Rate-temperature Superposition Research Articles

Using Deposition Rate and Substrate Temperature to Manipulate Liquid Crystal-Like Order in a Vapor-Deposited Hexagonal Columnar Glass.

We investigate vapor-deposited glasses of a phenanthroperylene ester, known to form an equilibrium hexagonal columnar phase, and show that liquid crystal-like order can be manipulated by the choice of deposition rate and substrate temperature during deposition. We find that rate-temperature superposition (RTS)-the equivalence of lowering the deposition rate and increasing the substrate temperature-can be used to predict and control the molecular orientation in vapor-deposited glasses over a wide range of substrate temperatures (0.75-1.0 Tg). This work extends RTS to a new structural motif, hexagonal columnar liquid crystal order, which is being explored for organic electronic applications. By several metrics, including the apparent average face-to-face nearest-neighbor distance, physical vapor deposition (PVD) glasses of the phenanthroperylene ester are as ordered as the glass prepared by cooling the equilibrium liquid crystal. By other measures, the PVD glasses are less ordered than the cooled liquid crystal. We explain the difference in the maximum attainable order with the existence of a gradient in molecular mobility at the free surface of a liquid crystal and its impact upon different mechanisms of structural rearrangement. This free surface equilibration mechanism explains the success of the RTS principle and provides guidance regarding the types of order most readily enhanced by vapor deposition. This work extends the applicability of RTS to include molecular systems with a diverse range of higher-order liquid-crystalline morphologies that could be useful for new organic electronic applications.

Effect of high strain rates and temperature on the micromechanical properties of 3D-printed polymer structures made by two-photon lithography

Two-photon lithography (TPL) enables the fabrication of metamaterials exhibiting unprecedented mechanical performance. Such lattice structures consist of micron-scale geometric features and are designed to achieve their envisioned properties through stretching and bending of individual trusses. Here, we present for the first time a comprehensive study of the effect of temperature and strain rate on the mechanical properties of micron-sized 3D printed polymers made by TPL using Nanoscribe’s negative tone photoresist IP-Dip. A strong strain rate dependency was identified for the yield strength, which increased fourfold in compression from 68 MPa to 230 MPa as the strain rate was raised from 7e−4 s−1 to 600 s−1. This trend was also seen in tension. The elastic modulus was found to increase with strain rate in the quasistatic regime but was constant at 3.2 GPa for strain rates of 0.7 s−1 and above. Even slightly elevated temperatures of 80°C led to a significant drop in the yield strength and elastic modulus, though the observed reduction was less severe at higher strain rates. Further, a model based on the strain rate-temperature superposition principle is reported that allows the extrapolation of the mechanical properties even beyond the conditions tested here.

Intermediate pyrolysis of organic fraction of municipal solid waste and rheological study of the pyrolysis oil for potential use as bio-bitumen

This work presents a study on intermediate pyrolysis of the organic fraction of municipal solid waste (OFMSW) and characterisation of organic liquid product (pyrolysis oils) with particular focus on aging and rheological characteristics. The feedstock was a real municipal waste sample received from a local waste treatment plant. Shredded into small particles, it contained a high amount of moisture (51.2%) and ash (17.4%). A pilot-scale intermediate pyrolysis system was used to process the material. The process mass balance showed that the yield pyrolysis oil was 10.6%. GC-MS and FTIR experiments showed that the accelerated aging (80 °C for 24 h) did not cause an obvious change in the liquid chemical composition, but led to a significant reduction in the solids and moisture contents. The dynamic viscosity tests demonstrated that the intermediate pyrolysis oil derived from OFMSW is a non-Newtonian fluid. The dynamic viscosity of the pyrolysis oil reduced with the increase of temperature or shear rate, which can be modelled by WLF function and the Carreau model, respectively. A shear rate-temperature superposition method was proposed to construct the viscosity master curve at a wide range of shear rate, where WLF function was employed to model the shear rate-temperature shift factor. The accelerated aging caused an obvious reduction in dynamic viscosity, resulting from the decomposition of the semisolid organic agglomerates in the solids content during the aging of the OFMSW intermediate pyrolysis oil. The relatively high viscosity and reduced viscosity after aging of the OFMSW pyrolysis oil has indicated its potential for application as a substitute of the light fraction in the bitumen for road construction.

Molecular based temperature and strain rate dependent yield criterion for anisotropic elastomeric thin films

A molecular formulation of the onset of plasticity is proposed to assess temperature and strain rate effects in anisotropic semi-crystalline rubbery films. The presented plane stress criterion is based on the strain rate-temperature superposition principle and the cooperative theory of yielding, where some parameters are assumed to be material constants, while others are considered to depend on specific modes of deformation. An orthotropic yield function is developed for a linear low density polyethylene thin film. Uniaxial and biaxial inflation experiments were carried out to determine the yield stress of the membrane via a strain recovery method. It is shown that the 3% offset method predicts the uniaxial elastoplastic transition with good accuracy. Both the tensile yield points along the two principal directions of the film and the biaxial yield stresses are found to obey the superposition principle. The proposed yield criterion is compared against experimental measurements, showing excellent agreement over a wide range of deformation rates and temperatures.

Compression yielding of epoxy: Strain rate and temperature effect

The response to compressive mechanical loading of the thermosetting system epoxy polymer based on diglycidyl ether of bisphenol A (DGEBA) and an aromatic amine hardener, 4,4′-methylenedianiline (DDM) has been experimentally investigated as a function of strain rate and temperature, ranging from 0.0025 to 2500s−1 and from room temperature to 100°C, all below the glass transition temperature. Results showed that both the elastic modulus and the compression strength of the material significantly increased with strain rate and decreased with temperature. Quasi-static tests, performed in electromechanical machines, and dynamic tests, carried out in a Hopkinson bar, were compared and simultaneously analyzed to generate a master curve representative of the material yielding, assuming the strain rate-temperature superposition principle. Experimental data were fitted to equations based on thermally activated models for polymers.

Compression yielding of polypropylenes above glass transition temperature

Abstract Yielding behaviour under compressive loading of two materials based on polypropylene, an isotactic homopolymer and an ethylene–propylene block copolymer, is studied at different strain rates and temperatures. Quasi-static tests, performed in electromechanical machines, and dynamic tests, carried out in a Hopkinson bar, were compared and simultaneously analyzed to generate a master curve representative of the material yielding, assuming the strain rate-temperature superposition principle. Experimental data were fitted to equations based on the cooperative model for semi-crystalline polymers.

Effects of strain rate and temperature on tensile properties of CPVC pipe material

In this study, crosshead speed and temperature dependence of tensile properties of CPVC pipe material is investigated. Standard ASTM tensile specimens are used. Temperatures ranging from –10 to 70°C and three crosshead speeds (5, 50 and 500 mm min-1) are considered in the testing programme. The tensile test results showed that a reduction in temperature and an increase in crosshead speed have favoured brittle fracture. The yield stress and elastic modulus decreased logarithmically with decreasing crosshead speed and linearly with increasing temperature. The strain rate–temperature superposition held for both yield stress and modulus of elasticity and master curves were developed for both properties using a temperature based shift factor. A single power law relationship is developed between the shear strength and the shear modulus for all temperatures and strain rates. The value of fracture strain remained almost constant with crosshead speed at 50°C and below. At 70°C however, an enhancement of fracture strain occurred with increasing crosshead speed between 5 and 50 mm min-1 then stabilised with further increasing crosshead speed.

Shear induced crystallization in poly(ε-caprolactone): effect of shear rate

We have studied the effect of shear rate on the very early stages of poly(ε-caprolactone) crystallization with in situ rheology and ex situ optical microscopy. The effect of shear rate is mainly to increase the nucleation density and to speed-up the crystallization process. Thermally-induced crystallization is separated from strain-induced crystallization allowing the construction of a ‘master curve’ indicating that the system obeys the shear rate—temperature—superposition. It was found that the free enthalpy difference in the melt Gm(γ̇,T)−Gm(γ̇=0,T) increases with the shear rate (γ̇) that can be qualitatively rationalized by the reduced entropy of the melt prior to crystallization.

Rheological behaviour of thermoplastic elastomers from polypropylene/acrylonitrile–butadiene rubber blends: effect of blend ratio, reactive compatibilization and dynamic vulcanization

The rheological behaviour of polypropylene (PP)/acrylonitrile–butadiene rubber (NBR) blends has been investigated with special reference to the effect of blend ratio, compatibilization and dynamic vulcanization. The morphology of the extrudates and the size and distribution of domains were examined. PP/NBR blends show pseudoplastic behaviour, which is evidenced by the decrease in viscosity with increasing shear rate. The melt viscosity of these blends increased with increasing nitrile rubber concentration and shows negative deviation from the theoretically predicted values. Compatibilization of the blends with phenolic-modified polypropylene (Ph-PP) increases the melt viscosity at low concentrations of Ph-PP. The rheological behaviour of PP/BR blends was correlated with the blend morphology. Dynamic vulcanization has a decreasing effect on die swell values. The effect of temperature on the melt viscosity of the blends was examined and attempts have been made to construct a shear rate–temperature superposition master curve. The melt-flow index values for these blends were measured and have been correlated with rheometer data to develop a master curve for different blend compositions. The morphological analysis indicates that compatibilization of the blends prevents the coalescence of dispersed NBR domains during annealing.

Strain rate and temperature dependence of shear properties of epoxy resin with various molecular weight between cross-linkings

Epoxy resins with various molecular weights between cross-linkings were prepared. In order to examine the strain rate and temperature dependence of the shear yield strength and the shear strength, test specimens were subjected to shear deformation at various strain rates and temperatures. The shear yield strength and the shear strength increased almost linearly as the logarithm of the strain rate increased. The strain rate-temperature superposition held for these shear properties. In particular, an experimental equation of the strain rate-temperature superposition for the shear yield strength was found. The shift factor to obtain a master curve was given with the temperature dependence of an Arrhenius type. Furthermore, the strain rate-temperature-molecular weight between cross-linkings superposition held for the shear properties.

Melt rheological behaviour of natural rubber/poly(methyl methacrylate)/natural rubber-g-poly(methyl methacrylate) blends

Melt rheological behaviour of natural rubber (NR) and poly(methyl methacrylate) (PMMA) blends has been studied with reference to the effect of blend ratio, processing conditions and graft copolymer concentration as a function of shear stress and temperature. The viscosity of the blend increases as the amount of NR increases. The observed values of viscosity are found to be higher than that calculated from additivity rule. In the case of solution cast samples the melt viscosity increases as the percentage of graft copolymer increases and the blend becomes more sensitive to temperature in the presence of graft copolymers. The influence of dynamic vulcanization of the rubber phase on the flow behaviour of the blends has been analysed. The die swell of the NR/PMMA blends was also evaluated in the presence and absence of graft copolymers. The morphology of extrudates has been analysed as a function of composition, graft copolymer concentration and shear stress conditions. Addition of compatibilizer reduces the domain size of the dispersed phase and stabilizes the morphology. It is also noted that state of dispersion are affected by conditions of blend preparation and extrusion. A shear rate-temperature super position master curve is constructed to predict the melt viscosities of the system as a function of temperature. The rheograms of 50/50 NR/PMMA with varying graft copolymer concentrations are found to coalesce into a master curve using modified viscosity and shear rate functions.

Adhesion and Failure Mechanisms of a Model Hot Melt Adhesive Bonded to Polypropylene

A model hot melt adhesive (HMA) based on an ethylene/vinyl acetate copolymer (EVA), an Escorez® hydrocarbon tackifier, and a wax has been used to bond together polypropylene (PP) films to give equilibrium bonding. Peel strengths were determined over a broad range of peel rates and test temperatures. Contrary to the peel behavior of joints with simple rubbery adhesives [1], peel strengths with this semi-crystalline adhesive are not rate-temperature superposable, and multiple transitions in failure locus occur. The semi-crystalline structure of the HMA also prevents rate-temperature superposition of its dynamic moduli. At different test temperatures, the dependence of peel strength on peel rate shows some resemblance to the dependence of the loss tangent of the bulk adhesive on frequency. This is consistent with a previous result [2] that the HMA debonding term. D, varies with the loss tangent of a HMA at the T-peel debonding frequency. This model HMA, similar to block copolymer/tackifier blends [3], consists of two phases: an EVA-rich and a tackifier-rich phase, in its amorphous region. At a low peel rate of 8.33 × 10−5 m/s, the peel strength shows a maximum at a temperature that corresponds to the transition temperature of the tackifier-rich phase (T 1). At a higher peel rate of 8.33 × 10−3 m/s, the peel strength rises with increasing test temperature, but becomes essentially constant at temperature T 1'. It is believed that, to optimize the peel strength of a HMA at ambient temperature, it is advantageous to formulate the EVA polymer (or other semi-crystalline polyolefins) with a compatible tackifier that yields a tackifier-rich phase with a transition temperature (T 1') in the vicinity of room temperature.

Relation between shear strength at the fibre-matrix interphase and shear properties of resin matrix

The shear yield strength and the shear strength of a resin matrix increase almost linearly as the logarithm of the strain rate increases. This increasing tendency is almost the same at various temperatures. The strain rate temperature superposition held and an experimental equation was found to estimate the strain rate and temperature dependence of these shear properties. The strain rate and temperature dependence of the shear yield strength at the fibre-matrix interphase can be also estimated by the same equation. A strong quantitative relation was observed between the strain rate and temperature dependence of the shear properties of a resin matrix and that of the shear yield strength at the fibre-matrix interphase.

Strain rate and temperature dependence of shear properties of epoxy resin

Epoxy resins with various ratios from two kinds of curing agents, ethylenediamine (EDA) and N,N′-dimethylethylenediamine (MeEDA), were prepared. In order to examine the strain rate and temperature dependence of the shear yield strength and the shear strength, test specimens were subjected to shear deformation at various strain rates and temperature. The shear yield strength and the shear strength increased almost linearly as the logarithm of the strain rate increased. The strain rate-temperature superposition held for these shear properties. In particular, an experimental equation of the strain rate-temperature superposition for the shear yield strength was found. The shift factor to obtain a master curve was given with the temperature dependence of an Arrhenius type. Furthermore, the strain rate-temperature-mixing ratio of curing agent superposition held for the shear properties, and the shift factor for these superpositions increased with the increase in MeEDA.

Shear viscosity behavior of flocculated suspensions

Considering that agglomerates are subject to breakdown into individual flocs under shear, a theoretical model is developed to describe the shear viscosity behavior of flocculated suspensions using the Robinson equation η a/μ 0 = 1 + kφ/(1 − φ/φ max). The rheological properties of coal—oil mixtures (COM) have been investigated using a concentric cylinder rotational viscometer in the coal volume fraction range of 0.268–0.375 at 55–85 °C. The model successfully accounts for the effects of solid concentration and shear rate on rheological behavior of flocculated suspensions of irregularly shaped particles. The experimental results show that the agglomerates are densely packed with flocs, and are broken into small fragments under shear. The effect of temperature on the rheological behavior is also examined. By applying the shear rate—temperature superposition method, it was found that the bulkiness of agglomerates increases above 75 °C, leading to a change in the rheological properties of COMs.

Strain rate and temperature dependence of tensile strength for carbon/glass fibre hybrid composites

The tensile strength of epoxy resin reinforced with a random planar orientation of short carbon and glass fibres increased as the strain rate increased, and the increase in tensile strength became slightly remarkable with increasing temperature. The strain rate-temperature superposition was held for each composite. The strain rate and temperature dependence of tensile strength of composites could be estimated based on the dependence of the mechanical properties of the matrix resin, the interfacial yield shear strength and the critical fibre length. The strain rate and temperature dependence of the tensile strength of the hybrid composite could be estimated by the additive rule of hybrid mixtures, using the strain rate and temperature dependence of the tensile strength of both composites. The experimental values at a higher rate were lower than the calculated values. It was hypothesized that this may have been caused by the ineffective fibres formed during preparation of the specimen.

Rheological properties of a star-shaped butadiene-styrene block copolymer

Flow curves and viscosity functions at 160, 180, 200 and 220 °C have been determined for a star-shaped butadiene-styrene block copolymer, by means of a slit die rheometer. At a critical shear stress, a transition occurs in flow curves, which suggests a modification in the melt structure. The flow activation energy is deduced by a classical shear rate—temperature superposition method, and the obtained value suggests an aggregates flow mechanism for the star-shaped product. Exit pressure and entrance pressure drop are obtained and show a dependence on the slit depth.

Cavitation in Viscoelastic Media

A theoretical model for cavitation in viscoelastic media is developed which incorporates both interfacial and rheological arguments. The properties of interfacial and bulk cavities are correlated, and the influence of time-dependent cavity vapor pressure Pν(t) or hydrostatic tension σ̄(t) are correlated with the extension ratio λ=r/r0 of the cavity surface in a stress relaxation model. Experimental data are analysed for cavitation by peeling, tensile loading of butt joints, and superheating of swollen elastomers. For each case an agreement between theory and experiments is obtained which identifies the specific mode of cavitation and indicates for all cases the presence of prenucleated cavities with radii r0⩾1.0 microns. The direct correlation of peeling stresses with viscoelastic restraints to interfacial cavitation provides a new type of experimental confirmation for the applicability of rate-temperature superposition to peeling data. The implicit assumption that the interfacial failure stress depends upon frequency or strain rate in the same fashion as cohesive failure stress is confirmed by cavitation theory for the general case where surface tension constraints are negligible compared to viscoelastic constraints.

Flow properties of polymeric systems. XX. Shear rate–temperature superposition for low molecular weight polyethylenes

Flow properties of polymeric systems. XX. Shear rate–temperature superposition for low molecular weight polyethylenes