Ep22 Mechanical properties of polymers & viscoelastic models NANO 134 UCSD Darren Lipomi
TLDRThis script explores the viscoelastic properties of polymers, using a Rubio's plastic bag as a practical example. It delves into the elastic and plastic deformations, stress-strain behavior, and the different regimes of polymer response. It also discusses concepts like tensile modulus, yield points, and resilience, comparing polymers to metals and introducing models like Voigt and Maxwell to explain viscoelastic behavior. The lecture aims to prepare students for an upcoming exam and mentions guest lectures on polymers in various applications.
Takeaways
- 😀 The script discusses the properties and behavior of polymers, specifically using a piece of a Rubio's bag made of polyethylene as a demonstration.
- 🔍 It explains the elastic regime of the polymer where entropic elasticity is the dominant restoring force up to about 10% strain.
- 🔄 The script describes the process of stretching the polymer beyond the elastic limit, leading to the breaking of crystalline domains and eventual fracture.
- ⏳ The concept of creep is introduced, showing how polymers exhibit viscoelasticity by slowly deforming under a constant load over time.
- 📈 Stress-strain curves for polymers are detailed, illustrating the transition from elastic to plastic deformation and the various points such as the elastic limit, yield point, and ultimate tensile strength.
- 🔆 The differences between the mechanical behavior of metals and polymers are highlighted, with metals experiencing strain hardening due to dislocation accumulation and polymers due to chain alignment.
- 💪 The script explains the terms resilience and toughness in the context of materials science, describing the energy absorbed before permanent deformation and fracture, respectively.
- 📚 It introduces the terms tensile modulus (Young's modulus) and Poisson's ratio, which are fundamental to understanding a material's response to stress and strain.
- 🌡️ The effect of temperature on the modulus of a material is discussed, showing how it changes across different states from glassy to leathery to rubbery flow.
- 🕒 The time-dependent behavior of materials under constant stress (creep test) and constant strain (relaxation test) is explored, emphasizing the viscoelastic nature of polymers.
- 🔧 The script concludes with an explanation of viscoelastic models, including the Voigt-Kelvin and Maxwell models, which help to understand complex material behavior under varying conditions.
Q & A
What is the elastic regime in the context of the script?
-The elastic regime is the initial phase when a material is stretched and it returns to its original shape once the force is removed. It's characterized by entropic elasticity, where the polymer chains straighten out to reduce their entropic freedom and then return to their original state, producing a restoring force.
What happens when a polymer is stretched beyond the elastic regime?
-When a polymer is stretched beyond the elastic regime, it enters the plastic deformation phase. The crystalline domains within the material start to break apart and realign along the strained axis, leading to permanent deformation and eventually breaking if stretched further.
What is the significance of the stress-strain curve in understanding polymer behavior?
-The stress-strain curve is a graphical representation of the relationship between the stress applied to a material and the deformation it undergoes. It helps in understanding the material's elastic behavior, yield point, ultimate tensile strength, and fracture point, which are crucial for determining the material's mechanical properties and performance under different conditions.
What is creep and how is it related to viscoelasticity?
-Creep is a phenomenon where a material deforms permanently over time under a constant stress. It is associated with viscoelasticity, which is the combination of elasticity (a solid-like property where energy is stored) and viscosity (a liquid-like property where energy is dissipated) in polymers.
What are the units of Young's modulus or tensile modulus?
-Young's modulus, also known as tensile modulus, is measured in Newtons per square meter, commonly expressed in Pascals (Pa).
What is the difference between the elastic limit and the yield point in the context of a stress-strain curve?
-The elastic limit is the point up to which the material's deformation is reversible and the stress is proportional to strain. The yield point is where the material starts to deform permanently, and it is very close to the elastic limit. The terms are often used interchangeably, but the yield point emphasizes the onset of permanent deformation.
What is the modulus of resilience and how is it calculated?
-The modulus of resilience is the energy density that represents the amount of energy a material can absorb up to the proportionality limit while remaining elastic. It is calculated as the area under the stress-strain curve up to the proportionality limit, and mathematically, it can be expressed as the stress at the proportionality limit squared over twice the tensile modulus.
How does the behavior of metals differ from that of polymers in a stress-strain curve?
-While both metals and polymers can exhibit similar stress-strain curves, the underlying mechanisms are different. Metals can show an increase in strength with strain past the yield point due to the accumulation of dislocations in the crystal lattice, a phenomenon known as strain hardening. In contrast, polymers strengthen by aligning the polymer chains along the strain axis, increasing the order in the system.
What are the different regimes of viscoelastic behavior in polymers?
-Polymers exhibit different viscoelastic regimes depending on temperature and other factors. These include the glassy state (very elastic and solid-like), the leathery state (some time-dependent elasticity), the rubbery state (partially elastic with less time dependence), and the viscous state (liquid-like with no elastic recovery).
What is the purpose of the Voigt-Kelvin model and the Maxwell model in viscoelasticity?
-The Voigt-Kelvin model and the Maxwell model are simplified representations used to understand viscoelastic behavior. The Voigt-Kelvin model consists of a spring and a dashpot in parallel, representing immediate elastic response and time-dependent viscous flow, respectively. The Maxwell model consists of a spring and a dashpot in series, representing a quick elastic response followed by permanent deformation and slow recovery.
Outlines
🌯 Elasticity and Deformation of Polymers
The speaker begins by demonstrating the elastic properties of a polyethylene plastic bag from Rubio's, explaining the concept of entropic elasticity in the material's amorphous regions. They describe the behavior of polymer chains under strain, the transition from elastic to plastic deformation, and the eventual fracture of the material. The importance of strain rate is emphasized, with a rapid pull leading to immediate breakage, while a slow, sustained load can result in creep, a time-dependent deformation characteristic of viscoelastic materials.
📈 Stress-Strain Behavior and Polymer Characteristics
This paragraph delves into the stress-strain curve of a generic polymer, detailing the phases of elastic behavior, crystalline domain alignment, and the strengthening of the material due to covalent bond alignment. The speaker discusses the points of proportionality limit, elastic limit, and yield point, and how these relate to the material's response to stress. The concepts of tensile modulus, ultimate tensile strength, and energy density terms such as resilience and toughness are introduced, highlighting the significance of these properties in material performance.
🔗 Chain Alignment and Material Failure
The discussion continues with the effects of chain alignment and bond fission on the mechanical properties of polymers. The speaker explains how the stress-strain curve can vary depending on the material's structure and the presence of crystallites. The mechanisms behind the rupture of covalent bonds and the sliding of chains past each other are described, leading to catastrophic failure. The differences between stress-strain curves of metals and polymers are also highlighted, emphasizing the unique strengthening mechanisms of each.
🛡️ Modulus of Materials and Poisson's Ratio
The speaker introduces the concept of modulus, akin to the spring constant for a three-dimensional solid, and Poisson's ratio, which describes the transverse contraction of a material under tension. Examples of various materials, including diamond, graphene, steel, glass, polyethylene, and latex, are given to illustrate the wide range of moduli and Poisson's ratios. The dependence of modulus on temperature and the glass transition temperature is also discussed, along with the concept of the relaxation modulus.
📉 Temperature Dependence and Viscoelasticity
This section explores the temperature dependence of a material's modulus and its transition through different mechanical states: glassy, leathery, rubbery, and viscous. The speaker describes the relaxation modulus experiments and how they reveal the material's behavior at various temperatures. The concept of creep tests, where constant stress is applied and the resulting strain is measured over time, is introduced to illustrate the time-dependent nature of viscoelastic materials.
⏱ Time Dependence in Viscoelastic Materials
The speaker explains the time-dependent behavior of viscoelastic materials under constant stress, known as creep tests, and constant strain, which measures the stress relaxation over time. The differences in material response based on temperature and the mechanical state (glassy, leathery, rubbery, or viscous) are discussed. The use of viscoelastic models, such as the Voigt-Kelvin and Maxwell models, to represent these behaviors is introduced, providing a framework for understanding complex material responses.
🔄 Complex Viscoelastic Behavior and Modeling
The paragraph discusses more complex viscoelastic behavior where materials exhibit a combination of immediate and time-dependent deformation, followed by partial recovery. The speaker proposes a model combining the Voigt-Kelvin and Maxwell models to account for this behavior, illustrating how different components of the model respond to stress application and removal. The concept of a purely viscous regime, where no elastic recovery occurs, is also introduced.
📚 Upcoming Lectures and Exam Preparation
In the final paragraph, the speaker informs the audience about upcoming guest lectures on polymers in drug delivery, cancer therapy, and bioengineering, as well as a special lecture on computational modeling. They also mention an office hour for exam-related questions and encourage attendance at the lectures for a deeper understanding of the topics discussed in class.
Mindmap
Keywords
💡Polyethylene
💡Elastic Regime
💡Strain
💡Stress
💡Yield Point
💡Creep
💡Viscoelasticity
💡Stress-Strain Curve
💡Elastic Modulus (Young's Modulus)
💡Ultimate Tensile Strength
💡Resilience
💡Toughness
💡Voiget-Kelvin Model
💡Maxwell Model
Highlights
Demonstration of the elastic and plastic deformation of a polyethylene bag, illustrating the concepts of entropic elasticity and the breaking of crystalline domains.
Explanation of the stress-strain relationship in polymers, including the elastic regime, yield point, and ultimate tensile strength.
The phenomenon of creep in polymers, where a heavy weight suspended from a bag leads to slow elongation over time, demonstrating visco-elasticity.
The distinction between elasticity and viscosity in polymers, with a plot of stress versus strain to illustrate the behavior of a generic polymer.
Identification of the proportionality limit, elastic limit, and yield point in a stress-strain curve, and their significance in material deformation.
Comparison of the stress-strain behavior of metals and polymers, highlighting the different mechanisms of strain hardening.
Calculation of the modulus of resilience and toughness, which represent the energy absorbed elastically and the total energy absorbable prior to fracture, respectively.
Discussion of the time-dependent nature of modulus in relation to temperature, particularly around the glass transition temperature.
Introduction of the relaxation modulus, a measure of the time-dependent response of a material to stress, and its significance in viscoelastic materials.
Description of the creep test, a constant stress experiment that measures the elongation of a material over time under a given load.
Explanation of viscoelastic models, such as the Voigt-Kelvin and Maxwell models, and their representation of material behavior in different temperature regimes.
Illustration of the glassy, leathery, rubbery, and viscous states of a material, and how they relate to the modulus and Poisson's ratio.
The concept of modulus as a measure of a material's stiffness and its dependence on temperature and the glass transition temperature.
Use of marshmallow fluff as an example to explain the behavior of a viscoelastic liquid and its properties of partial elastic recovery.
Complex viscoelastic models that combine elements of different states to represent more intricate material behavior under stress.
Upcoming guest lectures on polymers in drug delivery, cancer therapy, and bioengineering, and computational modeling with visual aids.
Transcripts
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