I'll try to answer this succinctly:
When the polymer is below LCST, water in the first hydration shell forms relatively strong hydrogen bonds with the amide group of PNIPAAm, resulting in solubility. As the temperature increases, movement of the water molecules makes the hydrogen bonding of the amide to the polymer less energetically favorable than than amide-amide hydrogen bonding. When the amide-amide hydrogen bonds form, water is excluded from the first hydration shell, and the polymer precipitates due to the exposure of the hydrophobic backbone and isopropyl group. From a thermodynamic perspective, we have a negative deltaH from hydrogen bonding with water. We also have a negative deltaS, due to the degree of order formed by the hydrogen bonds, and the hydrophobic effect of the polymer's backbone and isopropyl group. So, when the temperature is raised, the entropy term begins to dominate and resulting in a positive deltaG, corresponding to the precipitation of the polymer.
I hope that helps.
I'll try to answer this succinctly:
When the polymer is below LCST, water in the first hydration shell forms relatively strong hydrogen bonds with the amide group of PNIPAAm, resulting in solubility. As the temperature increases, movement of the water molecules makes the hydrogen bonding of the amide to the polymer less energetically favorable than than amide-amide hydrogen bonding. When the amide-amide hydrogen bonds form, water is excluded from the first hydration shell, and the polymer precipitates due to the exposure of the hydrophobic backbone and isopropyl group. From a thermodynamic perspective, we have a negative deltaH from hydrogen bonding with water. We also have a negative deltaS, due to the degree of order formed by the hydrogen bonds, and the hydrophobic effect of the polymer's backbone and isopropyl group. So, when the temperature is raised, the entropy term begins to dominate and resulting in a positive deltaG, corresponding to the precipitation of the polymer.
I hope that helps.
PNIPA shows LCST at 32C. Below LCST it absorbs water molecules, do these H2O molecules chemically attached there or just entraped ?
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