Mechanical properties of thin confined polymer films close to the glass transition in the linear regime of deformation: Theory and simulations

Over the past twenty years experiments performed on thin polymer films deposited on substrates have shown that the glass transition temperature T(g) can either decrease or increase depending on the strength of the interactions. Over the same period, experiments have also demonstrated that the dynamics in liquids close to the glass transition temperature is strongly heterogeneous, on the scale of a few nanometers. A model for the dynamics of non-polar polymers, based on percolation of slow subunits, has been proposed and developed over the past ten years. It proposes a unified mechanism regarding these two features. By extending this model, we have developed a 3D model, solved by numerical simulations, in order to describe and calculate the mechanical properties of polymers close to the glass transition in the linear regime of deformation, with a spatial resolution corresponding to the subunit size. We focus on the case of polymers confined between two substrates with non-negligible interactions between the polymer and the substrates, a situation which may be compared to filled elastomers. We calculate the evolution of the elastic modulus as a function of temperature, for different film thicknesses and polymer-substrate interactions. In particular, this allows to calculate the corresponding increase of glass transition temperature, up to 20 K in the considered situations. Moreover, between the bulk T(g) and T(g) + 50 K the modulus of the confined layers is found to decrease very slowly in some cases, with moduli more than ten times larger than that of the pure matrix at temperatures up to T(g) + 50 K. This is consistent with what is observed in reinforced elastomers. This slow decrease of the modulus is accompanied by huge fluctuations of the stress at the scale of a few tens of nanometers that may even be negative as compared to the solicitation, in a way that may be analogous to mechanical heterogeneities observed recently in molecular dynamics simulations. As a consequence, confinement may result not only in an increase of the glass transition temperature, but in a huge broadening of the glass transition.     

Influence of stress and temperature on damping behavior of amorphous Pd₇₇.₅Cu₆.₀Si ₁₆.₅ below Tg

In this work we analyzed the mechanical damping behavior of amorphous Pd(77.5)Cu(6.0)Si(16.5) below the glass transition temperature (T(g)) with creep/recovery measurements. Here a correlation between temperature stimulation and external stress is found in an exponential, multiplicative way. This demonstrates that not only is the yield stress of the material influenced by temperature variation (mechanical melting) but also the secondary relaxation is modified under stress and temperature.     

Two simultaneous mechanisms causing glass transition temperature reductions in high molecular weight freestanding polymer films as measured by transmission ellipsometry

We study the glass transition in confined polymer films and present the first experimental evidence indicating that two separate mechanisms can act simultaneously on the film to propagate enhanced mobility from the free surface into the material. Using transmission ellipsometry, we have measured the thermal expansion of ultrathin, high molecular-weight (MW), freestanding polystyrene films over an extended temperature range. For two different MWs, we observed two distinct reduced glass transition temperatures (T(g)'s), separated by up to 60 K, within single films with thicknesses h less than 70 nm. The lower transition follows the expected MW dependent, linear T(g)(h) behavior previously seen in high MW freestanding films. We also observe a much stronger upper transition with no MW dependence that exhibits the same T(g)(h) dependence as supported and low MW freestanding polymer films.     

Dielectric investigation of the temperature dependence of the nonexponentiality of the dynamics of polymer melts

Using broad band dielectric spectroscopy (10(-5)-10(9) Hz), combining time domain and frequency domain techniques, we study the temperature dependence of the non-Debye character of the alpha relaxation of polymer melts in the glass transition temperature T(g) range. The alpha relaxation process is described in terms of the Kohlrausch-Williams-Watts relaxation function which has a single parameter beta to characterize the nonexponentiality of the relaxation. At high temperatures, beta remains nearly insensitive to temperature changes, whereas in the vicinity of T(g) a nearly linear increasing of beta with temperature is found. The temperature range where the change of the beta(T) behavior occurs is located for all the polymers investigated around 1.2T(g). Moreover, our results indicate a common value of beta approximately equal to 1/3 at the temperature where the relaxation time diverges. The beta(T) behavior near T(g) is discussed in terms of a "rugged landscape" phase space which allows us to rationalize both the beta(T) behavior observed as well as the similarities of our findings near T(g) with the results reported in simulations on Ising spin glasses and other model systems.     

Three-dimensional athermal waveguide at 1.3 μm wavelength for temperature independent lightwave devices

To eliminate the temperature dependence of narrow-band filter, a three-dimensional athermal waveguide of which optical path length is independent of temperature was developed at 1.3 μm wavelength. The temperature coefficient of the refractive index of films made of glass materials was measured at this wavelength, and a strip-loaded athermal waveguide was designed using a scalar finite element method. The temperature coefficient of optical path length was successfully decreased to 9.65X10^-9[/K] (0.1% of the conventional waveguide).     

Specific heats of the charge density wave compounds o-TaS and (TaSe ) I

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     

Quantum critical scaling and the origin of non-fermi-liquid behavior in Sc1-xUxPd3

We used inelastic neutron scattering to study magnetic excitations of Sc1-xUxPd3 for U concentrations (x=0.25, 0.35) near the spin glass quantum critical point (QCP). The excitations are spatially incoherent, broad in energy (E=variant Planck's over 2piomega), and follow omega/T scaling at all wave vectors investigated. Since similar omega/T scaling has been observed for UCu5-xPdx and CeCu6-xAux near the antiferromagnetic QCP, we argue that the observed non-Fermi-liquid behavior in these f-electron materials arises from the critical phenomena near a T=0 K phase transition, irrespective of the nature of the transition.     

Reductions of the glass transition temperature in thin polymer films: probing the length scale of cooperative dynamics

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.     

Activated dynamics and effective temperature in a steady state sheared glass

We conduct nonequilibrium molecular dynamics simulations to measure the shear stress sigma, the average inherent structure energy E{IS}, and the effective temperature T{eff} of a sheared model glass as a function of bath temperature T and shear strain rate gamma. For T above the glass transition temperature T0, the rheology approaches a Newtonian limit and T{eff}-->T as gamma-->0, while for T相似文献   

Glass transition and relaxation processes in supercooled water

Many of water's peculiar physical properties are still not well understood, and one of the most important unresolved questions is its glass transition related dynamics. The consensus has been to accept a glass transition temperature (T(g)) around 136 K, but this value has been questioned and reassigned to about 165 K. We find evidence that the dielectric relaxation process of confined water that has been associated with the long accepted T(g) of water (130-140 K) must be a local process which is not related to the actual glass transition. Rather, our data indicate a glass transition at 160-165 K for bulk water and about 175 K for confined water (depending on the confining system).     

Pressure, temperature, and thickness dependence of CO2-induced devitrification of polymer films

The glass transition temperature is known to increase with decreasing film thickness h for sufficiently thin poly(methyl methacrylate) films supported by silicon oxide substrates. We show that this system undergoes a CO2 pressure-induced devitrification transition, P(g), which is film thickness dependent, P(g)(h)=DeltaP(g)+P(bulk)(g). P(bulk)(g) is the bulk glass transition and DeltaP(g) can be positive or negative depending on T and P. The phenomenon of retrograde vitrification, wherein the polymer exhibits a rubbery-to-glassy-to-rubbery transition upon changing temperature isobarically, is also shown to occur in this system and it is film thickness dependent.     

大块金属玻璃Zr41Ti14Cu12.5Ni10Be22.5的流变行为研究

采用静态拉伸方法在连续升温条件下动态地测量了大块金属玻璃Zr41Ti14Cu12.5Ni10Be22.5（Vit1）的黏度随温度的变化关系.在应变速率与温度的关系曲线中，观测到了与玻璃转变和晶化过程相联系的多个应变速率峰.在玻璃转变温度Tg以上，大块金属玻璃Zr41Ti14Cu12.5Ni10Be22.5的过冷液体呈现Newton流体特征，其黏度与温度的关系符合Vogel Fulcher-Tammann (VFT)关系式，拟合得到脆度D*＝36，VFT温度T0＝319K，脆度参数m＝30，这说明Zr41T 关键词： 大块金属玻璃 应变速率 剪切黏度 自由体积     

Dynamics and internal stress at the nanoscale related to unique thermomechanical behavior in polymer nanocomposites

A small amount of alumina nanoparticles in polymethylmethacrylate causes a sharp depression of the glass transition temperature (Tg) accompanied by a toughening of the composite. We investigated this phenomenon using multispeckle x-ray photon correlation spectroscopy. Measurements reveal a dynamic structure factor that has the form exp[-(t/taua)beta], with beta greater than 1. We show for the first time that beta(T) tracks the internal stress at the polymer-particle interface. The internal stress, which we propose arises due to the entropic penalty that the polymer faces in the presence of the nanoparticles, engenders temporally heterogeneous dynamics. In the jammed glassy state, we show that the dominant fast relaxation mode--taumax--aided by a weak dewetting interface relieves the stress and follows the variations in Tg.     

Dielectric relaxations in ultrathin isotactic PMMA films and PS-PMMA-PS trilayer films

The local and cooperative dynamics of supported ultrathin films ( L = 6.4 - 120 nm) of isotactic poly(methyl methacrylate) (i-PMMA, Mn = 118 x 10(3) g/mol) was studied using dielectric relaxation spectroscopy for a wide range of frequencies (0.1 Hz to 10(6) Hz) and temperatures (250 - 423 K). To assess the influence of the PMMA film surfaces on the glass transition dynamics, two different sample geometries were employed: a single layer PMMA film with the film surfaces in direct contact with aluminum films which act as attractive, hard boundaries; and a stacked polystyrene-PMMA-polystyrene trilayer film which contains diffuse PMMA-PS interfaces. For single layer films of i-PMMA, a decrease of the glass transition temperature T(g) by up to 10 K was observed for a film thickness L < 25 nm (comparable to R(EE)), indicated by a decrease of the peak temperature T(alpha) in the loss epsilon(")(T) at low and high frequencies and by a decrease in the temperature corresponding to the maximum in the apparent activation energy E(a)(T) of the alpha-process. In contrast, measurements of i-PMMA sandwiched between PS-layers revealed a slight (up to 5 K) increase in T(g) for PMMA film thickness values less than 30 nm. The slowing down of the glass transition dynamics for the thinnest PMMA films is consistent with an increased contribution from the less mobile PMMA-PS interdiffusion regions.     

Mechanical, vibrational, and dynamical properties of amorphous systems near jamming

Amorphous systems undergo the jamming transition when the density increases, temperature drops, or external shear stress decreases, as described by the jamming phase diagram which was proposed to unify different processes such as the glass transition, random close packing, and yielding under shear stress. At zero temperature and shear stress, the jamming transition occurs at a critical density at Point J. In this paper, we review recent studies of the material properties of marginally jammed solids and the glassy dynamics in the vicinity of Point J. As the only singular point in the jamming phase diagram, Point J exhibits special criticality in both mechanical and vibrational quantities. Dynamics approaching the glass transition in the vicinity of Point J show critical scalings, suggesting that the molecular glass transition and the colloidal glass transition are equivalent in the hard sphere limit. All these studies shed light on the long-standing puzzles of the glass transition and unusual properties of amorphous solids.     

Glass transitions and dynamics in thin polymer films: dielectric relaxation of thin films of polystyrene

The glass transition temperature T(g) and the temperature T(alpha) corresponding to the peak in the dielectric loss due to the alpha process have been simultaneously determined as functions of film thickness d through dielectric measurements for polystyrene thin films supported on glass substrate. The dielectric loss peaks have also been investigated as functions of frequency for a given temperature. A decrease in T(g) was observed with decreasing film thickness, while T(alpha) was found to remain almost constant for d>d(c) and to decrease drastically with decreasing d for d相似文献   

Magnetic, structural, and thermal properties of CoV?O?

In this paper, we investigate the electric, magnetic, structural, and thermal properties of spinel CoV(2)O(4). The temperature dependence of magnetization shows that, in addition to the paramagnetic-to-ferrimagnetic transition at T(C) = 142 K, two magnetic anomalies exist at 100 K, T(1) = 59 K. Consistent with the anomalies, the thermal conductivity presents two valleys at 100 K and T(1). At the temperature T(1), the heat capacity shows one peak, which cannot be attributed to the structural transition as revealed by the x-ray diffraction patterns for CoV(2)O(4). Below the transition temperature T(1), the ac susceptibility displays the characteristics of a glass. The series of phenomena at T(1) and the orbital state on V(3+) sites are discussed.     

Finite-size scaling analysis of the glass transition

We show that finite-size scaling techniques can be employed to study the glass transition. Our results follow from the postulate of a diverging dynamical correlation length at the glass transition whose physical manifestation is the presence of dynamical heterogeneities. We introduce a parameter B(T,L) whose temperature, T, and system size, L, dependences permit a precise location of the glass transition. We discuss the finite-size scaling behavior of a diverging susceptibility chi(L,T). These new techniques are successfully used to study two lattice models. The analysis straightforwardly applies to any glass-forming system.     

Lactone polymers. V. Viscoelastic properties of poly-ε-caprolactone and poly-ε-thiocaprolactone

The viscoelastic properties of crystalline poly-ε-caprolactone and poly-ε-thiocaprolactone have been characterized and compared by stress relaxation and dynamic mechanical studies. The glass transition temperature of poly-ε-thiocaprolactone was shown to be -40°C at 1 Hz and appeared to be independent of the degree of crystallinity. The rate of viscoelastic relaxation for each polymer was independent of linear strain rate of a decade range. The density of each polymer over a wide temperature range was used to reduce the individual time-dependent modulus values to an arbitrary reference temperature. This reduction of stress relaxation data to a standard mechanical state obviated the requirement of vertical shift factors for construction of the respective master curves. The distribution of relaxation times was correlated with the glass transition and the crystalline melt temperature range for each polymer.     

Ageing of natural rubber under stress

We report a dynamical-mechanical study of stress relaxation at small deformation in a natural (polyisoprene) rubber well above its glass transition temperature . We find that an almost complete relaxation of stress takes place over very long relaxation periods, even though the elastic network is retained. The relaxation rate and the long-time equilibrium modulus are sensitive functions of temperature which do not follow time-temperature superposition. Many characteristic features of non-ergodic ageing response are apparent at both short and very long times. We interpret the observed behaviour in terms of the nature of rubber cross-links, capable of isomerisation under stress, and relate the results to recent models of slow glassy rheology. Received 22 November 1999 and Received in final form 18 January 2000