Shape Memory Polymers: Viscoelastic Thermomechanical Constitutive Modeling

Shape memory polymers (SMPs) are known to change their elastic stiffness as they respond to change in induced stimulus such as temperature. Under appropriate loading and pre-deformation, a shape memory effect can be captured as the stimulus change. From the nature of polymers, the pre-deformation can tend to be large and can in turn be memorized by SMPs. Due to this characteristic of SMPs, it makes a great candidate for morphing structures. To analyze complex structures a simple but yet practical constitutive model needs to be developed for commercial engineering application. In this paper, a thermomechanical constitutive model is proposed making use of the standard linear viscoelastic model. The total strain during the shape memorization process is defined by mechanical, thermal and storage strains. The rheological model defined is an elastic element in parallel with a Maxwell element, which in turn are both in series with storage and thermal element. Inclusion of a storage strain within the model reveals the internal strain storage mechanism as the temperature of the material drops. Similar work done in the past requires material parameters that can be arduous to determine in the laboratory. This model proposes a simplified approximate material parameter called a binding factor which accounts for the polymer’s molecular architecture and morphology as the temperature changes. Finally, the model is applied to a four step shape memorization and stress-free recovery process. For this study, the four steps considered are a) Pre-loading of the material at high temperature b) Constant strain fixity c) unconstrained relaxation at low temperature d) unconstrained free strain recovery. The developed model is validated by comparing the predictions to experimental results in literature.

20#,60Si2Mn,9Cr18MoV Steel Ferrite Area Dynamic Recovery and Recrystallization Research

By using the Gleeble - 1500 hot simulation test machine we studied 20#, 60Si2Mn, 9Cr18MoV the three kinds of low, medium and high carbon steels to observe the dynamic recovery and recrystallization of ferrite while in the process of thermoplastic deformation. We calculated the hot deformation activation energy of each kind of steel by combining the stress-strain curve we got in the experiment and the theoretical model of Z parameter. It turns out Q9Cr18MoV<Q20#<Q60Si2Mn. In the meanwhile, microstructure observation through transmission electron microscope shows that the dynamic recrystallization of ferrite is more likely to happen in 9Cr18MoV steel than in 20# steel and in 60Si2Mn steel. These results indicate that the dynamic recrystallization of ferrite is not only determined by stacking fault energy but also closely related with the strain storage energy release degree.

Rheological Study of Steady Shear and Linear Viscoelastic Characteristics of Polyethylene and Polycarbonate Nanocomposites

The addition of nano-sized particles into a polymer matrix is an excellent way to manipulate polymer properties. Our current efforts try to understand how a material’s properties are influenced by parameters such as temperature, particle size, and particle concentrations of additives. This paper presents the rheological behavior of polycarbonate (PC) and low density polyethylene (LDPE) containing at least 2.5% and 5% weight fractions of functionalized and un-functionalized carbon nanotubes (CNTs). These materials were investigated using steady shear and oscillatory rheometry over a range of processing temperatures. The properties examined through rheological experimentation include: viscosity, critical strain, storage modulus (G′), and loss modulus (G ). The nanocomposites samples were examined using scanning electron microscopy (SEM) and UV light microscopy.

Thermomechanics of the Shape Memory Effect in Polymers

ABSTRACTShape memory polymers (SMPs) have the capacity to store and recover relatively large strains when subjected to a unique thermomechanical cycle. In this study, the thermomechanics of strain storage and strain/stress recovery are investigated in a shape memory polymer deformed under uniaxial tension and compression. During heated recovery, three cases of constraint are examined: unconstrained (free) strain recovery, stress recovery under pre-strain constraint, and stress recovery under fixed-strain constraint. Based on the experimental results, a one-dimensional SMP constitutive model is developed, which is motivated by the shape memory mechanism of the polymer network. The foundation of the model is that the entropy change is gradually stored during cooling and released during reheating as free recovery strain or constrained recovery stress. When fit to free strain recovery data, the model can predict the trends of the stress evolution during shape fixation and constrained strain/stress recovery under various thermomechanical conditions.

Survival of Escherichia coliO157:H7 in Synthetic Gastric Fluid after Cold and Acid Habituation in Apple Juice or Trypticase Soy Broth Acidified with Hydrochloric Acid or Organic Acids

Extreme acid tolerance of Escherichia coli O157:H7 has raised doubts about the safety of acidic foods. This study examined whether prior storage in acidic and/or cold conditions enhanced survival of E. coli O157 :H7 in synthetic gastric fluid (SGF). Three E. coli O157:H7 strains were stored in trypticase soy broth (TSB; acidified with HCl, malic acid, citric acid, or lactic acid) or pH 3.5 and 6.5 (nonacidic control) apple juice at 4 and 21°C for ≤7 days and then were incubated in pH 2.5 SGF at 37°C for 4 h. Cells survived better in apple juice than in TSB containing organic acids, suggesting that juice constituents other than organic acids protect E. coli O157:H7. Refrigeration combined with low pH best protected cells in apple juice and acidified TSB, but, compared to the nonacidic control, only acidified TSB enhanced subsequent survival in pH 2.5 SGF. Equal survival in SGF occurred after storage in pH 3.5 or 6.5 apple juice at 4°C, suggesting that low temperature alone in apple juice enhanced acid tolerance. Two strains stored at 4°C in TSB containing malic or citric acid subsequently survived better in SGF than cells stored in nonacidified TSB but poorer than cells stored in the presence of HCl. These differences reflect the higher pKa of these organic acids. However, subsequent survival of these strains in SGF was poorer after refrigerated storage in apple juice than in TSB containing citric or malic acids. Cells stored in lactic acid were most likely to be completely eliminated upon transfer to SGF. Differences in survival in storage media or SGF related to strain, storage conditions, or acidifier were consistent and often statistically significant (P &lt; 0.05). Although the survival of E. coli O157:H7 in refrigerated acidic beverages may not be affected by the type of acidifier used, the subsequent survival in SGF of this pathogen may be critically dependent on this factor.

Effect of Hydrocarbon and Amine Diluents on the Properties of Dicumyl Peroxide Cured Styrene-Butadiene Rubber, Part I: Gum Compositions

Gum SBR compositions containing dicumyl peroxide and either alkane (nonadecane) or amine (octadecylamine or oleylamine) diluent were prepared. Dilution caused a reduction in rheometer torque upon curing and in the resulting crosslink density based on solvent swelling. Results for each diluent were indistinguishable. However, tensile strengths and especially storage moduli were different for nonadecane- and octadecylamine-diluted vulcanizates. Added octadecylamine mildly reinforced the gum, while nonadecane lowered strength and stiffness. Furthermore, quite strikingly, octadecylamine-diluted samples had a large, low-strain storage modulus, which decreased markedly with strain amplitude (Payne effect), while vulcanizates diluted with nonadecane showed no Payne effect.

THE INTERIOR QUALITY DECLINE OF SPRAY-OILED VERSUS UNOILED EGGS, STORED AT 70°F., AMONG THREE STRAINS OF LEGHORNS

A total of 3,600 eggs were collected from three Leghorn strains for use in this experiment. Two of the strains were commercial strains and the third was the Ottawa control strain. Each of the commercial strains was obtained as hatching eggs from nine supply flocks through three franchised hatcheries.Haugh-unit values, the criteria of internal quality, were determined for fresh and stored eggs. Comparable lots of oil-treated and unoiled eggs were stored at 70°F. for 4, 8, 12 and 16 days.The decline of internal quality of unoiled eggs was markedly more rapid when compared with oiled eggs. A low magnitude strain × storage-time interaction was evident with oiled eggs but was due to the Ottawa control strain.No parent supply flock or hatchery effects were demonstrated.