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We present a detailed study of free polymer surfaces and their effects on the measured glass transition temperature (Tg) 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( > Tg), the embedding of the spheres is driven by geometrical considerations arising from the wetting of the gold spheres by the PS. At temperatures below Tg ( 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 Tg 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 Tg 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 Tg 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 Tg 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 Tg reductions in thin PS films. 相似文献
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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
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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
6.
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
7.
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
8.
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
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Erichsen J Shiferaw T Zaporojtchenko V Faupel F 《The European physical journal. E, Soft matter》2007,24(3):243-246
Using the cluster-embedding method of V. Zaporojchenko et al. (Macromolecules 34, 1125 (2000)), we measured the glass transition temperature T
g at the polystyrene/vacuum interface of bimodal mixtures of monodisperse polystyrenes of 3.5k and 1000k. Embedding of ≈ 1
nm Au clusters was monitored in situ by X-ray photoelectron spectroscopy (XPS). The clusters were formed by evaporation of Au onto the polymer surface. Only one
glass transition was observed in the mixtures. The surface glass transition temperatures are correlated to but are below the
bulk values of the mixtures and obey the Gordon-Taylor equation. The results suggest that the earlier reported molecular-weight
dependence of the surface glass transition is not due to segregation of short chains to the surface. 相似文献
10.
Ellison CJ Kim SD Hall DB Torkelson JM 《The European physical journal. E, Soft matter》2002,8(2):155-166
Fluorescence intensity measurements of chromophore-doped or -labeled polymers have been used for the first time to determine
the effects of decreasing film thickness on glass transition temperature, T
g, the relative strength of the glass transition, and the relative rate of physical aging below T
g in supported, ultrathin polymer films. The temperature dependence of fluorescence intensity measured in the glassy state
of thin and ultrathin films of pyrene-doped polystyrene (PS), poly(isobutyl methacrylate) (PiBMA), and poly(2-vinylpyridine)
(P2VP) differs from that in the rubbery state with a transition at T
g. Positive deviations from bulk T
g are observed in ultrathin PiBMA and P2VP films on silica substrates while substantial negative deviations from bulk T
g are observed in ultrathin PS films on silica substrates. The relative difference in the temperature dependences of fluorescence
intensity in the rubbery and glassy states is usually reduced with decreasing film thickness, indicating that the strength
of the glass transition is reduced in thinner films. The temperature dependence of fluorescence intensity also provides useful
information on effects of processing history as well as on the degree of polymer-substrate interaction. In addition, when
used as a polymer label, a mobility-sensitive rotor chromophore is demonstrated to be useful in measuring relative rates of
physical aging in films as thin as 10 nm.
Received 21 August 2001 相似文献
11.
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
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U. Schneider R. Brand P. Lunkenheimer A. Loidl 《The European physical journal. E, Soft matter》2000,1(1):67-73
The Nagel scaling and the modified scaling procedure proposed recently by Dendzik et al. have been applied to broadband dielectric data on two glass-forming liquids (glycerol and propylene carbonate) and three
plastic crystals (ortho-carborane, meta-carborane, and 1-cyano-adamantane). Our data extend the upper limit of the abscissa
range to considerably higher values than in previously published analyses. At the highest frequencies investigated, deviations
from a single master curve show up which are most pronounced in the Dendzik scaling plot. The loss curves of the plastic crystals
do not scale in the Nagel plot, but they fall onto a separate master curve in the Dendzik plot. In addition, we address the
question of a possible divergence of the static susceptibility near the Vogel-Fulcher temperature. For this purpose, the low-temperature
evolution of the high-frequency wing of the dielectric loss peaks is investigated in detail. No convincing proof for such
a divergence can be deduced from the present broadband data.
Received 14 June 1999 and Received in final form 4 October 1999 相似文献
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S. Moreno R.G. Rubio G. Luengo F. Ortega M.G. Prolongo 《The European physical journal. E, Soft matter》2001,4(2):173-182
The complex dielectric permittivity has been measured for three poly(ethylenglycol)-b-poly(propylenglycol)-b-poly(ethylenglycol)
copolymers with different content of poly(ethylenglycol) (15%, 33% and 80%), and increasing degree of crystallinity (0%, 10%
and 20%, respectively). Only the non-crystalline sample shows the normal mode relaxation together with the segmental (α-relaxation)
and the Johari-Goldstein (β-relaxation) modes. The crystalline samples show also polarization contributions due to the existence
of interfaces between the crystallites and the amorphous phase. The relaxation times of the (α and normal modes can be described
by a VFT equation with the same value of T0. There is a slowing-down of the segmental mode due to the presence of crystallites. The temperature dependence of the α and
β relaxations in the copolymers is very similar to that found in pure PPG, while there are significant differences in the
case of the normal mode of the non-crystalline sample. The size of the cooperatively rearranging regions CRR, and the width
of the glass transition region increase slightly with the degree of crystallinity. The temperature dependence of the size
of CRRs is compatible with the prediction of fluctuation theory. No systematic effect of the degree of crystallinity on the
β-relaxation has been found. Near T
g the β-relaxation time is close to the primitive time of the coupling model.
Received: 31 May 2000 相似文献
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Hutchings LR Richards RW Thompson RL Bucknall DG 《The European physical journal. E, Soft matter》2002,8(2):121-128
High-molecular-weight heterotelechelic deuteriopolystyrene, NDPSF, possessing an amine functional group at one end of the
chain and a fluorocarbon group at the other was tethered to a silicon substrate by its amine functional group. These layers
were coated with an unfunctionalised polystyrene matrix, HPS, such that the total film thickness covered a range from 2.2
to 9 times the radius of gyration of NDPSF. The detailed distribution of the polymers after annealing for times much greater
than the reptation period of either of the components, was obtained using neutron reflectometry. No evidence for bridging
of the two interfaces was found for the thicker films, but the finite concentration of the NDPSF polymer observed for the
thinnest films may be due to bridging since the energy gain of the fluorocarbon end is just greater than the loss due to configurational
entropy losses. A linear increase in the ellipsometric thickness of the excess of NDPSF at the substrate was discovered and
we attribute this to the NDPSF slowly being leached out of the layer initially at the substrate followed by diffusion into
the bulk of the film. The concentration profiles obtained are consistent with hindered relaxation of the large NDPSF molecules,
when they are tethered at the substrate or at the vacuum surface.
Received 21 August 2001 and Received in final form 7 January 2002 相似文献
16.
A. Martin O. Rossier A. Buguin P. Auroy F. Brochard-Wyart 《The European physical journal. E, Soft matter》2000,3(4):337-341
We study theoretically the behavior of nanoscopic liquid films L (thickness e) intercalated between a solid S and a rubber R (elastic modulus μ). Thickness modulations involve a healing length , which results from a competition between elastic and disjoining pressure. With van der Waals interactions, , where a is a molecular size and h0 the rubber capillary length ( , interfacial tension). If the Hamaker constant of the intercalated liquid is negative, the film dewets by amplification of
peristaltic fluctuations (“spinodal dewetting”). The typical size of the contacts is predicted to scale like for films of thicknesses . The rise time of the fastest mode, predicted to scale like , should be very sensitive to the film thickness.
Received 11 February 2000 and Received in final form 22 May 2000 相似文献
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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
19.
Sciortino F La Nave E Scala A Stanley HE Starr FW 《The European physical journal. E, Soft matter》2002,9(3):233-237
We present an overview of the recent studies on the properties of the potential energy surface for a simple model of water.
We emphasize the relations between PES properties and dynamics in supercooled states for the model and discuss possible future
application of the PES studies.
Received 13 March 2002 相似文献
20.
W. Peng P.M. Goldbart 《The European Physical Journal B - Condensed Matter and Complex Systems》2001,19(3):461-466
Certain density correlators, measurable via various experimental techniques, are studied in the context of the vulcanization transition. It is shown that these correlators
contain essential information about both the vulcanization transition and the emergent amorphous solid state. Contact is made
with various physical ingredients that have featured in experimental studies of amorphous colloidal and gel systems and in
theoretical studies of the glassy state.
Received 30 September 2000 相似文献