Periodic surface photoreliefs in glassy and hyperelastic polymers

The formation and degradation of periodic photoreliefs on the surface of polymer layers having significantly different glass-transition temperatures are investigated for various process activation temperatures. It is established that the main factor limiting the resolution of periodic relaxation photoreliefs at the surface of glassy polymer layers containing dimerizing anthracene derivatives is the presence of shear stresses. Their action is suppressed by the thermal decomposition of dimers, a process which gives rise to inverted reliefs of higher spatial frequencies. It is shown that the resolution can be enhanced by more than an order of magnitude by using a polymer matrix in the hyperelastic state. Zh. Tekh. Fiz. 69, 79–83 (August 1999)     

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.     

Creeping friction dynamics and molecular dissipation mechanisms in glassy polymers

The dissipation mechanism of nanoscale kinetic friction between an atomic force microscopy tip and a surface of amorphous glassy polystyrene has been studied as a function of two parameters: the scanning velocity and the temperature. Superposition of the friction results using the method of reduced variables revealed the dissipative behavior as an activated relaxation process with a potential barrier height of 7.0 kcal/mol, corresponding to the hindered rotation of phenyl groups around the C-C bond with the backbone. The velocity relationship with friction F(v) was found to satisfy simple fluctuation surface potential models with F proportional to const-ln(v) and F proportional to const-ln(v)2/3.     

Mössbauer studies of polymers in the glassy state and near their glass transition

Mössbauer absorption spectra of glassy block and copolymers with lowT _G containing ferrocene have been recorded in a wide temperature range (4–330 K) to investigate the dynamical processes of the system, in the glassy state and around the glass transition. An inhomogeneous broadening due to conformational substates with different hyperfine parameters found at lower temperatures becomes motionally narrowed above 40 K. Whereas in the glassy state both the block and the copolymers reveal the same vibrational behaviour, we found differences in the vicinity of their respective glass transitions. For the block polymer an anomalous decrease of the recoilfree fractionf _A was found above the static glass transition. In agreement with neutron data on other polymers we attribute this behaviour to an onset of motions of the ferrocene units of higher than 10¹⁰ Hz. For the copolymers, however, a departure of lnf _A from linearity was detected already far below the staticT _G. We propose that this is caused by residual solvents which allow an easier cooperative rearrangement of the segments even belowT _G. First attempts to interpret the Mössbauer data by means of the predictions of the mode coupling theory are reported.     

On the size dependence of the surface tension

The general form of a differential equation that deduces a size dependence of the surface tension is derived. The well-known Gibbs-Tolman-Koenig-Buff equation for the spherical surface is a particular case of the newly derived one. Analytical solutions to this equation for the spherical, cylindrical, parabolic, and conical surfaces are found.     

Influence of the size dependence of surface tension on the dynamics of a bubble in a liquid

The Gibbs dividing surface method is used to derive the differential equation defining the dependence of the surface tension of a bubble in a nonpolar single-component liquid on its radius. Exact and asymptotic solutions of this equation have been obtained. It follows from the calculations that the bubble surface tension increases with decreasing radius. The Rayleigh-Plesset equation describing the bubble collapse dynamics is solved numerically by taking into account the size dependence of surface tension. The size dependence of surface tension is shown to affect significantly the final bubble collapse stage and, on the whole, accelerates this process.     

Some remarks on the chain segment dynamics in confined polymers

Recently, the chain dynamics of molten polymers in confined geometries has been investigated, using NMR. Some of these experiments have been performed on poydimethylsiloxane (PDMS) confined in planar nanolayers involving non-adsorbant solid surfaces. Here the data obtained on various PDMS systems are compared. The common property to these systems is that the local dynamics is anisotropic and the chain segments are undergoing uniaxial fluctuations around the normal n to the layers. Our data clearly show that the sign of the degree of orientational order S ( i.e. the direction of the fluctuations around n) and the broadening of the order distribution P( S) both result from the relative influence of the impenetrable interfaces and anchoring junctions on the segment behaviour.     

Some remarks on the chain segment dynamics in confined polymers

The European Physical Journal E - Recently, the chain dynamics of molten polymers in confined geometries has been investigated, using NMR. Some of these experiments have been performed on...     

The fracture mechanics of glassy polymers

The application of fracture mechanics to glassy polymers, in particular crack growth in PMMA, is discussed. Particular attention is paid to two processes which modulate the energy supply to the crack tip: viscoelastic dissipation at slow crack speeds and specimen inertia at large crack speeds. The relation between fracture energy and crack speed is reviewed, and, where possible, fracture surface observations are correlated with dynamic behavior.     

Specific features of the temperature dependence of the conformational dynamics of polymers

Physics of the Solid State - The dielectric and mechanical losses in the amorphous polar polymer poly(vinyl acetate) (PVA) are measured over wide ranges of frequencies and temperatures. It is...     

Molecular conformation in molten and glassy polymers

As a consequence of enthalpy fluctuations in a small thermodynamic system, the conformation in molten and glassy polymers is predicted to be more nearly an irregularly folded molecule with 5 to 10 nearest neighbors than a random coil with 50 to 100 nearest neighbors.     

氦在铝表面辐照和积聚的团簇动力学模拟

我们利用团簇动力学模型(IRadMat)研究了keV-He离子辐照金属铝的缺陷动力学和氦的聚集行为.通过对不同俘获类型(团簇、晶界和位错)俘获He浓度的定量分析,我们发现大多数He原子被晶界吸收,这成为铝在低辐照通量下发生脆化的主要原因.随着辐照能量的增加,He滞留峰的位置会变得更深.然而,随着辐照通量的增加,He在体内的滞留量会变得更多,但滞留深度的峰值位置不变.我们的结果表明,晶界的影响在He的滞留分布以及铝的脆化行为中起着关键作用,这也有助于我们理解He在金属中的动力学行为和损伤的分布.     

Temperature and spatial dependence of the magnetization near a surface of a ferromagnet

The temperature and spatial dependence of the magnetization in a semi-infinite, single tight-binding band model ferromagnet is calculated. It is assumed that the spatial dependence of the magnetization arises primarily because of the downward shift of the ‘atomic’ energy level in the surface layer. This shift was found to occur in Ni in a previous calculation. The effects of the surface are approximated by considering a plane of defects in an otherwise translationally invariant metal. The surface layer is furthermore assumed to be paramagnetic as was previously suggested to be the case for Ni. It is demonstrated that the range of the magnetic disturbance arising from the surface increases with temperature and that the deviation in the magnetization of a given layer from the bulk magnetization is a damped oscillatory function of the distance of the layer from the surface. while the present theory assumes a highly over simplified model for the band structure and it is, therefore, not reasonable to apply these calculations directly to real metals, the results found here are qualitatively consistent with experiments on Ni.     

WIM/WISE NMR studies of chain dynamics in solid polymers and blends

Wideline separation (WISE) NMR with window-less isotropic mixing (WIM) is developed as a method to study the dynamics of polymers and blends. This experiment is designed to measure the dynamics of polymers through the proton lineshapes that are correlated with the carbon chemical shifts in two-dimensional NMR experiments. If the atoms experience large amplitude fluctuations that are fast compared to the dipolar broadening, then the proton lines will be narrowed relative to rigid solids. We have modified the WISE experiment by replacing the cross polarization step with WIM to quench spin diffusion during the cross polarization so that the proton linewidths can be directly related to the chain dynamics. Two-dimensional WIM/WISE has been used to measure the main-chain and side-chain dynamics in poly(n-butyl methacrylate) and blends of polystyrene and poly(vinyl methyl ether).     

Partial crystallinity in alkyl side chain polymers dictates surface freezing

We have studied the structure of a novel crystalline surface monolayer on top of a disordered melt of the same material [poly(n-alkyl acrylate)s] using grazing incidence x-ray diffraction. The grazing incidence x-ray diffraction, surface tension, and bulk latent heat results show that side chains crystallize except the nine methylene units of the alkyl side chains closest to the polymer backbone. The partial crystallinity along with a thicker surface layer, due to the additional length of the linker group, explains why the difference between the surface order-to-disorder transition temperature and bulk melting temperature increases with a decrease in the length of the alkyl side chain.     

Glassy dynamics of model colloidal polymers:Effect of controlled chain stiffness

Colloidal polymers with tunable chain stiffness have been successfully assembled in experiments recently.Similar to molecular polymers,chain stiffness is an important feature which can distinctly affect the dynamical behaviors of colloidal polymers.Hence,we model colloidal polymers with controlled chain stiffness and study the effect of chain stiffness on glassy behaviors.For stiff chains,there are long-ranged periodic intrachain correlations besides two incompatible local length scales,i.e.,monomer size and bond length.The mean square displacement of monomers exhibits sub-diffusion at intermediate time/length scale and the sub-diffusive exponent increases with chain stiffness.The data of localization length of stiff polymers versus rescaled volume fraction for different monomer sizes can gather close to an exponential curve and decay slower than those of flexible polymers.The increase of chain stiffness linearly increases the activation energy of the colloidal-polymer system and thus makes the colloidal polymers vitrify at lower volume fraction.Static and dynamic equivalences between stiff colloidal polymers of different monomer sizes have been checked.     

Rheological observation of glassy dynamics of dilute polymer solutions near the coil-stretch transition in elongational flows

It has long been conjectured that the macroscopic dynamics of dilute polymer solutions may exhibit a glasslike slowdown caused by ergodicity breaking, in the vicinity of the coil-stretch transition in elongational flows. We report experimental observations using a filament stretching rheometer that confirm the existence of such glassy states. It is observed that different time-dependent elongational strain-rate profiles lead to a pronounced history dependence and aging effects within a narrow range of strain rates. The results have a direct bearing on the analysis and design of processes employing dilute polymer solutions, such as ink-jet printing, surface coating, and turbulent-drag reduction.     

The nuclear size dependence of large momentum transfer scattering and quark interaction dynamics

A simple dynamical picture for hadron interactions is used to discuss the A dependence of large p_⊥ scattering and dilepton production. Observations in large p_⊥ experiments are explained. Important corrections to dilepton production cross sections are found, leading to acceptable agreement between colored-quark theory and experiment.     

Wave packet dynamics of the photodetachment of H~- near dielectric surface

<正>The wave packet dynamics of the photodetachment of H~- near dielectric surface are studied by using the wave packet evolution and the autocorrelation function.The results show that the evolutions of the autocorrelation function and the wave packet in the time domain correspond well with each other. Besides,we consider the influence of the electronic state lifetime on the wave packet evolution and the autocorrelation function.Numerical simulation shows that the evolution of the photodetached electronic wave packet near the dielectric surface exhibits some properties similar to the time-resolved two-photon photoemission intensity of surface electron.