Thermomechanical studies of polymer deformation II. A thermodynamic analysis of a viscoelastic material in sinusoidal deformation

In this study, a linearly viscoelastic polyurethane film was subjected to continuous, sinusoidal deformation in a new isothermal deformation calorimeter, whose design details were recently reported (1). Internal energy and entropy of the polymer at each state in the deformation cycle were computed from heat rate and work rate data. This was made possible by using linear viscoelasticity theory to predict the irreversible entropy production. Thermal data were corrected for instrument time lag.     

Effects of stretch induced softening to the free recovery behavior of shape memory polymer composites

During a cyclic tension test, many elastomeric materials exhibit an appreciable softening in their mechanical properties resulting from the previous stretch, known as the Mullins effect. This paper explores the influence of the stretch induced softening effect to the free recovery behavior of an acrylate shape memory polymer (SMP) composite by incorporating carbon black (CB) as filler materials. The observed softening effect in this SMP composite is considered to be a consequence of stretch induced alternation of filler–polymer interactions inside the composite. Further experiments find that a larger prior stretch gives a larger increase in material softening, which in turn decreases the shape recovery speed. To capture the experimental observations, a multi-branch one dimensional (1D) model is applied, where the modulus in the equilibrium branch is modeled to decrease with stretching deformation following a damage-like softening function. It is found that the loss in modulus due to softening consequently reduces the driving force for recovery and thus results in a slow recovery. Parametric studies further demonstrate that the discounted shape recovery speed will finally reach a saturated level when gradually increasing the programmed strain level in a shape memory cycle.     

In-plane deformation of shape memory polymer sheets programmed using only scissors

This paper describes an unconventional, yet simple method to program sheets of shape memory polymer into a variety of two dimensional (2D) structures. The final shape is “encoded” by physically cutting an initial design out of a pre-strained film. The orientation of the initial cut-out relative to the direction of strain and the subsequent relaxation of strain via heating defines the final shape. The appeal of the approach described here is that an easy, low-cost cutting method can achieve a similar shape memory effect attained by more complex processing techniques. Unlike conventional methods, where the final shape of a shape memory polymer must be defined a priori, the direction of cutting of the polymer defines its final shape without any complex pre-programmed strain profiles. A geometric model relating the resolved 2D polymer shape to the initial shape and strain orientation reveals linear correlation between the model-predicted and experimentally-observed shapes. In addition to demonstrating the principle with simple rectangular shapes, we suggest geometries related to encryption and high aspect ratio fibers.     

Blade-coating of a viscoelastic fluid

A theory is presented which describes the dynamics of blade-coating of a viscoelstic fluid onto a moving sheet. The method begins with the usual “lubrication” approximation, and develops the solution as a perturbation about the Newtonian case. Viscoelasticity is described by an empirical constitutive equation which shows non-Newtonian viscosity and finite normal stress behavior consistent with typical observations of polymeric fluids. Theoretical results indicate a small increase in coating thickness due to departure from Newtonian behavior, and a significant decrease in the magnitude of the pressure developed under the blade. Consequently, the blade loading can be reduced significantly by viscoelastic effects. The results for the loading may be an artifact of the specific constitutive model, since it can be shown that some viscoelastic fluids, specifically an “elastic Newtonian” fluid, would exhibit increased loading relative to the inelastic Newtonian case.     

Some viscoelastic properties of ABS polymer

The discrete relaxation spectrum of an ABS (acrylonitrile–butadiene–styrene) polymer at 190°C. was calculated by using results from tensile relaxation moduli and the principle of reduced variables. The shift factor was found to conform well to the WLF equation, and the free volume fraction at T_g was calculated to be 0.026 in good agreement with the universal value. The values of the thermal expansion coefficient of free volume were calculated to be 9.8 X 10^-4 deg.^?1 and 7.0 × 10^?4 deg.^?1, respectively, from the WLF coefficients and from dilatometric results. The width of the entanglement plateau of the relaxation spectrum was observed to be a factor of approximately 2 larger than that calculated from molecular weights between entanglement couplings determined either from rubber elasticity theory or from an assumed molecular model which discounts the presence of the butadiene in the ABS system. By using Pao's theory, flow curves at 190°C. were calculated both from the discrete relaxation spectrum and from the dynamic modulus. These curves were essentially identical. However, the stress values of these curves were found to be about a decade higher than those experimentally determined from capillary flow measurements. Nevertheless, the shapes of the curves are in good agreement, and an explanation is suggested for existing discrepancies. Flow instability, processing variables, and residual strains are discussed in light of the flow curves and the calculated recoverable shear strains.     

Coupling between homogeneous rate processes and fluid deformation rate: Brownian particle coagulation in a rapidly dilating solvent

P. Curie's principle applied to an isotropic medium of arbitrary EOS does not preclude coupling between homogeneous (chemical,…) rate processes and local fluid dilation rate. Yet, practical examples of this coupling have largely remained unexplored. Using recently studied supercritical “antisolvent” (SAS) examples for precipitating high‐value particles (e.g., pharmaceuticals), we suggest that the characteristic dilation time t_V of the swelling solvent can be small enough to noticeably reduce the operative coagulation rate “constant,” β. Moreover, we expect that this coupling can occur under conditions in which postnucleation Brownian coagulation must be accounted for in predicting the efficacy of such micron‐sized powder production methods. Accordingly, a rational approximate theory for this rate constant “correction factor,” β/β(0), is proposed here, emphasizing the applicable limit of continuum Brownian diffusion control. We also present a preliminary assessment of the particle size distribution (PSD) consequences of these “corrections,” implying strategies to reduce both mean particle size and PSD spread. Possible generalizations are indicated. © 2010 American Institute of Chemical Engineers AIChE J, 2011     

Gas permeation and viscoelastic deformation of prepregs in composite manufacturing processes

Gas permeation and creep deformation of a commercial prepreg, which exhibits viscoelastic characteristics, were investigated as a function of time, temperature, and consolidation pressure. Experiments using a prepreg stack demonstrated that the material exhibited a linear viscoelastic bulk deformation under vacuum/autoclave pressure and furthermore, the in-plane gas flow exhibited non-Darcian flow behavior with a permeation hysteresis. This behavior was viewed and analyzed by two viscoelastic relaxation processes: (1) bulk dimensional relaxation, and (2) microscopic pore structure rearrangement. A modified standard linear solid (SLS) viscoelastic model was used to interpret the creep compliance and dynamic gas permeability utilizing two independent relaxation parameters. By visual investigation of pore sizes and their distribution, air permeation was found to take place mostly through the interlaminar porosity network for the prepreg system examined.     

The viscoelastic relaxation of cross-linked polymer networks

Summary Theoretical interpretations of the viscoelastic relaxation behavior of cross-linked elastomers are discussed. The dangling chain retracing mechanisms of deGennes and Pearson-Helfand, which assume that the stress contribution of a dangling chain decreases as it assumes successively lower entropy configurations, are replaced by an alternative relaxation mechanism, based on the hopping model of hindered diffusion.     

Transient behavior of viscoelastic fluid in an extruder

A simplified model is used for calculating the time-dependent velocity of polymeric fluid in an extruder. The flow properties of the fluid are characterized by a simple constitutive equation based on two parameters: a constant viscosity μ and a constant elasticity modulus G. It was found that the transient velocity fluctuates periodically, and the time t_t needed to restore the steady-state velocity from a disturbance varies with the ratio G/μ and the dimensionless group _ρH²G/μ², where ρ is the density of the fluid and H is the screw depth of the extruder.     

Compressive large deformation viscoelastic behaviour of a polycarbonate

The effect of deformation level and loading rate on compression creep and stress-relaxation behaviour of a polycarbonate has been studied. The possibility of obtaining master curves has been examined throughout. Satisfactory results were obtained for the stress-relaxation data by considering only the relaxable part of stress and by using a time shift factor proportional to both the inverse of deformation rate just prior to the test and the strain. The same shift factor allowed us to obtain a single master curve for the creep data.     

Slow bubble growth and dissolution in a viscoelastic fluid

A simple equation is derived for the time dependence of the bubble radius for the diffusion-induced slow growth or dissolution of a spherical gas bubble in a viscoelastic fluid of infinite extent. The constitutive equation for a first-order fluid and a surface–volume perturbation scheme are used to develop the solution, and the effect of viscosity level and elasticity on the bubble dynamics is considered. © 1998 John Wiley & Sons, Inc. J Appl Polym Sci 67:2093–2103, 1998     

The response of a glassy polymer in a loading/unloading deformation: The stress memory experiment

A ‘stress memory’ experiment was designed to expose the nonlinear viscoelastic relaxation processes in a glassy epoxy polymer. The stress memory experiment consists of (i) constant strain rate uniaxial loading to a pre-yield, yield or post-yield condition, (ii) unloading at the same strain rate to zero stress, (iii) holding the strain constant and (iv) monitoring the subsequent stress memory response, where the stress first increases to a maximum and then relaxes to an equilibrium value for that strain. This is an analog to the classic volume memory experiment by Kovacs (Fortschr Hochpolym Forsch, 3, 394, 1964). The stress memory response showed a strong dependence on the loading/unloading strain rate which cannot be predicted by linear viscoelasticity and also provides a significant challenge to a current nonlinear constitutive models. A recently developed Stochastic Constitutive Model (J Rheol, 57(3), 949, 2013) qualitatively predicts the effect of strain rate on the stress memory response.     

Phase inversion and viscoelastic properties of phase-separated polymer blends

Summary The morphologies and viscoelastic properties of the phase-separated poly(styrene-co-maleic anhydride)/poly(methyl methacrylate) bends have been investigated using TEM and rotational rheometry. Various rheological criteria based on the viscoelastic properties of the blends have been used to evaluate the phase inversion. By correlating the rheological results to data from morphological analysis by TEM, it is found that the maximum of the storage modulus and the viscosity at low shear rate are most suitable for determining the phase-inversion composition of the present phase-separated polymer blends. While the data from the maximum of the shear viscosity at high shear rate and from the shear-thinning extent proposed by Ziegler et al. slightly deviate from that from TEM micrograph, which indicates that shear-induced structure lie. Moreover, the prediction using various rheological models, as the viscosity ratio of the two coexisting phases is substituted for that of the pure components, is in nearly good agreement with that from TEM observation.     

The viscoelastic behavior of polymer/oligomer blends

The viscoelastic properties of poly(α-methyl styrene), its hexamer, and their athermal blends at various concentrations are studied. Master curves for the dynamic shear responses, G′ and G″, are successfully constructed for both the pure materials and the blends, indicating the validity of the time-temperature superposition principle for these systems. The temperature dependence of the shift factor follows the Vogel-Fulcher behavior over the temperature range studied, and the temperature dependence is slightly weaker for the blends. The rubbery plateau modulus scales with the polymer concentration as ; the terminal relaxation time scales with the polymer concentration as . The shape of the segmental dispersion appears unchanged by concentration, which differs from our calorimetric studies where mixtures show obviously temperature-broadened glass transitions and depressed enthalpy overshoots. The TNM (Tool-Narayanaswamy-Moynihan) model indicates that the change in the temperature dependence is not sufficient to account for the observed calorimetric broadening. We conclude that the temperature broadening of the glass transition for our blends is not due to a broadening of the dynamic spectrum or to changes in its temperature dependence. The possibility that the broadening is due to changes in the non-linearity parameter x in the TNM model is also considered. While the broadening could be due to a decreasing value of x, we found that this same decrease would lead to increasing enthalpy overshoots on heating, contrary to the experimental observations. The combination of the calorimetric results with the rheological measurements further indicates that the fundamental basis of the TNM-type of model of structural kinetics in glasses is potentially wrong.     

Melt solidification in polymer processing

This paper treats two cases of polymer melt solidification in rectangular geometry. The cases treated are the one of static solidification and that of solidification during flow in a narrow gap channel. Both cases are solved using the method of Dussinberre, which reduces the two-phase moving boundary case to a single phase problem, simplifying the mathematics considerably. The numerical solutions are based on a combination of the concept of flow analysis network (FAN), a finite element method developed for solving polymer flow problems, with a Crank-Nicolson implicit finite difference scheme. The methods may be used in computing the cooling down period and preventing “short” conditions in injection molding dies. Examples of solidification of high density polyethylene illustrate the applicability of the method.     

粘弹性清洁压裂液的性能评价与应用

采用一种长链脂肪酸季铵盐类阳离子表面活性剂配制成粘弹性清洁压裂液,室内性能评价结果表明,该压裂液的最佳使用浓度为1.5%².0%;60℃时静态滤失系数为3.15×10-4m/min1/2;静态悬砂速度为0.25 mm/s,常温下与原油混合可迅速破胶;具有良好的耐温性能(65℃)、稳定性和抗剪切性能,与地层水配伍性良好;岩心伤害率9.9%2.0%;60℃时静态滤失系数为3.15×10-4m/min1/2;静态悬砂速度为0.25 mm/s,常温下与原油混合可迅速破胶;具有良好的耐温性能(65℃)、稳定性和抗剪切性能,与地层水配伍性良好;岩心伤害率9.9%¹0.3%,比瓜胶压裂液下降了65%。在同类井况条件下,粘弹性清洁压裂液和瓜胶压裂液现场应用对比实验结果表明,该清洁压裂液在增油效果上具有一定优势,推广应用前景良好。     

Gas-assisted displacement of a viscoelastic fluid in capillary geometries

The effect of fluid viscoelasticity on the fraction of liquid deposited on the walls of capillary geometry and the pressure drop at the capillary static region are theoretically investigated using the Criminale-Ericksen-Filbey (CEF) constitutive equation to describe a non-Newtonian fluid displaced by the pressurized gas in a capillary. The singular perturbation method is used to determine the residual liquid film thickness of a viscoelastic fluid on the walls of a circular tube or a rectangular channel when displaced by another immiscible fluid. Inner and outer expansions are developed in terms of both a small parameter Ca^1/3 and a small parameter De/Ca^1/3. The method of matched asymptotic expansion is used to match the inner and outer solutions by means of a transition region between the advancing meniscus and the entrained film where the fluid rheology has its greatest effect. A detailed analysis indicates that the residual liquid film thickness of the viscoelastic fluid tends to decrease and the pressure drop across the bubble front tends to increase as the fluid becomes more viscoelastic. The theoretical results presented in this paper are in agreement with some of the experimental data and theoretical analyses available in the literature.