PDI is described as Mw/Mn. Mw can only be larger or equal to Mn, so it is always equal to or greater than 1.

If you go logicall Mw is the weighted average and Mn is the number average. Take Mn as you balls moving inside the neck bottle in which you are only concerned about the number of balls moving and in case of Mw you are concerned about the size of balls moving irrespective of size of the balls. So i am not able to think logically and mathematically also there are discrepancies.

according the PDI formula, weight average molecular weight always bigger than number average molecular weight

I am not expert but I have seen many data with less than 1 PI in nanoparticles. You may need to check more about nanoparticles. I am also trying to find out why is less than 1 when it says it should be equal or higher than 1 according to definition.

By definition, PDI = weight average molecular weight / number average molecular weight. It is a measure of the breadth of molecular mass distribution and it is called the heterogeneity index indicating that the synthesized macromolecules come out in different chain lengths in most cases.So this ratio is >1. Monodisperse polymers are rare and include specific proteins & nucleic acids. Many natural polymers are polydisperse, e.g. cellulose & natural rubber.

Living polymers, made by anionic polymerization, could have PDI in the range 1.01- 1.05 & hence considered to be monodisperse (approximately).

PDI (or dispersity Ð) is simply defined to be a ratio that is always > or = 1. The definition is Mw/Mn. Mw is a weight average molecular weight for the sample: Mw=(Σ M^2 N)/(Σ M N) where N is the number of polymer chains with mass M. For a sample Mw represents the molecular weight above and below which there is an equal MASS of polymer chains. Mn is a number average molecular weight for the sample: Mn=(Σ M N)/(Σ N) and represents the molecular weight above and below which there is an equal NUMBER of polymer chains.

From the definitions you can see that Mw is always > or = Mn, so PDI is always > or = 1

Dear Mathew you are right, even i totally understand it my this mathematical defination but i am having promlem when it comes to physically perceive it.

Hi Shashank. This is how I visualize Mn and Mw.

There are an even number of repeat units (balls) above and below Mw, and an even number of chains (connected balls) above and below Mn.

**Formula for PDI = Averaged Molecular weight (weight average mode)/ Average Molecular weight(number average mode).

Both Numerator and denominator are Molecular weights but different way of statistical average such as weight in first and number in later case.

I got PDI for microspheres 13.28, it is correct value or not? please tell me.

Hi,

In Gel Permeation Chromatography and Size Exclusion Chromatography we are interested in the molecular weight of the sample. The distribution obtained from these techniques is typically a molecular weight distribution describing how much material there is present of the various molecular weight “slices.”  The distribution is traditionally described by two numbers derived from it:

• Mw – the mass weighted molecular weight  and
• Mn – the number weighted molecular weight

However, When using Dynamic Light Scattering technique the size distribution of molecules or particles is the property of interest.  The distribution describes how much material there is present of the different size “slices.”  In DLS, the native distribution is the intensity distribution which indicates how much light is scattered from the various size “slices” or “bins.” The mean size and the standard deviation from that mean can be obtained directly from the statistics of the distribution. Here, the (absolute) standard deviation (or “halfwidth”) of the distribution can be compared to the mean, and a relative polydispersity = standard deviation / mean can be obtained. Historically, instead of requiring a distribution, a simpler forced single exponential fitting scheme (the cumulant method) has been used to find an overall mean size (by intensity) and an overall polydispersity (the normalized second cumulant). For a theoretical Gaussian distribution the overall polydispersity would be the relative polydispersity of the distribution. Traditionally, this overall polydispersity has also been converted into an overall polydispersity index PDI which is the square of the light scattering polydispersity. For a perfectly uniform sample, the PDI would be 0.0

http://www.materials-talks.com/blog/2014/10/23/polydispersity-what-does-it-mean-for-dls-and-chromatography/