Calorimetric Tg Research Articles

The effect of mechanical glass transition temperature on the oxidation rates of omega fatty acids in condensed biopolymer matrices

The concept of glass transition has been used in food products to study their stability, extending shelf life and enhancing organoleptic desirability. This investigation evaluates the effect of three-dimensional structure as a barrier to oxidation of omega fatty acids in condensed hydrocolloid-based matrices. Two high-solid preparations were employed: κ-carrageenan with glucose syrup and genipin-crosslinked gelatin with polydextrose. They supported discontinuous microscopic inclusions of linoleic and linolenic acids within the rubber-to-glass transition region of the condensed mixtures. Glass transition temperatures (Tg) were estimated using differential scanning calorimetry and in-shear dynamic oscillation. The rate of lipid oxidation was monitored by analysing hydroperoxide (ROOH) production during each oxidation phase. The structural transformation of the supporting matrices as a function of temperature significantly affects the oxidation processes. The mechanical or network Tg exhibited higher values than the calorimetric Tg, supporting reduced lipid oxidation rates by suppressing ROOH accumulation in the densified glassy matrices.

Segmental mobility in linear polylactides of various molecular weights

In this work we synthesized and studied a series of linear polylactides (PLA) with a fixed L/d -lactide stereoisomer ratio, covering together the wide range of molecular weights, Mn, from ∼6 to ∼80 kg/mol. The basic aims are the segmental mobility mapping, in terms of time scale and fragility, the recording of the Mn dependences of the glass transition temperature and fragility, and, finally, the comparison with corresponding findings from the literature. Obviously, the direct Mn effects on Tg are assessed by preserving PLA at the amorphous state, i.e., via melting and relatively fast cooling. The segmental mobility is studied by conventional and temperature modulated calorimetry, whereas the molecular dynamics is assessed employing the technique of broadband dielectric spectroscopy in combination with proper critical analysis and evaluation routes. The calorimetric and dielectric Tg(Mn) trends were found consistent between each other as well as with previous findings in similar systems, both with respect to experimental findings and theoretic predictions for PLAs of similar L/D ratio. The Mn threshold of entanglements seems to lay between 13 and 28 kg/mol. The fragility index of the α relaxation, i.e., the dielectric analogue of the glass transition, exhibits a similar dependence to that of Tg(Mn). When decreasing Mn to very low values, the high fragility (∼150) drops down to 0 (Mñ6 kg/mol), denoting the vanishing of the polymer chains’ cooperativity. Qualitatively interesting alternations are recorded in the dielectric strength of the α relaxation and the relaxation times width. The effects are discussed in terms of the various parameters co-affecting the polymer dynamics, namely, chain lengths, free volume and dynamical heterogeneities.

Influence of ice formation on the dynamic and thermodynamic properties of aqueous solutions

Water dynamics in solutions with biological or non-biological solutes has been intensely studied when both components (solvent and solute) are amorphous. Here, we apply broadband dielectric spectroscopy combined with calorimetric measurements to analyze the dynamics of the aqueous solutions tri-propylene glycol (3PG) and ε-poly (lysine) (ε-PLL), after their water becomes semi-crystalline. Various crystallization levels were explored by conducting experiments with different annealing times at temperatures above the glass transition temperature (Tg). We find that the amount of ice depends on both the time and temperature of the annealing, and that this, in turn, affects Tg and dynamics of the amorphous part of the samples. However, it should be noted that the observed differences are relatively small for the degrees of crystallinity we have studied (up to about 26 wt% of the water). This also implies that the dynamic crossover of the water relaxation from a high temperature non-Arrhenius behavior to a low temperature Arrhenius dependence is unaffected by the partial crystallization and still occurs as a single crossover at the calorimetric Tg. Thus, we cannot detect two different crossovers, as commonly observed for other types of two-component systems, such as two glass formers.

Molecular mobility investigation of the biobased Poly(ethylene vanillate) and Poly(propylene vanillate)

In this work, the molecular mobility of two relatively new biobased polyesters poly(ethylene vanillate) (PEV) and poly(propylene vanillate) (PPV) is investigate. The importance of these polymers lays on their renewable character, a desired property of today's global needs, along with the fact that are derived from vanillin, trough vanillic acid, a product of lignin. The latter is the second most abundant polymer on earth. Broadband dielectric spectroscopy (BDS) supplemented by differential scanning calorimetry (DSC, TMDSC), polarized light microscopy (PLM) and nuclear magnetic resonance are the main investigation tools. The molecular mobility mapping, for both local and segmental dynamics of PEV and PPV, is shown here for the first time. Beginning with calorimetry, in the amorphous state, the glass transition temperature, Tg, of PEV and PPV was found 69 and 62 °C, respectively, while the corresponding heat capacity change was found 0.54 and 0.47 J/g∙K. A variety of relaxation processes were recorded in BDS, namely the local γ, β1 and β2, the segmental α relaxation, the dielectric analogue of glass transition, and additional processes revealed by the employed analysis. We propose possible molecular origins for the local γ, β1 and β2 relaxations, namely, on side groups rotation and crankshaft motions on the backbone. The latter is suggested by the recorded changes in time scale at specific temperature regions, for example, around Tg for β2, which seems to be a common characteristic between some polyesters. Segmental α relaxation was found to be in accordance with the calorimetric Tg, i.e., faster for PPV, while the dielectric Tgs were found 64 °C and 59 °C, for PEV and PPV, respectively. In the amorphous state, α is less fragile and slightly stronger in PEV, whereas its time-scale is less affected by cold crystallization, in contrast to that of PPV. From the overall results, indications for differences in the interchain distances and chain-chain interactions, along with the effects by the chain length were revealed. These parameters were correlated with the retarded crystals nucleation in the case of the short PEV chains as compared to PPV.

Decoupling between calorimetric and dynamical glass transitions in high-entropy metallic glasses

Glass transition is one of the unresolved critical issues in solid-state physics and materials science, during which a viscous liquid is frozen into a solid or structurally arrested state. On account of the uniform arrested mechanism, the calorimetric glass transition temperature (Tg) always follows the same trend as the dynamical glass transition (or α-relaxation) temperature (Tα) determined by dynamic mechanical analysis (DMA). Here, we explored the correlations between the calorimetric and dynamical glass transitions of three prototypical high-entropy metallic glasses (HEMGs) systems. We found that the HEMGs present a depressed dynamical glass transition phenomenon, i.e., HEMGs with moderate calorimetric Tg represent the highest Tα and the maximum activation energy of α-relaxation. These decoupled glass transitions from thermal and mechanical measurements reveal the effect of high configurational entropy on the structure and dynamics of supercooled liquids and metallic glasses, which are associated with sluggish diffusion and decreased dynamic and spatial heterogeneities from high mixing entropy. The results have important implications in understanding the entropy effect on the structure and properties of metallic glasses for designing new materials with plenteous physical and mechanical performances.

Molecular Dynamics in Nanocomposites Based on Renewable Poly(butylene 2,5-furan-dicarboxylate) In Situ Reinforced by Montmorillonite Nanoclays: Effects of Clay Modification, Crystallization, and Hydration.

This study deals with poly(butylene 2,5-furan-dicarboxylate), PBF, a renewable bio-based polyester expected to replace non-eco-friendly fossil-based homologues. PBF exhibits excellent gas barrier properties, which makes it promising for packaging applications; however, its rather low and slow crystallinity affects good mechanical performance. The crystallization of this relatively new polymer is enhanced here via reinforcement by introduction in situ of 1 wt % montmorillonite, MMT, nanoclays of three types (functionalizations). We study PBF and its nanocomposites (PNCs) also from the basic research point of view, molecular dynamics. For this work, we employ the widely used combination of techniques, differential scanning calorimetry (DSC) with broad-band dielectric relaxation spectroscopy (BDS), supplemented by polarized light microscopy (PLM) and thermogravimetric analysis (TGA). In the PNCs, the crystalline rate and fraction, CF, were found to be strongly enhanced as these fillers act as additional crystallization nuclei. The improvements in crystallization here correlate quite well with those on the mechanical performance recorded recently; moreover, they occur in the same filler order, in particular, with increasing MMT interlayer distance (from ∼1 to ∼3 nm). In the amorphous fraction of the polymer, the chain diffusion (calorimetric Tg and dynamic α process) is easier in the PNCs due to their slightly smaller length, while in the semicrystalline state, it decelerates by crystal-induced constraints. The local polymer dynamics (β process, below Tg) was found to be independent of the PNC composition, however, sensitive to structural changes of the matrix. Finally, a filler-induced dynamics was additionally recorded in the PNCs (α* process), arising possibly from the polymer located at the MMT surfaces. α* follows the changes in polymer chain length and decelerates with crystallization, whereas its activation energy decreases with mild hydration. The combined results on α* with the DSC and TGA findings, provide proof for weak MMT-PBF interactions. Overall, our results, along with data from the literature, suggest that such furan-based polyesters reinforced with properly chosen nanofillers could potentially serve well as tailor-made PNCs for targeted applications.

Glass transition at the polystyrene/polyethylene glycol interface observed via contact angle measurements

We investigated the possibility of detecting the interfacial glass transition of a polymer with static contact angle measurements of a liquid polyethylene glycol on the polymer surface. The observed contact angle θ reflects the deviation from an equilibrium state at low temperatures, and the temperature dependence of cos θ changes discontinuously at the interfacial glass transition temperature Tg, manifesting a change in the interfacial entropy. This outcome was demonstrated experimentally for a liquid of polyethylene glycol (PEG) on an atactic polystyrene (PS) surface. The evaluated Tg was ca. 362 K. This value is lower than a calorimetric Tg for a bulk PS. This difference reflects molecular interactions at the interface, indicating that the Tg obtained from the contact angle measurement is sensitive to the dynamics near the polymer/liquid interface. The possibility of detecting the interfacial glass transition of polystyrene with contact angle measurements of a liquid polyethylene glycol on the polymer surface was investigated. The observed contact angle reflects the deviation from an equilibrium state at low temperatures, exhibiting a discontinuous change in the temperature dependence of the interfacial tension. The evaluated Tg was ca. 362 K, which is lower than a calorimetric Tg for a bulk polystyrene. The interfacial glass transition appears to be detected when the polymer/liquid interactions affect the wetting process.

The dynamic fragility and apparent activation energy of bitumens as expressed by a modified Kaelble equation

The temperature dependence of the dynamics of glass-forming liquids can be characterized by the dynamic fragility (m) and apparent activation energy (Ea) at the glass transition temperature Tg. In this study, we derive analytical expressions that allow the calculation of these parameters from a modified Kaelble equation which divides the temperature dependence into two regimes above and below a characteristic temperature Td. Special emphasis is given to the analysis of the Td parameter that can be considered as the rheological glass transition temperature. Rheological characterization is performed on twenty-seven bitumens originating from various crude oil sources and refining processes. Their dynamic fragilities and apparent activation energies are calculated at the calorimetric Tg and at Td. Bitumen can be classified as a strong glass-forming liquid, dynamic fragilities varying in the range of m(Tg) = 26 … 52 for the individual bitumen samples. The results indicate that m(Tg) and Ea(Tg) are linearly correlated with Tg, and these Tg-dependences are unusually strong in comparison to other classes of glass-forming liquids. However, dynamic fragilities and apparent activation energies evaluated at Td are nearly independent of the type of bitumen and show only a weak dependence on Td.

Polyurethanes with POSS pendent on flexible hard segments: Morphology and glass transition

Polyhedral Oligomeric Silsesquioxanne (POSS) moieties are tethered on a polyurethane chain, as side groups on the hard domains, which are based on the flexible hexamethylene diisocyanate (HDI). Their effect on morphology, and consequently on molecular dynamics, is studied in detail by microscopy, scattering, spectroscopic, thermal, and dielectric techniques. Albeit the PU micromorphology does not change radically, POSS-rich domains are formed. The segmental dynamics is severely slowed down in a continuous manner, i.e. without the emergence of a new, distinct relaxation, as was the case in a similar system based on the aromatic methylene diisocyanate (MDI). Strikingly, the slowed down components do not seem to have any calorimetric footprint, and hence the calorimetric Tg remains practically unaffected. The linear HDI systems have faster dynamics in the soft phase, as compared to the aromatic MDI ones, presumably owing to higher flexibility and better microphase separation.

Ion Transport in Cyclopropenium-Based Polymerized Ionic Liquids

Ion transport in polymerized ionic liquids (poly-ILs) occurs via a fundamentally different mechanism than in monomeric ionic liquids, and recently progress has been made toward understanding ion conduction in poly-ILs. To gain insight into the nature of ionic conductivity in ionic polymers, we investigate the physical properties of the trisaminocyclopropenium (TAC) ion, as it is an aromatic carbocation with unique structural and electronic properties. Herein, we characterize the thermal properties, local morphology, and dielectric response of a series of monomeric and polymeric TAC ionic liquids with different counterions. We have found that the extent of a “superionic” mechanism depends on the nature of the ion pair and can result in anomalously high conductivity at the calorimetric Tg. Our results suggest that the molecular volumes of the cationic and anionic species are important parameters that impact ion conductivity in polymerized ionic liquids.

Physicochemical properties and relaxation time in strength analysis of amorphous poly (vinyl-pyrrolidone) and maltodextrin: Effects of water, molecular weight, and lactose addition

Fundamental knowledge of physicochemical properties of polymeric food materials is of practical importance in food and pharmaceutical industry. Effects of water, molecular weight, and lactose addition on glass transition temperature (Tg), α-relaxations, and relaxation time (τ) of amorphous poly (vinyl pyrrolidone) (PVP) and maltodextrin (MD) were studied at various aw and 25 °C. Water sorption behaviour of amorphous PVP and MD was Mwavg–dependent at aw ≤ 0.44, whereas the molecular size effects dominated the sorption process of MD at aw ≥ 0.56 and 25 °C. Fractional water sorption was confirmed in studied polymers/lactose mixtures at aw ≤ 0.44 in molecular scale, whilst collapsed structure of above mixtures were observed at aw ≥ 0.44 during storage up to 20 days at 25 °C. Despite of the calorimetric onset Tg of amorphous polymers/lactose mixtures showed a composition-independent at aw ≤ 0.44, Tg, α-relaxation, and τ of PVP and MD were disturbed by water, molecular weight and size, and adding lactose. Structural strength (S) of PVP was determined by its molecular weight at aw ≤ 0.44, whereas molecular size dominated the S of MD at 0.44 ≤ aw ≤ 0.76 due to water migration. The changes of aw and lactose addition significantly disturbed the S of studied polymers. Moreover, a combined use of water sorption isotherm and the relationship of S and aw gave a quantitative measurement of compositional effects and could be used to control the physicochemical properties of amorphous food polymers, such as collapse.

A study on the progress of mutarotation above and below the Tg and the relationship between constant rates and structural relaxation times.

Comprehensive FTIR studies on the progress of mutarotation in d-fructose mixed with maltitol have been carried out over a wide range of temperatures, both above and below the glass transition temperature Tg. In addition to the analysis of single bands, we have developed a completely new approach considering the full spectral range to follow the overall progress of the reaction. We have found that at the calorimetric Tg, there is a clear change in the temperature dependence of constant rates. The activation barrier for mutarotation changes from around 59 kJ mol-1 (the supercooled state) to around 249 kJ mol-1 (the glassy state). This dramatic variation in the activation barrier is consistent with the change in the mechanism of this specific chemical conversion, as theoretically considered by Wlodarczyk et al. [Phys. Chem. Chem. Phys., 2014, 16, 4694-4698]. Alternatively, it can also be connected to the change in the viscosity of the sample. Additionally, we investigated the relationship between constant rates (k) of mutarotation, structural relaxation times (τα), and dc conductivity (σdc) above and below the glass transition temperature. It was found that there was a linear correlation between all these quantities; they scale with various exponents changing at Tg. Our results also indicate that a single activation barrier might not be sufficient to describe the mutarotation process.

Glass Transition and Dynamics of the Polymer and Water in the Poly(vinylpyrrolidone)-Water Mixtures Studied by Dielectric Relaxation Spectroscopy.

In this study, broadband dielectric spectroscopy and differential scanning calorimetry (DSC) measurements are performed to study the dynamics of water and polymers in an aqueous solution of poly(vinylpyrrolidone) (PVP) with concentrations of 60, 65, and 70 wt % PVP in a temperature range of 123-298 K. Two distinct relaxation processes, l- and h-processes, which originate from the segmental chain motion of PVP and the primary relaxation process of water, respectively, are observed simultaneously. The relationship between l- and h-processes and their temperature dependences mimic those of the α-process and Johari-Goldstein β-process, which are observed in ordinal glass formers. The relaxation time of the l-process, τl, obeys the Vogel-Fulcher (VF)-type temperature dependence, and the glass-transition temperature of the l-process, Tg,l, which is defined by the temperature that is reached in a τl of 100-1000 s, shows good agreement with the calorimetric Tg obtained by DSC. The temperature dependence of the relaxation time of the h-process, τh, exhibits a crossover from VF to Arrhenius behavior at the so-called fragile-to-strong transition (FST) of water at Tg,l. The temperature dependence of the relaxation strength of the h-process, Δεh, increases with a decrease in temperature from 298 K to Tg,l. Below Tg,l, Δεh is nearly constant or slightly decreases with decreasing temperature. According to previous studies on aqueous solutions of sugars and alcohols, the Δε of the ν-process, which originates from local motion of water, decreases with decreasing temperature above the Tg of the α-process, which originates from the cooperative motion of the solute and water. Therefore, the l-process in the PVP-water mixture is not a result of the cooperative motion of PVP and water but rather a result of the polymer-polymer cooperative motion of PVP. In addition, agreement among Tg,l, the temperature of the FST of water, and calorimetric Tg suggests that the FST of water occurs at Tg.

Unambiguous Evidence for a Highly Mobile Surface Layer in Ultrathin Polymer Films by Specific Heat Spectroscopy on Blends

Despite the decade long controversial discussion on the effect of nanometer confinement on the glass transition temperature (Tg) of ultrathin polymer films, there is still no consistent picture. Here, the dynamic calorimetric glass transition of ultrathin films of a blend, which is miscible in the bulk, is directly investigated by specific heat spectroscopy. By a self-assembling process, a nanometer thick surface layer with a higher molecular mobility is formed at the polymer/air interface. By measuring the dynamic calorimetric Tg in dependence on the film thickness, it was shown that the Tg of the whole film was strongly influenced by that nanometer thick surface layer, with a lower Tg. Since the observed thickness dependence of the dynamic Tg is similar to the thickness dependence of the Tg for thin films of homopolymers, it is concluded that also for homopolymer a highly mobile surface layer is relevant for the widely observed Tg depression.

Selective determination of glass transition temperature and vibrational properties at the chain end of polystyrene by Fourier transform infrared measurement in combination with deuterium-labeling

Glass transition temperature (Tg) and vibrational properties at the chain end of polystyrene (PS) were selectively determined by a novel method utilizing Fourier transform infrared (FT-IR) spectroscopy. In this method, PSs having almost the same number average molecular weight (Mn) but selectively deuterated at different sites, midchain (M-PS, Mn = 10.7 kDa) or chain end (E-PS, Mn = 10.3 kDa), were synthesized and the asymmetric stretching vibration of the main chain CD2 (νas(CD2)) were analyzed in detail. We found that the temperature dependence of the νas(CD2) frequency shows a clear inflection at the calorimetric Tg of PS. The inflection temperature was defined as Tg,FTIR, which represents the Tg at the local region around the deuterated sites. Although higher segmental mobility at the chain end had been shown above the Tg in our earlier work, this study found that the Tg,FTIRs of the M-PS and E-PS are the same. From this result, it was concluded that the chain ends move cooperatively with numerous neighboring other segments, averaging out those mobilities at temperatures around the Tg. On the other hand, the νas(CD2) frequency of the E-PS was by a few cm−1 higher than that of the M-PS for any temperature. This was explained in terms of the discontinuity of the repeat unit at the chain end resulting in reduced interactions between repeat units along the chain.

Effect of thermal denaturation on the mechanical glass transition temperature of globular protein/co-solute systems

The work prepared high-solid mixtures of whey protein or bovine serum albumin with an amorphous co-solute (glucose syrup) and examined their glass transition behaviour at subzero temperatures. The interest in these condensed matrices was in relation to what extent thermal denaturation and subsequent aggregation of the proteinaceous molecules affects vitrification and, therefore, they were subjected to distinct heating regimes followed by cooling. Small-deformation dynamic oscillation in shear is known to respond to changes in network formation as a function of thermal treatment, albeit published reports thus far focused on low-solid aqueous hydrocolloid samples, and it has been chosen presently to examine the viscoelasticity of their high-solid counterparts. Results were further compared with those from a micromolecular technique, i.e. modulated differential scanning calorimetry. It appears that thermally induced cross-linking is readily recorded in what is known in the literature as the mechanical or network glass transition temperature, whereas the calorimetric Tg is not affected by the extent of polymeric associations in these mixtures. Further, the thermal protocol employed presently results in considerable differences in predictions of the mechanical Tg, which should reflect distinct three dimensional morphologies in these systems of globular protein and co-solute.

Segmental dynamics and physical aging of polystyrene/silver nanocomposites

We report the complicated variation trend of calorimetric Tg and physical aging in PS/Ag nanocomposites, despite the invariant segmental dynamics with increasing silver nanoparticle loading.

Why is there no clear glass transition of confined water?

To overcome the problem of crystallization of supercooled bulk water and water rich solutions we have studied water–glycerol mixtures confined in 21Å pores of the silica matrix MCM-41 C10. The results from the differential scanning calorimetry (DSC) measurements shows an almost concentration independent glass transition temperature, Tg, at about 176K for water concentrations up to 80wt%, suggesting that the confined water has no influence on Tg in this concentration range. Rather, the findings indicate that the water molecules in the solutions have a stronger preference to coordinate to the hydroxyl surface groups than the glycerol molecules, which results in a micro-phase separation of the two liquids. The water phase does not give any sign of a Tg and therefore the observed Tg should be associated with the glass transition of the glycerol phase. Finally, we discuss why the confined water does not exhibit any clear calorimetric Tg.

Locating Malleable Bulk Metallic Glasses in Zr–Ti–Cu–Al Alloys with Calorimetric Glass Transition Temperature as an Indicator

We defined the plastic deformability under constrained loading conditions as malleability for bulk metallic glass (BMG) materials. Quaternary Zr–Ti–Cu–Al alloys in the Zr-rich composition range are selected to investigate the compositional dependence of malleability assessed by bending testing and glass transition temperature (Tg). As indicated, increasing the Al or Cu concentration in the alloys leads to the rise of Tg. The Zr61 Ti2Cu25Al12 (ZT1) and Zr61.6Ti4.4Cu24Al10 (ZT3) alloys exhibit an optimal combination of lower Tg and higher glass-forming ability. The malleable BMGs such as ZT1 manifests two characters during deformation, the stable propagation of a single shear band indicated by large shear offsets and easy proliferation of shear bands. With increasing the Tg of BMG, the yield strength σy, Young's modulus and shear modulus simultaneously increase as well, while the Poisson's ratio decreases. The σy of ZT1 BMG is about 1680 MPa in compression and 1600 MPa in tension. In tensile loading, no any visible plasticity appears even when the strain rate increases up to the order of magnitude of 10−1 s−1. In consistent with the Tg, malleability of Zr–Ti–Cu–Al BMGs manifests significant compositional dependence. The malleable BMG is associated with lower Tg, as well as lower shear modulus or higher Poisson's ratio, which can be understood on the basis of the correlation of Tg with shear energy barrier in metallic glass. Thus, the calorimetric Tg can be used as an indicator to screen malleable BMG-forming composition, with advantage of experimental accessibility.

Mechanical spectra and calorimetric evaluation of gelatin–xanthan gum systems with high levels of co-solutes in the glassy state

Mechanical properties of gelatin–xanthan gum (XG) mixtures with high levels of co-solutes were examined by dynamic mechanical analysis (DMA). The mechanical spectra of the samples were modeled according to the Williams–Landel–Ferry (WLF) equation/free-volume theory, which requires an entropic lightly cross-linked network. For the α dispersion, E′ and E′′ superposed with the horizontal shift factor aT, which was temperature-dependent according to the WLF equation; no other secondary dispersion mechanism was detected. The addition of XG to gelatin networks with high levels of co-solutes changed the glass transition temperature (Tg) and kinetics of glass transition and glassy states. In the glassy state, the WLF equation was unable to follow progress in the mechanical properties, which were better described by the Andrade equation. The calorimetric measurements of the gelatin–XG systems were made using a modulated temperature differential scanning calorimetry (MTDSC) to improve the determination of Tg. The samples were exposed to two cooling and heating cycles to provide a controlled recent thermal history in the temperature range of 40 °C to −70 °C. The Tg values of the samples were determined from the second heating cycle in the reversing heat signal. The calorimetric Tg values increased with increasing glucose syrup:sucrose ratio due to increased crosslinking, whereas mechanical Tg decreased with increased XG content due to network formation.