Crystalline Polymers

Highly crystalline polymers are rigid, high melting, and less affected by solvent penetration. Crystallinity makes a polymers strong, but also lowers their impact resistance. As an example, samples of polyethylene prepared under high pressure (5000 atm) have high crystallinities (95 – 99%) but are extremely brittle.

Small molecules and ions form a three-dimensional lattice with an extended regular structure that makes large crystals possible.

We usually describe such lattices with a unit cell – the smallest repeating unit in the lattice . In the case of NaCl, the unit cell is said to be face-centered cubic.

Polymer molecules are very large so it might seem that they could not pack together regularly and form a crystal. It now is known that regular polymers may form lamellar (plate-like) crystals with a thickness of 10 to 20 nm in which the parallel chains are perpendicular to the face of the crystals.

Pros

• Good Strength
• Good Flexibility
• Good High Temperature Performance
• Good Chemical Resistance

Cons

• High Shrinkage. Warp
• Smaller Processing Window
• Poor Impact
• Good Memory (For Living Hinges)
• Good Creep Resistance
• Not Usually Solvent Bondable or Paintable

Specifications