Effect of physical ageing in thin glassy polymer films

We have studied the effect of physical ageing in thin supported glassy polystyrene films by using ellipsometry to detect overshooting in the expansivity-temperature curve upon heating of aged samples. Films with thickness 10-200 nm have been aged at 70^° C and 80^° C (below the bulk glass transition temperature). We observe clear relaxation peaks in the expansivity-temperature curve for films thicker than 18 nm but not for the 10 nm film. The intensity of the relaxation peak is inversely proportional to the film thickness, while the temperatures characteristic to the relaxation peak are almost independent of the film thickness. These observations are successfully interpreted by the idea that the surface layer of the order of 10 nm has liquid-like thermal properties. Received 28 October 2002 / Published online: 1 April 2003 RID="a" ID="a"Present address: Yokohama Research Center, Mitsubishi Chemical Corporation, 1000 Kamoshida-chou, Aoba-ku, Yokohama 227-8502, Japan; e-mail: kawana@rc.m-kagaku.co.jp     

Relaxation times and energy barriers of rubbing-induced birefringence in glass-forming polymers

The relaxations of rubbing-induced birefringence (RIB) in several glass-forming polymers, including polycarbonate and polystyrene (PS) derivatives with various modifications to the phenyl ring side group, are studied. Significant relaxations of RIB are observed at temperatures well below the glass transition temperature T _g . The relaxation times span a wide range from ∼ 10 s to probably geological time scale. Physical aging effects are absent in the RIB relaxations. The model proposed for the interpretation of RIB in PS describes well the RIB relaxations in all the polymers investigated here. The energy barriers are of the order of a few hundred kJ/mol and decrease with decreasing temperature, in opposition to the trend of Vogel-Fulcher form for polymer segmental relaxations above T _g . The relaxation behaviors of different polymers are qualitatively similar but somewhat different in quantitative details, such as in the values of the saturated birefringence, the shape of the initial barrier density distribution functions, the rates of barrier decrease with decreasing temperature, and the dependence of relaxation times on temperature and parameter , etc. The RIB relaxations are different from any of the other relaxations below T _g that have been reported in the literature, such as dielectric relaxations or optical probe relaxations. A microscopic model for the relaxations of RIB is much desired.     

Anisotropy analysis of the surface stress and surface energy in Cu surfaces with the modified embedded atom method

Taking Cu as an example, the surface stress and surface energy in three low index surfaces and two families of representative surfaces and belong to [0 0 1]- and -rotating axis respectively, have been calculated using MEAM. For the three low index surfaces, the decrease in the surface energy is small after relaxation, while the surface stresses in the surface planes τ_xx and τ_yy show opposite changes (decreasing and increasing) for inward and outward relaxations. The resulting relaxation direction is related to the normal stress τ_zz before relaxation. For the surfaces of the and families, with the increasing angle α (between the and (1 0 0) planes, and between and (0 0 1) planes, respectively), the surface stress and surface energy go through an oscillatory change. The surface stress and surface energy are symmetric about the planes (1 0 0), (1 1 0) and (0 1 0) at α=0^°, 45^° and 90^°, and about the planes (0 0 1) and (1 1 0) at α=0^° and 90^° respectively, due to crystal symmetry.     

The absence of physical aging effects on the relaxation of rubbing-induced birefringence in polystyrene

We found, through extensive experimental studies, that the physical aging effects are absent in the relaxation of rubbing-induced birefringence (RIB) in polystyrene (PS), and the relaxation involves very small length scale. A phenomenological model based on individual birefringence elements is proposed for the RIB relaxation. The relaxation times (RTs) of the elements are found to be independent of the thermal or stress history of the samples, either before or after the formation of the birefringence. The RTs are also independent of the molecular weight, rubbing conditions, and film thickness, while the RTs distribution function does depend on the molecular weight and rubbing conditions. The model provides quantitative interpretations that agree very well with all the reported experimental results, and sheds important light on the novel behaviors of the RIB relaxation. The absence of physical aging effects is probably due to the combined effects of small length scale of the RIB relaxation, and the accelerated aging speed in the near surface region in which the RIB concentrates.     

SiC film growth on Si(111) by supersonic beams of C 60

The development of electronic devices based on Silicon Carbide (SiC) has been strongly limited by the difficulties in growing high quality crystalline bulk materials and films. We have recently elaborated a new technique for the synthesis of SiC on clean Si substrates by means of supersonic beams of C₆₀: the electronic and structural properties of the film can be controlled by monitoring the beam parameters, i.e. flux and particles energy and aggregation state. SiC films were grown in Ultra High Vacuum on Si(111)-7×7, at substrates temperatures of 800 ^° C, using two different supersonic beams of C₆₀: He and H₂ have been used as seeding gases, leading to particles energy of 5 eV and 20 eV, respectively. Surface characterisation was done in situ by Auger and X-Ray photoelectron spectroscopy, as well as by low energy electron diffraction and ex situ by atomic force microscopy technique. SiC films exhibited good structural and electronic properties, with presence of defects different from the typical triangular voids. Received 20 November 2001     

Structural relaxation investigations of glassy polystyrene by radial distribution functions

The structural relaxation of an atactic polystyrene under sub-T _g annealing at 50°C and at 70°C was investigated by the radial distribution function (RDF) derived from its wide-angle x-ray scattering (WAXS). By recording the changes in RDF (i.e., ΔH(r), where r is the radial distance from an arbitrary reference atom) after it had been annealed for a certain period of days, and taking the integration of ΔAH(r) ²dr, we found that the atom density within structural domains in a size below 15 Å was changed dramatically, but that the tendency leveled off as annealing proceeded. However, the atom density outside the domains is barely changed by sub-T _g annealing. The size of the domain is similar to the statistical segment length reported in the literature. The behavior of the domains—that the segmental relaxation inside the domains in the initial sub-T _g annealing is unconstrained by their neighbors outside the domains—is also similar to the behavior of the statistical segments. The domains are believed to be composed of a statistical segment across the center, which has about 6 styrene repeat units, and 6 equal-distanced parallel segments tangent to the edge of the domain. On the other hand, as the annealing temperature is closer to the glass transition temperature, the frozen unstable chain segments reach equilibrium sooner and with less disturbance in their conformation.     

Mass-transport driven by surface instabilities in metals under reactive plasma/ion beam treatment at moderate temperature

This paper presents a generalized approach to the mechanisms of oxidation, hydrogenation and nitriding of metals under ion irradiation with reactive particles at elevated temperatures. Experimental results on the plasma oxidation of bilayered Y/Zr films, the plasma hydrogenation of Mg films and the ion beam (1.2 keV N ₂ ⁺ ) nitriding of stainless steel are presented and discussed. We make special emphasis on the analysis of surface effects and their role in the initiation of mixing of bilayered films, the ingress of reactive species in the bulk and the restructuring of the surface layers. It is suggested that primary processes driving reactive atoms from the surface into the bulk are surface instabilities induced by thermal and ballistic surface atom relocations under reactive adsorption and ion irradiation, respectively. The diffusion of adatoms and vacancies, at temperature when they become mobile, provide the means to relax the surface energy. It is recognized that the stabilizing effect of surface adatom diffusion is significant at temperatures below 300–350°C. As the temperature increases, the role of surface adatom diffusion decreases and processes in the bulk become dominant. The atoms of subsurface monolayers occupy energetically favorable sites on the surface, and result in reduced surface energy.     

Thermal stability and diffusion processes in Mo x Si y /Si multilayers studied with high-resolution RBS

Mo _x Si _y /Si multilayers with a period thickness of ∼7.5 nm and bilayers Mo _x Si _y /Si have been fabricated by e⁻-beam evaporation in UHV at a deposition temperature of 150°C [1]. The composition of the as-deposited layer systems and changes in the composition after baking the samples have been studied with high-resolution RBS. For a multilayer with a mixing ratioy/x≃2, no interdiffusion is observed up to a baking temperature of 830°C. For samples with a mixing ratioy/x≃1, diffusion is observed up to a baking temperature of 630°C, resulting in a mixing ratio close toy/x≃2. This mixing ratio remains almost stable up to ∼830°C, and considerable interdiffusion is only observed in those systems where regions with a mixing ratio smaller than 2 still exist. Possible reasons for the high thermal stability of the samples are the lack of a concentration gradient for Si in the system and/or the crystallization of MoSi₂.     

Enhancement of second harmonic intensity in thermally poled ferroelectric nanocrystallized glasses in the BaO-TiO2-SiO2 system

The transparent nanocrystallized (heat-treatment: 750 ^°C, 1 h) glasses consisting of ferroelectric Ba₂TiSi₂O₈ nanocrystals (size: 100-200 nm) have been prepared in 40BaO-20TiO₂-40SiO₂ glass, and the effect of a thermal poling (DC electric voltage: 8.8 kV/cm, temperature: 110-300 ^°C, time: 1 h) on the second harmonic (SH) intensity has been examined. It is found that the formation behavior of nanocrystals at the surface differs from that in the interior of glass, giving a new insight into the well-known concept for nanocrystallization or homogeneous nucleation in glass. The Maker fringe patterns with fine structures are observed, indicating a high orientation of the polarization axes of Ba₂TiSi₂O₈ crystals formed at the surface. The prominent enhancement in the SH intensity is observed due to thermal poling, demonstrating that thermal poling is an effective method in enhancing anisotropic polarization of ferroelectric Ba₂TiSi₂O₈ nanocrystals in crystallized glasses. The present study proposes that the Maker fringe pattern for SH intensity of nanocrystallized glasses is very sensitive to the anisotropic polarization of nanocrystals at the surface, indicating the importance of the Maker fringe technique for the characterization of nanocrystals in materials.     

TSC study of electric dipole relaxations in chlorapatite

Electric dipole relaxations in chlorapatite, Ca₅(PO₄)₃Cl, have been studied with the fractional polarization mode of the thermally stimulated currents (TSC) method. Fifty-one of the fifty-seven sets of data obtained in the range 10–443°K fell naturally into four groups yielding compensation temperatures T_C of T_C1, = 202°C, T_C2: = 202°C, T_C3 = 420°C and T_C4= 644°C, with estimated error < 10°C, and characteristic relaxation times τ_C of τ_C1 = 1.3 × 10^?7s, τ_C2 = 3.2 × 10^?6s, τ_C3 = 8.8 × 10^?5s and τ_C4 = 2.3 × 10^?4s. Atomic-scale physical models involving Cl^? ion motion are offered for the 202°C compensation at the temperature of the reported monoclinic-to-hexagonal phase transition and for the 420°C compensation, at which temperature the Cl^? ions individually are thought to have enough thermal energy to maintain the hexagonal form dynamically.     

NMR at single crystal surfaces

15 sites on 1 cm². To overcome this for the important class of alkali adsorbates on metals and semiconductors, two methods are presented. Common to both is the preparation of a highly nuclear spin-polarized atomic beam of ⁶Li in the one case and ⁸Li in the other. The latter isotope is radioactive and undergoes a β-decay with a half-life of 0.84 s. Li adsorbed on the close-packed Ru(001) surface is investigated. The longitudinal relaxation time, T₁, is the main observable and is used to deduce the local electronic density of states [LDOS(E_F,r=0)] and Li diffusion barriers. The second experiment uses ⁶Li as an adsorbate, also studied on Ru(001). The nuclear polarization is measured by beam foil spectroscopy. A novel particle detected (photon counting) Fourier transform NMR technique is demonstrated. This is done by observing the time-dependent flux of circularly polarized light emitted behind the foil after a 90° pulse has been employed at the surface. Electric field gradients and transverse relaxation times, T₂, are thus determined. A large difference between T₁ and T₂ is traced to the dimensionality of the system. Received: 21 March 1997/Accepted: 12 August 1997     

Microphase separation in weakly charged hydrophobic polyelectrolytes

Aqueous solutions of a well-defined poly(N-isopropylacrylamide-co-sodium 2-acrylamido-methylpropanesulfonate) (NIPAM/NaAMPS in a 95/5 molar ratio) have been investigated by means of small-angle neutron scattering (SANS) and rheological experiments as a function of temperature ( 25^° C T 60^° C) and polymer concentration ( 0.5wt% C 12wt%). The solutions remain optically transparent and isotropic over the whole temperature range, in contrast with the homopolyNIPAM which precipitates above its lower critical solution temperature (LCST = 32^° C). Upon addition of salt, the systems undergo a micro-macrophase separation. At temperatures above 45^° C, the SANS spectra exhibit a sharp peak at a scattering wave vector, q _max, which increases slightly with temperature. At high temperature ( T∼ 60^° C), the scattered intensity follows a power law I(q) ∼q ^-4 in the asymptotic regime, characteristic of two-density media with sharp interfaces, and q _max is found to vary with polymer concentration as q _max∼C ^0.22. Estimates of the typical sizes give values between 40 ? and 200 ?. These results provide a strong evidence of a thermally induced microphase separation, which is corroborated by the very sharp increases of the viscosity (over 2 decades) and of the stress relaxation time of the solutions, occurring in the temperature range where the scattering peak is observed. The results are discussed and compared with the theoretical models proposed for weakly charged polyelectrolytes in a poor solvent. Received 1 October 2001     

The properties of free polymer surfaces and their influence on the glass transition temperature of thin polystyrene films

We present a detailed study of free polymer surfaces and their effects on the measured glass transition temperature (T_g) of thin polystyrene (PS) films. Direct measurements of the near-surface properties of PS films are made by monitoring the embedding of 10 and 20 nm diameter gold spheres into the surface of spin-cast PS films. At a temperature T = 378K( > T_g), the embedding of the spheres is driven by geometrical considerations arising from the wetting of the gold spheres by the PS. At temperatures below T_g ( 363K < T < 370K), both sets of spheres embed 3-4 nm into the PS films and stop. These studies suggest that a liquid-like surface layer exists in glassy PS films and also provide an estimate for the lower bound of the thickness of this layer of 3-4 nm. This qualitative idea is supported by a series of calculations based upon a previously developed theoretical model for the indentation of nanoscale spheres into linear viscoelastic materials. Comparing data with simulations shows that this surface layer has properties similar to those of a bulk sample of PS having a temperature of 374 K. Ellipsometric measurements of the T_g are also performed on thin spin-cast PS films with thicknesses in the range 8nm < h < 290nm. Measurements are performed on thin PS films that have been capped by thermally evaporating 5 nm thick metal (Au and Al) capping layers on top of the polymer. The measured T_g values (as well as polymer metal interface structure) in such samples depend on the metal used as the capping layer, and cast doubt on the general validity of using evaporative deposition to cover the free surface. We also prepared films that were capped by a new non-evaporative procedure. These films were shown to have a T_g that is the same as that of bulk PS (370±1 K) for all film thicknesses measured (> 7 nm). The subsequent removal of the metal layer from these films was shown to restore a thickness-dependent T_g in these samples that was essentially the same as that observed for uncapped PS films. An estimate of the thickness of the liquid-like surface layer was also extracted from the ellipsometry measurements and was found to be 5±1 nm. The combined ellipsometry and embedding studies provide strong evidence for the existence of a liquid-like surface layer in thin glassy PS films. They show that the presence of the free surface is an important parameter in determining the existence of T_g reductions in thin PS films.     

Measuring surface and bulk relaxation in glassy polymers

We present a comprehensive study of gold nanoparticle embedding into polystyrene (PS) surfaces at temperatures ranging from T _g + 8 K to T _g − 83 K and times as long as 10⁵ minutes. This range in times and temperatures allows the first concurrent observation of and differentiation between surface and bulk behavior in the 20nm region nearest the free surface of the polymer film. Of particular importance is the temperature region near the bulk glass transition temperature where both surface and bulk processes can be measured. The results indicate that for the case of PS, enhanced surface mobility only exists at temperatures near or below the bulk T _g value. The surface relaxation times are only weakly temperature dependent and near T _g , the enhanced mobility extends less than 10nm into the bulk of the film. The results suggest that both the concept of a “surface glass transition” and the use of glass transition temperatures to measure local mobility near interfaces may not universally apply to all polymers. The results can also be used to make a quantitative connection to molecular dynamics simulations of polymer films and surfaces.     

Morphology of Fractured Polymer‐Polymer Interfaces Bonded at Ambient Temperature as Studied by Atomic Force Microscopy

The amorphous polymer surfaces of polystyrene (PS, M _n=200 kg/mol, M _w/M _n=1.05) and poly(methyl methacrylate) (PMMA, M _n=51.9 kg/mol, M _w/M _n≤1.07) were brought into contact at 21°C to form PS‐PS (for 54 days) and PMMA‐PMMA auto‐adhesive joints (for 11 days). After contact at that temperature corresponding to T _g‐bulk ?81°C for PS and to T _g‐bulk–88°C for PMMA, where T _g‐bulk is the calorimetric glass transition temperature of the bulk sample, the bonded interfaces were fractured and their surfaces were analyzed by atomic force microscopy (AFM). The surface roughness, R _q, of the fractured interfaces was larger by a factor of 3–4 than was that of the free PS and PMMA surfaces aged for the same period of time. A similar increase in R _q was found by comparison of the free PS surface aged at T _g‐bulk+15°C for 1 h and of the surface of the PS‐PS interface fractured after healing at T _g‐bulk+15°C for 1 h. These observations, indicative of the deformation of the fractured interfaces, suggest the occurrence of some mass transfer across the interface even below T _g‐bulk ?80°C.     

Relaxation and electronic states of Au(100), (110) and (111) surfaces

Using a first-principles method based on density functional theory, we investigate the surface relaxation and electronic states of Au(100), (110) and (111) surfaces. The calculated results show that the relaxations of the (100) and (110) surfaces of the metal are inward relaxations. However, the Au(111) surface shows an ‘anomalous’ outward relaxation, although several previous theoretical studies have predicted inward relaxations that are contrary to the experimental measurements. Electronic densities of states and the respective charge density distribution along the Z-axis of the relaxed surfaces are analyzed, and the origin of inward and outward relaxation is discussed in detail.     

Viscosity and stress autocorrelation function in supercooled water: a molecular dynamics study

Following Guo, G.-J., and Zhang, Y.-G., 2001, Molec. Phys., 99, 283, which calculates the bulk and shear viscosities of SPC/E water at 30°C and 0.999 g cm^?3, further molecular dynamics simulations have been performed at state points of 0°C, ?20°C, ?40°C, and ?60°C along an approximate isobar with the previous state point. SACF and BACF (stress autocorrelation functions related to shear and bulk viscosities, respectively) of high precision have been obtained and compared for their similarities and differences. Shear and bulk viscosities calculated from them showed an increased deviation from real water with decreasing temperature. These correlation functions were then fitted using a uniform two-step relaxation function including a fast oscillatory Kohlrausch law and a slow straightforward Kohlrausch law. The fitting parameters of SACF and BACF have been analysed in detail, and several interesting dynamic phenomena were observed. (1) The oscillation frequency of SACF (44 ~ 48ps^?1) for short time intervals agrees with the stretching mode of hydrogen bonds, while that of BACF (7 ~ 12ps^?1) agrees with the bending mode of hydrogen bonds. (2) With decreasing temperature, the slow relaxation fraction of the BACF increases, while that of the SACF remains constant. (3) The exponents β in the Kohlrausch laws with values greater than 1 are obtained for BACF at ambient temperatures. (4) With regard to both shear and bulk viscosities, the slow relaxation time largely increases with decreasing temperature, while the fast relaxation time slightly decreases. These phenomena are qualitatively explained and discussed.     

The influence of the semiconductor properties on the Mössbauer emission spectra of 57Co cobalt oxide

⁵⁷CO_1?xO Mössbauer sources were prepared at 1000°C and different oxygen pressures. The number x of Co vacancies, which depend on the oxygen pressure during the preparation, determine the Fe³⁺ fraction in the emission spectrum of the source at room-temperature. A theoretical model allows a correlation between these quantities.Measurements on ⁵⁷Co_1?xO sources between room-temperature and 1000°C proved that at temperatures above 500°C electron relaxations between the ⁵⁷Fe^m impurity, introduced in the lattice as a consequence of the ⁵⁷Co decay, and the valence band take place. A theoretical model was developed which describes these relaxation phenomena reasonably well.     

The effects of thermal annealing on the relaxation of rubbing-induced birefringence in polystyrene

We have conducted a systematic study on the effects of post rubbing annealing on the relaxation of rubbing-induced birefringence of polystyrene. It is found that annealing at T₀ only affects the relaxation up to T₀ + T_Lag, where T_Lag is proportional to the logarithm of the annealing time t_A. A theoretical model based on the distribution of relaxation times due to the individual birefringence elements is proposed. To remove its contribution to the net birefringence each element must overcome an energy barrier E = (317 + 1.17ξ)×10³ J/mol, and therefore must have a characteristic relaxation time τ which depends on temperature T and a barrier height which ranges from 340.4 kJ/mol to 445.7 kJ/mol. The relaxation of birefringence is expressed by the equation NB(T, t) = N(ξ)e^-t/τ(T,ξ)dξ, in which both the relaxation time τ(T,ξ) and the distribution function N(ξ) can be extracted from experimental data. The predictions of the model agree well with all the experimental results presented in this work. The differences and similarities of the relaxation of birefringence with respect to the physical aging of quenched PS are discussed. In particular, similarities in terms of the general temperature lag phenomena are noted.