Kovacs effect in PMMA observed by low-frequency Raman scattering (boson peak)

The physical aging of the poly(methyl methacrylate) glass (PMMA) is described by its effects on the refractive index, that reflects the change of mean specific volume, and, on the other hand, on the low-frequency Raman scattering, i.e., on the boson peak. The boson peak depends mainly on the cohesion fluctuations. The memory or so-called Kovacs effect is observed by the appearance of a minimum of refractive index (i.e. a maximum of volume) as a function of the aging time at a higher temperature subsequent to an aging at lower temperature. The minimum of refractive index corresponds to a maximum of the boson peak intensity. However, the cohesion is not directly related to the volume, so that the evolution of the refractive index does not mimic exactly that of the boson peak intensity. Information on the change of cohesion by aging is obtained by enthalpy measurement. The obtained experimental results are discussed in the frame of the heterogeneous cohesion or elasticity at the nanometric scale. This clarifies the phenomenon of physical aging and the consequent Kovacs effect.     

Physical aging in PMMA/silica nanocomposites: Enthalpy and dielectric relaxation

We have investigated the physical aging below the glass transition temperature, namely the slow evolution occurring in non-equilibrium glasses, of poly(methyl methacrylate)/silica (PMMA/silica) nanocomposites. To do so we have followed the time evolution of the enthalpy and that of the dielectric strength of PMMA β process during isothermal annealing. The results indicate that physical aging is generally accelerated in all nanocomposites in comparison to pure PMMA, despite the lack of effect of the nanoparticles on PMMA molecular dynamics. Furthermore, the shorter is the interparticle distance, and hence the higher is the area/volume of silica in PMMA, the more pronounced is such acceleration. The acceleration of the physical aging together with the invariance of PMMA dynamics in the nanocomposites in comparison to pure PMMA poses serious questions on the idea that the molecular mobility is the only responsible parameter for the rapidity of physical aging. Thus, an interpretation based on the free volume holes diffusion towards the external surface, in this case represented by the polymer/silica interface, is provided.     

Formation and stability of amorphous molecular systems: As the prototype of functional amorphous organic materials

Structural changes in vapor-deposited amorphous states of simple organic compounds were studied by Raman scattering, X-ray diffraction, and light interference in the sample. Two types of processes, namely direct crystallization and structural relaxation leading to glass transition, were observed depending on the flexibility of the molecule. Gradual relaxations were also found to occur in amorphous states of butyronitrile and 1,2-dichloroethane at temperatures much lower than their T_g or crystallization temperature. The observed structural changes are discussed by referring to the high enthalpy of the vapor-deposited amorphous systems.     

Polyamorphous transition in amorphous fullerites C<Subscript>70</Subscript>

Samples of amorphous fullerites C₇₀ have been obtained by mechanical activation (grinding in a ball mill). The structure of the samples has been investigated by neutron and X-ray diffraction. The high-temperature (up to 1200°C) annealing of amorphous fullerites revealed a polyamorphous transition from molecular to atomic glass, which is accompanied by the disappearance of fullerene halos at small scattering angles. Possible structural versions of the high-temperature amorphous phase are discussed.     

Low-energy excitations in polyvinyl chloride: Raman scattering and thermal properties

The low-energy excitations of a very fragile glass-former (in the sense of Angell), poly(vinyl chloride), have been studied by low-frequency Raman scattering and by measurements of the low-temperature heat capacity and thermal conductivity. Two different samples were investigated : one with a crystallinity of about 15%, the other quenched and amorphous, as measured by small-angle neutron scattering. The boson peak in Raman scattering was observed in both samples even at room temperature. A clear correspondance between the Raman boson peak, the excess of heat capacity and the plateau of thermal conductivity was shown. It is confirmed that the boson peak or the heat capacity excess are relatively small in this very fragile glass-former. However it is deduced that the high concentration of tunnelling systems in the amorphous sample is the result of a rapid quenching rather than an intrinsic property of fragile glass-formers.     

Physical aging effect on the boson peak and heterogeneous nanostructure of a silicate glass

The effect of physical aging on a silicate glass has been investigated by low-frequency Raman scattering. It was observed that the low-frequency side of the excess of Raman scattering, or boson peak, due to harmonic vibration modes decreases in intensity with the thermal annealing (aging) at a temperature lower than the glass transition temperature (T_g), after quenching (rejuvenation) from a temperature higher than T_g. Moreover, it was found that the lowering of the very low-frequency scattering mainly due to anharmonic modes becomes more pronounced when the aging temperature is decreased. These observations are interpreted in the frame of the energy landscape, and by considering the model of the glass heterogeneous cohesion at the nanometric scale.     

The structure of phosphate glass evidenced by small angle X-ray scattering

Numerous phosphate glass systems with compositions covering the entire glass forming range have been examined by small angle X-ray scattering. The experiments were carried out in a widely extended interval of s values between 0.055 and 31 nm⁻¹. A small angle scattering effect was detected for all samples investigated with the exception of the aluminium metaphosphate glass. The small angle scattering recorded indicates the presence of two differently-sized heterogeneity regions of electron density. The small-scale heterogeneities are present in magnesium and zinc phosphate glasses only. Their size is about 1 nm diameter in the magnesium and about 2 nm in the zinc phosphate glasses. The species do not result from liquid–liquid phase separation. Examinations to extract information on the nature of these heterogeneity regions are described in detail. There is no fundamental knowledge on the large-scale heterogeneities established. The small angle scattering suggests that their occurrence is related to water contamination. Anomalous small angle X-ray scattering experiments of a series of zinc polyphosphate glasses and a strontium metaphosphate glass sample are performed to examine the distribution of the Zn or Sr atoms, respectively.     

Wide and small angle X-ray scattering measurements for expanded fluid Se accompanying the semiconductor–metal and metal–nonmetal transition

Wide and small angle X-ray scattering measurements for supercritical fluid Se in the wide density region from liquid to dense vapor have been carried out using synchrotron radiation at SPring-8. The large background for the small angle spectra with the previous high-pressure vessel was much suppressed using a new vessel with a large diamond window. We carefully measured wide angle X-ray scattering near the critical density and obtained the structure factor, S(k), and the pair distribution functions, g(r). By small angle X-ray scattering measurements, we succeeded in observing a medium-range fluctuation in the semiconductor–metal transition in expanded fluid Se.     

Enthalpy recovery, creep and creep-recovery measurements during physical aging of amorphous selenium

The physical aging behavior of amorphous selenium has been investigated using differential scanning calorimetry and conventional and interrupted creep experiments. As a result of physical aging, enthalpy decreases and the creep and recovery curves shift to longer times. The times required to reach equilibrium for enthalpy recovery and creep appear to have different temperature dependences resulting in enthalpy reaching equilibrium before creep at aging temperatures a few degrees below the nominal glass temperature. In the nominal glass transformation range, however, the times required to reach equilibrium are approximately the same. A general picture of aging behavior has emerged from our data on selenium coupled with past work on polyetherimide and polystyrene.     

Influence of CoO addition on phase separation and crystallization of glasses of the ZnO–Al2O3–SiO2–TiO2 system

The evolution of structure, phase composition and spectroscopic properties of CoO-doped (up to 5 mol%) titania-containing zinc aluminosilicate glasses with their heat-treatment has been studied using Raman scattering, small angle X-ray scattering, X-ray diffraction analysis and optical absorption spectra. Addition of cobalt oxide was observed to facilitate amorphous phase separation of the parent glass and gahnite, ZnAl₂O₄, crystallization. Cobalt oxide entered phases formed during low-temperature heat-treatments (720 °C), i.e., amorphous phase, enriched in ZnO, Al₂O₃ and TiO₂ and crystalline phase of gahnite. The absorption of these glass-ceramics was defined mainly by tetrahedral Co²⁺ ions located in gahnite nanocrystals. As the temperature was increased further, traces of anosovite solid solution appeared and then decomposed. Even after high-temperature heat-treatments, a certain portion of Co²⁺ ions remained in amorphous zinc aluminotitanate phase and in octahedral sites of inversed gahnite spinel. In glass-ceramics, the residual high silica amorphous phase contained a small quantity of [TiO₄] centers, which content was smaller in Co:ZAS samples as compared with non-doped glass-ceramics.     

The effect of structural relaxation on the local structure of Cu40Zr60 amorphous alloys

Structural relaxation phenomena associated with heat treatments near the glass transition temperature range were investigated in liquid-quenched amorphous Cu₄₀Zr₆₀ alloys by means of differential scanning calorimetry (DSC), small angle X-ray scattering (SAXS) and EXAFS. A kinetic study was performed by DSC in order to determine the evolution of the configurational enthalpy ($\text{3 H}{}_{\mathrm{\sigma }}$) against temperature (upon continuous heating or during isothermal annealing treatments) and to define the heat treatment suitable to obtain highly relaxed samples.SAXS results show that the structure of the relaxed samples remains homogeneous at the intermediate range of order up to at least 100 Å, whatever be the previous heat treatment. EXAFS detects a slight change in the local structure only for the most highly relaxed samples; this change could be interpreted by a variation of 0.5 atom in the coordination numbers (probably of the ZrZr pairs) or by a change of the disorder parameter of about 0.01 Å.No phase separation is detected in the relaxed amorphous Cu₄₀Zr₆₀ specimens.     

Differential anomalous X-ray scattering studies of amorphous Cd59As41 and Cd26As74

The local structure of amorphous cadmium arsenide semiconducting films has been studied by differential anomalous X-ray scattering. Intensity measurements were carried out on two samples, containing 41 and 74 at.% As, in the vicinity of the absorption K edges of both constituents using synchrotron radiation. The computational procedure, similar to that proposed by Warren for an amorphous sample with more than one kind of atom, was applied to obtain the structural parameters from the experimental data. It has been found that atoms in the amorphous CdAs films remain almost tetrahedrally coordinated and that the investigated films are chemically ordered. The structural changes going from cadmium- to arsenic-rich composition have been revealed. The differential anomalous X-ray scattering technique proved to be effective, providing the evidence for the CdCd and AsAs near neighbour correlations in Cd₅₉As₄₁ and Cd₂₆As₇₄, respectively. The simulations of the differential radial distribution functions have shown that for the amorphous film containing 41 at.% As the distorted tetrahedral structure, intermediate between the CdAs and Si III type structures, is adequate to account for the experimental data. At 74 at.% As, the atomic arrangement can be described satisfactorily by the structural model based on the tetragonal CdAs₂ structure. The structural parameters obtained from the present study and those previously derived using the extended X-ray absorption fine structure and conventional large angle X-ray scattering techniques are compared.     

AIT玻璃衬底非晶硅薄膜的固相晶化研究

本实验在铝诱导织构的基础上,对以AIT浮法玻璃为衬底沉积非晶硅薄膜固相晶化进行了初步研究.采用拉曼散射、X射线衍射等手段对生成多晶硅薄膜的结构和光学性能进行了表征和分析.研究结果表明:热处理10h,薄膜的晶化率达到80;以上,同时具有良好的(111)择优取向;同平板玻璃衬底对比,AIT玻璃上制备的多晶硅薄膜具有良好的陷光作用.     

X-ray scattering studies of polymeric zirconium species in aqueous xerogels

X-ray scattering in the small angle range was used to investigate the structure and size of the polymeric species present in zirconium acetate (ZrAc)-based gel powders (GP) and thin films (TF). The formation of amorphous zirconia aggregates/linear polymers was studied by measuring their gyration radii and their correlation and hydrodynamic lengths via Guinier, `longrods', Zimm and Porod plots. Scattering data obtained in this investigation suggest that both amorphous ZrAc-based GP and TF contain `tetrameric' units, arranged in a cylindrical fashion, that polymerize upon thermal treatment. Housdorff fractal dimensionality, D, of amorphous zirconia GP (D∼2.25) and TF (D∼2.5) indicates a diffusion-limited aggregation mechanism. Tricyclic cluster aggregates (Porod D∼1.85) with a M_w∼12 000 were found within amorphous ZrAc-based GP. This structure is preserved over the entire curing range. In contrast, a linear polymeric chain (Porod D∼1.52), with a M_w∼20 000 below 165 °C and ∼13 000 at higher temperatures, was suggested for amorphous ZrAc-based TF.     

From small molecules to polymers: Relaxation behavior of n-butyl methacrylate based systems

Calorimetric and dielectric data for a series of n-butyl methacrylate (nBMA) based systems are presented. Dynamic glass transition (α) and secondary relations (β, β′, γ) are studied for monomer, dimer, oligomers as well as polymers with a narrow molecular weight distribution. Fragility m and cooperativity N_α increase with decreasing degree of polymerization P indicating that the cooperative character of the α motions increases. All glass transition parameters change smoothly with P without clear peculiarities that would indicate a transition from small molecules to polymers. Relations between α dynamics and the different secondary relaxations in these series are discussed. The results are compared with the predictions of different approaches aimed to understand the nature of the dynamic glass transition and the interaction between localized and cooperative dynamics in glass-forming liquids.     

Brillouin light scattering study of polymeric glassy sulfur

We present here a study of the acoustic dynamics of polymeric glassy sulfur. The data have been collected in the GHz frequency range by means of Brillouin light scattering for different scattering geometries. The choice of the experimental setup allows us to obtain information on the refractive index, n, that comes out to be close to that corresponding to the high temperature polymeric liquid phase. The longitudinal acoustic excitations have been investigated as a function of temperature from deep into the glassy state up to the glass transition temperature. The temperature dependence of the sound velocity is compared to the one measured in the liquid phase. We find that the sound velocity of the glass can be linearly extrapolated from that of the polymeric liquid measured in the THz frequency range with inelastic X-ray scattering.     

Broadband terahertz time-domain spectroscopy and low-frequency Raman scattering of glassy polymers: Boson peak of PMMA

The low-energy vibrational properties of polymethyl methacrylate (PMMA) were studied by terahertz time-domain spectroscopy and Raman scattering. The real and imaginary parts of a complex dielectric constant were accurately determined in the frequency range from 0.2 to 5.0 THz. The imaginary part of the dielectric constant shows a line-shape similar to the imaginary part of the Raman susceptibility measured by Raman scattering. These two spectra show broad peaks at about 2.3 THz. We also separately determined the Raman and far-infrared light-vibration coupling constants of the PMMA using the vibrational density of states determined by cold neutron inelastic scattering in the literature.     

Morphological fluctuations in discharge-produced μc-Si:H studied by small angle X-ray scattering

Structural inhomogeneities of μc-Si:H in the range 20≈500 A are observed for the first time by small angle X-ray scattering. Inhomogeneous distributions of bonded hydrogen in the amorphous tissue are suggested to be a possible origin of the SAXS yield. Such inhomogeneities can be reduced when the average grain size and volume fraction of microcrystallites are simultaneously increased.     

Structure formation in liquid and amorphous metallic alloys

Bulk metallic glasses developed in last 15 years represent a new class of amorphous metallic alloys. These multi-component metallic alloys can be obtained at relatively low cooling rates, which allow the production of large-scale materials by conventional casting processes. Furthermore, bulk metallic glasses show a glass transition well below the crystallization temperature enabling hot deformation, but also to investigate the glass transition phenomenon in a metallic system. The thermal behavior of Zr- and Pd-based bulk metallic glasses was studied by in situ X-ray diffraction at elevated temperatures. The temperature dependence of the X-ray structure factor of the glassy state can be well described by the Debye theory. At the caloric glass transition the temperature dependence of the structure alters, pointing to a continuous development of structural changes in the liquid state. The short-range order of the glass, of the super-cooled liquid, and of the equilibrium melt is found to be very similar. The existence of complex chemically ordered clusters in the melt is supposed to be related to the high glass-forming ability of the alloys. The microstructure of metallic glasses consisting of elements with negative enthalpy of mixing is homogeneous at dimensions above 1 nm. Phase separation in the liquid state appears in metallic systems with large positive enthalpy of mixing of the elements like Nb–Y. Thermodynamic calculations of the Ni–Nb–Y phase diagram show that the miscibility gap of the monotectic binary Nb–Y system extends into the ternary up to large Ni content. Experimental evidence of the phase separation in ternary Ni–Nb–Y melts is obtained by in situ X-ray diffraction at elevated temperatures and differential scanning calorimetry. The phase separated melt can be frozen into a two-phase amorphous metallic alloy by rapid quenching from the liquid. The microstructure depends on the chemical composition and consists of two amorphous regions, one Nb-enriched and the other Y-enriched, with a size distribution from several nanometers up to micrometer dimension. The experimental results confirm the close relationship between the structure of metallic glasses and the corresponding under-cooled liquids.     

An anomalous X-ray structural study of an amorphous La55Al25Ni20 alloy with a wide supercooled liquid region

The atomic structures of amorphous La₅₅Al₂₅Ni₂₀ alloys which have a wide supercooled liquid region and a high reduced glass transition temperature have been studied using anomalous X-ray scattering (AXS) at the Ni K-absorption edge as well as the ordinary X-ray diffraction with Mo K radiation. The interference functions and the radial distribution functions were determined for the amorphous alloys as-quenched and after annealing at various temperatures and also for a fully crystallized sample. These systematic structural studies revealed a drastic change in Ni environment upon crystallization. The need for such atomic rearrangements around the Ni atoms during crystallization may be the reason why the amorphous phase is thermally stable.