Enthalpy Relaxation Research Articles

Relationship between Dynamic Viscoelasticity and Amorphous Structural Changes Associated with Enthalpy Relaxation in Polystyrene Injection Moldings

Relationship between Dynamic Viscoelasticity and Amorphous Structural Changes Associated with Enthalpy Relaxation in Polystyrene Injection Moldings

Fracture toughness of a rejuvenated β-Ti reinforced bulk metallic glass matrix composite

• Cryothermal cycling is performed on embrittled bulk metallic glass composite. • Plasticity of BMGC examined using nanoindentation, DSC and compression tests. • Mode I notch toughness of CTC treated BMGC is same as brittle BMGC. • Rejuvenation is hampered by chemical short range ordering of constituents in amorphous matrix. A β-Ti dendrite reinforced Zr-based bulk metallic glass composite (BMGC) was found to be brittle when cast in a large size. The reasons for the embrittlement and the effectiveness of the cryothermal cycling (CTC) treatment in restoring the mode I fracture toughness are examined. Plasticity in all the CTC treated BMGC is estimated from the distribution and occurrence of pop-ins in nanoindentation tests and by measuring the magnitude of enthalpy of relaxation (Δ H rel ) via differential scanning calorimetry (DSC). This is further validated by examining the strain-to-failure ( ε f ) in compression tests. Mode I fracture behaviour of the as-cast embrittled BMGC and the CTC treated BMGC, which exhibits maximum plasticity, is examined. Results show that both BMGCs are equally brittle and exhibit 5 times lower notch toughness ( K QJ ) than their tougher counterpart. Post-facto imaging of the side surfaces reveals the absence of notch-tip plasticity in both BMGCs. The lack of notch tip plasticity of CTC treated BMGC, despite exhibiting signatures of plasticity in nanoindentation and higher Δ H rel is rationalized by reassessing the origin of pop-ins in nanoindentation tests and describing the variations in chemical and topological short range ordering during CTC, respectively. Implications of these results in terms of improving the fracture toughness of structurally relaxed BMGCs via CTC are discussed.

Stress-induced gradient rejuvenation framework and memory effect in a metallic glass

Rejuvenation of metallic glasses (MGs) brings them in higher-energy states, which improves their mechanical properties. However, how to rejuvenate MGs through a fast and simple approach is still a challenge. In the present work, a nonuniform structure consisting of a center-relaxation region surrounded by an edge-gradient-rejuvenation framework was created in a Zr-based bulk metallic glass (BMG) by lateral elastostatic preloading. Such a nonuniform structure greatly suppresses the rapid propagation of shear bands and has the potential to overcome the inverted relationship between strength and plasticity. More interestingly, we found a stress-induced memory effect, viz., the recovery of relaxation enthalpy during elastostatic loading, which is quite different from the thermal-induced memory effect in previous works. This indicates a certain degree of equivalence between stress and temperature in the dynamics of MGs.

Electrospun Fibrous Membrane with Confined Chain Configuration: Dynamic Relaxation and Glass Transition.

Thermodynamic glass transition processes of electrospun membranes were first introduced to study their dynamic relaxation nature, which is not constantly in equilibrium. The relaxation modes of electrospun membranes are slow but measurable near and above the Tg, given the stretched chain over long distances. Based on differential scanning calorimetry (DSC) experiments and the general principle of mode-coupling theory (MCT), endothermic peak temperature and relaxation enthalpy were used to analyze the relaxation process by capturing these instantaneous “arrested” structures. The short- and long-wavelength relaxation modes could be identified with different annealing times and temperatures relative to DSC-measured Tg for electrospun membranes with different molecular weights. Results clearly showed the dynamic nature of a glass transition in polymeric materials. Tp and enthalpy loss initially increased and then directly decreased with the increase in annealing time. When Ta > Tg, regardless of the size of the molecular weight, the Tp and enthalpy loss of the PLGA fibers would directly decrease, and the curves would shift toward the melted one. Combination of electrospinningand normal DSC instrument can be used to investigating the dynamic relax process through an adequately designed kinetic scanning procedure. This result can be explained by the general principle of MCT-type dynamic theory.

Assessment of activation energy of enthalpy relaxation in sucrose-water system: effects of DSC cycle type and sample thermal history

The purpose of this study is to critically analyze different methods of calculation of activation energy of relaxation in sucrose-water system from differential scanning calorimetry data. We consider the use of different thermal cycles for calculations together with Moynihan and Kissinger equations. We study the effect of two methods of glass transition temperature determination (half-step and inflection point) on the activation energy values. Along with experimental DSC data, we use the data simulated using Tool–Narayanaswamy–Moynihan model to validate the use of cooling and heating curves and to check the reproducibility of the activation energy calculations. The obtained results show that the thermal cycle with equal cooling and heating rates provides the most reliable data set and the glass transition temperature definition using inflection point rather than half step can be recommended for calculations. Moreover, due to technical reasons, heating rather than cooling scans provide the most reliable results of activation energy calculations. Furthermore, a simple method based on the width of the glass transition region shows reasonable results for single scan experiments. The activation energies of the glass transition in sucrose-water system with different water contents and different thermal histories were studied. Since it is impossible to apply traditional methods based on Moynihan equation for the activation energy evaluation for freeze-dried samples, we propose using another method based on the properties of the recovery peak. Combining the results obtained by different methods, we present a dependence of activation energy in sucrose-water system on water content. The results show that water decreases the activation energy of relaxation process in sucrose matrix.

Effect of modifiers on bending recovery of fully degradable brush wire monofilament

In this paper, the effects of three modifiers, such as PEG, ATBC, and DOP, on the modification of PBAT/PLA-based fully degradable brush monofilaments were studied. The microstructure and the mechanical properties of the PBAT/PLA brush wire before and after modification were characterized by Fourier-transform infrared spectroscopy, scanning electron microscope, and universal material testing machine. The effect of modifiers on bending recovery was discussed in detail and was analyzed combined with the results of differential scanning calorimetry and low-field nuclear magnetic resonance spectroscopy. The results showed that with an increase in the content of modifiers, the tensile strength and the impact strength of the PBAT/PLA brush wire would decrease, and the bending recovery rate would increase. The effect of PEG and ATBC on bending recovery is weaker than that of DOP, and the bending recovery rate of the PBAT/PLA brush wire modified by DOP is close to 80%. With an increase in the content of modifiers, the melting enthalpy and transverse relaxation time T2 of PBAT/PLA would increase. This research has guiding significance for the preparation of fully degradable brush wires.

Morphologies and mechanical properties of basalt fibre processed at elevated temperature

In this study, the morphologies and properties of basalt fibre (BF) treated at different temperatures in air atmosphere were studied. The results showed that the heat treatment caused the formation of crystals on fibre surface, which resulted in the alteration of BF from the paramagnetic material to the ferromagnetic one. The tensile strength of BF was reduced significantly upon thermal treatment, and such reduction occurred in three temperature ranges: i) Decomposition of organic sizing on fibre surface, which led to the exposure of more defect structures (100–400 °C), and the structural changes of BF may also occur; ii) Simultaneous relaxation of excessive enthalpy and structural anisotropy, which were responsible for the phase separation in the amorphous matrix in the fibre (500–600 °C); iii) Structural modification due to the formation of multifarious crystals in the fibre (700–1000 °C). The crystallization in BF began with spontaneous spinel phase formation, which was favored by the initial phase separation and the oxidation of Fe2+. The spinel crystals became nucleation sites for the formation of pyroxene structure, and this process was accompanied by the enrichment of Ca, Mg and Fe on fibre surface. Therefore, the inhibition of crystallization for BF was important to maintain the mechanical performance of filament under the continuous thermal conditions for practical applications.

Fast increase in ductility and strength of Zr-based bulk amorphous alloys induced by intermittent high-frequency vibration loading

Intermittent high-frequency vibration loading is introduced into the Zr41.2Ti13.8Cu12.5Ni10Be22.5 bulk amorphous alloy as an ultrashort time, easy-to-obtain, nondestructive physical method to adjust its atomic arrangement and shear deformation behavior. It is found that the method of intermittent high-frequency vibration loading can make the ductility and strength of the bulk amorphous alloys increase quickly within 4 s, increasing to 5.3% and 2240 MPa, respectively. And there is a mechanical power threshold of approximately 0.43 kJ/mol. Apart from the as-cast sample, when it is less than this threshold, with the increase of amplitude and pre-pressure, the relaxation enthalpy of IHF-treated samples increases, the ductility increases, but the compressive yield strength is basically unchanged, and there is basically no precipitation of nanocrystals at the same time. When it is greater than the threshold, as the amplitude and pre-pressure increase, the relaxation enthalpy decreases, nanocrystals precipitate, the ductility and the compressive yield strength begin to decrease. Both the increase in free volume content and the appearance of nanocrystals will lead to an increase in the critical stress of the first ejection event, thereby simultaneously increasing the ductility and yield strength. It is also found that the increase in free volume content is the main factor for the increase in ductility and strength, and the appearance of nanocrystals is a secondary factor. Our findings provide a new ultrashort time method for overcoming the strength-ductility trade-off dilemma.

Influence of ionic liquids on the chain dynamics and enthalpy relaxation of poly(methyl methacrylate).

Imidazolium ionic liquids (ILs) with various alkyl chain lengths on the cations ([Cnmim]+, n = 2, 4 and 8) and different combined anions ([TFSI]- and [PF6]-) were blended with poly(methyl methacrylate) (PMMA), and the effects of the IL structure on the chain dynamics of PMMA were experimentally investigated by rheology and DSC measurements combined with a simulation method. The results indicate that the interaction between PMMA and ILs becomes stronger as the alkyl chain length on the imidazolium ring increases or the anion changes from [PF6]- to [TFSI]-. As a result, a higher critical entanglement concentration and a larger entanglement molecular weight of PMMA were found in [C8mim][TFSI] due to the stiffer conformation. Molecular dynamics (MD) simulations further demonstrated stronger interactions between PMMA and ILs with longer cationic alkyl chain lengths or [TFSI]- anions, which showed smaller Flory-Huggins interaction parameters and larger radii of gyration, Rg. However, the larger size of alkyl chains or [TFSI]- anions produced a larger free volume in the system as evidenced by positron annihilation lifetime spectroscopy (PALS), which competed with the molecular interaction and dominated the segmental motion. Therefore, a lower Tg and accelerated segmental relaxation were observed. Compared to alkyl chain length, the effect of anions on the interactions between ILs and PMMA is more prominent.

Structural evolution under elastic cyclic loading in a Ti-based metallic glass

Multiple characterization techniques including DSC, TEM, XRD, HV and SEM were utilized to investigate the structural evolution of metallic glass (MG) under elastic cyclic loading. It is found that the relaxation enthalpy (served as the reflection of free volume contents) in the fatigued specimen was raised by ∼5 times as compared with as-cast specimen. The cyclic loading induces the larger interatomic distance and smaller coordination number. Besides, the microhardness reduction from ∼545.6 ± 15.3 to ∼518.7 ± 4.8 was observed in the as-cast and fatigued specimens, indicating the cyclic loading induced softening behavior. Such results reveal that the elastic cyclic loading can cause the irreversible structural change with the increase of free volume. Then the key problem of shear band formation under cyclic stress level lower than yield strength was clarified by the free volume theory.

Microstructure and mechanical behavior evolution of Ti-based bulk metallic glass induced by sub-Tg isothermal annealing

In this study, the effects of sub-Tg isothermal annealing on the microstructure, mechanical and serrated flow behaviors of Ti33Zr30Cu9Ni5.5Be22.5 bulk metallic glass (BMG) were investigated. The change in free volume content of the BMG was characterized according to the structural relaxation enthalpy δHr and the onset structural relaxation temperature Tr, with the results indicating a monotonic decreasing trend of the free volume content with the extension in annealing time. A number of nanocrystals with a size of approximately 4 nm were precipitated on the glassy matrix when the annealing time was extended to 1.0 h, and then the size of these nanocrystals increased with further prolongation of the annealing time. Meanwhile, the room-temperature compressive plasticity of the Ti33Zr30Cu9Ni5.5Be22.5 BMG greatly improved from 1.4% to 4.2%, and the high strength was retained after annealing for 1.0 h, which can be largely attributed to the formation of the nanocrystals with the size of about 4 nm. With the extension of the annealing time, however, the further growth of the nanocrystals resulted in the degeneration of the compressive strength and the plasticity. The serrated rheology analysis revealed that the distribution histogram of the stress drop for the 1.0 h-annealed specimen represented a well characteristic of power-law distribution, whereas the 4.0 h-annealed showed a chaotic dynamics of jerk flow. Furthermore, the 1.0 h-annealed specimen possessed the lower average stress drop (12 MPa), formation energy per unit shear band (254.9 J/m2) and average shear band velocity (1.21 × 10−5 m s−1).

Correction to: Physical aging of multilayer polymer films—influence of layer thickness on enthalpy relaxation process, effect of confinement

Correction to: Physical aging of multilayer polymer films—influence of layer thickness on enthalpy relaxation process, effect of confinement

The evolution of the structure and mechanical properties of fully bioresorbable polymer-glass composites during degradation

Fully bioresorbable polymer matrix composites have long been considered as potential orthopaedic implant materials, however their combination of mechanical strength, stiffness, ductility and bioresorbability is also attractive for cardiac stent applications. This work investigated reinforcement of polylactide-based polymers with phosphate glasses, addressing key drawbacks of current polymer stents, and examined the often-neglected evolution of structure and mechanical properties during degradation. Incorporation of 15–30 wt% phosphate glass led to modulus increases of up to 80% under simulated body conditions, and 15 wt% glass composites retained comparable ductility to pure polymers, crucial for stent applications where ductility and stiffness are required. Two-stage degradation was observed, dominated by interfacial water absorption and glass dissolution. Polymer embrittlement mechanisms (crystallisation, enthalpy relaxation) were suppressed by glass addition, allowing composites to achieve a more controlled loss of mechanical properties during degradation, which could allow gradual transfer of loading to newly healed tissue. These results provide a valuable new system for understanding the structural and mechanical changes occurring during degradation of fully bioresorbable polymer matrix composites, providing important new data to underpin the design of effective cardiac stent materials.

Evidence for 3-D network of P-centered pyramidal P(Se1/2)3 and quasi-tetrahedral Se[dbnd]P(Se1/2)3 local structures and their 3-membered ring super structure counterparts decoupled from quasi 1D- ethylene-like P2Se2+x (x = 2,1,0) chains in PxSe100−x glasses

The molecular structure of binary PxSe100−x glasses over the 0 < x < 57% range is examined in Raman scattering, Modulated Differential Scanning Calorimetry (MDSC) and Volumetric measurements. Raman scattering and trends in glass transition temperature Tg(x), provide evidence of two network backbones (BB) that are decoupled from each other, with Tg in the 0 < x < 40% range fixed by BB1 and in the 40%< x < 54% range by BB2. The 3D Backbone, BB1, is composed of Sen chains cross-linked by P-centered pyramidal (PSe1/2)3 units and Quasi-Tetrahedral [Se-P(Se1/2) units and their 3-membered Pyramidal [(PSe1/2)3]3 ring, and Quasi-Tetrahedral [Se-P(Se1/2)3]3 ring counterparts. Backbone BB2, is composed of quasi-1D Ethylene-like P2Se2+x (x = 2,1,0) chains forming in the approximate 20%< x < 54% range. The enthalpy of relaxation at Tg shows a square-well like variation in the 28%< x < 40% range determined by BB1, thus fixing the Flexible- and Intermediate- phase (IP). Compositional trends in melt Fragility index, m(x), show anomalously low value of 13.1 at x = 15% and of 7.7 at x = 34%. These minima in m(x) coincide with minima in the liquidus near the same compositions x and are traced respectively to the formation of the 3-membered rings and the decoupling of BB1 from BB2.

Relationship between Molecular Orientation Relaxation during Physical Aging and Heat Resistance of Polystyrene Injection Moldings

Abstract This work examined the relationship between changes in molecular orientation and the heat resistance (heat distortion temperature) of polystyrene injection moldings following heat treatment below the glass transition temperature. Molecular orientation around the core layer of the injection moldings was found to be relaxed by the heat treatment. Also, in the untreated specimen, the molecular orientation around the core layer was relaxed from 60°C during the heating process. Since loss tangent (tanδ) also increased from 60°C during the heating process in the untreated specimen, it was considered that the increase in tanδ occurred with the molecular motion due to the relaxation of molecular orientation from 60°C. After the heat treatment, because of the relaxation of molecular orientation around the core layer by the heat treatment, the relaxation of molecular orientation from 60 °C did not occur during the subsequent heating process, and the tanδ of the polymer between 60 and 90 °C was decreased. Because this decrease in the tanδ over this temperature range improved the heat resistance of the material, the enhanced heat resistance by the heat treatment was attributed to the suppression of the relaxation of molecular orientation from 60°C during the heating process. Furthermore, relaxation of molecular orientation and enthalpy relaxation were related to improvement in the heat resistance.

Analysis of molecular mobility in corn and quinoa flours through 1H NMR and its relationship with water distribution, glass transition and enthalpy relaxation

Solids-water interactions of corn and quinoa flours were evaluated through 1H NMR, DSC, and water sorption isotherms. Glass transition temperature (Tg), observed by DSC, was better distinguished through FID signals, and correlated to water content through the Gordon and Taylor model. Enthalpy relaxations, identified by thermal analysis at 50–70 °C were studied through transverse relaxation times (T2) measured after Hahn spin-echo sequence, which revealed a rearrangement of the biopolymers structures that cause immobilization of polymer chains and reduced mobility of water molecules with weak interactions with solids (lower T22). The higher lipid content of quinoa flour was manifested after the CPMG sequence (T2 ≈ 100 ms) and caused reduced hygroscopicity and Tg values compared with corn flour systems. 1H NMR resulted efficient for assigning proton populations and understanding the changes in their distribution with temperature, analyzing glass transition and interpreting the implications of enthalpy relaxations processes in corn and quinoa flours.

Comparative characterization of gum rosins for their use as sustainable additives in polymeric matrices

AbstractThere is a growing interest in the use of non‐polluting compounds, which come from renewable sources, and which performance in their scope is equivalent to their synthetic similes. In this work, five types of rosins from different sources were studied, verifying the existence of differences that can be inferred in their subsequent use and application as material additives. For the study, rosins were analyzed using gas‐mass chromatographic techniques, infrared spectrophotometry (FTIR), differential scanning calorimetry (DSC), thermogravimetric analysis (TGA), and color characterization. The results showed that the samples are composed of either abietic acid or by its structural isomers in contents higher than 80%. FTIR shows that the main difference in the gum rosins is related to the proclivity to absorb environmental moisture and that this technique is not enough to differentiate them. Moreover, the DSC reveals that the gum rosins present enthalpy relaxation effects due to their manufacturing process. The TGA showed that gum rosins are thermally stable until 200°C, therefore they can be successfully blended with thermoplastic polymers. Finally, the color characterization shows little differences between the samples, being CA the gum rosin with the greatest total color differences.

Thermal behavior of newly developed Zr33Hf8Ti6Cu32Ni10Co5Al6 high-entropy bulk metallic glass

In this study, new non-equiatomic high entropy bulk metallic glass (HEBMG) Zr33Hf8Ti6Cu32Ni10Co5Al6 was developed according to the Cu47Zr47Al6 prototype BMG. Kinetic studies were performed with non-isothermal DSC for the BMG and the HEBMG, and their thermal properties were compared. Calculation of activation energies by different methods indicated that Eg is higher for the HEBMG than the BMG, while Ep is lower for the HEBMG than the BMG, and Ex is similar for both alloys. The values of structural relaxation enthalpy for the BMG and the HEBMG were 4.95 J/g and 8.19 J/g, respectively. Free volume was calculated at room temperature and Tg. By doing so, the direct correlation between fragility, structural relaxation enthalpy, diffusion, and free volume was shown. Using a compression test at room temperature, a 58% improvement in plastic strain was obtained for HEBMG, which confirmed the calculations.

Strain aging in metallic glasses

We investigate the evolution of strain and viscosity in metallic glasses during the cyclic tensile creep experiments at a constant temperature. The effects of cyclic creep result in increasing viscosity and structural relaxation, which are attributed to the shrinkage and annihilation of flow units. We further confirm the strain deformation effect via β-relaxation and relaxation enthalpy measurements. We find that the strain deformation also has the dramatic negative effect on the β-relaxation behavior and relaxation enthalpy of metallic glass, and leads to an aging phenomenon. The results indicate that the deformation accelerate the structural relaxation which is closely correlated with the inhomogeneous microstructure and the potential defects of flow units in metallic glasses.

Role of modifiers in the structural interpretation of the glass transition behavior in MgO/BaO-Al2O3-P2O5 glasses

Thermomechanical analysis, differential scanning calorimetry and Raman spectroscopy were used to study the role of the modifying oxides on the glass transition behavior of the MgO/BaO-Al2O3-P2O5 glasses. In case of the volumetric changes associated with the glass-liquid transformation, a complex description combining the effects of viscosity, thermal expansion and structural relaxation was successfully applied to the thermomechanical data based on non-linear optimization of the involved variables. For the enthalpy relaxation data, a novel simulation-comparative method was developed to accurately determine the kinetic parameters of the Tool-Narayanaswamy-Moynihan relaxation model, overcoming the difficulties associated with the commonly encountered instrumental distortions of the experimental data. In most cases, the modifying oxides had the dominant role, as compared to the primarily cross-linking effect of the Al2O3 oxide, in determination of the physico-chemical properties of the studied glasses. Differences between the magnesium and barium phosphates were consistent with the concept of larger size Ba2+ ions leading to the higher disorganization and diversification of the phosphate glass matrix. Compositional trends in the measured quantities were correlated with the representation of the particular structural units occurring in the MgO/BaO-Al2O3-P2O5 glasses.