I have synthesized PNIPAM microgels, and it undergoes a transition at around 32 degrees. When I do DLS of these microgel particles, the intensity(counts) is more for dehydrated state(after 32 degrees) than the swollen state(before 32 degrees). Shouldn't the counts be more for dehydrated state as less number of particles, so more scattering?
There are also other possibilities - particles may start aggregating, even settling, and the larger aggregates may cause more scattering.
Aggregation should be detectable by comparing the hydrodynamic radius below and above 32°C. However, one should be careful in comparing intensities. First of all, the scattering angle should be stated. It might be that you measure close to a form factor minimum, giving rise to intensity variations against "expectation". Here an angular dependent measurement (SLS) migh help to determine the form factor at each temperature. Further I recommend to extrapolate the scattering intensity to scattering angle 0. If no aggregation occurs, this scattering intensity should be basically the same below and above 32°C (slight variations in intensity might be caused by slight variations of the dn/dc value with temperature)....
The answer will to some degree depend on the size of your particles. Naively, the intensity should not change upon deswelling as both the number density of particles and the molecular weight don't change, and also the scattering contrast should not change as the dn/dc of the hydrated polymer does depend weakly on temperature only. However, if your microgels are big enough, then upon deswelling you probably go from simple Rayleigh-Gans-Debye scattering to Mie scattering (as the overall index of refraction of the swollen or deswollen particles are quite different), and this then results in a change of the scattering cross section of the particles and intensity of the suspension. Your observation is in fact standard for such systems.
If a particle (microgel) swell the scattering intensity remains the same at zero scattering angle. On swelling the mass concentration remains the same.
We write the Rayleigh scattering as proportional to (dn/dc)^2 *C*M *P(Q)
In actual fact all three proportionality factors (dn/dc)^2 *C*M remain the same on swelling. The mass concentration does not change!
What changes however is the form factor P(Q). If the measurement is at finite angles P(Q) will fall of faster with Q for a swollen particle and therefore the scattering intensity goes down at finite Q on swelling (Not at Q >>>0)
See e.g International Dairy Journal 18 (2008) 556–565 by Huppertz
for more details see: Biomacromolecules 2007, 8, 1300-1305 Huppertz et al
You may also run a SLS to confirm the MW and radius of gyration of the PNIPAM.
A quick response to Kees comment: You are absolutely right within the RGD picture, there it would only be the form factor that changes, and this is also what one observes in neutron scattering, where the contrast and the forward scattering I(0) does not change with temperature, while the angular denounce described by P(Q) does change. This is however not the case for sufficiently large microgels (say radii of a few hundred nanometer). Here in the swollen case they still fulfil conditions of RGD scattering, while in the collapsed state they are much denser and can no longer be treated within RGD, but require Mie scattering. This then results in a change of scattering contrast, and changes not only the angular dependence (P(Q)), but also the forward scattering, overall transmission or turbidity, which would not change in the RGD picture as the integrated intensity would still be the same.
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