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Difference Between Amorphous and Crystalline Polymers

May 26, 2017 Posted by Madhu

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.

Difference Between Amorphous and Crystalline Polymers

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

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Filed Under: General Chemistry Tagged With: Amorphous and Crystalline Polymers Differences, Amorphous Polymers, Amorphous Polymers Definition, Amorphous Polymers Features, Amorphous vs Crystalline Polymers, Atactic, Compare Amorphous and Crystalline Polymers, Crystalline Polymers, Crystalline Polymers Definition, Crystalline Polymers Features, Isotactic, Molecular Structures of Polymers, Polymer Structure, Semicrystalline, Syndiotactic

About the Author: Madhu

Madhu is a graduate in Biological Sciences with BSc (Honours) Degree and currently persuing a Masters Degree in Industrial and Environmental Chemistry. With a mind rooted firmly to basic principals of chemistry and passion for ever evolving field of industrial chemistry, she is keenly interested to be a true companion for those who seek knowledge in the subject of chemistry.

Comments

  1. TOPAS says

    January 22, 2018 at 9:13 pm

    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).

    Reply

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