Electro-optical properties of KNbO3

The half-wave voltages, the electro-optic (EO) and the polarization-optic (PO) coefficients of orthorhombic KNbO₃ have been evaluated. We used a method where the refractive index changes are induced by very small electric fields to prevent formation of domains. From the temperature dependence of the EO coefficients we can conclude that the PO coefficients are temperature independent and that the linear EO effect is a quadratic effect biased by the spontaneous polarization.     

Phase behavior and structure of an ABC triblock copolymer dissolved in selective solvent

A mean-field lattice theory is applied to predict the self-assembly into ordered structures of an ABC triblock copolymer in selective solvent. More specifically, the composition-temperature phase diagram has been constructed for the system (C)₁₄(PO)₁₂(EO)₁₇/water, where C stands for methylene, PO for propylene oxide and EO for ethylene oxide. The model predicts thermotropic phase transitions between the ordered hexagonal, lamellar, reverse hexagonal, and reverse cubic phases, as well as the disordered phase. The thermotropic behavior is a result of the temperature dependence of water interaction with EO- and PO-segments. The lyotropic effect (caused by changing the solvent concentration) on the formation of different structures has been found weak. The structure in the ordered phases is described by analyzing the species volume fraction profiles and the end segment and junction distributions. A “triple-layer” structure has been found for each of the ordered phases, with each layer rich in C-, PO-, and EO-segments, respectively. The blocks forming the layers are not stretched. The dependence of the domain spacing on polymer volume fraction and temperature is also considered. Received 17 April 2002 Published online: 21 January 2003     

Electro-optic study of acrylate based holographic polymer dispersed liquid crystals with broad band photosensitivity

We describe the electro-optic (EO) modulation of diffraction in holographic polymer dispersed liquid crystals, recently developed for broad band photosensitivity, ranging from visible to near infrared. More than 90% EO modulation achieved for relatively low electric fields confirms the phase separation and formation of liquid crystal sub-micrometer sized droplets in the polymer matrix. Molecular reorientation induced by the applied voltage and a residual electrical current induced heating are at the origin of the EO modulation.     

Structure and Properties of Poly(Oxypropylene) Diamine Intercalated Montmorillonite/Epoxy Composites

Abstract

A type of micro-multilayer particles with a structure similar to that of nacre was prepared by poly(oxypropylene) diamine (POPD) intercalating organic montmorillonite (OMMT). The prepared particles were then blended with epoxy resin (EP) to obtain high performance EP composites. The Fourier transform infrared spectroscopy, X-ray diffraction, thermogravimetric analysis and contact angle analysis of the OMMT showed that the (POPD) had been successfully intercalated into the OMMT and the micro-multilayer particles were obtained as expected. Transmission electron microscope observation of the cured composites further confirmed that the micro-multilayer particles were well maintained in the EP network. The tensile and bending strength and glass transition temperature of the OMMT/EP composites were all increased compared with those of the EP. All these showed that the addition of the OMMT was an effective way to obtain high performance EP composites.     

Effect of Structure of Graft on the Properties of Graft Copolymers of Acrylamide and Surfactant Macromonomers Dilute Aqueous Solutions

Several surfactant macromonomers (SMM_xyz) were prepared by using triblock polymers of ethylene oxide (EO) and propylene oxide (PO) (PO_x–EO_y–PO_z triblock polymer, x: y: z represent the weight ratio of the different blocks). Then graft copolymers of acrylamide and SMM_xyz (PAM-g-SMM_xyz) were synthesized. The effects of EO/PO mole ratio, length of grafted SMM_xyz and the three relative block sizes on the surface activity and thermo-associativity of PAM-g-SMM_xyz dilute solutions were studied. The results showed that the surface tension decreased with increasing PO in the graft at 30°C and decreased with the length of the graft when the temperature was higher than the cloud points of the grafted PO_x–EO_y–PO_z chains, regardless of EO/PO ratio. When the EO/PO ratio was the same in the SMM_xyz, the PAM-g-SMM_xyz with higher PO content at the graft end had higher surface activity. With increasing temperature, the hydrodynamic radius (R_h) of PAM-g-SMM_xyz in dilute aqueous solution increased. The polymer with shorter grafts may have much more intermolecular interaction points and there were more polymer chains in the polymeric aggregates, which led to R_h increasing with decreasing length of graft. In addition, when the EO/PO was not more than 1:1.74, the three relative block sizes had no great influence on the temperature-associativity of PAM-g-SMM_xyz. However, for PAM-g-SMM₂₁₇ and PAM-g-SMM₇₁₂, the R_h of PAM-g-SMM₂₁₇ was smaller than PAM-g- SMM_712.     

Intumescent Flame Retardancy and Thermal Degradation of Epoxy Resin Filled with Ammonium Polyphosphate Using Thermogravimetric Analysis–Fourier Transform Infrared Spectroscopy and Thermogravimetric Analysis–Mass Spectrometry

The flammability of epoxy resin (EP) and its composite with ammonium polyphosphate (EPAPP) was investigated with limiting oxygen index (LOI), UL 94, and cone calorimeter tests. A systematic and comparative evaluation of the thermal degradation of EP and EPAPP has been investigated using thermogravimetry coupled with Fourier transform infrared spectroscopy (TG–FTIR) and thermogravimetry coupled with mass spectrometry (TG–MS). The results showed that the flame retardant of ammonium polyphosphate (APP) can constitute an intumescent flame retardant (IFR) system with EP, and APP can effectively improve the LOI of EP; with 6 wt% addition level of APP, EPAPP can pass UL 94 V 0 test. The cone calorimeter test results showed that the flame retardancy and smoke suppression of EP were significantly improved by APP, and toxic gas products such as carbon monoxide and carbon dioxide obviously decreased. Thermogravimetry–Fourier transform infrared spectroscopy and TG–MS results showed that the degradation process of EP produces large amounts of gas products and mainly containing of water, carbon dioxide, methane, benzene and its derivatives, as well as phenol and its derivatives. Compared to EP, the kinds of decomposition products of EPAPP sample were not changed significantly, except that more ammonia gas was generated. For the EPAPP sample, the products of water, benzene, and phenol increased, whereas the carbon dioxide and the flammable hydrocarbon fragments C_xH_y decreased significantly during the decomposition.     

High dielectric anisotropy compound doped transmission gratings of HPDLC

Effects of high dielectric anisotropy compound (MLC) and oligomer molecular weight on the grating formation and electro-optical properties of transmission holographic polymer dispersed liquid crystal (HPDLC) have been studied. Addition of MLC reduced the response time and switching voltage with the increased formulation viscosity which introduced a sequence of phenomenon, i.e. slow diffusion, polymerization, phase separation, grating formation, and droplet coalescence giving small droplet size and enhanced diffraction efficiency of the film. Apparently, increase in oligomer molecular weight gave effects similar to those of increasing MLC content since both variables control droplet coalescence through different mechanisms.     

Tuning crack pattern by phase separation in the drying of binary colloid–polymer suspension

Drying the colloidal suspension leads to the versatile crack patterns, where the microstructure formed during the drying process determines crack patterns. Here, the polymer is introduced into a silica suspension to tune the crack arrangement in the drying deposit. Five dominated types of crack patterns are defined by varying the ratio of nanoparticles to polymer. Two phase separation processes are proposed to explain the spatial characteristics of the crack patterns: the depletion-induced phase separation of particles and polymer in their mixture; and the separation of water and particle (or polymer) clusters by the water drainage.     

Tribological Properties of Hydroxyl-Terminated Liquid Nitrile Rubber-Modified Polyurethane/Epoxy Interpenetrating Polymer Network Composites

A series of castor oil-based polyurethane (PU)/epoxy resin (EP) graft interpenetrating polymer network (IPN) composites modified by two kinds of hydroxy-terminated liquid nitrile rubber (HTLN) was prepared. A systematic investigation of the tribological properties of the two kinds of HTLN-modified PU/EP IPN composites was carried out through a pin-on-disk arrangement under dry sliding conditions. Experimental results revealed that the incorporation of HTLN can improve the friction and wear properties of PU/EP IPN significantly. Both the friction coefficient and wear loss decreased with increasing content of HTLN. The worn surfaces of the samples were analyzed using scanning electron microscope and a three-dimensional (3D) noncontact surface-mapping profiler; the results showed that the worn surfaces of the PU/EP IPN composites became smooth when the HTLN was added. The mechanisms for the improvement of tribological properties are discussed.     

Clustering of magnetic nanoparticles using a double hydrophilic block copolymer, poly(ethylene oxide)-b-poly(acrylic acid)

The use of a double hydrophilic block copolymer (DHBC), poly(ethylene oxide)-block-poly(acrylic acid) (PEO-b-PAA) to prepare magnetic nanoparticle (MNP) clusters was investigated. In this one-pot synthesis method, the DHBC controlled the particle growth and served as both stabilizer and clustering agent. Depending on the iron-to-polymer ratio, the synthesized particles can be in the form of colonies of small iron oxide particles or clusters of these particles with size larger than 100 nm. Compared to the previous reported result using random copolymers, the clusters prepared with DHBC were more compact and homogeneous. The yield of clusters increased when the amount of polymer added was limiting. Insufficient amounts of polymer resulted in the formation of bare patches on the magnetite surface, and the strong van der Waals attraction induced cluster formation.     

Damping,thermal, and mechanical properties of polyurethane based on poly(tetramethylene glycol)/epoxy interpenetrating polymer networks: effects of composition and isocyanate index

A series of polyurethane (PU) samples based on poly(tetramethylene glycol)/epoxy resin (EP) graft interpenetrating polymer networks (IPNs) were prepared and their damping, thermal, and mechanical properties were systematically studied in terms of composition and the value of the PU isocyanate index (R). The damping properties and thermal stability measurements revealed that the formation of PU/EP IPN could improve not only the damping capacity but also the thermal stability. Meanwhile, mechanical tests showed that the tensile strengths of the IPNs decreased while their impact strengths increased with increasing PU content. The value of R also had significant impacts on the properties of the IPNs when the PU and EP ratio was fixed, which could be an effective means for manipulating the fabrication of PU/EP IPNs. The morphologies of the PU/EP IPNs were observed by SEM and AFM characterization and the relationship between the morphologies and properties is discussed. With the results in hand, the PU/EP IPNs hold promise for use in structural damping materials.     

Metal-Filled Epoxy Composites: Mechanical Properties and Electrical/Thermal Conductivity

Abstract

The mechanical properties and the electrical and thermal conductivity of composites based on an epoxy polymer (EP) filled with dispersed copper (Cu) and nickel (Ni) were studied. It was shown that the electrical conductivity of the composites demonstrated percolation behavior with the values of the percolation threshold being 9.9 and 4.0?vol.% for the EP-Cu and EP-Ni composites, respectively. Using the Lichtenecker model, the thermal conductivity of the dispersed metal phase in the composites, λ_f, was estimated as being 35?W/mK for Cu powder and 13?W/mK for Ni powder. It was shown that introduction of the filler in EP led to a decrease in the intensity of the mechanical loss tangent (tan δ) peak that was caused by the existence of an immobilized polymer layer around the filler particles which did not contribute to mechanical losses. Using several models the thickness of this layer, ΔR, was estimated. The concept of an “excluded volume” of the polymer, V_ex, i.e. the volume of the immobilized polymer layer, which does not depend on the particle size and is determined solely by the value of the interaction parameter, B, was proposed.     

Competition between two forms of ordering in finite Coulomb clusters

The lowest-energy state of spherical clusters made up of single-species charged particles in a three-dimensional confining potential is investigated by molecular dynamics simulations for a system size of 5 x 10(3) to 1.2 x 10(5). The energy per particle is compared between shell-structured clusters and spherical finite-bcc lattices with relaxed surfaces. The shell structure in the interior is the lowest-energy configuration for ion numbers lower than about 10(4), while for higher ion numbers, an interior with bcc ordering surrounded by a few shells on the outside has lower energy. The formation of a small bcc lattice (nucleation) in the shell-structured cluster of 2 x 10(4) ions is observed.     

Microphase separation in polymer solutions containing surfactants

A microphase separation in solutions containing a polymer and a mixture of two solvents, one of which consists of amphiphilic molecules (surfactant), is considered theoretically in the weak-segregation regime. A surfactant molecule is described as a dimer consisting of hydrophobic and polar parts. The energy gain due to the orientation of surfactant molecules can lead to the appearance of non-homogeneities in the solution, where density fluctuations cause the orientational ordering of surfactant molecules. The difference in the interaction energies of hydrophobic and polar groups of a surfactant with solvent is considered as a main reason for orienting surfactant molecules. The free energy is calculated for various morphologies (lamellar, cylindrical hexagonal, spherical particles arranged at different cubic lattices). The phase diagrams are presented. With worsening the solvent quality, the transitions from disordered to a macro-separated state at low polymer and surfactant concentrations or to a body-centered-cubic, then hexagonal, and then lamellar structure at high polymer and surfactant concentrations are predicted. The amphiphilicity degree of surfactant molecules should exceed a certain critical value to make a microstructure formation possible. The period of the lamellar microstructure decreases with increasing the surfactant and polymer concentrations.     

Damping,Thermal, and Mechanical Properties of Polycaprolactone-Based PU/EP Graft IPNs: Effects of Composition and Isocyanate Index

A series of polycaprolactone (PCL)-based polyurethane (PU)/epoxy resin (EP) graft interpenetrating polymer networks (IPNs) were prepared and their damping propertiesand thermal stability, as well as mechanical properties, were systematically studied in terms of composition and the values of the PU isocyanate index (R). The morphologies of the PU/EP IPNs were observed by scanning electron microscope (SEM) and atomic force microscope (AFM) characterization and the relationship between the morphologies and the properties is also discussed. The damping properties and thermal stability measurements revealed that the formation of PU/EP IPNs could significantly improve not only the damping properties but also the thermal stability. Meanwhile, the mechanical tests showed that the tensile strengths of the IPNs decreased, while their impact strengths increased with increasing PU content. The value of the PU isocyanate index also had significant impacts on the properties of the IPNs when the PU to EP ratio was fixed, which could be an effective means for manipulating the fabrication of PU/EP IPNs. From the results obtained, the PCL-based PU/EP IPNs hold promise for use in structural damping materials.     

聚合物对非离子十二烷基聚氧乙烯聚氧丙烯醚浊点的影响

测定了水溶性高分子聚乙二醇（PEG1000、PEG2000、PEG6000）和聚乙烯吡咯烷酮（PVP－K30、PVP－K90）对三种非离子表面活性剂十二烷基聚氧乙烯聚氧丙烯醚C12H25O（EO ）m(PO)nH(LS36,m=3,n=6,LS45,m=4,n=5;LS54,m=5,n=4)浊点的影响。结果表明,聚乙二醇（PEG） 可使三种表面活性剂水溶液浊点降低;而聚乙烯吡咯烷酮（PVP）随其浓度增加,表面活性剂溶液浊点先升高然后又下降;浊点下降程度与聚合物浓度和分子量有关。     

Study of PEO—PPO—PEO Copolymers Conformational Changes: Viscosity and Dynamic Light Scattering Measurements

The self-assembly behaviour of poly(ethylene oxide)-b-poly(propylene oxide)-b-poly (ethylene oxide) copolymers, (EO)₁₃(PO)₃₀(EO)₁₃ (Pluronic L64), and (EO)₇₀ (PO)₃₀(EO)₇₀ (Pluronic F68) in water and in p-xylene has been elucidated by using viscosity and dynamic light scattering measurements to investigate the effects of hydrophilic chains length on their conformational changes. The viscosity measurements were performed for a range of temperature varying from 27°C to 60°C and concentration from 4 to 60 mg/ml. The variation of the viscosity and the conformational changes in aqueous solution depends on the kinds of interactions and the balance between excluded volume effects and hydrophobic interactions. Some dynamic light scattering measurements were also performed at room temperature for the same range of concentration to provide more information on the micellar structures in aqueous and organic solution.     

UV-radiation-induced formation of gold nanoparticles in a three-dimensional polymer matrix

The possibility of generation of a surface plasmon resonance by gold nanoparticles in a 3D network polymer matrix upon irradiation of a liquid acrylic composition containing a dissolved Au³⁺ salt is demonstrated for the first time. UV-irradiation of the solution simultaneously causes gold photoreduction to Au⁰ and acrylate photopolymerization. The possibility of preparing transparent colorless films that, when heated or exposed to light, give rise to a visible-spectrum absorption band belonging to a plasmon resonance of metal nanoparticles is demonstrated. The intensity and position of the plasmon resonance band depend on the parameters of the 3D polymer network, chemical structure of the oligomer, type of the support, and conditions of formation of Au⁰ particles. The formation of reduced gold nanoparticles responsible for the plasmon resonance band occurs more effectively in low-density networks with a large interchain length and containing oligomer block capable of complexation with the metal. The formation of nanoparticles is affected by the chlorine-containing products of the reduction of the gold salt.     

Pattern phenomena in an rf discharge dusty plasma system

Various dust patterns are observed in an rf discharge dusty plasma system. According to the dust growth process from small to large in size, the formation of different dust patterns can be divided into two stages: the small-particle stage (or dust cloud stage), and the large-particle stage (or dust crystal stage). The evolution relations between different dust patterns with gas pressure changing are investigated. Dust voids, dust acoustic waves and strong turbulence modes are presented at the small-particle stage. The self-organized dust lattices and dust clusters are investigated at the large-particle stage. The static structure of a dust lattice is characterized by means of the pair correlation function. Dust clusters formed by particles with different numbers and the regular evolution of the clusters with gas pressure are also investigated. The packing sequences of dust clusters are verified through two-dimensional confined molecular dynamics simulations.     

Shelf Life and Controlled Cure of Epoxies by Loaded Zeolite

Epoxy formulations based on the multi-functional amine hardener, dicyandiamide (Dicy), regularly contain a free accelerator for reducing the curing temperature and the time needed to complete the network formation. Unfortunately, all accelerators reduce the shelf life of these adhesives at 25°C. In order to solve this problem, accelerator-loaded zeolites fillers were developed, optimised with respect to host–guest interactions and characterised by Fraunhofer IFAM (Bremen, Germany) with regard to the release and curing behaviour in epoxy adhesive formulations. They are added to an epoxy adhesive (diglycidylether of bisphenol A (EP) and dicyandiamide (Dicy), mass ratio 100:6.7), stored at 25°C in regular air or cured (heated with β = 10 K/min to 170°C subsequent isothermal curing for 45 min). That shelf life and curing behaviour are investigated by FT-IR spectroscopy and modulated DSC. Compared to the EP containing free accelerator, the zeolite-filled EP possesses a threefold increase in shelf life at 25°C due to the immobilization of the accelerator in the pores of the zeolites. While the free accelerator acts steadily during heating, it is shown that the loaded zeolite releases the accelerator at about 76°C. Surprisingly, the released accelerator is not only involved in the chemical formation of the epoxy network but it accelerates the dissolution of Dicy considerably. As the result, network formation at 170°C finishes after not more than 19 min and the starting temperature for curing could be reduced to 140°C.