Influence of cross-linking functionality on the nonlinear optical properties of poled and cross-linked polymer films

We investigate the two potentially competing processes of molecular orientation and network formation in a functionalized guest-host system. We examine a pair of similar guest molecules, one difunctional, the other tetrafunctional. Using second-harmonic-generation data, we determine the average molecular orientation in each sample. At low concentrations of the guest molecules, the molecular orientation is well described by the oriented gas model. However, at concentrations high enough to permit the formation of a long-range network, the molecular orientation of the tetrafunctional molecules is seen to be compromised. This is in contrast with samples containing the difunctional molecules, which show no reduction in the induced orientation. However, the induced orientation is seen to relax more quickly in samples containing difunctional units than in those containing tetrafunctional units. Hence it would appear that increased stability in molecular orientation is achieved at the expense of the degree to which the molecules can be oriented.     

Prestressed double network thermoset: preparation and characterization

An innovative scheme to prepare Prestressed double network (PDN) epoxies is presented using a judicious combination of tetrafunctional curatives that have similar molecular weights but different reaction kinetics. A diglycidyl ether of bisphenol A epoxy monomer was reacted stoichiometrically and sequentially with various molar ratios of an aliphatic polyetheramine curing agent and an aromatic curing agent. Deformation was imposed on the partially cured resins after the formation of the first network, and postcuring was conducted at 50% compressive strain. Physical properties of the resulting PDN epoxies were examined using thermomechanical analysis, dynamic mechanical analysis, uniaxial tensile test, and plane-strain fracture toughness test. The application of prestress resulted in no changes in glass transition temperature, coefficient of linear thermal expansion, and Young’s modulus. However, a marked increase in fracture toughness is observed, accompanied by strong birefringence and visible roughness on the fracture surface.     

1,3,4-Oxadiazole epoxy resin-based liquid crystalline thermosets and their cure kinetics

Liquid crystalline thermosets were synthesized based on a difunctional liquid–crystal (LC) epoxy resin monomer, namely 2,5-bis(4-glycidyloxyphenyl)-1,3,4-oxadiazole with various tetrafunctional crosslinkers such as diaminodiphenyl methane, diaminodiphenyl sulfone, and diaminodiphenyl ether. The epoxy monomer was characterized by infrared and nuclear magnetic resonance spectroscopy. Cure kinetics of a stoichiometric mixture of epoxy monomer and diaminodiphenyl methane was investigated by differential scanning calorimetry (DSC) for specimens cured under various cure conditions. The nematic LC texture for the cured specimen was identified by polarized microscopy and confirmed by X-ray diffractometry. Phase diagram of cure time versus transition temperature was constructed based on the DSC data for epoxy/DDM system. The diagram displayed the changes of melting transition, isotropic transition, and glass transition temperatures as curing proceeds.     

Influence of voids on the stress distribution and deformation behaviour of epoxies under uniaxial deformation

A zero-order model is presented, which allows calculation of the stress distribution in porous epoxies by taking into account the interaction between randomly distributed voids. These results demonstrate that the mean value of the stress concentration factor increases with the volume fraction of voids. This leads to a decrease in sample yield strength. However, the generation of a porous morphology also creates a considerable number of regions where the stress is completely released. The theoretical predictions are in good agreement with experimental results obtained with macroporous epoxies, which were prepared based on the chemically induced phase separation technique, and tested under uniaxial compression.     

Transversely isotropic non-linear electro-active elastomers

Electro-active or electro-sensitive elastomers are ‘smart materials’, which are composed of a rubber-like basis material filled with electro-active particles, and as a result, their properties are able to change significantly by the application of electric fields. In this paper, we provide the theoretical basis of the non-linear properties for a special class of these materials, namely, the transversely isotropic electro-active elastomers, whose characteristic is that during the curing process, due to the presence of an external applied field, the electro-active particles are aligned in a preferred direction. The theory is applied to some boundary value problems. As well as this, a linear approximation is obtained from the general non-linear formulation, which is compared with the results of the classical theory for piezoelectric materials.     

Examination of the behavior of structures of elastomers with different laws of the state using the finite element method

A method of deriving the laws of the state of Peng-Landel and Lindley for nearly incompressible elastomers is based on the Finger form. An algorithm of solving nonlinear problems by the finite-element method in the form of the displacement method is described and numerical results obtained in free and constricted deformation of thin-layer structures, obtained on the basis of the KODETOM computing system, are presented.Translated from Problemy Prochnosti, No. 5, pp. 56–63, May, 1994.     

The influence of fibre surface treatment on the formation of an interphase in CFRP

In the scheme of a EURAM programme the influence of a wet oxidative surface treatment on the formation of an interphase, on the fibre-matrix bond strength and on the mechanical properties is investigated. The fibre CG 43–750 was supplied by Courtaulds treated to four different surface treatment levels (designated STL=0% or untreated, 10, 50 and 100%) sized (1% by weight) and impregnated with the resin HG 9106. This resin consists of di-, tri- and tetrafunctional epoxies with the hardener 3.3 DDS and also contains polyethersulphone. During the cure this resin separates in a continuous (thermoset-rich) phase which completely covers the fibres and a discontinuous (thermoplastic-rich) phase with a roughly globular structure. From water uptake experiments and matrix (interphase) sensitive composite properties (shear modulus G ₁₂, transverse modulus E ₂₂) it was concluded that the activated carbon fibre surface gives rise to a more fully crosslinked interphase, resulting in a reduction of the modulus of this interphase.     

Fiber-reinforced composites based on epoxy resins modified with elastomers and surface-modified silica nanoparticles

The properties of fiber-reinforced composites made using epoxy resin formulations can be improved using modified epoxy resins. As epoxies are inherently brittle, they are toughened with reactive liquid rubbers or core–shell elastomers. Surface-modified silica nanoparticles, 20 nm in diameter and with a very narrow particle size distribution, are available as concentrates in epoxy resins in industrial quantities for the past 10 years. Some of the drawbacks of toughening like lower modulus or a loss in strength can be compensated when using nanosilica together with these tougheners. Apparently, there exists a synergy as toughness and fatigue performance are increased significantly. Some of these improvements in bulk resin properties can be found for fiber-reinforced composites as well. In this article, the literature published in the last decade is studied with a focus on mechanical properties. Results are compared, and the mechanisms responsible for the property improvements are discussed. A relationship between the improvements of the fracture energy of the cured bulk epoxy resins and the fracture energy of the fiber-reinforced composites could be established.     

苯并噁嗪二苯醚树脂的固化行为研究EI

研究了苯并 口恶嗪二苯醚树脂固化时的体积收缩率、固化温度和固化焓变等。结果表明 ,这类新型树脂在室温下 ,固化收缩小于环氧树脂。在高温下 ,固化收缩率与环氧树脂相当。     

Single-wall carbon nanotubes/epoxy elastomers exhibiting high damping capacity in an extended temperature range

In nanocomposites containing single-wall or multi-wall carbon nanotubes (SWCNT and MWCNT) high damping can be achieved by taking advantage of the weak bonding and interfacial friction between individual nanotubes and the matrix. The increase in damping capacity has already been proved for stiff epoxies and in this study it is extended to epoxy elastomers. Variable amounts (0.5–3 wt%) of oxidized SWCNT were dispersed by ultrasonication in precursors of an epoxy elastomer based on the reaction of diglycidylether of bisphenol A (DGEBA) and a polyoxypropylene with average molar mass of 2000, end-capped with primary amine groups. The quality of the initial dispersion was assessed by the constancy of the storage modulus with frequency in the low-frequency range. A rheological percolation threshold of 0.41 wt% SWCNT was found. Cured elastomers exhibited a large increase of the loss modulus with increasing amounts of SWCNT. For 3 wt% SWCNT, the increase in loss modulus was 1400% at room temperature. When temperature was increased up to 140 °C the loss modulus of the nanocomposite was practically constant while the one of the matrix dropped to a negligible value. The damping capacity at high temperatures opens important practical applications.     

Stimuli-Triggered Multishape,Multimode, and Multistep Deformations Designed by Microfluidic 3D Droplet Printing

Elastomers generally possess low Young's modulus and high failure strain, which are widely used in soft robots and intelligent actuators. However, elastomers generally lack diverse functionalities, such as stimulated shape morphing, and a general strategy to implement these functionalities into elastomers is still challenging. Here, a microfluidic 3D droplet printing platform is developed to design composite elastomers architected with arrays of functional droplets. Functional droplets with controlled size, composition, position, and pattern are designed and implemented in the composite elastomers, imparting functional performances to the systems. The composited elastomers are sensitive to stimuli, such as solvent, temperature, and light, and are able to demonstrate multishape (bow- and S-shaped), multimode (gradual and sudden), and multistep (one- and two-step) deformations. Based on the unique properties of droplet-embedded composite elastomers, a variety of stimuli-responsive systems are developed, including designable numbers, biomimetic flowers, and soft robots, and a series of functional performances are achieved, presenting a facile platform to impart diverse functionalities into composite elastomers by microfluidic 3D droplet printing.     

Water vapor transport in liquid crystalline and non-liquid crystalline epoxies

This paper presents a comparison of moisture permeation in liquid crystalline and non-liquid crystalline epoxy systems. The permeability is obtained using a dynamic method. It is found that diffusion in both epoxy systems is Fickian. The liquid crystalline epoxy network exhibits higher barrier properties to moisture transport than the conventional epoxy network. The efficient chain packing of the smectic mesophase of the liquid crystalline epoxy is the main factor for this difference. The stoichiometry has a large effect on the moisture permeation. The diffusion coefficient decreases monotonically with increasing amine/epoxide functional ratio. The permeability (P) and solubility coefficient (S) reach a minimum for a functional ratio of one. The results are described on the basis of hydrogen bonding of water to the epoxy network and the two phase morphology of cured epoxies.     

Simultaneously Enhanced Cryogenic Tensile Strength,Ductility and Impact Resistance of Epoxy Resins by Polyethylene Glycol简

Rubbers have been well accepted for modifying brittle epoxies but rubber modified epoxies usually posses lowered tensile strength though enhanced ductility and fracture resistance. In this work, a polyethylene glycol （PEG-4000） is used to modify diglycidyl ether of bisphenol A/methyltetrahydrophthalic anhydride system for enhancing cryogenic tensile strength, ductility and impact resistance. The results display that the cryogenic tensile strength, ductility （failure strain） and fracture resistance （impact strength） are all enhanced for the modified epoxy system at proper PEG contents. The maximum tensile strength （127.8 MPa） at the cryogenic temperature （77 K） with an improvement of 30.1% is observed for the modified system with the 15 wt% PEG content. The ductility and impact resistance at both room temperature and cryogenic temperature are all improved for the modified epoxy system with proper PEG-4000 contents. These observations are explained by the positron annihilation lifetime spectroscopy results and scanning electron microscopy results. Moreover, the glass transition temperature decreases slightly with increasing PEG content.     

The microscopic failure processes and their relation to the structure of amine-cured bisphenol-A-diglycidyl ether epoxies

Electron and optical microscopy are used to study the relation between the structure and the microscopic flow and failure processes of diethylene triamine-cured bisphenol-A-diglycidyl ether epoxies. By straining films directly in the electron microscope, these epoxies are found to consist of 6 to 9 nm diameter particles which remain intact when flow occurs. It is suggested that these particles are intramolecularly crosslinked molecular domains which can interconnect to form larger network morphological entities. Epoxy films, either strained directly in the electron microscope or strained on a metal substrate, deform and fail by a crazing process. The flow processes that occur during deformation are dependent on the network morphology in which regions of either high or low crosslink density are the continuous phase. The fracture topographies of the epoxies are interpreted in terms of a crazing process. The coarse fracture topography initiation regions result from void growth and coalescence through the centre of a simultaneously growing poorly developed craze which consists of coarse fibrils. The surrounding smooth slow-crack growth mirror-like region results from crack propagation either through the centre or along the craze—matrix boundary interface of a thick, well developed craze consisting of fine fibrils.     

蒙脱土对硅橡胶基磁流变弹性体性能的影响

考察蒙脱土对硅橡胶基磁流变弹性的动态力学特性和磁流变性能的影响，通过调节体系中蒙脱土的含量优化硅橡胶磁流变弹性体的综合性能。研究结果表明，当弹性体中蒙脱土的含量在4．6％时，硅橡胶基磁流变弹性体／蒙脱土复合材料的模量比纯弹性体的模量提高了近345％，硅橡胶磁流变弹性体的磁致模量达到1.76MPa，磁流变效应为63．7％。     

取向磁场强度对磁流变弹性体力磁性能的影响

为探究制备过程中取向磁场对MR E力磁性能的影响,制备了相同磁敏颗粒夹杂的两种不同基体特性的MR E,分别对其微观组织和材料的力学性能进行了系统研究.结果表明:随着取向磁场与硅油含量的增大,磁敏颗粒在MR E中的链状排布更加明显;所制备材料的磁致剪切模量与磁流变效应均随着取向磁场的增大而增大,并且其增加趋势随着硅油含量...     

The structure,modes of deformation and failure,and mechanical properties of diaminodiphenyl sulphone-cured tetraglycidyl 4,4′diaminodiphenyl methane epoxy

The tensile mechanical properties of diaminodiphenyl sulphone (DDS) — cured tetraglycidyl 4,4diaminodiphenyl methane (TGDDM) epoxies [TGDDM-DDS (12 to 35 wt% DDS)] are reported as a function of temperature and strain rate. TGDDM-DDS (20 to 35 wt% DDS) epoxies, which exhibit broadT _gs near 250° C, are not highly cross-linked glasses because diffusional and steric restrictions limit their cross-link density. TGDDM-DDS (10 to 20wt% DDS) epoxies are more brittle with lowerT _gs as a result of lower molecular weights and/or lower cross-link densities. Electron diffraction and X-ray emission spectroscopy studies indicate that TGDDM-DDS (>25wt% DDS) epoxies contain crystalline regions of unreacted DDS which can be eliminated from these epoxies during cure resulting in microvoids. TGDDM-DDS (12 to 35wt% DDS) epoxies predominantly deform and fail in tension by crazing, as indicated by fracture topography studies. These glasses also deform by shear banding as indicated by right-angle steps in the fracture topography initiation region and mixed modes of deformation that involve both crazing and shear banding. No evidence was found for heterogeneous cross-link density distributions in TGDDM-DDS (15 to 35wt% DDS) epoxies on straining films in the electron microscope.Research sponsored by the Air Force Office of Scientific Research/AFSC, United States Air Force, under Contract No. F44620-76-0075. The United States Government is authorized to reproduce and distribute preprints for governmental purposes notwithstanding any copyright notation hereon.National Science Foundation Faculty Research Participant: NSF Grant No. SER 76-04721.     

Electromechanical instability in anisotropic dielectric elastomers

In this paper, we analyze the electromechanical instability of anisotropic dielectric elastomers. When an electric field and biaxial stress are applied to a dielectric elastomer, the homogeneous deformation of the dielectric elastomer may be unstable and pull-in instability and bifurcation instability may occur. Based on the previous investigations on the incompressible anisotropic elastic solids and dielectric elastomers, we outline the theory of anisotropic dielectrics. The electromechanical instability is considered for a thin layer of a dielectric elastomer sandwiched between two compliant electrodes. Analytic solutions are obtained for the classic neo-Hookean model of anisotropic materials. The results show that the stability of the dielectric elastomer is remarkably enhanced by anisotropy parameter.     

The effect of moisture on the physical and mechanical integrity of epoxies

The effect of specific combinations of moisture, heat, and stress on the physical structure, failure modes, and tensile mechanical properties of diaminodiphenyl sulphone (DDS)-cured tetraglycidyl 4,4diaminodiphenyl methane (TGDDM) epoxies [TGDDM-DDS (27 wt% DDS)] are reported. Sorbed moisture plasticizes TGDDM-DDS epoxies and deteriorates their mechanical properties in the range 23 to 150° C. Studies of the initiation cavity and mirror regions of the fracture topographies of these epoxies indicate that sorbed moisture enhances the craze initiation and propagation processes. The effect of tensile stress-level, applied for 1 h on dry epoxies, on the subsequent moisture sorption characteristics of the epoxies was also investigated. Such studies indicate that the initial stages of failure that involve both dilatational craze propagation and subsequent crack propagation enhance the accessibility of moisture to sorption sites within the epoxy to a greater extent than in the latter stages of failure which involve crack propagation alone. The amount of moisture sorbed by TGDDM-DDS epoxies is enhanced by 1.6 wt% after exposure to a 150° C thermal spike, as a result of moisture-induced free volume increases in the epoxies that involve rotational—isomeric population changes.Research sponsored in part by the McDonnell Douglas Independent Research and Development Program and in part by the Air Force Office of Scientific Research/AFSC, United States Air Force, under Contract No. F44620-76-C-0075. The United States Government is authorized to reproduce and distribute reprints for governmental purposes notwithstanding any copyright notation hereon.     

Toughening mechanisms in elastomer-modified epoxies

The role Of matrix ductility on the toughenability and toughening mechanism of elastomer-modified, diglycidyl ether of bisphenol A (DGEBA)-based epoxies is investigated. Matrix ductility is varied by using epoxide resins of varying epoxide monomer molecular weights. These epoxide resins are cured using 4,4 diaminodiphenyl sulphone (DDS) and, in some cases, modified with 10 vol% carboxyl-terminated copolymer of butadiene and acrylonitrile (CTBN). Fracture toughness values for the neat epoxies are found to be almost independent of the monomer molecular weight of the epoxide resin used. However, the fracture toughness of the elastomer-modified epoxies is found to be very dependent upon the epoxide monomer molecular weight. Tensile dilatometry indicates that the toughening mechanism, when present, is similar to the mechanism found for piperidine cured, elastomer-modified epoxies studied previously. Scanning electron microscopy and optical microscopy techniques corroborate this finding.