Influence of crosslinking and plasticizing on the viscoelasticity of highly filled elastomers

Solid propellants, like all highly filled elastomers, exhibit a complex nonlinear viscoelastic behavior. The aim of this study was to establish the relationships between the structure and properties, which is needed to construct a robust constitutive law for these materials. An extensive design of experiments approach allowed us to quantify the influence of the curing agents and plasticizer molecules on the microstructure of the propellant and its viscoelastic properties. Swelling and gel permeation chromatography measurements described the microstructure of the propellant and prestrained dynamic mechanical analysis (PDMA) characterized the viscoelastic behavior. The curing agents reacted with polymer chain ends participating in the network, in the sol fraction, or in filler–binder links. Consequently, the polymer network was incomplete even in stoichiometric conditions, and a minimum of 10% of the polymer was free in the microstructure. In addition, preswelling the polymer with plasticizer molecules before curing modified the obtained network by decreasing the crosslink density in the binder and increasing it in the vicinity of the filler surface. This study provided new insight into the local deformation mechanisms controlling nonlinearity as measured by PDMA. The nonlinear behavior appeared between 0 and 1.7% prestrain in both the elastic and viscous parts of the behavior. The network reached its maximum extensibility in the elastic part and constrained the sol fraction in this extended mesh for the viscous part. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40392.     

Influence of fillers and bonding agents on the viscoelasticity of highly filled elastomers

Highly filled elastomers such as solid propellants exhibit a complex nonlinear viscoelastic behavior. This work aimed at determining the influence of binder–filler and filler–filler interactions on the microstructure and the viscoelastic properties of the propellant using a design of experiments method. The influences of the filler fraction and of the filler–binder bonding agents (FBBA) were measured by swelling experiments and prestrained dynamic mechanical analyses. The results showed that FBBA react on the filler surface and concentrate the curing agents in the vicinity of the fillers. The nonlinearity of the viscoelastic behavior originated from filler–filler interactions that created high stress zones between fillers and therefore constrained the movements of the macromolecules of the binder. Filler–binder interactions induced by the FBBA increased the filler effective volume as well as the heterogeneous stress distribution in the microstructure. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40664.     

An original experimental approach showing that most nonlinearities expressed by filled elastomers relate to microscopic friction and cavitation

Filled elastomers present a highly nonlinear behavior when submitted to cyclic mechanical loads. Origins of these nonlinearities are still uncertain, but many models rely on micromechanisms such as friction or cavitation at the filler/binder interface. To substantiate these hypotheses, an experimental approach is proposed to assess the real effects of friction and cavitation on the macroscopic response of filled elastomers. A linear, viscoelastic, transparent material is used to create macroscopic samples in which mechanisms are activated, separately or jointly. The response of these samples to cyclic mechanical loads shows that each mechanism generates some typical nonlinearities. A deeper analysis of the resulting curves allows a correlation between the nonlinearities characteristics (shapes and amplitudes) and the ones expressed by filled elastomers, providing clues for the development of future models.     

Preparation and viscoelasticity of anisotropic polyurethane composites filled with TiO2 particles

An anisotropic structure arranged by fillers is an effective method to make composites possess special properties, but the conventional particle-reinforced polyurethane (PU) composites usually have an isotropic 0-3 structure. In this study, a precipitation method was used to synthesize TiO₂ particles. The particles were dispersed in a PU matrix, and the structures were observed by scanning electron microscopy. The results indicate that in the presence of an applied electric field, 1-3-like composites with TiO₂ particles in an oriented arrangement were prepared, while 0-3 PU composites were prepared without an electric field. Dynamic viscoelasticity test results show that the PU-TiO₂ composites with a 1-3-like structure have a higher storage and loss modulus. The creep properties of these two kinds of PU composites were measured and further fitted with a Findley power law and Weibull model. It was found that the creep resistance and recovery properties of the PU composites were enhanced by the anisotropic structures of the filler particles in the matrix. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2019 , 136, 47450.     

Electric‐field response behaviors of chitosan/barium titanate composite hydrogel elastomers

Chitosan/barium titanate (BaTiO₃) composite hydrogel elastomers were prepared in the presence or absence of an applied direct‐current (dc) electric field. Scanning electron microscopy was used to observe the microstructure of the elastomers and the dispersion of particles in it. Tests of the storage moduli (Gs) of the elastomers were investigated with a dynamic mechanical analyzer. On this basis, the G increment and increment sensitivity were explored. The results show that the particles were sequentially dispersed, and the values of the G values for the elastomer were higher under an external applied dc electric field; this indicated that the composite elastomers exhibited excellent electric field response. Furthermore, the electric‐field response of the composite elastomers changed with the particle concentration, and the maximum response occurred when the mass fraction of BaTiO₃ was 2.0%. The G value of the composite elastomer with a BaTiO₃ weight percentage of 2.0 increased with increasing electric field; this revealed that the composite elastomer had a positive electric field response. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42094.     

Influence of matrix viscosity on the dynamic mechanical performance of magnetorheological elastomers

Anisotropic magnetorheological elastomers (MREs) with different natural rubber matrix viscosities and industrial waste nickel zinc ferrite were prepared in order to assess the dynamic and mechanical performance of the materials. The curing characteristics of the anisotropic MREs were determined using a moving disk rheometer (MDR 2000). The loss tangent (tan δ) was measured through a parallel-plate rheometer over a frequency range of 1─100 Hz (Hz) and a strain amplitude range of 0.1─6%. It was found that tan δ increased with increasing matrix viscosity over the range of frequency and strain amplitude explored. Furthermore, as the matrix viscosity increased, the height of the tan δ peak also increased and the glass transition temperature (T_g) valued shifted to a higher temperature. It was also found that tensile strength and elongation at break increased with increasing matrix viscosity. The SEM micrographs revealed that the columnar structures became longer and thicker with a decrease in matrix viscosity. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48492.     

Understanding the role of isocyanate dilution toward spraying of polyurea

An experimental study into the consequences of diluting isocyanate toward processing of polyurea has been reported. The effect of introducing increasing amount of propylene carbonate into a representative isocyanate precursor has been established. Viscosity‐blending index based equations were found to be more reliable toward prediction of isocyanate precursor‐propylene carbonate blend viscosity. Spraying of undiluted isocyanate precursor with amine formulations led to formation of “noodle‐like” fibrous product, irrespective of the type of chain extender being used. Diluting isocyanate with propylene carbonate (10% v/v), lowered the viscosity of the precursor from 85 to 48 mPa s (at 70 °C), and the resulting formulations could be effectively sprayed to form polyurea films with excellent mechanical properties. Dilution with propylene carbonate also increased the “tack free” time appreciably from ～1 to ～4 s, which directly reflects on the improved processability. Rheological studies were performed to quantify the activation energy associated with the isocyanate‐amine reaction for polyurea preparation. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 45869.     

Influence of the morphological structure of carbon nanotubes on the viscoelasticity of PMMA‐based nanocomposites

The rheological behavior of polymeric nanocomposites provides major determination for their processability. In this work, three carbon nanotubes (CNTs) with varied geometries were adopted as nanofiller and then were introduced into poly(methyl methacrylate) (PMMA) matrix with different loadings (0.07–1.0 wt %). The different preparation routine led to varied CNTs dispersion states, on which the shear viscosity and the compressibility of their melts were proved to be sensitive. The technology for the preparation of their nanocomposites played a crucial role in controlling their rheological behavior. With melting blended bare CNTs, the dynamic shear viscosity of PMMA/CNTs increased with the increase of CNTs content, accompanied by aggregated CNTs in which no polymer matrix was entrapped. With the help of surface modification and pre‐mixing, well dispersed CNTs were obtained and a rather low aggregation rate ca. 0.029% was revealed. The well dispersed CNTs with an organic layer which was constructed by small molecules and presented lower viscosity. Such CNTs led to no remarkable clusters within polymer host and played the role of lubricant with an increased‐mobility layer, which can be reflected from the weighted relaxation time spectra. © 2018 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2018 , 135, 46444.     

Linear‐nonlinear dichotomy of the rheological response of particle‐filled polymers

Novel nanoparticles, polymer‐particle coupling agents, and functionalized polymers are being developed to enhance the performance of particle‐reinforced polymer systems such as advanced rubber compounds for automobile tires. Understanding the complex rheological behavior of rubber is critical to providing insights into both processability and end‐use properties. One unique aspect of the rheology of filled elastomers is that the incorporation of particles introduces a hysteretic softening (Payne effect) at small dynamic strains. This study demonstrates that this nonlinear viscoelastic behavior needs to be considered when attempting to correlate steady shear response (Mooney viscosity) to oscillatory shear measurements from test equipment such as the Rubber Process Analyzer (RPA). While a wide array of unfilled gum elastomers show good correlation between Mooney viscosity and dynamic torque from the RPA at all of the strain amplitudes used, rubber compounds containing silica and carbon black particles only exhibit good agreement between the two measures of processability when the oscillatory strain amplitude is high enough to sufficiently break up the filler network. Other features of the filler network and its influence on nonlinear rheology are considered in this investigation, including the effects of polymer–filler interactions on filler flocculation and the use of Fourier transform rheometry to illustrate the “linear‐nonlinear dichotomy” of the Payne effect. © 2014 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2014 , 131, 40818.     

Correlations in rheological behavior between large amplitude oscillatory shear and steady shear flow of silica-filled star-shaped styrene-butadiene rubber compounds: Experiment and simulation

The large amplitude oscillatory shear (LAOS) and steady shear behavior of star-shaped SSBR/silica 60 phr (21 vol%) compounds with various filler surface areas was measured and simulated. An SBR gum and SBR compounds containing four different silicas with surface areas of 55, 135, 160, and 195 m²/g were utilized. Rheological behavior indicated clear correlation with surface area. LAOS tests showed an increase in dynamic moduli, shear stress, and higher order harmonic contributions with surface area. Elastic and viscous Lissajous figures showed significant distortion at intermediate and higher strain amplitudes. Additionally, ratios of third and fifth order stress harmonics to the first stress harmonic (I_3/1 and I_5/1 , respectively) showed a ''bump'' at intermediate strain amplitudes for the three highest surface area compounds. With regards to steady shear, all materials showed strong shear thinning behavior, and an increase in shear viscosity with surface area. The Cox-Merz rule was shown to be valid for the SBR gum but not for the filled compounds. However, the complex viscosity as a function of shear rate amplitude at various frequencies at high strain amplitudes and the steady shear viscosity as a function of shear rate coincided. This correlation, referred to as the Philippoff approach, has important ramifications for the rubber industry, providing quick data for predicting processing behavior. The Simhambhatla-Leonov model was successfully employed to simulate rheological behavior for the SBR gum and the lowest surface area silica compound, but the model yielded mixed results for the higher surface area silica compounds.     

Rheological properties of neat epoxy exposed to in‐service aerospace contaminants

We present an experimental study on the rheological properties of a commonly used epoxy resin system (EPIKOTE‐862 resin and EPIKURE‐W curing agent), exposed to a variety of fluids typical of aerospace operations (jet fuel, hydraulic fluids, deicing, detergents, etc.), for a period of up to 6 months, at room temperature for most conditions, and with no concurrent mechanical loading or prior degradation. The specimens were subjected to stress and frequency sweeps with a shear rheometer, while a limited set received also a temperature sweep in a range consistent with aircraft operations. Results indicate that the treated resin samples are linear viscoelastic under these testing conditions. The resin has reasonable chemical resistance to most contaminants of this study, with the exception of two commonly used detergents: an aircraft surface cleaning compound, Penair C5572, and a nonionic detergent, Methyl Ethyl Ketone (MEK). The durability change of the first compound appears triggered by high temperatures only, while the second compound causes a very drastic stiffness loss under several conditions. This drop of performance occurs within a 3‐months period, with no apparent color change or fracture that could prompt visual inspection and repair. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 3961–3971, 2013     

Effect of electric field on storage modulus of dielectric composites

DCs composed by dielectric particles and elastomer matrix present electric field dependent deformation. Viscoelasticity dominates their electric field response, which has to be considered in their applications. Although the influence of strain amplitude and oscillation frequency on the viscoelasticity of DCs has been investigated, the effect of electric field has been seldom studied. In this study, DCs were prepared by dispersing TiO₂ particles with different concentrations and different distributions within silicone rubbers. The areal strain and the storage modulus of the DCs under different electric fields were tested. The results indicated the electric field has significant influence on the storage modulus of the DCs. Such an electric field dependent storage modulus is more significant for the DCs with higher particle fraction, or with aligned distributed particles. The enhanced electrostatic interaction between the adjacent particles by applying electric field is responsible for the phenomena.     

Enhanced dielectric and electrorheological properties of needle‐like TiO2/polyrhodanine core/shell hybrid nanostructure

In this study, antisedimentation, dielectric, electrorheological (ER) and creep–recovery properties of needle‐like TiO₂/polyrhodanine (PRh) nanocomposite were investigated. Antisedimentation ratio of needle‐like TiO₂/PRh was determined to be 45% after 30 days in silicone oil (SO). Polarizability and relaxation time of needle‐like TiO₂/PRh/SO system were determined to be 0.18 and 2.9 × 10^?5 s, respectively by the dielectric spectroscopy which was further used to evaluate the ER performance of the dispersion, and the data obtained were in good agreement with the overall ER results. ER properties of needle‐like TiO₂/PRh/SO system were determined by taking the effects of shear rate, shear stress, electric field strength, and temperature into account using a torque electrorheometer. Non‐Newtonian shear thinning behaviors were observed for the samples. Vibration damping capabilities of the dispersions were investigated by measuring their elastic and viscous moduli as functions of frequency, time, and electric field strengths. Enhanced and reversible viscoelastic deformations were recorded for needle‐like TiO₂/SO system from creep–recovery tests with 88% recovery under E = 3.5 kV mm^?1 condition; thus, the system was classified as a smart one and suitable for potential vibration damping applications. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43240.     

The study of short-shot water-assisted injection molding of short glass fiber reinforced polypropylene

water penetration length and fiber orientation (along the melt flow direction) are important indicators for water-assisted injection molding products of the fiber-reinforced polymer. The effects of melt short shot size, water injection delay time and water injection pressure on these two important indexes are analyzed theoretically and experimentally. The study found that with the increase of the melt short shot size, the extension of the water injection delay time and the increase of the water injection pressure, the water penetration length changed from 216 to 96 mm, 170 to 210 mm, and 215 to 180 mm, respectively. Therefore, it can be known that melt short shot size has the greatest influence on water penetration length, followed by water injection delay time, and finally water injection pressure. Meantime, due to the fiber orientation and change degree of water-assisted injection-molded products along the melt flow direction, the fiber orientation in the water channel layer along the melt flow direction has the highest and lowest change degree, followed by the wall layer and finally the core layer. It can be known that the melt short shot size has the greatest influence on the fiber orientation and the degree of change along the melt flow direction, followed by the water injection delay time, and finally the water injection pressure.     

Viscoelastic properties and filler dispersion in carbon black‐filled and silica‐filled cross‐linked natural rubbers

The activation energies (ΔE_{J ′} and ΔE_J″) calculated from the temperature dependence of the storage compliance (J′) and the loss compliance (J″) of carbon black (CB)‐filled, hydrophobic silica‐ and hydrophilic silica‐filled cross‐linked natural rubbers (NRs) were found to be less than 15 kJ mol^?1, which corresponds to the physical range of van der Waals interaction. The results of three‐dimensional‐transmission electron microscopy measurements indicate that the closest distance (d_p) between the two neighboring nanofiller aggregates centers decreased sharply with increasing nanofiller loading and tended to become constant at a nanofiller loading of around 30 phr or higher. For all samples examined, there were two regions related to the elastic deformation energy, and the critical d_p value between the two regions was in the order of CB > hydrophobic silica = hydrophilic silica. Additionally, ΔE_J′ developed in the region of longer d_p than that of ΔE_J″. On the other hand, ΔE_J″ occurred after the formation of the filler network and was larger than ΔE_J′. ΔE_J″ is assumed to be related to slippage of the junction and the rearrangement of the nanofiller network. Therefore, the dependence of ΔE_J′ and ΔE_J″ on d_p suggests that the interaction layer between the nanofiller and NR has at least two energy levels. © 2013 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 130: 2594–2602, 2013     

Melt rheology and interfacial properties of binary and ternary blends of PS,EOC, and SEBS

This works systematically investigates the interfacial properties of the binary and the ternary blends based on polystyrene (PS), ethylene octene copolymer (EOC), and styrene–ethylene–butylene–styrene (SEBS) by analyzing the melt linear rheological behavior of the blends and neat components. Moreover, the relationship between rheology, phase morphology, and mechanical properties of PS/EOC ternary blends with various quantities of SEBS were studied. The surface shear modulus (β) and interfacial tension values obtained by Palierne model indicated that the EOC/SEBS blend has the best interfacial properties, while the lowest interaction was found for PS/EOC blend. Based on the Palierne model and Harkin's spreading coefficients a core–shell type morphology with EOC phase encapsulated by the SEBS shell dispersed in the PS matrix was determined for the ternary blends. Scanning electron microscopy results revealed that both fibrillar and droplet forms of dispersed phase could be developed during the blending of PS and EOC in presence of SEBS. The extent of fibrillar morphology and interfacial interactions in PS/EOC/SEBS ternary blends was dependent on the SEBS content. The improvement of the mechanical properties of PS/EOC blends in the presence of SEBS was evidenced by the tensile and impact resistance experiments. The tensile strength reinforcement was more pronounced for the ternary blends with more fibrillar dispersed phase. © 2019 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2020 , 137, 48791.     

Linear viscoelasticity and stress growth in blood

This note complements a previous publication where we used first order kinetics to describe biofluid behavior, especially blood, in the cases of simple shear flow, hysteresis and yield stress. Here, we extend the model and consider the viscoelastic properties of blood. Specifically, we look at small amplitude oscillatory flow and stress growth. Successful comparisons of model predictions with blood data are produced.     

Preparation and characterization of hyperbranched poly(ether sulfone) and its application as a coating additive for linear poly(ether sulfone)

Hyperbranched poly(ether sulfone) (HPES), a suitable coating additive for improving the rheological properties of linear poly(ether sulfone) (LPES), was easily produced via polymerization of commercially available bisphenol S (A₂ monomer, BPS) and synthesized 2,4′,6‐trifluoro‐phenylsulfone (BB′₂ monomer, TF). During this reaction, fluoro‐ or phenolic‐terminated HPES (F‐HPES or OH‐HPES) could be facilely obtained by controlling the feed mole ratios of the two monomers. The polymerization mode A₂ + BB′₂ was confirmed by analyzing the model compounds and the degree of branching (DB) was calculated systematically. In addition, the synthesized polymers' chemical structures were exhibited by FTIR, ¹H NMR as well as ¹⁹F NMR spectroscopy. Notably, the addition of 1 wt % HPES reduced the melt viscosity and improved the high temperature liquidity of LPES because of its unique spherical shape. Furthermore, the addition of HPES did not have a negative impact on the performance of LPES, which was attributed to the good miscibility between HPES and LPES. © 2016 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2016 , 133, 43892.     

Melt linear viscoelastic rheological analysis to assess the microstructure of polyamide 6–acrylonitrile butadiene styrene terpolymer immiscible blends via the application of fractional Zener and Coran models

In this study, the melt linear viscoelastic rheological properties of polyamide 6 (PA6)–acrylonitrile butadiene styrene terpolymer (ABS) immiscible blends were analyzed with the help of Coran and fractional Zener models (FZMs) to assess the microstructure of the blends. For this purpose, dynamic shear flow experiments and scanning electron microscopy investigations were performed. The nonzero value of the elastic modulus of the spring element (G_e) of the FZM for ABS‐rich blends was explained by the formation of a networklike structure because of the agglomeration of the rubber phases of the ABS matrix, whereas for the PA6‐rich blends with a high content of ABS, the interactions and/or interconnectivity of the ABS dispersed phase led to a nonzero value of G_e. The value of the fitting parameter of the Coran model (f) was near to 0.5 for the 50/50 blend; this was fully in agreement with the formed cocontinuous morphology for this blend composition. On the other hand, the f value for the blends with a matrix–droplet‐type morphology was near to zero for the PA6‐rich blends; this indicated the lower continuity of the ABS dispersed phase as a harder phase compared to the PA6 soft matrix, whereas the f value was near to 1 for ABS‐rich blends. This confirmed the formation of an interconnected networklike structure for this series of blends. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017 , 134, 45423.     

Nonlinear viscoelastic modeling of soft polymers

A one‐dimensional phenomenological constitutive model, representing the nonlinear viscoelastic behavior of polymers is developed in this study. The proposed model is based on a modification of the well‐known three element standard solid model. The linear dashpot is replaced by an Eyring type one, while the nonlinearity is enhanced by a nonlinear, strain dependent spring constant. The new constitutive model was proved to be capable of capturing the main aspects of nonlinear viscoelastic response, namely, monotonic and cyclic loading, creep and stress relaxation, with the same parameter values. Model validation was tested on the experimental results at various modes of deformation for two elastomeric type materials, performed elsewhere. A very good agreement between model simulations and experimental data was obtained in all cases. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42141.