Main Difference – Amorphous vs Crystalline Polymers
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 single type of monomer (ex: polyvinyl chloride) and (b) heteropolymer, 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, namely; crystalline and amorphous. However, this classification is rather confusing because polymers can generally be both crystalline and amorphous. Although 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 the 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. Thus, 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.
This article describes,
1. What is Amorphous Polymer?
2. What is Crystalline Polymer?
3. Difference Between Amorphous and Crystalline Polymers
What is 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 (Tg), amorphous polymers exhibit glassy, hard and brittle properties. As the temperature is increased, while it passes the Tg, amorphous polymers form cross-links and show elastic properties. Tg is defined as the temperature at which the polymer becomes soft due to the long-range coordinated molecular motion. Natural rubber latex, styrene-butadiene rubber (SBR) are good examples of amorphous polymers below the glass transition temperature.
What is 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
Amorphous polymers are the polymers that contain amorphous regions where molecules are arranged randomly.
Crystalline polymers are the polymers with crystalline regions where molecules are arranged in a partial pattern.
Arrangement of Molecules
Amorphous polymers do not have uniformly packed molecules.
Crystalline polymers have uniformly packed molecules.
Amorphous polymers do not have a sharp melting point.
Crystalline polymers have a sharp melting point.
Amorphous polymers are transparent.
Crystalline polymers are opaque/ translucent.
Amorphous polymers have a low shrinkage.
Crystalline polymers have a high shrinkage.
Amorphous polymers have a poor chemical resistance.
Crystalline polymers have a good chemical resistance.
Amorphous polymers are soft.
Crystalline polymers are hard.
Energy to Melt
Amorphous polymers have low energy.
Crystalline polymers have high energy.
Unlike crystalline polymers, amorphous polymers are brittle and glassy below Tg, while elastomeric above Tg.
Amorphous polymers have a high gas permeability.
Crystalline polymers have a low gas permeability.
Unlike amorphous polymers, crystalline polymers exhibit a melting transition temperature (Tm), glass transition temperature (Tg) and crystalline order.