Confinement-induced differences between dielectric normal modes and segmental modes of an ion-conducting polymer

Dielectric measurement in the range 0.1 Hz to 1 MHz were used to study the motions of polymers and ions in an ion-conducting polymer, polypropylene oxide containing small quantities (on the order of 1%) of lithium ions (LiClO₄), confined as a sandwich of uniform thickness between parallel insulating mica surfaces. In the dielectric loss spectrum, we observed three peaks; they originated from the normal mode of the polymer, segmental mode of the polymer, and ion motions. With decreasing film thickness, the peak frequencies corresponding to the normal mode and ion motion shifted to lower frequencies, indicating retardation due to confinement above 30 nm. This was accompanied by diminished intensity of the dielectric normal-mode relaxation, suggesting that confinement diminished the fluctuations of the end-to-end vector of the chain dipole in the direction between the confining surfaces. On the contrary, the segmental mode was not affected at that thickness. Finally, significant retardation of the segmental mode was observed only for the thinnest film (14 nm). The different dynamical modes of the polymer (segmental and slowest normal modes) respond with different thickness and temperature dependence to confinement. Received 31 August 2001 and Received in final form 30 October 2001     

Dynamics of propylene glycol and its oligomers confined in clay

The dynamics of propylene glycol (PG) and its oligomers 7-PG and PPG, with (about 70 monomers), confined in a Na-vermiculite clay have been investigated by quasi-elastic neutron scattering and dielectric spectroscopy. The liquids are confined to a single molecular layer between the clay platelets, thus giving a true 2D liquid. The results show that the average relaxation time , deduced from neutron scattering at a momentum transfer Q of about , is in perfect agreement with the dielectric -relaxation time, although neutron scattering does not only probe the main ( -) relaxation, but all motions of hydrogens on the experimental time scale. At room temperature is proportional to Q ², indicating that the relaxations are mainly due to ordinary translational diffusion. The most unexpected finding is that (or the dielectric -relaxation time) is almost unaffected by the 2D confinement, in contrast to the dielectrically active normal mode of PPG which is substantially slower in the confinement. Only the 7-mer has a significantly slower segmental translational diffusion in the clay. The results suggest that the interactions to the clay surfaces are weak and that the present 2D confinement has a very small influence on the time scale of all our observed relaxation processes, except the normal-mode relaxation.Received: 1 January 2003, Published online: 8 October 2003PACS: 61.25.Em Molecular liquids - 68.35.Ja Surface and interface dynamics and vibrations - 61.12.-q Neutron diffraction and scattering     

Dynamics of an Optically Switched Bistable Semiconductor Laser

The switching time of an optically switched laser diode between a locked mode and free oscillation mode is studied. By starting with linear stability analysis, analytical expressions are developed to predict the switching time. It is shown that the switching of optical output between the locked and unlocked modes is governed by two relaxation processes with different time constants, and the change in the first relaxation process, dependent on the carrier density in the initial state, gives a strong effect on the total switching time, in contrast to the dynamic behavior of the carrier density dominated by a single relaxation process. Our analytical results show that larger detuning and power of optical input are effective in shortening the switching time and are believed useful to estimate the switching time of an optically switched bistable laser diode.     

Analysis of historical porous building materials by the NMR-MOUSE

Samples of sandstone with and without deposits of silicon oxide stone strengthener as well as samples of historical brick material were analyzed by transverse NMR relaxation and mercury intrusion porosimetry. Relaxation times and relaxation time distributions of the protons from the water saturated samples were measured by low-field NMR using homogeneous and inhomogeneous fields. The measurements in inhomogeneous fields were performed with two different NMR-MOUSE sensors, one with a field gradient of 2 T/m and the other with an average field gradient of about 20 T/m. In the sandstone samples the application of stone strengtheners was shown to result in a confinement of the large pores within the outer layer of a few millimeters depth. Depending on the ferromagnetic contamination of the brick samples, the relaxation time distributions can be affected. The agreement of T2 relaxation time distributions and pore size distributions from mercury intrusion porosimetry was found to be better for the NMR-MOUSE sensors than for the homogeneous field measurements. This is true even for different brick samples, unless the content in ferromagnetic particles is very strong.     

Dielectric Probing of Relaxation Behaviors in PMMA/Organoclay Nanocomposites: Effect of Organic Modification

Poly(methyl methacrylate) (PMMA)–clay nanocomposites (PCN) were prepared with PMMA/clay concentrates and POP-diamines modified clay via melt blending. Partially exfoliated clay layers were uniformly dispersed in the nanocomposites as evidenced from XRD study. The subglassβ relaxation and the segmental α relaxation were observed from the dynamic mechanical thermal analysis (DMTA). The PCNs at 5 wt% clay level from clay concentrate exhibited significantly higher glass-transition temperatures and dynamic storage moduli relative to the neat PMMA. The confinement effect from the tethering of polymer chain to the clay surface would contribute to such a dramatic difference. In addition to the normal-mode relaxations, the αβ-merging process above T _g and the subglass interfacial polarization were detected from the dielectric analysis. Significant increase in dielectric permittivities and dielectric losses due to interfacial polarization and ionic conduction at low frequency were observed for the PCNs. The intensity of the interfacial polarization process increases with the dispersion degree of clay layers and hence the process can be assigned to the space charge polarization of the ionic species in the clay intergalleries.     

纳米二氧化钛材料的相变和声子限制效应(英文)

拉曼光谱用于研究二氧化钛材料的相变和声子限制效应。化学溶液方法制备了TiO2纳米晶材料。其平均粒度为 6 8- 2 7 9nm。最低频率 1 5 2cm- 1 Eg 模随粒度减小出现蓝移和加宽。在声子限制模型下 ,理论上对不同粒度的TiO2 纳米晶 (6 8,1 0 3和 2 7 9nm)的频移和线宽进行计算 ,结果与实验吻合得很好。研究了TiO2 纳米晶锐钛矿 -金红石相变 ,其相变温度为 6 5 0 - 6 90℃ ,比体块TiO2 的相变温度 1 0 0 0℃低 ,表明了相变的尺寸效应     

Polymer chain dynamics under nanoscopic confinements

It is shown that the confinement of polymer melts in nanopores leads to chain dynamics dramatically different from bulk behavior. This so-called corset effect occurs both above and below the critical molecular mass and induces the dynamic features predicted for reptation. A spinodal demixing technique was employed for the preparation of linear poly(ethylene oxide) (PEO) confined to nanoscopic strands that are in turn embedded in a quasi-solid and impenetrable methacrylate matrix. Both the molecular weight of the PEO and the mean diameter of the strands were varied to a certain degree. The chain dynamics of the PEO in the molten state was examined with the aid of field-gradient NMR diffusometry (time scale, 10(-2)-10(0) s) and field-cycling NMR relaxometry (time scale, 10(-9)-10(-4) s). The dominating mechanism for translational displacements probed in the nanoscopic strands by either technique is shown to be reptation. On the time scale of spin-lattice relaxation time measurements, the frequency dependence signature of reptation (i.e., T1 approximately nu(3/4)) showed up in all samples. A "tube" diameter of only 0.6 nm was concluded to be effective on this time scale even when the strand diameter was larger than the radius of gyration of the PEO random coils. This corset effect is traced back to the lack of the local fluctuation capacity of the free volume in nanoscopic confinements. The confinement dimension is estimated at which the crossover from confined to bulk chain dynamics is expected.     

Dynamics of water in molecular sieves by dielectric spectroscopy

We present recent dielectric data on the dynamics of water confined in molecular sieves with pore sizes 5 and 10 A. The dielectric measurements in the frequency and temperature ranges 10(-2)-10(6) Hz and 120-300 K show three relaxation processes for both samples. In the case of the 10 A pore the slowest process shows an Arrhenius temperature dependence at low temperatures (<220 K), while at high temperatures the relaxation appears to follow a more Vogel-Fulcher-Tammann (VFT) like behaviour. The relaxation time for this process is 100 s at about 170 K. The second slowest process is at low temperatures very similar to the main process of (bulk-like) water in a fully hydrated clay, but also this process seems to exhibit some kind of dynamical transition, in this case at T approximately 185 K. All the three processes in the 5 A pore exhibit Arrhenius temperature dependence, and two of them are considerably slower than the main relaxation in the hydrated clay. Thus, dynamics of bulk-like water is only observed in the 10 A molecular sieves, and most of the water molecules in both 5 and 10 A pores have considerably slower dielectric relaxation than has been observed for water confined in clay, most likely due to strong interactions with the considerably more hydrophilic inner surfaces of molecular sieves.     

Measuring nanopore size from the spin-lattice relaxation of CF4 gas

The NMR 19F spin-lattice relaxation time constant T1 for CF4 gas is dominated by spin-rotation interaction, which is mediated by the molecular collision frequency. When confined to pores of approximately the same size or smaller than the bulk gas mean free path, additional collisions of molecules with the pore walls should substantially change T1. To develop a method for measuring the surface/volume ratio S/V by measuring how T1 changes with confinement, we prepared samples of known S/V from fumed silica of known mass-specific surface area and compressed to varying degrees into cylinders of known volume. We then measured T1 for CF4 in these samples at varying pressures, and developed mathematical models for the change in T1 to fit the data. Even though CF4 has a critical temperature below room temperature, we found that its density in pores was greater than that of the bulk gas and that it was necessary to take this absorption into account. We modeled adsorption in two ways, by assuming that the gas condenses on the pore walls, and by assuming that gas in a region near the wall is denser than the bulk gas because of a simplified attractive potential. Both models suggested the same two-parameter formula, to which we added a third parameter to successfully fit the data and thus achieved a rapid, precise way to measure S/V from the increase in T1 due to confinement in pores.     

Relaxation of stretched DNA in slitlike confinement

We experimentally observe two separate time scales governing the entropic recoil in the linear force-extension regime of single double-stranded DNA in slit confinement. We demonstrate the existence of two distinct relaxation regimes at different extensions during relaxation. Contrary to bulk measurements, the true longest relaxation time may only be probed very close to equilibrium. A simple model of the relaxation mechanism leads to a scaling analysis that correctly predicts the extension at the crossover between the two regimes.     

Dynamics of a polymer chain confined in a membrane

We present a Brownian dynamics theory with full hydrodynamics (Stokesian dynamics) for a Gaussian polymer chain embedded in a liquid membrane which is surrounded by bulk solvent and walls. The mobility tensors are derived in Fourier space for the two geometries, namely, a free membrane embedded in a bulk fluid, and a membrane sandwiched by the two walls. Within the preaveraging approximation, a new expression for the diffusion coefficient of the polymer is obtained for the free-membrane geometry. We also carry out a Rouse normal mode analysis to obtain the relaxation time and the dynamical structure factor. For large polymer size, both quantities show Zimm-like behavior in the free-membrane case, whereas they are Rouse-like for the sandwiched membrane geometry. We use the scaling argument to discuss the effect of excluded-volume interactions on the polymer relaxation time.     

Conformation and dynamics of DNA confined in slitlike nanofluidic channels

Using laser fluorescence microscopy, we study the shape and dynamics of individual DNA molecules in slitlike nanochannels confined to a fraction of their bulk radius of gyration. With a confinement size spanning 2 orders of magnitude, we observe a transition from the de Gennes regime to the Odijk regime in the scaling of both the radius of gyration and the relaxation time. The radius of gyration and the relaxation time follow the predicted scaling in the de Gennes regime, while, unexpectedly, the relaxation time shows a sharp decrease in the Odijk regime. The radius of gyration remains constant in the Odijk regime. Additionally, we report the first measurements of the effect of confinement on the shape anisotropy.     

Size dependent ultrasonic properties of InN nanowires

The ultrasonic properties of single crystalline indium nitride nanowires (InN NWs) are studied for wire size (diameter) 6–100 nm at 300 K following the interaction potential model. Ultrasonic attenuation, ultrasonic velocity, acoustic coupling constant and thermal relaxation time are calculated using higher order elastic constants and thermal conductivity of the nanowires. The analysis of size dependent thermal relaxation time and ultrasonic properties shows that above the 20 nm diameter, InN nanowire tends towards its bulk material property. The third order polynomial is found to be best fit for size variation of thermal relaxation time. The ultrasonic attenuation as a function of size of the nanowires is found to be mainly affected by the thermal conductivity of the nanowires of different sizes.     

Segmental dynamics of poly(methyl phenyl siloxane) confined to nanoporous glasses

The effect of a nanometer confinement on the molecular dynamics of poly(methyl phenyl siloxane) (PMPS) was studied by dielectric spectroscopy (DS), temperature modulated DSC (TMDSC) and neutron scattering (NS). Nanoporous glasses with pore sizes of 2.5–20 nm have been used. DS and TMDSC experiments show that for PMPS in 7.5 nm pores the molecular dynamics is faster than in the bulk which originates from an inherent length scale of the underlying molecular motions. For high temperatures the temperature dependence of the relaxation rates for confined PMPS crosses that of the bulk state. Besides finite states effects also the thermodynamic state of nano-confined PMPS is different from that of the bulk. At a pore size of 5 nm the temperature dependence of the relaxation times changes from a Vogel/Fulcher/Tammann like to an Arrhenius behavior where the activation energy depends on pore size. This is in agreement with the results obtained by NS. The increment of the specific heat capacity at the glass transition depends strongly on pore size and vanishes at a finite length scale between 3 and 5 nm which can be regarded as minimal length scale for glass transition to appear in PMPS.     

Enhanced photoinduced desorption from metal nanoparticles by photoexcitation of confined hot electrons using femtosecond laser pulses

Strong fluence dependence of photodesorption cross sections is observed in femtosecond laser photodesorption of NO from (NO)2 on silver nanoparticles, in contrast to femtosecond photodesorption on bulk metals. The time scale of excitation buildup is found to be equal or less than the pulse duration of ～100 fs; NO translational energies are independent of fluence and pulse duration. We propose a nanoparticle-specific nonlinear mechanism in which, due to confinement, strongly nonthermal hot-electron distributions are maintained during the femtosecond pulses, enhancing the normal desorption pathway.     

Substrate and chain size dependence of near surface dynamics of glassy polymers

We use nanohole relaxation to study the surface relaxation of films of glassy isotactic poly (methyl methacrylate) (i-PMMA) films. These measurements allow us to obtain the time dependent relaxation function at a number of different sample temperatures for the first 2-3 nm of the free surface in a system often used as a model system for the effect of the substrate on thin film dynamics. The surface is observed to relax at temperatures up to 42 K below the bulk Tg value, even on systems where the thin film Tg is known to be greater than the bulk value. We are able to determine the range over which the substrate directly affects the free surface relaxation, and determine a surprisingly large (Mw independent) limiting thickness of approximately 180 nm where the free surface relaxation is not affected by the substrate. For thick films (h>200 nm) we find an unexpected linear Mw dependence of the near surface relaxation time.     

Isointense model for the evaluation of tumor-specific MRI contrast agents

An isointense model has been developed to evaluate the applicability of putative tumor-specific MRI contrast agents. Data for tissue relaxation measurements in the presence of Mn(III)TPPS4 are used to illustrate the model. The concentration of contrast agent in tumor tissue required for a tumor/normal tissue signal difference-to-noise ratio of 5 (delta SNR = 5) is determined for a T1 weighted pulse sequence and several hypothetical tumor/normal tissue pairs. The impact of various contrast agent characteristics including initial tumor/normal tissue relaxation values, differential uptake of contrast agent, and in vivo relaxivity are considered. Isointense tumor/normal tissue with longer initial relaxation times are shown to be more affected by the presence of contrast agent. In addition those with initially longer relaxation times have less rigorous requirements for tumor specificity. Typically, a normal tissue/tumor uptake ratio of 1:2 increases the concentration required for delta SNR = 5 by a factor of two compared to that of exclusive uptake in tumor. For the T1 weighted pulse sequence employed, the concentration required for delta SNR = 5 is shown to be linear with the inverse of in vivo relaxivity for the hypothetical tissues considered. The isointense model is also extended to predict the field dependence of tumor-specific contrast enhancement by Mn(III)TPPS4.     

石墨烯的声子热学性质研究

声子是石墨烯导热过程中的主要载体,而声子的弛豫时间又是其中最基本、最重要的物理量.本文采用简正模式分解法研究了石墨烯声子的弛豫时间,并且借此分析了不同声子在导热过程中的贡献.该方法通过平衡分子动力学模拟实现,首先通过模拟得到单个声子的能量自相关函数衰减曲线,并进一步采用拟合和积分两种方法得到单个声子的弛豫时间.然后,研究了弛豫时间与波矢、频率和温度的关系.结果发现,弛豫时间随波矢的变化与对应的色散关系相近,弛豫时间与频率和温度的关系符合理论模型:1/τ=νnTm,其中声学支的n为1.56,而光学支结果较为发散,指数m对于不同声子支结果略有不同.最后,还研究了不同频率声子对导热的贡献,发现低频声子在态密度上占有绝对优势,并且其弛豫时间整体高于高频声子,所以低频声子对导热的贡献占据主导地位.     

Complex study of bulk screening processes in single crystals of lithium niobate and lithium tantalate family

Comprehensive study of depolarization field bulk screening was carried out in lithium niobate and lithium tantalate single crystals with various stoichiometry. Three used complementary methods are based on: 1) dependence of coercive field on delay time; 2) decrease of optical contrast of domain wall trace; 3) relaxation of light diffraction intensity on domain walls. The following parameters of bulk screening process were obtained: relaxation time constants, type of relaxation law, maximal value of bias field and “true” value of coercive field. Advantages and disadvantages of used experimental methods were analyzed.     

Dynamic NMR investigation of plastic crystals in bulk and confined in porous materials

In this work, the molecular dynamics of four organic compounds confined in silica pores of nominal diameter 6 and 20 nm are studied by high-field (9.4 T) nuclear magnetic resonance (NMR), and the results are discussed with reference to the bulk substances. By using organic compounds forming soft plastic crystals on freezing as adsorbates, damage to the pore structures can be avoided. NMR lineshapes, spin-lattice relaxation times (T ₁), spin-spin relaxation timesT ₂ and diffusivities are reported as a function of temperature. Since the porous grains are much greater than the distance travelled by the molecules during the experiment, intracrystalline NMR parameters were obtained. However, the shortT ₂ (∼1 ms) encountered in both the bulk and confined samples prohibited measurements ofT ₂ and the diffusivity in the low-temperature ordered phases. The confinement in the pores gives rise to substantial changes in the phase behavior and molecular dynamics. Thus, the¹H lineshape observations of the confined samples clearly reveal a narrow-line component superimposed on a broad resonance at temperatures well below the transition point of the bulk material. In the freezing region, the narrow-line component is attributed to the surface layer and the undercooled liquid in the smaller pores that remains unfrozen. In the two-component, low-temperature region, the narrow component corresponds to the surface layer, while the broad component originates from the crystalline phase at the center of the pores.