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.     

Segmental order and dynamics of polymer chains confined in block copolymer lamellar mesophases: NMR and dielectric relaxation studies

The European Physical Journal E - The PDMS lamellar sublayers of a poly(styrene)-poly(dimethylsiloxane) diblock (PS-PDMS) and PS-PDMS-PS triblocks are investigated by NMR and dielectric...     

Raman and infrared study of amorphous SeTe/CdSe superlattices

Structural modifications induced by the interfaces in the amorphous sublayers of SeTe/CdSe superlattices are studied by means of Raman and infrared spectroscopy. The prevalence of polymer chain molecules in the SeTe sublayers is explained in terms of the intermolecular interactions. The relation between the root mean square bond angle deviations induced by the interfaces, spectral band width of LO phonons in CdSe, and amorphization of CdSe sublayers is studied. The increase of the distortion energy on CdSe sublayers amorphization is evaluated.     

Interlayer Exchange Coupling Across Quasi-Amorphous Ti-Co and Zr-Co Spacers

Co/Ti and Co/Zr multilayers with wedge-shaped and constant-thickness Ti and Zr sublayers were prepared using UHV DC/RF magnetron sputtering. Results showed that the Co sublayers are ferromagnetically coupled up to Ti and Zr spacer thickness of about 1.9 and 2.4 nm, respectively. Furthermore, a weak antiferromagnetic coupling of the Co sublayers was observed for a Ti (Zr) thickness range between 1.9 and 2.7 nm (2.4 and 3.2 nm). The Co sublayers are very weakly exchange coupled or decoupled for d _Ti 2.7 nm and d _Zr 3.2 nm. The rapid decrease of the interlayer exchange coupling could be explained by its strong damping due to formation of a non-magnetic quasi-amorphous Ti-Co and Zr-Co alloy layer at the interfaces.     

High-Field Electron Paramagnetic Resonance Reveals a Stable Glassy Fraction up to Melting in Semicrystalline Poly(dimethylsiloxane)

The reorientation of the guest 4-methoxy-TEMPO (spin probe) in the disordered fraction of semicrystalline poly(dimethylsiloxane) (PDMS) is investigated by high-field electron paramagnetic resonance (HF-EPR) at 190 and 285 GHz. Accurate numerical simulations of the HF-EPR lineshapes evidence that the reorientation times of the spin probes are distributed between the melting temperature \(T_{\rm m}\) and \(T_{\rm m}\)—30 K. The distribution exhibits, in addition to a broad component, a narrow component with low mobility up to the PDMS melting point. It is shown that the temperature dependence of the reorientation time of the spin probes with low mobility is the same of the spin probes in glassy PDMS. The result suggests that the low-mobility fraction is localized in the so-called rigid amorphous fraction.     

Dynamic model of super-arrhenius relaxation rates in glassy materials

Super-Arrhenius relaxation rates in glassy materials can be associated with thermally activated rearrangements of increasing numbers of molecules at decreasing temperatures. We explore a model of such a mechanism in which stringlike fluctuations in the neighborhood of shear transformation zones provide routes along which rearrangements can propagate, and the entropy associated with critically long strings allows the rearrangement to be distributed stably in the surrounding material. We further postulate that, at low enough temperatures, these fluctuations are localized on the interfaces between frustration-limited domains, and in this way obtain a modified Vogel-Fulcher formula for the relaxation rate.     

Quantum competitions between the effects of the interlayer exchange couplings and the magnetically structural symmetry in a four-layer ferrimagnetic superlattice

The magnon energy spectra, the sublayer magnetization and the quantum fluctuations in a ferrimagnetic superlattice consisting of four different magnetic sublayers are studied by employing the linear spin-wave approach and Green's function technique. The effects of the interlayer exchange couplings and the spin quantum numbers on the sublayer magnetization and the quantum fluctuations of the systems are discussed for three different spin configurations. The roles of quantum competitions among the interlayer exchange couplings and the symmetry of the different spin configurations have been understood. The magnetizations of some sublayers increase monotonously, while those of others can exhibit their maximum, and the quantum fluctuations of the whole superlattice system can show a minimum when one of the antiferromagnetic interlayer exchange couplings increases. This is due to the quantum competition/transmission of effects of the interlayer exchange couplings. When the spin quantum number of sublayers varies, the system goes through from a quantum region of small spin numbers to a classical region of large spin numbers. The quantum fluctuations of the system exhibit a maximum as a function of the spin quantum number of a sublayer, which is related with higher symmetry of the system. It belongs to the type III Shubnikov group of magnetic groups. This magnetically structural symmetry consists of not only the symmetry of space group, but also the symmetry of the direction and strength of spins.     

Chain segment ordering in lamellar sublayers of block copolymers: A NMR study

The chain segment dynamics in the bulk lamellar phase of polystyrene-polydimethylsiloxane (PS-PDMS) block copolymers has been probed by NMR. The experiments were performed on a PS-PDMS diblock and on a PS-PDMS-PS triblock with twice the molecular weight. In the diblock, at room temperature, the PDMS block segments undergo uniaxial reorientations around the normal to the lamellae. In the triblock, the reorientational motions exhibit a lower degree of symmetry: deviations from a uniaxial dynamics are observed. Such a behaviour originates in the anchorage of both PDMS chain ends into the PS glassy layers. Received 27 September 2001 and Received in final form 18 January 2002     

Light scattering by multilayer nonconfocal ellipsoids in the Rayleigh approximation

The Rayleigh approximation of light scattering by multilayer nonconfocal coaxial ellipsoids is constructed. The ellipsoids that form a multilayer particle have coincident centers, and their principal axes are parallel. A particle in a constant electric field is considered, and the corresponding system of Laplace equations and boundary conditions is solved. Since the ellipsoidal surfaces of the interlayer interfaces are not confocal, the layers are divided into many sublayers, in each of which the potential is written in the proper ellipsoidal systems of coordinates. The potentials are sewn together at the sublayer interfaces by approximate matching conditions (the continuity of potentials and their normal derivatives). The polarizability of a multilayer particle is expressed as a 2×2 matrix in terms of the parameters of the core and the subsequent layers.     

Thermodynamic picture of the glassy state gained from exactly solvable models

A picture for thermodynamics of the glassy state was introduced recently by us [Phys. Rev. Lett. 79, 1317 (1997); 80, 5580 (1998)]. It starts by assuming that one extra parameter, the effective temperature, is needed to describe the glassy state. This approach connects responses of macroscopic observables to a field change with their temporal fluctuations, and with the fluctuation-dissipation relation, in a generalized, nonequilibrium way. Similar universal relations do not hold between energy fluctuations and the specific heat. In the present paper, the underlying arguments are discussed in greater length. The main part of the paper involves details of the exact dynamical solution of two simple models introduced recently: uncoupled harmonic oscillators subject to parallel Monte Carlo dynamics, and independent spherical spins in a random field with such dynamics. At low temperature, the relaxation time of both models diverges as an Arrhenius law, which causes glassy behavior in typical situations. In the glassy regime, we are able to verify the above-mentioned relations for the thermodynamics of the glassy state. In the course of the analysis, it is argued that stretched exponential behavior is not a fundamental property of the glassy state, though it may be useful for fitting in a limited parameter regime.     

Simple liquid crystal display backlight unit comprising only a single-sheet micropatterned polydimethylsiloxane (PDMS) light-guide plate

A simple liquid crystal display (LCD) backlight unit (BLU) comprising only a single-sheet polydimethylsiloxane (PDMS) light-guide plate (LGP) has been developed. The PDMS LGP, having micropatterns with an inverse-trapezoidal cross section, was fabricated by backside 3-D diffuser lithography followed by PDMS-to-PDMS replication. The fabricated BLU showed an average luminance of 2878 cd/m(2) with 73.3% uniformity when mounted in a 5.08 cm backlight module with four side view 0.85cd LEDs. The developed BLU can greatly reduce the cost and thickness of LCDs, and it can be applied to flexible displays as a flexible light source due to the flexible characteristic of the PDMS itself.     

Enhanced field emission properties of screen-printed doubled-walled carbon nanotubes by polydimethylsiloxane elastomer

Field emission (FE) properties of double-walled carbon nanotubes (DWCNTs) treated by polydimethylsiloxane (PDMS) elastomer with different heating temperature have been systematically studied. The current density of treated DWCNT films decreases with the increase of heating temperature. The screen-printed DWCNTs treated by PDMS elastomer with drying temperature 150 °C for 20 min have the best FE performance with a marvelous field enhancement factor (β = 20194). The optimized FE performance is attributed to the morphological change of DWCNT films after PDMS elastomer treatment and the change of separation energy for the CNT-substrate interface. It is proved that the PDMS treatment is a facile and effective method for field emission display (FED) application, especially for low-temperature FED preparation.     

非晶态合金表面的水润湿动力学

采用分子动力学模拟方法研究了改进的Simple pointcharge模型SPC/E水滴在Cu_(50)Zr_(50)非晶薄膜上的润湿行为和铺展过程.通过与CuZr(110)和(100)晶面对比研究发现,水滴在Cu_(50)Zr_(50)非晶薄膜表面上表现出较高的铺展速度.水滴在非晶合金表面的铺展过程中形成了明显的吸附层;而在晶态表面,水滴铺展前沿呈脚状形态.分析结果表明非晶表面的水分子在吸附层内呈现完全无序的单层排列方式,而在晶态表面,特别是(100)晶面,吸附层水分子呈双层有序排列.这种吸附层结构的差异导致了吸附层内水分子方向的差异:非晶表面吸附层内水分子方向倾向平行于表面,而晶态基底上吸附层内的水分子倾向于指向液滴内部.由此造成了非晶表面吸附层中的水分子与液滴内部以相对较弱的氢键相互作用,这使得上层水分子能够较容易扩散至吸附层前沿,促进液滴铺展.     

Spontaneous emission and elastic scattering of light from quantum-well excitons in a Fabry-Perot microcavity

A theory of spontaneous emission and elastic light scattering by quasi-two-dimensional excitons in a quantum well placed in a Fabry-Perot microcavity is developed. The problem is solved by means of electrodynamic Green’s functions with inclusion of fluctuations of the quantum-well width and cavity wall shape treated as a perturbation. General expressions are found in a zero approximation of perturbation theory (plane interfaces) for the radiative decay rates of quasi-two-dimensional excitons and for their energy shifts in the cavity. The boundary conditions for the electromagnetic field are taken into account through the coefficients of inward light reflection from the cavity walls. Resonance contributions to the scattering cross sections, which differ in the polarizations (p or s) of the incident and scattered waves, are derived in the lowest (Born) approximation in quantum-well width fluctuations. The spectral and angular dependences of elastic light scattering are studied numerically for Gaussian and exponential correlation functions. It is shown that the contribution from quantum-well width fluctuations to light scattering exceeds that due to single interfaces (surfaces) of a heterostructure by two orders of magnitude.     

Fast interfacial ionic conduction in nanostructured glass ceramics

The hopping movements of mobile ions in a nanostructured LiAlSiO4 glass ceramic are characterized by time-domain electrostatic force spectroscopy (TDEFS). While the macroscopic conductivity spectra are governed by a single activation energy, the nanoscopic TDEFS measurements reveal three different dynamic processes with distinct activation energies. Apart from the ion transport processes in the glassy and crystalline phases, we identify a third process with a very low activation energy, which is assigned to ionic movements at the interfaces between the crystallites and glassy phase. Such interfacial processes are believed to play a key role for obtaining high ionic conductivities in nanostructured solid electrolytes.     

Polydimethylsiloxane at the interfaces of fumed silica and zirconia/fumed silica

Polydimethylsiloxane (PDMS)/fumed silica A-300 and PDMS/ZrO₂/A-300 were studied using adsorption, thermogravimetry, temperature-programmed desorption (TPD) mass-spectrometry, infrared spectroscopy, XRD, and broadband dielectric relaxation spectroscopy. ZrO₂ was synthesized on fumed silica with zirconium acetylacetonate in CCl₄ at 350 K for 1 h and calcinated at 773 K for 1 h (1-4 reaction cycles). PDMS (5-40 wt.%) was adsorbed onto silica and zirconia/silica from hexane solution and then dried. Grafted zirconia changes the chemistry of the surface (because of its catalytic capability) and the topology of secondary particles (because of occupation of voids in aggregates of primary silica particles by zirconia nanoparticles) responsible for the textural porosity of the powders. Therefore, many properties (such as structural characteristics of the composites, reactions on heating in air and vacuum, interfacial relaxation phenomena, hydrophobicity as a function of treatment temperature, etc.) of PDMS/zirconia/silica strongly differ from those of PDMS/A-300. Broadening of the α-relaxation of PDMS at the interfaces of disperse oxides suggests both weakening of the PDMS-PDMS interaction and strengthening of the PDMS-oxide interaction.     

Thermal annealing of a-Si:H/a-SiNx:H multilayers

The crystallinity of Si/SiN_x multilayers annealed by a rapid thermal process and furnace annealing is investigated by a Raman-scattering technique and transmission electron microscopy. It is found that the crystallization temperature varies from 900 °C to 1000 °C when the thickness of a-Si:H decreases from 4.0 nm to 2.0 nm. Raman measurements imply that the high crystallization temperature for the a-Si:H sublayers originates from the confinement modulated by the interfaces between a-Si:H and a-SiN_x:H. In addition to the annealing temperature, the thermal process also plays an important role in crystallization of a-Si sublayers. The a-Si:H sublayers thinner than 4.0 nm can not be crystallized by furnace annealing for 30 min, even when the annealing temperature is as high as 1000 °C. In contrast, rapid thermal annealing is advantageous for nucleation and crystallization. The origin of process-dependent crystallization in constrained a-Si:H is briefly discussed. Received: 11 April 2001 / Accepted: 20 June 2001 / Published online: 30 August 2001     

Effect of substrate elasticity on evaporation kinetics and evaporative deposition of aqueous polystyrene nanoparticles droplets

Evaporation of aqueous polystyrene(PS) nanoparticles droplets on silicon and polydimethylsiloxane(PDMS) surfaces was studied. Experimental results showed that softer PDMS surfaces yielded a longer constant contact radius(CCR) stage, which could be ascribed to surface deformation of PDMS induced by the vertical component of liquid-vapor interfacial tension. Ringlike depositions of nanoparticles with different crack patterns were found on both silicon and PDMS surfaces. In-situ observation of crack formation showed that nanoparticle movement on the silicon surface was impeded, resulting in radial cracks with periodic distribution. In contrast, nanoparticles were shown to move easily on the PDMS surface. This observation indicated the difference in crack patterns on surfaces could be attributed to the friction force between nanoparticles and the substrate. A large friction force between nanoparticles and the substrate prevented cracks from moving, resulting in a radial crack pattern with periodic distribution, while a small friction force produced multiple large cracks.     

A flexible piezoelectric-triboelectric hybrid nanogenerator in one structure with dual doping enhancement effects

For achieving more effective mechanical energy conversion, based on low-cost and flexible polydimethylsiloxane (PDMS) and polyvinylidene fluoride (PVDF) polymer film as triboelectric and piezoelectric function layer respectively, a polymer-based hybrid piezoelectric-triboelectric nanogenerator (PT-NG) with a structure of Al/PVDF/Cu-PDMS/indium tin oxide (ITO)/polyethylene terephthalate (PET) has been demonstrated. The device has realized the simultaneous triboelectric and piezoelectric conversions in one structure. In addition, when PDMS and PVDF are further modified by graphene quantum dot (GQD) and titanium dioxide (TiO₂) nanoparticles respectively, both triboelectric and piezoelectric outputs of the modified device are greatly enhanced synchronously. The experimental results have indicated that the increase of triboelectric output is due to the improvement of dielectric properties of PDMS film doped with conductive GQDs as well as the enhancement of the effective contact caused by the change of PDMS surface microstructure. While the promotion of piezoelectric output is mainly attributed to the fact that PVDF film after TiO₂ modification induces more polarized β phase with a polarization-free process. Accordingly, the modified device converts mechanical energy into electricity more effectively, which shows a promising prospect in the fields of flexibility display, electronic skin, wearable electronic products and self-powered sensors.     

Inhomogeneous low frequency spin dynamics in La(1.65)Eu(0.2)Sr(0.15)CuO(4)

We report Cu and La nuclear magnetic resonance measurements in the title compound that reveal an inhomogeneous glassy behavior of the spin dynamics. A low temperature peak in the La spin lattice relaxation rate and the "wipeout" of Cu intensity both arise from these slow electronic spin fluctuations that reveal a distribution of activation energies. Inhomogeneous slowing of spin fluctuations appears to be a general feature of doped lanthanum cuprate.