation symmetry of two (or more) polymorphous modifications (PM) of these substances. The physicochemical essence of glass transition process is discussed.

Effects of Si addition on the glass-forming ability, glass transition and crystallization behaviors of Ti 40Zr 10Cu 36Pd 14 bulk glassy alloy

The effects of small amounts of Si on the glass-forming ability, glass transition and crystallization behaviors of a Ti 40Zr 10Cu 36Pd 14 bulk glassy alloy were investigated. The addition of Si caused the decrease of glass-forming ability and the increase of the supercooled liquid region. The fragility of the alloys containing Si was higher than that of the Si-free alloy. In the glass transition process, both generation and annihilation of free volumes are suppressed by the addition of 1–3 at% Si. The activation energy for crystallization increases with increasing Si content, indicating the improvement of thermal stability. The crystallization structure of the alloys containing Si contents less than 4% consisted of six phases of CuTi 2, Ti, Cu 8Zr 3, Cu 4Ti 3, TiZr and PdTi 2.

Application of fractional calculus to the modeling of the complex rheological behavior of polymers: From the glass transition to flow behavior. I. The theoretical model

AbstractA model based on the concept of fractional calculus is proposed for the description of the dynamic elastic modulus (E* = E′ + iE″, where E* is the complex modulus, E′ is the storage modulus, or real part of the complex modulus, and E″ is the loss modulus, or imaginary part of the complex modulus) under isothermal and isochronal conditions for amorphous polymers, including both the glass transition process and the flow behavior. The differential equations obtained for this model, which we call the extended fractional Zener model (EFZM), have differential and/or integral operators of fractional order between 0 and 1. The application of the Fourier transform to the fractional equation of the EFZM, the association of their parameters to the relaxation times of the cooperative or noncooperative molecular movements of polymer chains, and the isothermal and isochronal diagrams of E′ and tan δ = E″/E′ were evaluated. These theoretical diagrams were typical curves that clearly showed the glass‐transition (α‐relaxation) and flow behavior. The EFZM will enable us to analyze the complex rheological behavior of amorphous polymers. © 2008 Wiley Periodicals, Inc. J Appl Polym Sci, 2008

Experimental Investigation on the Damping Behaviors of Epoxies with Different Epoxy Value

In this article, five different epoxies including a new kind of flexible epoxy having low glass transition temperatures (11 ∼ 28°C) were prepared using polyamine as curing agent. Damping mechanical tests show that compared with other common available epoxies, the flexible epoxy has high loss factor over broad frequency and temperature range. Activation energy corresponding to glass transition process of different epoxies has been calculated from the temperature corresponding to tan δmax values, obtained at different measurement frequencies. The maximum value of loss factor is 0.71 and the Tg varies from 11 to 28°C, indicating the flexible epoxy can be used as damping polymer materials in common temperature or frequency range.

Some Properties of the Glass Transition in the Amorphous Sb-As-S-Se-I System

Transformations of glasses from the multicomponent pseudobinary system (As2Se3)100−x(SbSI)x were analyzed from the aspect of determining the glass transition temperature Tg, activation energy of the process Et, and characteristic changes of the specific heat. The established dependence of Tg on glass composition and heating rate served as the basis for determining the activation energy of glass transition process Et. An abrupt increase in the specific heat cp at the glass transition temperature was analyzed with the aim of classifying the materials according to the criterion of the so-called 'fragility'. It was found that the investigated glasses, i.e. their melts, belong to the group of thermodynamically 'strong' melts.

MD study of the glass transition in binary liquid metals: Ni 6Cu 4 and Ag 6Cu 4

The structure, thermodynamic and kinetic evolution of the binary Ni 6Cu 4 and Ag 6Cu 4 liquids during a rapid cooling process were studied in detail by using the molecular dynamics (MD) simulation method. The Gibbs free-energy difference between the supercooled liquid and the crystal (Δ G lc) for the pure metals Ni and Ag and for alloys Ni 6Cu 4 and Ag 6Cu 4 is calculated. A time–temperature-nucleation diagram for Ni 6Cu 4 is constructed based on the temperature dependence of the characteristic relaxation times. The results indicate that the driving force for the crystallization Δ G lc plays an important role in the glass transition process. The increase of the metallic glass forming ability (GFA) is in consistent with the decrease of Δ G lc.

Effects of La content on the glass transition and crystallization process of Al–Ni–La amorphous alloys

Effects of La content on the glass transition and crystallization process of Al 94− x Ni 6La x ( x = 3–9) amorphous alloys were investigated by X-ray diffraction and differential scanning calorimeter. The results show that the thermal stability increases with increasing the La content. The crystallization changes from a two-stage process without glass transition at x = 3–6 to a three-stage one with obvious glass transition at x = 7–9. The first crystallization process results in precipitation of single fcc-Al at x = 3–5, fcc-Al plus metastable phase(s) at x = 6 and 7, and single metastable phase at x = 8 and 9. The first crystallization process at x = 4 and 5 is the growth of quenched-in nuclei, whereas that at x = 6, 7 and 9 is the diffusion-controlled growth with a decreasing, constant and increasing nucleation rate, respectively. The activation energy for the first crystallization process is larger in the eutectic reaction than that in the primary reaction, and is the highest when the number of the products is the most.

Equivalence between polymer nanocomposites and thin polymer films: Effect of processing conditions and molecular origins of observed behavior

Recently we established a quantitative equivalence in thermomechanical properties between polystyrene-silica nanocomposites and planar freestanding polystyrene thin films. This equivalence was quantified by drawing a direct analogy between film thickness and an appropriate experimental particle spacing. Using these findings, here we unequivocally show that the glass transition process in confined geometries is controlled by the mean volume fraction of polymer that is affected by the presence of surfaces. Since separate signatures of the bulk and the surface layers are never found, we can clearly rule out any simple “two layer” model which postulates the existence of surfaces which are dynamically decoupled from the bulk. Rather, we argue that the modification of properties at the surfaces propagates into the bulk through a spatial gradient: macroscopic experimental techniques average over these gradients and yield a broadened signature relative to the bulk polymer. In a second aspect of this paper we focus on the role of processing conditions on the results obtained. We have developed a new method of processing the nanocomposites which results in a better dispersion of the nanoparticles in the matrix. However, these samples did not show the unique glass transition behavior seen in the first set of nanocomposites discussed above. This indicates that processing conditions can profoundly affect the nature of the particle-polymer interface which controls the macroscopic behavior of these important systems.

Glass Transition and Structure Relaxation of 1, 2-Propanediol Aqueous Solutions

In order to examine the effects of water contents on glass transition and structure relaxation parameters of 1, 2-propanediol (PD) aqueous glasses, apparent specific heat capacity during glass transition processes of five PD aqueous solutions with high concentrations (60%, 70%, 80%, 90%, and 100%, w) were determined using the differential scanning calorimetry (DSC). Five cooling rates (1, 2, 5, 10, 20 K·min-1) and one heating rate (10 K·min-1) were used to obtain the glass transition and structure relaxation parameters. Water seemed to be no obvious plasticization effect on 1, 2-propanediol (compared to some hydroxyl group linear polyalcohol) since the glass transition temperatures of the aqueous solutions were only slightly decreased with the increasing of water content. The structure relaxation activation energies were found to decrease as the water content increased. A comparison of thermodynamic fragility(ΔCp) and dynamic fragility (Angell′s fragility) was present and no similar trend was found with the changing of the component, since the former increased first and decreased last. The calculating results of dynamic fragility indicated that 1, 2-propanediol aqueous solutions behaved as though they were stronger than the pure solute. The sizes of cooperative rearrangement region (CRR) discussed in the term of glass transition characteristic length were determined using Donth's thermodynamic temperature fluctuant method. It was found that the sizes of CRR decreased as the water content increasing in 1,2-propanediol aqueous solutions.

Structure and Properties of Poly(ε‐caprolactone) Networks with Modulated Water Uptake

AbstractSummary: A PCL macromonomer was obtained by the reaction of PCL diol with methacrylic anhydride. The effective incorporation of the polymerizable end groups was assessed by FT‐IR and 1H NMR spectroscopy. PCL networks were then prepared by photopolymerization of the PCL macromonomer. Furthermore, the macromonomer was copolymerized with HEA, with the aim of tailoring the hydrophilicity of the system. A set of hydrophilic semicrystalline copolymer networks were obtained. The phase microstructure of the new system and the network architecture was investigated by DSC, IR, DMS, TG, dielectric spectroscopy and water sorption studies. The presence of the hydrophilic units in the system prevented PCL crystallization on cooling; yet there was no effect on the glass transition process. The copolymer networks showed microphase separation and the α relaxation of the HEA units moved to lower temperatures as the amount of PCL in the system increased.Ideal structure, compatible with the experimental results, for the hydrophilized poly(ε‐caprolactone) networks with modulated water uptake.magnified imageIdeal structure, compatible with the experimental results, for the hydrophilized poly(ε‐caprolactone) networks with modulated water uptake.

Comparative analysis of calorimetric studies in Se90M10 (M=In, Te, Sb) chalcogenide glasses

Abstract Calorimetric measurements have been performed in glassy Se90M10 (M=In, Te, Sb) alloys to study the effect of In, Te and Sb additives on the kinetics of glass transition and crystallization in glassy Se90M10 system. Kinetic parameters of glass transition and crystallization such as the activation energy of glass transition (Eg), the activation energy of crystallization (Mc), the order parameter (n), the rate constant (K), etc. have been determined using different non-isothermal methods. The composition dependence of the activation energies of glass transition and crystallization processes is also discussed.