Title: The effects of polymer chain mobility on the fragmentation mechanims of ToF-SIMS
PI: Chi Ming Chan; Co-I: Lu-Tao Weng and Kai Mo Ng
Time-of-flight secondary ion mass spectrometry (ToF-SIMS) is very useful in surface characterizations of polymers. ToF-SIMS can provide structural information of various polymers. Another very useful feature of this technique is ToF-SIMS imaging, which can provide spatial distribution of different species on a surface. The formation mechanisms of the secondary fragments under ion beam bombardment are very complex. Up to now, the fragmentation mechanisms are still not fully established. A good understanding of the relationship between the secondary ion structures and polymer chain structures is very important to the applications of ToF-SIMS to problems of polymer identification and quantification.
One of the possible ways for us to develop a better understanding of the SIMS fragmentation process is to examine the fragment ions of a polymer as a function of temperature. This experiment can easily be performed in a spectrometer which is equipped with a heating and cooling stage. It is reasonable to suspect that an increase of temperature could promote the formation of certain ions. A careful examination of the distribution of the fragment ions as a function of temperature may provide us a deeper understanding of the fragmentation process.
It is well known that the mobility of a polymer changes significantly as it goes through its glass transition temperature and melting point. We have obtained preliminary ToF-SIMS data of polystyrene at temperatures ranging from room temperature to about 160 oC. The data suggest that there are two temperature transitions. The low and high temperatures seem to be related to the glass transition temperature and the degradation temperature of the polymer respectively. Both theoretical and experimental studies on amorphous polymers have shown that the polymer chains at or near a surface behave differently from those of the bulk because of the enhanced surface mobility. Most of the studies that indicated the existence of a low-Tg layer at the surface were based on the empirical measurements of Tg by various techniques. It would be an exciting experiment to determine if we can use temperature-dependent ToF-SIMS data to locate the surface glass transition temperature of a polymer.