Dynamic Fragility Research Articles

Dynamic glass transition and fragility of lithium borate binary glass

The compositional variations of the fragility of lithium borate binary glasses have been investigated over a wide composition range up to 64 mol% of Li 2O. The frequency dependences of the dynamic heat capacity C p( ω) above the dynamic glass transition temperature T g have been observed by Temperature-Modulated DSC. The fragility or steepness index m has been determined. The composition dependence has been discussed on the basis of the fragility model proposed by Vilgis, and it has been revealed that the trends of the fragility and the fluctuation of the coordination number of boron atom have the correlation over a wide composition range. The result indicates the origin of the fragility relates to the degree of the fluctuation of the coordination number. The formation of the 4-coordinated boron atom plays the dominant role in the fragility of the borate glassy system.

Crystallinity and Crystalline Confinement of the Amorphous Phase in Polylactides

AbstractSummary: Stereorregular polylactides such as poly (L‐lactide) (PLLA) or poly (D‐lactide) result from polymerization of optically pure lactides and are semicrystalline. Optically non‐active poly(D,L‐lactide) (PDLLA) can be regarded as random or atactic copolymers, show a random moiety distribution, and are completely amorphous. In this work three phases, comprising mobile amorphous fraction (MAF, χMA), rigid amorphous fraction (RAF, χRA) and crystalline fraction (χc) were determined in PLLA. It will be shown that RAP fraction not only elevates Tg but also increases the dynamic fragility (m) of polylactide chains around the Tg. These results agree with reported cases in which topologycal constraints inhibit longer range dynamics and suggest a smaller length scale of cooperativity of chains in confined environments.

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.

Glass transition behavior and dynamic fragility in polylactides containing mobile and rigid amorphous fractions

The segmental dynamics of polylactide chains covering the Tg−30°C to Tg+30°C range was studied in absence and presence of a crystalline phase by dynamic mechanical analysis (DMA) using the framework provided by the WLF theory and the Angell's dynamic fragility concept. An appropriate selection of stereoisomers combined with a thermal conditioning strategy to promote crystallization (above Tg) or relaxation of chains (below Tg) was revealed as an efficient method to tune the ratio of the rigid and mobile amorphous phases in polylactides. A single bulklike mobile amorphous phase was taken for poly(d,l-lactide) (PDLLA). In turn three phases, comprising a mobile amorphous fraction (MAF, XMA), a rigid amorphous fraction (RAF, XRA) and a crystalline fraction (Xc) were determined in poly(l-lactide) (PLLA) by modulated differential scanning calorimetry (MDSC) according to a three-phase model. The analysis of results confirms that crystallinity and RAF not only elevate the Tg and the breadth of the glass transition region but also yields an increase in dynamic fragility parameter (m) which entails the existence of a smaller length-scale of cooperativity of polylactide chains in confined environments. Consequently it is proposed that crystallinity is acting in polymeric systems as a topological constraint that, preventing longer range dynamics, provides a faster segmental dynamics by the temperature dependence of relaxation times according to the strong–fragile scheme.

Influence of structural chemical characteristics on polymer chain dynamics

A comparative study of the dielectric relaxation behavior of two structurally close polymers containing aliphatic-aromatic side groups was carried out in order to get a better understanding on how slight differences in chemical structure affect the molecular responses to perturbation fields. Specifically, chain dynamics of poly(2-acryloyloxyethyl-2-naphthalene-2-ylacetate) and poly(2-methacryloyloxyethyl-2-naphthalene-2-ylacetate) were studied by broadband dielectric spectroscopy in the frequency range of 10(-2)-10(8) Hz and temperature window of 298-403 K. Also, the relaxation behavior of (2-acetyloxyethyl-2-naphthalene-2-ylacetate), model compound of the polymer side groups, was analyzed. The isotherms representing the dielectric loss in the frequency domain show important conductive contributions, especially at high temperature, which hide the low frequency side of the alpha relaxation. Conductivity also increases the real component of the complex permittivity in the low frequencies region. Retardation spectra were obtained by minimizing the sum of the squares of the difference between the experimental values of the complex permittivity for each frequency and the analytical ones, predicted by the linear phenomenological theory, using a Tikhonov regularization technique. The spectra present an apparent alpha peak with an excess wing at short time side resulting from the overlapping of the true alpha relaxation and a beta process. Three absorptions, named in increasing order of time gamma, beta, and alpha relaxations, are separated by deconvolution methods. The activation energies associated with the gamma process are 70.0+/-1.8, 68.0+/-1.4, and 74.8+/-0.8 kJ mol(-1) for (2-acetyloxyethyl-2-naphthalene-2-yl acetate), poly(2-acryloyloxyethyl-2-naphthalene-2-yl acetate) and poly(2-methacryloyloxyethyl-2-naphthalene-2-yl acetate), respectively. The respective activation energies associated with the beta relaxation are 121.7+/-2.4, 135.3+/-1.4, and 141.6+/-1.3 kJ mol(-1). Values of the shape parameters and the strengths of the relaxation processes were obtained as a function of temperature. The dynamic fragility of the polymers and the model compound was studied and compared with that reported for macromolecular and monomeric systems. Also, the evolution of the size of the correlated domains associated with the alpha relaxation was estimated. Finally, the gamma relaxation rather than the beta absorption obeys the criteria apparently held by the Johari-Goldstein beta processes.

Increasing Superplasticity and Strong Dynamic Behaviour of Zr—Cu—Ni—Al Bulk Metallic Glass

The plasticity and the dynamic fragility of bulk metallic glass of a Zr62Cu18Ni10Al10 alloy are studied by three-point beam bending methods. We find that the alloy behaves super plastic not only at room temperature, but also at high temperatures. More importantly, it is found that the superplasticity increases with increasing temperature. In addition, the dynamic fragility parameter m for the supercooled liquid of this alloy is measured to be 34.87 and the supercooled liquid of Zr62Cu18Ni10Al10 alloy behaves as a strong liquid.

Liquid polyamorphism: Possible relation to the anomalous behaviour of water

We present evidence from experiments and computer simulations supporting the hypothesis that water displays polyamorphism, i.e., water sepa- rates into two distinct liquid phases. This concept of a new liquid-liquid phase transition is finding application to other liquids as well as water, such as silicon and silica. Specifically, we investigate, the relation between changes in dynamic and thermodynamic anomalies arising from the presence of the liquid-liquid critical point in (i) Two models of water, TIP5P and ST2, which display a first order liquid-liquid phase transition at low temperatures; (ii) the Jagla model, a spherically symmetric two-scale potential known to possess a liquid-liquid critical point, in which the competition between two liquid structures is generated by repulsive and attractive ramp interactions; and (iii) A Hamiltonian model of water where the idea of two length/energy scales is built in. This model also displays a first order liquid-liquid phase transition at low temperatures besides the first order liquid-gas phase transition at high temperatures. We find a correlation between the dynamic fragility crossover and the locus of specific heat maxima C max ("Widom line") emanating from the critical point. Our findings are consistent with a possible relation between the previously hypothesised liquid-liquid phase transition and the transition in the dynamics recently observed in neutron scattering experiments on confined water. More generally, we argue that this connection between C max P and the dynamic crossover is not limited to the case of water, a hydrogen bonded network liquid, but is a more general feature of crossing the Widom line, an extension of the first-order coexistence line in the supercritical region. Dedicated to Armin Bunde on the occasion of his 60th birthday.

Optical and broadband dielectric investigations of photochromic polymethacrylates

The molecular dynamics and optical switching behavior (photo alignment) of novel azobenzene containing side-group (co)polymers were studied by time-resolved optical spectroscopy and broadband dielectric spectroscopy (BDS). To elucidate the effect of the molecular structure on the photochromic properties, two series of poly(methacrylates) with different aliphatic spacer units and four different concentrations of the chromophoric monomer were investigated. For both series, an inverse relation between the switching (retardation and relaxation) times and the chromophore content was found as well as a strong correlation between switching times and alignment efficiency. Dielectric spectroscopy on all materials revealed up to three relaxation processes (α, α*, δ) above the glass transition temperature that were assigned to the dynamic glass transition of the polymer backbone and the fast and slow fluctuation of the chromophores around their long and short molecular axes. In the glassy state, occasionally two Arrhenius-type relaxations were observed that were identified as local motions of the butyl side group and the chromophore in its anisotropic environment. Both materials series showed a monotonic increase in Tg and the dynamic fragility with increasing chromophore concentration, which was explained by an increasing effect of physical crosslinking provided by the increasingly dense packed side groups.

Complex nature of the β relaxation and fragility in aromatic polyesters

The following experimental study shows the dielectric relaxation behavior of random copolyesters formed by butylene terephthalate and butylene isophthalate comonomeric units. The experimental data were analyzed using two relaxation modes describing the sub-glass relaxation, characterized by different activation energies. We propose that the high frequency secondary relaxation β 1 is associated to the most flexible part of the repeat unit, whereas the low frequency mode β 2 has been assigned to atoms of the ester groups linked to the aromatic rings. A strong correlation between the dynamic fragility of these polymers and the monomer architecture was also demonstrated.

Segmental dynamics of reactively prepared polystyrene blends: Unsaturated polyester resin versus high impact polystyrene

AbstractSegmental dynamics of two phase‐separated reactively prepared polystyrene blends namely unsaturated polyester resin (UPR) and high impact polystyrene (HIPS) was investigated by dynamic–mechanical spectroscopy and calorimetric studies. The results showed a thermorheologically simple behavior for the HIPS, which could be quantified based on the KWW function with βKWW of 0.37. The UPR data, however, could not be evaluated using KWW function, even though lower βKWW than the HIPS was expected for it. Furthermore, the α‐dispersion of the UPR was considerably broader while its fragility index was comparable with the HIPS. Nonetheless, segmental dynamics comparison based on normalized ΔCp(Tg) by molecular weight of the structural units of the studied systems showed much greater differences. Accordingly, the UPR was both kinetically and thermodynamically more fragile than the HIPS. The higher fragility of the UPR could be attributed to its larger relative cooperativity size and topological constraints. Finally, enhanced contrast in dynamic fragilities of two studied systems could be achieved if similar overall and local compositions could be made experimentally. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007

Short time properties, dynamic fragility and pressure effects in deeply supercooled polymermelts

Our activated barrier hopping theory of segmental relaxation in deeply supercooledpolymer melts is applied to compute short time properties including the glassy shearmodulus, localization length and vibrational frequency. Numerical calculations for specificpolymers suggest the theory simultaneously predicts a reasonable elastic modulus, localizedstate vibrational frequency, dynamic fragility and dynamic crossover and glasstransition temperatures. The theory also provides explicit connections between shorttime-/length-scale properties and the slow alpha relaxation process. The extension of thetheory to elevated pressures is initiated. Pressure is found to broaden the deeplysupercooled regime and reduce the dynamic fragility. However, the predictedRossler–Sokolov universal supra-Arrhenius law for the temperature dependence of thealpha relaxation time remains accurate at all pressures. A common theme is theessential role played by the ratio of the dynamic crossover temperature (idealmode coupling critical temperature) and kinetic glass transition temperatureeven in the deeply supercooled regime where activated processes are dominant.

Dielectric relaxation spectrum of poly (ε-caprolactone) networks hydrophilized by copolymerization with 2-hydroxyethyl acrylate

The dielectric relaxation spectrum of polycaprolactone (PCL) networks hydrophilized with different amounts of 2-hydroxyethyl acrylate (HEA) is investigated. PCL is a semicrystalline polyester with a complex relaxation spectrum that includes the main alpha relaxation and two secondary modes (beta, gamma) at lower temperatures. The overlapping of the different relaxational modes was split by using several Havriliak-Negami functions. Crosslinking the material modifies the dynamics of the main relaxation process as reflected by the parameters that characterize the Vogel behavior of the process and the dynamic fragility. The incorporation of HEA units in the network results in a material with microphase separation: two alpha processes are detected, the one corresponding to the PCL chains and the new one associated to nanometric regions that contain different amount of both comonomers. The incorporation of the HEA units in the system involves the presence of a new beta(sw) relaxation due to the link of two side chains by water molecules through hydrogen bonding.

Physical crosslinking effects in α,ω-dihydroxy terminated polybutadienes

Structure and molecular dynamics of α,ω-dihydroxy terminated polybutadienes of varying number averaged molecular weight (1320–10 500g/mol) have been investigated by Fourier-transformed infrared (FTIR) spectroscopy, nuclear magnetic resonance (NMR) spectroscopy, dielectric relaxation spectroscopy (DRS), differential scanning calorimetry (DSC) and wide-angle X-ray scattering (WAXS). DSC and DRS revealed an increase in the glass transition temperature upon decrease of the molecular weight, accompanied by an increasing dynamic fragility m (or steepness index) of the dielectric α-process. This correlation between Tg and m for different molecular weights indicates the presence of a physical network, where H-bonded end-group clusters act as temporary crosslinks. From the dielectric relaxation strength Δɛα(Mn), the fraction of associated hydroxy groups (fbond) was estimated showing a peak value for the two but shortest polymers, a behaviour that strongly resembles the molecular weight dependence of the fragility. By considering the quantity fbond(Mn) in a modified Fox-Flory approach, the measured Tg(Mn) behaviour could be reproduced in a satisfying way. FTIR results support this general picture and show a considerable dependence of the extent of hydrogen bonding and formation of hydroxy groups associates on the molecular weight. Further, WAXS and DSC results disprove the idea of formation of pseudo-crystalline hydrophobic microdomains in these compounds as suggested by other authors.

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.

Fragility and Glass Transition Temperature in the Rheological Response of Azobenzene Copolymers

Two copolymers containing nonmesogenic methyl methacrylate (MMA) and nematogenic azobenzene methacrylate (MA4) side groups (30/70 and 40/60 copolymers) azobenzene were studied in the linear viscoelastic regime. The time-temperature superposition principle was found to hold over the whole investigated temperature range, even across the nematic-isotropic transition in the 30/70 copolymer. Rheological steady state and oscillatory measurements were used to obtain the temperature dependence of the zero shear viscosity. The strength and dynamic fragility of these two copolymers were compared with those of the corresponding PMA4 and PMMA homopolymers, which pointed out the role of the structure of the main chain in dictating the relaxational dynamics.

Variation in Activation Energy of the Glass Transition for Polymers of Different Dynamic Fragility

AbstractSummary: Variations of the effective activation energy E of glass transitions have been determined by applying an isoconversional method to DSC data on the glass transition in PVC, PBMA, PVP, and PEN. A variability parameter ΔE has been introduced to characterize the rate of decrease in E with increasing temperature. By using the present data and our previous results for PS, PET, and B2O3, it has been found that ΔE correlates with the values of dynamic fragility, increasing with the latter in approximately exponential fashion.Correlation of the variability parameter with the dynamic fragility.imageCorrelation of the variability parameter with the dynamic fragility.

Correlation between dynamic fragility and glass transition temperature for different classes of glass forming liquids

Here we compile literature data for dynamic fragility m for six types of glass forming liquids: polymers, small molecule organics, hydrogen bonding organics, inorganics, ionic and metallic glass formers. Our analysis of the data shows that different categories of glass forming liquids exhibit different behaviors in terms of the correlation between m and Tg, a correlation not previously examined. For example, for hydrogen bonding organics, polymeric and metallic glass formers, there is an approximately linear increase in m with increasing Tg. While for inorganic glass formers, m appears almost independent of Tg, remaining nearly constant over a wide range in Tg. At the same time, another important parameter, the apparent activation energy Eg at Tg has been investigated. It was found that Eg increases with Tg to the 2nd power for hydrogen bonding organics, polymeric and metallic glass forming liquids, while Eg of inorganic glasses has a linear dependence on Tg.

Thermodynamics and rheology of cycloolefin copolymers

Cycloolefin copolymers of ethylene and norbornene, with norbornene content in the range from 36 to 62 mol %, were studied with respect to the thermal, thermodynamic, and rheological properties using differential scanning calorimetry, pressure-volume-temperature, and dynamic mechanical measurements. All copolymers obey the principle of time-temperature superposition, i.e., they can be considered as thermorheologically simple except for a temperature range in the vicinity of T(g). Despite this, the results on (i) the ratio of activation energies E(V)(*)/H(*) used to quantify the origin of the liquid-to-glass transition, (ii) the pressure coefficient of the glass temperature T(g)(P), and (iii) the dynamic fragility m suggest increasing dynamic heterogeneity with increasing norbornene content that is driven by the structural heterogeneity along the backbone.

A thermodynamic approach to the fragility of glass-forming polymers

We have connected the dynamic fragility, namely, the steepness of the relaxation-time variation upon temperature reduction, to the excess entropy and heat capacity of a large number of glass-forming polymers. The connection was obtained in a natural way from the Adam-Gibbs equation, relating the structural relaxation time to the configurational entropy. We find a clear correlation for a group of polymers. For another group of polymers, for which this correlation does not work, we emphasize the role of relaxation processes unrelated to the alpha process in affecting macroscopic thermodynamic properties. Once the residual excess entropy at the Vogel temperature is removed from the total excess entropy, the correlation between dynamic fragility and thermodynamic properties is reestablished.

Liquid fragility and characteristic of the structure corresponding to the prepeak of AlNiCe amorphous alloys

Liquid fragility and its relation to characteristic of the structure corresponding to the prepeak have been investigated for the AlNiCe alloy system. Thermal analysis shows that in the Al-rich ternary system, the fragility strength exhibits a strong dependence on the Ce content. A close correlation between liquid fragility and the stability of the structure corresponding to the prepeak, as well as the correlation length, is observed for these alloys. Strong liquids correspond to stable medium-range order structure, which results from large energy of cluster breakage. It has been found that the influence of Ce additions on the stability of the structure corresponding to the peak is completely opposite to that on the thermal stability of the glasses. Besides, no obvious relationship exists between dynamic fragility and the glass-forming ability. These unexpected observations have been explained in terms of structural relaxation of the clusters in liquids exhibiting different dynamic fragilities.