Crystalline Polymer

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.

Overview

Pro's

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

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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