Morphology,electro-optical and dielectric properties of polymer network liquid crystals in visible wavelengths

The correlations among electrical, optical properties and polymer morphologies of polymer network liquid crystals (PNLCs) constructed with various curing parameters are investigated. The experimental results indicate that high UV curing intensity, low curing temperature and high monomer-dopant concentration reduce the sizes of liquid crystal (LC) domains, thereby decreasing field-off response time and light scattering and increasing phase retardation of the PNLC cells. Photoinitiator concentration affects the LC domain size as well. For instance, increase in photoinitiator concentration results in the acceleration of polymerisation and thus decreases LC domain size. This effect increases driving voltages of the PNLC cells. Notably, excessive amounts of photoinitiator increases the LC domain size of the PNLC cell. Furthermore, dielectric measurement reveals that decrease in the LC domain size generally increases the dielectric relaxation frequency of the PNLC cells. When the LC domain size is small enough, the dielectric relaxation frequency of the PNLC cell is further dominated by the monomer concentration owing to the increased densities of polymer networks that facilitate the alignment of LC molecules.     

Dynamics in ferro- and antiferroelectric phases of a liquid crystal with fluorinated molecules as studied by dielectric spectroscopy

For 1-[3-fluoro-4-(1-methylheptyloxycarbonyl)phenyl]-2-[4-2,2,3,3,4,4,4-heptafluorobutoxybutoxy)biphenyl-4-yl]ethane (1F7), built of chiral molecules, results of dielectric measurements of liquid-crystalline and solid phases are presented. Rich polymorphism of liquid-crystalline (SmC*, SmC*_A and SmI*_A) phases as well as of solid (Cr1 and Cr2) phases were observed down to –130°C. At a frequency range from 0.1 Hz to 3 MHz, the relaxation processes were detected in ferroelectric SmC*, antiferroelectric SmC*_A and highly ordered SmI*_A smectic phases. The mechanism of complex dynamics (moleculear and collective) was identified with the help of the bias field. Vitrification of conformationally disordered crystal phase Cr2 was found in accordance with calorimetric observations.     

Atomic level picture of stress relaxation in polymer melts

We have been developing a physical picture on the atomic level of stress relaxation in polymer melts by means of computer simulation of the process in model systems. In this article we treat a melt of freely jointed chains, each with N = 200 bonds and with excluded-volume interactions between all nonbonded atoms, that has been subjected to an initial constant-volume uniaxial extension. We consider both the stress relaxation history σ(t) based on atomic interactions, and the stress history σ_e(t; N_R) based on subdividing the chain into segments with N_R bonds each, with each segment regarded as an entropic spring. It is found that at early times σ(t) > σ_e(t; N_R) for all N_R, and that, for the remainder of the simulation, there is no value of N_R for which σ(t) = σ_e(t; N_R) for an extended period; by the end of the simulation σ(t) has fallen just below the value σ_e(t; 50). The decay of segment orientation, 〈P₂(t; N_R)〉, and of bond orientation 〈P₂(t; 1)〉, is computed during the simulation. It is found that the decay of the atom-based stress σ(t) is closely related to that of 〈P₂(t; 1)〉. This result may be understood through the concept of steric shielding. The change in local structure of the polymer melt during relaxation is also studied. © 1998 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 36 : 143–154, 1998     

Static and dynamic scaling behavior of a polymer melt model with triple-well bending potential

We perform molecular-dynamics simulations for polymer melts of the coarse-grained poly(vinyl alcohol) model that crystallizes upon slow cooling. To establish the properties of its high temperature, liquid state as a reference point, we characterize in detail the structural features of equilibrated polymer melts with chain lengths 5 ≤ N ≤ 1000 at a temperature slightly above their crystallization temperature. We find that the conformations of sufficiently long polymers with N > 50 obey essentially the Flory's ideality hypothesis. The chain length dependence of the end-to-end distance and the gyration radius follow the scaling predictions of ideal chains and the probability distributions of the end-to-end distance, and form factors are in good agreement with those of ideal chains. The intrachain correlations reveal evidences for incomplete screening of self-interactions. However, the observed deviations are small. Our results rule out any preordering or mesophase structure formation that are proposed as precursors of polymer crystallization in the melt. Moreover, we characterize in detail primitive paths of long entangled polymer melts and we examine scaling predictions of Rouse and the reptation theory for the mean squared displacement of monomers and polymers center of mass. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2018 , 56, 1376–1392     

Dielectric study of ferroelectric side-chain liquid crystalline polysiloxanes with broad temperature ranges of the chiral smectic C phase 1. Structure dependence of dielectric relaxation

The dielectric behavior of a series of ferroelectric side-chain liquid crystalline polysiloxanes containing 1–3 oligooxyethylene units as spacers, and 4-(S)-2-methyl-1-butyl[[[(4-hydroxy-biphenyl-4′-yl)]carbonyl]oxy] benzoate or 4-(S)-2-methyl-1-butyl[[(4-hydroxy-biphenyl-4′-yl)carbonyl]oxy]-3-fluoro benzoate side groups was studied by broadband dielectric spectroscopy. The increase of the spacer length, and incorporation of a strong dipole moment fluoro-substituent into the mesogenic group, resulted in a decrease of the relaxation activation energy and an increase in the intensity of the relaxation. Moreover, the relaxation peak of the Goldstone mode has only been observed for the FLCP with a lateral fluoro-substituent. The relationship between the thermal dynamic behavior and chemical structure is discussed in detail. Furthermore, analysis of the dielectric relaxation behavior of these FLCPs showed that the molecular relaxations could be described by the Cole-Cole equation. © 1996 John Wiley & Sons, Inc.     

Molecular model of phenolic polymer dissolution in photolithography

The resolution of photolithographic processes has advanced to the point that difficulties, such as line‐edge roughness, associated with phenomena occurring at molecular length scales are becoming important. In order to control these phenomena, it is necessary to understand them. To that end, a numerical model has been used to simulate the dissolution of phenolic polymers in aqueous base. The simulation applies the Critical Ionization Model to a rectangular‐lattice representation of the polymer matrix. The model has been adapted to describe the dissolution process that is responsible for photoresist function. Both continuum and molecular versions of the model are presented. The Continuum Model yields dissolution profiles that approximate the contours of the calculated spatial variations in chemical blocking (blocking profile). An algorithm has been developed to connect individual cells to form polymer chains, and to fill the lattice in a way that produces a Gaussian chain length distribution. The model employs only a single adjustable parameter, the time‐step correction factor (assuming one can measure the probability of ionization once a site encounters the developer). The Molecular Model predicts a dissolution rate that decreases non‐linearly with respect to degree of chemical blocking, as is observed experimentally. Dissolution profiles can be generated with the Molecular Model based either on this calculated dependence of the dissolution rate on blocking fraction or from direct application of the model to a blocking profile. The probabilistic nature of the model introduces edge roughness of the same degree as that observed experimentally. © 1999 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 37: 2103–2113, 1999     

Molecular Dynamics,Phase Transition and Frequency‐Tuned Dielectric Switch of an Ionic Co‐Crystal

Dielectric switches that can be converted between high and low dielectric states by thermal stimuli have attracted much interest owing to their many potential applications. Currently one main drawback for practical application lies in the non‐tunability of their switch temperatures (T_S). We report here an ionic co‐crystal (Me₃NH)₄[Ni(NCS)₆] that contains a multiply rotatable Me₃NH⁺ ion and a solely rotatable one due to a more spacious supramolecular cage for the former one. This compound undergoes an isostructural order–disorder phase transition and it can function as a frequency‐tuned dielectric switch with highly adjustable T_S, which is further revealed by the variable‐temperature structure analyses and molecular dynamics simulations. In addition, the distinct arrangements and molecular dynamics of two coexisting Me₃NH⁺ ions confined in different lattice spaces as well as the notable offset effect on the promoting/hindering of dipolar reorientation after dielectric transition provide a rarely observed but fairly good model for understanding and modulating the dipole motion in crystalline environment.     

Molecular Dynamics and Structure of Poly(Methyl Methacrylate) Chains Grafted from Barium Titanate Nanoparticles

Core−shell nanocomposites comprising barium titanate, BaTiO₃ (BTO), and poly(methyl methacrylate) (PMMA) chains grafted from its surface with varied grafting densities were prepared. BTO nanocrystals are high-k inorganic materials, and the obtained nanocomposites exhibit enhanced dielectric permittivity, as compared to neat PMMA, and a relatively low level of loss tangent in a wide range of frequencies. The impact of the molecular dynamics, structure, and interactions of the BTO surface on the polymer chains was investigated. The nanocomposites were characterized by broadband dielectric and vibrational spectroscopies (IR and Raman), transmission electron microscopy, differential scanning calorimetry, and nuclear magnetic resonance. The presence of ceramic nanoparticles in core–shell composites slowed down the segmental dynamic of PMMA chains, increased glass transition temperature, and concurrently increased the thermal stability of the organic part. It was also evidenced that, in addition to segmental dynamics, local β relaxation was affected. The grafting density influenced the self-organization and interactions within the PMMA phase, affecting the organization on a smaller size scale of polymeric chains. This was explained by the interaction of the exposed surface of nanoparticles with polymer chains.     

Quantification of solvent water and hydration dynamic of thermo‐sensitive microgel by dielectric spectroscopy

The hydration of natural or synthetic macromolecules is of fundamental importance in our understanding of their structure and stability. Quantification of hydration water can promote the understanding to many complex biological mechanisms such as protein folding, as well as the dynamics and conformation of polymers. An approach to quantification of solvent water was developed by dielectric spectroscopy. Dielectric behaviors of PNIPAM microgels with different crosslink density distribution were measured in the range of 0.5–40 GHz and 15–50. An obvious relaxation process caused by free and bound water was found. Dielectric parameters of free and bound water show that the crosslink density distribution does not affect the volume phase transition temperature of microgels, but significantly influence the orientation dynamics of the solvent water. We found that the three kinds of microgel can be distinguished by the dielectric parameters of the bound water. In addition, the number of water in and outside microgel during the volume phase transition process was quantitatively calculated for the first time. This study provides the possibility for the quantification of water in complex biological process. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55, 1859–1864     

Theoretical investigation of polymer molecular structure influence on dielectric properties and mechanical properties

Triboelectric nanogenerator (TENG) technologies have explosive development in the field of energy harvesting and self-powered sensing. As the key element of triboelectric devices, dielectric polymers have obtained much attention in recent years. The dielectric properties of polymer determine the output performance of TENG. In this paper, we take silicone rubber as an example of dielectric polymers, to study the properties of molecular structure influence on the dielectric properties and mechanical properties by the molecular dynamics simulation method. The free volume fraction, dielectric constant, and mechanical properties of silicone rubbers with different branch chains were calculated. The dielectric constant is highly related to the free volume distribution and the dipole moments of silicone rubbers with different amounts of branch chains. For fewer branch chains silicone rubber, the free volume distribution contributes most to the dielectric constant; for more branch chains silicone rubber, the dipole moment dominates the dielectric constant. Therefore, the silicone rubber ratio has a great influence on the dielectric constant of silicone rubber. With the increase of temperature, the dielectric constant of 2-chain silicone rubber increases at first and then decreases, and the maximum value is obtained near 300 K. Therefore, it is necessary to control the temperature when silicone rubber is used as a dielectric material. This work can be a guide for improving the dielectric properties of silicone rubber, and it provides a new approach to the optimal design of high-performance triboelectric nanogenerators.     

Molecular dynamics study of polymer crystallization in the presence of a particle

We have used molecular dynamics simulations with a coarse‐grained model to study the effect of a particle on the crystallization of polymer melt. We analyzed in particular a bond order parameter to characterize the nucleation and crystallization process. Our calculations show that the presence of a particle modifies the free energy landscape of polymer melts, locally induces the ordering of polymer melts near the particle surface, and thus enhances the polymer crystallization. Because the interaction between the particle and polymers is repulsive, our results suggest that the origin of the enhancement for polymer crystallization is entropic. © 2007 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 45: 2161–2166, 2007     

Dielectric properties and molecular mobility of organic/inorganic polymer composites

Microstructure-dielectric properties relationship and molecular mobility of organic/inorganic polymer composites (OIPCs), consisting of polyurethane (PU) and sodium silicate (NaSi), were investigated in this work. Broadband dielectric relaxation spectroscopy (DRS) and thermally stimulated depolarization current (TSDC) techniques were employed. Our interest was focused on the study of the glass transition mechanism and conductivity relaxation. The influence of the molecular weight of PU and inorganic phase content on the dielectric properties of the composites was of particular interest. Glass transition temperature shifts to higher temperatures with the addition of NaSi. The overall molecular mobility was found to increase in the composites, compared to the pure PU matrix. The results are more intense for the composites based on the PU with low molecular weight.     

Molecular relaxation of isotactic polystyrene: Real‐time dielectric spectroscopy and X‐ray scattering studies*

The molecular relaxation processes and structure of isotactic polystyrene (iPS) films were investigated with real‐time dielectric spectroscopy and simultaneous wide‐ and small‐angle X‐ray scattering. The purpose of this work was to explore the restrictions imposed on molecular mobility in the vicinity of the α relaxation (glass transition) for crystallized iPS. Isothermal cold crystallization at temperatures of T_c = 140 or 170 °C resulted in a sigmoidal increase of crystallinity with crystallization time. The glass‐transition temperature (T_g), determined calorimetrically, exhibited almost no increase during the first stage of crystal growth before impingement of spherulites. After impingement, the calorimetric T_g increased, suggesting that confinement effects occur in the latter stages of crystallization. For well‐crystallized samples, the radius of the cooperativity region decreased substantially as compared with the purely amorphous sample but was always smaller than the layer thickness of the mobile amorphous fraction. Dielectric experiments directly probed changes in the amorphous dipole mobility. The real‐time dielectric data were fitted to a Havriliak–Negami model, and the time dependence of the parameters describing the distribution of relaxation times and dielectric strength was obtained. The central dipolar relaxation time showed little variation before spherulite impingement but increased sharply during the second stage of crystal growth as confinement occurred. Vogel–Fulcher–Tammann analysis demonstrated that the dielectric reference temperature, corresponding to the onset of calorimetric T_g, did not vary for well‐crystallized samples. This observation agreed with a model in which constraints affect primarily the modes having longer relaxation times and thus broaden the glass‐transition relaxation process on the higher temperature side. © 2004 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 42: 777–789, 2004     

Metal particle adsorption and diffusion in a model polymer/metal composite system

We have developed a model polymer/metal composite system based on the adsorption of colloidal gold particles from a dilute aqueous suspension to the surface of poly(2-vinylpyr-idine) (PVP). Particle coverages and tracer diffusion coefficients for the particles within a PVP matrix phase were measured by Rutherford backscattering spectrometry. The adsorption process is quantitatively described by a diffusion-limited mechanism where gold particles irreversibly adsorb to the surface of the polymer film. Model dispersions produced in this way are excellent model systems for studying the fundamental properties of metal particle dispersions, since the particle size and the areal density of particles on the surface are well-controlled. Diffusion coefficients for the gold particles within PVP were also measured. The diffusion of the gold particles was found to be coupled to the bulk viscosity of the polymer, even though the size of the gold particles was only slightly larger than the mesh size of the entanglement network for PVP. © 1995 John Wiley & Sons, Inc.     

Slow Molecular Motions in Ionic Liquids Probed by Cross‐Relaxation of Nuclear Spins During Overhauser Dynamic Nuclear Polarization

Solution‐state Overhauser dynamic nuclear polarization (ODNP) at moderate fields, performed by saturating the electron spin resonance (ESR) of a free radical added to the sample of interest, is well known to lead to significant NMR signal enhancements in the steady state, owing to electron–nuclear cross‐relaxation. Here it is shown that under conditions which limit radical access to the molecules of interest, the time course of establishment of ODNP can provide a unique window into internuclear cross‐relaxation, and reflects relatively slow molecular motions. This behavior, modeled mathematically by a three‐spin version of the Solomon equations (one unpaired electron and two nuclear spins), is demonstrated experimentally on the ¹⁹F/¹H system in ionic liquids. Bulky radicals in these viscous environments turn out to be just the right setting to exploit these effects. Compared to standard nuclear Overhauser effect (NOE) work, the present experiment offers significant improvement in dynamic range and sensitivity, retains usable chemical shift information, and reports on molecular motions in the sub‐megahertz (MHz) to tens of MHz range—motions which are not accessed at high fields.     

Molecular simulation of structure formation and rheological properties of mixtures of telechelic and monofunctional associating polymer

We study the phase behavior, the characteristic times, and the rheological properties under the steady shear flow of the mixtures consisting of telechelic and monofunctional associating polymers by a coarse‐grained molecular dynamics simulation. The mixtures form the transient networks, the closely packed spherical micelles, and the wormlike micelles. We confirm the molecular origins of the several characteristic times of the mixtures. The dependencies of the characteristic times on the composition ratio between telechelic and monofunctional associating polymers show good agreement with reported experimental results. Under the steady shear flow, the mixtures show the shear thinning induced by the change of the spatial configuration of the micelles. The telechelic associating polymers especially play an important role in connecting the micelles at the shear thinning regime and enhance the steady shear viscosity. Furthermore, at the wormlike micellar region, the mixtures show the second shear thinning initiated by the transformation of the association conformation of the telechelic associating polymers.     

A study on the structural transition of a single polymer chain by parallel tempering molecular dynamics simulation

The structural transition of a single polymer chain with chain length of 100,200 and 300 beads was investigated by parallel tempering MD simulation.Our simulation results can capture the structural change from random coil to orientationally ordered structure with decreasing temperature.The clear transition was observed on the curves of radius of gyration and global orientational order parameter P as the function of temperature,which demonstrated structural formation of a single polymer chain.The linear relationships between three components of square radius of gyration R_gx²,R_gx²,R_gz² and global orientational order P can be obtained under the structurally transformational process.The slope of the linear relationship between x（or y-axis） component R_gx²(or R_gy²) and P is negative,while that of RL as the function of P is positive.The absolute value of slope is proportional to the chain length.Once the single polymer chain takes the random coil or ordered configuration,the linear relationship is invalid.The conformational change was also analyzed on microscopic scale.The polymer chain can be treated as the construction of rigid stems connecting by flexible loops.The deviation from exponentially decreased behavior of stem length distribution becomes prominent,indicating a stiffening of the chain arises leading to more and more segments ending up in the trans state with decreasing temperature.The stem length N_tr is about 21 bonds indicating the polymer chain is ordered with the specific fold length.So,the simulation results,which show the prototype of a liquid-crystalline polymer chain,are helpful to understand the crystallization process of crystalline polymers.