Degree of Crystallinity: Definition, Factors Affecting and Determination Methods

What is Degree of Crystallinity?

The degree of crystallinity of the polymer is defined as the fraction of the sample which is crystalline. It can be either expressed in terms of the mass fraction or the volume fraction.

The degree of crystallinity by a volume fraction is given by

Degree-of-crystallinity-by-volume-fraction

Where, 

Xₘ = Degree of crystallinity by mass

V = Specific volume of the sample

Vₐ = Specific volume of fully amorphous polymer

Vc = Specific volume of fully crystalline polymer

Factors affecting the degree of crystallinity

1. Molecular weight

With the increase in molecular weight of the polymer, the degree of crystallinity increases due to a large number of entanglement of the chain which restricts the growth of a crystallite.

2. Symmetry of repeating unit

A symmetrical repeating unit structure like CH₂ makes it easier for the formation of crystallites. Thus, increasing the degree of crystallinity. That's why random copolymers do not crystallize because there is no regularity of the repeating unit.

3. Chain Branching

Branched chains are less likely to crystallize, thus decreasing the degree of crystallinity. For example, low-density polythene has short and long-chain branches. So, it cannot be obtained in a highly crystalline state whereas high-density polypropylene has an almost perfectly linear structure. Thus, it can be obtained in a highly crystalline state.

4. Cross-Linking

A polymer with high cross-linked density has a lower degree of crystallinity due to the presence of a dense array of cross-links that effectively eliminate crystallinity.

5. Secondary Bonding

Both inter and intra-molecular hydrogen bonding play important roles in the Degree of crystallinity of the polymer. With the increased chain-to-chain secondary bonding, there is an increase in the degree of crystallinity of the polymer.

Affect of crystallinity on the properties of Polymer

1. With the increased degree of crystallinity, strength, and stiffness of polymer increase but brittleness also increases.

2. With the increase of degree of crystallinity, solubility, and permeability of polymer decreases.

3. With the increase in the degree of crystallinity, the density and melting point of polymer decreases.

4. With the increase of the degree of crystallinity, the opacity of the polymer also increases.

Determination of degree of crystallinity

There are many ways from which we can calculate the degree of crystallinity including common methods like the density measurement method and Differential Scanning Calorimetry (DSC) method and other methods like X-ray analysis, IR spectroscopy, and NMR spectral analysis.

1. Density measurement method

The density measurement method is one of the most common methods for measuring the degree of crystallinity of semi-crystalline polymers. This technique relies on the differences between the densities of completely amorphous (Pa) and entirely crystalline domains (Pc) of the same polymer. It estimates the degree of crystallinity (Wc) from the densities of the real specimen (Ps).

Assuming that the volumes of the crystalline and amorphous domains are additive, we can write the formula to determine the degree of crystallinity. To get the percentage of the degree of crystallinity, we will simply multiply it by 100.

Percentage-of-degree-of-crystallinity-formula

2. Differential scanning Calorimetry (DSC)

Differential Scanning Calorimetry (DSC) provides a quick method for determining the degree of crystallinity of a polymer-based on the heat required to melt the polymer.

DSC is a technique which measures heat flow into or out of a polymeric material as a function of time or temperature. So, to calculate the degree of crystallinity of a polymer, specific heat of fusion in J/g is determined from the DSC curve peak area and enthalpy of fusion of 100 % crystalline material is taken from scientific databases. Then, both values are put into the formula given below:

Determination-of-degree-of-crystallinity-by-DSC-method

Procedure

1. Take a polymer sample of polyethylene terephthalate of 15 mg weight and clean it and dry it.

2. Place the polyethylene into the sample pan whereas the reference pan is to be kept empty. Both pans are placed on the raised platforms on sensors.

3. Log into the computer and start the DSC 2920 system by pressing the power button on the instrument.

4. A heating rate of 5 ℃/min was used and nitrogen gas is pumped into the system at the rate of 35 ml/min.

5.Now, the sample of polythene is analyzed over the temperature range from 20 ℃ to 180 ℃ then it is cooled from 180 ℃ at 5 ℃/min.

6. After analyzing the sample, DSC software will give peak and curve peak area which will be used to calculate the degree of crystallinity.


A-DSC-plot-for-a-polyethylene-terephthalate-(PET)-sample


Popular Posts

Conducting Polymers: Definition, Examples, Properties and Applications

Image
What are Conducting Polymers? As the name suggests organic polymers that conduct electricity are known as conducting polymers. They are also known as intrinsically conducting polymers (ICPs) and they have alternating single and double bonds along the polymer backbone (conjugated bonds) or that are composed of aromatic rings such as Phenylene, naphthalene, anthracene, pyrrole, and thiophene which are connected through carbon-carbon single bonds. Examples: Polyacetylene, Polypyrrole, Polyaniline, etc What is the reason behind conducting nature of these polymers? Conducting polymers comes in two forms that are doped conducting polymers and non-doped conducting polymers. The conductivity of non-doped conjugated polymers is due to the existence of a conductivity band similar to a metal. In a conjugated polymer, three of the four valence electrons form strong sigma bonds through sp² hybridization where electrons are strongly localized. The remaining unpaired electron of each carbon atom re...
Read more

Crime Scene: Definition, Types and Characteristics

Image
What is a Crime Scene ? A place where the crime is committed or where the maximum physical evidence related to crime is found is known as a crime scene. A crime scene is a starting point of the investigation which provides information about the suspect and the victim. This helps to reconstruct the crime and fast resolution of the case. It is noted that the crime scene is not limited to a single place but may extend to a wider area depending upon the nature of the crime committed. For example, In a murder case where murder is done at one place and the body is disposed on another place. In this case, we have two crime scenes that give information about the crime. Types of Crime Scene Based on evidence found on the crime scene: 1. Primary Crime Scene The crime scene where the actual crime occurred or where more usable pieces of evidence were found is known as the primary crime scene. For example, A murder scene, theft, assault, etc. 2. Secondary Crime Scene The crime scene which is some...
Read more

Documentation of the Crime Scene: Step by Step

Image
What is documentation of the crime scene and Why it is done? After securing the crime scene, the next most important step is documentation of the crime scene because documentation permanently records the crime scene and its physical evidence. Documentation should be done without any wastage of time as the scene and its evidence may get altered over time. The documentation should be done innovatively and the originality of the crime scene should be maintained and documentation should not have pauses or breaks, it should remain constant till the whole crime scene is documented. There are four main tasks of documentation that is note-making, photography, videography , and sketching . It should be noted that all four tasks are necessary and none is a substitute for another. For example, notes are not substitutes for photography, video is not a substitute for sketching, etc. Following are the sequence for crime scene documentation: 1. Note making Documentation of crime scene in the f...
Read more