10-30 mins is my guess. if you want the effect to stay a long time, store the oxidized PMMA surfaces *under water* then the hydrophilic groups will stay in contact with the water molecules at the surface as described by Koberstein and coworkers.
The duration of the effect of plasma treatment depends on the temperature and environment where the sample is placed. The treated surface is temporarily hydrophilic, yet air is quite hydrophobic. Therefore, if the PMMA chain has mobility (it does not have as large a unit of glass transition movement. It can be local motion such as beta transition) the hydrophilic portion will hide inside the bulk and hydrophobic portion will be re-exposed. Therefore, you can keep your plasma-treated sample either at temperatures below the beta transition of PMMA (between 0 and 50oC) or with high humidity environment, including the extreme case of liquid water. Thus, the longevity of the plasma surface treatment all depends on polymer chain mobility (influenced by temperature) and environment. It is generally said that surface treated ultrahigh modulus polyethylene fiber loses its surface treatment effect in about two weeks.
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