Editorial Feature

Calculating the Polydispersity Index of a Polymer

The polydispersity index (PDI) is an important statistic for determining the breadth or dispersion of grain size distribution. This article explores the analysis of the polydispersity index of a polymer or polymeric materials.

molecular mass, PDI, polydispersity index, polymers

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Realistic PDI values may range from 0 to 1, with granules with PDIs less than 0.1 implying narrow size distribution and suspended particles with PDIs greater than 0.1 implying highly dispersed pore size distribution.

Basic Idea Behind PDI

Biomolecules are the "giant" structures that are generated when tiny organic compounds (monomers) combine to form complex molecules in a biological milieu. The basic biopolymers, which are composed of homologous sequences with a varied amount of monomer units, are responsible for the molar mass dispersion of macromolecules. Their mean data are divided into two categories: the average molar mass of the molecule and the weight-average molar mass of the particle. The Poly Dispersity Index is measured as the proportion of these average numbers.

It is a well-known fact that molecular mass and average molecular weight are two important aspects of monomers that influence their properties. We also recognize that the polydispersity index (PDI) is used to estimate the width of a polymer's degree of polymerization. Generally, it is believed that a polymeric specimen with a greater PDI would have a "wider" molecular mass, as indicated by a greater Sn.  PDI provides data on Sn/Mn, but not on Sn, which is a more accurate indicator of the breadth of a distribution curve. Here Mn is the number of the average molecular weight, while Sn is the number average of standard deviation.

Consider two polymeric materials with the same PDI, say 2, but with Mn values of 10 000 and 100 000 that are different. In this case, the Sn of the 2 specimens is 10 000 and 100 000, correspondingly. The fact that the polymeric specimens in the preceding table have the same PDI but have radically different degrees of separation is self-explanatory. According to this comparison, if specimens have differing Mn values, the specimen with the higher PDI may not have the bigger Sn value.

The molecular mass dispersion of a polymer substance is directly connected to the majority of its qualities. Traditionally, the dissolving of the specimen is required for the standard analytical procedures for estimating the molecular weight dispersion (MWD). Certain polymers, on the other hand, are either hard to break down or completely intractable in conventional solvents. Aside from that, if the existing technology does not have a gauge for measuring precise molecular mass, reference polymers must be used to recalibrate the apparatus.

However, other viscoelastic approaches are not affected by these issues at all. In terms of the molecular characterization of polymer composites, the rheological approach seems to be very strong. Likewise, many recent studies have focused on defining the link between polymeric rheological knowledge and molecular weight distribution in order to better understand polymer behavior.

Polydispersity Index of a Polymer

Often, it is not critical to identify the entire molecular mass in order to describe the substance from a molecular standpoint in technological applications; instead, understanding the molecular mass and the polydispersity index is adequate. The average molecular weight (Mw) and the polydispersity index are the two measures that are most often utilized. A typical measure of entangled densities among chains inside a substance is zero-shear rate viscosity, which is very sensitive to the length of the structure in the substance under consideration.

The stable conformance of melt elasticity is a factor that describes the network’s flexible deformability among attachments in relation to their static equilibrium location, and it is defined as the elastic energy that accumulates during flow and is recovered when the tension is eliminated. It is also measured by this parameter. As a result, it is very sensitive to the presence of high molecular weight fractions in the substance.

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The tube and reptation approach developed by de Gennes is the foundation for the majority of the chemical designs used to deduce the molecular mass of sample mixture from linearly elastic measurements. Although it is possible to add polydispersity into these systems, doing so has proven to be problematic since the influence of the polydisperse surroundings on the relaxation durations of the separate networks must be taken into consideration.

For polydisperse solid samples, one of the most effective ways to forecast their MWD is the double reptation mixing rule, which may be derived either directly or indirectly from the entangled notion or the double notoriety process, accordingly. The double reptation hypothesis has lately been improved by other writers, who have combined tube renewal with the double reptation process of the chains to provide a more refined version of the idea.

Earlier this year, a recent publication described and analyzed the molecular framework that was employed in the current investigation. This theory, like the vast majority of chemical models reported in the literature, is relevant to polymer electrolytes when all of the links are large enough to create an entangled copolymer, which is the scenario in most cases.

Latest Advances

In the latest study by Mr. Llorens presented in the journal Polymer, the Mellin transformation of the relaxation modulus of a basic molecular rheological model is used. One of the most important aspects of this method is that it allows for the immediate extraction of valuable MWD data from dynamic shear studies. For a broad range of relative molecular weights and polydispersity values, the technique demonstrates excellent concordance between the suggested conceptual framework and the practical polydispersity index values of numerous polymeric materials. It also applies to binary mixes. 

Further Reading

Danaei, M., Dehghankhold, M., Ataei, S., Davarani, F. H., Javanmard, R., & Dokhani, A. (2018). Impact of Particle Size and Polydispersity Index on the Clinical Applications of Lipidic Nanocarrier Systems. pharmaceuticals. https://www.mdpi.com/1999-4923/10/2/57

Llorens, J., E. R., & Marcos, R. M. (2015). POLYDISPERSITY INDEX FROM LINEAR VISCOELASTIC DATA: UNIMODAL. University of Barcelona. Available at:https://www.sciencedirect.com/science/article/pii/S0032386103000223?via%3Dihub

Shrivastava, A. (2018). Introduction to Plastics Engineering. In A. Shrivastava. Applied Science Publishers.

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Ibtisam Abbasi

Written by

Ibtisam Abbasi

Ibtisam graduated from the Institute of Space Technology, Islamabad with a B.S. in Aerospace Engineering. During his academic career, he has worked on several research projects and has successfully managed several co-curricular events such as the International World Space Week and the International Conference on Aerospace Engineering. Having won an English prose competition during his undergraduate degree, Ibtisam has always been keenly interested in research, writing, and editing. Soon after his graduation, he joined AzoNetwork as a freelancer to sharpen his skills. Ibtisam loves to travel, especially visiting the countryside. He has always been a sports fan and loves to watch tennis, soccer, and cricket. Born in Pakistan, Ibtisam one day hopes to travel all over the world.


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    The term "Polydispersity Index" (PDI) is outdated .  IUPAC has replaced PDI with the term DISPERSITY.

The opinions expressed here are the views of the writer and do not necessarily reflect the views and opinions of AZoM.com.

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