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 brittle21.
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.
Amorphous regions of a polymer are made up of a randomly coiled and entangled chains. They have been compared to a bucket containing a large number of entangled worms - each one 20-feet long and of 1/4-inch thickness1. The worms are so tangled that an entire worm cannot slide past the others, but small portions of the worms can twist around within the mass.
Amorphous polymers are softer, have lower melting points, and are penetrated more by solvents than are their crystalline counterparts.
Good High Temperature Performance
Good Chemical Resistance
High Shrinkage. Warp
Smaller Processing Window
Good Memory (For Living Hinges)
Good Creep Resistance
Not Usually Solvent Bondable or Paintable
|Polymer||Advantages||Drawbacks||Some Typical Applications|
|PEEK||Extreme chemical resistance, Wear, Stength||High cost||Wear plates, Semiconductor|
|LCP||Very fast flowing||Hot Water||Electronic/Electric connectors|
|PPS||Inherently FR, High Temperature||Nitric acid||Electronics, Reflectors|
|PPA||Low water absorption, Good strength||Phenols, Some acids||Electronics, Industrial, Cooling systems|
|PET||Appearance, Stiffness, Creep resistance||Sterilization, Hot water||Engine components, Appliance housings|
|PBT||Appearance, Moldability, Chemical resistance||Sterilization, Hot water||Connectors, Relays, Housings|
|NYLON||Fatigue resistance, Scuff resistance||Sodium Hydroxide, Salts||Cable ties, Velcro, Auto|
|POM||Snap-fits, Wear, Critical tolerances||Fumes, Tool corrosion||Clips, Knobs, Gears|
|PMP||Transparency, Chemical resistance||UV exposure, Chlorine||Labware, Cookware|
|PP||Living hinges, Flexible, Chemical resistant||Nitric acid, Warp, Low temperature||Consumer, Disposables|
|HDPE||Semi-rigid, Low temperature impact, Chemical resistance||Nitric acid, Acetone||Lids, Handles, Containers|
|LLDPE||Flexible, Low temperature impact, Chemical resistance||Nitric acid, Acetone||Lids, Handles, Containers|
|LDPE||Very Flexible, Low temperature impact, Chemical resistance||Nitric acid, Acetone||Lids, Handles, Containers|