Crystalline Polymer

Crystalline Polymers Definition:

Fun fact: not a single polymer is completely crystalline because all crystalline polymers contain considerable amounts of amorphous material. Therefore, crystalline polymers are more accurately called semi-crystalline polymers.

How can we define crystalline polymers then?

This can be seen in the figure below, which clearly demonstrates how materials and elements are always characterized by the presence of an amorphous element:

Fig. 1: Crystalline + Amorphous in the presence of an element: https://support.3devo.com/oval-flat-shaped-filament/

Pretty standard so far no? Let's find out about its properties!

Crystalline Polymers Properties:

Due to their structure and temperature, crystalline polymers have a wide range of mechanical and physical characteristics:

• High density.
• High melting point.
• High resistance to wear and tear.
• Low ductility and impact resistance.
• Opaque to visible light.

The biggest characteristics of crystalline polymers are: high melting point, rigidity, and resistance to solvent penetration. Although the crystallinity makes polymers robust, it also lowers their resistance to impacts.

Crystalline polymers furthermore exhibit x-ray diffraction patterns due to the existence of specific lattice patterns of molecules in polymer chains and exhibit a crystalline melting temperature. X-ray diffraction, density measurements and heat of fusion are measured to determine the fraction of crystalline substances present in a particular polymer.

On a general spectrum though, all of the properties of the semi-crystalline structures mentioned above are conditional to the degree of crystallization, to the structure of the lamellar crystallites (tiny, alternating layers of various materials), and to the size and distribution of these structures.

Crystalline Polymers Structure:

Crystalline polymers are characterized by a three-dimensional order. Now, assuming as a general rule that all molecules have a certain structural regularity, the molecules in the structures of crystalline polymers are for the most part aligned parallel to each other.

As we learned above concerning the properties of crystalline polymers, these structures can crystallize under certain conditions of temperature, pressure, voltage or by the influence of a medium, adopting fully extended forms (such as helical forms for instance).

X-ray diagrams have revealed the existence of two different parts of the structure of a crystalline polymer: an interfacial amorphous one and an ordered crystalline one, giving rise to the existence of a micellar pattern.

• The ordered crystalline regions are formed by sequences of the polymer chains in ordered conformations.
• The interfacial region is determined by the re-entry of a part of the chain into the crystallite.
  • the amorphous region is characterized by parts of the chain in disordered conformations (as amorphous polymers are disorderly).

Take a look at the image below to have a visual representation of what was explained above (notice how they are all aligned parallel to each other too):

Fig. 2: Structure of crystalline polymers: crystalline region vs. amorphous region https://www.victrex.com/en/blog/2017/polymer-crystallinity-hpp-explained-part-

Semi-Crystalline Polymers with Examples

Examples of crystalline polymers are, but not limited to:

• Polyethylene (PE), Polypropylene (PP), and Polybutylene terephthalate (PBT)
  • Polyethelyne (PE) is an element we use on a daily basis (and arguably the ruin of our environment). PE is the most common plastic.
  • Polypropylene (PP) is a plastic as well.
  • Polybutylene terephthalate (PBT) is a type of polymer used mainly in the engineering field - mostly as an insulator in the electric and electronic industry.

Now, all of these are just super long words that aren't super important, unless you are working with them daily. However, they are super cool because these types of materials are commonly used in the field of dentistry!

Fig. 3: Polymers used in chipped tooth repair: https://www.drjamescovan.com/treatment/cosmetic-dentistry/chipped-teeth

Semi-Crystalline polymers have mostly been used to build prosthetic items like denture bases. However, they are also employed in endodontic fillings, prosthetic teeth, restorations, cements, elastics, inlay patterns, implants, impression materials, and many more!

Look at the image here: notice the chipped teeth and then how they were fixed after bonding? The material used for bonding has a high quantity of polymers in it!

What about the polyethylene that we mentioned above? Given that it is one of the strongest currently available reinforcing fibers they are great for cosmetic dentistry as they have low density and modulus, are formed of matched polymer chains, and have high impact resistance.