Polymers and Their Properties

Polymers

• Polymers are large molecules composed of repeating units called monomers, joined by covalent bonds.
• Addition polymers are formed by the addition of monomers without the loss of atoms. An example includes polyethylene.
• Condensation polymers are formed by the reaction of monomers with the elimination of a small molecule. An example includes nylon.
• Polymer structures can be linear (long, straight chains), branched (chains with side branches), cross-linked (chains bonded together to form a network), or network polymers (3D structures with extensive cross-linking).

Polymer Properties

• Molecular Weight:
  • Number average molecular weight (Mn): Average molecular weight based on the number of chains.
  • Weight average molecular weight (Mw): Average molecular weight weighted by the mass fraction of each chain.
  • Polydispersity Index (PDI): A measure of the distribution of molecular weights, calculated as Mw/Mn.

Crystallinity

• Crystalline regions have ordered structures that provide strength and stiffness.
• Amorphous regions are disordered and offer flexibility.

Thermoplastics vs. Thermosets

• Thermoplastics soften when heated and harden when cooled. Polyethylene is an example.
• Thermosets harden permanently after heating and curing. Epoxy resin is an example

Phase Diagrams

• Phase diagrams are graphical representations showing the stable phases of a material at various temperatures and compositions.
• Key Concepts:
  • Single-phase region: Area where only one phase (solid, liquid, or gas) exists.
  • Two-phase region: Area where two phases coexist (e.g., liquid + solid).
  • Eutectic point: The composition and temperature at which the liquid phase transforms directly into two solid phases.
  • Solidus line: The boundary below which the material is completely solid.
  • Liquidus line: The boundary above which the material is completely liquid.
  • Tie line: Connects the compositions of coexisting phases in a two-phase region.
• Lever Rule: Used to calculate the weight fraction of each phase.
• Phase Compositions: Ca and Cβ represent the compositions of solid and liquid phases respectively.

Imperfections in Solids

• Imperfections (defects) in solids are deviations from the ideal atomic arrangement in a crystal structure.

• Point Defects:

  • Vacancies: Missing atoms from lattice positions.
  • Interstitials: Extra atoms positioned between lattice sites.
  • Substitutional Defects: Foreign atoms replacing host atoms.

• Line Defects:

  • Dislocations: Line defects enabling plastic deformation.
    • Edge Dislocation: Extra half-plane of atoms.
    • Screw Dislocation: Atoms spiral around a dislocation line.

• Area Defects:

  • Grain Boundaries: Interfaces between different crystal grains.
  • Stacking Faults: Errors in the stacking sequence of atomic planes.

• Vacancy Concentration: Fraction of vacant lattice sites calculated as Nv = N * e^(-Qv/kT), where N is total number of sites, Qv is activation energy, k is Boltzmann's constant, and T is temperature.

• Vacancy concentration influences mechanical properties like diffusion, strength, and electrical conductivity. Higher vacancy concentration leads to increased diffusion rates, affecting mechanical properties. Vacancy concentration rises with increasing temperature.