Friday, July 19, 2013

Molecular weight of polymers?

(image from http://www.putus.org/News/2009/0121/13.shtml)
Mw: Weight-average molecular weight
Mn: Number-average molecular weight
Mp: Peak molecular weight
PDI: Polydispersity, PDI= Mw/Mn

Note when polymers are made there is always a range of molecular weights generated with some chains shorter and others longer which generally fits into a normalized curve. For historical reasons molecular weight is typically described as weight average molecular weight (Mw) or number average molecular weight (Mn) based on whether the number of chains at a given MW or the weight of the chains at a given MW is considered in the averaging process. The Mn is always lower than the Mw and these two divided give the polydispersity index (PDI). For most polymers purchased from PolyScitech analysis is performed via GPC so this data is made available to customers.
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