I ve been doing some experiment on PNIPAAM dilute aqueous solution. However, when measuring the size of each polymer molecue, the DLS result always shows two peaks, namely a fast mode and a slow mode. I tried filtering the samples with passing through 0.45um membranes, but the slow mode peak still exist. Does anyone has some advice on it? Thanks.
Fast and slow decays physically correspond to small and large sized clusters, respectively, having different rates of diffusion. If you know the hydrodynamic sizes for the corresponding particles, then you can choose your filter cut-off more accurately. Otherwise, it could be contaminants introduced into your system at some point for e.g. dust in the cuvette/pipettes or air bubbles in the dispersion. One can remove air-bubbles by ultrasonicating the cuvette loaded with the dispersion, just prior to measurement. Dust can be avoided by careful sample preparation.
The samples under investigation should be monodisperse Mw/Mn< 1.1. Besides, you should take into consideration the answer of jAbhilash Sugunan.
I wish you every success!
I don't agree with Tatiana - no need for a monodisperse sample.
Instead of filtering, you could try centrifugation. For more viscous samples, I centrifuge directly the sealed cuvettes containing the polymer solution. Do your own tests to find conditions which do not break the cuvettes!
Coming back to filtering: 0.45microm is a large pore for a polymer solution. Maybe you could try 0.2microm or even my favorite Anotop filters, which have 20nm pores. But be careful not to filter out the polymer! And, of course, rinse your cuvettes carefully with filtered water before adding sample.
Finally, slow mode can be due to poorly dissolved polymer so be careful with your procedure for preparing a solution.
General comments on slow modes, not all necessarily relevant here:
Start with pure filtered water, and see that you do not have a mode there. If you do, the mode is in the instrument. Try next small polystyrene or silica spheres, at the photocount rate and hence laser power seen for your polymer. If you have small molecules in your aqueous solution, e.g., NaCl, urea, try the solvent without the polymer. Some "pure" salts are purified with activated charcoal or other such materials that leave microparticles behind -- a good friend, many years ago, spent months before he found this issue.. Prepare your samples. Let them sit for a day or two (an important reason for disposable cuvettes). Sometimes microbubbles form in the filtration process and take a few days to go away. Centrifugation can work. Some people have told me that filling the centrifuge cell with liquid, to match the level of liquid inside the cell, can help. Your mileage may vary. You may need an improved cell cleaning process. An ultrasonic cleaner at the front end may be good. DO NOT try chromic acid (sulphuric acid + potassium permanganate)...it will wreck the cell surface to you permanently have stuff coming off.
The filtration with 200nm would be good start, also measurements at a few different temperatures may give some insight. Certainly keep in mind that this could be self-association, and even if the larger objects are filtered out they could come back. Change pH? There may be reports of similar behavior, for example http://www.ncbi.nlm.nih.gov/pubmed/19618921
http://www.materials-talks.com/blog/2015/02/20/on-the-mystery-of-nanoparticles-in-sucrose/
http://www.materials-talks.com/blog/2016/04/21/aggregation-kinetics-with-dls/
Practical question: How dilute is "dilute"? I have discovered I am sometimes surprised by abbreviations, that turn out not too be the chemical I had assumed. What is PNIPAAM? Is your polymer charged? If so, "dilute" may be hard to reach.
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