Experimental,analytical, and numerical investigation of interphasial stress and strain fields in MWCNT polymer composites

In this investigation, the effect of polymer matrix‐MWCNT interphase on the stress and strain fields developed at the close vicinity of MWCNT was studied. The recently developed concept of the hybrid interphase (Papanicolaou et al., 2002) was applied. According to this concept, the interphase thickness depends on the property considered at the time. The parameter of imperfect bonding between the primary constituent materials is also introduced by means of the degree of adhesion. Experimental findings combined with analytical and numerical results gave a better understanding of the structural and mechanical performance of epoxy resin‐carbon nanotubes composites. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Modeling the Effect of a Soft Interlayer on the Stress Distribution around Fibers: Longitudinal and Transverse Loading

A model is presented which describes the effect of a soft interlayer with changing properties on the stress distribution around a fiber embedded into an infinite matrix. Transverse loading induces large deformations in the interlayer compared to the matrix and if the interphase is very soft mainly compressive deformations occur in it. Radial stress decreases considerably in this case and the stress maximum shifts to the surface of the fiber. However, stress concentration depends also on the stiffness of the interphase, increased stress concentrations may develop in the presence of interlayers having only slightly lower stiffness than the matrix. In parallel loading, deformations and shear stresses decrease in the presence of the soft interphase, the entire deformation is concentrated into a very narrow layer. Both shear yielding and debonding can take place in transverse loading, the dominating mechanism depends mainly on the properties of the interlayer. Very soft interlayers promote shear yielding. In parallel loading such interlayers result in low pull‐out forces, load transfer is impossible. Although the presence of a soft interlayer favorably changes stress concentration around the fiber, as expected, it is deleterious for reinforcement and stress transfer. The results clearly explain why soft interlayers are not employed in practice in spite of their apparent advantages.     

Dynamic stress/strain response of the interphase in polymer matrix composites

The interphases of various sized E‐glass‐fiber/epoxy‐amine systems were tested at displacement rates in the range of 230 to 2450 fxm/sec using a new experimental technique (dynamic micro‐debonding technique). The fiber systems include unsized, epoxy‐amine compatible sized, and epoxy‐amine incompatible sized glass fibers. A data reduction scheme was developed to relate the force vs. displacement response obtained from the dynamic micro‐debonding technique to interphase shear stress/strain response. The stress/strain curves and interphase shear modulus values were obtained from these composite systems under average shear strain rates (ASSR) in the range of 215–3278 (1/s). The results showed that the magnitude of the interphase shear modulus was sizing and strain rate dependent. In all cases, the shear modulus was found to be more compliant than the bulk matrix. The two sized fiber systems exhibited the highest strain rate sensitivity, with modulus increasing about threefold over the range studied. In addition, the rate dependent behavior of the model interphase materials were determined using the dynamic mechanical analysis (DMA) technique. The model interphase materials closely resemble the interphase that forms on unsized and compatible sized fibers. Master curves relating the flexural storage modulus to strain rate were constructed based on the time‐temperature superposition principle from DMA frequency sweep measurements. The DMA measured results are consistent with the dynamic micro‐debonding test results, providing confidence in the test method as a reliable technique for characterizing the high strain rate properties of the interphase in composites.     

New composites based on short melamine fiber reinforced epdm rubber

Melamine fibre is a new category of advanced synthetic fiber having superior heat and flame resistance with decomposition temperature above 350°C. It suitability as a reinforcing fiber for ethylene propylene diene terpolymer, abbreviated as EPDM rubber, where ‘M’ stands for polymethylene chain, was investigated. It has been observed that tensile strength and stress at 100% strain of EPDM‐melamine fiber composites increase with the addition of a three‐component dry bonding system, comprising hexamethylene tetramine (hexa), resorcinol, and hydrate silica, abbreviated HRH system. Moreover, the fiber‐filled composites anisotropy in stress‐strain properties due to preferential of the short fibers along the milling direction (longitudinal), which is substantiated by the results of swelling and fractography studies. Aging causes an increase in the modulus, tensile strength and hardness of the composites. The fractographs show an increase in interfacial adhesion between the fibers and the matrix during aging, which is further confirmed by the reduction in tan δ peak height of the aged composites during dynamic mechanical studies. Atomic Force Microscopy (AFM) studies reveal the formation of an interphase with the addition of bonding agents and a better fiber‐matrix adhesion due to aging. AFM images also confirm the role of dry bonding systems in improving the fiber‐matrix adhesion of the aged vulcanizates. The composite modulus has been theoretically calculated using the well‐known Halpin‐Tsai equation. It is found that in the transverse direction, observed modulus values are greater than the calculated values, while in the longitudinal direction, the experimental modulus values are found to be lower than the calculated values for both unaged and aged composites owing to some degree of anisotropy in fiber orientation.     

Characterization of interfacial toughness in carbon fiber/epoxy resin composite subjected to water aging using single‐fiber fragmentation method in an energy‐based model

The toughness of the interphase between carbon fibers and epoxy resin was characterized by the interfacial fracture energy, which was derived from the modified Wagner‐Nairn‐Detassis (WND) model, considering the moisture swelling stress. The characterization was used to evaluate the changes of interphase bonding before and after water aging, including boiling water and 70°C water immersion. The effects of the water aging on the parameters in WND model were analyzed. The mechanism of interphase degradation under water aging was interpreted considering the change of interphase thickness, which was measured using the dynamic nanomechanical mapping method. It is shown that the single‐fiber fragmentation test with the proposed energy‐based model can quantify the degradation of interphase toughness after water aging. For the studied system, the interphase thickness has a close relationship with the interfacial fracture energy, indicating that the swelling of the resin matrix and interphase results in an increase in interphase thickness and a decrease in interphase bonding property. POLYM. COMPOS., 2012. © 2012 Society of Plastics Engineers     

Neutral Polymeric Bonding Agents (NPBA) and Their Use in Smokeless Composite Rocket Propellants Based on HMX‐GAP‐BuNENA

Very few efficient bonding agents for use in solid rocket propellants with nitramine filler materials and energetic binder systems are currently available. In this work, we report the synthesis, detailed characterization, and use of neutral polymeric bonding agents (NPBA) in isocyanate‐cured and smokeless composite rocket propellants based on the nitramine octogen (HMX), the energetic binder glycidyl azide polymer (GAP), and the energetic plasticizer N‐butyl‐2‐nitratoethylnitramine (BuNENA). These polymeric bonding agents clearly influenced the viscosity of the uncured propellant mixtures and provided significantly enhanced mechanical properties to the cured propellants, even at low NPBA concentrations (down to 0.001 wt‐% of propellant). A modified NPBA more or less free of hydroxyl functionalities for interactions with isocyanate curing agent provided the same level of mechanical improvement as regular NPBA containing a substantial number of reactive hydroxyl groups. However, some degree of reactivity towards isocyanate is essential for function.     

The Properties of Organosiloxane/Polyester Interfaces at an E-Glass Fiber Surface

The role of vinylchlorosilane coupling agents in creating an interphase between a polyester matrix and a reinforcing E-glass fiber was investigated. Measurements of the cure kinetics of the polyester resin, using differential scanning calorimetry, revealed that the presence of untreated E-glass surfaces retarded the cure reaction of the polyester, while treatment of the glass with reactive silanes enhanced the cure relative to the unfilled resin. Internal reflection infrared spectroscopy was used to study the permeation of polyester into a polysiloxane coating and the chemical reactions of the vinylsilanes and polyesters. It was found that a vinyltrichlorosilane coupling agent forms a relatively impermeable siloxane film on the fiber surface that probably reacts with the polyester at the siloxane/polyester interface. Octenyltrichlorosilane forms a siloxane layer that is permeated by the polyester and coreacts with it. The resulting interphase is extremely weak and debonds readily from the fiber. Methacryloxypropyltrichlorosilane forms a siloxane layer that is easily permeated by the polyester and reacts with it to form a mechanically-strong interphase. It was also found that the silane surface treatments reduced the stress transmission to the glass fibers, as determined from fiber fragmentation tests, and that the optically observed modes of failure were consistent with the observations of the internal reflection infrared and fiber fragmentation experiments. A finite element analysis of a single fiber embedded in a polymer matrix was used to simulate the effects of interphase toughness and stiffness on the mode of crack propagation from a broken fiber end. While chemical bonding of the interphase to the fiber surface is a necessary condition for a strong, stable interface, it was found that the stress transmission to the fibers (i.e., the fiber efficiency) and the modes of crack propagation are controlled by the stiffness, fracture toughness and the thickness of the applied coatings.     

高能丁羟推进剂用键合剂分子设计与应用

针对不同丁羟推进剂配方特点 ,设计合成几种新型键合剂 ,并分别在这些推进剂中试用。结果表明 :键合剂结构不同 ,对推进剂高、低、常温力学性能的影响也不同。同时还初步探讨了这些键合剂的键合机理     

Preparation and characterization of polypropylene/serpentine composites

Composites of serpentine and polypropylene (PP) were prepared by twin‐screw extrusion. Serpentine was collected as rocks from the Ankara–Beynam region and ground into powder with an average particle size of about 3 μm for composite production. Both as‐received (rock) and powdered serpentine were characterized. A silane coupling agent (SCA), γ‐aminopropyl triethoxy silane, was used for the surface treatment of serpentine. Mechanical properties of the composites were measured in terms of impact strength, elastic modulus, stress at yield, stress at break, and percentage strain at break. The addition of serpentine was found to have a profound effect on the reinforcement of the PP matrix. Because of the stronger interactions at the interphase induced by SCA treatment, mechanical properties were improved further in comparison with the untreated composites. Similar thermal and morphological behaviors were recorded for the composites with and without surface treatment. Thermal studies showed an increase in both melting temperature and percentage crystallinity of the composites. Scanning electron microscopy analysis revealed that homogeneous distribution of filler was observed at low filler contents, but a certain extent of agglomeration was also seen at high filler loadings. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Isocyanate‐Free and Dual Curing of Smokeless Composite Rocket Propellants

Traditional composite rocket propellants are cured by treatment of hydroxyl‐terminated prepolymers with polyfunctional aliphatic isocyanates. For development of smokeless composite propellants containing nitramines and/or ammonium dinitramide (ADN), energetic binder systems using glycidyl azide polymer (GAP) are of particular interest. Polyfunctional alkynes are potential isocyanate‐free curing agents for GAP through thermal azide‐alkyne cycloaddition and subsequent formation of triazole crosslinkages. Propargyl succinate or closely related aliphatic derivatives have previously been reported for such isocyanate‐free curing of GAP. Herein, we present the synthesis and use of a new aromatic alkyne curing agent, the crystalline solid bisphenol A bis(propargyl ether) (BABE), as isocyanate‐free curing agent in smokeless propellants based on GAP, using either octogen (HMX) and/or prilled ADN as energetic filler materials. Thermal and mechanical properties, impact and friction sensitivity and ballistic characteristics were evaluated for these alkyne cured propellants. Improved mechanical properties could be obtained by combining isocyanate and alkyne curing agents (dual curing), a combination that imparted better mechanical properties in the cured propellants than either curing system did individually. The addition of a neutral polymeric bonding agent (NPBA) for improvement of binder‐filler interactions was also investigated using tensile testing and dynamic mechanical analysis (DMA). It was verified that the presence of isocyanates is essential for the NPBA to improve the mechanical properties of the propellants, further strengthening the attractiveness of dual cure systems.     

An efficient nanoscale representative volume element simulation including graded interphase for tensile behavior of epoxy/silica nanocomposites

The behavior of interphase-particle adhesion and interphase region around the nanoparticles can significantly affect the stress distribution and mechanical properties of polymeric nanocomposites. In this study, the elastic modulus of epoxy/silica nanocomposites is analyzed using the finite element method and different mathematical models. A nanoscale representative volume element including graded interphase, homogenous interphase, and no interphase model is implemented. Furthermore, the effect of interfacial adhesion is also considered. The final elastic modulus was clearly affected by the interphase modulus, especially at higher nanoparticle content. Under imperfect interfacial bonding, the existence of an interphase region leads to a slight increase in modulus, and in the absence of that area, the elastic modulus decreases to 3.28 GPa. In perfect bonding models, stress transferred from the matrix to interphase and, then, to nanoparticle, which led to a significant increase in elastic modulus. Unlike the imperfect bonding, the maximum stress was located in the elements along to the loading direction. A maximum 26% increase in elastic modulus for perfect bonding/graded interphase model with 6.54 vol% of nanosilica particles compared to bulk epoxy was achieved. Finally, on comparing the FEM analysis and theoretical results with the experimental data, good agreement between obtained results was found.     

硝胺类推进剂用键合剂的研究进展

综述了各类适用于硝胺类推进剂的键合剂的研究情况,认为中性聚合物键合剂（NPBA）提高硝胺类推进剂的力学性能效果较好,需根据具体配方情况合成具有适当溶度积参数的NPBA,并根据NPBA的临界温度确定合适的推进剂制备温度条件。酰胺类和硼化物类键合剂具有成为优良键合剂的潜力,应加强对其合成与应用的研究。通过文献分析,认为键合剂的作用效果是官能团因素和分子结构因素共同作用的结果。对今后用于硝胺类推进剂的键合剂的发展提出了一些建议。     

High‐density polyethylene/cycloolefin copolymer blends. Part 1: Phase structure,dynamic mechanical,tensile, and impact properties

High‐density polyethylene (HDPE) was blended with “reinforcing” cycloolefin copolymer (COC) in order to produce polyolefin materials with increased stiffness, yield and tensile strength. Experimental data on tensile modulus E_b, creep modulus E_bcr, storage modulus E_b′, loss modulus E_b″, yield strength S_yb, and tensile strength S_ub of blends are in plausible accord with their simultaneous prediction based on a predictive format that operates with a two‐parameter equivalent box model and the data on the phase continuity of components obtained from modified equations of the percolation theory. Dependencies of these mechanical properties on blend composition indicate the critical volume fraction v_2cr = 0.16 of COC. Interfacial adhesion in the HDPE/COC blends is strong enough to transmit acting stress up to the break point. Strain at break, tensile energy to break and tensile impact strength show conspicuous drops in the interval 15–25% of COC in the blends, during which COC starts to form a co‐continuous brittle component. Further growth of COC fraction accounts for reduction of blend ultimate properties to values typical of brittle polymers. However, tensile impact strength shows a local maximum at HDPE/COC = 25/75, which probably corresponds to COC toughened with HDPE particles. POLYM. ENG. SCI. 45:817–826, 2005. © 2005 Society of Plastics Engineers     

Study on the Synthesis and Interfacial Interaction Performance of Novel Dodecylamine‐Based Bonding Agents Used for Composite Solid Propellants

An effective pathway was explored to design and select proper bonding agents that could effectively improve the interfacial interactions between bonding agents and solid particles, with three novel synthesized alkyl bonding agents, dodecylamine‐N,N‐di‐2‐hydroxypropyl‐acetate (DIHPA), dodecylamine‐N,N‐di‐2‐hydroxypropyl‐hydroxy‐acetate (DIHPHA) and dodecylamine‐N,N‐di‐2‐hydroxypropyl‐cyano‐acetate (DIHPCA), as examples. Molecular dynamics simulation was applied to compare unit bond energies of these bonding agents with the [110] crystal face of ammonium perchlorate (AP) and the [120] crystal face of hexogen (RDX). The infrared test was used to characterize the interfacial interactions of these bonding agents with AP or RDX. XPS test was applied to calculate the adhesion percentage of the bonding agents on the surface of precoated AP or RDX particles. All of the above results indicated that these three bonding agents have strong interfacial interactions with AP or RDX in the order of DIHPCA>DIHPHA>DIHPA. The prepared three bonding agents were used in HTPB/AP/RDX/Al propellants, and their effects on tensile strength (σ), elongation under maximum tensile strength (ε_m), elongation at breaking point of the propellant (ε_b) and adhesion index (Φ) of the propellant were studied. The results show that the bonding agents improve the mechanical properties of the propellant in the order of DIHPCA>DIHPHA>DIHPA. The methods found from theoretical design, materials synthesis, and mechanistics studies up to practical application show effective guiding significance for choosing the proper bonding agent and improving the interfacial interactions between the solid particles and binder matrix.     

Effects of short fiber tip geometry and inhomogeneous interphase on the stress distribution of rubber matrix sealing composites

The normal and interfacial shear stress distributions with flat fiber tip of short‐fiber‐reinforced rubber matrix sealing composites (SFRC) compared with the shear lag model were investigated by using the finite element method (FEM). The results indicate that stress values do not agree with those calculated by the shear lag model. The effect of different geometrical shapes of fiber tip on the stress distributions of SFRC was also investigated. The geometrical shapes of fiber tip under present investigation are flat, semi‐elliptical, hemispherical, and circular cone, respectively. The results show that the hemispherical fiber tip transfers the load with less stress concentration and is contributed to controlling the interface debonding failure more effectively than other shapes of fiber tip. Further study on the effect of the inhomogeneous interphase properties on the normal and interfacial shear stresses of hemispherical fiber tip was also conducted. The results indicate that the normal stress increases with the increase of the interphase thickness and interfacial shear stress remains unchanged, and the normal stress values of SFRC with interphase are higher than those without interphase. The interphase elastic modulus has no influence on the stress distributions along the direction to the fiber axis. The stress distributions along the radial direction in the interphase end are largely dependent on the interphase elastic modulus, and the interfacial shear stress is larger than the normal stress, which reveals that a significant part of the external load is transferred from the fiber to the matrix through shear stresses within the interphase. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 41638.     

Thermophysical and Chemical Processes of Burning of Double‐Base Solid Propellants under External Irradiation

A piece‐wise linear function for the dependence of the burning rate on the radiant flux and its break point are important burning characteristics of double base propellants under external irradiation. The Fourier equation can be used to analyze the burning characteristics, but some early models resulted in discontinuous characteristic parameters at the break point. In this paper, a new model, where the loss of external irradiation energy was considered above the surface of condensed phase, would be used to resolve the contradiction. The burning characteristics of 1040, 1041, N‐5 and N propellants were analyzed.     

The role of the terminal functional group of self‐assembled monolayers on fiber matrix adhesion

Adhesion at the fiber‐matrix interface of a composite is often influenced by a combination of factors such as mechanical interlocking, physicochemical interactions, and chemical bonding in the fiber‐matrix interphase region. We demonstrate the use of an approach using self‐assembled monolayers (SAMs) for studying the impact of one of the factors, chemical bonding, on the overall adhesion of the glass‐fiber/matrix interface. Transformation of these monolayer surfaces using conventional chemistry with a focus on the creation of a terminal functional group that interacts with epoxy resin is reported. The modified surfaces were characterized by ellipsometry, X‐ray photoelectron spectroscopy, and contact angle techniques for chlorosilane coverage, and in situ conversion. The adhesion of diglycidyl ether of bisphenol‐A resin to modified SAMs on E‐glass fibers was measured by performing single‐fiber fragmentation test. The extent of adhesion between the fiber and matrix was found to be dependent on the type of functional group at the terminal end of the SAM in contact with the epoxy matrix. Methyl terminal group resulted in the least adhesion, while amine terminal groups resulted in the most adhesion. © 2007 Wiley Periodicals, Inc. J Appl Polym Sci, 2007     

Silane coupling agents: the role of the organofunctional group

Some highlights of recent research on silane coupling agents are reviewed. Studies of silanes in solution, of chemical bonding in the substrate/matrix interphase, and of the interfacial bond in composites have provided new insights into the mechanism of effectiveness of silane adhesion promoters. Examples are presented of new chemical structures and of new concepts of bonding designed for optimal performance of silane coupling agents in advanced composites.     

醇胺类键合剂在丁羟推进剂中的应用进展

介绍醇胺类键合剂的发展现状,分析键合剂在复合推进剂中的键合机理及其在丁羟推进剂中的应用,总结了醇胺类键合剂在应用中所存在的问题及发展趋势。认为醇胺类键合剂可明显改善丁羟推进剂的性能。     

Effects of carbon nanotubes and interphase properties on the interfacial conductivity and electrical conductivity of polymer nanocomposites

L_c is the minimum length of carbon nanotubes (CNTs) required for efficient transfer of filler conductivity to polymer matrix in polymer CNT nanocomposites (PCNTs). In this work, L_c is correlated with the dimensions of the CNTs and the interphase thickness. Subsequently, the interfacial conductivity as well as the effective length and concentration of CNTs are expressed by CNT and interphase properties. Moreover, a simple model for the tunneling conductivity of PCNTs is developed with these effective terms. The impacts of all parameters on L_c, the interfacial conductivity, the fraction of CNTs in the networks and the conductivity of the PCNT are explained and justified. In addition, the predictions of the percolation threshold and conductivity are compared with the experimental results of several samples. The desirable values of interfacial conductivity are achieved by thin, short and super‐conductive CNTs, high waviness and a thick interphase. However, thin and long CNTs, low waviness, a thick interphase, poor tunneling resistivity due to the polymer matrix and a short tunneling distance advantageously affect the conductivity of PCNTs, because they produce large conductive networks. The predictions also show good agreement with the experimental measurements of percolation threshold and conductivity, which confirms the developed equations. © 2020 Society of Chemical Industry