共查询到20条相似文献,搜索用时 31 毫秒
1.
Ngai KL 《The European physical journal. E, Soft matter》2002,8(2):225-235
Large increases of mobility of local segmental relaxation observed in polymer films as the film thickness is decreased, as
evidenced by decreases of the glass temperature, are not found for relaxation mechanisms that have longer length scales including
the Rouse relaxation modes and the diffusion of entire polymer chains. We show that the coupling model predictions, when extended
to consider polymer thin films, are consistent with a large increase of the mobility of the local segmental motions and the
lack of such a change for the Rouse modes and the diffusion of entire polymer chains. There are two effects that can reduce
the coupling parameter of the local segmental relaxation in thin films. One is the chain orientation that is induced parallel
to the surface when the film thickness h becomes smaller than the end-to-end distance of the chains and the other is a finite-size effect when h is no longer large compared to the cooperative length scale. Extremely thin ( ≈ 1.5 nm) films obtained by intercalating a
polymer into layered silicates have thickness significantly less than the cooperative length scale near the bulk polymer glass
transition temperature. As a result, the coupling parameter of the local segmental relaxation in such thin films is reduced
almost to zero. With this plausible assumption, we show the coupling model can explain quantitatively the large decrease of
the local segmental relaxation time found experimentally.
Received 1 August 2001 and Received in final form 1 December 2001 相似文献
2.
Hartmann L Gorbatschow W Hauwede J Kremer F 《The European physical journal. E, Soft matter》2002,8(2):145-154
The molecular dynamics in thin films (18 nm-137 nm) of isotactic poly(methyl methacrylate) (i-PMMA) of two molecular weights
embedded between aluminium electrodes are measured by means of dielectric spectroscopy in the frequency range from 50 mHz
to 10 MHz at temperatures between 273 K and 392 K. The observed dynamics is characterized by two relaxation processes: the
dynamic glass transition (α-relaxation) and a (local) secondary β-relaxation. While the latter does not depend on the dimensions
of the sample, the dynamic glass transition becomes faster (≤2 decades) with decreasing film thickness. This results in a
shift of the glass transition temperature T
g to lower values compared to the bulk. With decreasing film thickness a broadening of the relaxation time distribution and
a decrease of the dielectric strength is observed for the α-relaxation. This enables to deduce a model based on immobilized
boundary layers and on a region displaying a dynamics faster than in the bulk. Additionally, T
g was determined by temperature-dependent ellipsometric measurements of the thickness of films prepared on silica. These measurements
yield a gradual increase of T
g with decreasing film thickness. The findings concerning the different thickness dependences of T
g are explained by changes of the interaction between the polymer and the substrates. A quantitative analysis of the T
g shifts incorporates recently developed models to describe the glass transition in thin polymer films.
Received 12 August 2001 and Received in final form 16 November 2001 相似文献
3.
N. Keller J. Mistrík Š. Višňovský D.S. Schmool Y. Dumont P. Renaudin M. Guyot R. Krishnan 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,21(1):67-73
Orthoferrites present, as bulk materials, reorientation transitions of their magnetic moment alignment at temperatures depending
on the rare-earth (RE) ion. In particular, orthoferrites (REFeO3) with RE = Sm, Dy, present this transition at T
SRT = 443 K and 36 K, respectively. The spectra of the complex Kerr and Faraday angle have been measured on orthoferrite thin
films (RE = Sm, Dy, Y), which were prepared by pulsed laser deposition on amorphous quartz substrates. The obtained spectra
exhibit contributions of both surfaces and interfaces. Propagation effects of the polarized light in the magneto-optical medium
which is interpreted in terms of a simplified theoretical formalism, is also observed. For selected photon energies, temperature
dependent Faraday rotation measurements, , on orthoferrite thin films (RE = Sm, Dy, Y) have been performed. A quite different thermal variation compared to the bulk
magnetization has been observed. Curie temperatures are found to be close to the bulk values or slightly larger by 10 K to
20 K as in the case of DyFeO3 and YFeO3. For RE = Sm and Dy, increases with increasing temperature contrary to the saturation magnetization, passes through a maximum at about 460 K and
vanishes with a T
C of 647±18 K, 695 K for RE = Sm and Dy respectively.
Received 28 July 2000 相似文献
4.
Forrest JA Mattsson J 《Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics》2000,61(1):R53-R56
We report measurements of the glass transition temperature, T(g), in free standing polymer films in a low M(n) limit where chain confinement effects are not observed. The measured T(g) values are accurately described by a layer model incorporating a mobile surface layer with a size determined by the length scale of cooperative dynamics. The analysis leads to a surface T(g) value and length scale of cooperative motion near bulk T(g) which quantitatively agree with independently determined values. The model and parameters provide a framework within which all previous measurements of T(g) values in thin supported films may be understood and provides values for the length scale of cooperative motion over an extended range of temperatures below the bulk T(g) value. 相似文献
5.
Herminghaus S 《The European physical journal. E, Soft matter》2002,8(2):237-243
It is discussed how the proximity of a free surface or mobile interface may affect the strain relaxation behavior in a viscoelastic
material, such as a polymer melt. The eigenmodes of a viscoelastic film are thus derived, and applied in an attempt to explain
the experimentally observed substantial shift of the glass transition temperature of sufficiently thin polymer films with
respect to the bulk. Based on the idea that the polymer freezes due to memory effects in the material, and exploiting results
from mode-coupling theory, the experimental findings of several independent groups can be accounted for quantitatively, with
the elastic modulus at the glass transition temperature as the only fitting parameter. The model is finally applied discussing
the possibility of polymer surface melting. A surface molten layer is predicted to exist, with a thickness diverging as the
inverse of the reduced temperature. A simple model of thin polymer film freezing emerges which accounts for all features observed
experimentally so far.
Received 8 August 2001 相似文献
6.
Grohens Y Hamon L Reiter G Soldera A Holl Y 《The European physical journal. E, Soft matter》2002,8(2):217-224
We have measured, the thickness dependence of the glass transition temperature T(g)( h), using ellipsometry at variable temperature, for poly(methyl-methacrylate) (PMMA) of various tacticity in confined geometry. We report that several factors significantly affect T(g)( h): i) polymer microstructure (stereoregularity of PMMA) related to local dynamics; ii) interfacial interactions; iii) conformation of the polymer chains. These results raise many fundamental questions on the origin of the thickness-dependent glass transition. Why and how do the interactions with the substrate significantly affect T(g)( h)? Does T(g)( h) depend on the modifications of conformational parameters of the chains (their entropy)? What is the correlation between local dynamics and T(g)( h) in thin films? The aim of this paper is to summarise these open questions, which should stimulate further investigations in the thin polymer film scientific community. 相似文献
7.
Bernazzani P Simon SL Plazek DJ Ngai KL 《The European physical journal. E, Soft matter》2002,8(2):201-207
The process of spin-coating to fabricate thin polymer films with high molecular weight can produce samples with entanglement
concentrations that are far below the equilibrium value. It is not clear whether or not such low entanglement concentrations
are responsible for the depression of the glass temperature in thin polymer films. In this work, we measure the calorimetric
glass temperature and viscoelastic response of polystyrenes with molecular weights ranging from 3×103 to 43.7×106 g/mol, for both bulk material and for samples freeze-dried from dilute solution. We conclude that the reduction of the glass
temperature observed in thin polymer films cannot be due to the reduced entanglement concentration in the samples.
Received 15 August 2001 and Received in final form 2 March 2002 相似文献
8.
G. Reiter G. Castelein P. Hoerner G. Riess J.-U. Sommer G. Floudas 《The European physical journal. E, Soft matter》2000,2(4):319-334
Spin-coated thin films of about 100nm of low-molecular-weight hydrogenated poly(butadiene-b- ethyleneoxide) (PBh-PEO) diblock copolymers have been crystallized at various constant temperatures. Crystallization has been observed in real
time by light microscopy. Detailed structural information was obtained by atomic force microscopy, mainly enabled by the large
viscoelastic contrast between amorphous and crystalline regions. The behavior in thin films is compared to the bulk properties
of the polymer. Crystallization started from an annealed microphase separated melt where optical microscopy indicated a lamellar
orientation parallel to the substrate. A small difference in the length of the crystallizable block produced significantly
different crystallization behavior, both in the bulk and in thin films. For thin films of the shortest diblock copolymer (45%
PEO content) and for an undercooling larger than about 10 degrees, crystallization created vertically oriented lamellae. These
vertical lamellae could be preferentially aligned over several micrometers when crystallization occurred close to a three-phase
contact line. Annealing at temperatures closer to the melting point or keeping the sample at room temperature for several
months allowed the formation of a lamellar structure parallel to the substrate. A tentative interpretation based on kinetically
caused chain folding and relaxation within the crystalline state, with implications on general aspects of polymer crystallization,
is presented.
Received 19 March 1999 and Received in final form 14 December 1999 相似文献
9.
Higgins AM Sferrazza M Jones RA Jukes PC Sharp JS Dryden LE Webster J 《The European physical journal. E, Soft matter》2002,8(2):137-143
We investigate the dynamics of spinodal dewetting in liquid-liquid polymer systems. Dewetting of poly(methyl-methacrylate)
(PMMA) thin films on polystyrene (PS) “substrates” is followed in situ using neutron reflectivity. By following the development of roughness at the PS/PMMA interface and the PMMA surface we extract
characteristic growth times for the dewetting process. These characteristic growth times are measured as a function of the
molecular weight of the two polymers. By also carrying out experiments in the regime where the dynamics are independent of
the PS molecular weight, we are able to use dewetting to probe the scaling of the PMMA thin film viscosity with temperature
and molecular weight. We find that this scaling reflects bulk behaviour. However, absolute values are low compared to bulk
viscosities, which we suggest may be due in part to slippage at the polymer/polymer interface.
Received 25 June 2001 and Received in final form 5 December 2001 相似文献
10.
The T
g of organic liquids confined to nanoporous matrices and that of thin polymer films can decrease dramatically from the bulk
value. One possible explanation for this phenomenon is the development of hydrostatic tension during vitrification under confinement
that results in a concomitant increase in the free volume. Here we present experimental evidence and modeling results for
ortho-terphenyl (o-TP) confined in pores as small as 11.6 nm that indicate that, although there is an important hydrostatic
tension in the liquid in the pores, it does not develop until near the reduced T
g of the constrained material --well below the bulk T
g. Enthalpy recovery for the o-TP in the nanopores exhibits accelerated physical aging relative to the bulk, as well as a leveling
off of the fictive temperature at equilibrium values greater than the aging temperature. An adaptation of the structural recovery
model that incorporates vitrification under isochoric conditions is able to provide a quantitative explanation for the apparently
anomalous aging observed in nanopore confined liquids and in thin polymeric films. The results strongly support the existence
of an intrinsic size effect as the cause of the reduced T
g.
Received 3 September 2001 相似文献
11.
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 相似文献
12.
C. Caizer I. Hrianca 《The European Physical Journal B - Condensed Matter and Complex Systems》2003,31(3):391-400
We have studied the magnetization of a system of γ-Fe2O3 (0.68 vol.%) nanoparticles isolated in an SiO2 amorphous matrix placed in an alternating magnetic field with a frequency of 640 Hz and in the temperature range of (77-300)
K. Compared to temperatures closer to 300 K (where the system has a superparamagnetic behaviour), at lower temperatures, the
magnetization has a dynamic hysteresis loop due to the magnetization's phase shift between the field and the magnetization.
The delay of the magnetization (attributed to the Néel relaxation processes) increases with the decrease of temperature. It
has been shown that the relaxation time resulting from the Néel theory is determined by an effective anisotropy constant (
K
) that takes into account the magnetocrystalline anisotropy, as well as the shape, surface and strain anisotropies. In the
following we will show that the surface and strain anisotropy components have the most significant influence. When the temperature
decreases from 300 to 77 K, the relative increase of the saturation magnetization of the nanoparticles is much higher than
that of the (spontaneous) saturation magnetization of bulk γ-Fe2O3. This increase is due to the increase of the mean magnetic diameter of the particles attached to the core of aligned spins,
from 10.16 nm to 11.70 nm, as a result of the modification of the superexchange interaction in the surface layer.
Received 25 April 2002 / Received in final form 11 August 2002 Published online 14 February 2003
RID="a"
ID="a"e-mail: ccaizer@physics.uvt.ro 相似文献
13.
D. Starešinić A. Kiš K. Biljaković B. Emerling J. W. Brill J. Souletie H. Berger F. Lévy 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,29(1):71-77
Specific heats of the charge-density-wave compounds o-TaS3 and (TaSe4)2I have been measured over the wide temperature interval 10 K-300 K. Both systems exhibit strong non-Debye behavior. Very weak
and broad anomalies are observed at the Peierls transition temperatures. For o-TaS3, the change in the curvature of the specific heat occurs at temperature of 40 K where glass transition has been deduced from
dielectric measurements, and an extended scaling analysis suggests that the glass transition is associated with a dynamical
cross over in length scales. We briefly discuss the characteristics and physical origins of the anomalies at both the Peierls
and glass transitions.
Received 5 April 2002 / Received in final form 28 June 2002 Published online 17 September 2002 相似文献
14.
P. Orgiani C. Aruta G. Balestrino S. Lavanga P.G. Medaglia A. Tebano 《The European Physical Journal B - Condensed Matter and Complex Systems》2002,26(1):23-28
Transport properties of SrRuO3 thin films were studied as a function of the epitaxial strain. SrRuO3 films were grown on (100) SrTiO3 substrates by the Pulsed Laser Deposition technique equipped with Reflection High Energy Electrons Diffraction (RHEED). Samples
thickness has been varied from a few unit cells to above 1000 ? while monitoring RHEED intensity oscillations. In thicker
films epitaxial strain was found to be progressively relaxed. SrRuO3 relaxed films (thickness 1000 ?) show metallic behavior for the whole temperature range with a ferromagnetic ordering at about 150 K. For thinner
films, ferromagnetic ordering occurs at progressively lower temperatures, until in films thinner than 400 ? it disappears.
Films thinner than 80 ? show a semiconducting behavior at low temperatures. Our results provide direct evidence of the crucial
role of the strain effect for conducting and magnetic properties of SrRuO3.
Received 16 July 2001 and Received in final form 22 October 2001 相似文献
15.
Tseng KC Turro NJ Durning CJ 《Physical review. E, Statistical physics, plasmas, fluids, and related interdisciplinary topics》2000,61(2):1800-1811
Fluorescence recovery after photobleaching was used to measure in-plane dye-probe diffusion coefficients, D, in thin films of monodisperse polystyrene supported on fused quartz substrates. The substrates were prepared with a high density of surface hydroxyl groups which interact favorably with repeat units of the polymer. The effects of temperature and film thickness were investigated, at temperatures above the bulk glass transition of the polymer, T(g), and in the range of film thicknesses from 1-10(2) times the radius of gyration (R(g)) of individual polymer molecules. As the film thickness decreases towards R(g) the value of D increases above the bulk values, with significant effects first appearing in films approximately 20R(g). In the thinnest films studied, about 4R(g), the values of D lie as much as two orders over bulk values. At the same time, the temperature dependence of D becomes much weaker than in bulk. Analysis by free volume theory indicates that apparent values of both T(g) and the thermal expansion coefficient for liquid state, alpha(L), decrease as the film thickness decreases. The possible effects of surface segregation of the dye probe are discussed. 相似文献
16.
X. Wu A.K. Ray 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,19(3):345-351
The electronic and geometric structures of bulk PuO2 and its (110) surface have been studied using a periodic model within the generalized gradient approximation (GGA) of density
functional theory (DFT). The sixty core electrons of the Pu atom have been represented by a relativistic effective core potential
and scalar relativistic effects have been incorporated on the valence orbitals. For bulk PuO2, we predict an equilibrium lattice constant of 10.10 a.u. and a cohesive energy of 17.28 eV, in good agreement with experimental
data. For the (110) surface, upon relaxation, the distance between the top layer and the next layer is found to decrease by
0.12 ?, i.e. 5.3% of the corresponding interlayer distance in the bulk. The distance between the two oxygen atoms on the top layer is
found to increase by 0.15 ?, i.e. 5.6% of the corresponding bulk value. The small surface relaxation energy of 0.268 eV per unit cell indicates the fair stability
of this surface. The effective charges on Pu and O atoms show that the chemical bonding in this system is not purely ionic.
Together with the metallic feature of the density of states (DOS) on the surface, the effective charge distribution provides
some basis for understanding surface reactivity and corresponding support for catalysis.
Received 16 June 2000 相似文献
17.
W. Nörenberg G. Papp P. Rozmej 《The European Physical Journal A - Hadrons and Nuclei》2000,9(3):327-343
The diabatic approach to dissipative collective nuclear motion is reformulated in the local-density approximation in order
to treat the normal modes of a spherical nuclear droplet analytically. In a first application the adiabatic isoscalar modes
are studied and results for the eigenvalues of compressional (bulk) and pure surface modes are presented as function of density
and temperature inside the droplet, as well as for different mass numbers and for soft and stiff equations of state. We find
that the region of bulk instabilities (spinodal regime) is substantially smaller for nuclear droplets than for infinite nuclear
matter. For small densities below 30% of normal nuclear matter density and for temperatures below 5 MeV all relevant bulk
modes become unstable with similar growth rates. The surface modes have a larger spinodal region, reaching out to densities
and temperatures way beyond the spinodal line for bulk instabilities. Essential experimental features of multifragmentation,
like fragmentation temperatures and fragment-mass distributions (in particular the power-law behavior) are consistent with
the instability properties of an expanding nuclear droplet, and hence with a dynamical fragmentation process within the spinodal
regime of bulk and surface modes (spinodal decomposition).
Received: 4 September 2000 / Accepted: 14 November 2000 相似文献
18.
R. Dimova U. Seifert B. Pouligny S. Förster H.-G. Döbereiner 《The European physical journal. E, Soft matter》2002,7(3):241-250
Giant vesicles prepared from the diblock copolymer polybutadien-b-polyethyleneoxide (PB-PEO) exhibit a shear surface viscosity,
which is about 500 times higher than those found in common phospholipid bilayers. Our result constitutes the first direct
measurement of the shear surface viscosity of such polymersomes. At the same time, we measure bending and stretching elastic
constants, which fall in the range of values typical for lipid membranes. Pulling out a tether from an immobilized polymersome
and following its relaxation back to the vesicle body provides an estimate of the viscous coupling between the two monolayers
composing the polymer membrane. The detected intermonolayer friction is about an order of magnitude higher than the characteristic
one for phospholipid membranes. Polymersomes are tough vesicles with a high lysis tension. This, together with their robust
rheological properties, makes them interesting candidates for a number of technological applications.
Received 2 March 2001 and Received in final form 15 February 2002 相似文献
19.
The dynamic and static properties of a supercooled (non-entangled) polymer melt are investigated via molecular-dynamics (MD) simulations. The system is confined between two completely smooth and purely repulsive walls. The
wall-to-wall separation (film thickness), D, is varied from about 3 to about 14 times the bulk radius of gyration. Despite the geometric confinement, the supercooled
films exhibit many qualitative features which were also observed in the bulk and could be analyzed in terms of mode-coupling
theory (MCT). Examples are the two-step relaxation of the incoherent intermediate scattering function, the time-temperature
superposition property of the late time α-process and the space-time factorization of the scattering function on the intermediate
time scale of the MCT β-process. An analysis of the temperature dependence of the α-relaxation time suggests that the critical
temperature, T
c, of MCT decreases with D. If the confinement is not too strong ( D≥10monomer diameter), the static structure factor of the film coincides with that of the bulk when compared for the same distance,
T - T
c(D), to the critical temperature. This suggests that T - T
c(D) is an important temperature scale of our model both in the bulk and in the films.
Received 12 September 2001 相似文献
20.
It has been shown over the last few years that the dynamics close to the glass transition is strongly heterogeneous, both
by measuring the diffusion coefficient of tagged particles or by NMR studies. Recent experiments have also demonstrated that
the glass transition temperature of thin polymer films can be shifted as compared to the same polymer in the bulk. We propose
here first a thermodynamical model for van der Waals liquids, which accounts for experimental results regarding the bulk modulus
of polymer melts and the evolution of the density with temperature. This model allows us to describe the density fluctuations
in such van der Waals liquids. Then, by considering the thermally induced density fluctuations in the bulk, we propose that
the 3D glass transition is controlled by the percolation of small domains of slow dynamics, which allows to explain the heterogeneous
dynamics close to T
g. We show then that these domains percolate at a lower temperature in the quasi-2D case of thin suspended polymer films and
we calculate the corresponding glass transition temperature reduction, in quantitative agreement with experimental results
of Jones and co-workers. In the case of strongly adsorbed films, we show that the strong adsorption amounts to enhance the
slow domains percolation. This effect leads to 1) a broadening of the glass transition and 2) an increase of T
g in quantitative agreement with experimental results. For both strongly and weakly adsorbed films, the shift in T
g is given by a power law, the exponent being the inverse of that of the correlation length of 3D percolation.
Received 21 March 2000 and Received in final form 4 December 2000 相似文献