Viscoelastic properties of ternary in situ elastomer composites based on fluorocarbon, acrylic elastomers and thermotropic liquid crystalline polymer blends

Dynamic mechanical analysis was performed to characterize the viscoelastic properties of binary and ternary blends of fluorocarbon elastomer (FKM), acrylic elastomer (ACM) and liquid crystalline polymer (LCP). The results showed that the storage and loss modulus of all the blends increased significantly with the weight percentage of the LCP. The glass transition temperature evaluated at the loss modulus peak, were in the range of −10-+5 °C for all the blends. The time temperature superposition principle was applied for the FKM/ACM and 20% LCP filled FKM/ACM blend in order to evaluate the changes in the viscoelastic properties of FKM/ACM blend by the addition of LCP. The Arrhenius and William-Landel-Ferry (WLF) equations were used to quantify the viscoelastic behaviour at the glass transition region. Both the blends exhibited a single relaxation, which is glass transition, observed as a peek in the loss modulus at 1 Hz. The glassy moduli of these two systems were found to be comparable, but the rubbery moduli of the LCP filled FKM/ACM was much higher than the LCP unfilled system. However, the viscoelastic behaviour of these two systems and their sensitivity to time temperature may be considered to be quite similar.     

Aging of the Binders GAP-N100 and HTPB-IPDI Investigated by Torsion-DMA

The aging of the binders GAP-N100 and HTPB-IPDI was investigated by DMA in torsion mode to find out the changes in the storage shear modulus G′(ω), loss shear modulus G″(ω) and in the glass transition temperature T_G. The forced sinusodial deformation method was used with measuring frequencies between 0.1 Hz and 56.2 Hz. A measurement temperature range between −100°C and +50°C was applied. The DMA instrument was a Rheometrics Dynamical Spectrometer, type RDS II/7700. Non-aged GAP-N100 shows a well defined steep glass transition between −40°C and −25°C, which is found with aged samples also but shifted to higher temperatures. The glass transition of the binder HTPB-IPDI lies between −70°C and −10°C, but HTPB-IPDI has not a well defined glass transition. With aging it looses its glass transition, which can be seen by a smoothing out of the transition step in the curve G′(ω)=f(T). The behaviour and the differences of these binders are explainable on a molecular basis. The systematic shift of the glass transition temperature of the non-aged and aged GAP-N100 as well as of the non-aged HTPB-IPDI is describable by the Williams-Landel-Ferry equation. This equation was used to extrapolate the values of the glass transition temperatures to shear rates possible during operational use of propellants.     

Modeling of dynamic mechanical properties of vulcanized fluoroelastomer

The dynamic mechanical properties of a vulcanized fluoroelastomer (FKM) were studied over a range of temperatures and shear frequencies. Dynamic mechanical analysis and differential scanning calorimetry were used for the purpose of the study. A model was developed in order to describe FKM's viscoelastic behavior at various temperatures. The model was fitted to experimental data using an algorithm, which was developed for this purpose. As a result the FKM discrete relaxation spectrum at two reference temperatures was obtained, as well as the Williams‐Landel‐Ferry (WLF) equation parameters or the activation energy equivalent. Further on, the model was applied on storage modulus and loss tangent values obtained from the experiments, during which the temperature increased linearly. It was observed that the WLF equation fits well with the results during the glass transition, while the Arrhenius‐type relationship predicted too rapid decrease of the storage modulus during the glass transition. The master curves were constructed using the previously calculated WLF parameters and the activation energy equivalent. The developed model may be readily applied for the prediction of the numerous FKM compounds' frequency–temperature behavior using the dynamic mechanical properties obtained from either isothermal or low linear heating rate program measurements. POLYM. ENG. SCI., 47:2085–2094, 2007. © 2007 Society of Plastics Engineers     

Linking structure and nanomechanical properties via instrumented nanoindentations on well-defined and fine-tuned morphology poly(ethylene)

Several poly(ethylene) samples with a broad range of morphologies were studied in this work using nanoindentations. The samples had degrees of crystallinity ranging from 30 to 100% while their Young's modulus ranged from few tens of MPa up to several GPa. Experimental conditions for the correct evaluation of Young's modulus were at first identified, choosing a suitable loading rate in order to minimize viscoelastic effects on the unloading. The force curves, i.e., plots of applied load vs. penetration depth, were then analyzed following two common procedures available in the literature. None of these procedures leads to satisfying results when compared to other experimental techniques. However, it was found that a recently proposed correction factor to the Oliver and Pharr procedure allows to evaluate reliably Young's modulus of the poly(ethylene) samples exhibiting very different, fine-tuned morphologies.     

Explanation of tackifier effect on the viscoelastic properties of polyolefin‐based pressure sensitive adhesives

The effects of three tackifiers on the glass transition temperature, terminal relaxation time, plateau modulus, and steady shear viscosity of polyolefin‐based pressure‐sensitive adhesives (PSAs) were investigated. Free volume theory and the Gordon‐Taylor equation are used to explain the special effects of tackifiers on the glass transition temperature of the PSA systems. The plateau modulus and zero shear viscosities were determined from which entanglements and monomeric friction coefficients were calculated. The terminal relaxation time (related to the whole molecular chain relaxations) was calculated from the plateau modulus and zero shear viscosity. Explanations were offered as to why tackifiers have “paradoxical” effects on the viscoelastic properties of the polyolefin‐based PSA, such as increasing the glass transition temperature but decreasing the plateau modulus of the base polymer. © 2005 Wiley Periodicals, Inc. J Appl Polym Sci, 2006     

Effect of viscoelasticity on the spherical and flat adhesion characteristics of photopolymerizable acrylate polymer networks

This research is the first of its kind to study the comparison between spherical and flat probe adhesion behavior as a function of viscoelasticity. Viscoelastic properties were tailored through the use of acrylate networks synthesized from tert-butyl acrylate and poly(ethylene glycol) dimethacrylate (PEGDMA) solutions. The molecular weight and the weight fraction of PEGDMA crosslinker was altered to maintain a constant glass transition temperature of approximately 57 °C, but systematically vary the viscoelastic properties and the rubbery moduli (1–62 MPa). Dynamic mechanical analysis was performed to characterize the low-strain thermo-mechanical behavior of the materials. Viscoelastic behavior of the materials was characterized by creep testing and was observed to inversely correlate with crosslinking density. The samples tested with the spherical probe exhibited low pull-off forces at temperatures well above and well below the glass transition temperature of the material. A maximum in pull-off force was observed in the vicinity of the glass transition temperature owing to the viscoelastic energy dissipative processes. The peak in pull-off force was observed to decrease with an increase in crosslinking density and modulus. Adhesion measurements using the flat probe demonstrated a strong dependence of pull-off force on the modulus of the material above the glass transition temperature. It is concluded that viscoelasticity is a dominating factor in increasing the pull-off force values in the vicinity of the glass transition, while it plays a little or no role for temperatures +/−20 °C away from transition region , opening the possibility of thermally switchable adhesives.     

Cooperative behavior of poly(vinyl alcohol) and water as revealed by molecular dynamics simulations

An aqueous poly(vinyl alcohol) (PVA) model has been extensively studied by using the molecular dynamics (MD) simulation method. The employed molecular and force field models are validated against the available data in the literature. In particular, the glass transition temperature (T_g) is determined from the specific volume versus temperature, which compares well with the experimental observations. The diffusion coefficients of water (H₂O) through the PVA matrix follow the Arrhenius equations at both temperature regions separated by T_g, indicating the existence of free and bound water defined by hydrogen bonds (HBs). It has also been confirmed that HBs occur between PVA and H₂O, between PVA and PVA, between H₂O and H₂O, and all of them play the key roles in the glass transition. The local dynamics suggested by the decorrelations of various bond vectors can be well described by the Williams-Landel-Ferry (WLF) equation. This work demonstrates the cooperative behavior of PVA and H₂O which is responsible for the glass transition of the whole binary system.     

The diffusion of nonionic penetrants in polyamide

The diffusion behavior of nonionic penetrants in aqueous solution into nylon 6 was examined in the temperature range 5°–95°C. The Arrhenius plot of the diffusion coefficients is linear and its slope changes at 30–40°C higher than the glass transition temperature in water, as determined by dilatometry and viscoelastic measurements. The results are discussed in relation to the molecular size of the penetrant and the segmental motion of polymer chains.     

Temperature Dependence of Anelastic Deformation in Polycrystalline Silicon Nitride

The influence of grain-boundary sliding on the evaluation of the apparent Young's modulus and plastic-deformation (flow) stress was investigated by bending tests for two types of silicon nitrides sintered with Y₂O₃-based additives The apparent Young's moduli measured at high temperatures are consistent with those predicated from a theory based on polycrystalline anelasticity due to grain-boundary sliding. The temperature dependence of the critical bending stress for the onset of plastic deformation shows viscoplastic properties of the interglanular glass. The ductile-to-brittle transition of fracture is discussed by the bending strengths normalized by the measured Young's modulus.     

Thermorheological properties of asphalt binders

The rheological properties of different types of asphalt binders were studied and compared considering their constituents and physical characteristics. The saturate, aromatic, resin, and asphaltene (SARA) analysis and differential scanning calorimetry (DSC) have shown their individual constituents and two distinct glass transition temperatures, indicating the phase changes of the two main components of the asphalt binders, namely asphaltenes and maltenes. Rheological characterization was performed over a wide range of temperatures (−10°C-60°C) showing that these materials may exhibit viscoelastic solid to viscous liquid behaviour. Master curves of complex viscosity, storage modulus, and loss modulus were constructed by applying the time-temperature superposition principle, which was found applicable over the temperature range considered. Stress relaxation and steady-shear test were applied to the samples in order to determine their rheological behaviour in the nonlinear viscoelastic regime (viscosity and nonlinear relaxation modulus). The rheological results were modelled and revealed that the Kaye-Bernstein-Kearsley-Zappas (K-BKZ) constitutive equation is suitable in representing the rheological behaviour of asphalts. The SARA analysis and rheological measurements were found to be compatible.     

Sorption and diffusion of hydrocarbon vapors in glassy polymers

Sorption isotherms in the region of low relative pressures have been determined at several temperatures for methane, propane, and chlorodifluoromethane in polystyrene and for propane in bisphenol-A polycarbonate and poly(vinylacetate). The results are well represented by the isotherm equation of Dual Sorption Theory as applied to glassy polymers. The temperature dependence of the isotherm parameters is examined and discussed; the Langmuir component to sorption decreases as the glass transition temperature is approached and measurements with poly(vinylacetate) confirm that this component is absent above the transition. Average diffusion coefficients were obtained from sorption (desorption) rate curves at constant pressure for propane in polystyrene and polycarbonate and a procedure developed for their analysis to yield the diffusion coefficients of the two sorbed species of penetrant. For the polycarbonate there is evidence of mobility in that fraction of the penetrant: population exhibiting Langmuir-type sorption.     

On shift and resolution of relaxation maxima in two-phase polymeric systems

The results of calculations of viscoelastic properties of filled polymers assuming “boundary layer” and the polymer matrix having their own and different glass transition temperatures are presented. The calculations have been made on the basis of the models in series and parallel coupling of viscoelastic bodies as well as Takayanagi's model. Using specified temperature, dependences of viscoelastic properties of the polymer and the “boundary layer” temperature dependences of the real part G′ of the complex shear modulus and tan δ for such a two-phase system with varied differences in glass transition temperatures and concentration of the components were derived. The degree of maxima shift observed experimentally for the filled polymer owing to variation of properties of the “boundary layer” (its concentration and glass transition temperature) were evaluated. Conditions for the appearance of two maxima tan δ for the respective glass transtion temperatures of the polymer and “boundary layer” on the curves tan δ = f(T) were determined.     

Nanoconfined segmental dynamics in miscible polymer blend nanocomposites: the influence of the geometry of nanoparticles

The influence of nanoconfinement on segmental relaxation behavior of poly(methyl methacrylate) and poly(styrene-ran-acrylonitrile) miscible blend and its nanocomposites with spherical and layered nanoparticles have been investigated. Dynamic mechanical analysis was employed to examine the effect of geometry of nanoparticles on the temperature dependence and relaxation function breadth of segmental dynamics (α-relaxation) in the glass transition region. The maxima of the loss modulus curves were used to fit to the Vogel–Fulcher–Tamman equation to describe the temperature dependence of the characteristic relaxation times. Furthermore, the T _g-normalized semi-logarithmic Arrhenius plots (fragility plots) were exploited to indicate the changes in cooperative segmental motions across the glass transition. The master curves for relaxation modulus were also constructed for each sample as a function of time using the time–temperature superposition principle. The investigated nanocomposites showed a narrower segmental dispersion in the glass transition region compared to the neat systems. The relaxation modulus master curves were fitted by the Kohlrausch–Williams–Watts (KWW) function. It was observed that the distribution parameter of segmental relaxation time increased with addition of nanoparticles which was correlated with a decrease in fragility index. In addition, the increase of the KWW distribution parameter (β _KWW) for spherical silica nanocomposites was less than that for nanocomposites prepared with layered silicates (organoclay).     

Viscoelastic characterization of high performance epoxy matrix composites

Dynamic mechanical analysis, stress relaxation, and creep experiments were performed to characterize the viscoelastic properties of a basic unmodified epoxy (Hercules 3501-6) and a modified multiphase epoxy (Hercules 8551-7), which are commonly used as matrices in high performance composites. The Arrhenius and WLF equations and a modified Standard Linear Solid (SLS) Model were used to quantify the viscoelastic behavior below, above, and at the glass transition temperatures, Tg. Both the modified and the unmodified epoxy systems exhibited a single relaxation peak above ambient temperature. The Tg of the modified epoxy at full cure was 170°C, which is 50°C lower than that of the basic epoxy. The glassy moduli of these two resin systems were found to be comparable, but the rubbery modulus of the modified epoxy was much lower than that of the basic epoxy system. However, the viscoelastic behavior of these two systems and their sensitivity to time-temperature may be considered to be quite similar if they are compared with respect to their corresponding glass transition temperatures.     

Microindentation studies at the near surface of glassy polymers: Influence of molecular weight

The viscoelastic‐plastic properties of various amorphous, glassy polymers [polystyrene (PS), poly(styrene‐acrylonitrile) copolymer (SAN), poly(methyl methacrylate) (PMMA), poly(vinyl chloride) (PVC), polycarbonate (PC)] in the micron and submicron range were investigated by means of load‐displacement analysis from depth‐sensing experiments. Hardness and Young's modulus values decrease rapidly with increasing depth up to a few microns. New data on the glass transition temperature correlation with microhardness are presented. The influence of annealing below the glass transition temperature upon the microhardness for various glassy polymers is pointed out. For PS, the influence of the molecular weight variation and molecular weight distribution on the microhardness is reported. Results are discussed on the basis of an entanglement network model, recently developed to explain the fine structure of crazes in amorphous polymers. © 2004 Wiley Periodicals, Inc. J Appl Polym Sci 93: 1951–1956, 2004     

Viscoelastic behavior of Cytec FM73 adhesive during cure

In this work, the viscoelastic properties of Cytec FM73 structural film adhesive were characterized. Several resin plates were cured using various process cycles to achieve a range of final cure states. Specimens cut from these plates were tested using a dynamic mechanical analyzer (DMA) and the glass‐transition temperature at each degree of cure was determined. Stress relaxation tests at different temperatures were then performed using DMA in stress relaxation mode and time‐temperature superposition was used to generate master stress relaxation curves and associated shift functions for each degree of cure. Several different constitutive models were examined for their ability to describe relaxation modulus development during cure. A simple three‐parameter model consisting of a stretched exponential with cure‐dependent terms was found to provide the best results. The results indicate that of the parameters used in the model, relaxation time strongly depends on cure state. The empirical DiBenedetto equation was used to obtain an expression for glass‐transition temperature as a function of degree of cure. This expression was in turn used to derive a new relation to describe stress relaxation time as a function of degree of cure. The shift function was modeled using a simplified form of the Vogel equation with cure‐dependent coefficients. Good correlation between measured relaxation modulus and model predictions was observed. © 2003 Wiley Periodicals, J Appl Polym Sci 91: 2548–2557, 2004     

Time and temperature dependence of the mechanical properties of polystyrene bead foam

Tensile and compressive properties of polystyrene bead (PSB) foams at room temperature for strain rates from 10^?3 to 10⁵ min^?1 can be represented as nearly linearly increasing functions of modulus or stress versus the logarithm of the strain rate. The shear modulus and tensile data, including failure properties, on 0.054 g/cc PSB foam at various temperatures and strain rates can be represented by master curves of log (stress or modulus) versus log (reduced strain rate). These master curves are formed by a time and temperature superposition method, wherein data at one temperature are superposed on data at another temperature by a shift along the log (strain rate) axis. These time–temperature shift factors are calculated using a form of the Arrhenius equation.     

Rheological behavior of cellulose/monohydrate of N-methylmorpholine N-oxide solutions. Part 2. Glass transition domain

Dynamic mechanical experiments performed with original conditions allowed the analysis of a solution containing 15% cellulose dissolved in a monohydrate of N-methylmorpholine N-oxide (NMMO) in the amorphous state. The glass transition zone is studied by dynamic tensile experiments, while dynamic torsion technique is used to determine the viscoelastic behavior in the glassy state. A master curve of the storage and loss modulus versus frequency can be deduced from the isochronal curves measured by both techniques. This work allows one to complete the corresponding master curve obtained for the ‘liquid’ state and presented in a previous work [Rheol. Acta 1998;37: 107]. The measurements below the glass transition temperature exhibit two secondary relaxations. A modeling of the overall viscoelastic behavior, using the Nowick and Berry approach and the quasi-point defect theory, is proposed. Physical parameters deduced from this modeling are then discussed.     

Fracture behavior of polypropylene/ ethylene- diene-terpolymer blends : Effect of temperatures,notch radius and rubber content

The mechanical fracture and ductile-brittle transition (DBT) behavior, hysteresis phenomenon and the plastic zone size of polypropylene (PP) / ethylene-propylene-diene terpolymer (PP/EPDM) blends were investigated by varying EPDM content and notch radius under different temperatures. An increase in test temperature or rubber content in the PP/EPDM blend results in lower yield stress and Young's modulus. The ductile-brittle transition temperature (DBTT) of the notched impact strength decreases with the increase of the EPDM content. However, the DBTT is fairly independent of the notch radius. SEM morphologies of the fracture surfaces indicate that two separate modes, localized and mass shear yielding, work simultaneously in these blends. The plane-strain localized shear yielding dominates the brittle failure at lower temperatures, whereas the plane stress mass shear yielding dominates the ductile fracture at higher temperatures. The presence of EPDM rubber decreases the yield stress of the PP/EPDM blend due to the overlapping stress fields of adjacent particles, resulting in higher hysteresis energy. The relationships among the test temperature, hysteresis loss energy and the size of plastic zone are discussed in detail.     

Indentation Creep of Hydrated Soda–Lime Silicate Glass Determined by Nanoindentation

Time-dependent deformation behavior was investigated for soda–lime silicate glass with various water contents, using a nanoindentation technique. The complete indentation curve, loading and unloading part, is analyzed. It is shown that this deformation behavior may be represented in terms of a simple mechanical model analogous to a viscoelastic system. Values for Young's modulus were derived, a retardation spectrum was deduced, and apparent viscosity values were calculated. Structural rearrangements of the glass appear to be responsible for the observed changes of the viscoelastic properties. Water in the glass reduces Young's modulus and yield stress and thus promotes viscous flow.