I'm trying to make a case for purifying a set of polymers using published extraction methods. The polymers I'm interested in have similar physicochemical properties to what has been published, but the published polymers are much higher molecular weight (around 100 kDa), compared to what I'm interested in (10-20 kDa).
I've done some fairly extensive searching, but haven't had any luck yet. I'm just looking for any citations that (hopefully) provide support of polymer molecular weight not having a huge impact on extractability or solubility in a given organic solvent. The polymer types I'd be most interested in would be similar to PNIPAM, PEG, or PLA.
Thanks!
Hi Ross, Generally speaking, the larger the molecule the longer/harder to solubilize it. at least in the case of natural polymer which I have worked with. but parameters such structure of polymer chain, cross-linking within the polymer chain and temperature are very important.
Molecular weight for sure affect the solubility. Please have in mind that solubilizing polymers is usually slow process compared to pure materials. You may need to heat for hours.
When solubility of the polymer in a given solvent is low, then it decreases exponentially with the molecular weight. Hence the larger polymers would have a exp(10) times lower solubility than the smaller. This implies that polymers are only soluble in solvents that do very closely match their solubility parameter (very similar chemistry). When the solvent is very good the exponential factor does not hold. The physics becomes much more subtle. However, if a solvent is good for the extraction of the larger molecules, it will be also good for the smaller. If you are unsuccessful you should indeed check problems such as purity, temperature...
Regards, Mico
High molecular weight polymers dissolve in two stages. First the polymer is swelled by the solvent then the solvent swelled polymer dissolves. This process takes longer as MW goes up. If the quantities are not too large putting the solvent-polymer mixture in a sonicator can speed up the process significantly.
Thanks, everyone, for the feedback and some of the references!
It seems as though the general trend is that a lower MW polymer is more soluble than a higher MW verision of the same material in the same solvent. If anyone happens to have any good citations/papers showing a trend of this data with an example polymer, that would be great, but I should otherwise have enough move forward. Thanks again!
Dear all,
the increase in molecular weight in most cases leads to a decrease in solubility due maily to two reasons:
the first is that the increase in molecular weight increases the cohesive energy density (CED), where, according to J. Hildbrand the solubility parameter is the square root of the CED.
the second is the arising of entanglements with increasing MW after a threshold value of MW (somhow a form of physical crosslinks). Regards
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