共查询到20条相似文献,搜索用时 15 毫秒
1.
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
2.
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
3.
Recent experiments have demonstrated that the dynamics in liquids close to the glass transition temperature is strongly heterogeneous.
The characteristic size of these heterogeneities has been measured to be a few nanometers at T
g. We extend here a recent model for describing the heterogeneous nature of the dynamics which allows both to derive this length
scale and the right orders of magnitude of the heterogeneities of the dynamics close to the glass transition. Our model allows
then to interpret quantitatively small probes diffusion experiments.
Received 29 March 2002 and Received in final form 11 November 2002
RID="a"
ID="a"e-mail: long@lps.u-psud.fr 相似文献
4.
L. Saviot E. Duval J.F. Jal A.J. Dianoux 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,17(4):661-666
Relaxations in amorphous bis-phenol A polycarbonate are studied by neutron scattering, as a function of temperature below
the glass transition. Two different processes are observed. One is very fast, with a characteristic time (∼ 0.3 ps), that
is independent of temperature and momentum transfer. Conversely the other is slower, with a time which is dependent on temperature
and momentum transfer. The very fast localized anharmonic motion is interpreted by the overdamping of low-frequency vibrational
modes, by nearby dynamic holes. The slower relaxation is thermally activated and momentum transfer dependent. It corresponds
to molecular group motions and possibly to the short-time regime of the segmental relaxation.
Received 29 February 2000 and Received in final form 13 June 2000 相似文献
5.
Shiqi Zhou 《The European physical journal. E, Soft matter》2000,3(4):343-353
A density functional theory is proposed for nonuniform freely jointed tangential hard sphere polymer melts in which the bonding
interaction is treated on the basis of the properties of the Dirac δ-function, thus avoiding the use of the single chain simulation
in the theory. The excess free energy is treated by making use of the universality of the free energy density functional and
the Verlet-modified (VM) bridge function. To proceed numerically, one of the input parameters, the second-order direct correlation
function of a uniform polymer melt is obtained by solving numerically the Polymer-RISM integral equation with the Percus-Yevick
(PY) closure. The predictions of the present theory for the site density distribution, the partition coefficient and the adsorption
isotherm, near a hard wall or between two hard walls are compared with computer simulation results and with those of previous
theories. Comparison indicates that the present approach is more accurate than the previous integral equation theory and the
most accurate Monte Carlo density functional theories. The predicted oscillations of the medium-induced force between two
hard walls immersed in polymer melts are consistent with the experimental results available in the literature.
Received 18 April 2000 相似文献
6.
H.J. Angerman G. ten Brinke J.J.M. Slot 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,12(3):397-404
In this paper we investigate in a systematic way the influence of polydispersity in the block lengths on the phase behavior
of AB-multiblock copolymer melts. As model system we take a polydisperse multiblock copolymer for which both the A-blocks and the B-blocks satisfy a Schultz-Zimm distribution. In the limit of low polydispersity the expressions for the vertex functions are
clarified by using simple physical arguments. For various values of the polydispersity the phase diagram is presented, which
shows that the region of stability of the bcc phase increases considerably with increasing polydispersity. The strong dependence
of the periodicity of the microstructure on the polydispersity and on the interaction strength is presented.
Received 2 July 1998 相似文献
7.
T. Albrecht S. Armbruster S. Keller G. Strobl 《The European physical journal. E, Soft matter》2001,6(3):237-243
Poly(ethylene oxide) (PEO) in the semi-crystalline state shows a reversible surface crystallization and melting; a temperature
decrease leads to a certain crystal thickening, a temperature increase reversely to an expansion of the amorphous intercrystallite
layers. Dynamic calorimetry provides a means to investigate the kinetics of the process. The structural rearrangement in the
region of the crystalline-amorphous interface can only be accomplished if the chains can slide through the crystallites. One
therefore expects the associated time to change with the crystallite thickness. Variations of the crystal thickness of PEO
can be achieved by choosing different crystallization temperatures. We studied the effect of the crystal thickness employing
temperature-modulated differential scanning calorimetry and heat wave spectroscopy, and by carrying out small-angle X-ray
scattering experiments for the structural characterization. The effect of the crystal thickness is clearly observed. Results
indicate that the sliding diffusion through the crystallites takes place by helical jumps of whole stems. Data yield the activation
energy per unit length of the stems.
Received 20 April 2001 and Received in final form 13 August 2001 相似文献
8.
B. Lotz 《The European physical journal. E, Soft matter》2000,3(2):185-194
Contrary to most or all other materials, crystallization of chiral but racemic polymers such as isotactic polypropylene is
accompanied by a conformational rearrangement which leads to helical geometries: the building units of the crystal are helical
stems, -20nm long, which can be either right-handed or left-handed. Helical hand cannot be reversed within the crystal structure: it is therefore a permanent marker and an indicator of molecular processes (in particular segregation/selection of helical hands) which take place during crystal
growth, and more precisely during the crucial step of “efficient” helical stem deposition. The issue of proper helical hand
selection during polymer crystal growth is mainly illustrated with isotactic polypropylene. Its various crystalline polymorphs
(, , and smectic) display virtually all possible combinations of helical hands, azimuthal settings and even non-parallel orientation
of helix axes in space. Furthermore, a specific homoepitaxy which generates a lamellar branching in the phase “quadrites” and composite structures makes it possible a) to determine the helical hand and associated azimuthal setting of every stem in
the crystalline entities and b) to determine the impact on the crystal structure and morphology of “mistakes” in helical hand
of the depositing stem. Analysis of these morphologies demonstrates that the crystallization of isotactic polypropylene (and
by implication of other achiral, helical polymers) is a highly sequential and “substrate-determined” process, i.e. that the depositing stem probes the topography of the growth face prior to attachment. These observations appear difficult to reconcile with crystallization schemes in which molecules (helical segments) are
prearranged in a kind of pseudo-crystalline bundle (and as such, are not subjected to the high constraints of crystal symmetry)
before deposition as a preassembled entity on the substrate.
Received: 5 May 2000 相似文献
9.
Serghei A Mikhailova Y Huth H Schick C Eichhorn KJ Voit B Kremer F 《The European physical journal. E, Soft matter》2005,17(2):199-202
Broadband Dielectric Spectroscopy is employed to investigate the molecular dynamics in thin films of hyperbranched polyesters (type AB1B2, with -OH and -OCOCH3 as terminal groups). Three relaxation processes are detected: alpha, beta and gamma. While the latter two are not influenced by the confinement, a pronounced effect is observed on the alpha relaxation: with decreasing film thickness the slower relaxation modes of the dynamic glass transition are gradually suppressed, resulting in an increase of the average relaxation rate and in a linear decrease of the dielectric strength. This is attributed to an immobilization in confinement of the polymeric segments located at the periphery of the hyperbranched macromolecular structures. 相似文献
10.
Recently, we (P.A. O'Connell, G.B. McKenna, Science 307, 1760 (2005)) introduced a novel nano-bubble inflation method to measure the absolute creep compliance of nanometer thick
polymer films. In that work it was shown that even at film thicknesses as small as 27.5nm the glass temperature was unchanged
for poly(vinyl acetate) (PVAc). Perhaps more importantly, and the subject of the present work, was the observation that these
ultrathin films show a dramatic stiffening in the rubbery plateau regime, i.e., the compliance was reduced by over two orders of magnitude compared to the bulk material. In the present work we substantiate
the previous results in a study of the thickness dependence of the rubbery compliance of PVAc and polystyrene (PS) films for
thicknesses from 13nm to 276nm. We show the substantial stiffening of the plateau region for both materials. Furthermore,
the rubbery compliance (inverse of stiffness) scales with approximately the second power ( 1.8±0.2) in the film thickness
for both materials. 相似文献
11.
C. Monthus T. Garel H. Orland 《The European Physical Journal B - Condensed Matter and Complex Systems》2000,17(1):121-130
We consider two different problems involving the localization of a single polymer chain: (i) a periodic AB copolymer at a
selective fluid-fluid interface, with the upper (resp. lower) fluid attracting A (resp. B) monomers (ii) a homopolymer chain
attracted to a hard wall (wetting). Self avoidance is neglected in both models, which enables us to study their localization
transition in a grand canonical approach. We recover the results obtained in previous studies via transfer matrix methods. Moreover, we calculate in this way the loop length distribution functions in the localized phase.
Some finite size effects are also determined and tested numerically.
Received 13 April 2000 相似文献
12.
E. Orlandini T. Garel 《The European Physical Journal B - Condensed Matter and Complex Systems》1998,6(1):101-110
We study a single self avoiding hydrophilic hydrophobic polymer chain, through Monte-Carlo lattice simulations. The affinity
of monomer i for water is characterized by a (scalar) charge , and the monomer-water interaction is short-ranged. Assuming incompressibility yields an effective short ranged interaction
between monomer pairs (i,j), proportional to . In this article, we take (resp. ()) for hydrophilic (resp. hydrophobic) monomers and consider a chain with (i) an equal number of hydro-philic and -phobic
monomers (ii) a periodic distribution of the along the chain, with periodicity 2p. The simulations are done for various chain lengths N, in d=2 (square lattice) and d=3 (cubic lattice). There is a critical value p
c
(d,N) of the periodicity, which distinguishes between different low temperature structures. For p >p
c
, the ground state corresponds to a macroscopic phase separation between a dense hydrophobic core and hydrophilic loops. For
p <p
c
(but not too small), one gets a microscopic (finite scale) phase separation, and the ground state corresponds to a chain
or network of hydrophobic droplets, coated by hydrophilic monomers. We restrict our study to two extreme cases, and to illustrate the physics of the various phase transitions. A tentative variational approach is also presented.
Received: 10 March 1998 / Received in final form: 25 June 1998 / Accepted: 1st July 1998 相似文献
13.
N. Rehse C. Wang M. Hund M. Geoghegan R. Magerle G. Krausch 《The European physical journal. E, Soft matter》2001,4(1):69-76
We study the wetting behaviour of thin polystyrene (PS) films on regularly corrugated silicon substrates. Below a critical
film thickness the PS films are unstable and dewet the substrates. The dewetting process leads to the formation of nanoscopic
PS channels filling the grooves of the corrugated substrates. Films thicker than the critical thickness appear stable and
follow the underlying corrugation pattern. The critical thickness is found to scale with the radius of gyration of the unperturbed
polymer chains.
Received 6 April 2000 and Received in final form 24 August 2000 相似文献
14.
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
15.
L. Benguigui F. Boué 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,11(3):439-444
Small angle neutron scattering of polyacrylamide solutions and gels is presented for different polymer and cross link concentrations.
The gels are analyzed in connection with their elastic properties. For less than 11% of polymer concentration and for small
cross link contents , it was found that the gels are no more heterogeneous than the solutions. This corresponds to the range of where the elastic modulus increases with . For larger cross link contents when the elastic modulus decreases if increases, the sample appears to be strongly heterogeneous with a large size distribution of the inhomogeneities. The results
are compared with the recent model of Oshmyan and Benguigui of the elastic properties of the polyacrylamide gels. Thus a relation
can be proposed between the observation of the inhomogeneities by scattering and the mechanical properties
Received 30 December 1997 and Received in final form 26 June 1998 相似文献
16.
P.I.C. Teixeira 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,9(3):471-477
A model is proposed for the reorientation dynamics of a confined nematic liquid crystal elastomer, where the effect of crosslinks
is to couple the director to deformations of the elastic matrix. The model combines the (equilibrium) `neo-classical' theory
of liquid crystal rubber elasticity with the simplest time evolution equations for a system described by two coupled, non-conserved
order parameters. Relaxation from an orientation imposed by an electric field is studied as a function of elastic softness,
starting angle, surface pretilt, and the relative mobilities of director and strain. Most importantly, the absence of a `semi-soft'
elastic threshold changes the long-time behaviour of the effective refractive index of the medium from exponential to inverse
power law decay. Predictions are compatible with recent experimental results by Chang, Chien and Meyer [Phys. Rev. E 56, 595 (1997)].
Received 22 June 1998 相似文献
17.
J.-C. Castaing C. Allain P. Auroy L. Auvray 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,10(1):61-70
Suspensions of nanosized hairy grains have been prepared by grafting long polydimethylsiloxane chains (molecular weight ) onto silica particles (radius ), dispersed into a good solvent of PDMS. Depending on the particle volume fraction, different rheological behaviors are observed.
In the very dilute regime, the suspensions are perfectly stable and the particles behave almost as hard spheres: flow penetration
inside the corona is then very weak. When the particle volume fraction goes to the close packing volume fraction, the suspension
viscosity does not diverge as for hard spheres due to the increase of flow penetration inside the corona and to corona entanglements.
The particles have then the same behavior as polymer stars having an intermediate number of arms (). Finally, in the concentrated regime (), the suspensions form irreversible gels. We shown that this unexpected gelation phenomenon is related to the presence of
the silica cores: grafted PDMS chains can adsorb onto different particles and form irreversible bonds between the cores. The
viscosity and elastic modulus evolutions during gelation are well described by the scalar percolation model of sol-gel transition.
Received 23 March 1998 相似文献
18.
Several semicrystalline polymers show a recrystallization after melting during a heating scan. We have studied the mechanisms
of such recrystallization processes for two different polymers, namely syndiotactic polypropylene (sPP) and isotactic polystyrene
(iPS). This was done by monitoring the structure evolution during the recrystallization process and its changes during a subsequent
heating scan via time- and temperature-dependent SAXS measurements, respectively. The results of this study showed that the
sPP samples exhibited a recrystallization mechanism similar to the multi-stage route found upon initial crystallization of
semicrystalline polymers from an entangled melt. Meanwhile, a different recrystallization mechanism was shown by the iPS samples.
In this case, the recrystallization process proceeded as a direct growth into the melt in a one-step process. This is the
first time we have observed such a mechanism which resembles the picture presented by the classical models for crystallization
from an entangled polymer melt. The reason for such different mechanisms may be related to the initial melt state prior to
crystallization. It seems as though, when crystallization sets in an entangled polymer melt, it follows the multi-stage route,
whereas if the melt is locally disentangled, it proceeds by a direct growth mechanism.
Received 23 July 2001 and Received in final form 4 October 2001 相似文献
19.
D.J. Read 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,12(3):431-449
There are many experimental situations in which polymer chains are constrained or localised into a small region of space (e.g. melt chains confined to a “tube”, network chains pinned by crosslinks). We show that detailed consideration of the quenched
variables is vital in these experiments. This paper provides a crucial link between microscopic models with localising constraints
and scattering patterns by a generalisation of the Random Phase Approximation (RPA) which allows for quenched translational
variables. A method is developed which deals with correlations between the quenched variables brought about by incompressiblity
(for example, in a polymer melt there are correlations between tubes because of the interaction between chains). As an example,
the generalised RPA is applied to models based on the Warner-Edwards picture of the tube. Theoretical results for a melt of
H-shaped copolymers are compared with experimental scattering. Early results suggest that to fit the scattering we may be
forced to relax one of the central assumptions of the tube model; that the tube deforms affinely, that all chains retract
by the same amount or that the tube diameter does not couple to the strain.
Received 26 October 1998 and Received in final form 19 March 1999 相似文献
20.
J.J. Zanna H.T. Nguyen J.P. Parneix G. Ruffié M. Mauzac 《The European Physical Journal B - Condensed Matter and Complex Systems》1999,10(2):345-351
Dielectric response, from 1 Hz to 10 MHz, of liquid crystalline side chain elastomers has been compared to the one of analogous
uncrosslinked materials. Results are discussed in terms of mobility of the mesogens. Two different systems have been investigated:
smectic A elastomers allow to determine the influence of crosslinking on the relaxation (i.e. the reorientation of the whole mesogen around the chain), a S
C
* elastomer shows the drastic influence of the polymer network on the Goldstone mode.
Received 12 October 1998 相似文献