Thanks for your attention.
I have some question about the expansion mechanism of the hydrogel film, like expansion behaviors of the dry hydrogel in the saturate vapor pressure condition.( Macromolecules, 41(9), 3209–3218. http://doi.org/10.1021/ma7027775)
There was already some discussion about the expansion of hydrogel in water solution:
In the solution condition, some chemical group in the polymers, like carboxylic acid or amine, would be hydrolysed with the water. Then the changed group could repel each other and the polymers would be forced to expand.
However, i cannot make sure whether this theory can be applied in the previous paper. In this paper, the expansion mechanism is not detailed. But it have pointed "pure PNIPAM homopolymer films miss the possibility of internal cross-links to build up a gel". I'm confusing about this point. For the thin film, dose it mean that the pure PNIPAM homopolymer film cannot form the hydrogel structure, or lose the expansion ability? If the discussion about the expansion mechanism, as noted above, is true, i think the cross-links structure would not be necessary for the expansion in the thin film condition.
Anyway, i'm just the beginner in this field. If possible, could you share me some discussion about this topic?
Thanks very much for your time.
Dear Liu
The hydrogel is a crosslinked polymer which its functional groups are different depend on the application. For example for hydrophilic hydrogel with higher expansion, there are so many hydrophilic functional groups which can interact with water lead to expansion. The amount of expansion is swelling index which related to the degree of crosslinking in polymer network and kind of functional groups.
Dear Reze
Thanks very much for your answer. And I still have some questions about this discussion.
How to prepare the cross-linked polymer(hydrogel)? For the cross-link, one kind would be chemical interaction, and the other would be the physics interaction. I have the question about the physics cross-link. If i choice the spin coating for the polymer thin film preparation, which factor will affect the cross-link degree? The ended group of the polymer, or other factors? For example, if i just use the pure PNIPAM (C6H11NO)n dilute solution, without any ended modification, to prepare the film by spin coating, it is possible to build the cross-link, like the gel? If the film is not the gel, does it possible to absorb the water and swell with the hydrophilic functional group?
Thanks very much for your time.
role of crosslinking is to prevent full dissolution. Also, the higher degree of crosslinking, which limits swelling mechanically, the smaller rate and extent of swelling. If you keep saturated vapor conditions for long enough time hydrophilic polymer will eventually dissolve. If you start with saturated vapor conditions hydrated polymer film will get in equilibrium with a defined vapor pressure.
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