A coupled theory of fluid permeation and large deformations for elastomeric materials

An elastomeric gel is a cross-linked polymer network swollen with a solvent (fluid). A continuum-mechanical theory to describe the various coupled aspects of fluid permeation and large deformations (e.g., swelling and squeezing) of elastomeric gels is formulated. The basic mechanical force balance laws and the balance law for the fluid content are reviewed, and the constitutive theory that we develop is consistent with modern treatments of continuum thermodynamics, and material frame-indifference. In discussing special constitutive equations we limit our attention to isotropic materials, and consider a model for the free energy based on a Flory-Huggins model for the free energy change due to mixing of the fluid with the polymer network, coupled with a non-Gaussian statistical-mechanical model for the change in configurational entropy — a model which accounts for the limited extensibility of polymer chains. As representative examples of application of the theory, we study (a) three-dimensional swelling-equilibrium of an elastomeric gel in an unconstrained, stress-free state; and (b) the following one-dimensional transient problems: (i) free-swelling of a gel; (ii) consolidation of an already swollen gel; and (iii) pressure-difference-driven diffusion of organic solvents across elastomeric membranes.National Science Foundation (U.S.) (grant DMI-0517966)Singapore-MIT Allianc

Mechanics of amorphous polymers and polymer gels

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2011.Cataloged from PDF version of thesis.Includes bibliographical references (p. 345-356).Many applications of amorphous polymers require a thermo-mechanically coupled large-deformation elasto-viscoplasticity theory which models the strain rate and temperature dependent response of amorphous polymeric materials in a temperature range which spans the glass transition temperature of the material. We have formulated such a theory, and also numerically implemented our theory in a finite element program. The material parameters in the theory have been calibrated for poly(methyl methacrylate), polycarbonate, and Zeonex - a cyclo-olefin polymer. The predictive capabilities of the constitutive theory and its numerical implementation have been validated by comparing the results from a suite of validation experiments against corresponding results from numerical simulations. Amorphous chemically-crosslinked polymers form a relatively new class of thermallyactuated shape-memory polymers. Several biomedical applications for thermally-actuated shape-memory polymers have been proposed/demonstrated in the recent literature. However, actual use of such polymers and devices made from these materials is still quite limited. For the variety of proposed applications to be realized with some confidence in their performance, it is important to develop a constitutive theory for the thermo-mechanical response of these materials and a numerical simulation-based design capability which, when supported with experimental data, will allow for the prediction of the response of devices made from these materials under service conditions. We have developed such a theory and a numerical simulation capability, and demonstrated its utility for modeling the thermo-mechanical response of the shape-memory polymer tBA-PEGDMA. An elastomeric gel is a cross-linked polymer network swollen with a solvent, and certain thermally-responsive gels can undergo large reversible volume changes as they are cycled about a critical temperature. We have developed a thermodynamically-consistent continuum-level theory to describe the coupled mechanical-deformation, fluid permeation, and heat transfer of such gels. We have numerically implemented our theory in a finite element program by writing thermo-chemo-mechanically coupled elements. We show that our theory is capable of simulating swelling, squeezing of fluid by applied mechanical forces, and thermally-responsive swelling/de-swelling of such materials.by Shawn Alexander Chester.Ph.D

Thermally actuated shape-memory polymers: Experiments, theory, and numerical simulations

With the aim of developing a thermo-mechanically-coupled large-deformation constitutive theory and a numerical-simulation capability for modeling the response of thermally-actuated shape-memory polymers, we have (i) conducted large strain compression experiments on a representative shape-memory polymer to strains of approximately unity at strain rates of 10[superscript −3] s[superscript −1] and 10[superscript −1] s[superscript −1], and at temperatures ranging from room temperature to approximately 30C above the glass transition temperature of the polymer; (ii) formulated a thermo-mechanically-coupled large-deformation constitutive theory; (iii) calibrated the material parameters appearing in the theory using the stress-strain data from the compression experiments; (iv) numerically implemented the theory by writing a user-material subroutine for a widely-used finite element program; and (v) conducted representative experiments to validate the predictive capability of our theory and its numerical implementation in complex three-dimensional geometries. By comparing the numericallypredicted response in these validation simulations against measurements from corresponding experiments, we show that our theory is capable of reasonably accurately reproducing the experimental results. As a demonstration of the robustness of the three-dimensional numerical capability, we also show results from a simulation of the shape-recovery response of a stent made from the polymer when it is inserted in an artery modeled as a compliant elastomeric tube.National Science Foundation (U.S.) (grant DMI-0517966)Singapore-MIT Allianc

A thermo-mechanically-coupled large-deformation theory for amorphous polymers in a temperature range which spans their glass transition

Amorphous thermoplastic polymers are important engineering materials; however, their non-linear, strongly temperature- and rate-dependent elastic-viscoplastic behavior is still not very well understood, and is modeled by existing constitutive theories with varying degrees of success. There is no generally agreed upon theory to model the large-deformation, thermo-mechanically-coupled, elastic-viscoplastic response of these materials in a temperature range which spans their glass transition temperature. Such a theory is crucial for the development of a numerical capability for the simulation and design of important polymer processing operations, and also for predicting the relationship between processing methods and the subsequent mechanical properties of polymeric products. In this paper we extend our recently published theory [Anand, L., Ames, N. M., Srivastava, V., Chester, S. A., 2009. A thermo-mechanically-coupled theory for large deformations of amorphous polymers. Part I: formulation. International Journal Plasticity 25, 1474–1494; Ames, N. M., Srivastava, V., Chester, S. A., Anand, L., 2009. A thermo-mechanically coupled theory for large deformations of amorphous polymers. Part II: applications. International Journal of Plasticity 25, 1495–1539] to fill this need. We have conducted large strain compression experiments on three representative amorphous polymeric materials – a cyclo-olefin polymer (Zeonex-690R), polycarbonate (PC), and poly(methyl methacrylate) (PMMA) – in a temperature range from room temperature to approximately 50 °C above the glass transition temperature, ϑg [theta subscript g], of each material, in a strain-rate range of ≈10-4 [10 superscript -4]to 10-1 s-1 [10 superscript -1 s superscript -1], and compressive true strains exceeding 100%. We have specialized our constitutive theory to capture the major features of the thermo-mechanical response of the three materials studied experimentally. We have numerically implemented our thermo-mechanically-coupled constitutive theory by writing a user material subroutine for a widely used finite element program. In order to validate the predictive capabilities of our theory and its numerical implementation, we have performed the following validation experiments: (i) a plane-strain forging of PC at a temperature below ϑg [theta subscript g], and another at a temperature above ϑg [theta subscript g]; (ii) blow-forming of thin-walled semi-spherical shapes of PC above ϑg [theta subscript g]; and (iii) microscale hot-embossing of channels in Zeonex and PMMA above ϑg [theta subscript g]. By comparing the results from this suite of validation experiments of some key features, such as the experimentally-measured deformed shapes and the load-displacement curves, against corresponding results from numerical simulations, we show that our theory is capable of reasonably accurately reproducing the experimental results obtained in the validation experiments.National Science Foundation (U. S.) (Grant no. DMI-0517966)Singapore MIT Alliance Programme in Manufacturing Systems and Technolog

Omecamtiv mecarbil in chronic heart failure with reduced ejection fraction, GALACTIC‐HF: baseline characteristics and comparison with contemporary clinical trials

Aims: The safety and efficacy of the novel selective cardiac myosin activator, omecamtiv mecarbil, in patients with heart failure with reduced ejection fraction (HFrEF) is tested in the Global Approach to Lowering Adverse Cardiac outcomes Through Improving Contractility in Heart Failure (GALACTIC‐HF) trial. Here we describe the baseline characteristics of participants in GALACTIC‐HF and how these compare with other contemporary trials. Methods and Results: Adults with established HFrEF, New York Heart Association functional class (NYHA) ≥ II, EF ≤35%, elevated natriuretic peptides and either current hospitalization for HF or history of hospitalization/ emergency department visit for HF within a year were randomized to either placebo or omecamtiv mecarbil (pharmacokinetic‐guided dosing: 25, 37.5 or 50 mg bid). 8256 patients [male (79%), non‐white (22%), mean age 65 years] were enrolled with a mean EF 27%, ischemic etiology in 54%, NYHA II 53% and III/IV 47%, and median NT‐proBNP 1971 pg/mL. HF therapies at baseline were among the most effectively employed in contemporary HF trials. GALACTIC‐HF randomized patients representative of recent HF registries and trials with substantial numbers of patients also having characteristics understudied in previous trials including more from North America (n = 1386), enrolled as inpatients (n = 2084), systolic blood pressure < 100 mmHg (n = 1127), estimated glomerular filtration rate < 30 mL/min/1.73 m2 (n = 528), and treated with sacubitril‐valsartan at baseline (n = 1594). Conclusions: GALACTIC‐HF enrolled a well‐treated, high‐risk population from both inpatient and outpatient settings, which will provide a definitive evaluation of the efficacy and safety of this novel therapy, as well as informing its potential future implementation

A Thermo-Mechanically Coupled Large-Deformation Theory for Amorphous Polymers Across the Glass Transition Temperature

Amorphous thermoplastic polymers are important engineering materials; however, their nonlinear, strongly temperature- and rate-dependent elastic-viscoplastic behavior is still not very well understood, and is modeled by existing constitutive theories with varying degrees of success. There is no generally agreed upon theory to model the large-deformation, thermo-mechanically-coupled, elastic-viscoplastic response of these materials in a temperature range which spans their glass transition temperature. Such a theory is crucial for the development of a numerical capability for the simulation and design of important polymer processing operations, and also for predicting the relationship between processing methods and the subsequent me- chanical properties of polymeric products. In this manuscript we briefly summarize a few results from our own recent research [1–4] which is intended to fill this need. We have conducted large strain compression experiments on three representative amorphous polymeric materials a cyclo-olefin polymer (Zeonex-690R), polycarbonate (PC), and poly(methyl methacrylate) (PMMA) in a temperature range from room temperature to approximately 50C above the glass transi- tion temperature, θ g, of each material, in a strain-rate range of roughly 0.0001 s⁻¹ to 0.1 s⁻¹, and compressive true strains exceeding 100%. We have specialized our constitutive theory to capture the major features of the thermo-mechanical response of the three materials studied experimentally. We have numerically implemented our thermo- mechanically-coupled constitutive theory by writing a user material subroutine for a widely used finite element program Abaqus/Standard. In order to validate the predictive capabilities of our theory and its numerical implementation, we present the following validation experiments: (i) a plane-strain forging of PC at a temperature below θg, and another at a temperature above Tg; (ii) blow-forming of thin-walled semi-spherical shapes of PC above θg; and (iii) microscale hot-embossing of channels in PMMA above θ g. By comparing the results from this suite of validation experiments of some key features, such as the experimentally-measured deformed shapes and the load-displacement curves, against corresponding results from numerical simulations, we show that our theory is capable of reasonably accurately reproducing the experimental results obtained in the validation experiments

A thermo-mechanically coupled theory for large deformations of amorphous polymers. Part I: Formulation

In this Part I, of a two-part paper, we present a detailed continuum-mechanical development of a thermomechanically coupled elasto-viscoplasticity theory to model the strain rate and temperature dependent largedeformation response of amorphous polymeric materials. Such a theory, when further specialized (Part II) should be useful for modeling and simulation of the thermo-mechanical response of components and structures made from such materials, as well as for modeling a variety of polymer processing operations.National Science Foundation (U.S.) (grant DMI-0517966)Singapore-MIT Allianc

A thermo-mechanically coupled theory for large deformations of amorphous polymers. Part II: Applications

We have conducted large-strain compression experiments on three representative amorphous polymeric materials: poly(methyl methacrylate) (PMMA), polycarbonate (PC), and a cyclo-olefin polymer (Zeonex-690R), in a temperature range spanning room temperature to slightly below the glass transition temperature of each material, in a strain rate range of View the MathML source to View the MathML source, and compressive true strains exceeding 100%. The constitutive theory developed in Part I [Anand, L., Ames, N.M., Srivastava, V., Chester, S., 2009. A thermo-mechanically coupled theory for large deformations of amorphous polymers. Part 1: Formulation. International Journal of Plasticity] is specialized to capture the salient features of the thermo-mechanically coupled strain rate and temperature dependent large deformation mechanical response of amorphous polymers. For the three amorphous polymers studied experimentally, the specialized constitutive model is shown to perform well in reproducing the following major intrinsic features of the macroscopic stress–strain response of these materials: (a) the strain rate and temperature dependent yield strength; (b) the transient yield-peak and strain-softening which occurs due to deformation-induced disordering; (c) the subsequent rapid strain-hardening due to alignment of the polymer chains at large strains; (d) the unloading response at large strains; and (e) the temperature rise due to plastic-dissipation and the limited time for heat-conduction for the compression experiments performed at strain rates [View the MathML source]. We have implemented our thermo-mechanically coupled constitutive model by writing a user material subroutine for the finite element program [Abaqus/Explicit, 2007. SIMULIA, Providence, RI]. In order to validate the predictive capabilities of our constitutive theory and its numerical implementation, we have performed the following validation experiments: (i) isothermal fixed-end large-strain reversed-torsion tests on PC; (ii) macro-scale isothermal plane-strain cold- and hot-forming operations on PC; (iii) macro-scale isothermal, axi-symmetric hot-forming operations on Zeonex; (iv) micro-scale hot-embossing of Zeonex; and (v) high-speed normal-impact of a circular plate of PC with a spherical-tipped cylindrical projectile. By comparing the results from this suite of validation experiments of some key macroscopic features, such as the experimentally-measured deformed shapes and the load-displacement curves, against corresponding results from numerical simulations, we show that our theory is capable of reasonably accurately reproducing the experimental results obtained in the validation experiments.National Science Foundation (U.S.) (grant number DMI-0517966)Singapore-MIT Allianc

A Large-Deformation Theory for Thermally-Actuated Shape-Memory Polymers and its Application

The most common shape-memory polymers are those in which the shape-recovery is thermally-induced. A body made from such a material may be subjected to large deformations at an elevated temperature above its glass transition temperature θg . Cooling the deformed body to a temperature below θ g under active kinematical constraints fixes the deformed shape of the body. The original shape of the body may be recovered if the material is heated back to a temperature above θ g without the kinematical constraints. This phenomenon is known as the shape-memory effect. If the shape recovery is partially constrained, the material exerts a recovery force and the phenomenon is known as constrained-recovery . As reviewed by [1], one of the first widespread applications of shape-memory polymers was as heat-shrinkable tubes. Such rudimentary early applications did not necessitate a detailed understanding or modeling of the thermomechanical behavior of these materials. However, in recent years shape-memory polymers are beginning to be used for critical biomedical applications, microsystems, re-writable media for data storage, and self-deployable space structures. In order to develop a robust simulation-based capability for the design of devices for such critical applications, one requires an underlying accurate thermo- mechanically-coupled constitutive theory and an attendant vali- dated numerical implementation of the theory. In the past few years several efforts at experimental characterization of the thermo-mechanical stress-strain response of a wide variety of shape-memory polymers have been published in the literature [1, 2] to name a few. Significant modeling efforts have also been published [2, 3] to name a few. However, at this point in time, a thermo-mechanically-coupled large-deformation constitutive theory for odeling the response of thermally-actuated shape-memory polymers is not widely agreed upon — the field is still in its infancy. The purpose of this manuscript is to present results from of our own recent [4], and ongoing research in this area. Specifically, with the aim of developing a thermo-mechanically-coupled large-deformation constitutive theory and a numerical simulation capability for modeling the response of thermally-actuated shape-memory polymers, we have (i) conducted large strain compression experiments on a representative shape-memory polymer to strains of approximately unity at strain rates of 0⁻³ s⁻¹ and 10⁻¹ s⁻¹, and at temperatures ranging from room temperature to approximately 30 C above the glass transition temperature of the polymer; (ii) formulated a thermo-mechanically-coupled large deformation constitutive theory; (iii) calibrated the material parameters appearing in the theory using the stress-strain data from the compression exper- iments; (iv) numerically implemented the theory by writing a user-material subroutine for a widely-used finite element program; and (v) conducted representative experiments to validate the predictive capability of our theory and its numerical implementation in complex three-dimensional geometries. By comparing the numerically-predicted response in these validation simulations against measurements from corresponding experi- ments, we show that our theory is capable of reasonably accurately reproducing the experimental results. Also, as a demonstration of the robustness of the three-dimensional numerical ca- pability, we show results from a simulation of the shape-recovery response of a stent made from the polymer when it is inserted in an artery modeled as a compliant elastomeric tube. Furthermore, as is well known, when the shape-memory polymer recovers from its temporary shape it returns stored en- ergy and can serve as a thermally-activated actuator. Due to the low rubbery modulus of the polymer above the glass transition temperature, its actuation force is limited and the material is thus restricted form numerous applications where “high” actu- ation forces are required. To combat the problem of low actuation force of shape-memory polymers, we have synthesized re- inforced shape-memory polymer composites using superelastic nitinol wires. Specifically with these reinforced composites we have (i) conducted thermo-mechanical three-point bend experiments on samples with and without nitinol wires. In these exper- iments the deformation is constrained so as to measure the ac- tuation force; (ii) numerically simulated the thermo-mechanical response of the shape-memory polymer composite in the afore- mentioned condition of constrained recovery using the simula- tion capability developed. The numerical predictions are in good agreement with the experimental results of constrained-recovery of the reinforced shape-memory composites

Microstructured barbs on the North American porcupine quill enable easy tissue penetration and difficult removal

North American porcupines are well known for their specialized hairs, or quills that feature microscopic backward-facing deployable barbs that are used in self-defense. Herein we show that the natural quill’s geometry enables easy penetration and high tissue adhesion where the barbs specifically contribute to adhesion and unexpectedly, dramatically reduce the force required to penetrate tissue. Reduced penetration force is achieved by topography that appears to create stress concentrations along regions of the quill where the cross sectional diameter grows rapidly, facilitating cutting of the tissue. Barbs located near the first geometrical transition zone exhibit the most substantial impact on minimizing the force required for penetration. Barbs at the tip of the quill independently exhibit the greatest impact on tissue adhesion force and the cooperation between barbs in the 0–2 mm and 2–4 mm regions appears critical to enhance tissue adhesion force. The dual functions of barbs were reproduced with replica molded synthetic polyurethane quills. These findings should serve as the basis for the development of bio-inspired devices such as tissue adhesives or needles, trocars, and vascular tunnelers where minimizing the penetration force is important to prevent collateral damage.National Institutes of Health (U.S.) (Grant GM086433)American Heart Association (Grant 0835601D)National Science Foundation (U.S.) (Grant NIRT 0609182)National Institutes of Health (U.S.) (NIH Grant DE013023)National Research Foundation of Korea (Grant NRF-2010-357-D00277)National Science Foundation (U.S.) (Graduate Research Fellowship Program)National Natural Science Foundation (China) (no: 51273159)National Natural Science Foundation (China) (no. 51072159)China. Ministry of Education (Program for New Century Excellent Talents in Universities, 2301G107aaa)China. Ministry of Education (Program for New Century Excellent Talents in Universities, NCET-08-0444)China Scholarship CouncilMassachusetts Institute of Technology. Undergraduate Research Opportunities Progra