Difference Between Amorphous and Crystalline Polymers

The main difference between amorphous and crystalline polymers is that amorphous polymers do not have uniformly packed molecules, whereas crystalline polymers have uniformly packed molecules.

Polymers are large macromolecules formed by joining a large number of smaller units called monomers. There are various types of polymer classifications. The most primary classification is based on the types of monomers that form the polymer. According to this classification, there are two types of polymers: (a) homopolymers, formed by the joining of a single type of monomer (ex: polyvinyl chloride) and (b) heteropolymers, formed by the joining of two or more different types of monomer molecules (ex: poly (ethylene, propylene) copolymer). Another classification is based on the material properties of the polymer: crystalline and amorphous. However, this classification is rather confusing because polymers can generally be both crystalline and amorphous.

There are no 100% crystalline polymers; some polymers may be 100% amorphous under certain conditions. Polymer chains are arranged in crystallized regions with specific patterns. Since there are no completely crystallised polymers, polymers with more crystalline regions are called semicrystalline polymers. Amorphous polymers are polymers that have no crystalline regions and no uniformly packed molecules. Instead, amorphous regions of a polymer have randomly packed molecules with no sharp melting point.

Key Areas Covered

1. What is an Amorphous Polymer
      – Definition, Features
2. What is a Crystalline Polymer
      – Definition, Features
3. Difference Between Amorphous and Crystalline Polymers
      – Comparison of Key Differences

Key Terms

Amorphous Polymers, Crystalline Polymers, Semicrystalline Polymers

What is an Amorphous Polymer

Amorphous polymers are polymers that are composed of amorphous regions where molecules are randomly arranged. Polymers can be either completely amorphous or mixed with both amorphous and crystalline regions. Amorphous polymers possess widely different mechanical and physical properties owing to their structure and temperature. Below glass transition temperature (T_g), amorphous polymers exhibit glassy, hard and brittle properties. As the temperature is increased, while it passes the T_g, amorphous polymers form cross-links and show elastic properties. T_g is defined as the temperature at which the polymer becomes soft due to the long-range coordinated molecular motion. Natural rubber latex and styrene-butadiene rubber (SBR) are good examples of amorphous polymers below the glass transition temperature.

What is a Crystalline Polymer

Not a single polymer is crystalline because all the crystalline polymers contain considerable amounts of amorphous material. Thus, crystalline polymers are generally called semicrystalline polymers. Crystalline polymers show X-ray diffraction patterns due to the existence of specific partial patterns of molecules in the polymer chains and exhibit a crystalline melting temperature. X-ray diffraction, density measurements and heat of fusion are detected in order to determine the fraction of crystalline substances present in a particular polymer.

Difference Between Amorphous and Crystalline Polymers

Definition

Amorphous polymers are polymers that contain amorphous regions where molecules are arranged randomly, whereas crystalline polymers are polymers with crystalline regions where molecules are arranged in a partial pattern.

Arrangement of Molecules

Amorphous polymers do not have uniformly packed molecules, while crystalline polymers have uniformly packed molecules.

Melting Point

While amorphous polymers do not have a sharp melting point, crystalline polymers have a sharp melting point.

Clarity

Moreover, amorphous polymers are transparent, but crystalline polymers are opaque/ translucent.

Shrinkage

Amorphous polymers have a low shrinkage, while crystalline polymers have a high shrinkage.

Chemical Resistance

Amorphous polymers have poor chemical resistance, but crystalline polymers have good chemical resistance.

Hardness

Furthermore, amorphous polymers are soft, whereas crystalline polymers are hard.

Energy to Melt

While amorphous polymers have low energy, crystalline polymers have high energy. Unlike crystalline polymers, amorphous polymers are brittle and glassy below T_g, while elastomeric above T_g.

Gas Permeability

Amorphous polymers have a high gas permeability, while crystalline polymers have a low gas permeability.

Conclusion

Amorphous polymers do not have uniformly packed molecules, whereas crystalline polymers have uniformly packed molecules. This is the main difference between amorphous and crystalline polymers. In addition, amorphous polymers lack a sharp melting point, are transparent, and have low shrinkage, whereas crystalline polymers exhibit a clear melting point, are opaque or translucent, and experience high shrinkage.