I have a result from DSC thermal analysis of lignin with 180 degree centigrade glass transition , This lignin has a molecular weight less than 1,000 . How do I ensure the accuracy of the data. While lignin with low molecular weight should have a low glass transition temperature.
Your right the lignin should show low glass transition temperature.
You can confirm the Tg with DMA.
A decrease in Molecular weight will not necessarily reduce the Tg, depending on how high molecular weight you specimens are. The relationship is typically compared using the Flory–Fox equation.
"In polymer chemistry, the Flory–Fox equation is a simple empirical formula that relates molecular weight to the glass transition temperature of a polymer system. The equation was first proposed in 1950 by Paul J. Flory and Thomas G. Fox while at Cornell University."
To ensure accuracy of data- make sure your Tg is on 2nd heating and do repeats.
Wikipedia gives a nice summary of lignin, calling it a complex, cross-linked phenolic material, without a defined primary structure: https://en.wikipedia.org/wiki/Lignin
The picture offered for the structure suggests that the material is highly branched and crosslinked, and contains a variety of functional moieties that could interact to affect various physical properties.
First, I'd like to see the actual curve you obtained from DSC. The previous respondents have raised several important points: the need to differentiate Tg from Tm and T-decomposition. In addition, because of the strongly polar nature of lignin (caused by separation procedures which commonly produce lignosulfonates), there are other possible complex associations that can have their own "transition" temperatures (cf. ionomers). In short, we need to first determine what the material is that you're calling "lignin."
My next concern is that at a MW of around 1000 (how measured?), you are not likely to be measuring anything at all related to polymeric structure. Commonly, we see MW effects in high polymers on Tg up to around 50,000 MW (via light scattering in solution), after which it levels off. However that number varies considerably among different polymer types. At 1000 you are measuring molecular properties, and possibly polar or ionic properties. I couldn't even claim oligomeric behavior at this low a MW.
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