Dynamics of glass forming materials
Arellano, Astrid Karina Torres
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Abstract The liquid dewetting method was used to study the Tg dependence of ultrathin polymer films on thickness and on molecular architecture. Studies of linear polystyrene (PS), 3 arm star PS, short branched and long branched 8 arm star PS on a liquid substrate (glycerol) were performed by using two built dewetting apparatus. Results show that the Tg vs. film thickness for all of the PS films on glycerol are more similar to that of supported on rigid substrate films than for freely standing films. Additionally, a temperature step (T-step) dewetting method which is an extension of the original liquid dewetting technique idea was investigated. In the temperature step method one single polymer sample can provide the same Tg dependence on thickness information that multiple samples of different thicknesses would give. The idea behind the T-step method test is to control the operating temperature settings and use one polymeric thin film sample within an individual experimental run. It was found that linear and 3 arm star PS films have a similar Tg vs. film-thickness dewetting response, while the long and the short branched 8 arm star PS films have a more complex behavior. The dynamic fragility of glass former materials can be obtained by means of calorimetric methods from the Tg dependence on the cooling rate and by rheological methods. Calorimetry is used to investigate reports of extreme fragility values for polysulfone (PSF), bisphenol-A-polycarbonate (PC), and polyvinyl chloride (PVC). The polymers were used as-received and after a wash-precipitation treatment. Results for the fragilities for several materials using both a broad range of cooling rates and a small range of high cooling rates were compared. Results show that the dynamic fragilities of as received and wash-precipitated polymer samples were consistent with each other. The results were not in agreement with C. Evans, H. Deng, W. Jager, J. Torkelson, Macromolecules 2013, 46 (15), 6091-6103 of extremely high fragility for the mentioned polymers. The wash-precipitation treatment does not play an important role in the determination of the dynamic fragility of the polymers. However, the range of cooling rates does have an effect on the magnitude of m. Results show that the dynamic fragility obtained from the broader range of cooling rates tend to give a somewhat lower value of m. Calorimetric and rheological methods were used on D,L-lactic acid to investigate the speculation that this lactic acid should have a very extreme fragility. Results show that the dynamic fragility of D,L-lactic acid and D-lactic acid are not in the range for an extreme fragile material.