The Tg values detected by DSC at two different heating rates of 5 ºC/ min and 10 ºC/ min
were presented in Table 2. At heating rate of 5ºC/ min, the Tg value seems to increase
accordingly except for SPSU1 membrane. Meanwhile the Tg value of PSU at 10ºC/ min
heating rate were found much lower than the Tg value achieved at 5ºC/ min. This slight
decrease in Tg value might be due to a trace amount of organic solvent remaining in the
membrane sample during casting which shift Tg to lower temperatures. Similar observation
might be seen at slow heating rate of 10ºC/ min, where the Tg values of SPSU samples were
found to increase by 50% but surprisingly, sudden decline of Tg value for SPSU4 samples
were being observed. This rapid drop off Tg value might be due to the abovementioned similar
effects. One of the plausible cause that also contribute to this drop off temperature for SPSU4
was that polymers having the hydrophilic components such as hydroxyl or amide groups form
intermolecular bonds in the presence of water which strongly affect the characteristic of the
glass transition. Introducing a small amount of water to a hydrophilic polymer may disrupt the
intermolecular bonds, thereby enhancing the main-chain motion. In this case Tg shifts to lower
temperatures in presence of water 14. The water content studies conducted using DSC at
similar heating rate supported this phenomenon