2. Atomic Bond Stiffness

Discussion on stiffness of various atomic bonds continued from previous lecture on the origins of elastic modulus.

Primary Bond Types:
- Covalent Bonds:
  - Highest range of elastic modulus: 200 to 1000 gigapascals.
- Metallic Bonds:
  - Elastic modulus typically ranging from 60 to 300 gigapascals depending on the bonded metallic elements.
- Ionic Bonds:
  - Lowest modulus range but still relatively high compared to secondary bonds.
Secondary Bond Types:
- Thermoplastic Polymers:
  - Rely on Van der Waals or hydrogen bonding, resulting in elastic moduli of 2 to 12 gigapascals.

Higher atomic bond stiffness correlates with higher melting points.
- Example: Tungsten has the highest melting point, and osmium has the highest elastic modulus.
- Rhenium closely follows osmium in modulus.
Observations from charts:
- Materials with the highest melting points generally have the highest elastic modulus.

Engineering Materials Chart: Shows elastic moduli for various materials.
- Ceramics:
  - Dominated by covalent and ionic bonds, thus have the highest moduli.
- Metals:
  - High elastic moduli with examples such as osmium and tungsten.
- Polymers:
  - Typically characterized by lower elastic moduli.
Notes:
- Logarithmic scale indicates osmium and tungsten have moduli around 400 to 500 gigapascals.
- Refractory metals (osmium, tungsten, molybdenum) exhibit significant covalent bond character increasing stiffness.
- Minimum predicted elastic modulus from secondary bond types is around 2 gigapascals.

States of Matter:
- Table lists various states (solid, liquid) and conditions of atomic bonds.
- Liquids: No primary bonds—only weak secondary attractions.
- Solids: Primary bonds are solid and determinate of structure.
Characteristics of Engineering Materials:
- Polycrystalline nature is prevalent in metals and engineering ceramics.
- Glasses and Crystals: Solid materials with frozen primary bonds.
- Rubbers and Thermoplastic Polymers: Mix of frozen primary bonds and molten secondary bonds.

Composition of thermoplastic polymers: long-chain molecules with carbon backbone and side chains.
Molecular Structure of Polyethylene:
- Exhibits dipole structure with separation of charge centers.
- Secondary Van der Waals attractions help hold chains together.
Bonding Characteristics:
- In polyethylene, primary covalent bonds are strong, but secondary bonds are relatively weak.
- Polar Side Chains: Can lead to stronger secondary bonding, resulting in higher melt temperatures and stiffness.
Glass Transition Temperature:
- Polymers can have transition temperatures below room temperature.
- At room temperature, secondary bonds may be molten, affecting overall stiffness.
Below glass transition temperature, secondary bonds freeze, increasing stiffness and brittleness.