Samples Exposed to High Humidity Instead of Water Immersion

SAMPLES EXPOSED TO HIGH HUMIDITY (INSTEAD OF WATER IMMERSION) 

The fact that the very fast disappearance of surface fluorine atoms on water immersion was observed for the highly crystalline cellulose rather than the amorphous cellulose acetate suggests that the hydrophilicity or swelling capability (by water) of a polymer is more important than the degree of crystallinity. 

The changes observed with water vapor-exposed films are much slower than those for liquid water-immersed films. The time scale is in order of days (minutes for water-immersed films), and the change levels off at 70% (cellulose) and 85% (cellulose acetate). 

These experiments with water vapor exposure also confirm that the decrease of fluorine atoms observed by XPS is indeed mainly due to the migration of fluorine-containing segments away from the top surface region because no washing can occur in these experiments. 

It is important to emphasize that the rate of change in water vapor (20°C) is roughly four orders of magnitude (1/10,000) smaller than that in liquid water (40°C). Within the time scale of a liquid water immersion experiment, no change occurred in the case of water vapor. This is in accordance with the difference of liquid water and water vapor in the y interaction discussed earlier. 

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