Tacticity as a factor contributing to the thermal stability of polystyrene

TGA has been used to study the thermal degradation of PS in atactic (aPS), syndiotactic (sPS), and isotactic (iPS) forms. Thermal stability of sPS is similar to that of aPS, whereas iPS degrades at a notably higher temperature. TGA-FT-IR data do not show any obvious difference in the degradation products of the three materials. Isoconversional kinetic analysis of non-isothermal TGA data yields somewhat higher activation energy for degradation of iPS than for aPS that is found to result in a smaller rate constant for iPS. Another important factor contributing to slower degradation rate of iPS is found to be the reaction model that has been experimentally determined from isothermal TGA measurements. The enhanced thermal stability of iPS correlates with its hindered molecular mobility that appears to originate from intramolecular confinement. The hindered mobility has been detected by comparing the glass transition dynamics in amorphous iPS and aPS samples. By using multi-frequency temperature modulated DSC (TOPEM (R)) it has been demonstrated that the activation energy of the glass transition is noticeably larger for iPS than for aPS that supports the hypothesis of the slowed down molecular mobility.