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.     

Modeling viscoelastic flow with discrete methods

The hydrodynamics of viscoelastic materials (for example, polymer melts and solutions) presents interesting and complex phenomena, for example, instabilities and turbulent flow at very low Reynolds numbers due to normal stress effects and the existence of a finite stress relaxation time. This present work is motivated by renewed interest in instabilities in polymer flow. The majority of currently used numerical methods discretize a constitutive equation on a grid with finite difference or similar methods. We present work in progress in which we simulate viscoelastic flow with dissipative particle dynamics. The advantage of this approach is that many of the numerical instabilities of conventional methods can be avoided, and that the model gives clear physical insight into the origins of many viscoelastic flow instabilities.     

Studies of relaxation processes in organic glasses at low temperatures using the hole-burning spectroscopy technique: Verification of applicability limits of the model of two-level systems

A vitreous state is originally nonequilibrium. Because of this, attempts to classify relaxation processes in glasses as equilibrium and nonequilibrium are, strictly speaking, incorrect. This classification is, however, possible and useful at low temperatures when the model of two-level systems (TLS) appears to be sufficient to describe properties of glasses. Until now, the question of the applicability limits of the TLS model has remained unclear both in the temperature and temporal domains. A number of deviations from the so-called standard TLS model, observed experimentally, can be easily accounted for with allowance for nonequilibrium effects considered as a result of the nonequilibrium state of the TLS ensemble. There are some effects, however, that cannot be consistently explained within the framework of the TLS model. In this paper, we briefly consider the results of studying the relaxation processes in organic glasses at low temperatures in a wide time range using the spectral hole-burning technique. The experimental data are compared with predictions of the TLS model, and spectral criteria for the limits of its applicability are proposed.     

Natural line shape

The observable line shape of the spontaneous emission depends on the procedure of atom’s excitation. The spectrum of radiation emitted by a two-level atom excited from the ground state by a π pulse of the resonant pump field is calculated for the case when the Rabi frequency is much larger than the relaxation rate. It is shown that the central part of the spectral distribution has a standard Lorentzian form, whereas for detunings from the resonance that are larger than the Rabi frequency the spectral density falls off faster. The shape of the wings of the spectral line is sensitive to the form of the π pulse. The implications for the quantum Zeno effect theory and for the estimates of the duration of quantum jumps are discussed. The article is published in the original.     

Relaxation of strains in fibrillar porous carbon dioxide saturated media near their critical point

The isothermal relaxation of strains in deformed fibrillar porous layers saturated with subcritical or supercritical carbon dioxide has been studied at different temperatures by full-field speckle correlometry. The relaxation of strains is produced by the porous layer—fluid transition of a system from one equilibrium thermodynamic state to another due to a jump-like change in pressure. It has been established that the relaxation time attains a maximum near a critical point and rapidly decreases with increasing deviation of the system temperature from its critical value. The mechanisms of hydrodynamic relaxation in the density of a fluid layer and viscoelastic relaxation in a porous matrix are discussed. It is shown that the experimentally observed slow relaxation of a system is due to the viscoelastic relaxation of a porous fluid saturated layer at equal fluid densities inside and outside the layer.     

Viscoelastic dynamics of polymer thin films and surfaces

The strain relaxation behavior in a viscoelastic material, such as a polymer melt, may be strongly affected by the proximity of a free surface or mobile interface. In this paper, the viscoelastic surface modes of the material are discussed with respect to their possible influence on the freezing temperature and dewetting morphology of thin polymer films. In particular, the mode spectrum is connected with mode coupling theory assuming memory effects in the melt. Based on the idea that the polymer freezes due to these memory effects, surface melting is predicted. As a consequence, the substantial shift of the glass transition temperature of thin polymer films with respect to the bulk is naturally explanied. The experimental findings of several independent groups can be accounted for quantitatively, with the elastic modulus at the glass transition temperature as the only fitting parameter. Finally, a simple model is put forward which accounts for the occurrence of certain generic dewetting morphologies in thin liquid polymer films. It demonstrates that by taking into account the viscoelastic properties of the film, a morphological phase diagram may be derived which describes the observed structures of dewetting fronts. It is demonstrated that dewetting morphologies may also serve to determine nanoscale rheological properties of liquids.Received: 1 January 2003, Published online: 14 October 2003PACS: 47.50. + d Non-Newtonian fluid flows - 68.47.Mn Polymer surfaces - 68.60.Dv Thermal stability; thermal effects     

On the homogeneous nucleation and propagation of dislocations under shock compression

The dynamic response of crystalline materials subjected to extreme shock compression is not well understood. The interaction between the propagating shock wave and the material’s defect occurs at the sub-nanosecond timescale which makes in situ experimental measurements very challenging. Therefore, computer simulation coupled with theoretical modelling and available experimental data is useful to determine the underlying physics behind shock-induced plasticity. In this work, multiscale dislocation dynamics plasticity (MDDP) calculations are carried out to simulate the mechanical response of copper reported at ultra-high strain rates shock loading. We compare the value of threshold stress for homogeneous nucleation obtained from elastodynamic solution and standard nucleation theory with MDDP predictions for copper single crystals oriented in the [0 0 1]. MDDP homogeneous nucleation simulations are then carried out to investigate several aspects of shock-induced deformation such as; stress profile characteristics, plastic relaxation, dislocation microstructure evolution and temperature rise behind the wave front. The computation results show that the stresses exhibit an elastic overshoot followed by rapid relaxation such that the 1D state of strain is transformed into a 3D state of strain due to plastic flow. We demonstrate that MDDP computations of the dislocation density, peak pressure, dynamics yielding and flow stress are in good agreement with recent experimental findings and compare well with the predictions of several dislocation-based continuum models. MDDP-based models for dislocation density evolution, saturation dislocation density, temperature rise due to plastic work and strain rate hardening are proposed. Additionally, we demonstrated using MDDP computations along with recent experimental reports the breakdown of the fourth power law of Swegle and Grady in the homogeneous nucleation regime.     

Evidence for an interphase relaxation in a ternary polymer blend through a reverse mechanical modeling

The viscoelastic properties of a compatibilized epoxy/thermoplastic polymer blend were investigated. Mechanical spectrometry revealed an additional loss peak at about 333 K at 1 Hz in the compatibilized ternary blend spectrum that is detailed as a “micromechanical transition.” Such an interphase relaxation does not correspond to a relaxation of one of the pure components. Mechanical modeling was used in a reverse mode to investigate the actual characteristics of the interphase (i.e., volume content and mechanical properties).

In a “direct” mode, mechanical modeling permits prediction of the viscoelastic properties of a blend from knowledge of the characteristics of the pure components. In the reverse mode, this new approach for mechanical modeling appears to be a well-suited tool for separating the viscoelastic characteristics of one phase from others in multiphase systems.     

A study of the relaxation dynamics in a quadrupolar NMR system using Quantum State Tomography

This article reports a relaxation study in an oriented system containing spin 3/2 nuclei using quantum state tomography (QST). The use of QST allowed evaluating the time evolution of all density matrix elements starting from several initial states. Using an appropriated treatment based on the Redfield theory, the relaxation rate of each density matrix element was measured and the reduced spectral densities that describe the system relaxation were determined. All the experimental data could be well described assuming pure quadrupolar relaxation and reduced spectral densities corresponding to a superposition of slow and fast motions. The data were also analyzed in the context of Quantum Information Processing, where the coherence loss of each qubit of the system was determined using the partial trace operation.     

Rheology and DWS microrheology of concentrated suspensions of the semiflexible filamentous fd virus

Microrheology measurements were performed on suspensions of bacteriophage fd with diffusive wave spectroscopy in the concentrated regime, at different values of ionic strength. Viscosity vs. shear rate was also measured, and the effect of bacteriophage concentration and salt addition on shear thinning was determined, as well as on the peaks in the viscosity vs. shear curves corresponding to a transition from tumbling to wagging flow. The influence of concentration and salt addition on the mean square displacement of microspheres embedded in the suspensions was determined, as well as on their viscoelastic moduli up to high angular frequencies. Our results were compared with another microrheology technique previously reported where the power spectral density of thermal fluctuations of embedded micron-sized particles was evaluated. Although both results in general agree, the diffusive wave spectroscopy results are much less noisy and can reach larger frequencies. A comparison was made between measured and calculated shear modulus. Calculations were made employing the theory for highly entangled isotropic solutions of semiflexible polymers using a tube model, where various ways of calculating the needed parameters were used. Although some features are captured by the model, it is far from the experimental results mainly at high frequencies.     

尼古丁分子的能级结构和光谱计算

研究尼古丁分子的能级结构与光谱特征,对更好地了解尼古丁分子的毒性和药性有理论指导作用.基于密度泛函理论(DFT),本文利用Gaussian 09软件在B3LYP/6-311G(d,p)基组水平上对尼古丁分子进行结构优化,再采用含时密度泛函理论(TD-DFT)在乙醇溶剂中计算尼古丁分子的15个激发态.使用Multiwfn波谱分析软件对分子前线轨道进行计算,并绘制出分子的红外谱图和紫外谱图.通过前线轨道分析可知,尼古丁分子的亲核位点是吡啶环上的C3和N6、亲电位点是吡咯环上的N22.吡啶环上的C-H、N-H键面内伸缩振动峰主要集中在3049～3079 cm~(-1),吡咯环上的甲基、亚甲基的伸缩振动峰主要集中在2796～3005 cm~(-1),其中在2816 cm~(-1)处甲基上C-H键振动峰最为明显,占比43.3%;吡咯环与吡啶环的摆动峰主要集中在1027～1455 cm~(-1),吡啶环的面内振动峰主要集中在1008～1027 cm~(-1),在800 cm~(-1)以下吸收峰都为吡啶环的面外摆动峰.紫外光谱的最大吸收峰位于173.46571 nm处,主要是由基态S0跃迁到激发态S5、S6、S10、S11、S12、S13、S14形成的,其中基态S0跃迁到激发态S11的贡献最大,其余激发态跃迁振子强度小于0.03,为禁阻跃迁.     

Dynamic instability of thin viscoelastic films under lateral stress

We study the influence of lateral stress on the stability of thin viscoelastic films. The free surface of a deposited film under stress is shown to undergo an instability initiated by an anisotropic diffusion of the polymer molecules. This instability ultimately results in the formation of holes provided that the initial surface fluctuations are larger than a critical value. The latter is found to decrease when increasing the stress. An increase of the holes number density with the stress is therefore predicted. Most interestingly, the holes number density is also predicted to increase when increasing the molecular weight of the polymers. Additionally, we demonstrate that the friction of the substrate suppresses any spatial coherence between holes on large length scales. These predictions explain recent experimental observations made on thin spin-coated polystyrene films [G. Reiter, Nat. Mater. 4, 754 (2005)10.1038/nmat1484].     

Simple model for frozen-in viscoelastic stress in optical fibers

We have measured residual stress in a commercial single mode optical fiber and observed its relaxation in an annealing process with a polariscopic stress measurement method. After annealing the fiber for an hour at various high temperatures in a specially made image furnace we have measured the temperature dependent relaxation of frozen-in viscoelastic stress in the fiber. We have proposed a simple physical model to explain recently observed relaxation of frozen-in viscoelastic stress in optical fibers based on the Kelvin-Voigt model of viscoelastic materials. The experimental results are explained with our model of frozen-in viscoelastic stress by introducing simplified four-step procedures for fiber drawing.     

Polymer Transport in Random Flow

The dynamics of polymers in a random smooth flow is investigated in the framework of the Hookean dumbbell model. The analytical expression of the time-dependent probability density function of polymer elongation is derived explicitly for a Gaussian, rapidly changing flow. When polymers are in the coiled state the pdf reaches a stationary state characterized by power-law tails both for small and large arguments compared to the equilibrium length. The characteristic relaxation time is computed as a function of the Weissenberg number. In the stretched state the pdf is unstationary and exhibits multiscaling. umerical simulations for the two-dimensional Navier–Stokes flow confirm the relevance of theoretical results obtained for the -correlated model.     

Polymer thin films and surfaces: Possible effects of capillary waves

It is discussed how the proximity of a free surface or mobile interface may affect the strain relaxation behavior in a viscoelastic material, such as a polymer melt. The eigenmodes of a viscoelastic film are thus derived, and applied in an attempt to explain the experimentally observed substantial shift of the glass transition temperature of sufficiently thin polymer films with respect to the bulk. Based on the idea that the polymer freezes due to memory effects in the material, and exploiting results from mode-coupling theory, the experimental findings of several independent groups can be accounted for quantitatively, with the elastic modulus at the glass transition temperature as the only fitting parameter. The model is finally applied discussing the possibility of polymer surface melting. A surface molten layer is predicted to exist, with a thickness diverging as the inverse of the reduced temperature. A simple model of thin polymer film freezing emerges which accounts for all features observed experimentally so far. Received 8 August 2001     

Hyperpolarized long-lived states in solution NMR: three-spin case study in low field

Recent work has shown that singlet states in two-spin systems can possess lifetimes exceeding the T(1) relaxation time, provided that the system is kept under conditions that minimize the effects of the chemical shift Hamiltonian (for instance under low magnetic field or RF irradiation). Similar observations have been made in hyperpolarized states of multi-spin systems prepared via parahydrogen-induced polarization (PHIP). However, lifetime prolongation mechanisms in multi-spin systems are still under investigation. Here we present experimental observations of a long-lived state in a three-spin system prepared by PHIP and stored at low field. The observed lifetime of the long-lived state is 144s, about twice as long as the longest T(1) measured in the system at high field. The results are analyzed using a recently proposed theory of lifetime prolongation in multi-spin systems in low field. It is shown that quantum mechanical selection rules governing intramolecular dipolar relaxation in low field account for the enhanced lifetime and spectral features of this state.     

An Ising model for the stress relaxation of solids

A kinetic theory of stress relaxation of solids as a cooperative process is proposed. The theory is based on a two-state model for the relaxation. It is shown that the conventional mean field approximation leads to an exponential dependence of the rate of stress relaxation on the stress while the multiplicative approximation of Vol'kenstein et al. leads to a power law. It is argued that the exponential law should be valid initially in the relaxation process while the power law is appropriate when the system is nearer equilibrium, which is in qualitative agreement with recent experimental findings.     

Local deformation and the structure of the Stark splitting of rare-earth ions

A model for describing the relaxation of ligand ions close to a defect when impurity ions are introduced into a crystal is proposed and verified. The approach assumes that ionic displacements into new equilibrium positions can be regarded as fundamental parameters of impurity crystals that can be determined from experimental data concerning the energy structure of the impurity ion. Direct calculations for rare-earth impurity ions using crystal-field theory showed that the energy spectrum of these ions strongly depends on the equilibrium positions of the nearest matrix ions surrounding them. The results of the calculations are compared with the available experimental data. The parameters of the theory are determined. The possibility of applying this approach to the study of other systems is discussed. Fiz. Tverd. Tela (St. Petersburg) 41, 1194–1199 (July 1999)     

准静态颗粒介质的弹性势能弛豫分析

颗粒体系是典型的多体相互作用体系, 具有多重的能量亚稳态. 对于准静态颗粒体系, 引入构型颗粒温度T_c描述弹性势能涨落. 本文认为平衡的体系具有一定的构型颗粒温度T_a, 其量值反映了其结构特征. 当外界扰动激发的构型颗粒温度超出T_a时, 产生不可逆过程. 通过对应力松弛过程的分析, 发现(T_c-T_a)激发了弹性弛豫, 且(T_c-T_a)越大则松弛过程中应力变化越大, 最终构型颗粒温度T_c→T_a时,宏观应力松弛结束,体系达到新的能量亚稳态.     

Optical spectroscopy of azomethine-base (H2msalpn-1,3) polymer films

A comparative study of the optical properties of azomethine-base polymer structures in the oxidized and reduced states obtained by electrochemical synthesis have been performed. A difference in the character of the spectral dependence of optical density in the wavelength range λ = 550–700 nm and a higher transparency of the Poly(H₂msalpn-1,3) structure in the reduced state in the spectral range under consideration have been revealed. Analysis of the differential spectrum of optical density of the oxidized state and the values of calculated band gaps suggests that the polymer system in the oxidized state has a higher electroactivity.