Dear Rajib,

swelling of polyelectrolytes or hydrogels is
investigated e.g. by spectral ellipsometry and x-ray
or neutron-scattering. Dr. Markus Biesalski did such
studies. For publications see his webpage:
http://www.imtek.uni-freiburg.de/cpi/people.php?ls=5&nr=30&pb=1

If you want to study the swelling as a funtion of
humidity; a controlled humidity can be easily obtained
by putting a sample in a closed champer with a
reservoir with a saturated salt solution.

Best regards

Harald
(CSEM Zurich)