Key Difference – Amorphous vs Crystalline Polymers
The word “polymer” can be defined as a material made out of a large number of repeating units which are linked to each other through chemical bonding. A single polymer molecule may contain millions of small molecules or repeating units which are called monomers. Polymers are very large molecules having high molecular weights. Monomers should have a double bond or at least two functional groups in order to be arranged as a polymer. This double bond or two functional groups help the monomer to attach two more monomers, and these attached monomers also have functional groups to attract more monomers. A polymer is made in this way and this process is known as polymerization. The result of polymerization is a macromolecule or a polymer chain. These polymer chains can be arranged in different ways to make the molecular structure of a polymer. The arrangement can be amorphous or crystalline. The main difference between amorphous and crystalline polymers is their molecular arrangement. Amorphous polymers have no particular arrangement or a pattern whereas crystalline polymers are well arranged molecular structures.
CONTENTS
1. Overview and Key Difference
2. What are Molecular Structures of Polymers
3. What are Amorphous Polymers
4. What are Crystalline Polymers
5. Side by Side Comparison – Amorphous vs Crystalline Polymers
6. Summary
What are Molecular Structures of Polymers
It is important to know some facts about the molecular structure of polymers before reading further about the difference between amorphous and crystalline polymers. Polymer chains can be arranged in three ways known as syndiotactic, isotactic or atactic manner. Syndiotactic means the side groups of the polymer chain is arranged alternatively. In isotactic arrangement, side groups are located on the same side. But atactic arrangement shows a random arrangement of side groups along the polymer chain.
What is an Amorphous Polymer?
An amorphous polymer does not have an organized pattern in its molecular structure. Amorphous polymers are made out mainly of atactic polymer chains. This causes absence of crystallinity. Therefore, it is a weak structure. Since the degree of crystallinity is absent or crystallinity is absent is amorphous polymers, they have a low density compared to crystalline polymers. Therefore, chemical resistance is low and is transparent. There are weak attractions between polymer chains due to the absence of a patterned structure.
Examples of amorphous polymers include polyethylene, PVC, etc. The degree of crystallinity is affected by the polymerization and production process. Amorphous polymers may have crystallinity with the formation of crystallites or ordered areas. These are softer and are less resistant to solvent penetration.
What is a Crystalline Polymer?
Crystalline structure shows a regular line-up polymer molecules. Crystalline polymers have an ordered structure which is made out of syndiotactic and isotactic polymer chains. This ordered structure cause the polymer to become translucent. There are also strong attraction forces between molecules. Hence, it is chemical resistant and has a high density compared to amorphous polymers. Although crystalline polymers are well ordered, there can be amorphous areas too. Therefore, these polymers are called semi-crystalline materials.
Plastic material, such as nylon and other polyamides have crystallized structures. Other examples include linear polyethylene, PET (polyethylene terephthalate), polypropylene, etc. These are rigid structures and are less affected by solvent penetration.
Figure 01: Molecular chains in amorphous and semicrystalline polymers
What is the difference between Amorphous Polymers and Crystalline Polymers?
Amorphous vs Crystalline Polymers |
|
| Amorphous polymers are polymers that have no ordered pattern in its molecular structure. | Crystalline polymers are polymers that have a well-organized structure. |
| Morphology | |
| Amorphous polymers are made out of atactic polymer chains. | Crystalline polymers are made out of syndiotactic and isotactic polymer chains. |
| Attraction Forces | |
| Amorphous polymers have weak attraction forces between polymer chains. | Crystalline polymers have strong attraction forces between polymer chains. |
| Density | |
| Amorphous polymers have a low density. | Crystalline polymers have a high density. |
| Chemical Resistance | |
| Amorphous polymers have a low chemical resistance. | Crystalline polymers have a high chemical resistance. |
| Polymer Chains | |
| Polymer chains are arranged in an atactic manner in amorphous polymers. | Polymer chains are arranged in a syndiotactic and isotactic manner in crystalline polymers. |
| Appearance | |
| Amorphous polymers are transparent. | Crystalline polymers are translucent |
Summary – Amorphous Polymers vs Crystalline Polymers
All polymers have some crystallinity which is the main difference between amorphous and crystalline polymers. Amorphous polymers have a low degree of crystallinity whereas crystalline polymers have a high degree of crystallinity. The physical and chemical properties of a polymer will depend on the degree of crystallinity.
Reference:
1. “Types of Polymers.” Chemical Education Division Groups. Purdue University, n.d. Web. 25 May 2017.<<http://chemed.chem.purdue.edu/genchem/topicreview/bp/1polymer/types.html>>
2. “The Basics: Polymer Definition and Properties.” American Chemistry Council. N.p., n.d. Web. 25 May 2017.<<https://plastics.americanchemistry.com/plastics/The-Basics/>>
3. Chanda, M., 2000. Advanced polymer chemistry. 2nd ed. New York: Marcel Dekker
Image Courtesy:
1.”Polymerketten – amorph und kristallinEN” By Polymerketten_-_amorph_und_kristallin.svg: Rainer Ziel.Original uploader was Salino01 at de.wikipediaderivative work: Materialscientist (talk) – Polymerketten_-_amorph_und_kristallin.svg via Commons Wikimedia
It is not necessarily true that amorphous resins are atactic; steric hindrance is utilized to create amorphous plastics of various tacticity. Meanwhile, it is possible for amorphous polymers to provide exceptional chemical resistance (and barrier).